Thyroid hormone signaling specifies cone subtypes in human retinal organoids

CRISPR/Cas9 mediated specific ablation of vegfa in retinal pigment epithelium efficiently regresses choroidal neovascularization

The CRISPR/Cas9 system easily edits target genes in various organisms and is used to treat human diseases. In most therapeutic CRISPR studies, ubiquitously expressed promoters, such as CMV, CAG, and EF1α, are used; however, gene editing is sometimes necessary only in specific cell types relevant to the disease. Therefore, we aimed to develop a retinal pigment epithelium (RPE)-specific CRISPR/Cas9 system. We developed a CRISPR/Cas9 system that operates only in retinal pigment epithelium (RPE) by expressing Cas9 under the RPE-specific vitelliform macular dystrophy 2 promoter (pVMD2). This RPE-specific CRISPR/pVMD2-Cas9 system was tested in human retinal organoid and mouse model. We confirmed that this system works specifically in the RPE of human retinal organoids and mouse retina. In addition, the RPE-specific Vegfa ablation using the novel CRISPR-pVMD2-Cas9 system caused regression of choroidal neovascularization (CNV) without unwanted knock-out in the neural retina in laser-induced CNV mice, which is a widely used animal model of neovascular age-related macular degeneration. RPE-specific Vegfa knock-out (KO) and ubiquitous Vegfa KO were comparable in the efficient regression of CNV. The promoter substituted, cell type-specific CRISPR/Cas9 systems can be used in specific 'target cell' therapy, which edits genes while reducing unwanted off- 'target cell' effects.

Elimination of the color discrimination impairment along the blue-yellow axis in patients with hypothyroidism after treatment with levothyroxine as assessed by the Farnsworth-Munsell 100 hue test

Our previous study has shown that individuals with untreated hypothyroidism display significantly higher partial error scores (P E S) along the blue-yellow axis compared to the red-green axis than normal individuals using the Farnsworth-Munsell 100 hue test [J. Opt. Soc. Am. A37, A18 (2020)JOAOD60740-323210.1364/JOSAA.382390]. We wished to determine how color discrimination may change when hypothyroidism has been treated to the point of euthyroidism. Color discrimination was reassessed for 17 female individuals who had undergone treatment for hypothyroidism, and the results were compared with 22 female individuals without thyroid dysfunction. No statistically significant difference was found in the total error score (T E S) for the first and second measurements for both groups (p>0.45). The P E S for the hypothyroid group improved significantly in the previously impaired color regions after the treatment. Color discrimination defects found in untreated hypothyroidism can be negated with treatment of the condition over an appropriate time period.

Sunitinib efficacy with minimal toxicity in patient-derived retinoblastoma organoids

BACKGROUND

Recurrence of retinoblastoma (RB) following chemoreduction is common and is often managed with local (intra-arterial/intravitreal) chemotherapy. However, some tumors are resistant to even local administration of maximum feasible drug dosages, or effective tumor control and globe preservation may be achieved at the cost of vision loss due to drug-induced retinal toxicity. The aim of this study was to identify drugs with improved antitumor activity and more favorable retinal toxicity profiles via screening of potentially repurposable FDA-approved drugs in patient-derived tumor organoids.

METHODS

Genomic profiling of five RB organoids and the corresponding parental tissues was performed. RB organoids were screened with 133 FDA-approved drugs, and candidate drugs were selected based on cytotoxicity and potency. RNA sequencing was conducted to generate a drug signature from RB organoids, and the effects of drugs on cell cycle progression and proliferative tumor cone restriction were examined. Drug toxicity was assessed with human embryonic stem cell-derived normal retinal organoids. The efficacy/toxicity profiles of candidate drugs were compared with those of drugs in clinical use.

RESULTS

RB organoids maintained the genomic features of the parental tumors. Sunitinib was identified as highly cytotoxic against both classical RB1-deficient and novel MYCN-amplified RB organoids and inhibited proliferation while inducing differentiation in RB. Sunitinib was a more effective suppressor of proliferative tumor cones in RB organoids and had lower toxicity in normal retinal organoids than either melphalan or topotecan.

CONCLUSION

The efficacy and retinal toxicity profiles of sunitinib suggest that it could potentially be repurposed for local chemotherapy of RB.

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.

Retinal Ganglion Cells in a Dish: Current Strategies and Recommended Best Practices for Effective In Vitro Modeling of Development and Disease

The ability to derive retinal ganglion cells (RGCs) from human pluripotent stem cells (hPSCs) provides an extraordinary opportunity to study the development of RGCs as well as cellular mechanisms underlying their degeneration in optic neuropathies. In the past several years, multiple approaches have been established that allow for the generation of RGCs from hPSCs, with these methods greatly improved in more recent studies to yield mature RGCs that more faithfully recapitulate phenotypes within the eye. Nevertheless, numerous differences still remain between hPSC-RGCs and those found within the human eye, with these differences likely explained at least in part due to the environment in which hPSC-RGCs are grown. With the ultimate goal of generating hPSC-RGCs that most closely resemble those within the retina for proper studies of retinal development, disease modeling, as well as cellular replacement, we review within this manuscript the current effective approaches for the differentiation of hPSC-RGCs, as well as how they have been applied for the investigation of RGC neurodegenerative diseases such as glaucoma. Furthermore, we provide our opinions on the characteristics of RGCs necessary for their use as effective in vitro disease models and importantly, how these current systems should be improved to more accurately reflect disease states. The establishment of characteristics in differentiated hPSC-RGCs that more effectively mimic RGCs within the retina will not only enable their use as effective models of RGC development, but will also create a better disease model for the identification of mechanisms underlying the neurodegeneration of RGCs in disease states such as glaucoma, further facilitating the development of therapeutic approaches to rescue RGCs from degeneration in disease states.

Blimp-1/PRDM1 and Hr3/RORβ specify the blue-sensitive photoreceptor subtype in Drosophila by repressing the hippo pathway

During terminal differentiation of the mammalian retina, transcription factors control binary cell fate decisions that generate functionally distinct subtypes of photoreceptor neurons. For instance, Otx2 and RORβ activate the expression of the transcriptional repressor Blimp-1/PRDM1 that represses bipolar interneuron fate and promotes rod photoreceptor fate. Moreover, Otx2 and Crx promote expression of the nuclear receptor Nrl that promotes rod photoreceptor fate and represses cone photoreceptor fate. Mutations in these four transcription factors cause severe eye diseases such as retinitis pigmentosa. Here, we show that a post-mitotic binary fate decision in Drosophila color photoreceptor subtype specification requires ecdysone signaling and involves orthologs of these transcription factors: Drosophila Blimp-1/PRDM1 and Hr3/RORβ promote blue-sensitive (Rh5) photoreceptor fate and repress green-sensitive (Rh6) photoreceptor fate through the transcriptional repression of warts/LATS, the nexus of the phylogenetically conserved Hippo tumor suppressor pathway. Moreover, we identify a novel interaction between Blimp-1 and warts, whereby Blimp-1 represses a warts intronic enhancer in blue-sensitive photoreceptors and thereby gives rise to specific expression of warts in green-sensitive photoreceptors. Together, these results reveal that conserved transcriptional regulators play key roles in terminal cell fate decisions in both the Drosophila and the mammalian retina, and the mechanistic insights further deepen our understanding of how Hippo pathway signaling is repurposed to control photoreceptor fates for Drosophila color vision.

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.

Quantitative fate mapping: A general framework for analyzing progenitor state dynamics via retrospective lineage barcoding

Natural and induced somatic mutations that accumulate in the genome during development record the phylogenetic relationships of cells; whether these lineage barcodes capture the complex dynamics of progenitor states remains unclear. We introduce quantitative fate mapping, an approach to reconstruct the hierarchy, commitment times, population sizes, and commitment biases of intermediate progenitor states during development based on a time-scaled phylogeny of their descendants. To reconstruct time-scaled phylogenies from lineage barcodes, we introduce Phylotime, a scalable maximum likelihood clustering approach based on a general barcoding mutagenesis model. We validate these approaches using realistic in silico and in vitro barcoding experiments. We further establish criteria for the number of cells that must be analyzed for robust quantitative fate mapping and a progenitor state coverage statistic to assess the robustness. This work demonstrates how lineage barcodes, natural or synthetic, enable analyzing progenitor fate and dynamics long after embryonic development in any organism.

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.

Bioengineering Human Pluripotent Stem Cell-Derived Retinal Organoids and Optic Vesicle-Containing Brain Organoids for Ocular Diseases

Retinal organoids are three-dimensional (3D) structures derived from human pluripotent stem cells (hPSCs) that mimic the retina's spatial and temporal differentiation, making them useful as in vitro retinal development models. Retinal organoids can be assembled with brain organoids, the 3D self-assembled aggregates derived from hPSCs containing different cell types and cytoarchitectures that resemble the human embryonic brain. Recent studies have shown the development of optic cups in brain organoids. The cellular components of a developing optic vesicle-containing organoids include primitive corneal epithelial and lens-like cells, retinal pigment epithelia, retinal progenitor cells, axon-like projections, and electrically active neuronal networks. The importance of retinal organoids in ocular diseases such as age-related macular degeneration, Stargardt disease, retinitis pigmentosa, and diabetic retinopathy are described in this review. This review highlights current developments in retinal organoid techniques, and their applications in ocular conditions such as disease modeling, gene therapy, drug screening and development. In addition, recent advancements in utilizing extracellular vesicles secreted by retinal organoids for ocular disease treatments are summarized.

Foveal Cone Structure in Patients With Blue Cone Monochromacy

Purpose

Blue cone monochromacy (BCM) is a rare inherited cone disorder in which both long- (L-) and middle- (M-) wavelength sensitive cone classes are either impaired or nonfunctional. Assessing genotype-phenotype relationships in BCM can improve our understanding of retinal development in the absence of functional L- and M-cones. Here we examined foveal cone structure in patients with genetically-confirmed BCM, using adaptive optics scanning light ophthalmoscopy (AOSLO).

Methods

Twenty-three male patients (aged 6-75 years) with genetically-confirmed BCM were recruited for high-resolution imaging. Eight patients had a deletion of the locus control region (LCR), and 15 had a missense mutation-Cys203Arg-affecting the first two genes in the opsin gene array. Foveal cone structure was assessed using confocal and non-confocal split-detection AOSLO across a 300 × 300 µm area, centered on the location of peak cell density.

Results

Only one of eight patients with LCR deletions and 10 of 15 patients with Cys203Arg mutations had analyzable images. Mean total cone density for Cys203Arg patients was 16,664 ± 11,513 cones/mm2 (n = 10), which is, on average, around 40% of normal. Waveguiding cone density was 2073 ± 963 cones/mm2 (n = 9), which was consistent with published histological estimates of S-cone density in the normal eye. The one patient with an LCR deletion had a total cone density of 10,246 cones/mm2 and waveguiding density of 1535 cones/mm2.

Conclusions

Our results show that BCM patients with LCR deletions and Cys203Arg mutations have a population of non-waveguiding photoreceptors, although the spectral identity and level of function remain unknown.

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.

Selective deficits of S-cone in thyroid-associated ophthalmopathy patients without clinical signs of dysthyroid optic neuropathy

Purpose

We explored whether thyroid-associated ophthalmopathy (TAO) patients without clinical signs of dysthyroid optic neuropathy (DON) would have a selective deficit mediated by S-cone.

Methods

Thirty-two TAO patients without clinical signs of DON (non-DON, 42.03 ± 9.59 years old) and 27 healthy controls (41.46 ± 6.72 years old) participated in this prospective, cross-sectional study. All observers were tested psychophysically after passing color screening tests and a comprehensive ocular examination. Isolated L-, M-, and S-cone contrast thresholds were measured at 0.5 cyc/deg using Gabor patches. We calculated the area under the receiver operating characteristic (ROC) curve to quantify the ability of chromatic contrast sensitivity to detect the early visual function changes in non-DON patients.

Results

S-cone contrast sensitivity in non-DON patients was found to be lower than that of healthy controls (P < 0.001), whereas the sensitivities to L- and M-cone Gabor patches were similar between these two groups (P = 0.297, 0.666, respectively). Our analysis of the ROC curve revealed that the sensitivity to S-cone had the highest index to discriminate non-DON patients from healthy controls (AUC = 0.846, P < 0.001). The deficit of S-cone was significantly correlated with muscle index in non-DON patients (R = 0.576, P = 0.001).

Conclusion

There is a selective S-cone deficit in the early stage of TAO. S-cone contrast sensitivity could serve as a sensitive measure of visual impairments associated with early DON in patients with TAO.

Azole-Induced Color Vision Deficiency Associated with Thyroid Hormone Signaling: An Integrated In Vivo, In Vitro, and In Silico Study

Azoles that are used in pesticides, pharmaceuticals, and personal care products can have toxic effects on fish. However, there is no information regarding azole-induced visual disorder associated with thyroid disruption. We evaluated changes in retinal morphology, optokinetic response, transcript abundance of the genes involved in color perception and hypothalamic-pituitary-thyroid (HPT) axis, and thyroid hormone (TH) levels in zebrafish larvae exposed to common azoles, such as climbazole (CBZ, 0.1 and 10 μg/L) and triadimefon (TDF, 50 and 500 μg/L), at environmentally relevant and predicted worst-case environmental concentrations. Subsequently, the effect of azoles on TH-dependent GH3 cell proliferation and thyroid receptor (TR)-regulated transcriptional activity, as well as the in silico binding affinity between azoles and TR isoforms, was investigated. Azole exposure decreased cell densities of the ganglion cell layer, inner nuclear layer, and photoreceptor layer. Zebrafish larvae exposed to environmentally relevant concentrations of CBZ and TDF showed a decrease in optokinetic response to green-white and red-white stripes but not blue-white stripes, consistent with disturbance in the corresponding opsin gene expression. Azole exposure also reduced triiodothyronine levels and concomitantly increased HPT-related gene expression. Molecular docking analysis combined with in vitro TR-mediated transactivation and dual-luciferase reporter assays demonstrated that CBZ and TDF exhibited TR antagonism. These results are comparable to those obtained from a known TR antagonist, namely, TR antagonist 1, as a positive control. Therefore, damage to specific color perception by azoles appears to result from lowered TH signaling, indicating the potential threat of environmental TH disruptors to the visual function of fish.

Timed Notch Inhibition Drives Photoreceptor Fate Specification in Human Retinal Organoids

Purpose

Transplanting photoreceptors from human pluripotent stem cell–derived retinal organoids have the potential to reverse vision loss in affected individuals. However, transplantable photoreceptors are only a subset of all cells in the organoids. Hence, the goal of our current study was to accelerate and synchronize photoreceptor differentiation in retinal organoids by inhibiting the Notch signaling pathway at different developmental time-points using a small molecule, PF-03084014 (PF).

Methods

Human induced pluripotent stem cell– and human embryonic stem cells–derived retinal organoids were treated with 10 µM PF for 3 days starting at day 45 (D45), D60, D90, and D120 of differentiation. Organoids were collected at post-treatment days 14, 28, and 42 and analyzed for progenitor and photoreceptor markers and Notch pathway inhibition by immunohistochemistry (IHC), quantitative PCR, and bulk RNA sequencing (n = 3–5 organoids from three independent experiments).

Results

Retinal organoids collected after treatment showed a decrease in progenitor markers (KI67, VSX2, PAX6, and LHX2) and an increase in differentiated pan-photoreceptor markers (OTX2, CRX, and RCVRN) at all organoid stages except D120. PF-treated organoids at D45 and D60 exhibited an increase in cone photoreceptor markers (RXRG and ARR3). PF treatment at D90 revealed an increase in cone and rod photoreceptors markers (ARR3, NRL, and NR2E3). Bulk RNA sequencing analysis mirrored the immunohistochemistry data and quantitative PCR confirmed Notch effector inhibition.

Conclusions

Timing the Notch pathway inhibition in human retinal organoids to align with progenitor competency stages can yield an enriched population of early cone or rod photoreceptors.

An enhancer located in a Pde6c intron drives transient expression in the cone photoreceptors of developing mouse and human retinas

How cone photoreceptors are formed during retinal development is only partially known. This is in part because we do not fully understand the gene regulatory network responsible for cone genesis. We reasoned that cis-regulatory elements (enhancers) active in nascent cones would be regulated by the same upstream network that controls cone formation. To dissect this network, we searched for enhancers active in developing cones. By electroporating enhancer-driven fluorescent reporter plasmids, we observed that a sequence within an intron of the cone-specific Pde6c gene acted as an enhancer in developing mouse cones. Similar fluorescent reporter plasmids were used to generate stable transgenic human induced pluripotent stem cells that were then grown into three-dimensional human retinal organoids. These organoids contained fluorescently labeled cones, demonstrating that the Pde6c enhancer was also active in human cones. We observed that enhancer activity was transient and labeled a minor population of developing rod photoreceptors in both mouse and human systems. This cone-enriched pattern argues that the Pde6c enhancer is activated in cells poised between rod and cone fates. Additionally, it suggests that the Pde6c enhancer is activated by the same regulatory network that selects or stabilizes cone fate choice. To further understand this regulatory network, we identified essential enhancer sequence regions through a series of mutagenesis experiments. This suggested that the Pde6c enhancer was regulated by transcription factor binding at five or more locations. Binding site predictions implicated transcription factor families known to control photoreceptor formation and families not previously associated with cone development. These results provide a framework for deciphering the gene regulatory network that controls cone genesis in both human and mouse systems. Our new transgenic human stem cell lines provide a tool for determining which cone developmental mechanisms are shared and distinct between mice and humans.

The Prospects for Retinal Organoids in Treatment of Retinal Diseases

Retinal degeneration (RD) is a significant cause of incurable blindness worldwide. Photoreceptors and retinal pigmented epithelium are irreversibly damaged in advanced RD. Functional replacement of photoreceptors and/or retinal pigmented epithelium cells is a promising approach to restoring vision. This paper reviews the current status and explores future prospects of the transplantation therapy provided by pluripotent stem cell-derived retinal organoids (ROs). This review summarizes the status of rodent RD disease models and discusses RO culture and analytical tools to evaluate RO quality and function. Finally, we review and discuss the studies in which RO-derived cells or sheets were transplanted. In conclusion, methods to derive ROs from pluripotent stem cells have significantly improved and become more efficient in recent years. Meanwhile, more novel technologies are applied to characterize and validate RO quality. However, opportunity remains to optimize tissue differentiation protocols and achieve better RO reproducibility. In order to screen high-quality ROs for downstream applications, approaches such as noninvasive and label-free imaging and electrophysiological functional testing are promising and worth further investigation. Lastly, transplanted RO-derived tissues have allowed improvements in visual function in several RD models, showing promises for clinical applications in the future.

Causal Associations of Thyroid Function and Age-Related Macular Degeneration: A Two-Sample Mendelian Randomization Study

PURPOSE

To determine whether causal association lies between thyroid function and age-related macular degeneration (AMD) risk in human beings.

DESIGN

Two-sample Mendelian randomization (MR) study.

METHODS

The single-nucleotide polymorphisms associated with free thyroxine (FT4) and thyroid-stimulating hormone (TSH) were selected from a genome-wide association study (GWAS) of 72,167 individuals of European descent. Summary-level data for AMD were obtained from a GWAS published by the International Age-related Macular Degeneration Genomics Consortium of 33,526 individuals (16,144 cases and 17,832 controls). An inverse-variance-weighted (IVW) method was the main MR analysis. Maximum likelihood, weighted median, MR-Egger, MR-pleiotropy residual sum outlier methods were used for the sensitivity analysis.

RESULTS

An increase of 1 SD in genetically predicted FT4 levels was found to be significantly associated with an 18.9 % increase in the overall AMD risk (P = .005). In the multivariable MR analysis controlling for TSH level, the causal effect of FT4 level on the risk of AMD remained (odds ratio [OR] = 1.207, P = .004). A 1-SD increase in TSH levels was nominally associated with a 10.0% decrease in the overall AMD risk (P = .032). After adjusting for FT4 level by multivariable MR analysis, no direct causal relationship was found between TSH level and AMD risk (95% CI = 0.810, 1.125, P = .582).

CONCLUSIONS

Genetic variants predisposing to higher FT4 levels within the normal range were associated with higher AMD risk. Further studies are required to understand the mechanism underlying this putative causal relationship.

Self-Organization of the Retina during Eye Development, Retinal Regeneration In Vivo, and in Retinal 3D Organoids In Vitro

Self-organization is a process that ensures histogenesis of the eye retina. This highly intricate phenomenon is not sufficiently studied due to its biological complexity and genetic heterogeneity. The review aims to summarize the existing central theories and ideas for a better understanding of retinal self-organization, as well as to address various practical problems of retinal biomedicine. The phenomenon of self-organization is discussed in the spatiotemporal context and illustrated by key findings during vertebrate retina development in vivo and retinal regeneration in amphibians in situ. Described also are histotypic 3D structures obtained from the disaggregated retinal progenitor cells of birds and retinal 3D organoids derived from the mouse and human pluripotent stem cells. The review highlights integral parts of retinal development in these conditions. On the cellular level, these include competence, differentiation, proliferation, apoptosis, cooperative movements, and migration. On the physical level, the focus is on the mechanical properties of cell- and cell layer-derived forces and on the molecular level on factors responsible for gene regulation, such as transcription factors, signaling molecules, and epigenetic changes. Finally, the self-organization phenomenon is discussed as a basis for the production of retinal organoids, a promising model for a wide range of basic scientific and medical applications.

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.

Thyroid hormone tinkering elicits integrated phenotypic changes potentially explaining rapid adaptation of color vision in cichlid fish

Vision is critical for most vertebrates, including fish. One challenge that aquatic habitats pose is the high variability in spectral properties depending on depth and the inherent optical properties of the water. By altering opsin gene expression and chromophore usage, cichlid fish modulate visual sensitivities to maximize sensory input from the available light in their respective habitat. Thyroid hormone (TH) has been proposed to play a role in governing adaptive diversification in visual sensitivity in Nicaraguan Midas cichlids, which evolved in less than 4000 generations. As suggested by indirect measurements of TH levels (i.e., expression of deiodinases), populations adapted to short wavelength light in clear lakes have lower TH levels than ones inhabiting turbid lakes enriched in long-wavelength light. We experimentally manipulated TH levels by exposing 2-week-old Midas cichlids to exogenous TH or a TH inhibitor and measured opsin gene expression and chromophore usage (via cyp27c1 expression). Although exogenous TH induces long-wavelength sensitivity by changing opsin gene expression and chromophore usage in a concerted manner, TH-inhibited fish exhibit a visual phenotype with sensitivities shifted to shorter wavelengths. Tinkering with TH levels in eyes results in concerted phenotypic changes that can provide a rapid mechanism of adaptation to novel light environments.

Generation of a Cone Photoreceptor-specific GNGT2 Reporter Line in Human Pluripotent Stem Cells

Fluorescent reporter lines generated in human pluripotent stem cells are a highly useful tool to track, isolate, and analyze cell types and lineages in live cultures. Here, we generate the first human cone photoreceptor reporter cell line by CRISPR/Cas9 genome editing of a human embryonic stem cell (hESC) line to tag both alleles of the Guanine nucleotide-binding protein subunit gamma-T2 (GNGT2) gene with a mCherry reporter cassette. Three-dimensional optic vesicle-like structures were produced to verify reporter fidelity and track cones throughout their development in culture. The GNGT2-T2A-mCherry hESC line faithfully and robustly labels GNGT2-expressing cones throughout the entirety of their differentiation in vitro, recapitulating normal fetal expression of this gene. Our observations indicate that human cones undergo significant migratory activity during the course of differentiation in vitro. Consistent with this, our analysis of human fetal retinae from different stages of development finds positional differences of the cone population depending on their state of maturation. This novel reporter line will provide a useful tool for investigating human cone development and disease.

Retinal Organoids and Retinal Prostheses: An Overview

Despite the progress of modern medicine in the last decades, millions of people diagnosed with retinal dystrophies (RDs), such as retinitis pigmentosa, or age-related diseases, such as age-related macular degeneration, are suffering from severe visual impairment or even legal blindness. On the one hand, the reprogramming of somatic cells into induced pluripotent stem cells (iPSCs) and the progress of three-dimensional (3D) retinal organoids (ROs) technology provide a great opportunity to study, understand, and even treat retinal diseases. On the other hand, research advances in the field of electronic retinal prosthesis using inorganic photovoltaic polymers and the emergence of organic semiconductors represent an encouraging therapeutical strategy to restore vision to patients at the late onset of the disease. This review will provide an overview of the latest advancement in both fields. We first describe the retina and the photoreceptors, briefly mention the most used RD animal models, then focus on the latest RO differentiation protocols, carry out an overview of the current technology on inorganic and organic retinal prostheses to restore vision, and finally summarize the potential utility and applications of ROs.

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.

Genetic and Neurological Deficiencies in the Visual System of mct8 Mutant Zebrafish

Thyroid hormones (THs; T3 and T4) enter cells using specific transporters and regulate development and metabolism. Mutation in the TH transporter monocarboxylate transporter 8 (MCT8, SLC16A2) is associated with brain hypothyroidism and neurological impairment. We established mct8 mutant (mct8-/-) zebrafish as a model for MCT8 deficiency, which causes endocrinological, neurological, and behavioral alterations. Here, we profiled the transcriptome of mct8-/- larvae. Among hundreds of differentially expressed genes, the expression of a cluster of vision-related genes was distinct. Specifically, the expression of the opsin 1 medium wave sensitive 2 (opn1mw2) decreased in two mct8 mutants: mct8-/- and mct8^-25bp-/- larvae, and under pharmacological inhibition of TH production. Optokinetic reflex (OKR) assays showed a reduction in the number of conjugated eye movements, and live imaging of genetically encoded Ca²⁺ indicator revealed altered neuronal activity in the pretectum area of mct8^-25bp-/- larvae. These results imply that MCT8 and THs regulate the development of the visual system and suggest a mechanism to the deficiencies observed in the visual system of MCT8-deficiency patients.

Mnemonic-opto-synaptic transistor for in-sensor vision system

A mnemonic-opto-synaptic transistor (MOST) that has triple functions is demonstrated for an in-sensor vision system. It memorizes a photoresponsivity that corresponds to a synaptic weight as a memory cell, senses light as a photodetector, and performs weight updates as a synapse for machine vision with an artificial neural network (ANN). Herein the memory function added to a previous photodetecting device combined with a photodetector and a synapse provides a technical breakthrough for realizing in-sensor processing that is able to perform image sensing and signal processing in a sensor. A charge trap layer (CTL) was intercalated to gate dielectrics of a vertical pillar-shaped transistor for the memory function. Weight memorized in the CTL makes photoresponsivity tunable for real-time multiplication of the image with a memorized photoresponsivity matrix. Therefore, these multi-faceted features can allow in-sensor processing without external memory for the in-sensor vision system. In particular, the in-sensor vision system can enhance speed and energy efficiency compared to a conventional vision system due to the simultaneous preprocessing of massive data at sensor nodes prior to ANN nodes. Recognition of a simple pattern was demonstrated with full sets of the fabricated MOSTs. Furthermore, recognition of complex hand-written digits in the MNIST database was also demonstrated with software simulations.

Sex-Dependent Environmental Health Risk Analysis of Flupyradifurone

Pesticides are used in agricultural production worldwide, resulting in widespread environmental pollution. Many diseases are closely related to exposure to pesticide residues. In this study, the association between exposure to the pesticide flupyradifurone (FPF), a substitute for neonicotinoids, and sex-dependent thyroid dysfunction was explored for the first time. Exposure using rat models revealed that the FPF metabolism is sex-dependent, with males preferring N-dealkylation and hydrolytic metabolism and females preferring hydroxylation. In particular, novel chloropyridine-site hydroxylation I and II metabolic pathways of FPF were discovered. More importantly, differential metabolic pathways of FPF induced sex-based dysregulation of the hypothalamic-pituitary-thyroid axis, in which females exhibited subclinical hyperthyroidism, while males displayed abnormal hypothyroidism. This may be attributed to the potential agonistic or antagonistic effect of FPF sex-dependent metabolites on liver thyroid hormone receptors. Furthermore, FPF exposure further mediated sex-specific dysregulation of cellular lipid homeostasis, with abnormal fatty acid β-oxidation and excessive energy expenditure in females and the risk of excessive accumulation of triglycerides in males. These results illustrate the potential risk of sex-related thyroid metabolic diseases caused by FPF and provide an important basis and support for further studies of FPF on human health and as an environmental pollutant.

Toxic effects of triclocarban on larval zebrafish: A focus on visual dysfunction

Triclocarban (TCC) is considered an endocrine disruptor and shows antagonist activity on thyroid receptors. In view of the report that thyroid hormone signaling mediates retinal cone photoreceptor specification, we hypothesize that TCC could impair visual function, which is vital to wildlife. In order to verify our hypothesis, we assessed alteration in the retinal structure (retinal layer thickness and cell density), visually-mediated behavior, cone and rod opsin gene expression, and photoreceptor immunostaining in zebrafish larvae exposed to TCC at environmentally realistic concentrations (0.16 ± 0.005 µg/L, L-group) and one-fifth of the median lethal concentrations (25.4 ± 1.02 µg/L, H-group). Significant decrease in eye size, ganglion cell density, optokinetic response, and phototactic response can be observed in the L-group, while the thickness of outer nuclear layer, where the cell bodies of cone and rod cells are located, was significantly reduced with the down-regulation of critical opsin gene (opn1sw2, opn1mw1, opn1mw3, opn1lw1, opn1lw2, and rho) expression and rhodopsin immunofluorescence in the H-group. It should be noted that TCC could affect the sensitivity of zebrafish larvae to red and green light according to the results of behavioral and opsin gene expression analysis. These findings provide the first evidence to support our hypothesis that the visual system, a novel toxicological target, is affected by TCC. Consequently, we urgently call for a more in-depth exploration of TCC-induced ocular toxicity to aquatic organisms and even to humans.

Multiocular organoids from human induced pluripotent stem cells displayed retinal, corneal, and retinal pigment epithelium lineages

BACKGROUND

Recently, great efforts have been made to design protocols for obtaining ocular cells from human stem cells to model diseases or for regenerative purposes. Current protocols generally focus on isolating retinal cells, retinal pigment epithelium (RPE), or corneal cells and fail to recapitulate the complexity of the tissue during eye development. Here, the generation of more advanced in vitro multiocular organoids from human induced pluripotent stem cells (hiPSCs) is demonstrated.

METHODS

A 2-step method was established to first obtain self-organized multizone ocular progenitor cells (mzOPCs) from 2D hiPSC cultures within three weeks. Then, after the cells were manually isolated and grown in suspension, 3D multiocular organoids were generated to model important cellular features of developing eyes.

RESULTS

In the 2D culture, self-formed mzOPCs spanned the neuroectoderm, surface ectoderm, neural crest, and RPE, mimicking early stages of eye development. After lifting, mzOPCs developed into different 3D multiocular organoids composed of multiple cell lineages including RPE, retina, and cornea, and interactions between the different cell types and regions of the eye system were observed. Within these organoids, the retinal regions exhibited correct layering and contained all major retinal cell subtypes as well as retinal morphological cues, whereas the corneal regions closely resembled the transparent ocular-surface epithelium and contained of corneal, limbal, and conjunctival epithelial cells. The arrangement of RPE cells also formed organoids composed of polarized pigmented epithelial cells at the surface that were completely filled with collagen matrix.

CONCLUSIONS

This approach clearly demonstrated the advantages of the combined 2D-3D construction tissue model as it provided a more ocular native-like cellular environment than that of previous models. In this complex preparations, multiocular organoids may be used to model the crosstalk between different cell types in eye development and disease.

CRISPR Generated SIX6 and POU4F2 Reporters Allow Identification of Brain and Optic Transcriptional Differences in Human PSC-Derived Organoids

Human pluripotent stem cells (PSCs) represent a powerful tool to investigate human eye development and disease. When grown in 3D, they can self-assemble into laminar organized retinas; however, variation in the size, shape and composition of individual organoids exists. Neither the microenvironment nor the timing of critical growth factors driving retinogenesis are fully understood. To explore early retinal development, we developed a SIX6-GFP reporter that enabled the systematic optimization of conditions that promote optic vesicle formation. We demonstrated that early hypoxic growth conditions enhanced SIX6 expression and promoted eye formation. SIX6 expression was further enhanced by sequential inhibition of Wnt and activation of sonic hedgehog signaling. SIX6 + optic vesicles showed RNA expression profiles that were consistent with a retinal identity; however, ventral diencephalic markers were also present. To demonstrate that optic vesicles lead to bona fide "retina-like" structures we generated a SIX6-GFP/POU4F2-tdTomato dual reporter line that labeled the entire developing retina and retinal ganglion cells, respectively. Additional brain regions, including the hypothalamus and midbrain-hindbrain (MBHB) territories were identified by harvesting SIX6 + /POU4F2- and SIX6- organoids, respectively. Using RNAseq to study transcriptional profiles we demonstrated that SIX6-GFP and POU4F2-tdTomato reporters provided a reliable readout for developing human retina, hypothalamus, and midbrain/hindbrain organoids.

Characterization and staging of outer plexiform layer development in human retina and retinal organoids

The development of the first synapse of the visual system between photoreceptors and bipolar cells in the outer plexiform layer (OPL) of the human retina is critical for visual processing but poorly understood. By studying the maturation state and spatial organization of photoreceptors, depolarizing bipolar cells, and horizontal cells in the human fetal retina, we establish a pseudo-temporal staging system for OPL development that we term OPL-Stages 0 to 4. This was validated through quantification of increasingly precise subcellular localization of Bassoon to the OPL with each stage (p<0.0001). By applying these OPL staging criteria to human retinal organoids (HROs) derived from human embryonic and induced pluripotent stem cells, we observed comparable maturation from OPL-Stage 0 at day 100 in culture up to OPL-Stage 3 by day 160. Quantification of presynaptic protein localization confirmed progression from OPL-Stage 0 to 3 (p<0.0001). Overall, this study defines stages of human OPL development through mid-gestation and establishes HROs as a model system that recapitulates key aspects of human photoreceptor-bipolar cell synaptogenesis in vitro.

Retinal organoids as models for development and diseases

The evolution of pluripotent stem cell-derived retinal organoids (ROs) has brought remarkable opportunities for developmental studies while also presenting new therapeutic avenues for retinal diseases. With a clear understanding of how well these models mimic native retinas, such preclinical models may be crucial tools that are widely used for the more efficient translation of studies into novel treatment strategies for retinal diseases. Genetic modifications or patient-derived ROs can allow these models to simulate the physical microenvironments of the actual disease process. However, we are currently at the beginning of the three-dimensional (3D) RO era, and a general quantitative technology for analyzing ROs derived from numerous differentiation protocols is still missing. Continued efforts to improve the efficiency and stability of differentiation, as well as understanding the disparity between the artificial retina and the native retina and advancing the current treatment strategies, will be essential in ensuring that these scientific advances can benefit patients with retinal disease. Herein, we briefly discuss RO differentiation protocols, the current applications of RO as a disease model and the treatments for retinal diseases by using RO modeling, to have a clear view of the role of current ROs in retinal development and diseases.

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.

Human brain organoids assemble functionally integrated bilateral optic vesicles

During embryogenesis, optic vesicles develop from the diencephalon via a multistep process of organogenesis. Using induced pluripotent stem cell (iPSC)-derived human brain organoids, we attempted to simplify the complexities and demonstrate formation of forebrain-associated bilateral optic vesicles, cellular diversity, and functionality. Around day 30, brain organoids attempt to assemble optic vesicles, which develop progressively as visible structures within 60 days. These optic vesicle-containing brain organoids (OVB-organoids) constitute a developing optic vesicle's cellular components, including primitive corneal epithelial and lens-like cells, retinal pigment epithelia, retinal progenitor cells, axon-like projections, and electrically active neuronal networks. OVB-organoids also display synapsin-1, CTIP-positive myelinated cortical neurons, and microglia. Interestingly, various light intensities could trigger photosensitive activity of OVB-organoids, and light sensitivities could be reset after transient photobleaching. Thus, brain organoids have the intrinsic ability to self-organize forebrain-associated primitive sensory structures in a topographically restricted manner and can allow interorgan interaction studies within a single organoid.

A look into retinal organoids: methods, analytical techniques, and applications

Inherited retinal diseases (IRDs) cause progressive loss of light-sensitive photoreceptors in the eye and can lead to blindness. Gene-based therapies for IRDs have shown remarkable progress in the past decade, but the vast majority of forms remain untreatable. In the era of personalised medicine, induced pluripotent stem cells (iPSCs) emerge as a valuable system for cell replacement and to model IRD because they retain the specific patient genome and can differentiate into any adult cell type. Three-dimensional (3D) iPSCs-derived retina-like tissue called retinal organoid contains all major retina-specific cell types: amacrine, bipolar, horizontal, retinal ganglion cells, Müller glia, as well as rod and cone photoreceptors. Here, we describe the main applications of retinal organoids and provide a comprehensive overview of the state-of-art analysis methods that apply to this model system. Finally, we will discuss the outlook for improvements that would bring the cellular model a step closer to become an established system in research and treatment development of IRDs.

Human neural organoids: Models for developmental neurobiology and disease

Human organoids stand at the forefront of basic and translational research, providing experimentally tractable systems to study human development and disease. These stem cell-derived, in vitro cultures can generate a multitude of tissue and organ types, including distinct brain regions and sensory systems. Neural organoid systems have provided fundamental insights into molecular mechanisms governing cell fate specification and neural circuit assembly and serve as promising tools for drug discovery and understanding disease pathogenesis. In this review, we discuss several human neural organoid systems, how they are generated, advances in 3D imaging and bioengineering, and the impact of organoid studies on our understanding of the human nervous system.

ATF6 is essential for human cone photoreceptor development

Endoplasmic reticulum (ER) stress and Unfolded Protein Response (UPR) signaling promote the pathology of many human diseases. Loss-of-function variants of the UPR regulator Activating Transcription Factor 6 (ATF6) cause severe congenital vision loss diseases such as achromatopsia by unclear pathomechanisms. To investigate this, we generated retinal organoids from achromatopsia patient induced pluripotent stem cells carrying ATF6 disease variants and from gene-edited ATF6 null hESCs. We found that achromatopsia patient and ATF6 null retinal organoids failed to form cone structures concomitant with loss of cone phototransduction gene expression, while rod photoreceptors developed normally. Adaptive optics retinal imaging of achromatopsia patients carrying ATF6 variants also showed absence of cone inner/outer segment structures but preserved rod structures, mirroring the defect in cone formation observed in our retinal organoids. These results establish that ATF6 is essential for human cone development. Interestingly, we find that a selective small molecule ATF6 signaling agonist restores the transcriptional activity of some ATF6 disease-causing variants and stimulates cone growth and gene expression in patient retinal organoids carrying these variants. These findings support that pharmacologic targeting of the ATF6 pathway can promote human cone development and should be further explored for blinding retinal diseases.

Retinal Organoid Technology: Where Are We Now?

It is difficult to regenerate mammalian retinal cells once the adult retina is damaged, and current clinical approaches to retinal damages are very limited. The introduction of the retinal organoid technique empowers researchers to study the molecular mechanisms controlling retinal development, explore the pathogenesis of retinal diseases, develop novel treatment options, and pursue cell/tissue transplantation under a certain genetic background. Here, we revisit the historical background of retinal organoid technology, categorize current methods of organoid induction, and outline the obstacles and potential solutions to next-generation retinal organoids. Meanwhile, we recapitulate recent research progress in cell/tissue transplantation to treat retinal diseases, and discuss the pros and cons of transplanting single-cell suspension versus retinal organoid sheet for cell therapies.

Pluripotent stem cell therapy for retinal diseases

Pluripotent stem cells (PSCs), which include human embryonic stem cells (hESCs) and induced pluripotent stem cell (iPSC), have been used to study development of disease processes, and as potential therapies in multiple organ systems. In recent years, there has been increasing interest in the use of PSC-based transplantation to treat disorders of the retina in which retinal cells have been functionally damaged or lost through degeneration. The retina, which consists of neuronal tissue, provides an excellent system to test the therapeutic utility of PSC-based transplantation due to its accessibility and the availability of high-resolution imaging technology to evaluate effects. Preclinical trials in animal models of retinal diseases have shown improvement in visual outcomes following subretinal transplantation of PSC-derived photoreceptors or retinal pigment epithelium (RPE) cells. This review focuses on preclinical studies and clinical trials exploring the use of PSCs for retinal diseases. To date, several phase I/II clinical trials in patients with age-related macular degeneration (AMD) and Stargardt disease (STGD1) have demonstrated the safety and feasibility of PSC-derived RPE transplantation. Additional phase I/II clinical trials using PSC-derived RPE or photoreceptor cells for the treatment of AMD, STGD1, and also retinitis pigmentosa (RP) are currently in the pipeline. As this field continues to evolve, additional technologies may enhance PSC-derived cell transplantation through gene-editing of autologous cells, transplantation of more complex cellular structures such as organoids, and monitoring of transplanted cells through novel imaging technologies.

Human retinal model systems: Strengths, weaknesses, and future directions

The retina is a complex neuronal structure that converts light energy into visual perception. Many specialized aspects of the primate retina, including a cone rich macula for high acuity vision, ocular size, and cell type diversity are not found in other animal models. In addition, the unique morphologies and distinct laminar positions of cell types found in the retina make this model system ideal for the study of neuronal cell fate specification. Many key early events of human retinal development are inaccessible to investigation as they occur during gestation. For these reasons, it has been necessary to develop retinal model systems to gain insight into human-specific retinal development and disease. Recent advances in culturing retinal tissue have generated new systems for retinal research and have moved us closer to generating effective regenerative therapies for vision loss. Here, we describe the strengths, weaknesses, and future directions for different human retinal model systems including dissociated primary tissue, explanted primary tissue, retinospheres, and stem cell-derived retinal organoids.

Characterization and allogeneic transplantation of a novel transgenic cone-rich donor mouse line

OBJECTIVES

Cone photoreceptor transplantation is a potential treatment for macular diseases. The optimal conditions for cone transplantation are poorly understood, partly because of the scarcity of cones in donor mice. To facilitate allogeneic cone photoreceptor transplantation studies in mice, we aimed to create and characterize a donor mouse model containing a cone-rich retina with a cone-specific enhanced green fluorescent protein (EGFP) reporter.

METHODS

We generated OPN1LW-EGFP/NRL-/- mice by crossing NRL-/- and OPN1LW-EGFP mice. We characterized the anatomical phenotype of OPN1LW-EGFP/NRL-/- mice using multimodal confocal scanning laser ophthalmoscopy (cSLO) imaging, immunohistology, and transmission electron microscopy. We evaluated retinal function using electroretinography (ERG), including 465 and 525 nm chromatic stimuli. Retinal sheets and cell suspensions from OPN1LW-EGFP/NRL-/- mice were transplanted subretinally into immunodeficient Rd1 mice.

RESULTS

OPN1LW-EGFP/NRL-/- retinas were enriched with OPN1LW-EGFP+ and S-opsin+ cone photoreceptors in a dorsal-ventral distribution gradient. Cone photoreceptors co-expressing OPNL1W-EGFP and S-opsin significantly increased in OPN1LW-EGFP/NRL-/- compared to OPN1LW-EGFP mice. Temporal dynamics of rosette formation in the OPN1LW-EGFP/NRL-/- were similar as the NRL-/- with peak formation at P15. Rosettes formed preferentially in the ventral retina. The outer retina in P35 OPN1LW-EGFP/NRL-/- was thinner than NRL-/- controls. The OPN1LW-EGFP/NRL-/- ERG response amplitudes to 465 nm stimulation were similar to, but to 535 nm stimulation were lower than, NRL-/- controls. Three months after transplantation, the suspension grafts showed greater macroscopic degradation than sheet grafts. Retinal sheet grafts from OPN1LW-EGFP/NRL-/- mice showed greater S-opsin + cone survival than suspension grafts from the same strain.

CONCLUSIONS

OPN1LW-EGFP/NRL-/- retinae were enriched with S-opsin+ photoreceptors. Sustained expression of EGFP facilitated the longitudinal tracking of transplanted donor cells. Transplantation of cone-rich retinal grafts harvested prior to peak rosette formation survived and differentiated into cone photoreceptor subtypes. Photoreceptor sheet transplantation may promote greater macroscopic graft integrity and S-opsin+ cone survival than cell suspension transplantation, although the mechanism underlying this observation is unclear at present. This novel cone-rich reporter mouse strain may be useful to study the influence of graft structure on cone survival.

Organoid technology for tissue engineering

For centuries, attempts have been continuously made to artificially reconstitute counterparts of in vivo organs from their tissues or cells. Only in the recent decade has organoid technology as a whole technological field systematically emerged and been shown to play important roles in tissue engineering. Based on their self-organizing capacities, stem cells of versatile organs, both harvested and induced, can form 3D structures that are structurally and functionally similar to their in vivo counterparts. These organoid models provide a powerful platform for elucidating the development mechanisms, modeling diseases, and screening drug candidates. In this review, we will summarize the advances of this technology for generating various organoids of tissues from the three germ layers and discuss their drawbacks and prospects for tissue engineering.

Regulatory Mechanisms of Retinal Photoreceptors Development at Single Cell Resolution

Photoreceptors are critical components of the retina and play a role in the first step of the conversion of light to electric signals. With the discovery of the intrinsically photosensitive retinal ganglion cells, which regulate non-image-forming visual processes, our knowledge of the photosensitive cell family in the retina has deepened. Photoreceptor development is regulated by specific genes and proteins and involves a series of molecular processes including DNA transcription, post-transcriptional modification, protein translation, and post-translational modification. Single-cell sequencing is a promising technology for the study of photoreceptor development. This review presents an overview of the types of human photoreceptors, summarizes recent discoveries in the regulatory mechanisms underlying their development at single-cell resolution, and outlines the prospects in this field.

Thyroid Hormone Deiodinases: Dynamic Switches in Developmental Transitions

Thyroid hormones exert pleiotropic, essential actions in mammalian, including human, development. These actions depend on provision of thyroid hormones in the circulation but also to a remarkable extent on deiodinase enzymes in target tissues that amplify or deplete the local concentration of the primary active form of the hormone T3 (3,5,3'-triiodothyronine), the high affinity ligand for thyroid hormone receptors. Genetic analyses in mice have revealed key roles for activating (DIO2) and inactivating (DIO3) deiodinases in cell differentiation fates and tissue maturation, ultimately promoting neonatal viability, growth, fertility, brain development, and behavior, as well as metabolic, endocrine, and sensory functions. An emerging paradigm is how the opposing activities of DIO2 and DIO3 are coordinated, providing a dynamic switch that controls the developmental timing of a tissue response, often during neonatal and maturational transitions. A second paradigm is how cell to cell communication within a tissue determines the response to T3. Deiodinases in specific cell types, often strategically located near to blood vessels that convey thyroid hormones into the tissue, can regulate neighboring cell types, suggesting a paracrine-like layer of control of T3 action. We discuss deiodinases as switches for developmental transitions and their potential to influence tissue dysfunction in human thyroid disorders.

Complementary Photo-Synapses Based on Light-Stimulated Porphyrin-Coated Silicon Nanowires Field-Effect Transistors (LPSNFET)

Neuromorphic computing has emerged as the high-energy-efficiency and intelligent solution for processing sensory data. As a potential alternative to neuromorphic computing, photo-excited synaptic systems can integrate the functions of optoelectronic sensing and synaptic computing to realize the low-power and high-performance visual perception. However, one major challenge in high-efficient photo-excited synaptic system is to realize the complementarily enhanced and inhibited synaptic behaviors with small hardware cost as possible. Another challenge is to fabricate the photo-synapse devices with complementary metal oxide semiconductor (CMOS)-compatible process to achieve high enough integration density for practical application. Here, a CMOS-compatible Light-stimulated Porphyrin-coated Silicon Nanowire Field Effect Transistor (LPSNFET) technology is proposed and developed to form the complementary photo-synapses with only two CMOS-like transistors. LPSNFET exhibits fivefold improvement in photo-sensitivity compared to the bare silicon nanowire (SiNW) devices, and can still show obvious responses when incident illumination power is as low as 0.1 mW cm^-2 . Moreover, it enables tunable dynamic synaptic plasticity and versatile synaptic functions. Especially, the complementarily enhanced and inhibited behaviors can be realized by modulating SiNW/porphyrin interface via simply changing the MOS type of LPSNFET, which acts like the photonic counterpart of CMOS technology to provide the basic brick for building complex neuromorphic circuits efficiently and economically. Finally, the CMOS process compatibility of LPSNFET provides potential application in future large scale in-sensor computing.