Retinoic acid guides eye morphogenetic movements via paracrine signaling but is unnecessary for retinal dorsoventral patterning

Effect of vitamin A on maternal, fetal, and neonatal outcomes: An overview of deficiency, excessive intake, and intake recommendations

Vitamin A imbalance during pregnancy and lactation is a global public health concern with potentially negative consequences for fetuses and neonates. Inadequate vitamin A intake during this critical period can lead to anemia, weakened immune function, night blindness, and increased susceptibility to infections. Conversely, excessive intake of vitamin A can result in birth defects, hypercalcemia, and psychiatric symptoms. This review aims to identify risk factors contributing to vitamin A deficiency in pregnant women and its impact on maternal, fetal, and neonatal outcomes. It also examines the effects of high-dose vitamin A supplementation during pregnancy on offspring health. By analyzing existing literature and recommendations, the review emphasizes the significance of vitamin A in the development of various body systems and organs. It provides a comprehensive overview of the effects of vitamin A during pregnancy and lactation, encompassing deficiencies, excessive intake, and supplementation guidelines. The need for further research in this field is highlighted. In conclusion, maintaining a balanced vitamin A status is crucial during pregnancy to promote better outcomes for fetuses and newborns. Effective monitoring and intervention strategies are essential to address vitamin A deficiency and excess in pregnant women, thereby improving fetal and neonatal health.

Molecular mechanisms controlling vertebrate retinal patterning, neurogenesis, and cell fate specification

This review covers recent advances in understanding the molecular mechanisms controlling neurogenesis and specification of the developing retina, with a focus on insights obtained from comparative single cell multiomic analysis. We discuss recent advances in understanding the mechanisms by which extrinsic factors trigger transcriptional changes that spatially pattern the optic cup (OC) and control the initiation and progression of retinal neurogenesis. We also discuss progress in unraveling the core evolutionarily conserved gene regulatory networks (GRNs) that specify early- and late-state retinal progenitor cells (RPCs) and neurogenic progenitors and that control the final steps in determining cell identity. Finally, we discuss findings that provide insight into regulation of species-specific aspects of retinal patterning and neurogenesis, including consideration of key outstanding questions in the field.

Retinoic acid, RARs and early development

Vitamin A (retinol) is an important nutrient for embryonic development and adult health. Early studies identified retinoic acid (RA) as a metabolite of retinol, however, its importance was not apparent. Later, it was observed that RA treatment of vertebrate embryos had teratogenic effects on limb development. Subsequently, the discovery of nuclear RA receptors (RARs) revealed that RA controls gene expression directly at the transcriptional level through a process referred to as RA signaling. This important discovery led to further studies demonstrating that RA and RARs are required for normal embryonic development. The determination of RA function during normal development has been challenging as RA gain-of-function studies often lead to conclusions about normal development that conflict with RAR or RA loss-of-function studies. However, genetic loss-of-function studies have identified direct target genes of endogenous RA/RAR that are required for normal development of specific tissues. Thus, genetic loss-of-function studies that eliminate RARs or RA-generating enzymes have been instrumental in revealing that RA signaling is required for normal early development of many organs and tissues, including the hindbrain, posterior body axis, somites, spinal cord, forelimbs, heart, and eye.

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.

All-trans retinoic acid modulates pigmentation, neuroretinal maturation, and corneal transparency in human multiocular organoids

BACKGROUND

All-trans retinoic acid (ATRA) plays an essential role during human eye development, being temporally and spatially adjusted to create gradient concentrations that guide embryonic anterior and posterior axis formation of the eye. Perturbations in ATRA signaling can result in severe ocular developmental diseases. Although it is known that ATRA is essential for correct eye formation, how ATRA influences the different ocular tissues during the embryonic development of the human eye is still not well studied. Here, we investigated the effects of ATRA on the differentiation and the maturation of human ocular tissues using an in vitro model of human-induced pluripotent stem cells-derived multiocular organoids.

METHODS

Multiocular organoids, consisting of the retina, retinal pigment epithelium (RPE), and cornea, were cultured in a medium containing low (500 nM) or high (10 µM) ATRA concentrations for 60 or 90 days. Furthermore, retinal organoids were cultured with taurine and T3 to further study photoreceptor modulation during maturation. Histology, immunochemistry, qPCR, and western blot were used to study gene and protein differential expression between groups.

RESULTS

High ATRA levels promote the transparency of corneal organoids and the neuroretinal development in retinal organoids. However, the same high ATRA levels decreased the pigmentation levels of RPE organoids and, in long-term cultures, inhibited the maturation of photoreceptors. By contrast, low ATRA levels enhanced the pigmentation of RPE organoids, induced the opacity of corneal organoids-due to an increase in collagen type IV in the stroma- and allowed the maturation of photoreceptors in retinal organoids. Moreover, T3 promoted rod photoreceptor maturation, whereas taurine promoted red/green cone photoreceptors.

CONCLUSION

ATRA can modulate corneal epithelial integrity and transparency, photoreceptor development and maturation, and the pigmentation of RPE cells in a dose-dependent manner. These experiments revealed the high relevance of ATRA during ocular tissue development and its use as a potential new strategy to better modulate the development and maturation of ocular tissue through temporal and spatial control of ATRA signaling.

Retinoic acid signaling mediates peripheral cone photoreceptor survival in a mouse model of retina degeneration

Retinitis Pigmentosa (RP) is a progressive, debilitating visual disorder caused by mutations in a diverse set of genes. In both humans with RP and mouse models of RP, rod photoreceptor dysfunction leads to loss of night vision, and is followed by secondary cone photoreceptor dysfunction and degeneration, leading to loss of daylight color vision. A strategy to prevent secondary cone death could provide a general RP therapy to preserve daylight color vision regardless of the underlying mutation. In mouse models of RP, cones in the peripheral retina survive long-term, despite complete rod loss. The mechanism for such peripheral cone survival had not been explored. Here, we found that active retinoic acid (RA) signaling in peripheral Muller glia is necessary for the abnormally long survival of these peripheral cones. RA depletion by conditional knockout of RA synthesis enzymes, or overexpression of an RA degradation enzyme, abrogated the extended survival of peripheral cones. Conversely, constitutive activation of RA signaling in the central retina promoted long-term cone survival. These results indicate that RA signaling mediates the prolonged peripheral cone survival in the rd1 mouse model of retinal degeneration, and provide a basis for a generic strategy for cone survival in the many diseases that lead to loss of cone-mediated vision.

Long-read genome sequencing resolves a complex 13q structural variant associated with syndromic anophthalmia

Microphthalmia, anophthalmia, and coloboma (MAC) are a heterogeneous spectrum of anomalous eye development and degeneration with genetic and environmental etiologies. Structural and copy number variants of chromosome 13 have been implicated in MAC; however, the specific loci involved in disease pathogenesis have not been well-defined. Herein we report a newborn with syndromic degenerative anophthalmia and a complex de novo rearrangement of chromosome 13q. Long-read genome sequencing improved the resolution and clinical interpretation of a duplication-triplication/inversion-duplication (DUP-TRP/INV-DUP) and terminal deletion. Sequence features at the breakpoint junctions suggested microhomology-mediated break-induced replication (MMBIR) of the maternal chromosome as the origin. Comparing this rearrangement to previously reported copy number alterations in 13q, we refine a putative dosage-sensitive critical region for MAC that might provide new insights into its molecular etiology.

Transmural pressure signals through retinoic acid to regulate lung branching

During development, the mammalian lung undergoes several rounds of branching, the rate of which is tuned by the relative pressure of the fluid within the lumen of the lung. We carried out bioinformatics analysis of RNA-sequencing of embryonic mouse lungs cultured under physiologic or sub-physiologic transmural pressure and identified transcription factor-binding motifs near genes whose expression changes in response to pressure. Surprisingly, we found retinoic acid (RA) receptor binding sites significantly overrepresented in the promoters and enhancers of pressure-responsive genes. Consistently, increasing transmural pressure activates RA signaling, and pharmacologically inhibiting RA signaling decreases airway epithelial branching and smooth muscle wrapping. We found that pressure activates RA signaling through the mechanosensor Yap. A computational model predicts that mechanical signaling through Yap and RA affects lung branching by altering the balance between epithelial proliferation and smooth muscle wrapping, which we test experimentally. Our results reveal that transmural pressure signals through RA to balance the relative rates of epithelial growth and smooth muscle differentiation in the developing mouse lung and identify RA as a previously unreported component in the mechanotransduction machinery of embryonic tissues.

Towards a Better Vision of Retinoic Acid Signaling during Eye Development

Retinoic acid (RA) functions as an essential signal for development of the vertebrate eye by controlling the transcriptional regulatory activity of RA receptors (RARs). During eye development, the optic vesicles and later the retina generate RA as a metabolite of vitamin A (retinol). Retinol is first converted to retinaldehyde by retinol dehydrogenase 10 (RDH10) and then to RA by all three retinaldehyde dehydrogenases (ALDH1A1, ALDH1A2, and ALDH1A3). In early mouse embryos, RA diffuses to tissues throughout the optic placode, optic vesicle, and adjacent mesenchyme to stimulate folding of the optic vesicle to form the optic cup. RA later generated by the retina is needed for further morphogenesis of the optic cup and surrounding perioptic mesenchyme; loss of RA at this stage leads to microphthalmia and cornea plus eyelid defects. RA functions by binding to nuclear RARs at RA response elements (RAREs) that either activate or repress transcription of key genes. Binding of RA to RARs regulates recruitment of transcriptional coregulators such as nuclear receptor coactivator (NCOA) or nuclear receptor corepressor (NCOR), which in turn control binding of the generic coactivator p300 or the generic corepressor PRC2. No genes have been identified as direct targets of RA signaling during eye development, so future studies need to focus on identifying such genes and their RAREs. Studies designed to learn how RA normally controls eye development in vivo will provide basic knowledge valuable for determining how developmental eye defects occur and for improving strategies to treat eye defects.

Genetic Control of MAP3K1 in Eye Development and Sex Differentiation

The MAP3K1 is responsible for transmitting signals to activate specific MAP2K-MAPK cascades. Following the initial biochemical characterization, genetic mouse models have taken center stage to elucidate how MAP3K1 regulates biological functions. To that end, mice were generated with the ablation of the entire Map3k1 gene, the kinase domain coding sequences, or ubiquitin ligase domain mutations. Analyses of the mutants identify diverse roles that MAP3K1 plays in embryonic survival, maturation of T/B cells, and development of sensory organs, including eye and ear. Specifically in eye development, Map3k1 loss-of-function was found to be autosomal recessive for congenital eye abnormalities, but became autosomal dominant in combination with Jnk and RhoA mutations. Additionally, Map3k1 mutation increased eye defects with an exposure to environmental agents such as dioxin. Data from eye developmental models reveal the nexus role of MAP3K1 in integrating genetic and environmental signals to control developmental activities. Here, we focus the discussions on recent advances in understanding the signaling mechanisms of MAP3K1 in eye development in mice and in sex differentiation from human genomics findings. The research works featured here lead to a deeper understanding of the in vivo signaling network, the mechanisms of gene-environment interactions, and the relevance of this multifaceted protein kinase in disease etiology and pathogenesis.

A master of all trades - linking retinoids to different signalling pathways through the multi-purpose receptor STRA6

Retinoids are a group of vitamin A-related chemicals that are essential to chordate mammals. They regulate a number of basic processes, including embryogenesis and vision. From ingestion to metabolism and the subsequent cellular effects, retinoid levels are tightly regulated in the organism to prevent toxicity. One component of this network, the membrane receptor STRA6, has been shown to be essential in facilitating the cellular entry and exit of retinol. However, recent data suggests that STRA6 may not function merely as a retinoid transporter but also act as a complex signalling hub in its own right, being able to affect cell fate through the integration of retinoid signalling with other key pathways, such as those involving p53, JAK/STAT, Wnt/β catenin and calcium. This may open new therapeutic strategies in diseases like cancer, where these pathways are often compromised. Here, we look at the growing evidence regarding the novel roles of STRA6 beyond its well characterized classic functions.

Retinaldehyde Dehydrogenase Inhibition-Related Adverse Outcome Pathway: Potential Risk of Retinoic Acid Synthesis Inhibition during Embryogenesis

Retinoic acid (RA) is one of the factors crucial for cell growth, differentiation, and embryogenesis; it interacts with the retinoic acid receptor and retinoic acid X receptor to eventually regulate target gene expression in chordates. RA is transformed from retinaldehyde via oxidization by retinaldehyde dehydrogenase (RALDH), which belongs to the family of oxidoreductases. Several chemicals, including disulphiram, diethylaminobenzaldehyde, and SB-210661, can effectively inhibit RALDH activity, potentially causing reproductive and developmental toxicity. The modes of action can be sequentially explained based on the molecular initiating event toward key events, and finally the adverse outcomes. Adverse outcome pathway (AOP) is a conceptual and theoretical framework that describes the sequential chain of casually liked events at different biological levels from molecular events to adverse effects. In the present review, we discussed a recently registered AOP (AOP₂₉₇; inhibition of retinaldehyde dehydrogenase leads to population decline) to explain and support the weight of evidence for RALDH inhibition-related developmental toxicity using the existing knowledge.

Conditional Deletion of AP-2β in the Periocular Mesenchyme of Mice Alters Corneal Epithelial Cell Fate and Stratification

The cornea is an anterior eye structure specialized for vision. The corneal endothelium and stroma are derived from the periocular mesenchyme (POM), which originates from neural crest cells (NCCs), while the stratified corneal epithelium develops from the surface ectoderm. Activating protein-2β (AP-2β) is highly expressed in the POM and important for anterior segment development. Using a mouse model in which AP-2β is conditionally deleted in the NCCs (AP-2β NCC KO), we investigated resulting corneal epithelial abnormalities. Through PAS and IHC staining, we observed structural and phenotypic changes to the epithelium associated with AP-2β deletion. In addition to failure of the mutant epithelium to stratify, we also observed that Keratin-12, a marker of the differentiated epithelium, was absent, and Keratin-15, a limbal and conjunctival marker, was expanded across the central epithelium. Transcription factors PAX6 and P63 were not observed to be differentially expressed between WT and mutant. However, growth factor BMP4 was suppressed in the mutant epithelium. Given the non-NCC origin of the epithelium, we hypothesize that the abnormalities in the AP-2β NCC KO mouse result from changes to regulatory signaling from the POM-derived stroma. Our findings suggest that stromal pathways such as Wnt/β-Catenin signaling may regulate BMP4 expression, which influences cell fate and stratification.

The retinal pigment epithelium: Development, injury responses, and regenerative potential in mammalian and non-mammalian systems

Diseases that result in retinal pigment epithelium (RPE) degeneration, such as age-related macular degeneration (AMD), are among the leading causes of blindness worldwide. Atrophic (dry) AMD is the most prevalent form of AMD and there are currently no effective therapies to prevent RPE cell death or restore RPE cells lost from AMD. An intriguing approach to treat AMD and other RPE degenerative diseases is to develop therapies focused on stimulating endogenous RPE regeneration. For this to become feasible, a deeper understanding of the mechanisms underlying RPE development, injury responses and regenerative potential is needed. In mammals, RPE regeneration is extremely limited; small lesions can be repaired by the expansion of adjacent RPE cells, but large lesions cannot be repaired as remaining RPE cells are unable to functionally replace lost RPE tissue. In some injury paradigms, RPE cells proliferate but do not regenerate a morphologically normal monolayer, while in others, proliferation is pathogenic and results in further disruption to the retina. This is in contrast to non-mammalian vertebrates, which possess tremendous RPE regenerative potential. Here, we discuss what is known about RPE formation during development in mammalian and non-mammalian vertebrates, we detail the processes by which RPE cells respond to injury, and we describe examples of RPE-to-retina and RPE-to-RPE regeneration in non-mammalian vertebrates. Finally, we outline barriers to RPE-dependent regeneration in mammals that could potentially be overcome to stimulate a regenerative response from the RPE.

Absence of Cytochrome P450-1b1 Increases Susceptibility of Pressure-Induced Axonopathy in the Murine Retinal Projection

Mutations in the cytochrome P450-1B1 (Cyp1b1) gene is a common genetic predisposition associated with various human glaucomas, most prominently in primary congenital glaucoma (PCG). The role of Cyp1b1 in the eye is largely unknown, however, its absence appears to drive the maldevelopment of anterior eye structures responsible for aqueous fluid drainage in murine models. Nevertheless, vision loss in glaucoma ultimately results from the structural and functional loss of retinal ganglion cells (RGCs). Cyp1b1’s influence in the development and support of retinal ganglion cell structure and function under normal conditions or during stress, such as elevated ocular pressure; the most common risk factor in glaucoma, remains grossly unknown. Thus, to determine the role of Cyp1b1 in normal retinal projection development we first assessed the strucutrual integrity of RGCs in the retina, optic nerve, and superior colliculus in un-manipulated (naïve) Cyp1b1-knockout (Cyp1b1^–/–) mice. In addition, in a separate cohort of Cyp1b1^–/– and wildtype mice, we elevated and maintained intraocular pressure (IOP) at glaucomatous levels for 5-weeks, after which we compared RGC density, node of Ranvier morphology, and axonal transport between the genotypes. Our results demonstrate that naïve Cyp1b1^–/– mice develop an anatomically intact retinal projection absent of overt glaucomatous pathology. Following pressure elevation, Cyp1b1^–/– accelerated degradation of axonal transport from the retina to the superior colliculus and altered morphology of the nodes of Ranvier and adjacent paranodes in the optic nerves. Together this data suggests the absence Cyp1b1 expression alone is insufficient to drive murine glaucomatous pathology, however, may increase the vulnerability of retinal axons to disease relevant elevations in IOP.

Local retinoic acid signaling directs emergence of the extraocular muscle functional unit

Coordinated development of muscles, tendons, and their attachment sites ensures emergence of functional musculoskeletal units that are adapted to diverse anatomical demands among different species. How these different tissues are patterned and functionally assembled during embryogenesis is poorly understood. Here, we investigated the morphogenesis of extraocular muscles (EOMs), an evolutionary conserved cranial muscle group that is crucial for the coordinated movement of the eyeballs and for visual acuity. By means of lineage analysis, we redefined the cellular origins of periocular connective tissues interacting with the EOMs, which do not arise exclusively from neural crest mesenchyme as previously thought. Using 3D imaging approaches, we established an integrative blueprint for the EOM functional unit. By doing so, we identified a developmental time window in which individual EOMs emerge from a unique muscle anlage and establish insertions in the sclera, which sets these muscles apart from classical muscle-to-bone type of insertions. Further, we demonstrate that the eyeballs are a source of diffusible all-trans retinoic acid (ATRA) that allow their targeting by the EOMs in a temporal and dose-dependent manner. Using genetically modified mice and inhibitor treatments, we find that endogenous local variations in the concentration of retinoids contribute to the establishment of tendon condensations and attachment sites that precede the initiation of muscle patterning. Collectively, our results highlight how global and site-specific programs are deployed for the assembly of muscle functional units with precise definition of muscle shapes and topographical wiring of their tendon attachments.

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.

Corneal development: Role of the periocular mesenchyme and bi-directional signaling

The cornea is a highly specialized transparent tissue located at the anterior most surface of the eye. It consists of three main layers, the outer stratified squamous epithelium, the inner endothelium, and the intermediate stroma. Formation of these layers during development involves a complex interaction between ectodermal-derived structures, such as the overlying head ectoderm with the periocular mesenchyme (POM), the latter of which is comprised of neural crest cells (NCC) and mesoderm-derived progenitor cells. Regulation of corneal epithelial development, including both epithelial cell fate and stratification, has been shown to depend on numerous bi-directional mesenchymal-epithelial signaling pathways. In this review we pay particular attention to the genes and signaling pathways that involve the POM.

The Cdx transcription factors and retinoic acid play parallel roles in antero-posterior position of the pectoral fin field during gastrulation

The molecular regulators that determine the precise position of the vertebrate limb along the anterio-posterior axis have not been identified. One model suggests that a combination of hox genes in the lateral plate mesoderm (LPM) promotes formation of the limb field, however redundancy among duplicated paralogs has made this model difficult to confirm. In this study, we identify an optimal window during mid-gastrulation stages when transient mis-regulation of retinoic acid signaling or the caudal related transcription factor, Cdx4, both known regulators of hox genes, can alter the position of the pectoral fin field. We show that increased levels of either RA or Cdx4 during mid-gastrulation are sufficient to rostrally shift the position of the pectoral fin field at the expense of surrounding gene expression in the anterior lateral plate mesoderm (aLPM). Alternatively, embryos deficient for both Cdx4 and Cdx1a (Cdx-deficient) form pectoral fins that are shifted towards the posterior and reveal an additional effect on size of the pectoral fin buds. Prior to formation of the pectoral fin buds, the fin field in Cdx-deficient embryos is visibly expanded into the posterior LPM (pLPM) region at the expense of surrounding gene expression. The effects on gene expression immediately post-gastrulation and during somitogenesis support a model where RA and Cdx4 act in parallel to regulate the position of the pectoral fin. Our transient method is a potentially useful model for studying the mechanisms of limb positioning along the AP axis.

Ocular coloboma: Genetic variants reveal a dynamic model of eye development

Ocular coloboma is a congenital disorder of the eye where a gap exists in the inferior retina, lens, iris, or optic nerve tissue. With a prevalence of 2-19 per 100,000 live births, coloboma, and microphthalmia, an associated ocular disorder, represent up to 10% of childhood blindness. It manifests due to the failure of choroid fissure closure during eye development, and it is a part of a spectrum of ocular disorders that include microphthalmia and anophthalmia. Use of genetic approaches from classical pedigree analyses to next generation sequencing has identified more than 40 loci that are associated with the causality of ocular coloboma. As we have expanded studies to include singleton cases, hereditability has been very challenging to prove. As such, researchers over the past 20 years, have unraveled the complex interrelationship amongst these 40 genes using vertebrate model organisms. Such research has greatly increased our understanding of eye development. These genes function to regulate initial specification of the eye field, migration of retinal precursors, patterning of the retina, neural crest cell biology, and activity of head mesoderm. This review will discuss the discovery of loci using patient data, their investigations in animal models, and the recent advances stemming from animal models that shed new light in patient diagnosis.

Retinoic acid synthesis and autoregulation mediate zonal patterning of vestibular organs and inner ear morphogenesis

Retinoic acid (RA), a vitamin A (retinol) derivative, has pleiotropic functions during embryonic development. The synthesis of RA requires two enzymatic reactions: oxidation of retinol into retinaldehyde by alcohol dehydrogenases (ADHs) or retinol dehydrogenases (RDHs); and oxidation of retinaldehyde into RA by aldehyde dehydrogenases family 1, subfamily A (ALDH1as), such as ALDH1a1, ALDH1a2 and ALDH1a3. Levels of RA in tissues are regulated by spatiotemporal expression patterns of genes encoding RA-synthesizing and -degrading enzymes, such as cytochrome P450 26 (Cyp26 genes). Here, we show that RDH10 is important for both sensory and non-sensory formation of the vestibule of the inner ear. Mice deficient in Rdh10 exhibit failure of utricle-saccule separation, otoconial formation and zonal patterning of vestibular sensory organs. These phenotypes are similar to those of Aldh1a3 knockouts, and the sensory phenotype is complementary to that of Cyp26b1 knockouts. Together, these results demonstrate that RDH10 and ALDH1a3 are the key RA-synthesis enzymes involved in vestibular development. Furthermore, we discovered that RA induces Cyp26b1 expression in the developing vestibular sensory organs, which generates the differential RA signaling required for zonal patterning.

Periocular neural crest cell differentiation into corneal endothelium is influenced by signals in the nascent corneal environment

During ocular development, periocular neural crest cells (pNC) migrate into the region between the lens and presumptive corneal epithelium to form the corneal endothelium and stromal keratocytes. Although defects in neural crest cell development are associated with ocular dysgenesis, very little is known about the molecular mechanisms involved in this process. This study focuses on the corneal endothelium, a monolayer of specialized cells that are essential for maintaining normal hydration and transparency of the cornea. In avians, corneal endothelial cells are first to be specified from the pNC during their migration into the presumptive corneal region. To investigate the signals required for formation of the corneal endothelium, we utilized orthotopic and heterotopic injections of dissociated quail pNC into chick ocular regions. We find that pNC are multipotent and that the nascent cornea is competent to induce differentiation of ectopically injected pNC into corneal endothelium. Injected pNC downregulate expression of multipotency transcription factors and upregulate genes that are consistent with ontogenesis of the chick corneal endothelium. Importantly, we showed that TGFβ2 is expressed by the nascent lens and the corneal endothelium, and that TGFβ signaling plays a critical role in changing the molecular signature of pNC in vitro. Collectively, our results demonstrate the significance of the ocular environmental cues towards pNC differentiation, and have potential implications for clinical application of stem cells in the anterior segment.

Retinoic acid synthesis by ALDH1A proteins is dispensable for meiosis initiation in the mouse fetal ovary

In mammals, the timing of meiosis entry is regulated by signals from the gonadal environment. All-trans retinoic acid (ATRA) signaling is considered the key pathway that promotes Stra8 (stimulated by retinoic acid 8) expression and, in turn, meiosis entry. This model, however, is debated because it is based on analyzing the effects of exogenous ATRA on ex vivo gonadal cultures, which not accurately reflects the role of endogenous ATRA. Aldh1a1 and Aldh1a2, two retinaldehyde dehydrogenases synthesizing ATRA, are expressed in the mouse ovaries when meiosis initiates. Contrary to the present view, here, we demonstrate that ATRA-responsive cells are scarce in the ovary. Using three distinct gene deletion models for Aldh1a1;Aldh1a2;Aldh1a3, we show that Stra8 expression is independent of ATRA production by ALDH1A proteins and that germ cells progress through meiosis. Together, these data demonstrate that ATRA signaling is dispensable for instructing meiosis initiation in female germ cells.

Molecular components affecting ocular carotenoid and retinoid homeostasis

The photochemistry of vision employs opsins and geometric isomerization of their covalently bound retinylidine chromophores. In different animal classes, these light receptors associate with distinct G proteins that either hyperpolarize or depolarize photoreceptor membranes. Vertebrates also use the acidic form of chromophore, retinoic acid, as the ligand of nuclear hormone receptors that orchestrate eye development. To establish and sustain these processes, animals must acquire carotenoids from the diet, transport them, and metabolize them to chromophore and retinoic acid. The understanding of carotenoid metabolism, however, lagged behind our knowledge about the biology of their receptor molecules. In the past decades, much progress has been made in identifying the genes encoding proteins that mediate the transport and enzymatic transformations of carotenoids and their retinoid metabolites. Comparative analysis in different animal classes revealed how evolutionary tinkering with a limited number of genes evolved different biochemical strategies to supply photoreceptors with chromophore. Mutations in these genes impair carotenoid metabolism and induce various ocular pathologies. This review summarizes this advancement and introduces the involved proteins, including the homeostatic regulation of their activities.

A Systematic Analysis Revealed the Potential Gene Regulatory Processes of ATRA-Triggered Neuroblastoma Differentiation and Identified a Novel RA Response Sequence in the NTRK2 Gene

Retinoic acid- (RA-) triggered neuroblastoma cell lines are widely used cell modules of neuronal differentiation in neurodegenerative disease studies, but the gene regulatory mechanism underlying differentiation is unclear now. In this study, system biological analysis was performed on public microarray data from three neuroblastoma cell lines (SK-N-SH, SH-SY5Y-A, and SH-SY5Y-E) to explore the potential molecular processes of all-trans retinoic acid- (ATRA-) triggered differentiation. RT-qPCR, functional genomics analysis, western blotting, chromatin immunoprecipitation (ChIP), and homologous sequence analysis were further performed to validate the gene regulation processes and identify the RA response element in a specific gene. The potential disturbed biological pathways (111 functional GO terms in 14 interactive functional groups) and gene regulatory network (10 regulators and 71 regulated genes) in neuroblastoma differentiation were obtained. 15 of the 71 regulated genes are neuronal projection-related. Among them, NTRK2 is the only one that was dramatically upregulated in the RT-qPCR test that we performed on ATRA-treated SH-SY5Y-A cells. We further found that the overexpression of the NTRK2 gene can trigger differentiation-like changes in SH-SY5Y-A cells. Functional genomic analysis and western blotting assay suggested that, in neuroblastoma cells, ATRA may directly regulate the NTRK2 gene by activating the RA receptor (RAR) that binds in its promoter region. A novel RA response DNA element in the NTRK2 gene was then identified by bioinformatics analysis and chromatin immunoprecipitation (ChIP) assay. The novel element is sequence conservation and position variation among different species. Our study systematically provided the potential regulatory information of ATRA-triggered neuroblastoma differentiation, and in the NTRK2 gene, we identified a novel RA response DNA element, which may contribute to the differentiation in a human-specific manner.

Retinoic acid degradation shapes zonal development of vestibular organs and sensitivity to transient linear accelerations

Each vestibular sensory epithelium in the inner ear is divided morphologically and physiologically into two zones, called the striola and extrastriola in otolith organ maculae, and the central and peripheral zones in semicircular canal cristae. We found that formation of striolar/central zones during embryogenesis requires Cytochrome P450 26b1 (Cyp26b1)-mediated degradation of retinoic acid (RA). In Cyp26b1 conditional knockout mice, formation of striolar/central zones is compromised, such that they resemble extrastriolar/peripheral zones in multiple features. Mutants have deficient vestibular evoked potential (VsEP) responses to jerk stimuli, head tremor and deficits in balance beam tests that are consistent with abnormal vestibular input, but normal vestibulo-ocular reflexes and apparently normal motor performance during swimming. Thus, degradation of RA during embryogenesis is required for formation of highly specialized regions of the vestibular sensory epithelia with specific functions in detecting head motions.

Genetics and functions of the retinoic acid pathway, with special emphasis on the eye

Retinoic acid (RA) is a potent morphogen required for embryonic development. RA is formed in a multistep process from vitamin A (retinol); RA acts in a paracrine fashion to shape the developing eye and is essential for normal optic vesicle and anterior segment formation. Perturbation in RA-signaling can result in severe ocular developmental diseases—including microphthalmia, anophthalmia, and coloboma. RA-signaling is also essential for embryonic development and life, as indicated by the significant consequences of mutations in genes involved in RA-signaling. The requirement of RA-signaling for normal development is further supported by the manifestation of severe pathologies in animal models of RA deficiency—such as ventral lens rotation, failure of optic cup formation, and embryonic and postnatal lethality. In this review, we summarize RA-signaling, recent advances in our understanding of this pathway in eye development, and the requirement of RA-signaling for embryonic development (e.g., organogenesis and limb bud development) and life.

The Molecular Basis of Human Anophthalmia and Microphthalmia

Human eye development is coordinated through an extensive network of genetic signalling pathways. Disruption of key regulatory genes in the early stages of eye development can result in aborted eye formation, resulting in an absent eye (anophthalmia) or a small underdeveloped eye (microphthalmia) phenotype. Anophthalmia and microphthalmia (AM) are part of the same clinical spectrum and have high genetic heterogeneity, with >90 identified associated genes. By understanding the roles of these genes in development, including their temporal expression, the phenotypic variation associated with AM can be better understood, improving diagnosis and management. This review describes the genetic and structural basis of eye development, focusing on the function of key genes known to be associated with AM. In addition, we highlight some promising avenues of research involving multiomic approaches and disease modelling with induced pluripotent stem cell (iPSC) technology, which will aid in developing novel therapies.

Retinoic acid signaling pathways

Retinoic acid (RA), a metabolite of retinol (vitamin A), functions as a ligand for nuclear RA receptors (RARs) that regulate development of chordate animals. RA-RARs can activate or repress transcription of key developmental genes. Genetic studies in mouse and zebrafish embryos that are deficient in RA-generating enzymes or RARs have been instrumental in identifying RA functions, revealing that RA signaling regulates development of many organs and tissues, including the body axis, spinal cord, forelimbs, heart, eye and reproductive tract. An understanding of the normal functions of RA signaling during development will guide efforts for use of RA as a therapeutic agent to improve human health. Here, we provide an overview of RA signaling and highlight its key functions during development.

What's retinoic acid got to do with it? Retinoic acid regulation of the neural crest in craniofacial and ocular development

Retinoic acid (RA), the active derivative of vitamin A (retinol), is an essential morphogen signaling molecule and major regulator of embryonic development. The dysregulation of RA levels during embryogenesis has been associated with numerous congenital anomalies, including craniofacial, auditory, and ocular defects. These anomalies result from disruptions in the cranial neural crest, a vertebrate-specific transient population of stem cells that contribute to the formation of diverse cell lineages and embryonic structures during development. In this review, we summarize our current knowledge of the RA-mediated regulation of cranial neural crest induction at the edge of the neural tube and the migration of these cells into the craniofacial region. Further, we discuss the role of RA in the regulation of cranial neural crest cells found within the frontonasal process, periocular mesenchyme, and pharyngeal arches, which eventually form the bones and connective tissues of the head and neck and contribute to structures in the anterior segment of the eye. We then review our understanding of the mechanisms underlying congenital craniofacial and ocular diseases caused by either the genetic or toxic disruption of RA signaling. Finally, we discuss the role of RA in maintaining neural crest-derived structures in postembryonic tissues and the implications of these studies in creating new treatments for degenerative craniofacial and ocular diseases.

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.

Transcriptomic analysis of differential gene expression during chick periocular neural crest differentiation into corneal cells

BACKGROUND

Multipotent neural crest cells (NCC) contribute to the corneal endothelium and keratocytes during ocular development, but the molecular mechanisms that underlie this process remain poorly understood. We performed RNA-Seq analysis on periocular neural crest (pNC), corneal endothelium, and keratocytes and validated expression of candidate genes by in situ hybridization.

RESULTS

RNA-Seq profiling revealed enrichment of genes between pNC and neural crest-derived corneal cells, which correspond to pathways involved in focal adhesion, ECM-receptor interaction, cell adhesion, melanogenesis, and MAPK signaling. Comparisons of candidate NCC genes to ocular gene expression revealed that majority of the NCC genes are expressed in the pNC, but they are either differentially expressed or maintained during corneal development. Several genes involved in retinoic acid, transforming growth factor-β, and Wnt signaling pathways and their modulators are also differentially expressed. We identified differentially expressed transcription factors as potential downstream candidates that may instruct expression of genes involved in establishing corneal endothelium and keratocyte identities.

CONCLUSION

Combined, our data reveal novel changes in gene expression profiles as pNC differentiate into highly specialized corneal endothelial cells and keratocytes. These data serve as platform for further analyses of the molecular networks involved in NCC differentiation into corneal cells and provide insights into genes involved in corneal dysgenesis and adult diseases.

Expression of the heparin-binding growth factors Midkine and pleiotrophin during ocular development

Midkine (MDK) and Pleiotrophin (PTN) belong to a group of heparin-binding growth factors that has been shown to have pleiotropic functions in various biological processes during development and disease. Development of the vertebrate eye is a multistep process that involves coordinated interactions between neuronal and non-neuronal cells, but very little is known about the potential function of MDK and PTN in these processes. In this study, we demonstrate by section in situ hybridization, the spatiotemporal expression of MDK and PTN during ocular development in chick and mouse. We show that MDK and PTN are expressed in dynamic patterns that overlap in a few non-neuronal tissues in the anterior eye and in neuronal cell layers of the posterior eye. We show that the expression patterns of MDK and PTN are only conserved in a few tissues in chick and mouse but they overlap with the expression of some of their receptors LRP1, RPTPZ, ALK, NOTCH2, ITGβ1, SDC1, and SDC3. The dynamic expression patterns of MDK, PTN and their receptors suggest that they function together during the multistep process of ocular development and they may play important roles in cell proliferation, adhesion, and migration of neuronal and non-neuronal cells.

Lens-regulated retinoic acid signalling controls expansion of the developing eye

Absence of the developing lens results in severe eye defects, including substantial reductions in eye size. How the lens controls eye expansion and the underlying signalling pathways are very poorly defined. We identified RDH10, a gene crucial for retinoic acid synthesis during embryogenesis, as a key factor downregulated in the peripheral retina (presumptive ciliary body region) of lens-removed embryonic chicken eyes prior to overt reductions in eye size. This is associated with a significant decrease in retinoic acid synthesis by lens-removed eyes. Restoring retinoic acid signalling in lens-removed eyes by implanting beads soaked in retinoic acid or retinal, but not vitamin A, rescued eye size. Conversely, blocking retinoic acid synthesis decreased eye size in lens-containing eyes. Production of collagen II and collagen IX, which are major vitreal proteins, is also regulated by the lens and retinoic acid signalling. These data mechanistically link the known roles of both the lens and retinoic acid in normal eye development, and support a model whereby retinoic acid production by the peripheral retina acts downstream of the lens to support vitreous production and eye expansion.

Protagonist or antagonist? The complex roles of retinoids in the regulation of hematopoietic stem cells and their specification from pluripotent stem cells

Hematopoietic stem cells (HSCs) are multipotent cells responsible for the maintenance of the hematopoietic system throughout life. Dysregulation of the balance in HSC self-renewal, death, and differentiation can have serious consequences such as myelodysplastic syndromes or leukemia. All-trans retinoic acid (ATRA), the biologically active metabolite of vitamin A/RA, has been shown to have pleiotropic effects on hematopoietic cells, enhancing HSC self-renewal while also increasing differentiation of more mature progenitors. Furthermore, ATRA has been shown to have key roles in regulating the specification and formation of hematopoietic cells from pluripotent stem cells including embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs). Here, we summarize the known roles of vitamin A and RA receptors in the regulation of hematopoiesis from HSCs, ES, and iPSCs.

Diverse Evolutionary Origins and Mechanisms of Lens Regeneration

In this review, we compare and contrast the three different forms of vertebrate lens regeneration: Wolffian lens regeneration, cornea-lens regeneration, and lens regeneration from lens epithelial cells. An examination of the diverse cellular origins of these lenses, their unique phylogenetic distribution, and the underlying molecular mechanisms, suggests that these different forms of lens regeneration evolved independently and utilize neither conserved nor convergent mechanisms to regulate these processes.

Neural retina-specific Aldh1a1 controls dorsal choroidal vascular development via Sox9 expression in retinal pigment epithelial cells

VEGF secreted from retinal pigment epithelial (RPE) cells is responsible for the choroidal vascular development; however, the molecular regulatory mechanism is unclear. We found that Aldh1a1^-/- mice showed choroidal hypoplasia with insufficient vascularization in the dorsal region, although Aldh1a1, an enzyme that synthesizes retinoic acids (RAs), is expressed in the dorsal neural retina, not in the RPE/choroid complex. The level of VEGF in the RPE/choroid was significantly decreased in Aldh1a1^-/- mice, and RA-dependent enhancement of VEGF was observed in primary RPE cells. An RA-deficient diet resulted in dorsal choroidal hypoplasia, and simple RA treatment of Aldh1a1^-/- pregnant females suppressed choroid hypoplasia in their offspring. We also found downregulation of Sox9 in the dorsal neural retina and RPE of Aldh1a1^-/- mice and RPE-specific disruption of Sox9 phenocopied Aldh1a1^-/- choroidal development. These results suggest that RAs produced by Aldh1a1 in the neural retina directs dorsal choroidal vascular development via Sox9 upregulation in the dorsal RPE cells to enhance RPE-derived VEGF secretion.

CHD7 represses the retinoic acid synthesis enzyme ALDH1A3 during inner ear development

CHD7, an ATP-dependent chromatin remodeler, is disrupted in CHARGE syndrome, an autosomal dominant disorder characterized by variably penetrant abnormalities in craniofacial, cardiac, and nervous system tissues. The inner ear is uniquely sensitive to CHD7 levels and is the most commonly affected organ in individuals with CHARGE. Interestingly, upregulation or downregulation of retinoic acid (RA) signaling during embryogenesis also leads to developmental defects similar to those in CHARGE syndrome, suggesting that CHD7 and RA may have common target genes or signaling pathways. Here, we tested three separate potential mechanisms for CHD7 and RA interaction: (a) direct binding of CHD7 with RA receptors, (b) regulation of CHD7 levels by RA, and (c) CHD7 binding and regulation of RA-related genes. We show that CHD7 directly regulates expression of Aldh1a3, the gene encoding the RA synthetic enzyme ALDH1A3 and that loss of Aldh1a3 partially rescues Chd7 mutant mouse inner ear defects. Together, these studies indicate that ALDH1A3 acts with CHD7 in a common genetic pathway to regulate inner ear development, providing insights into how CHD7 and RA regulate gene expression and morphogenesis in the developing embryo.

Cell Behaviors during Closure of the Choroid Fissure in the Developing Eye

Coloboma is a defect in the morphogenesis of the eye that is a consequence of failure of choroid fissure fusion. It is among the most common congenital defects in humans and can significantly impact vision. However, very little is known about the cellular mechanisms that regulate choroid fissure closure. Using high-resolution confocal imaging of the zebrafish optic cup, we find that apico-basal polarity is re-modeled in cells lining the fissure in proximal to distal and inner to outer gradients during fusion. This process is accompanied by cell proliferation, displacement of vasculature, and contact between cells lining the choroid fissure and periocular mesenchyme (POM). To investigate the role of POM cells in closure of the fissure, we transplanted optic vesicles onto the yolk, allowing them to develop in a situation where they are depleted of POM. The choroid fissure forms normally in ectopic eyes but fusion fails in this condition, despite timely apposition of the nasal and temporal lips of the retina. This study resolves some of the cell behaviors underlying choroid fissure fusion and supports a role for POM in choroid fissure fusion.

Generating retinoic acid gradients by local degradation during craniofacial development: One cell's cue is another cell's poison

Retinoic acid (RA) is a vital morphogen for early patterning and organogenesis in the developing embryo. RA is a diffusible, lipophilic molecule that signals via nuclear RA receptor heterodimeric units that regulate gene expression by interacting with RA response elements in promoters of a significant number of genes. For precise RA signaling, a robust gradient of the morphogen is required. The developing embryo contains regions that produce RA, and specific intracellular concentrations of RA are created through local degradation mediated by Cyp26 enzymes. In order to elucidate the mechanisms by which RA executes precise developmental programs, the kinetics of RA metabolism must be clearly understood. Recent advances in techniques for endogenous RA detection and quantification have paved the way for mechanistic studies to shed light on downstream gene expression regulation coordinated by RA. It is increasingly coming to light that RA signaling operates not only at precise concentrations but also employs mechanisms of degradation and feedback inhibition to self-regulate its levels. A global gradient of RA throughout the embryo is often found concurrently with several local gradients, created by juxtaposed domains of RA synthesis and degradation. The existence of such local gradients has been found especially critical for the proper development of craniofacial structures that arise from the neural crest and the cranial placode populations. In this review, we summarize the current understanding of how local gradients of RA are established in the embryo and their impact on craniofacial development.

Genes and pathways in optic fissure closure

Embryonic development of the vertebrate eye begins with the formation of an optic vesicle which folds inwards to form a double-layered optic cup with a fissure on the ventral surface, known as the optic fissure. Closure of the optic fissure is essential for subsequent growth and development of the eye. A defect in this process can leave a gap in the iris, retina or optic nerve, known as a coloboma, which can lead to severe visual impairment. This review brings together current information about genes and pathways regulating fissure closure from human coloboma patients and animal models. It focuses especially on current understanding of the morphological changes and processes of epithelial remodelling occurring at the fissure margins.

A novel mouse model of anterior segment dysgenesis (ASD): conditional deletion of Tsc1 disrupts ciliary body and iris development

Development of the cornea, lens, ciliary body and iris within the anterior segment of the eye involves coordinated interaction between cells originating from the ciliary margin of the optic cup, the overlying periocular mesenchyme and the lens epithelium. Anterior segment dysgenesis (ASD) encompasses a spectrum of developmental syndromes that affect these anterior segment tissues. ASD conditions arise as a result of dominantly inherited genetic mutations and result in both ocular-specific and systemic forms of dysgenesis that are best exemplified by aniridia and Axenfeld-Rieger syndrome, respectively. Extensive clinical overlap in disease presentation amongst ASD syndromes creates challenges for correct diagnosis and classification. The use of animal models has therefore proved to be a robust approach for unravelling this complex genotypic and phenotypic heterogeneity. However, despite these successes, it is clear that additional genes that underlie several ASD syndromes remain unidentified. Here, we report the characterisation of a novel mouse model of ASD. Conditional deletion of Tsc1 during eye development leads to a premature upregulation of mTORC1 activity within the ciliary margin, periocular mesenchyme and lens epithelium. This aberrant mTORC1 signalling within the ciliary margin in particular leads to a reduction in the number of cells that express Pax6, Bmp4 and Msx1 Sustained mTORC1 signalling also induces a decrease in ciliary margin progenitor cell proliferation and a consequent failure of ciliary body and iris development in postnatal animals. Our study therefore identifies Tsc1 as a novel candidate ASD gene. Furthermore, the Tsc1-ablated mouse model also provides a valuable resource for future studies concerning the molecular mechanisms underlying ASD and acts as a platform for evaluating therapeutic approaches for the treatment of visual disorders.

Optic Nerve Hypoplasia Is a Pervasive Subcortical Pathology of Visual System in Neonates

Purpose

Optic nerve hypoplasia (ONH) is the most common cause of childhood congenital blindness in developed nations, yet the fundamental pathobiology of ONH remains unknown. The objective of this study was to employ a ‘face validated' murine model to determine the timing of onset and the pathologic characteristics of ONH.

Methods

Based on the robust linkage between X-linked CASK haploinsufficiency and clinically diagnosed ONH, we hypothesized that heterozygous deletion of CASK (CASK^(+/−)) in rodents will produce an optic nerve pathology closely recapitulating ONH. We quantitatively analyzed the entire subcortical visual system in female CASK^(+/−) mice using immunohistochemistry, anterograde axonal tracing, toluidine blue staining, transmission electron microscopy, and serial block-face scanning electron microscopy.

Results

CASK haploinsuffiency in mice phenocopies human ONH with complete penetrance, thus satisfying the ‘face validity'. We demonstrate that the optic nerve in CASK^(+/−) mice is not only thin, but is comprised of atrophic retinal axons and displays reactive astrogliosis. Myelination of the optic nerve axons remains unchanged. Moreover, we demonstrate a significant decrease in retinal ganglion cell (RGC) numbers and perturbation in retinothalamic connectivity. Finally, we used this mouse model to define the onset and progression of ONH pathology, demonstrating for the first time that optic nerve defects arise at neonatally in CASK^(+/−)mice.

Conclusions

Optic nerve hypoplasia is a complex neuropathology of the subcortical visual system involving RGC loss, axonopathy, and synaptopathy and originates at a developmental stage in mice that corresponds to the late third trimester development in humans.

Signaling and Gene Regulatory Networks in Mammalian Lens Development

Ocular lens development represents an advantageous system in which to study regulatory mechanisms governing cell fate decisions, extracellular signaling, cell and tissue organization, and the underlying gene regulatory networks. Spatiotemporally regulated domains of BMP, FGF, and other signaling molecules in late gastrula-early neurula stage embryos generate the border region between the neural plate and non-neural ectoderm from which multiple cell types, including lens progenitor cells, emerge and undergo initial tissue formation. Extracellular signaling and DNA-binding transcription factors govern lens and optic cup morphogenesis. Pax6, c-Maf, Hsf4, Prox1, Sox1, and a few additional factors regulate the expression of the lens structural proteins, the crystallins. Extensive crosstalk between a diverse array of signaling pathways controls the complexity and order of lens morphogenetic processes and lens transparency.