Identification of RALDH-3, a novel retinaldehyde dehydrogenase, expressed in the ventral region of the retina

Similarities in DSG1 and KRT3 Downregulation through Retinoic Acid Treatment and PAX6 Knockdown Related Expression Profiles: Does PAX6 Affect RA Signaling in Limbal Epithelial Cells?

Congenital PAX6-aniridia is a rare panocular disease resulting from limbal stem cell deficiency. In PAX6-aniridia, the downregulation of the retinol-metabolizing enzymes ADH7 (All-trans-retinol dehydrogenase 7) and ALDH1A1/A3 (Retinal dehydrogenase 1, Aldehyde dehydrogenase family 1 member A3) have been described in limbal epithelial cells (LECs) and conjunctival epithelial cells. The aim of this study was to identify the role of retinol derivates in the differentiation of human LEC and its potential impact on aniridia-associated keratopathy development. Human LEC were isolated from healthy donor corneas and were cultured with retinol, retinoic acid, or pan-retinoic acid receptor antagonist (AGN 193109) acting on RARα, β, γ (NR1B1, NR1B2 NR1B3) or were cultured with pan-retinoid X receptor antagonist (UVI 3003) acting on RXR α, β, γ (retinoid X receptor, NR2B1, NR2B2, BR2B3). Using qPCR, differentiation marker and retinoid-/fatty acid metabolism-related mRNA expression was analysed. DSG1 (Desmoglein 1), KRT3 (Keratin 3), and SPINK7 (Serine Peptidase Inhibitor Kazal Type 7) mRNA expression was downregulated when retinoid derivates were used. AGN 193109 treatment led to the upregulation of ADH7, KRT3, and DSG1 mRNA expression and to the downregulation of KRT12 (Keratin 12) and KRT19 (Keratin 19) mRNA expression. Retinol and all-trans retinoic acid affect some transcripts of corneal LEC in a similar way to what has been observed in the LEC of PAX6-aniridia patients with the altered expression of differentiation markers. An elevated concentration of retinol derivatives in LEC or an altered response to retinoids may contribute to this pattern. These initial findings help to explain ocular surface epithelia differentiation disorders in PAX6-aniridia and should be investigated in patient cells or in cell models in the future in more detail.

Antiepileptogenic Effect of Retinoic Acid

Retinoic acid, a metabolite of vitamin A, acts through either genomic or nongenomic actions. The genomic action of retinoids exerts effects on gene transcription through interaction with retinoid receptors such as retinoic acid receptors (RARα, β, and γ) and retinoid X receptors (RXRα, β, and γ) that are primarily concentrated in the amygdala, pre-frontal cortex, and hippocampal areas in the brain. In response to retinoid binding, RAR/RXR heterodimers undergo major conformational changes and orchestrate the transcription of specific gene networks. Previous experimental studies have reported that retinoic acid exerts an antiepileptogenic effect through diverse mechanisms, including the modulation of gap junctions, neurotransmitters, long-term potentiation, calcium channels and some genes. To our knowledge, there are no previous or current clinical trials evaluating the use of retinoic acid for seizure control.

Why Does the Face Predict the Brain? Neural Crest Induction, Craniofacial Morphogenesis, and Neural Circuit Development

Mesenchephalic and rhombencephalic neural crest cells generate the craniofacial skeleton, special sensory organs, and subsets of cranial sensory receptor neurons. They do so while preserving the anterior-posterior (A-P) identity of their neural tube origins. This organizational principle is paralleled by central nervous system circuits that receive and process information from facial structures whose A-P identity is in register with that in the brain. Prior to morphogenesis of the face and its circuits, however, neural crest cells act as "inductive ambassadors" from distinct regions of the neural tube to induce differentiation of target craniofacial domains and establish an initial interface between the brain and face. At every site of bilateral, non-axial secondary induction, neural crest constitutes all or some of the mesenchymal compartment for non-axial mesenchymal/epithelial (M/E) interactions. Thus, for epithelial domains in the craniofacial primordia, aortic arches, limbs, the spinal cord, and the forebrain (Fb), neural crest-derived mesenchymal cells establish local sources of inductive signaling molecules that drive morphogenesis and cellular differentiation. This common mechanism for building brains, faces, limbs, and hearts, A-P axis specified, neural crest-mediated M/E induction, coordinates differentiation of distal structures, peripheral neurons that provide their sensory or autonomic innervation in some cases, and central neural circuits that regulate their behavioral functions. The essential role of this neural crest-mediated mechanism identifies it as a prime target for pathogenesis in a broad range of neurodevelopmental disorders. Thus, the face and the brain "predict" one another, and this mutual developmental relationship provides a key target for disruption by developmental pathology.

Structural and kinetic features of aldehyde dehydrogenase 1A (ALDH1A) subfamily members, cancer stem cell markers active in retinoic acid biosynthesis

Aldehyde dehydrogenases catalyze the NAD(P)⁺-dependent oxidation of aldehydes to their corresponding carboxylic acids. The three-dimensional structures of the human ALDH1A enzymes were recently obtained, while a complete kinetic characterization of them, under the same experimental conditions, is lacking. We show that the three enzymes, ALDH1A1, ALDH1A2 and ALDH1A3, have similar topologies, although with decreasing volumes in their substrate-binding pockets. The activity with aliphatic and retinoid aldehydes was characterized side-by-side, using an improved HPLC-based method for retinaldehyde. Hexanal was the most efficient substrate. ALDH1A1 displayed lower K_m values with hexanal, trans-2-hexenal and citral, compared to ALDH1A2 and ALDH1A3. ALDH1A2 was the best enzyme for the lipid peroxidation product, 4-hydroxy-2-nonenal, in terms of k_cat/K_m. The catalytic efficiency towards all-trans and 9-cis-retinaldehyde was in general lower than for alkanals and alkenals. ALDH1A2 and ALDH1A3 showed higher catalytic efficiency for all-trans-retinaldehyde. The lower specificity of ALDH1A3 for 9-cis-retinaldehyde against the all-trans- isomer might be related to the smaller volume of its substrate-binding pocket. Magnesium inhibited ALDH1A1 and ALDH1A2, while it activated ALDH1A3, which is consistent with cofactor dissociation being the rate-limiting step for ALDH1A1 and ALDH1A2, and deacylation for ALDH1A3, with hexanal as a substrate. We mutated both ALDH1A1 (L114P) and ALDH1A2 (N475G, A476V, L477V, N478S) to mimic their counterpart substrate-binding pockets. ALDH1A1 specificity for citral was traced to residue 114 and to residues 458 to 461. Regarding retinaldehyde, the mutants did not show significant differences with their respective wild-type forms, suggesting that the mutated residues are not critical for retinoid specificity.

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.

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.

GLI3 repressor but not GLI3 activator is essential for mouse eye patterning and morphogenesis

Since 1967, it is known that the loss of GLI3 causes very severe defects in murine eye development. GLI3 is able to act as a transcriptional activator (GLI3-A) or as a transcriptional repressor (GLI3-R). Soon after the discovery of these GLI3 isoforms, the question arose which of the different isoforms is involved in eye formation - GLI3-A, GLI3-R or even both. For several years, this question remained elusive. By analysing the eye morphogenesis of Gli3^XtJ/XtJ mouse embryos that lack GLI3-A and GLI3-R and of Gli3^Δ699/Δ699 mouse embryos in which only GLI3-A is missing, we revealed that GLI3-A is dispensable in vertebrate eye formation. Remarkably, our study shows that GLI3-R is sufficient for the creation of morphologically normal eyes although the molecular setup deviates substantially from normality. In depth-investigations elucidated that GLI3-R controls numerous key players in eye development and governs lens and retina development at least partially via regulating WNT/β-CATENIN signalling.

Evolutionary Origins of Pax6 Control of Crystallin Genes

The birth of novel genes, including their cell-specific transcriptional control, is a major source of evolutionary innovation. The lens-preferred proteins, crystallins (vertebrates: α- and β/γ-crystallins), provide a gateway to study eye evolution. Diversity of crystallins was thought to originate from convergent evolution through multiple, independent formation of Pax6/PaxB-binding sites within the promoters of genes able to act as crystallins. Here, we propose that αB-crystallin arose from a duplication of small heat shock protein (Hspb1-like) gene accompanied by Pax6-site and heat shock element (HSE) formation, followed by another duplication to generate the αA-crystallin gene in which HSE was converted into another Pax6-binding site. The founding β/γ-crystallin gene arose from the ancestral Hspb1-like gene promoter inserted into a Ca2+-binding protein coding region, early in the cephalochordate/tunicate lineage. Likewise, an ancestral aldehyde dehydrogenase (Aldh) gene, through multiple gene duplications, expanded into a multigene family, with specific genes expressed in invertebrate lenses (Ω-crystallin/Aldh1a9) and both vertebrate lenses (η-crystallin/Aldh1a7 and Aldh3a1) and corneas (Aldh3a1). Collectively, the present data reconstruct the evolution of diverse crystallin gene families.

Complement component C3aR constitutes a novel regulator for chick eye morphogenesis

Complement components have been implicated in a wide variety of functions including neurogenesis, proliferation, cell migration, differentiation, cancer, and more recently early development and regeneration. Following our initial observations indicating that C3a/C3aR signaling induces chick retina regeneration, we analyzed its role in chick eye morphogenesis. During eye development, the optic vesicle (OV) invaginates to generate a bilayer optic cup (OC) that gives rise to the retinal pigmented epithelium (RPE) and neural retina. We show by immunofluorescence staining that C3 and the receptor for C3a (the cleaved and active form of C3), C3aR, are present in chick embryos during eye morphogenesis in the OV and OC. Interestingly, C3aR is mainly localized in the nuclear compartment at the OC stage. Loss of function studies at the OV stage using morpholinos or a blocking antibody targeting the C3aR (anti-C3aR Ab), causes eye defects such as microphthalmia and defects in the ventral portion of the eye that result in coloboma. Such defects were not observed when C3aR was disrupted at the OC stage. Histological analysis demonstrated that microphthalmic eyes were unable to generate a normal optic stalk or a closed OC. The dorsal/ventral patterning defects were accompanied by an expansion of the ventral markers Pax2, cVax and retinoic acid synthesizing enzyme raldh-3 (aldh1a3) domains, an absence of the dorsal expression of Tbx5 and raldh-1 (aldh1a1) and a re-specification of the ventral RPE to neuroepithelium. In addition, the eyes showed overall decreased expression of Gli1 and a change in distribution of nuclear β-catenin, suggesting that Shh and Wnt pathways have been affected. Finally, we observed prominent cell death along with a decrease in proliferating cells, indicating that both processes contribute to the microphthalmic phenotype. Together our results show that C3aR is necessary for the proper morphogenesis of the OC. This is the first report implicating C3aR in eye development, revealing an unsuspected hitherto regulator for proper chick eye morphogenesis.

Differences in neural crest sensitivity to ethanol account for the infrequency of anterior segment defects in the eye compared with craniofacial anomalies in a zebrafish model of fetal alcohol syndrome

BACKGROUND

Ethanol (ETOH) exposure during pregnancy is associated with craniofacial and neurologic abnormalities, but infrequently disrupts the anterior segment of the eye. In these studies, we used zebrafish to investigate differences in the teratogenic effect of ETOH on craniofacial, periocular, and ocular neural crest.

METHODS

Zebrafish eye and neural crest development was analyzed by means of live imaging, TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) assay, immunostaining, detection of reactive oxygen species, and in situ hybridization.

RESULTS

Our studies demonstrated that foxd3-positive neural crest cells in the periocular mesenchyme and developing eye were less sensitive to ETOH than sox10-positive craniofacial neural crest cells that form the pharyngeal arches and jaw. ETOH increased apoptosis in the retina, but did not affect survival of periocular and ocular neural crest cells. ETOH also did not increase reactive oxygen species within the eye. In contrast, ETOH increased ventral neural crest apoptosis and reactive oxygen species production in the facial mesenchyme. In the eye and craniofacial region, sod2 showed high levels of expression in the anterior segment and in the setting of Sod2 knockdown, low levels of ETOH decreased migration of foxd3-positive neural crest cells into the developing eye. However, ETOH had minimal effect on the periocular and ocular expression of transcription factors (pitx2 and foxc1) that regulate anterior segment development.

CONCLUSION

Neural crest cells contributing to the anterior segment of the eye exhibit increased ability to withstand ETOH-induced oxidative stress and apoptosis. These studies explain the rarity of anterior segment dysgenesis despite the frequent craniofacial abnormalities in fetal alcohol syndrome. Birth Defects Research 109:1212-1227, 2017. © 2017 Wiley Periodicals, Inc.

Acetaldehyde and retinaldehyde-metabolizing enzymes in colon and pancreatic cancers

Colorectal cancer (CRC) and pancreatic cancer are two very significant contributors to cancer-related deaths. Chronic alcohol consumption is an important risk factor for these cancers. Ethanol is oxidized primarily by alcohol dehydrogenases to acetaldehyde, an agent capable of initiating tumors by forming adducts with proteins and DNA. Acetaldehyde is metabolized by ALDH2, ALDH1B1, and ALDH1A1 to acetate. Retinoic acid (RA) is required for cellular differentiation and is known to arrest tumor development. RA is synthesized from retinaldehyde by the retinaldehyde dehydrogenases, specifically ALDH1A1, ALDH1A2, ALDH1A3, and ALDH8A1. By eliminating acetaldehyde and generating RA, ALDHs can play a crucial regulatory role in the initiation and progression of cancers. ALDH1 catalytic activity has been used as a biomarker to identify and isolate normal and cancer stem cells; its presence in a tumor is associated with poor prognosis in colon and pancreatic cancer. In summary, these ALDHs are not only biomarkers for CRC and pancreatic cancer but also play important mechanistic role in cancer initiation, progression, and eventual prognosis.

Leukocyte homing, fate, and function are controlled by retinoic acid

Although vitamin A was recognized as an "anti-infective vitamin" over 90 years ago, the mechanism of how vitamin A regulates immunity is only beginning to be understood. Early studies which focused on the immune responses in vitamin A-deficient (VAD) animals clearly demonstrated compromised immunity and consequently increased susceptibility to infectious disease. The active form of vitamin A, retinoic acid (RA), has been shown to have a profound impact on the homing and differentiation of leukocytes. Both pharmacological and genetic approaches have been applied to the understanding of how RA regulates the development and differentiation of various immune cell subsets, and how RA influences the development of immunity versus tolerance. These studies clearly show that RA profoundly impacts on cell- and humoral-mediated immunity. In this review, the early findings on the complex relationship between VAD and immunity are discussed as well as vitamin A metabolism and signaling within hematopoietic cells. Particular attention is focused on how RA impacts on T-cell lineage commitment and plasticity in various diseases.

Comparative and evolutionary studies of vertebrate ALDH1A-like genes and proteins

Vertebrate ALDH1A-like genes encode cytosolic enzymes capable of metabolizing all-trans-retinaldehyde to retinoic acid which is a molecular 'signal' guiding vertebrate development and adipogenesis. Bioinformatic analyses of vertebrate and invertebrate genomes were undertaken using known ALDH1A1, ALDH1A2 and ALDH1A3 amino acid sequences. Comparative analyses of the corresponding human genes provided evidence for distinct modes of gene regulation and expression with putative transcription factor binding sites (TFBS), CpG islands and micro-RNA binding sites identified for the human genes. ALDH1A-like sequences were identified for all mammalian, bird, lizard and frog genomes examined, whereas fish genomes displayed a more restricted distribution pattern for ALDH1A1 and ALDH1A3 genes. The ALDH1A1 gene was absent in many bony fish genomes examined, with the ALDH1A3 gene also absent in the medaka and tilapia genomes. Multiple ALDH1A1-like genes were identified in mouse, rat and marsupial genomes. Vertebrate ALDH1A1, ALDH1A2 and ALDH1A3 subunit sequences were highly conserved throughout vertebrate evolution. Comparative amino acid substitution rates showed that mammalian ALDH1A2 sequences were more highly conserved than for the ALDH1A1 and ALDH1A3 sequences. Phylogenetic studies supported an hypothesis for ALDH1A2 as a likely primordial gene originating in invertebrate genomes and undergoing sequential gene duplication to generate two additional genes, ALDH1A1 and ALDH1A3, in most vertebrate genomes.

N-type voltage-dependent Ca2+ channel in non-excitable microglial cells in mice is involved in the pathophysiology of neuropathic pain

Peripheral nerve injury induces neuropathic pain which is characterized by tactile allodynia and thermal hyperalgesia. N-type voltage-dependent Ca(2+) channel (VDCC) plays pivotal roles in the development of neuropathic pain, since mice lacking Cav2.2, the pore-forming subunit of N-type VDCC, show greatly reduced symptoms of both tactile allodynia and thermal hyperalgesia. Our study on gene expression profiles of the Cav2.2 knockout (KO) spinal cord after spinal nerve ligation (SNL)-injury revealed altered expression of genes known to be expressed in microglia, raising an odd idea that N-type VDCC may function in not only excitable (neurons) but also non-excitable (microglia) cells in neuropathic pain state. In the present study, we have tested this idea by using a transgenic mouse line, in which suppression of Cav2.2 expression can be achieved specifically in microglia/macrophage by the application of tamoxifen. We found SNL-operated transgenic mice exhibited greatly reduced signs of tactile allodynia, whereas the degree of thermal hyperalgesia was almost the same as that of control. Immunohistochemical analysis of the transgenic lumbar spinal cord revealed reduced accumulation of Iba1-positive cells (microglia/macrophage) around the injured neurons, indicating microglial N-type VDCC is important for accumulation of microglia at the lesion sites. Although the mechanism of its activation is not clear at present, activation of N-type VDCC expressed in non-excitable microglial cells contributes to the pathophysiology of neuropathic pain.

Retinoic acid signalling regulates the development of tonotopically patterned hair cells in the chicken cochlea

Precise frequency discrimination is a hallmark of auditory function in birds and mammals and is required for distinguishing similar sounding words, like 'bat,' 'cat' and 'hat.' In the cochlea, tuning and spectral separation result from longitudinal differences in basilar membrane stiffness and numerous individual gradations in sensory hair cell phenotypes, but it is unknown what patterns the phenotypes. Here we used RNA-seq to compare transcriptomes from proximal, middle and distal regions of the embryonic chicken cochlea, and found opposing longitudinal gradients of expression for retinoic acid (RA)-synthesizing and degrading enzymes. In vitro experiments showed that RA is necessary and sufficient to induce the development of distal-like hair cell phenotypes and promotes expression of the actin-crosslinking proteins, Espin and Fscn2. These and other findings highlight a role for RA signalling in patterning the development of a longitudinal gradient of frequency-tuned hair cell phenotypes in the cochlea.

The rhythm of retinoids in the brain

The retinoids are a family of compounds that in nature are derived from vitamin A or pro-vitamin A carotenoids. An essential part of the diet for mammals, vitamin A has long been known to be essential for many organ systems in the adult. More recently, however, they have been shown to be necessary for function of the brain and new discoveries point to a central role in processes ranging from neuroplasticity to neurogenesis. Acting in several regions of the central nervous system including the eye, hippocampus and hypothalamus, one common factor in its action is control of biological rhythms. This review summarizes the role of vitamin A in the brain; its action through the metabolite retinoic acid via specific nuclear receptors, and the regulation of its concentration through controlled synthesis and catabolism. The action of retinoic acid to regulate several rhythms in the brain and body, from circadian to seasonal, is then discussed to finish with the importance of retinoic acid in the regular pattern of sleep. We review the role of vitamin A and retinoic acid (RA) as mediators of rhythm in the brain. In the suprachiasmatic nucleus and hippocampus they control expression of circadian clock genes while in the cortex retinoic acid is required for delta oscillations of sleep. Retinoic acid is also central to a second rhythm that keeps pace with the seasons, regulating function in the hypothalamus and pineal gland.

SPIG1 negatively regulates BDNF maturation

We previously identified SPARC-related protein-containing immunoglobulin domains 1 (SPIG1, also known as Follistatin-like protein 4) as one of the dorsal-retina-specific molecules expressed in the developing chick retina. We here demonstrated that the knockdown of SPIG1 in the retinal ganglion cells (RGCs) of developing chick embryos induced the robust ectopic branching of dorsal RGC axons and failed to form a tight terminal zone at the proper position on the tectum. The knockdown of SPIG1 in RGCs also led to enhanced axon branching in vitro. However, this was canceled by the addition of a neutralizing antibody against brain-derived neurotrophic factor (BDNF) to the culture medium. SPIG1 and BDNF were colocalized in vesicle-like structures in cells. SPIG1 bound with the proform of BDNF (proBDNF) but very weakly with mature BDNF in vitro. The expression and secretion of mature BDNF were significantly decreased when SPIG1 was exogenously expressed with BDNF in HEK293T or PC12 cells. The amount of mature BDNF proteins as well as the tyrosine phosphorylation level of the BDNF receptor, tropomyosin-related kinase B (TrkB), in the hippocampus were significantly higher in SPIG1-knockout mice than in wild-type mice. Here the spine density of CA1 pyramidal neurons was consistently increased. Together, these results suggest that SPIG1 negatively regulated BDNF maturation by binding to proBDNF, thereby suppressing axonal branching and spine formation.

Ocular aldehyde dehydrogenases: protection against ultraviolet damage and maintenance of transparency for vision

Aldehyde dehydrogenase (ALDH) enzymes catalyze the NAD(P)(+)-dependent oxidation of a wide variety of endogenous and exogenous aldehydes to their corresponding acids. Some members of the ALDH superfamily of enzymes are abundantly expressed in the mammalian cornea and lens in a taxon-specific manner. Considered to be corneal and lens crystallins, they confer protective and transparent properties upon these ocular tissues. ALDH3A1 is highly expressed in the cornea of most mammals, with the exception of rabbit that expresses exclusively ALDH1A1 in the cornea. ALDH1A1 is present in both the cornea and lens of several animal species. As a result of their catalytic and non-catalytic functions, ALDH3A1 and ALDH1A1 proteins protect inner ocular tissues from ultraviolet radiation and reactive oxygen-induced damage. In addition, these corneal crystallins contribute to cellular transparency in corneal stromal keratocytes, supporting a structural role of these ALDH proteins. A putative regulatory function of ALDH3A1 on corneal cell proliferation has also been proposed. Finally, the three retinaldehyde dehydrogenases cooperatively mediate retinoic acid signaling during the eye development.

Retinoic acid signalling during development

Retinoic acid (RA) is a vitamin A-derived, non-peptidic, small lipophilic molecule that acts as ligand for nuclear RA receptors (RARs), converting them from transcriptional repressors to activators. The distribution and levels of RA in embryonic tissues are tightly controlled by regulated synthesis through the action of specific retinol and retinaldehyde dehydrogenases and by degradation via specific cytochrome P450s (CYP26s). Recent studies indicate that RA action involves an interplay between diffusion (morphogen-like) gradients and the establishment of signalling boundaries due to RA metabolism, thereby allowing RA to finely control the differentiation and patterning of various stem/progenitor cell populations. Here, we provide an overview of the RA biosynthesis, degradation and signalling pathways and review the main functions of this molecule during embryogenesis.

Pax6: a multi-level regulator of ocular development

Eye development has been a paradigm for the study of organogenesis, from the demonstration of lens induction through epithelial tissue morphogenesis, to neuronal specification and differentiation. The transcription factor Pax6 has been shown to play a key role in each of these processes. Pax6 is required for initiation of developmental pathways, patterning of epithelial tissues, activation of tissue-specific genes and interaction with other regulatory pathways. Herein we examine the data accumulated over the last few decades from extensive analyses of biochemical modules and genetic manipulation of the Pax6 gene. Specifically, we describe the regulation of Pax6's expression pattern, the protein's DNA-binding properties, and its specific roles and mechanisms of action at all stages of lens and retinal development. Pax6 functions at multiple levels to integrate extracellular information and execute cell-intrinsic differentiation programs that culminate in the specification and differentiation of a distinct ocular lineage.

Retinoic acid synthesis and functions in early embryonic development

Retinoic acid (RA) is a morphogen derived from retinol (vitamin A) that plays important roles in cell growth, differentiation, and organogenesis. The production of RA from retinol requires two consecutive enzymatic reactions catalyzed by different sets of dehydrogenases. The retinol is first oxidized into retinal, which is then oxidized into RA. The RA interacts with retinoic acid receptor (RAR) and retinoic acid X receptor (RXR) which then regulate the target gene expression. In this review, we have discussed the metabolism of RA and the important components of RA signaling pathway, and highlighted current understanding of the functions of RA during early embryonic development.

A zebrafish model of axenfeld-rieger syndrome reveals that pitx2 regulation by retinoic acid is essential for ocular and craniofacial development

PURPOSE

The homeobox transcription factor PITX2 is a known regulator of mammalian ocular development, and human PITX2 mutations are associated with Axenfeld-Rieger syndrome (ARS). However, the treatment of patients with ARS remains mostly supportive and palliative.

METHODS

The authors used molecular genetic, pharmacologic, and embryologic techniques to study the biology of ARS in a zebrafish model that uses transgenes to mark neural crest and muscle cells in the head.

RESULTS

The authors demonstrated in vivo that pitx2 is a key downstream target of retinoic acid (RA) in craniofacial development, and this pathway is required for coordinating neural crest, mesoderm, and ocular development. pitx2a knockdown using morpholino oligonucleotides disrupts jaw and pharyngeal arch formation and recapitulates ocular characteristics of ARS, including corneal and iris stroma maldevelopment. These phenotypes could be rescued with human PITX2A mRNA, demonstrating the specificity of the knockdown and evolutionary conservation of pitx2a function. Expression of the ARS dominant negative human PITX2A K50E allele also caused ARS-like phenotypes. Similarly, inhibition of RA synthesis in the developing eye (genetic or pharmacologic) disrupted craniofacial and ocular development, and human PITX2A mRNA partially rescued these defects.

CONCLUSIONS

RA regulation of pitx2 is essential for coordinating interactions among neural crest, mesoderm, and developing eye. The marked evolutionary conservation of Pitx2 function in eye and craniofacial development makes zebrafish a potentially powerful model of ARS, amenable to in vivo experimentation and development of potential therapies.

Pax6 regulates craniofacial form through its control of an essential cephalic ectodermal patterning center

Normal patterning and morphogenesis of the complex skeletal structures of the skull requires an exquisite, reciprocal cross-talk between the embryonic cephalic epithelia and mesenchyme. The mesenchyme associated with the jaws and the optic and olfactory capsules is derived from a Hox-negative cranial neural crest (CNC) population that acts much as an equivalence group in its interactions with specific local cephalic epithelial signals. Craniofacial pattern and morphogenesis is therefore controlled in large part through the regulation of these local cephalic epithelial signals. Here, we demonstrate that Pax6 is essential to the formation and maturation of the complex cephalic ectodermal patterning centers that govern the development and morphogenesis of the upper jaws and associated nasal capsules. Previous examinations of the craniofacial skeletal defects associated with Pax6 mutations have suggested that they arise from an optic-associated blockage in the migration of a specific subpopulation of midbrain CNC to the lateral frontonasal processes. We have addressed an alternative explanation for the craniofacial skeletal defects. We show that in Pax6(SeyN/SeyN) mutants regional CNC is present by E9.25 while there is already specific disruption in the early ontogenetic elaboration of cephalic ectodermal expression, associated with the nascent lambdoidal junction, of secreted signaling factors (including Fgf8 and Bmp4) and transcription factors (including Six1 and Dlx5) essential for upper jaw and/or nasal capsular development. Pax6 therefore regulates craniofacial form, at stages when CNC has just arrived in the frontonasal region, through its control of surface cephalic ectodermal competence to form an essential craniofacial patterning center.

Vax2 regulates retinoic acid distribution and cone opsin expression in the vertebrate eye

Vax2 is an eye-specific homeobox gene, the inactivation of which in mouse leads to alterations in the establishment of a proper dorsoventral eye axis during embryonic development. To dissect the molecular pathways in which Vax2 is involved, we performed a transcriptome analysis of Vax2(-/-) mice throughout the main stages of eye development. We found that some of the enzymes involved in retinoic acid (RA) metabolism in the eye show significant variations of their expression levels in mutant mice. In particular, we detected an expansion of the expression domains of the RA-catabolizing enzymes Cyp26a1 and Cyp26c1, and a downregulation of the RA-synthesizing enzyme Raldh3. These changes determine a significant expansion of the RA-free zone towards the ventral part of the eye. At postnatal stages of eye development, Vax2 inactivation led to alterations of the regional expression of the cone photoreceptor genes Opn1sw (S-Opsin) and Opn1mw (M-Opsin), which were significantly rescued after RA administration. We confirmed the above described alterations of gene expression in the Oryzias latipes (medaka fish) model system using both Vax2 gain- and loss-of-function assays. Finally, a detailed morphological and functional analysis of the adult retina in mutant mice revealed that Vax2 is necessary for intraretinal pathfinding of retinal ganglion cells in mammals. These data demonstrate for the first time that Vax2 is both necessary and sufficient for the control of intraretinal RA metabolism, which in turn contributes to the appropriate expression of cone opsins in the vertebrate eye.

Signaling "cross-talk" is integrated by transcription factors in the development of the anterior segment in the eye

Extracellular signaling "cross-talk" between tissues is an important requirement for development of many organs yet the underlying mechanisms generally remain poorly understood. The anterior segment of the eye, which is constructed from four embryonic lineages, provides a unique opportunity to genetically dissect developmental processes such as signaling "cross-talk" without fear of inducing lethality. In the current review, we summarize recent data showing that PITX2, a homeodomain transcription factor, integrates retinoic acid and canonical Wnt/beta-catenin signaling during anterior segment development. Because the requirements for retinoic acid signaling, canonical Wnt/beta-catenin signaling, and PITX2 are not unique to the eye, this newly identified pathway may have relevance elsewhere during development and in tissue homeostasis.

APC2 plays an essential role in axonal projections through the regulation of microtubule stability

Growth cones at the tip of growing axons are key cellular structures that detect guidance cues and mediate axonal growth. An increasing number of studies have suggested that the dynamic regulation of microtubules in the growth cone plays an essential role in growth cone steering. The dynamic properties of microtubules are considered to be regulated by variegated cellular factors but, in particular, through microtubule-interacting proteins. Here, we examined the functional role of adenomatous polyposis coli-like molecule 2 (APC2) in the development of axonal projections by using the chick retinotectal topographic projection system. APC2 is preferentially expressed in the nervous system from early developmental stages through to adulthood. Immunohistochemical analysis revealed that APC2 is distributed along microtubules in growth cones as well as axon shafts of retinal axons. Overexpression of APC2 in cultured cells induced the stabilization of microtubules, whereas the knockdown of APC2 in chick retinas with specific short hairpin RNA reduced the stability of microtubules in retinal axons. APC2 knockdown retinal axons showed abnormal growth attributable to a reduced response to ephrin-A2 in vitro. Furthermore, they showed drastic alterations in retinotectal projections without making clear target zones in the tectum in vivo. These results suggest that APC2 plays a critical role in the development of the nervous system through the regulation of microtubule stability.

Consequences of lineage-specific gene loss on functional evolution of surviving paralogs: ALDH1A and retinoic acid signaling in vertebrate genomes

Genome duplications increase genetic diversity and may facilitate the evolution of gene subfunctions. Little attention, however, has focused on the evolutionary impact of lineage-specific gene loss. Here, we show that identifying lineage-specific gene loss after genome duplication is important for understanding the evolution of gene subfunctions in surviving paralogs and for improving functional connectivity among human and model organism genomes. We examine the general principles of gene loss following duplication, coupled with expression analysis of the retinaldehyde dehydrogenase Aldh1a gene family during retinoic acid signaling in eye development as a case study. Humans have three ALDH1A genes, but teleosts have just one or two. We used comparative genomics and conserved syntenies to identify loss of ohnologs (paralogs derived from genome duplication) and to clarify uncertain phylogenies. Analysis showed that Aldh1a1 and Aldh1a2 form a clade that is sister to Aldh1a3-related genes. Genome comparisons showed secondarily loss of aldh1a1 in teleosts, revealing that Aldh1a1 is not a tetrapod innovation and that aldh1a3 was recently lost in medaka, making it the first known vertebrate with a single aldh1a gene. Interestingly, results revealed asymmetric distribution of surviving ohnologs between co-orthologous teleost chromosome segments, suggesting that local genome architecture can influence ohnolog survival. We propose a model that reconstructs the chromosomal history of the Aldh1a family in the ancestral vertebrate genome, coupled with the evolution of gene functions in surviving Aldh1a ohnologs after R1, R2, and R3 genome duplications. Results provide evidence for early subfunctionalization and late subfunction-partitioning and suggest a mechanistic model based on altered regulation leading to heterochronic gene expression to explain the acquisition or modification of subfunctions by surviving ohnologs that preserve unaltered ancestral developmental programs in the face of gene loss.

Functional mode of FoxD1/CBF2 for the establishment of temporal retinal specificity in the developing chick retina

Two winged-helix transcription factors, FoxG1 (previously called chick brain factor1, CBF1) and FoxD1 (chick brain factor2, CBF2), are expressed specifically in the nasal and temporal regions of the developing chick retina, respectively. We previously demonstrated that FoxG1 controls the expression of topographic molecules including FoxD1, and determines the regional specificity of the nasal retina. FoxD1 is known to prescribe temporal specificity, however, molecular mechanisms and downstream targets have not been elucidated. Here we addressed the genetic mechanisms for establishing temporal specificity in the developing retina using an in ovo electroporation technique. Fibroblast growth factor (Fgf) and Wnt first play pivotal roles in inducing the region-specific expression of FoxG1 and FoxD1 in the optic vesicle. Misexpression of FoxD1 represses the expression of FoxG1, GH6, SOHo1, and ephrin-A5, and induces that of EphA3 in the retina. GH6 and SOHo1 repress the expression of FoxD1. In contrast to the inhibitory effect of FoxG1 on bone morphogenic protein (BMP) signaling, FoxD1 does not alter the expression of BMP4 or BMP2. Studies with chimeric mutants of FoxD1 showed that FoxD1 acts as a transcription repressor in controlling its downstream targets in the retina. Taken together with previous findings, our data suggest that FoxG1 and FoxD1 are located at the top of the gene cascade for regional specification along the nasotemporal (anteroposterior) axis in the retina, and FoxD1 determines temporal specificity.

Retinoic acid signaling in mammalian eye development

Retinoic acid (RA) is a biologically active metabolite of vitamin A (retinol) that serves as a signaling molecule during a number of developmental and physiological processes. RA signaling plays multiple roles during embryonic eye development. RA signaling is initially required for reciprocal interactions between the optic vesicle and invaginating lens placode. RA signaling promotes normal development of the ventral retina and optic nerve through its activities in the neural crest cell-derived periocular mesenchyme. RA coordinates these processes by regulating biological activities of a family of non-steroid hormone receptors, RARalpha/beta/gamma, and RXRalpha/beta/gamma. These DNA-binding transcription factors recognize DNA as RAR/RXR heterodimers and recruit multiprotein transcriptional co-repressor complexes. RA-binding to RAR receptors induces a conformational change in the receptor, followed by the replacement of co-repressor with co-activator complexes. Inactivation of RARalpha/beta/gamma receptors in the periocular mesenchyme abrogates anterior eye segment formation. This review summarizes recent genetic studies of RA signaling and progress in understanding the molecular mechanism of transcriptional co-activators that function with RAR/RXR.

Identification of pax6-dependent gene regulatory networks in the mouse lens

Lineage-specific DNA-binding transcription factors regulate development by activating and repressing particular set of genes required for the acquisition of a specific cell type. Pax6 is a paired domain and homeodomain-containing transcription factor essential for development of central nervous, olfactory and visual systems, as well as endocrine pancreas. Haploinsufficiency of Pax6 results in perturbed lens development and homeostasis. Loss-of-function of Pax6 is incompatible with lens lineage formation and results in abnormal telencephalic development. Using DNA microarrays, we have identified 559 genes expressed differentially between 1-day old mouse Pax6 heterozygous and wild type lenses. Of these, 178 (31.8%) were similarly increased and decreased in Pax6 homozygous embryonic telencephalon [Holm PC, Mader MT, Haubst N, Wizenmann A, Sigvardsson M, Götz M (2007) Loss- and gain-of-function analyses reveals targets of Pax6 in the developing mouse telencephalon. Mol Cell Neurosci 34: 99–119]. In contrast, 381 (68.2%) genes were differently regulated between the lens and embryonic telencephalon. Differential expression of nine genes implicated in lens development and homeostasis: Cspg2, Igfbp5, Mab21l2, Nrf2f, Olfm3, Spag5, Spock1, Spon1 and Tgfb2, was confirmed by quantitative RT-PCR, with five of these genes: Cspg2, Mab21l2, Olfm3, Spag5 and Tgfb2, identified as candidate direct Pax6 target genes by quantitative chromatin immunoprecipitation (qChIP). In Mab21l2 and Tgfb2 promoter regions, twelve putative individual Pax6-binding sites were tested by electrophoretic mobility shift assays (EMSAs) with recombinant Pax6 proteins. This led to the identification of two and three sites in the respective Mab21l2 and Tgfb2 promoter regions identified by qChIPs. Collectively, the present studies represent an integrative genome-wide approach to identify downstream networks controlled by Pax6 that control mouse lens and forebrain development.

Metalloproteinase- and gamma-secretase-mediated cleavage of protein-tyrosine phosphatase receptor type Z

Protein-tyrosine phosphatase receptor type Z (Ptprz) is preferentially expressed in the brain as a major chondroitin sulfate proteoglycan. Three splicing variants, two receptor isoforms and one secretory isoform, are known. Here, we show that the extracellular region of the receptor isoforms of Ptprz are cleaved by metalloproteinases, and subsequently the membrane-tethered fragment is cleaved by presenilin/gamma-secretase, releasing its intracellular region into the cytoplasm; of note, the intracellular fragment of Ptprz shows nuclear localization. Administration of GM6001, an inhibitor of metalloproteinases, to mice demonstrated the metalloproteinase-mediated cleavage of Ptprz under physiological conditions. Furthermore, we identified the cleavage sites in the extracellular juxtamembrane region of Ptprz by tumor necrosis factor-alpha converting enzyme and matrix metalloproteinase 9. This is the first evidence of the metalloproteinase-mediated processing of a receptor-like protein-tyrosine phosphatase in the central nervous system.

Retrovirus vector-mediated gene transfer into the chick optic vesicle by in ovo electroporation

Owing to its external position in the embryo, the chick eye has been used as a readily accessible model for studying the molecular mechanisms behind the patterning of the central nervous system. Although methods of genetic analysis have not been established as in the mouse, the chick is convenient for analyzing the functions of genes by in ovo electroporation of retroviral vectors. In this review, we describe the retroviral vector-mediated transfer of genes into the chick optic vesicle by in ovo electroporation. A rapid, efficient, and sustained expression of transgenes is achieved by this approach.

Ectopic Pax2 expression in chick ventral optic cup phenocopies loss of Pax2 expression

Pax2 is essential for the development of the urogenital system, neural tube, otic vesicle, optic cup and optic tract [Dressler, G.R., Deutsch, U., et al., 1990. PAX2, a new murine paired-box-containing gene and its expression in the developing excretory system. Development 109 (4), 787-795; Nornes, H.O., Dressler, G.R., et al., 1990. Spatially and temporally restricted expression of Pax2 during murine neurogenesis. Development 109 (4), 797-809; Eccles, M.R., Wallis, L.J., et al., 1992. Expression of the PAX2 gene in human fetal kidney and Wilms' tumor. Cell Growth Differ 3 (5), 279-289]. Within the visual system, a loss-of-function leads to lack of choroid fissure closure (known as a coloboma), a loss of optic nerve astrocytes, and anomalous axonal pathfinding at the optic chiasm [Favor, J., Sandulache, R., et al., 1996. The mouse Pax2(1Neu) mutation is identical to a human PAX2 mutation in a family with renal-coloboma syndrome and results in developmental defects of the brain, ear, eye, and kidney. Proc. Natl. Acad. Sci. U. S. A. 93 (24), 13870-13875; Torres, M., Gomez-Pardo, E., et al., 1996. Pax2 contributes to inner ear patterning and optic nerve trajectory. Development 122 (11), 3381-3391]. This study is directed at determining the effects of ectopic Pax2 expression in the chick ventral optic cup past the normal developmental period when Pax2 is found. In ovo electroporation of Pax2 into the chick ventral optic cup results in the formation of colobomas, a condition typically associated with a loss of Pax2 expression. While the overexpression of Pax2 appears to phenocopy a loss of Pax2, the mechanism of the failure of choroid fissure closure is associated with a cell fate switch from ventral retina and retinal pigmented epithelium (RPE) to an astrocyte fate. Further, ectopic expression of Pax2 in RPE appears to have non-cell autonomous effects on adjacent RPE, creating an ectopic neural retina in place of the RPE.

Iris development in vertebrates; genetic and molecular considerations

The iris plays a key role in visual function. It regulates the amount of light entering the eye and falling on the retina and also operates in focal adjustment of closer objects. The iris is involved in circulation of the aqueous humor and hence functions in regulation of intraocular pressure. Intriguingly, iris pigmented cells possess the ability to transdifferentiate into different ocular cell types of retinal pigmented epithelium, photoreceptors and lens cells. Thus, the iris is considered a potential source for cell-replacement therapies. During embryogenesis, the iris arises from both the optic cup and the periocular mesenchyme. Its interesting mode of development includes specification of the peripheral optic cup to a non-neuronal fate, migration of cells from the surrounding periocular mesenchyme and an atypical formation of smooth muscles from the neuroectoderm. This manner of development raises some interesting general topics concerning the early patterning of the neuroectoderm, the specification and differentiation of diverse cell types and the interactions between intrinsic and extrinsic factors in the process of organogenesis. In this review, we discuss iris anatomy and development, describe major pathologies of the iris and their molecular etiology and finally summarize the recent findings on genes and signaling pathways that are involved in iris development.

Mechanism of asymmetric ovarian development in chick embryos

In most animals, the gonads develop symmetrically, but most birds develop only a left ovary. A possible role for estrogen in this asymmetric ovarian development has been proposed in the chick, but the mechanism underlying this process is largely unknown. Here, we identify the molecular mechanism responsible for this ovarian asymmetry. Asymmetric PITX2 expression in the left presumptive gonad leads to the asymmetric expression of the retinoic-acid (RA)-synthesizing enzyme, RALDH2, in the right presumptive gonad. Subsequently, RA suppresses expression of the nuclear receptors Ad4BP/SF-1 and estrogen receptor alpha in the right ovarian primordium. Ad4BP/SF-1 expressed in the left ovarian primordium asymmetrically upregulates cyclin D1 to stimulate cell proliferation. These data suggest that early asymmetric expression of PITX2 leads to asymmetric ovarian development through up- or downregulation of RALDH2, Ad4BP/SF-1, estrogen receptor alpha and cyclin D1.

Expression of zebrafish aldh1a3 (raldh3) and absence of aldh1a1 in teleosts

The vitamin A-derived morphogen retinoic acid (RA) plays important roles during the development of chordate animals. The Aldh1a-family of RA-synthesizing enzymes consists of three members, Aldh1a1-3 (Raldh1-3), that are dynamically expressed throughout development. We have searched the known teleost genomes for the presence of Raldh family members and have found that teleost fish possess orthologs of Aldh1a2 and Aldh1a3 only. Here we describe the expression of aldh1a3 in the zebrafish, Danio rerio. Whole mount in situ hybridization shows that aldh1a3 is expressed during eye development in the retina flanking the optic stalks and later is expressed ventrally, opposite the expression domain of aldh1a2. During inner ear morphogenesis, aldh1a3 is expressed in developing sensory epithelia of the cristae and utricular macula and is specifically up-regulated in epithelial projections throughout the formation of the walls of the semicircular canals and endolymphatic duct. In contrast to the mouse inner ear, which expresses all three Raldhs, we find that only aldh1a3 is expressed in the zebrafish otocyst, while aldh1a2 is present in the periotic mesenchyme. During larval stages, additional expression domains of aldh1a3 appear in the anterior pituitary and the swim bladder. Our analyses provide a starting point for genetic studies to examine the role of RA in these organs and emphasize the suitability of the zebrafish inner ear in dissecting the contribution of RA signaling to the development of the vestibular system.

Role of a transcription factor Pax6 in the developing vertebrate olfactory system

The olfactory system is responsible for capturing and processing odorant information, which significantly influences a variety of behaviors in animals. The vertebrate olfactory system consists of several neuronal components including the olfactory epithelium, olfactory bulb and olfactory cortex, which originate from distinct embryonic tissues. The transcription factor Pax6 is strongly expressed in the embryonic and postnatal olfactory systems, and regulates neuronal specification, migration and differentiation. Here we review classical and recent studies focusing on the role of Pax6 in the developing olfactory system, and highlight the cellular and molecular mechanisms underlying the highly coordinated developmental processes of the vertebrate olfactory system.

Distribution of the cellular retinoic acid binding protein CRABP-I in the developing chick optic tectum

Vitamin A is a major morphogen for the visual system. Most of its effects are mediated by retinoic acid (RA), whose developmental functions include pattern formation, neuronal differentiation and possibly axonal guidance. Although RA has been suggested to regulate development of the retina and its central projection, little is known about the distribution of retinoid receptors and binding proteins in the optic tectum, which in birds is the direct target of most retinofugal axons. We investigated the spatial and temporal distribution of the cellular retinoic acid binding protein-I (CRABP-I) in the chick midbrain. While the precise role of CRABP-I is still unknown, this is an intracellular transport protein for RA, which tends to be expressed in cells that are responsive to retinoic acid. Our data show immunoreactivity of CRABP-I in the tectal anlage at E2.5 and during the entire period of embryonic development. It was found in differentiating neurons of the generative zone, in migrating cells of the prospective stratum griseum et fibrosum superficiale and in mature neurons in this layer. In addition, we detected retinoid receptors RARalpha, RARbeta, RXRalpha, RXRbeta and RXRgamma in the developing tectum. Cell culture experiments demonstrate CRABP-I expression in a subpopulation of tectal neurons as they differentiate in vitro. These results are consistent with a regulatory role of RA in tectal neurogenesis and physiology.

Retinoids control anterior and dorsal properties in the developing forebrain

We have previously shown that retinoic acid (RA) synthesized by the retinaldehyde dehydrogenase 2 (RALDH2) is required in forebrain development. Deficiency in RA due to inactivation of the mouse Raldh2 gene or to complete absence of retinoids in vitamin-A-deficient (VAD) quails, leads to abnormal morphogenesis of various forebrain derivatives. In this study we show that double Raldh2/Raldh3 mouse mutants have a more severe phenotype in the craniofacial region than single null mutants. In particular, the nasal processes are truncated and the eye abnormalities are exacerbated. It has been previously shown that retinoids act mainly on cell proliferation and survival in the ventral forebrain by regulating SHH and FGF8 signaling. Using the VAD quail model, which survives longer than the Raldh-deficient mouse embryos, we found that retinoids act in maintaining the correct position of anterior and dorsal boundaries in the forebrain by modulating FGF8 anteriorly and WNT signaling dorsally. Furthermore, BMP4 and FGF8 signaling are affected in the nasal region and BMP4 is ventrally expanded in the optic vesicle. At the optic cup stage, Pax6, Tbx5 and Bmp4 are ectopically expressed in the presumptive retinal pigmented epithelium (RPE), while Otx2 and Mitf are not induced, leading to a dorsal transdifferentiation of RPE to neural retina. Therefore, besides being required for survival of ventral structures, retinoids are involved in restricting anterior identity in the telencephalon and dorsal identity in the diencephalon and the retina.

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

Optic vesicle formation, transformation into an optic cup and integration with neighboring tissues are essential for normal eye formation, and involve the coordinated occurrence of complex cellular and molecular events. Perhaps not surprisingly, these complex phenomena have provided fertile ground for controversial and even contradictory results and conclusions. After presenting an overview of current knowledge of optic vesicle development, we will address conceptual and methodological issues that complicate research in this field. This will be done through a review of the pertinent literature, as well as by drawing on our own experience, gathered through recent studies of both intra- and extra-cellular regulation of optic vesicle development and patterning. Finally, and without attempting to be exhaustive, we will point out some important aspects of optic vesicle development that have not yet received enough attention.

Pax6 is required for delta-catenin/neurojugin expression during retinal, cerebellar and cortical development in mice

The transcription factor Pax6 regulates multiple aspects of central nervous system (CNS) development. At the cellular level, the Pax6 mutation was reported to affect homophilic and heterophilic cellular adhesion, neuron polarity and neurite outgrowth. These abnormalities were observed in multiple regions of Pax6-mutant CNS, suggesting a common function for Pax6 in regulating cytoskeletal dynamics. However, target genes mediating Pax6 function in cytoskeletal dynamics remain largely unknown. Using DNA microarrays, we identified delta-catenin (delta-catenin /neurojugin) as a potential direct target of Pax6 in the CNS. delta-catenin encodes a large cytoskeletal protein that localizes at adherens junction in the CNS and that can modulate neurite outgrowth and N-cadherin turnover. delta-catenin was found to be co-expressed with Pax6 in several regions of the developing CNS. In Pax6 mutant embryos, delta-catenin expression was severely reduced in the optic vesicle neural ectoderm, in the ventricular zone of the neocortex and in the external granule layer of the cerebellum. We identified a Pax6 binding site in delta-catenin promoter that is conserved between mice and humans and which is effectively bound by Pax6 in vitro. Our results suggest that Pax6 regulates delta-catenin expression during CNS development in mice.

Role of bone morphogenic protein 2 in retinal patterning and retinotectal projection

It has been long believed that the anteroposterior (A-P) and dorsoventral (D-V) axes in the developing retina are determined independently and also that the retinotectal projection along the two axes is controlled independently. However, we recently demonstrated that misexpression of Ventroptin, a bone morphogenic protein (BMP) antagonist, in the developing chick retina alters the retinotectal projection not only along the D-V (or mediolateral) axis but also along the A-P axis. Moreover, the dorsal-high expression of BMP4 is relieved by the dorsotemporal-high expression of BMP2 at embryonic day 5 (E5) in the retina, during which Ventroptin continuously counteracts the two BMPs keeping on the countergradient expression pattern, respectively. Here, we show that the topographic molecules so far reported to have a gradient only along the D-V axis and ephrin-A2 so far only along the A-P axis are both controlled by the BMP signal, and that they are expressed in a gradient manner along the tilted axis from E6 on in the developing chick retina: the expression patterns of these oblique-gradient molecules are all changed, when BMP2 expression is manipulated in the developing retina. Furthermore, in both BMP2 knockdown embryos and ephrin-A2-misexpressed embryos, the retinotectal projection is altered along the two orthogonal axes. The expressional switching from BMP4 to BMP2 thus appears to play a key role in the retinal patterning and topographic retinotectal projection by tilting the D-V axis toward the posterior side during retinal development. Our results also indicate that BMP2 expression is essential for the maintenance of regional specificity along the revised D-V axis.

Is retinoic acid genetic machinery a chordate innovation?

Development of many chordate features depends on retinoic acid (RA). Because the action of RA during development seems to be restricted to chordates, it had been previously proposed that the "invention" of RA genetic machinery, including RA-binding nuclear hormone receptors (Rars), and the RA-synthesizing and RA-degrading enzymes Aldh1a (Raldh) and Cyp26, respectively, was an important step for the origin of developmental mechanisms leading to the chordate body plan. We tested this hypothesis by conducting an exhaustive survey of the RA machinery in genomic databases for twelve deuterostomes. We reconstructed the evolution of these genes in deuterostomes and showed for the first time that RA genetic machinery--that is Aldh1a, Cyp26, and Rar orthologs--is present in nonchordate deuterostomes. This finding implies that RA genetic machinery was already present during early deuterostome evolution, and therefore, is not a chordate innovation. This new evolutionary viewpoint argues against the hypothesis that the acquisition of gene families underlying RA metabolism and signaling was a key event for the origin of chordates. We propose a new hypothesis in which lineage-specific duplication and loss of RA machinery genes could be related to the morphological radiation of deuterostomes.

Abnormal migration and distribution of neural crest cells in Pax6 heterozygous mutant eye, a model for human eye diseases

PAX6/Pax6 gene encodes a transcription factor that is crucially required for eye development. Pax6 heterozygous mutant mouse (Pax6(Sey/+)) shows various ocular defects, especially in the anterior segment. It has been well known that the induction of the lens and development of the cornea and retina are dependent on PAX6/Pax6 in a cell-autonomous fashion, although the influence of PAX6/Pax6 on the other tissues derived from the ocular mesenchyme is largely unknown. Using transgenic mouse lines in which neural crest cells are genetically marked by LacZ or EGFP, we revealed the extensive contribution of neural crest derived cells (NCDCs) to the ocular tissues. Furthermore, various eye defects in Pax6(Sey/+) mouse were accompanied by abnormal distribution of NCDCs from early developmental stages to the adult. In Pax6(Sey/+) mouse mice, neural crest cells abnormally migrated into the developing eye in a cell nonautonomous manner at early embryonic stages. These results indicate that normal distribution and integration of NCDCs in ocular tissues depend on a proper dosage of Pax6, and that Pax6(Sey/+) eye anomalies are caused by cell autonomous and nonautonomous defects due to Pax6 haploinsufficiency.