Cone photoreceptor types in zebrafish are generated by symmetric terminal divisions of dedicated precursors

Cone photoreceptor differentiation regulated by thyroid hormone transporter MCT8 in the retinal pigment epithelium

The key role of a thyroid hormone receptor in determining the maturation and diversity of cone photoreceptors reflects a profound influence of endocrine signaling on the cells that mediate color vision. However, the route by which hormone reaches cones remains enigmatic as cones reside in the retinal photoreceptor layer, shielded by the blood-retina barrier. Using genetic approaches, we report that cone differentiation is regulated by a membrane transporter for thyroid hormone, MCT8 (SLC16A2), in the retinal pigment epithelium (RPE), which forms the outer blood-retina barrier. Mct8-deficient mice display hypothyroid-like cone gene expression and compromised electroretinogram responses. Mammalian color vision is typically facilitated by cone types that detect medium-long (M) and short (S) wavelengths of light but Mct8-deficient mice have a partial shift of M to S cone identity, resembling the phenotype of thyroid hormone receptor deficiency. RPE-specific ablation of Mct8 results in similar shifts in cone identity and hypothyroid-like gene expression whereas reexpression of MCT8 in the RPE in Mct8-deficient mice partly restores M cone identity, consistent with paracrine-like control of thyroid hormone signaling by the RPE. Our findings suggest that in addition to transport of essential solutes and homeostatic support for photoreceptors, the RPE regulates the thyroid hormone signal that promotes cone-mediated vision.

The generation gap in endocrine disruption: Can the integrated fish endocrine disruptor test (iFEDT) bridge the gap by assessing intergenerational effects of thyroid hormone system disruption?

Thyroid hormones (THs) act early in ontogenesis, even prior to the differentiation of thyrocytes. Maternal transfer of THs is therefore known to play an essential role in early development. Current OECD test guidelines for the assessment of TH system disruption (THSD) do not address inter- or transgenerational effects. The integrated fish endocrine disruptor test (iFEDT), a test combining parental and developmental exposure of filial fish, may fill this gap. We tested the ability of the iFEDT to detect intergenerational effects in zebrafish (Danio rerio): Parental fish were exposed to propylthiouracil (PTU), an inhibitor of TH synthesis, or not exposed. The offspring was submitted to a crossed experimental design to obtain four exposure scenarios: (1) no exposure at all, (2) parental exposure only, (3) embryonic exposure only, and (4) combined parental and embryonic exposure. Swim bladder inflation, visual motor response (VMR) and gene expression of the progeny were analysed. Parental, but not embryonic PTU exposure reduced the size of the swim bladder of 5 d old embryos, indicating the existence of intergenerational effects. The VMR test produced opposite responses in 4.5 d old embryos exposed to PTU vs. embryos derived from exposed parents. Embryonic exposure, but not parental exposure increased gene expression of thyroperoxidase, the target of PTU, most likely due to a compensatory mechanism. The gene expression of pde-6h (phosphodiesterase) was reduced by embryonic, but not parental exposure, suggesting downregulation of phototransduction pathways. Hence, adverse effects on swim bladder inflation appear more sensitive to parental than embryonic exposure and the iFEDT represents an improvement in the testing strategy for THSD.

Male germ cell-associated kinase is required for axoneme formation during ciliogenesis in zebrafish photoreceptors

Vertebrate photoreceptors are highly specialized retinal neurons that have cilium-derived membrane organelles called outer segments, which function as platforms for phototransduction. Male germ cell-associated kinase (MAK) is a cilium-associated serine/threonine kinase, and its genetic mutation causes photoreceptor degeneration in mice and retinitis pigmentosa in humans. However, the role of MAK in photoreceptors is not fully understood. Here, we report that zebrafish mak mutants show rapid photoreceptor degeneration during embryonic development. In mak mutants, both cone and rod photoreceptors completely lacked outer segments and underwent apoptosis. Interestingly, zebrafish mak mutants failed to generate axonemes during photoreceptor ciliogenesis, whereas basal bodies were specified. These data suggest that Mak contributes to axoneme development in zebrafish, in contrast to mouse Mak mutants, which have elongated photoreceptor axonemes. Furthermore, the kinase activity of Mak was found to be critical in ciliary axoneme development and photoreceptor survival. Thus, Mak is required for ciliogenesis and outer segment formation in zebrafish photoreceptors to ensure intracellular protein transport and photoreceptor survival.

Ancient origin of the rod bipolar cell pathway in the vertebrate retina

Vertebrates rely on rod photoreceptors for vision in low-light conditions. The specialized downstream circuit for rod signalling, called the primary rod pathway, is well characterized in mammals, but circuitry for rod signalling in non-mammals is largely unknown. Here we demonstrate that the mammalian primary rod pathway is conserved in zebrafish, which diverged from extant mammals ~400 million years ago. Using single-cell RNA sequencing, we identified two bipolar cell types in zebrafish that are related to mammalian rod bipolar cell (RBCs), the only bipolar type that directly carries rod signals from the outer to the inner retina in the primary rod pathway. By combining electrophysiology, histology and ultrastructural reconstruction of the zebrafish RBCs, we found that, similar to mammalian RBCs, both zebrafish RBC types connect with all rods in their dendritic territory and provide output largely onto amacrine cells. The wiring pattern of the amacrine cells postsynaptic to one RBC type is strikingly similar to that of mammalian RBCs and their amacrine partners, suggesting that the cell types and circuit design of the primary rod pathway emerged before the divergence of teleost fish and mammals. The second RBC type, which forms separate pathways, was either lost in mammals or emerged in fish.

Liquid crystal monomers disrupt photoreceptor patterning of zebrafish larvae via thyroid hormone signaling

Liquid crystal monomers (LCMs) are the raw material for liquid crystal displays, and their use is steadily increasing in electronic products. Recently, LCMs have been reported to be novel endocrine disrupting chemicals, however, the mechanisms underlying their potential for thyroid hormone disruption and visual toxicity are not well understood. In this study, six widely used fluorinated LCMs (FLCMs) were selected to determine putative mechanisms underlying FLCM-induced toxicity to the zebrafish thyroid and visual systems. Exposure to FLCMs caused damage to retinal structures and reduced cell density of ganglion cell layer, inner nuclear layer, and photoreceptor layer approximately 12.6-46.1%. Exposure to FLCMs also disrupted thyroid hormone levels and perturbed the hypothalamic-pituitary-thyroid axis by affecting key enzymes and protein in zebrafish larvae. A thyroid hormone-dependent GH3 cell viability assay supported the hypothesis that FLCMs act as thyroid hormone disrupting chemicals. It was also determined that FLCMs containing aliphatic ring structures may have a higher potential for T3 antagonism compared to FLCMs without an aliphatic ring. Molecular docking in silico suggested that FLCMs may affect biological functions of thyroxine binding globulin, membrane receptor integrin, and thyroid receptor beta. Lastly, the visual motor response of zebrafish in red- and green-light was significantly inhibited following exposure to FLCMs. Taken together, we demonstrate that FLCMs can act as thyroid hormone disruptors to induce visual dysfunction in zebrafish via several molecular mechanisms.

Development of the integrated fish endocrine disruptor test-Part B: Implementation of thyroid-related endpoints

Given the vital role of thyroid hormones (THs) in vertebrate development, it is essential to identify chemicals that interfere with the TH system. Whereas, among nonmammalian laboratory animals, fish are the most frequently utilized test species in endocrine disruptor research, for example, in guidelines for the detection of effects on the sex hormone system, there is no test guideline (TG) using fish as models for thyroid-related effects; rather, amphibians are used. Therefore, the objective of the present project was to integrate thyroid-related endpoints for fish into a test protocol combining OECD TGs 229 (Fish Short-Term Reproduction Assay) and 234 (Fish Sexual Development Test). The resulting integrated Fish Endocrine Disruption Test (iFEDT) was designed as a comprehensive approach to covering sexual differentiation, early development, and reproduction and to identifying disruption not only of the sexual and/or reproductive system but also the TH system. Two 85-day exposure tests were performed using different well-studied endocrine disruptors: 6-propyl-2-thiouracil (PTU) and 17α-ethinylestradiol (EE2). Whereas the companion Part A of this study presents the findings on effects by PTU and EE2 on endpoints established in existing TGs, the present Part B discusses effects on novel thyroid-related endpoints such as TH levels, thyroid follicle histopathology, and eye development. 6-Propyl-2-thiouracil induced a massive proliferation of thyroid follicles in any life stage, and histopathological changes in the eyes proved to be highly sensitive for TH system disruption especially in younger life stages. For measurement of THs, further methodological development is required. 17-α-Ethinylestradiol demonstrated not only the well-known disruption of the hypothalamic-pituitary-gonadal axis, but also induced effects on thyroid follicles in adult zebrafish (Danio rerio) exposed to higher EE2 concentrations, suggesting crosstalk between endocrine axes. The novel iFEDT has thus proven capable of simultaneously capturing endocrine disruption of both the steroid and thyroid endocrine systems. Integr Environ Assess Manag 2024;20:830-845. © 2023 The Authors. Integrated Environmental Assessment and Management published by Wiley Periodicals LLC on behalf of Society of Environmental Toxicology & Chemistry (SETAC).

Lineage motifs as developmental modules for control of cell type proportions

In multicellular organisms, cell types must be produced and maintained in appropriate proportions. One way this is achieved is through committed progenitor cells or extrinsic interactions that produce specific patterns of descendant cell types on lineage trees. However, cell fate commitment is probabilistic in most contexts, making it difficult to infer these dynamics and understand how they establish overall cell type proportions. Here, we introduce Lineage Motif Analysis (LMA), a method that recursively identifies statistically overrepresented patterns of cell fates on lineage trees as potential signatures of committed progenitor states or extrinsic interactions. Applying LMA to published datasets reveals spatial and temporal organization of cell fate commitment in zebrafish and rat retina and early mouse embryonic development. Comparative analysis of vertebrate species suggests that lineage motifs facilitate adaptive evolutionary variation of retinal cell type proportions. LMA thus provides insight into complex developmental processes by decomposing them into simpler underlying modules.

Imidacloprid affects the visual behavior of adult zebrafish (Danio rerio) by mediating the expression of opsin and phototransduction genes and altering the metabolism of neurotransmitters

Imidacloprid poses a significant threat to aquatic ecosystems. In this study, we investigated the visual toxicity of imidacloprid and the underlying molecular mechanisms in adult zebrafish. After exposure to imidacloprid at environmental relevant concentrations (10 and 100μg/L) for 21 days, the detectable contents of imidacloprid were 23.0 ± 0.80 and 121 ± 1.56 ng/mg in eyes of adult zebrafish, respectively. The visual behavior of adult zebrafish was impaired including a reduced ability to track smoothly visual stimuli and visually guided self-motion. The immunofluorescence experiment showed that the content of Rhodopsin (Rho) in the retina of zebrafish was changed significantly. The expression rhythm of genes played key roles in capturing photons in dim (rho) and bright (opn1mw3, opn1lw2 and opn1sw2) light, and in phototransduction (gnb3b, arr3a and rpe65a), was disrupted significantly throughout a 24-h period in adult zebrafish. Targeted metabolomics analysis showed that the content of 16 metabolites associated with neurotransmitter function changed significantly, and were enriched in top three metabolism pathways including Arginine biosynthesis, Alanine, aspartate and glutamate metabolism, and Tryptophan metabolism. These results indicated that imidacloprid exposure at environmentally relevant concentrations could cause optical toxicity through disturbing the expression of opsins and affecting the phototransduction in the retina of zebrafish adults.

Bisphenol A alters retinal morphology, visually guided behavior, and thyroid hormone levels in zebrafish larvae

Bisphenols are industrial chemicals that are produced in large quantities and have been detected in all parts of the environment as well as in a multitude of different organisms including humans and fish. Several bisphenols, such as bisphenol A (BPA) and bisphenol F, have been shown to disrupt endocrine systems thereby affecting development and reproduction. While numerous studies investigated the effect of bisphenols on estrogen signaling, their impact on the thyroid hormone system (THS), which is vital for neurodevelopment including sensory development, has been explored to a lesser extent. The present work selected BPA as a representative for structurally similar bisphenols and assessed its impact on the THS as well as sensory development and function in zebrafish. To this end, zebrafish were exposed to BPA until up to 8 days post fertilization (dpf) and thyroid hormone levels, eye morphology, and sensory-mediated behaviors were analyzed. Zebrafish larvae exposed to BPA showed altered retinal layering, decreased motility across varying light conditions, and a loss of responsiveness to red light. Furthermore, whole-body levels of the thyroid hormones thyroxine (T4) and 3,5-diiodothyronine (3,5-T2) were significantly decreased in 5 dpf zebrafish. Taken together, BPA disrupted THS homeostasis and compromised visual development and function, which is pivotal for the survival of fish larvae. This work underlines the necessity for ongoing research on BPA and its numerous substitutes, particularly concerning their effects on the THS and neurodevelopment, to ensure a high level of protection for the environment and human health.

Cell behaviors that pattern developing tissues: the case of the vertebrate nervous system

Morphogenesis from cells to tissue gives rise to the complex architectures that make our organs. How cells and their dynamic behavior are translated into functional spatial patterns is only starting to be understood. Recent advances in quantitative imaging revealed that, although highly heterogeneous, cellular behaviors make reproducible tissue patterns. Emerging evidence suggests that mechanisms of cellular coordination, intrinsic variability and plasticity are critical for robust pattern formation. While pattern development shows a high level of fidelity, tissue organization has undergone drastic changes throughout the course of evolution. In addition, alterations in cell behavior, if unregulated, can cause developmental malformations that disrupt function. Therefore, comparative studies of different species and of disease models offer a powerful approach for understanding how novel spatial configurations arise from variations in cell behavior and the fundamentals of successful pattern formation. In this chapter, I dive into the development of the vertebrate nervous system to explore efforts to dissect pattern formation beyond molecules, the emerging core principles and open questions.

Single-cell RNA sequencing unravels the transcriptional network underlying zebrafish retina regeneration

In the lesioned zebrafish retina, Müller glia produce multipotent retinal progenitors that generate all retinal neurons, replacing lost cell types. To study the molecular mechanisms linking Müller glia reactivity to progenitor production and neuronal differentiation, we used single-cell RNA sequencing of Müller glia, progenitors and regenerated progeny from uninjured and light-lesioned retinae. We discover an injury-induced Müller glia differentiation trajectory that leads into a cell population with a hybrid identity expressing marker genes of Müller glia and progenitors. A glial self-renewal and a neurogenic trajectory depart from the hybrid cell population. We further observe that neurogenic progenitors progressively differentiate to generate retinal ganglion cells first and bipolar cells last, similar to the events observed during retinal development. Our work provides a comprehensive description of Müller glia and progenitor transcriptional changes and fate decisions in the regenerating retina, which are key to tailor cell differentiation and replacement therapies for retinal dystrophies in humans.

Progenitor division and cell autonomous neurosecretion are required for rod photoreceptor sublaminar positioning

Migration is essential for the laminar stratification and connectivity of neurons in the central nervous system. In the retina, photoreceptors (PRs) migrate to positions according to birthdate, with early-born cells localizing to the basal-most side of the outer nuclear layer. It was proposed that apical progenitor mitoses physically drive these basal translocations non-cell autonomously, but direct evidence is lacking, and whether other mechanisms participate is unknown. Here, combining loss- or gain-of-function assays to manipulate cell cycle regulators (Sonic hedgehog, Cdkn1a/p21) with an in vivo lentiviral labelling strategy, we demonstrate that progenitor division is one of two forces driving basal translocation of rod soma. Indeed, replacing Shh activity rescues abnormal rod translocation in retinal explants. Unexpectedly, we show that rod differentiation also promotes rod soma translocation. While outer segment function or formation is dispensable, Crx and SNARE-dependent synaptic function are essential. Thus, both non-cell and cell autonomous mechanisms underpin PR soma sublaminar positioning in the mammalian retina.

Ancient origin of the rod bipolar cell pathway in the vertebrate retina

Vertebrates rely on rod photoreceptors for vision in low-light conditions. Mammals have a specialized downstream circuit for rod signaling called the primary rod pathway, which comprises specific cell types and wiring patterns that are thought to be unique to this lineage. Thus, it has been long assumed that the primary rod pathway evolved in mammals. Here, we challenge this view by demonstrating that the mammalian primary rod pathway is conserved in zebrafish, which diverged from extant mammals ~400 million years ago. Using single-cell RNA-sequencing, we identified two bipolar cell (BC) types in zebrafish that are related to mammalian rod BCs (RBCs) of the primary rod pathway. By combining electrophysiology, histology, and ultrastructural reconstruction of the zebrafish RBCs, we found that, like mammalian RBCs, both zebrafish RBC types connect with all rods in their dendritic territory, and provide output largely onto amacrine cells. The wiring pattern of the amacrine cells post-synaptic to one RBC type is strikingly similar to that of mammalian RBCs, suggesting that the cell types and circuit design of the primary rod pathway have emerged before the divergence of teleost fish and amniotes. The second RBC type, which forms separate pathways, is either lost in mammals or emerged in fish.

Ancient origin of the rod bipolar cell pathway in the vertebrate retina

Vertebrates rely on rod photoreceptors for vision in low-light conditions1. Mammals have a specialized downstream circuit for rod signaling called the primary rod pathway, which comprises specific cell types and wiring patterns that are thought to be unique to this lineage2-6. Thus, it has been long assumed that the primary rod pathway evolved in mammals3,5-7. Here, we challenge this view by demonstrating that the mammalian primary rod pathway is conserved in zebrafish, which diverged from extant mammals ~400 million years ago. Using single-cell RNA-sequencing, we identified two bipolar cell (BC) types in zebrafish that are related to mammalian rod BCs (RBCs) of the primary rod pathway. By combining electrophysiology, histology, and ultrastructural reconstruction of the zebrafish RBCs, we found that, like mammalian RBCs8, both zebrafish RBC types connect with all rods and red-cones in their dendritic territory, and provide output largely onto amacrine cells. The wiring pattern of the amacrine cells post-synaptic to one RBC type is strikingly similar to that of mammalian RBCs. This suggests that the cell types and circuit design of the primary rod pathway may have emerged before the divergence of teleost fish and amniotes (mammals, bird, reptiles). The second RBC type in zebrafish, which forms separate pathways from the first RBC type, is either lost in mammals or emerged in fish to serve yet unknown roles.

Plasticity of cone photoreceptors in adult zebrafish revealed by thyroid hormone exposure

Vertebrate color vision is predominantly mediated by the presence of multiple cone photoreceptor subtypes that are each maximally sensitive to different wavelengths of light. Thyroid hormone (TH) has been shown to be essential in the spatiotemporal patterning of cone subtypes in many species, including cone subtypes that express opsins that are encoded by tandemly replicated genes. TH has been shown to differentially regulate the tandemly replicated lws opsin genes in zebrafish, and exogenous treatments alter the expression levels of these genes in larvae and juveniles. In this study, we sought to determine whether gene expression in cone photoreceptors remains plastic to TH treatment in adults. We used a transgenic lws reporter line, multiplexed fluorescence hybridization chain reaction in situ hybridization, and qPCR to examine the extent to which cone gene expression can be altered by TH in adults. Our studies revealed that opsin gene expression, and the expression of other photoreceptor genes, remains plastic to TH treatment in adult zebrafish. In addition to retinal plasticity, exogenous TH treatment alters skin pigmentation patterns in adult zebrafish after 5 days. Taken together, our results show a remarkable level of TH-sensitive plasticity in the adult zebrafish.

Neuronal migration prevents spatial competition in retinal morphogenesis

The concomitant occurrence of tissue growth and organization is a hallmark of organismal development1-3. This often means that proliferating and differentiating cells are found at the same time in a continuously changing tissue environment. How cells adapt to architectural changes to prevent spatial interference remains unclear. Here, to understand how cell movements that are key for growth and organization are orchestrated, we study the emergence of photoreceptor neurons that occur during the peak of retinal growth, using zebrafish, human tissue and human organoids. Quantitative imaging reveals that successful retinal morphogenesis depends on the active bidirectional translocation of photoreceptors, leading to a transient transfer of the entire cell population away from the apical proliferative zone. This pattern of migration is driven by cytoskeletal machineries that differ depending on the direction: microtubules are exclusively required for basal translocation, whereas actomyosin is involved in apical movement. Blocking the basal translocation of photoreceptors induces apical congestion, which hampers the apical divisions of progenitor cells and leads to secondary defects in lamination. Thus, photoreceptor migration is crucial to prevent competition for space, and to allow concurrent tissue growth and lamination. This shows that neuronal migration, in addition to its canonical role in cell positioning4, can be involved in coordinating morphogenesis.

The multi-level regulation of clownfish metamorphosis by thyroid hormones

Most marine organisms have a biphasic life cycle during which pelagic larvae transform into radically different juveniles. In vertebrates, the role of thyroid hormones (THs) in triggering this transition is well known, but how the morphological and physiological changes are integrated in a coherent way with the ecological transition remains poorly explored. To gain insight into this question, we performed an integrated analysis of metamorphosis of a marine teleost, the false clownfish (Amphiprion ocellaris). We show how THs coordinate a change in color vision as well as a major metabolic shift in energy production, highlighting how it orchestrates this transformation. By manipulating the activity of liver X regulator (LXR), a major regulator of metabolism, we also identify a tight link between metabolic changes and metamorphosis progression. Strikingly, we observed that these regulations are at play in the wild, explaining how hormones coordinate energy needs with available resources during the life cycle.

Expanding the phenotype of THRB: a range of macular dystrophies as the major clinical manifestations in patients with a dominant splicing variant

Inherited retinal dystrophies (IRDs) are a clinically and genetically heterogeneous group of disorders that often severely impair vision. Some patients manifest poor central vision as the first symptom due to cone-dysfunction, which is consistent with cone dystrophy (COD), Stargardt disease (STGD), or macular dystrophy (MD) among others. Here, we aimed to identify the genetic cause of autosomal dominant COD in one family. WGS was performed in 3 affected and 1 unaffected individual using the TruSeq Nano DNA library kit and the NovaSeq 6,000 platform (Illumina). Data analysis identified a novel spliceogenic variant (c.283 + 1G>A) in the thyroid hormone receptor beta gene (THRB) as the candidate disease-associated variant. Further genetic analysis revealed the presence of the same heterozygous variant segregating in two additional unrelated dominant pedigrees including 9 affected individuals with a diagnosis of COD (1), STGD (4), MD (3) and unclear phenotype (1). THRB has been previously reported as a causal gene for autosomal dominant and recessive thyroid hormone resistance syndrome beta (RTHβ); however, none of the IRD patients exhibited RTHβ. Genotype-phenotype correlations showed that RTHβ can be caused by both truncating and missense variants, which are mainly located at the 3' (C-terminal/ligand-binding) region, which is common to both THRB isoforms (TRβ1 and TRβ2). In contrast, the c.283 + 1G>A variant is predicted to disrupt a splice site in the 5'-region of the gene that encodes the N-terminal domain of the TRβ1 isoform protein, leaving the TRβ2 isoform intact, which would explain the phenotypic variability observed between RTHβ and IRD patients. Interestingly, although monochromacy or cone response alterations have already been described in a few RTHβ patients, herein we report the first genetic association between a pathogenic variant in THRB and non-syndromic IRDs. We thereby expand the phenotype of THRB pathogenic variants including COD, STGD, or MD as the main clinical manifestation, which also reflects the extraordinary complexity of retinal functions mediated by the different THRB isoforms.

Deterministic and probabilistic fate decisions co-exist in a single retinal lineage

Correct nervous system development depends on the timely differentiation of progenitor cells into neurons. While the output of progenitor differentiation is well investigated at the population and clonal level, how stereotypic or variable fate decisions are during development is still more elusive. To fill this gap, we here follow the fate outcome of single neurogenic progenitors in the zebrafish retina over time using live imaging. We find that neurogenic progenitor divisions produce two daughter cells, one of deterministic and one of probabilistic fate. Interference with the deterministic branch of the lineage affects lineage progression. In contrast, interference with fate probabilities of the probabilistic branch results in a broader range of fate possibilities than in wild-type and involves the production of any neuronal cell type even at non-canonical developmental stages. Combining the interference data with stochastic modelling of fate probabilities revealed that a simple gene regulatory network is able to predict the observed fate decision probabilities during wild-type development. These findings unveil unexpected lineage flexibility that could ensure robust development of the retina and other tissues.

Long wavelength-sensing cones of zebrafish retina exhibit multiple layers of transcriptional heterogeneity

Introduction

Understanding how photoreceptor genes are regulated is important for investigating retinal development and disease. While much is known about gene regulation in cones, the mechanism by which tandemly-replicated opsins, such as human long wavelength-sensitive and middle wavelength-sensitive opsins, are differentially regulated remains elusive. In this study, we aimed to further our understanding of transcriptional heterogeneity in cones that express tandemly-replicated opsins and the regulation of such differential expression using zebrafish, which express the tandemly-replicated opsins lws1 and lws2.

Methods

We performed bulk and single cell RNA-Seq of LWS1 and LWS2 cones, evaluated expression patterns of selected genes of interest using multiplex fluorescence in situ hybridization, and used exogenous thyroid hormone (TH) treatments to test selected genes for potential control by thyroid hormone: a potent, endogenous regulator of lws1 and lws2 expression.

Results

Our studies indicate that additional transcriptional differences beyond opsin expression exist between LWS1 and LWS2 cones. Bulk RNA-Seq results showed 95 transcripts enriched in LWS1 cones and 186 transcripts enriched in LWS2 cones (FC > 2, FDR < 0.05). In situ hybridization results also reveal underlying heterogeneity within the lws1- and lws2-expressing populations. This heterogeneity is evident in cones of mature zebrafish, and further heterogeneity is revealed in transcriptional responses to TH treatments.

Discussion

We found some evidence of coordinate regulation of lws opsins and other genes by exogenous TH in LWS1 vs. LWS2 cones, as well as evidence of gene regulation not mediated by TH. The transcriptional differences between LWS1 and LWS2 cones are likely controlled by multiple signals, including TH.

Lineage motifs: developmental modules for control of cell type proportions

In multicellular organisms, cell types must be produced and maintained in appropriate proportions. One way this is achieved is through committed progenitor cells that produce specific sets of descendant cell types. However, cell fate commitment is probabilistic in most contexts, making it difficult to infer progenitor states and understand how they establish overall cell type proportions. Here, we introduce Lineage Motif Analysis (LMA), a method that recursively identifies statistically overrepresented patterns of cell fates on lineage trees as potential signatures of committed progenitor states. Applying LMA to published datasets reveals spatial and temporal organization of cell fate commitment in zebrafish and rat retina and early mouse embryo development. Comparative analysis of vertebrate species suggests that lineage motifs facilitate adaptive evolutionary variation of retinal cell type proportions. LMA thus provides insight into complex developmental processes by decomposing them into simpler underlying modules.

Reversibility of Thyroid Hormone System-Disrupting Effects on Eye and Thyroid Follicle Development in Zebrafish (Danio rerio) Embryos

Early vertebrate development is partially regulated by thyroid hormones (THs). Environmental pollutants that interact with the TH system (TH system-disrupting chemicals [THSDCs]) can have massively disrupting effects on this essential phase. Eye development of fish is directly regulated by THs and can, therefore, be used as a thyroid-related endpoint in endocrine disruptor testing. To evaluate the effects of THSDC-induced eye malformations during early development, zebrafish (Danio rerio) embryos were exposed for 5 days postfertilization (dpf) to either propylthiouracil, a TH synthesis inhibitor, or tetrabromobisphenol A, which interacts with TH receptors. Subsequently, one half of the embryos were exposed further to the THSDCs until 8 dpf, while the other half of the embryos were raised in clean water for 3 days to check for reversibility of effects. Continued THSDC exposure altered eye size and pigmentation and induced changes in the cellular structure of the retina. This correlated with morphological alterations of thyroid follicles as revealed by use of a transgenic zebrafish line. Interestingly, effects were partly reversible after a recovery period as short as 3 days. Results are consistent with changes in TH levels measured in different tissues of the embryos, for example, in the eyes. The results show that eye development in zebrafish embryos is very sensitive to THSDC treatment but able to recover quickly from early exposure by effective repair mechanisms. Environ Toxicol Chem 2023;42:1276-1292. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC.

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

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

Thyroid hormone as a temporal switch in mouse development

Thyroid hormones are known to trigger metamorphosis in an amphibian. This review discusses the hypothesis according to which they act in a similar manner to synchronize the post-natal development of mice, using brain, brown adipose tissue, and heart as examples.

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.

Developmental exposure to triclosan and benzophenone-2 causes morphological alterations in zebrafish (Danio rerio) thyroid follicles and eyes

Thyroid hormones (THs) regulate a multitude of developmental and metabolic processes, which are responsible for vertebrate development, growth, and maintenance of homeostasis. THs also play a key role in neurogenesis of vertebrates and thus affect eye development, which is vital for foraging efficiency and for effective escape from predation. Currently, there are no validated test guidelines for the assessment of TH system-disrupting chemicals (THSDCs) in fish. Consequently, the present study was designed to demonstrate the suitability of novel thyroid-related endpoints in early life-stages of fish. Embryos of a transgenic zebrafish (Danio rerio) line expressing the reporter gene tg:mCherry in their thyrocytes were used to investigate the effects of the environmental THSDCs triclosan (TCS, antibacterial agent) and benzophenone-2 (BP-2, UV filter) on thyroid follicle and eye development. Both BP-2 and TCS caused thyroid follicle hyperplasia in transgenic zebrafish, thus confirming their role as THSDCs. The effect intensity on follicle size and fluorescence was comparable with a 1.7-fold increase for BP-2 and 1.6-fold for TCS. Alterations of the cellular structures of the retina indicate an impact of both substances on eye development, with a stronger impact of TCS. With respect to guideline development, results provide further evidence for the suitability of morphological changes in thyroid follicles and the eyes as novel endpoints for the sensitive assessment of THSD-related effects in fish.

Transcription factors underlying photoreceptor diversity

During development, retinal progenitors navigate a complex landscape of fate decisions to generate the major cell classes necessary for proper vision. Transcriptional regulation is critical to generate diversity within these major cell classes. Here, we aim to provide the resources and techniques required to identify transcription factors necessary to generate and maintain diversity in photoreceptor subtypes, which are critical for vision. First, we generate a key resource: a high-quality and deep transcriptomic profile of each photoreceptor subtype in adult zebrafish. We make this resource openly accessible, easy to explore, and have integrated it with other currently available photoreceptor transcriptomic datasets. Second, using our transcriptomic profiles, we derive an in-depth map of expression of transcription factors in photoreceptors. Third, we use efficient CRISPR-Cas9 based mutagenesis to screen for null phenotypes in F0 larvae (F0 screening) as a fast, efficient, and versatile technique to assess the involvement of candidate transcription factors in the generation of photoreceptor subtypes. We first show that known phenotypes can be easily replicated using this method: loss of S cones in foxq2 mutants and loss of rods in nr2e3 mutants. We then identify novel functions for the transcription factor Tbx2, demonstrating that it plays distinct roles in controlling the generation of all photoreceptor subtypes within the retina. Our study provides a roadmap to discover additional factors involved in this process. Additionally, we explore four transcription factors of unknown function (Skor1a, Sall1a, Lrrfip1a, and Xbp1), and find no evidence for their involvement in the generation of photoreceptor subtypes. This dataset and screening method will be a valuable way to explore the genes involved in many other essential aspects of photoreceptor biology.

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.

Noncoding Mutations in a Thyroid Hormone Receptor Gene That Impair Cone Photoreceptor Function

The function of a hormone receptor requires mechanisms to control precisely where, when, and at what level the receptor gene is expressed. An intriguing case concerns the selective induction of thyroid hormone receptor β2 (TRβ2), encoded by Thrb, in the pituitary and also in cone photoreceptors, in which it critically regulates expression of the opsin photopigments that mediate color vision. Here, we investigate the physiological significance of a candidate enhancer for induction of TRβ2 by mutagenesis of a conserved intron region in its natural context in the endogenous Thrb gene in mice. Mutation of e-box sites for bHLH (basic-helix-loop-helix) transcription factors preferentially impairs TRβ2 expression in cones whereas mutation of nearby sequences preferentially impairs expression in pituitary. A deletion encompassing all sites impairs expression in both tissues, indicating bifunctional activity. In cones, the e-box mutations disrupt chromatin acetylation, blunt the developmental induction of TRβ2, and ultimately impair cone opsin expression and sensitivity to longer wavelengths of light. These results demonstrate the necessity of studying an enhancer in its natural chromosomal context for defining biological relevance and reveal surprisingly critical nuances of level and timing of enhancer function. Our findings illustrate the influence of noncoding sequences over thyroid hormone functions.

The evolutionary history and spectral tuning of vertebrate visual opsins

Many animals depend on the sense of vision for survival. In eumetazoans, vision requires specialized, light-sensitive cells called photoreceptors. Light reaches the photoreceptors and triggers the excitation of light-detecting proteins called opsins. Here, we describe the story of visual opsin evolution from the ancestral bilaterian to the extant vertebrate lineages. We explain the mechanisms determining color vision of extant vertebrates, focusing on opsin gene losses, duplications, and the expression regulation of vertebrate opsins. We describe the sequence variation both within and between species that has tweaked the sensitivities of opsin proteins towards different wavelengths of light. We provide an extensive resource of wavelength sensitivities and mutations that have diverged light sensitivity in many vertebrate species and predict how these mutations were accumulated in each lineage based on parsimony. We suggest possible natural and sexual selection mechanisms underlying these spectral differences. Understanding how molecular changes allow for functional adaptation of animals to different environments is a major goal in the field, and therefore identifying mutations affecting vision and their relationship to photic selection pressures is imperative. The goal of this review is to provide a comprehensive overview of our current understanding of opsin evolution in vertebrates.

Disease-causing mutations in genes encoding transcription factors critical for photoreceptor development

Photoreceptor development of the vertebrate visual system is controlled by a complex transcription regulatory network. OTX2 is expressed in the mitotic retinal progenitor cells (RPCs) and controls photoreceptor genesis. CRX that is activated by OTX2 is expressed in photoreceptor precursors after cell cycle exit. NEUROD1 is also present in photoreceptor precursors that are ready to specify into rod and cone photoreceptor subtypes. NRL is required for the rod fate and regulates downstream rod-specific genes including the orphan nuclear receptor NR2E3 which further activates rod-specific genes and simultaneously represses cone-specific genes. Cone subtype specification is also regulated by the interplay of several transcription factors such as THRB and RXRG. Mutations in these key transcription factors are responsible for ocular defects at birth such as microphthalmia and inherited photoreceptor diseases such as Leber congenital amaurosis (LCA), retinitis pigmentosa (RP) and allied dystrophies. In particular, many mutations are inherited in an autosomal dominant fashion, including the majority of missense mutations in CRX and NRL. In this review, we describe the spectrum of photoreceptor defects that are associated with mutations in the above-mentioned transcription factors, and summarize the current knowledge of molecular mechanisms underlying the pathogenic mutations. At last, we deliberate the outstanding gaps in our understanding of the genotype-phenotype correlations and outline avenues for future research of the treatment strategies.

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

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

Transcriptional control of cone photoreceptor diversity by a thyroid hormone receptor

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

The Developmental Progression of Eight Opsin Spectral Signals Recorded from the Zebrafish Retinal Cone Layer Is Altered by the Timing and Cell Type Expression of Thyroxin Receptor β2 (trβ2) Gain-Of-Function Transgenes

Zebrafish retinal cone signals shift in spectral shape through larval, juvenile, and adult development as expression patterns of eight cone-opsin genes change. An algorithm extracting signal amplitudes for the component cone spectral types is developed and tested on two thyroxin receptor β2 (trβ2) gain-of-function lines crx:mYFP-2A-trβ2 and gnat2:mYFP-2A-trβ2, allowing correlation between opsin signaling and opsin immunoreactivity in lines with different developmental timing and cell-type expression of this red-opsin-promoting transgene. Both adult transgenics became complete, or nearly complete, "red-cone dichromats," with disproportionately large long-wavelength-sensitive (LWS)1 opsin amplitudes as compared with controls, where LWS1 and LWS2 amplitudes were about equal, and significant signals from SWS1, SWS2, and Rh2 opsins were detected. But in transgenic larvae and juveniles of both lines it was LWS2 amplitudes that increased, with LWS1 cone signals rarely encountered. In gnat2:mYFP-2A-trβ2 embryos at 5 d postfertilization (dpf), red-opsin immunoreactive cone density doubled, but red-opsin amplitudes (LWS2) increased <10%, and green-opsin, blue-opsin, and UV-opsin signals were unchanged, despite co-expressed red opsins, and the finding that an sws1 UV-opsin reporter gene was shut down by the gnat2:mYFP-2A-trβ2 transgene. By contrast both LWS2 red-cone amplitudes and the density of red-cone immunoreactivity more than doubled in 5-dpf crx:mYFP-2A-trβ2 embryos, while UV-cone amplitudes were reduced 90%. Embryonic cones with trβ2 gain-of-function transgenes were morphologically distinct from control red, blue or UV cones, with wider inner segments and shorter axons than red cones, suggesting cone spectral specification, opsin immunoreactivity and shape are influenced by the abundance and developmental timing of trβ2 expression.

AOP Report: Thyroperoxidase Inhibition Leading to Altered Visual Function in Fish Via Altered Retinal Layer Structure

Thyroid hormones (THs) are involved in the regulation of many important physiological and developmental processes, including vertebrate eye development. Thyroid hormone system-disrupting chemicals (THSDCs) may have severe consequences, because proper functioning of the visual system is a key factor for survival in wildlife. However, the sequence of events leading from TH system disruption (THSD) to altered eye development in fish has not yet been fully described. The development of this adverse outcome pathway (AOP) was based on an intensive literature review of studies that focused on THSD and impacts on eye development, mainly in fish. In total, approximately 120 studies (up to the end of 2021) were used in the development of this AOP linking inhibition of the key enzyme for TH synthesis, thyroperoxidase (TPO), to effects on retinal layer structure and visual function in fish (AOP-Wiki, AOP 363). In a weight-of-evidence evaluation, the confidence levels were overall moderate, with ample studies showing the link between reduced TH levels and altered retinal layer structure. However, some uncertainties about the underlying mechanism(s) remain. Although the current weight-of-evidence evaluation is based on fish, the AOP is plausibly applicable to other vertebrate classes. Through the re-use of several building blocks, this AOP is connected to the AOPs leading from TPO and deiodinase inhibition to impaired swim bladder inflation in fish (AOPs 155-159), together forming an AOP network describing THSD in fish. This AOP network addresses the lack of thyroid-related endpoints in existing fish test guidelines for the evaluation of THSDCs. Environ Toxicol Chem 2022;41:2632-2648. © 2022 SETAC.

Cell Fate of Retinal Progenitor Cells: In Ovo UbC-StarTrack Analysis

Clonal cell analysis outlines the ontogenic potential of single progenitor cells, allowing the elucidation of the neural heterogeneity among different cell types and their lineages. In this work, we analyze the potency of retinal stem/progenitor cells through development using the chick embryo as a model. We implemented in ovo the clonal genetic tracing strategy UbC-StarTrack for tracking retinal cell lineages derived from individual progenitors of the ciliary margin at E3.5 (HH21-22). The clonal assignment of the derived-cell progeny was performed in the neural retina at E11.5-12 (HH38) through the identification of sibling cells as cells expressing the same combination of fluorophores. Moreover, cell types were assessed based on their cellular morphology and laminar location. Ciliary margin derived-cell progenies are organized in columnar associations distributed along the peripheral retina with a limited tangential dispersion. The analysis revealed that, at the early stages of development, this region harbors multipotent and committed progenitor cells.

Jag2b-Notch3/1b-mediated neuron-to-glia crosstalk controls retinal gliogenesis

In the developing central nervous systems (CNS), neural progenitor cells generate neurons and glia in sequential order. However, the influence of neurons on glia generation remains elusive. Here, we report that photoreceptor cell-derived Jag2b is required for Notch-dependent Müller glia (MG) generation in the developing zebrafish retina. In jab2b-/- mutants, differentiating MGs are re-specified into lineage-related bipolar neuron fate at the expense of mature MG. Single-cell transcriptome analysis and knock-in animals reveal that jab2b is specifically expressed in crx+ -photoreceptor cells during MG generation. Crx promoter-driven jag2b, but not other Notch ligands, is sufficient to rescue the loss of MGs observed in jag2b-/- mutants. Furthermore, we observe a severe and moderate decrease in the number of MGs in notch3-/- and notch1b-/- mutants, respectively, and the activation of Notch3 or Notch1b rescues the MG loss in jag2b-/- mutants. Together, our findings reveal that the interaction of Jag2b and Notch3/Notch1b mediates the crosstalk between neurons and glial cells to ensure the irreversible differentiation of MG, providing novel mechanistic insights into the temporal specification of glial cell fate in a developing vertebrate CNS structure.

Cellular and Molecular Determinants of Retinal Cell Fate

The vertebrate retina is regarded as a simple part of the central nervous system (CNS) and thus amenable to investigations of the determinants of cell fate. Its five neuronal cell classes and one glial cell class all derive from a common pool of progenitors. Here we review how each cell class is generated. Retinal progenitors progress through different competence states, in each of which they generate only a small repertoire of cell classes. The intrinsic state of the progenitor is determined by the complement of transcription factors it expresses. Thus, although progenitors are multipotent, there is a bias in the types of fates they generate during any particular time window. Overlying these competence states are stochastic mechanisms that influence fate decisions. These mechanisms are determined by a weighted set of probabilities based on the abundance of a cell class in the retina. Deterministic mechanisms also operate, especially late in development, when preprogrammed progenitors solely generate specific fates.

Eyes on CHARGE syndrome: Roles of CHD7 in ocular development

The development of the vertebrate visual system involves complex morphogenetic interactions of cells derived from multiple embryonic lineages. Disruptions in this process are associated with structural birth defects such as microphthalmia, anophthalmia, and coloboma (collectively referred to as MAC), and inherited retinal degenerative diseases such as retinitis pigmentosa and allied dystrophies. MAC and retinal degeneration are also observed in systemic congenital malformation syndromes. One important example is CHARGE syndrome, a genetic disorder characterized by coloboma, heart defects, choanal atresia, growth retardation, genital abnormalities, and ear abnormalities. Mutations in the gene encoding Chromodomain helicase DNA binding protein 7 (CHD7) cause the majority of CHARGE syndrome cases. However, the pathogenetic mechanisms that connect loss of CHD7 to the ocular complications observed in CHARGE syndrome have not been identified. In this review, we provide a general overview of ocular development and congenital disorders affecting the eye. This is followed by a comprehensive description of CHARGE syndrome, including discussion of the spectrum of ocular defects that have been described in this disorder. In addition, we discuss the current knowledge of CHD7 function and focus on its contributions to the development of ocular structures. Finally, we discuss outstanding gaps in our knowledge of the role of CHD7 in eye formation, and propose avenues of investigation to further our understanding of how CHD7 activity regulates ocular and retinal development.

Zebrafish as an emerging tool for drug discovery and development for thyroid diseases

Zebrafish is a useful model for understanding human genetics and diseases and has evolved into a prominent scientific research model. The genetic structure of zebrafish is 70% identical to that of humans. Its small size, low cost, and transparent embryo make it a valuable tool in experimentation. Zebrafish and mammals possess the same molecular mechanism of thyroid organogenesis and development. Thus, thyroid hormone signaling, embryonic development, thyroid-related disorders, and novel genes involved in early thyroid development can all be studied using zebrafish as a model. Here in this review, we emphasize the evolving role of zebrafish as a possible tool for studying the thyroid gland in the context of physiology and pathology. The transcription factors nkx2.1a, pax2a, and hhex which contribute a pivotal role in the differentiation of thyroid primordium are discussed. Further, we have described the role of zebrafish as a model for thyroid cancer, evaluation of defects in thyroid hormone transport, thyroid hormone (TH) metabolism, and as a screening tool to study thyrotoxins. Hence, the present review highlights the role of zebrafish as a novel approach to understand thyroid development and organogenesis.

MYCN induces cell-specific tumorigenic growth in RB1-proficient human retinal organoid and chicken retina models of retinoblastoma

Retinoblastoma is a rare, intraocular paediatric cancer that originates in the neural retina and is most frequently caused by bi-allelic loss of RB1 gene function. Other oncogenic mutations, such as amplification and increased expression of the MYCN gene, have been found even with proficient RB1 function. In this study, we investigated whether MYCN over-expression can drive carcinogenesis independently of RB1 loss-of-function mutations. The aim was to elucidate the events that result in carcinogenesis and identify the cancer cell-of-origin. We used the chicken retina, a well-established model for studying retinal neurogenesis, and established human embryonic stem cell-derived retinal organoids as model systems. We over-expressed MYCN by electroporation of piggyBac genome-integrating expression vectors. We found that over-expression of MYCN induced tumorigenic growth with high frequency in RB1-proficient chicken retinas and human organoids. In both systems, the tumorigenic cells expressed markers for undifferentiated cone photoreceptor/horizontal cell progenitors. The over-expression resulted in metastatic retinoblastoma within 7–9 weeks in chicken. Cells expressing MYCN could be grown in vitro and, when orthotopically injected, formed tumours that infiltrated the sclera and optic nerve and expressed markers for cone progenitors. Investigation of the tumour cell phenotype determined that the potential for neoplastic growth was embryonic stage-dependent and featured a cell-specific resistance to apoptosis in the cone/horizontal cell lineage, but not in ganglion or amacrine cells. We conclude that MYCN over-expression is sufficient to drive tumorigenesis and that a cell-specific resistance to apoptosis in the cone/horizontal cell lineage mediates the cancer phenotype.

BDE-99 Disrupts the Photoreceptor Patterning of Zebrafish Larvae via Transcription Factor six7

Proper visual function is essential for collecting environmental information and supporting the decision-making in the central nervous system and is therefore tightly associated with wildlife survival and human health. Polybrominated diphenyl ethers (PBDEs) were reported to impair zebrafish vision development, and thyroid hormone (TH) signaling was suspected as the main contributor. In this study, a pentabrominated PBDE, BDE-99, was chosen to further explore the action mechanism of PBDEs on the disruption of zebrafish color vision. The results showed that BDE-99 could impair multiple photoreceptors in the retina and disturb the behavior guided by the color vision of zebrafish larvae at 120 h post-fertilization. Although the resulting alteration in photoreceptor patterning highly resembled the effects of 3,3',5-triiodo-l-thyroine, introducing the antagonist for TH receptors was unable to fully recover the alteration, which suggested the involvement of other potential regulatory factors. By modulating the expression of six7, a key inducer of middle-wavelength opsins, we demonstrated that six7, not THs, dominated the photoreceptor patterning in the disruption of BDE-99. Our work promoted the understanding of the regulatory role of six7 in the process of photoreceptor patterning and proposed a novel mechanism for the visual toxicity of PBDEs.

Patterning and Development of Photoreceptors in the Human Retina

Humans rely on visual cues to navigate the world around them. Vision begins with the detection of light by photoreceptor cells in the retina, a light-sensitive tissue located at the back of the eye. Photoreceptor types are defined by morphology, gene expression, light sensitivity, and function. Rod photoreceptors function in low-light vision and motion detection, and cone photoreceptors are responsible for high-acuity daytime and trichromatic color vision. In this review, we discuss the generation, development, and patterning of photoreceptors in the human retina. We describe our current understanding of how photoreceptors are patterned in concentric regions. We conclude with insights into mechanisms of photoreceptor differentiation drawn from studies of model organisms and human retinal organoids.

The Role of Estrogen and Thyroid Hormones in Zebrafish Visual System Function

Visual system development is a highly complex process involving coordination of environmental cues, cell pathways, and integration of functional circuits. Consequently, a change to any step, due to a mutation or chemical exposure, can lead to deleterious consequences. One class of chemicals known to have both overt and subtle effects on the visual system is endocrine disrupting compounds (EDCs). EDCs are environmental contaminants which alter hormonal signaling by either preventing compound synthesis or binding to postsynaptic receptors. Interestingly, recent work has identified neuronal and sensory systems, particularly vision, as targets for EDCs. In particular, estrogenic and thyroidogenic signaling have been identified as critical modulators of proper visual system development and function. Here, we summarize and review this work, from our lab and others, focusing on behavioral, physiological, and molecular data collected in zebrafish. We also discuss different exposure regimes used, including long-lasting effects of developmental exposure. Overall, zebrafish are a model of choice to examine the impact of EDCs and other compounds targeting estrogen and thyroid signaling and the consequences of exposure in visual system development and function.

Spatiotemporal patterns of neuronal subtype genesis suggest hierarchical development of retinal diversity

How do neuronal subtypes emerge during development? Recent molecular studies have profiled existing neuronal diversity, but neuronal subtype genesis remains elusive. The 15 types of mouse retinal bipolar interneurons are characterized by distinct functions, morphologies, and transcriptional profiles. Here, we develop a comprehensive spatiotemporal map of bipolar subtype genesis in the murine retina. Combining multiplexed detection of 16 RNA markers with timed delivery of 5-ethynyl uridine (EdU) and bromodeoxyuridine (BrdU), we analyze more than 30,000 single cells in full retinal sections to classify all bipolar subtypes and their birthdates. We find that bipolar subtype birthdates are ordered and follow a centrifugal developmental axis. Spatial analysis reveals a striking wave pattern of bipolar subtype birthdates, and lineage analyses suggest clonal restriction on homotypic subtype production. These results inspire a hierarchical developmental model, with ordered subtype genesis within lineages. Our results provide insight into neuronal subtype development and establish a framework for studying subtype diversification.

Notch-mediated re-specification of neuronal identity during central nervous system development

Neuronal identity has long been thought of as immutable, so that once a cell acquires a specific fate, it is maintained for life.¹ Studies using the overexpression of potent transcription factors to experimentally reprogram neuronal fate in the mouse neocortex^2,3 and retina^4,5 have challenged this notion by revealing that post-mitotic neurons can switch their identity. Whether fate reprogramming is part of normal development in the central nervous system (CNS) is unclear. While there are some reports of physiological cell fate reprogramming in invertebrates,^6,7 and in the vertebrate peripheral nervous system,⁸ endogenous fate reprogramming in the vertebrate CNS has not been documented. Here, we demonstrate spontaneous fate re-specification in an interneuron lineage in the zebrafish retina. We show that the visual system homeobox 1 (vsx1)-expressing lineage, which has been associated exclusively with excitatory bipolar cell (BC) interneurons,^9-12 also generates inhibitory amacrine cells (ACs). We identify a role for Notch signaling in conferring plasticity to nascent vsx1 BCs, allowing suitable transcription factor programs to re-specify them to an AC fate. Overstimulating Notch signaling enhances this physiological phenotype so that both daughters of a vsx1 progenitor differentiate into ACs and partially differentiated vsx1 BCs can be converted into ACs. Furthermore, this physiological re-specification can be mimicked to allow experimental induction of an entirely distinct fate, that of retinal projection neurons, from the vsx1 lineage. Our observations reveal unanticipated plasticity of cell fate during retinal development.

Trilogy Development of Proopiomelanocortin Neurons From Embryonic to Adult Stages in the Mice Retina

Proopiomelanocortin-positive amacrine cells (POMC ACs) were first discovered in adult mouse retinas in 2010; however, the development of POMC-ACs has not been studied. We bred POMC-EGFP mice to label POMC-positive cells and investigated the development of POMC neurons from embryonic to adult stages. We found that POMC neuron development is mainly divided into three stages: the embryonic stage, the closed-eye stage, and the open-eye stage. Each stage has unique characteristics. In the embryonic stage, POMC neurons appeared in the retina at about E13. There was a cell number developmental peak at E15, followed by a steep decline at E16. POMC neurons showed a large soma and increased spine numbers at the closed-eye stage, and two dendritic sublaminas formed in the inner plexiform layer (IPL). The appearance and increased soma size and dendrite numbers did not occur continuously in space. We found that the soma number was asymmetric between the superior and inferior retinas according to the developmental topographic map. Density peaked in the superior retina, which existed persistently in the retinal ganglion cell layer (GCL), but disappeared from the inner nuclear layer (INL) at about P6. At the same time, the soma distribution in the INL was the most regular. At the open-eye stage, the development of POMC neurons was nearly stable only with only an increase in the IPL width, which increased the soma–dendrite distance.

Ancestral circuits for vertebrate color vision emerge at the first retinal synapse

For color vision, retinal circuits separate information about intensity and wavelength. In vertebrates that use the full complement of four “ancestral” cone types, the nature and implementation of this computation remain poorly understood. Here, we establish the complete circuit architecture of outer retinal circuits underlying color processing in larval zebrafish. We find that the synaptic outputs of red and green cones efficiently rotate the encoding of natural daylight in a principal components analysis–like manner to yield primary achromatic and spectrally opponent axes, respectively. Blue cones are tuned to capture most remaining variance when opposed to green cones, while UV cone present a UV achromatic axis for prey capture. We note that fruitflies use essentially the same strategy. Therefore, rotating color space into primary achromatic and chromatic axes at the eye’s first synapse may thus be a fundamental principle of color vision when using more than two spectrally well-separated photoreceptor types.

Foxq2 determines blue cone identity in zebrafish

Most vertebrate lineages retain a tetrachromatic visual system, which is supported by a functional combination of spectrally distinct multiple cone photoreceptors, ultraviolet (UV), blue, green, and red cones. The blue cone identity is ensured by selective expression of blue (sws2) opsin, and the mechanism is poorly understood because sws2 gene has been lost in mammalian species such as mouse, whose visual system has been extensively studied. Here, we pursued loss-of-function studies on transcription factors expressed predominantly in zebrafish cone photoreceptors and identified Foxq2 as a blue cone–specific factor driving sws2 gene expression. Foxq2 has dual functions acting as an activator of sws2 transcription and as a suppressor of UV (sws1) opsin transcription in blue cones. A wide range of vertebrate species retain both foxq2 and sws2 genes. We propose that Foxq2-dependent sws2 expression is a prevalent regulatory mechanism that was acquired at the early stage of vertebrate evolution.

Ganglion cells in larval zebrafish retina integrate inputs from multiple cone types

We recently showed the presence of seven physiological cone opsins-R1 (575 nm), R2 (556 nm), G1 (460 nm), G3 (480 nm), B1 (415 nm), B2 (440 nm), and UV (358 nm)-in electroretinogram (ERG) recordings of larval zebrafish (Danio rerio) retina. Larval ganglion cells (GCs) are generally thought to integrate only four cone opsin signals (red, green, blue, and UV). We address the question as to whether they may integrate seven cone spectral signals. Here we examined the 127 possible combinations of seven cone signals to find the optimal representation, as based on impulse discharge data sets from GC axons in the larval optic nerve. We recorded four varieties of light-response waveform, sustained-ON, transient-ON, ON-OFF, and OFF, based on the time course of mean discharge rates to all stimulus wavelengths combined. Modeling of GC responses revealed that each received 1-6 cone opsin signals, with a mean of 3.8 ± 1.3 cone signals/GC. Most onset or offset responses were opponent (ON, 80%; OFF, 100%). The most common cone signals were UV (93%), R2 (50%), G3 (55%), and G1 (60%). Seventy-three percent of cone opsin signals were excitatory, and 27% were inhibitory. UV signals favored excitation, whereas G3 and B2 signals favored inhibition. R1/R2, G1/G3, and B1/B2 opsin signals were selectively associated along a nonsynergistic/opponent axis. Overall, these results suggest that larval zebrafish GC spectral responses are complex and use inputs from the seven expressed opsins.NEW & NOTEWORTHY Ganglion cells in larval zebrafish retina have complex spectral responses driven by seven different cone opsin types. UV cone inputs are significant and excitatory to ganglion cells, whereas green and blue cone inputs favor inhibition. Most dramatic are the pentachromatic cells. These responses were identified at 5-6 days after fertilization, reflecting an impressive level of color processing not seen in older fish or mammals.