Molecular mechanisms of optic vesicle development: complexities, ambiguities and controversies

NMNAT1 Is Essential for Human iPS Cell Differentiation to the Retinal Lineage

Purpose

The gene encoding nicotinamide mononucleotide adenylyltransferase 1 (NMNAT1), a nicotinamide adenine dinucleotide synthetase localized in the cell nucleus, is a causative factor in Leber's congenital amaurosis, which is the earliest onset type of inherited retinal degeneration. We sought to investigate the roles of NMNAT1 in early retinal development.

Methods

We used human induced pluripotent stem cells (hiPSCs) and established NMNAT1-knockout (KO) hiPSCs using CRISPR/cas9 technology to reveal the roles of NMNAT1 in human retinal development.

Results

NMNAT1 was not essential for the survival and proliferation of immature hiPSCs; therefore, we subjected NMNAT1-KO hiPSCs to retinal organoid (RO) differentiation culture. The expression levels of immature hiPSC-specific genes decreased in a similar manner after organoid culture initiation up to 2 weeks in the control and NMNAT1-KO. Neuroectoderm-specific genes were induced in the control and NMNAT1-KO organoids within a few days after starting the organoid culture; PAX6 and TUBB3 were higher in NMNAT1-KO organoids up to 7 days than in the control organoids. However, the induction of genes involving retinal early development, such as RAX, which was induced at around day 10 in this culture, was considerably reduced in NMNAT1-KO organoids. Morphological examination also showed failure of retinal primordial structure formation, which became visible at around 2 weeks of the control culture, in the NMNAT1-KO organoids. Decreased intracellular NAD levels and poly(ADP-ribosyl)ation were observed in NMNAT1-KO organoids at 7 to 10 days of the culture. Mass spectrometry analysis of inhibited proteins in the poly(ADP-ribosyl)ation pathway identified poly(ADP-ribosyl)ation of poly(ADP-ribose) polymerase 1 (PARP1) as a major protein.

Conclusions

These results indicate that NMNAT1 was dispensable for neural lineage differentiation but essential for the commitment of retinal fate differentiation in hiPSCs. The NMNAT1-NAD-PARP1 axis may play a critical role in the appropriate development of human retinal lineage differentiation.

Congenital anterior segment ocular disorders: Genotype-phenotype correlations and emerging novel mechanisms

Development of the anterior segment of the eye requires reciprocal sequential interactions between the arising tissues, facilitated by numerous genetic factors. Disruption of any of these processes results in congenital anomalies in the affected tissue(s) leading to anterior segment disorders (ASD) including aniridia, Axenfeld-Rieger anomaly, congenital corneal opacities (Peters anomaly, cornea plana, congenital primary aphakia), and primary congenital glaucoma. Current understanding of the genetic factors involved in ASD remains incomplete, with approximately 50% overall receiving a genetic diagnosis. While some genes are strongly associated with a specific clinical diagnosis, the majority of known factors are linked with highly variable phenotypic presentations, with pathogenic variants in FOXC1, CYP1B1, and PITX2 associated with the broadest spectrum of ASD conditions. This review discusses typical clinical presentations including associated systemic features of various forms of ASD; the latest functional data and genotype-phenotype correlations related to 25 ASD factors including newly identified genes; promising novel candidates; and current and emerging treatments for these complex conditions. Recent developments of interest in the genetics of ASD include identification of phenotypic expansions for several factors, discovery of multiple modes of inheritance for some genes, and novel mechanisms including a growing number of non-coding variants and alleles affecting specific domains/residues and requiring further studies.

Profiling IOP-Responsive Genes in the Trabecular Meshwork and Optic Nerve Head in a Rat Model of Controlled Elevation of Intraocular Pressure

Purpose

The rat controlled elevation of intraocular pressure (CEI) model allows study of in vivo responses to short-term exposure to defined intraocular pressures (IOP). In this study, we used NanoString technology to investigate in vivo IOP-related gene responses in the trabecular meshwork (TM) and optic nerve head (ONH) simultaneously from the same animals.

Methods

Male and female rats (N = 35) were subjected to CEI for 8 hours at pressures simulating mean, daytime normotensive rat IOP (CEI-20), or 2.5× IOP (CEI-50). Naïve animals that received no anesthesia or surgical interventions served as controls. Immediately after CEI, TM and ONH tissues were dissected, RNA was isolated, and samples were analyzed with a NanoString panel containing 770 genes. Postprocessing, raw count data were uploaded to ROSALIND for differential gene expression analyses.

Results

For the TM, 45 IOP-related genes were significant in the CEI-50 versus CEI-20 and CEI-50 versus naïve comparisons, with 15 genes common to both comparisons. Bioinformatics analysis identified Notch and transforming growth factor beta (TGFβ) pathways to be the most up- and downregulated Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, respectively. For ONH, 22 significantly differentially regulated genes were identified in the CEI-50 versus naïve comparison. Pathway analysis identified defense response and immune response as two significantly upregulated biological process pathways.

Conclusions

This study demonstrated the ability to assay short-term IOP-responsive genes in both TM and ONH tissues simultaneously. In the TM, downregulation of TGFβ pathway genes suggests that TM responses may reduce TGFβ-induced extracellular matrix synthesis. For ONH, the initial response to short-term elevated IOP may be protective.

Profiling IOP-responsive genes in anterior and posterior ocular tissues in the rat CEI glaucoma model

Purpose

The rat Controlled Elevation of Intraocular pressure (CEI) model allows study of in vivo responses to defined intraocular pressures (IOP). In this study, we use Nanostring technology to investigate in vivo IOP-related gene responses in the trabecular meshwork (TM) and optic nerve head (ONH) simultaneously from the same animals.

Methods

Male and female rats (N=35) were subject to CEI for 8-hours at pressures simulating mean, daytime normotensive rat IOP (CEI-20), or 2.5x IOP (CEI-50). Naïve animals, receiving no anesthesia or surgical interventions, served as controls. Immediately after CEI, TM and ONH tissues were dissected, RNA isolated, and samples were analyzed with a Nanostring panel containing 770 genes. Post-processing, raw count data were uploaded to Rosalind® for differential gene expression analyses.

Results

For the TM, 45 IOP-related genes were significant in the "CEI-50 vs. CEI-20" and "CEI-50 vs. naïve" comparisons, with 15 genes common to both comparisons. Bioinformatics analysis identified Notch and TGFβ pathways to be the most up- and down-regulated KEGG pathways, respectively. For ONH, 22 significantly regulated genes were identified in the "CEI-50 vs. naïve" comparison. Pathway analysis identified 'defense response' and 'immune response' as two significantly upregulated biological process pathways.

Conclusions

This study demonstrates the ability to assay IOP-responsive genes in both TM and ONH tissues simultaneously. In the TM, downregulation of TGFβ pathway genes suggest that TM responses may prevent TGFβ-induced extracellular matrix synthesis. For ONH, the initial response to elevated IOP may be protective, with astrocytes playing a key role in these gene responses.

Generating Retinas through Guided Pluripotent Stem Cell Differentiation and Direct Somatic Cell Reprogramming

Retinal degeneration diseases affect millions of people worldwide but are among the most difficult eye diseases to cure. Studying the mechanisms and developing new therapies for these blinding diseases requires researchers to have access to many retinal cells. In recent years there has been substantial advances in the field of biotechnology in generating retinal cells and even tissues in vitro, either through programmed sequential stem cell differentiation or direct somatic cell lineage reprogramming. The resemblance of these in vitro-generated retinal cells to native cells has been increasingly utilized by researchers. With the help of these in vitro retinal models, we now have a better understanding of human retinas and retinal diseases. Furthermore, these in vitro-generated retinal cells can be used as donor cells which solves a major hurdle in the development of cell replacement therapy for retinal degeneration diseases, while providing a promising option for patients suffering from these diseases. In this review, we summarize the development of pluripotent stem cell-to-retinal cell differentiation methods, the recent advances in generating retinal cells through direct somatic cell reprogramming, and the translational applications of retinal cells generated in vitro. Finally, we discuss the limitations of the current protocols and possible future directions for improvement.

Development of a simplified human embryonic stem cell-based retinal pre-organoid model for toxicity evaluations of common pollutants

OBJECTIVE

To explore the retinal toxicity of pharmaceuticals and personal care products (PPCPs), flame retardants, bisphenols, phthalates, and polycyclic aromatic hydrocarbons (PAHs) on human retinal progenitor cells (RPCs) and retinal pigment epithelial (RPE) cells, which are the primary cell types at the early stages of retinal development, vital for subsequent functional cell type differentiation, and closely related to retinal diseases.

MATERIALS AND METHODS

After 23 days of differentiation, human embryonic stem cell (hESC)-based retinal pre-organoids, containing RPCs and RPE cells, were exposed to 10, 100, and 1000 nM pesticides (butachlor, terbutryn, imidacloprid, deltamethrin, pendimethalin, and carbaryl), flame retardants (PFOS, TBBPA, DBDPE, and TDCIPP), PPCPs (climbazole and BHT), and other typical pollutants (phenanthrene, DCHP, and BPA) for seven days. Then, mRNA expression changes were monitored and compared.

RESULTS

(1) The selected pollutants did not show strong effects at environmental and human-relevant concentrations, although the effects of flame retardants were more potent than those of other categories of chemicals. Surprisingly, some pollutants with distinct structures showed similar adverse effects. (2) Exposure to pollutants induced different degrees of cell detachment, probably due to alterations in extracellular matrix and/or cell adhesion.

CONCLUSIONS

In this study, we established a retinal pre-organoid model suitable for evaluating multiple pollutants' effects, and pointed out the potential retinal toxicity of flame retardants, among other pollutants. Nevertheless, the potential mechanisms of toxicity and the effects on cell detachment are still unclear and deserve further exploration. Additionally, this model holds promise for screening interventions aimed at mitigating the detrimental effects of these pollutants.

Continuous non-adherent culture promotes transdifferentiation of human adipose-derived stem cells into retinal lineage

Non-adherent culture is critical for the transdifferentiation of stem cells from mesoderm to neuroectoderm. Sphere culture has been reported to directly induce the adipose tissue cells to neural stem cells. Here we aimed to evaluate continuous non-adherent culture on the transdifferentiation potential of human adipose-derived stem cells (ASCs) into retinal lineage. Human ASCs were induced into retinal lineage by the treatment of noggin, dickkopf-related protein 1, and IGF-1 (NDI) under adherent and non-adherent culture. The NDI induction treatment with the adherent culture for 21 days promoted robust expression of retinal markers in the induced ASCs as compared to those without NDI induction on the adherent culture. With continuous non-adherent culture for 21 days, human ASCs could highly express retinal marker genes even without NDI induction treatment as compared to those on the adherent culture. The combination of continuous non-adherent culture with the NDI induction did not show a significant upregulation of the retinal marker expression as compared to either NDI induction with the adherent culture or continuous non-adherent culture without NDI induction treatment. In summary, both non-adherent culture and NDI induction medium could independently promote the transdifferentiation of human ASCs into retinal lineage. Yet, their combination did not produce an enhancement effect.

Human pluripotent stem cell (hPSC) and organoid models of autism: opportunities and limitations

Autism spectrum disorder (ASD) is a complex neurodevelopmental disorder caused by genetic or environmental perturbations during early development. Diagnoses are dependent on the identification of behavioral abnormalities that likely emerge well after the disorder is established, leaving critical developmental windows uncharacterized. This is further complicated by the incredible clinical and genetic heterogeneity of the disorder that is not captured in most mammalian models. In recent years, advancements in stem cell technology have created the opportunity to model ASD in a human context through the use of pluripotent stem cells (hPSCs), which can be used to generate 2D cellular models as well as 3D unguided- and region-specific neural organoids. These models produce profoundly intricate systems, capable of modeling the developing brain spatiotemporally to reproduce key developmental milestones throughout early development. When complemented with multi-omics, genome editing, and electrophysiology analysis, they can be used as a powerful tool to profile the neurobiological mechanisms underlying this complex disorder. In this review, we will explore the recent advancements in hPSC-based modeling, discuss present and future applications of the model to ASD research, and finally consider the limitations and future directions within the field to make this system more robust and broadly applicable.

Generation of iPSC-derived human forebrain organoids assembling bilateral eye primordia

Induced pluripotent stem cell-derived brain organoids enable the developmental complexities of the human brain to be deconstructed. During embryogenesis, optic vesicles (OVs), the eye primordium attached to the forebrain, develop from diencephalon. However, most 3D culturing methods generate either brain or retinal organoids individually. Here we describe a protocol to generate organoids with both forebrain entities, which we call OV-containing brain organoids (OVB organoids). In this protocol, we first induce neural differentiation (days 0-5) and collect neurospheres, which we culture in a neurosphere medium to initiate their patterning and further self-assembly (days 5-10). Then, upon transfer to spinner flasks containing OVB medium (days 10-30), neurospheres develop into forebrain organoids with one or two pigmented dots restricted to one pole, displaying forebrain entities of ventral and dorsal cortical progenitors and preoptic areas. Further long-term culture results in photosensitive OVB organoids constituting complementary cell types of OVs, including primitive corneal epithelial and lens-like cells, retinal pigment epithelia, retinal progenitor cells, axon-like projections and electrically active neuronal networks. OVB organoids provide a system to help dissect interorgan interactions between the OVs as sensory organs and the brain as a processing unit, and can help model early eye patterning defects, including congenital retinal dystrophy. To conduct the protocol, experience in sterile cell culture and maintenance of human induced pluripotent stem cells is essential; theoretical knowledge of brain development is advantageous. Furthermore, specialized expertise in 3D organoid culture and imaging for the analysis is needed.

Unravelling the link between embryogenesis and adult stem cell potential in the ciliary marginal zone: A comparative study between mammals and teleost fish

The discovery and study of adult stem cells have revolutionized regenerative medicine by offering new opportunities for treating various medical conditions. Anamniote stem cells, which retain their full proliferative capacity and full differentiation range throughout their lifetime, harbour a greater potential compared to mammalian adult stem cells, which only exhibit limited stem cell potential. Therefore, understanding the mechanisms underlying these differences is of significant interest. In this review, we examine the similarities and differences of adult retinal stem cells in anamniotes and mammals, from their embryonic stages in the optic vesicle to their residence in the postembryonic retinal stem cell niche, the ciliary marginal zone located in the retinal periphery. In anamniotes, developing precursors of retinal stem cells are exposed to various environmental cues during their migration in the complex morphogenetic remodelling of the optic vesicle to the optic cup. In contrast, their mammalian counterparts in the retinal periphery are primarily instructed by neighbouring tissues once they are in place. We explore the distinct modes of optic cup morphogenesis in mammals and teleost fish and highlight molecular mechanisms governing morphogenesis and stem cells instruction. The review concludes with the molecular mechanisms of ciliary marginal zone formation and offers a perspective on the impact of comparative single cell transcriptomic studies to reveal the evolutionary similarities and differences.

Retinal Pigment Epithelium Cell Development: Extrapolating Basic Biology to Stem Cell Research

The retinal pigment epithelium (RPE) forms an important cellular monolayer, which contributes to the normal physiology of the eye. Damage to the RPE leads to the development of degenerative diseases, such as age-related macular degeneration (AMD). Apart from acting as a physical barrier between the retina and choroidal blood vessels, the RPE is crucial in maintaining photoreceptor (PR) and visual functions. Current clinical intervention to treat early stages of AMD includes stem cell-derived RPE transplantation, which is still in its early stages of evolution. Therefore, it becomes essential to derive RPEs which are functional and exhibit features as observed in native human RPE cells. The conventional strategy is to use the knowledge obtained from developmental studies using various animal models and stem cell-based exploratory studies to understand RPE biogenies and developmental trajectory. This article emphasises such studies and aims to present a comprehensive understanding of the basic biology, including the genetics and molecular pathways of RPE development. It encompasses basic developmental biology and stem cell-based developmental studies to uncover RPE differentiation. Knowledge of the in utero developmental cues provides an inclusive methodology required for deriving RPEs using stem cells.

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.

Cell fate decisions, transcription factors and signaling during early retinal development

The development of the vertebrate eyes is a complex process starting from anterior-posterior and dorso-ventral patterning of the anterior neural tube, resulting in the formation of the eye field. Symmetrical separation of the eye field at the anterior neural plate is followed by two symmetrical evaginations to generate a pair of optic vesicles. Next, reciprocal invagination of the optic vesicles with surface ectoderm-derived lens placodes generates double-layered optic cups. The inner and outer layers of the optic cups develop into the neural retina and retinal pigment epithelium (RPE), respectively. In vitro produced retinal tissues, called retinal organoids, are formed from human pluripotent stem cells, mimicking major steps of retinal differentiation in vivo. This review article summarizes recent progress in our understanding of early eye development, focusing on the formation the eye field, optic vesicles, and early optic cups. Recent single-cell transcriptomic studies are integrated with classical in vivo genetic and functional studies to uncover a range of cellular mechanisms underlying early eye development. The functions of signal transduction pathways and lineage-specific DNA-binding transcription factors are dissected to explain cell-specific regulatory mechanisms underlying cell fate determination during early eye development. The functions of homeodomain (HD) transcription factors Otx2, Pax6, Lhx2, Six3 and Six6, which are required for early eye development, are discussed in detail. Comprehensive understanding of the mechanisms of early eye development provides insight into the molecular and cellular basis of developmental ocular anomalies, such as optic cup coloboma. Lastly, modeling human development and inherited retinal diseases using stem cell-derived retinal organoids generates opportunities to discover novel therapies for retinal diseases.

[Are mini-brains watching you?]

iPSC-derived brain and retinal organoids represent biologically relevant 3D models. A new step in the field of brain and retinal organoids was reached a few months ago with the simultaneous development of brain and eye structures from human iPS cells within the same organoid. Single-cell mRNA sequencing analyses allowed the identification of various ocular and cerebral neuronal populations and electrophysiological recordings confirm the presence of functional and electrically active neurons. The coexistence within the same organoid of different cell types from visual and brain regions and the establishment of connections between these regions raise the intriguing question of its real or potential functionality and its ability to process higher-level visual information. This unique model could also be used to further understand the development of the human visual system and associated developmental diseases.

Loss of zebrafish dzip1 results in inappropriate recruitment of periocular mesenchyme to the optic fissure and ocular coloboma

Cilia are essential for the development and function of many different tissues. Although cilia machinery is crucial in the eye for photoreceptor development and function, a role for cilia in early eye development and morphogenesis is still somewhat unclear: many zebrafish cilia mutants retain cilia at early stages due to maternal deposition of cilia components. An eye phenotype has been described in the mouse Arl13 mutant, however, zebrafish arl13b is maternally deposited, and an early role for cilia proteins has not been tested in zebrafish eye development. Here we use the zebrafish dzip1 mutant, which exhibits a loss of cilia throughout stages of early eye development, to examine eye development and morphogenesis. We find that in dzip1 mutants, initial formation of the optic cup proceeds normally, however, the optic fissure subsequently fails to close and embryos develop the structural eye malformation ocular coloboma. Further, neural crest cells, which are implicated in optic fissure closure, do not populate the optic fissure correctly, suggesting that their inappropriate localization may be the underlying cause of coloboma. Overall, our results indicate a role for dzip1 in proper neural crest localization in the optic fissure and optic fissure closure.

Extracellular vesicles released by human retinal pigment epithelium mediate increased polarised secretion of drusen proteins in response to AMD stressors

Age-related macular degeneration (AMD) is a leading cause of blindness worldwide. Drusen are key contributors to the etiology of AMD and the ability to modulate drusen biogenesis could lead to therapeutic strategies to slow or halt AMD progression. The mechanisms underlying drusen biogenesis, however, remain mostly unknown. Here we demonstrate that under homeostatic conditions extracellular vesicles (EVs) secreted by retinal pigment epithelium (RPE) cells are enriched in proteins associated with mechanisms involved in AMD pathophysiology, including oxidative stress, immune response, inflammation, complement system and drusen composition. Furthermore, we provide first evidence that drusen-associated proteins are released as cargo of extracellular vesicles secreted by RPE cells in a polarised apical:basal mode. Notably, drusen-associated proteins exhibited distinctive directional secretion modes in homeostatic conditions and, differential modulation of this directional secretion in response to AMD stressors. These observations underpin the existence of a finely-tuned mechanism regulating directional apical:basal sorting and secretion of drusen-associated proteins via EVs, and its modulation in response to mechanisms involved in AMD pathophysiology. Collectively, our results strongly support an active role of RPE-derived EVs as a key source of drusen proteins and important contributors to drusen development and growth.

The eyeball's connected to the brain ball

How does the human eye develop in concert with the brain to create a functioning visual system? In this issue of Cell Stem Cell, Gabriel et al. (2021) report the development of eye-like structures from forebrain organoids with light sensitivity, signal processing, and connectivity, which moves us toward answering this complex question.

Elastic Instabilities Govern the Morphogenesis of the Optic Cup

Because the normal operation of the eye depends on sensitive morphogenetic processes for its eventual shape, developmental flaws can lead to wide-ranging ocular defects. However, the physical processes and mechanisms governing ocular morphogenesis are not well understood. Here, using analytical theory and nonlinear shell finite-element simulations, we show, for optic vesicles experiencing matrix-constrained growth, that elastic instabilities govern the optic cup morphogenesis. By capturing the stress amplification owing to mass increase during growth, we show that the morphogenesis is driven by two elastic instabilities analogous to the snap through in spherical shells, where the second instability is sensitive to the optic cup geometry. In particular, if the optic vesicle is too slender, it will buckle and break axisymmetry, thus, preventing normal development. Our results shed light on the morphogenetic mechanisms governing the formation of a functional biological system and the role of elastic instabilities in the shape selection of soft biological structures.

Pax2a, but not pax2b, influences cell survival and periocular mesenchyme localization to facilitate zebrafish optic fissure closure

Background

Pax2 is required for optic fissure development in many organisms, including humans and zebrafish. Zebrafish loss‐of‐function mutations in pax2a display coloboma, yet the etiology of the morphogenetic defects is unclear. Further, pax2 is duplicated in zebrafish, and a role for pax2b in optic fissure development has not been examined.

Results

Using a combination of imaging and molecular genetics, we interrogated a potential role for pax2b and examined how loss of pax2 affects optic fissure development. Although optic fissure formation appears normal in pax2 mutants, an endothelial‐specific subset of periocular mesenchyme (POM) fails to initially localize within the optic fissure, yet both neural crest and endothelial‐derived POM ectopically accumulate at later stages in pax2a and pax2a; pax2b mutants. Apoptosis is not up‐regulated within the optic fissure in pax2 mutants, yet cell death is increased in tissues outside of the optic fissure, and when apoptosis is inhibited, coloboma is partially rescued. In contrast to pax2a, loss of pax2b does not appear to affect optic fissure morphogenesis.

Conclusions

Our results suggest that pax2a, but not pax2b, supports cell survival outside of the optic fissure and POM abundance within it to facilitate optic fissure closure.

Zebrafish pax2a null mutants display a defect in optic fissure closure and coloboma

Loss of pax2b does not affect optic fissure development

An endothelial‐specific subset of periocular mesenchyme cells fails to initially localize to the optic fissure in pax2a mutants

At a later stage of optic fissure development both neural crest and endothelial‐derived periocular mesenchyme ectopically accumulate within the optic fissure

Pax2a mutants have increased apoptosis in surrounding tissues, but not within the optic fissure margin cells, and apoptosis in part underlies the coloboma phenotype

Pathophysiology of aniridia-associated keratopathy: Developmental aspects and unanswered questions

Aniridia, a rare congenital disease, is often characterized by a progressive, pronounced limbal insufficiency and ocular surface pathology termed aniridia-associated keratopathy (AAK). Due to the characteristics of AAK and its bilateral nature, clinical management is challenging and complicated by the multiple coexisting ocular and systemic morbidities in aniridia. Although it is primarily assumed that AAK originates from a congenital limbal stem cell deficiency, in recent years AAK and its pathogenesis has been questioned in the light of new evidence and a refined understanding of ocular development and the biology of limbal stem cells (LSCs) and their niche. Here, by consolidating and comparing the latest clinical and preclinical evidence, we discuss key unanswered questions regarding ocular developmental aspects crucial to AAK. We also highlight hypotheses on the potential role of LSCs and the ocular surface microenvironment in AAK. The insights thus gained lead to a greater appreciation for the role of developmental and cellular processes in the emergence of AAK. They also highlight areas for future research to enable a deeper understanding of aniridia, and thereby the potential to develop new treatments for this rare but blinding ocular surface disease.

Itpr1 regulates the formation of anterior eye segment tissues derived from neural crest cells

Mutations in ITPR1 cause ataxia and aniridia in individuals with Gillespie syndrome (GLSP). However, the pathogenic mechanisms underlying aniridia remain unclear. We identified a de novo GLSP mutation hotspot in the 3'-region of ITPR1 in five individuals with GLSP. Furthermore, RNA-sequencing and immunoblotting revealed an eye-specific transcript of Itpr1, encoding a 218amino acid isoform. This isoform is localized not only in the endoplasmic reticulum, but also in the nuclear and cytoplasmic membranes. Ocular-specific transcription was repressed by SOX9 and induced by MAF in the anterior eye segment (AES) tissues. Mice lacking seven base pairs of the last Itpr1 exon exhibited ataxia and aniridia, in which the iris lymphatic vessels, sphincter and dilator muscles, corneal endothelium and stroma were disrupted, but the neural crest cells persisted after completion of AES formation. Our analyses revealed that the 218-amino acid isoform regulated the directionality of actin fibers and the intensity of focal adhesion. The isoform might control the nuclear entry of transcriptional regulators, such as YAP. It is also possible that ITPR1 regulates both AES differentiation and muscle contraction in the iris.

Build me up optic cup: Intrinsic and extrinsic mechanisms of vertebrate eye morphogenesis

The basic structure of the eye, which is crucial for visual function, is established during the embryonic process of optic cup morphogenesis. Molecular pathways of specification and patterning are integrated with spatially distinct cell and tissue shape changes to generate the eye, with discrete domains and structural features: retina and retinal pigment epithelium enwrap the lens, and the optic fissure occupies the ventral surface of the eye and optic stalk. Interest in the underlying cell biology of eye morphogenesis has led to a growing body of work, combining molecular genetics and imaging to quantify cellular processes such as adhesion and actomyosin activity. These studies reveal that intrinsic machinery and spatiotemporally specific extrinsic inputs collaborate to control dynamics of cell movements and morphologies. Here we consider recent advances in our understanding of eye morphogenesis, with a focus on the mechanics of eye formation throughout vertebrate systems, including insights and potential opportunities using organoids, which may provide a tractable system to test hypotheses from embryonic models.

4-Dimensional Imaging of Zebrafish Optic Cup Morphogenesis

Visual system function requires the establishment of precise tissue and organ structures. In the vertebrate eye, structural defects are a common cause of visual impairment, yet mechanisms of eye morphogenesis are still poorly understood. The basic organization of the embryonic eye is conserved throughout vertebrates, thus live imaging of zebrafish embryos has become a powerful approach to directly observe eye development at real time under normal and pathological conditions. Dynamic cell processes including movements, morphologies, interactions, division, and death can be visualized in the embryo. We have developed methods for uniform labeling of subcellular structures and timelapse confocal microscopy of early eye development in zebrafish. This protocol outlines the method of generating capped mRNA for injection into the 1-cell zebrafish embryo, mounting embryos at optic vesicle stage (~12 hours post fertilization, hpf), and performing multi-dimensional timelapse imaging of optic cup morphogenesis on a laser scanning confocal microscope, such that multiple datasets are acquired sequentially in the same imaging session. Such an approach yields data that can be used for a variety of purposes, including cell tracking, volume measurements, three-dimensional (3D) rendering, and visualization. Our approaches allow us to pinpoint the cellular and molecular mechanisms driving optic cup development, in both wild type and genetic mutant conditions. These methods can be employed directly by other groups or adapted to visualize many additional aspects of zebrafish eye development.

Organoids for the Study of Retinal Development and Developmental Abnormalities

The cumulative knowledge of retina development has been instrumental in the generation of retinal organoid systems from pluripotent stem cells; and these three-dimensional organoid models, in turn, have provided unprecedented opportunities for retinal research and translational applications, including the ability to model disease in a human setting and to apply these models to the development and validation of therapeutic drugs. In this review article, we examine how retinal organoids can also contribute to our understanding of retinal developmental mechanisms, how this knowledge can be applied to modeling developmental abnormalities, and highlight some of the avenues that remain to be explored.

Mesenchymal Stem Cells: Signaling Pathways in Transdifferentiation Into Retinal Progenitor Cells

Several signaling pathways and transcription factors control the cell fate in its in vitro development and differentiation. The orchestrated use of these factors results in cell specification. In coculture methods, many of these factors secrete from host cells but control the process. Today, transcription factors required for retinal progenitor cells are well known, but the generation of these cells from mesenchymal stem cells is an ideal goal. The purpose of the paper is to review novel methods for retinal progenitor cell production and selecting a set of signaling molecules in the presence of adult retinal pigment epithelium and extraocular mesenchyme acting as inducers of retinal cell differentiation.

Animal and cellular models of microphthalmia

Microphthalmia is a rare developmental eye disorder affecting 1 in 7000 births. It is defined as a small (axial length ⩾2 standard deviations below the age-adjusted mean) underdeveloped eye, caused by disruption of ocular development through genetic or environmental factors in the first trimester of pregnancy. Clinical phenotypic heterogeneity exists amongst patients with varying levels of severity, and associated ocular and systemic features. Up to 11% of blind children are reported to have microphthalmia, yet currently no treatments are available. By identifying the aetiology of microphthalmia and understanding how the mechanisms of eye development are disrupted, we can gain a better understanding of the pathogenesis. Animal models, mainly mouse, zebrafish and Xenopus, have provided extensive information on the genetic regulation of oculogenesis, and how perturbation of these pathways leads to microphthalmia. However, differences exist between species, hence cellular models, such as patient-derived induced pluripotent stem cell (iPSC) optic vesicles, are now being used to provide greater insights into the human disease process. Progress in 3D cellular modelling techniques has enhanced the ability of researchers to study interactions of different cell types during eye development. Through improved molecular knowledge of microphthalmia, preventative or postnatal therapies may be developed, together with establishing genotype-phenotype correlations in order to provide patients with the appropriate prognosis, multidisciplinary care and informed genetic counselling. This review summarises some key discoveries from animal and cellular models of microphthalmia and discusses how innovative new models can be used to further our understanding in the future.

Plain language summary

Animal and Cellular Models of the Eye Disorder, Microphthalmia (Small Eye) Microphthalmia, meaning a small, underdeveloped eye, is a rare disorder that children are born with. Genetic changes or variations in the environment during the first 3 months of pregnancy can disrupt early development of the eye, resulting in microphthalmia. Up to 11% of blind children have microphthalmia, yet currently no treatments are available. By understanding the genes necessary for eye development, we can determine how disruption by genetic changes or environmental factors can cause this condition. This helps us understand why microphthalmia occurs, and ensure patients are provided with the appropriate clinical care and genetic counselling advice. Additionally, by understanding the causes of microphthalmia, researchers can develop treatments to prevent or reduce the severity of this condition. Animal models, particularly mice, zebrafish and frogs, which can also develop small eyes due to the same genetic/environmental changes, have helped us understand the genes which are important for eye development and can cause birth eye defects when disrupted. Studying a patient's own cells grown in the laboratory can further help researchers understand how changes in genes affect their function. Both animal and cellular models can be used to develop and test new drugs, which could provide treatment options for patients living with microphthalmia. This review summarises the key discoveries from animal and cellular models of microphthalmia and discusses how innovative new models can be used to further our understanding in the future.

Retinal organoids: a window into human retinal development

Retinal development and maturation are orchestrated by a series of interacting signalling networks that drive the morphogenetic transformation of the anterior developing brain. Studies in model organisms continue to elucidate these complex series of events. However, the human retina shows many differences from that of other organisms and the investigation of human eye development now benefits from stem cell-derived organoids. Retinal differentiation methods have progressed from simple 2D adherent cultures to self-organising micro-physiological systems. As models of development, these have collectively offered new insights into the previously unexplored early development of the human retina and informed our knowledge of the key cell fate decisions that govern the specification of light-sensitive photoreceptors. Although the developmental trajectories of other retinal cell types remain more elusive, the collation of omics datasets, combined with advanced culture methodology, will enable modelling of the intricate process of human retinogenesis and retinal disease in vitro.

Summary: Retinal organoid systems derived from human pluripotent stem cells are micro-physiological systems that offer new insights into previously unexplored human retina development.

Genetics Underlying the Interactions between Neural Crest Cells and Eye Development

The neural crest is a unique, transient stem cell population that is critical for craniofacial and ocular development. Understanding the genetics underlying the steps of neural crest development is essential for gaining insight into the pathogenesis of congenital eye diseases. The neural crest cells play an under-appreciated key role in patterning the neural epithelial-derived optic cup. These interactions between neural crest cells within the periocular mesenchyme and the optic cup, while not well-studied, are critical for optic cup morphogenesis and ocular fissure closure. As a result, microphthalmia and coloboma are common phenotypes in human disease and animal models in which neural crest cell specification and early migration are disrupted. In addition, neural crest cells directly contribute to numerous ocular structures including the cornea, iris, sclera, ciliary body, trabecular meshwork, and aqueous outflow tracts. Defects in later neural crest cell migration and differentiation cause a constellation of well-recognized ocular anterior segment anomalies such as Axenfeld–Rieger Syndrome and Peters Anomaly. This review will focus on the genetics of the neural crest cells within the context of how these complex processes specifically affect overall ocular development and can lead to congenital eye diseases.

Human Organoids for the Study of Retinal Development and Disease

Recent advances in stem cell engineering have led to an explosion in the use of organoids as model systems for studies in multiple biological disciplines. Together with breakthroughs in genome engineering and the various omics, organoid technology is making possible studies of human biology that were not previously feasible. For vision science, retinal organoids derived from human stem cells allow differentiating and mature human retinal cells to be studied in unprecedented detail. In this review, we examine the technologies employed to generate retinal organoids and how organoids are revolutionizing the fields of developmental and cellular biology as they pertain to the retina. Furthermore, we explore retinal organoids from a clinical standpoint, offering a new platform with which to study retinal diseases and degeneration, test prospective drugs and therapeutic strategies, and promote personalized medicine. Finally, we discuss the range of possibilities that organoids may bring to future retinal research and consider their ethical implications.

Limitations and Promise of Retinal Tissue From Human Pluripotent Stem Cells for Developing Therapies of Blindness

The self-formation of retinal tissue from pluripotent stem cells generated a tremendous promise for developing new therapies of retinal degenerative diseases, which previously seemed unattainable. Together with use of induced pluripotent stem cells or/and CRISPR-based recombineering the retinal organoid technology provided an avenue for developing models of human retinal degenerative diseases "in a dish" for studying the pathology, delineating the mechanisms and also establishing a platform for large-scale drug screening. At the same time, retinal organoids, highly resembling developing human fetal retinal tissue, are viewed as source of multipotential retinal progenitors, young photoreceptors and just the whole retinal tissue, which may be transplanted into the subretinal space with a goal of replacing patient's degenerated retina with a new retinal "patch." Both approaches (transplantation and modeling/drug screening) were projected when Yoshiki Sasai demonstrated the feasibility of deriving mammalian retinal tissue from pluripotent stem cells, and generated a lot of excitement. With further work and testing of both approaches in vitro and in vivo, a major implicit limitation has become apparent pretty quickly: the absence of the uniform layer of Retinal Pigment Epithelium (RPE) cells, which is normally present in mammalian retina, surrounds photoreceptor layer and develops and matures first. The RPE layer polarize into apical and basal sides during development and establish microvilli on the apical side, interacting with photoreceptors, nurturing photoreceptor outer segments and participating in the visual cycle by recycling 11-trans retinal (bleached pigment) back to 11-cis retinal. Retinal organoids, however, either do not have RPE layer or carry patches of RPE mostly on one side, thus directly exposing most photoreceptors in the developing organoids to neural medium. Recreation of the critical retinal niche between the apical RPE and photoreceptors, where many retinal disease mechanisms originate, is so far unattainable, imposes clear limitations on both modeling/drug screening and transplantation approaches and is a focus of investigation in many labs. Here we dissect different retinal degenerative diseases and analyze how and where retinal organoid technology can contribute the most to developing therapies even with a current limitation and absence of long and functional outer segments, supported by RPE.

Retinal defocus and form-deprivation induced regional differential gene expression of bone morphogenetic proteins in chick retinal pigment epithelium

We previously reported bidirectional gene expression regulation of the Bone Morphogenetic Proteins (BMP2, 4, and 7) in chick retinal pigment epithelium (RPE) in response to imposed optical defocus and form-deprivation (FD). This study investigated whether there are local (regional) differences in these effects. 19-day old White-Leghorn chicks wore monocular +10 or - 10 D lenses, or diffusers (FD) for 2 or 48 hr, after which RPE samples were collected from both eyes, from a central circular zone (3 mm radius), and 3 mm wide annular mid-peripheral and peripheral zones in all cases. BMP2, 4, and 7 gene expression levels in RPE from treated and fellow control eyes were compared as well as differences across zones. With the +10 D lens, increased expression of both BMP2 and BMP4 genes was observed in central and mid-peripheral zones but not the peripheral zone after 2 and 48 hr. In contrast, with the -10 D lens BMP2 gene expression was significantly decreased in all three zones after 2 and 48 hr. Similar patterns of BMP2 gene expression were observed in all three zones after 48 hr of FD. Smaller changes were recorded for BMP4 and BMP7 gene expression for both myopia-inducing treatments. That optical defocus- and FD-induced changes in BMP gene expression in chick RPE show treatment-dependent local (regional) differences suggest important differences in the nature and contributions of local retinal and underlying RPE regions to eye growth regulation.

Optic vesicle morphogenesis requires primary cilia

Arl13b is a gene known to regulate ciliogenesis. Functional alterations in this gene's activity have been associated with Joubert syndrome. We found that in Arl13 null mouse embryos the orientation of the optic cup is inverted, such that the lens is abnormally surrounded by an inverted optic cup whose retina pigmented epithelium is oddly facing the surface ectoderm. Loss of Arl13b leads to the disruption of optic vesicle's patterning and expansion of ventral fates. We show that this phenotype is consequence of miss-regulation of Sonic hedgehog (Shh) signaling and demonstrate that the Arl13b-/- eye phenotype can be rescued by deletion of Gli2, a downstream effector of the Shh pathway. This work identified an unexpected role of primary cilia during the morphogenetic movements required for the formation of the eye.

OTX2 represses sister cell fate choices in the developing retina to promote photoreceptor specification

During vertebrate retinal development, subsets of progenitor cells generate progeny in a non-stochastic manner, suggesting that these decisions are tightly regulated. However, the gene-regulatory network components that are functionally important in these progenitor cells are largely unknown. Here we identify a functional role for the OTX2 transcription factor in this process. CRISPR/Cas9 gene editing was used to produce somatic mutations of OTX2 in the chick retina and identified similar phenotypes to those observed in human patients. Single cell RNA sequencing was used to determine the functional consequences OTX2 gene editing on the population of cells derived from OTX2-expressing retinal progenitor cells. This confirmed that OTX2 is required for the generation of photoreceptors, but also for repression of specific retinal fates and alternative gene regulatory networks. These include specific subtypes of retinal ganglion and horizontal cells, suggesting that in this context, OTX2 functions to repress sister cell fate choices.

Retina Development in Vertebrates: Systems Biology Approaches to Understanding Genetic Programs: On the Contribution of Next-Generation Sequencing Methods to the Characterization of the Regulatory Networks Controlling Vertebrate Eye Development

The ontogeny of the vertebrate retina has been a topic of interest to developmental biologists and human geneticists for many decades. Understanding the unfolding of the genetic program that transforms a field of progenitors cells into a functionally complex and multi-layered sensory organ is a formidable challenge. Although classical genetic studies succeeded in identifying the key regulators of retina specification, understanding the architecture of their gene network and predicting their behavior are still a distant hope. The emergence of next-generation sequencing platforms revolutionized the field unlocking the access to genome-wide datasets. Emerging techniques such as RNA-seq, ChIP-seq, ATAC-seq, or single cell RNA-seq are used to characterize eye developmental programs. These studies provide valuable information on the transcriptional and cis-regulatory profiles of precursors and differentiated cells, outlining the trajectories that connect each intermediate state. Here, recent systems biology efforts are reviewed to understand the genetic programs shaping the vertebrate retina.

Recapitulating developmental mechanisms for retinal regeneration

Degeneration of specific retinal neurons in diseases like glaucoma, age-related macular degeneration, and retinitis pigmentosa is the leading cause of irreversible blindness. Currently, there is no therapy to modify the disease-associated degenerative changes. With the advancement in our knowledge about the mechanisms that regulate the development of the vertebrate retina, the approach to treat blinding diseases through regenerative medicine appears a near possibility. Recapitulation of developmental mechanisms is critical for reproducibly generating cells in either 2D or 3D culture of pluripotent stem cells for retinal repair and disease modeling. It is the key for unlocking the neurogenic potential of Müller glia in the adult retina for therapeutic regeneration. Here, we examine the current status and potential of the regenerative medicine approach for the retina in the backdrop of developmental mechanisms.

LongAxis: A MATLAB-based program for 3D quantitative analysis of epithelial cell shape and orientation

Epithelial morphogenesis, a fundamental aspect of development, generates 3-dimensional tissue structures crucial for organ function. Underlying morphogenetic mechanisms are, in many cases, poorly understood, but mutations that perturb organ development can affect epithelial cell shape and orientation - difficult features to quantify in three dimensions. The basic structure of the eye is established via epithelial morphogenesis: in the embryonic optic cup, the retinal progenitor epithelium enwraps the lens. We previously found that loss of the extracellular matrix protein laminin-alpha1 (lama1) led to mislocalization of apical polarity markers and apparent misorientation of retinal progenitors. We sought to visualize and quantify this phenotype, and determine whether loss of the apical polarity determinant pard3 might rescue the phenotype. To this end, we developed LongAxis, a MATLAB-based program optimized for the retinal progenitor neuroepithelium. LongAxis facilitates 3-dimensional cell segmentation, visualization, and quantification of cell orientation and morphology. Using LongAxis, we find that retinal progenitors in the lama1^-/- optic cup are misoriented and slightly less elongated. In the lama1;MZpard3 double mutant, cells are still misoriented, but larger. Therefore, loss of pard3 does not rescue loss of lama1, and in fact uncovers a novel cell size phenotype. LongAxis enables population-level visualization and quantification of retinal progenitor cell orientation and morphology. These results underscore the importance of visualizing and quantifying cell orientation and shape in three dimensions within the retina.

Loss of the mitochondrial i-AAA protease YME1L leads to ocular dysfunction and spinal axonopathy

Disturbances in the morphology and function of mitochondria cause neurological diseases, which can affect the central and peripheral nervous system. The i‐AAA protease YME1L ensures mitochondrial proteostasis and regulates mitochondrial dynamics by processing of the dynamin‐like GTPase OPA1. Mutations in YME1L cause a multi‐systemic mitochondriopathy associated with neurological dysfunction and mitochondrial fragmentation but pathogenic mechanisms remained enigmatic. Here, we report on striking cell‐type‐specific defects in mice lacking YME1L in the nervous system. YME1L‐deficient mice manifest ocular dysfunction with microphthalmia and cataracts and develop deficiencies in locomotor activity due to specific degeneration of spinal cord axons, which relay proprioceptive signals from the hind limbs to the cerebellum. Mitochondrial fragmentation occurs throughout the nervous system and does not correlate with the degenerative phenotype. Deletion of Oma1 restores tubular mitochondria but deteriorates axonal degeneration in the absence of YME1L, demonstrating that impaired mitochondrial proteostasis rather than mitochondrial fragmentation causes the observed neurological defects.

Dissecting the transcriptome landscape of the human fetal neural retina and retinal pigment epithelium by single-cell RNA-seq analysis

The developmental pathway of the neural retina (NR) and retinal pigment epithelium (RPE) has been revealed by extensive research in mice. However, the molecular mechanisms underlying the development of the human NR and RPE, as well as the interactions between these two tissues, have not been well defined. Here, we analyzed 2,421 individual cells from human fetal NR and RPE using single-cell RNA sequencing (RNA-seq) technique and revealed the tightly regulated spatiotemporal gene expression network of human retinal cells. We identified major cell classes of human fetal retina and potential crucial transcription factors for each cell class. We dissected the dynamic expression patterns of visual cycle- and ligand-receptor interaction-related genes in the RPE and NR. Moreover, we provided a map of disease-related genes for human fetal retinal cells and highlighted the importance of retinal progenitor cells as potential targets of inherited retinal diseases. Our findings captured the key in vivo features of the development of the human NR and RPE and offered insightful clues for further functional studies.

Regional Gene Expression Profile Comparison Reveals the Unique Transcriptome of the Optic Fissure

Purpose

The optic fissure (OF) is a transient opening in the ventral optic cup (OC) that acts as a passage for blood vessels and retinal ganglion cell axons during early eye development. Failure to close the OF is the developmental basis for uveal coloboma, a congenital blinding eye disease that significantly contributes to childhood blindness. Genes specifically expressed in the OF region may play important roles in OF development and function. The aim of this study was to characterize the transcriptome of OC cells in the OF region and investigate the function of OF-specific genes during OF closure.

Methods

Laser-assisted microdissection was used to collect different regions of OC tissues. Microarray analysis was used to obtain and compare gene expression profiles of different OC regions. RNA in situ hybridization (ISH) was used to further characterize OF-specific gene expression patterns. Morpholino knockdown in zebrafish was used to study the function of a newly discovered OF-specific gene during OF closure.

Results

Microarray comparison revealed that the OC at the OF region exhibited a unique gene expression profile. OC expression patterns of a number of newly discovered OF-specific genes were confirmed by ISH. Morpholino knockdown and downstream target expression and function analysis demonstrated that afap1l2, a newly discovered OF-specific gene, controls OF closure by regulating pax2a expression.

Conclusions

Our study characterized the unique transcriptome of the OF region of the OC and demonstrated the essential role of a newly discovered OF-specific gene in OF closure. This study provides a valuable foundation for future mechanism dissection in OF development and physiology, and for human coloboma etiology exploration.

Retinal Ganglion Cell Replacement: Current Status and Challenges Ahead

The neurons of the retina can be affected by a wide variety of inherited or environmental degenerations that can lead to vision loss and even blindness. Retinal ganglion cell (RGC) degeneration is the hallmark of glaucoma and other optic neuropathies that affect millions of people worldwide. Numerous strategies are being trialed to replace lost neurons in different degeneration models, and in recent years, stem cell technologies have opened promising avenues to obtain donor cells for retinal repair. Stem cell-based transplantation has been most frequently used for the replacement of rod photoreceptors, but the same tools could potentially be used for other retinal cell types, including RGCs. However, RGCs are not abundant in stem cell-derived cultures, and in contrast to the short-distance wiring of photoreceptors, RGC axons take a long and intricate journey to connect with numerous brain nuclei. Hence, a number of challenges still remain, such as the ability to scale up the production of RGCs and a reliable and functional integration into the adult diseased retina upon transplantation. In this review, we discuss the recent advancements in the development of replacement therapies for RGC degenerations and the challenges that we need to overcome before these technologies can be applied to the clinic. Developmental Dynamics 248:118-128, 2019. © 2018 Wiley Periodicals, Inc.

Hedgehog signaling regulates cell motility and optic fissure and stalk formation during vertebrate eye morphogenesis

Establishment of precise three-dimensional tissue structure is vital for organ function. In the visual system, optic fissure and stalk morphogenesis is a crucial yet poorly understood process, disruptions of which can lead to coloboma, a birth defect causing visual impairment. Here, we use four-dimensional imaging, cell tracking, and molecular genetics in zebrafish to define the cell movements underlying normal optic fissure and stalk formation. We determine how these events are disrupted in a coloboma model in which the Hedgehog (Hh) receptor ptch2 is lost, resulting in overactive Hh signaling. In the ptch2 mutant, cells exhibit defective motile behaviors and morphology. Cells that should contribute to the fissure do not arrive at their correct position, and instead contribute to an ectopically large optic stalk. Our results suggest that overactive Hh signaling, through overexpression of downstream transcriptional targets, impairs cell motility underlying optic fissure and stalk formation, via non-cell-autonomous and cell-autonomous mechanisms. More broadly, our cell motility and morphology analyses provide a new framework for studying other coloboma-causing mutations that disrupt optic fissure or stalk formation.

Summary: Multidimensional imaging of ptch2 mutant zebrafish uncovers a role for the Hh signaling pathway in regulating the cell and tissue dynamics underlying early eye morphogenesis.

The peripheral eye: A neurogenic area with potential to treat retinal pathologies?

Numerous degenerative diseases affecting visual function, including glaucoma and retinitis pigmentosa, are produced by the loss of different types of retinal cells. Cell replacement therapy has emerged as a promising strategy for treating these and other retinal diseases. The retinal margin or ciliary body (CB) of mammals has been proposed as a potential source of cells to be used in degenerative conditions affecting the retina because it has been reported it might hold neurogenic potential beyond embryonic development. However, many aspects of the origin and biology of the CB are unknown and more recent experiments have challenged the capacity of CB cells to generate different types of retinal neurons. Here we review the most recent findings about the development of the marginal zone of the retina in different vertebrates and some of the mechanisms underlying the proliferative and neurogenic capacity of this fascinating region of the vertebrates eye. In addition, we performed experiments to isolate CB cells from the mouse retina, generated neurospheres and observed that they can be expanded with a proliferative ratio similar to neural stem cells. When induced to differentiate, cells derived from the CB neurospheres start to express early neural markers but, unlike embryonic stem cells, they are not able to fully differentiate in vitro or generate retinal organoids. Together with previous reports on the neurogenic capacity of CB cells, also reviewed here, our results contribute to the current knowledge about the potentiality of this peripheral region of the eye as a therapeutic source of functional retinal neurons in degenerative diseases.

Genetics of congenital eye malformations: insights from chick experimental embryology

Embryological manipulations in chick embryos have been pivotal in our understanding of many aspects of vertebrate eye formation. This research was particularly important in uncovering the role of tissue interactions as drivers of eye morphogenesis and to dissect the function of critical genes. Here, we have highlighted a few of these past experiments to endorse their value in searching for hitherto unknown causes of rare congenital eye anomalies, such as microphthalmia, anophthalmia and coloboma. We have also highlighted a number of similarities between the chicken and human eye, which might be exploited to address other eye pathologies, including degenerative ocular diseases.

Ciliary margin-derived BMP4 does not have a major role in ocular development

Heterozygous Bmp4 mutations in humans and mice cause severe ocular anterior segment dysgenesis (ASD). Abnormalities include pupil displacement, corneal opacity, iridocorneal adhesions, and variable intraocular pressure, as well as some retinal and vascular defects. It is presently not known what source of BMP4 is responsible for these defects, as BMP4 is expressed in several developing ocular and surrounding tissues. In particular, BMP4 is expressed in the ciliary margins of the optic cup which give rise to anterior segment structures such as the ciliary body and iris, making it a good candidate for the required source of BMP4 for anterior segment development. Here, we test whether ciliary margin-derived BMP4 is required for ocular development using two different conditional knockout approaches. In addition, we compared the conditional deletion phenotypes with Bmp4 heterozygous null mice. Morphological, molecular, and functional assays were performed on adult mutant mice, including histology, immunohistochemistry, in vivo imaging, and intraocular pressure measurements. Surprisingly, in contrast to Bmp4 heterozygous mutants, our analyses revealed that the anterior and posterior segments of Bmp4 conditional knockouts developed normally. These results indicate that ciliary margin-derived BMP4 does not have a major role in ocular development, although subtle alterations could not be ruled out. Furthermore, we demonstrated that the anterior and posterior phenotypes observed in Bmp4 heterozygous animals showed a strong propensity to co-occur, suggesting a common, non-cell autonomous source for these defects.

A Conserved Developmental Mechanism Builds Complex Visual Systems in Insects and Vertebrates

The visual systems of vertebrates and many other bilaterian clades consist of complex neural structures guiding a wide spectrum of behaviors. Homologies at the level of cell types and even discrete neural circuits have been proposed, but many questions of how the architecture of visual neuropils evolved among different phyla remain open. In this review we argue that the profound conservation of genetic and developmental steps generating the eye and its target neuropils in fish and fruit flies supports a homology between some core elements of bilaterian visual circuitries. Fish retina and tectum, and fly optic lobe, develop from a partitioned, unidirectionally proliferating neurectodermal domain that combines slowly dividing neuroepithelial stem cells and rapidly amplifying progenitors with shared genetic signatures to generate large numbers and different types of neurons in a temporally ordered way. This peculiar 'conveyor belt neurogenesis' could play an essential role in generating the topographically ordered circuitry of the visual system.

Coordinated Morphogenetic Mechanisms Shape the Vertebrate Eye

The molecular bases of vertebrate eye formation have been extensively investigated during the past 20 years. This has resulted in the definition of the backbone of the gene regulatory networks controlling the different steps of eye development and has further highlighted a substantial conservation of these networks among vertebrates. Yet, the precise morphogenetic events allowing the formation of the optic cup from a small group of cells within the anterior neural plate are still poorly understood. It is also unclear if the morphogenetic events leading to eyes of very similar shape are indeed comparable among all vertebrates or if there are any species-specific peculiarities. Improved imaging techniques have enabled to follow how the eye forms in living embryos of a few vertebrate models, whereas the development of organoid cultures has provided fascinating tools to recapitulate tissue morphogenesis of other less accessible species. Here, we will discuss what these advances have taught us about eye morphogenesis, underscoring possible similarities and differences among vertebrates. We will also discuss the contribution of cell shape changes to this process and how morphogenetic and patterning mechanisms integrate to assemble the final architecture of the eye.

An Eye Organoid Approach Identifies Six3 Suppression of R-spondin 2 as a Critical Step in Mouse Neuroretina Differentiation

Recent advances in self-organizing, 3-dimensional tissue cultures of embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) provided an in vitro model that recapitulates many aspects of the in vivo developmental steps. Using Rax-GFP-expressing ESCs, newly generated Six3-/- iPSCs, and conditional null Six3delta/f;Rax-Cre ESCs, we identified Six3 repression of R-spondin 2 (Rspo2) as a required step during optic vesicle morphogenesis and neuroretina differentiation. We validated these results in vivo by showing that transient ectopic expression of Rspo2 in the anterior neural plate of transgenic mouse embryos was sufficient to inhibit neuroretina differentiation. Additionally, using a chimeric eye organoid assay, we determined that Six3 null cells exert a non-cell-autonomous repressive effect during optic vesicle formation and neuroretina differentiation. Our results further validate the organoid culture system as a reliable and fast alternative to identify and evaluate genes involved in eye morphogenesis and neuroretina differentiation in vivo.

Proper ciliary assembly is critical for restricting Hedgehog signaling during early eye development in mice

Patterning of the vertebrate eye into optic stalk, retinal pigment epithelium (RPE) and neural retina (NR) territories relies on a number of signaling pathways, but how these signals are interpreted by optic progenitors is not well understood. The primary cilium is a microtubule-based organelle that is essential for Hedgehog (Hh) signaling, but it has also been implicated in the regulation of other signaling pathways. Here, we show that the optic primordium is ciliated during early eye development and that ciliogenesis is essential for proper patterning and morphogenesis of the mouse eye. Ift172 mutants fail to generate primary cilia and exhibit patterning defects that resemble those of Gli3 mutants, suggesting that cilia are required to restrict Hh activity during eye formation. Ift122 mutants, which produce cilia with abnormal morphology, generate optic vesicles that fail to invaginate to produce the optic cup. These mutants also lack formation of the lens, RPE and NR. Such phenotypic features are accompanied by strong, ectopic Hh pathway activity, evidenced by altered gene expression patterns. Removal of GLI2 from Ift122 mutants rescued several aspects of optic cup and lens morphogenesis as well as RPE and NR specification. Collectively, our data suggest that proper assembly of primary cilia is critical for restricting the Hedgehog pathway during eye formation in the mouse.