Photoreceptor morphology is severely affected in the beta,beta-carotene-15,15'-oxygenase (bcox) zebrafish morphant

Mice Lacking the Systemic Vitamin A Receptor RBPR2 Show Decreased Ocular Retinoids and Loss of Visual Function

Simple Summary

This work represents an initial evaluation of the second RBP4-vitamin A receptor RBPR2 in a mammalian model. We provide evidence that the membrane localized RBPR2 protein, under variable conditions of dietary vitamin A intake, plays an important role for dietary vitamin A transport to the eye for ocular retinoid homeostasis and visual function. These findings are of general interest, as disturbances in blood and ocular vitamin A homeostasis are linked to retinal degenerative diseases, which are blinding diseases. The animal model described here could also serve as an in vivo tool to study mechanisms related to retinal cell degeneration that are associated with vitamin A deficiency.

Abstract

The systemic transport of dietary vitamin A/all-trans retinol bound to RBP4 into peripheral tissues for storage is an essential physiological process that continuously provides visual chromophore precursors to the retina under fasting conditions. This mechanism is critical for phototransduction, photoreceptor cell maintenance and survival, and in the support of visual function. While the membrane receptor STRA6 facilitates the blood transport of lipophilic vitamin A into the eye, it is not expressed in most peripheral organs, which are proposed to express a second membrane receptor for the uptake of vitamin A from circulating RBP4. The discovery of a novel vitamin A receptor, RBPR2, which is expressed in the liver and intestine, but not in the eye, alluded to this long-sort non-ocular membrane receptor for systemic RBP4-ROL uptake and transport. We have previously shown in zebrafish that the retinol-binding protein receptor 2 (Rbpr2) plays an important role in the transport of yolk vitamin A to the eye. Mutant rbpr2 zebrafish lines manifested in decreased ocular retinoid concentrations and retinal phenotypes. To investigate a physiological role for the second vitamin A receptor, RBPR2, in mammals and to analyze the metabolic basis of systemic vitamin A transport for retinoid homeostasis, we established a whole-body Rbpr2 knockout mouse (Rbpr2^−/−) model. These mice were viable on both vitamin A-sufficient and -deficient diets. Rbpr2^−/− mice that were fed a vitamin A-sufficient diet displayed lower ocular retinoid levels, decreased opsins, and manifested in decrease visual function, as measured by electroretinography. Interestingly, when Rbpr2^−/− mice were fed a vitamin A-deficient diet, they additionally showed shorter photoreceptor outer segment phenotypes, altogether manifesting in a significant loss of visual function. Thus, under conditions replicating vitamin A sufficiency and deficiency, our analyses revealed that RBPR2-mediated systemic vitamin A transport is a regulated process that is important for vitamin A delivery to the eye when RBP4-bound ROL is the only transport pathway in the fasting condition or under vitamin A deficiency conditions.

Growth Factors as Tools in Photoreceptor Cell Regeneration and Vision Recovery

Photoreceptor loss is a major cause of blindness around the world. Stem cell therapy offers a new strategy in retina degenerative disease. Retinal progenitors can be derived from embryonic stem cells (ESC) in vitro, but cannot be processed to a mature state. In addition, the adult recipient retina presents a very different environment than the photoreceptor precursor donor. It seems that modulation of the recipient environment by ectopic development regulated growth factors for transplanted cells could generate efficient putative photoreceptors. The purpose of this review article was to investigate the signaling pathway of growth factors including: insulin-like growth factors (IGFs), fibroblast growth factors (FGF), Nerve growth factor (NGF), Brain-derived neurotrophic factor (BDNF), Taurin and Retinoic acid (RA) involved in the differentiation of neuroretina cell, like; photoreceptor and retinal progenitor cells. Given the results available in the related literature, the differentiation efficacy of ESCs toward the photoreceptor and retinal neurons and the important role of growth factors in activating signaling pathways such as Akt, Ras/Raf1/ and ERKs also inhibit the ASK1/JNK apoptosis pathway. Manipulating differentiated culture, growth factors can influence photoreceptor transplantation efficiency in retinal degenerative disease.

Animal and cellular models of microphthalmia

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

Plain language summary

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

A Functional Binding Domain in the Rbpr2 Receptor Is Required for Vitamin A Transport, Ocular Retinoid Homeostasis, and Photoreceptor Cell Survival in Zebrafish

Dietary vitamin A/all-trans retinol/ROL plays a critical role in human vision. ROL circulates bound to the plasma retinol-binding protein (RBP4) as RBP4-ROL. In the eye, the STRA6 membrane receptor binds to circulatory RBP4 and internalizes ROL. STRA6 is, however, not expressed in systemic tissues, where there is high affinity RBP4 binding and ROL uptake. We tested the hypothesis that the second retinol binding protein 4 receptor 2 (Rbpr2), which is highly expressed in systemic tissues of zebrafish and mouse, contains a functional RBP4 binding domain, critical for ROL transport. As for STRA6, modeling and docking studies confirmed three conserved RBP4 binding residues in zebrafish Rbpr2. In cell culture studies, disruption of the RBP4 binding residues on Rbpr2 almost completely abolished uptake of exogenous vitamin A. CRISPR-generated rbpr2-RBP4 domain zebrafish mutants showed microphthalmia, shorter photoreceptor outer segments, and decreased opsins, which were attributed to impaired ocular retinoid content. Injection of WT-Rbpr2 mRNA into rbpr2 mutant or all-trans retinoic acid treatment rescued the mutant eye phenotypes. In conclusion, zebrafish Rbpr2 contains a putative extracellular RBP4-ROL ligand-binding domain, critical for yolk vitamin A transport to the eye for ocular retinoid production and homeostasis, for photoreceptor cell survival.

The Retinol Binding Protein Receptor 2 (Rbpr2) is required for Photoreceptor Outer Segment Morphogenesis and Visual Function in Zebrafish

Vitamin A (all-trans retinol) plays critical roles in mammalian development and vision. Since vitamin A is food-derived, tissue-specific uptake and storage mechanism are needed. In the eye, uptake of RBP4-retinol is mediated by the receptor Stra6, whereas the receptor mediating RBP4 binding and retinol transport into the liver has just recently been discovered. Here we examined the role of zebrafish retinol binding protein receptor 2 (Rbpr2) for RBP4-retinol uptake in developing embryos, using eye development and vision as sensitive readouts. In cultured cells, Rbpr2 localized to membranes and promoted RBP4-retinol uptake. In larvae, Rbpr2 expression was detected in developing intestinal enterocytes and liver hepatocytes. Two rbpr2 mutant zebrafish lines, each resulting in Rbpr2 deficiency, exhibit a small eye defect, and systemic malformations including hydrocephaly and cardiac edema, phenotypes associated with vitamin A deficiency. In the retina, Rbpr2 loss resulted in shorter photoreceptor outer segments, mislocalization and decrease in visual pigments, decreased expression of retinoic acid-responsive genes and photoreceptor cell loss, overall leading to a reduction of visual function. Together, these results demonstrate that Rbpr2-mediated RBP4-retinol uptake in developing liver and intestine is necessary to provide sufficient substrate for ocular retinoid production required for photoreceptor cell maintenance and visual function.

Development of the Vertebrate Eye and Retina

The mature, functional, and healthy eye is generated by the coordinated regulatory interaction of numerous and diverse developing tissues. The neural retina of the eye must undergo the neurogenesis of multiple retinal cell types in the correct ratios and spatial patterns. This chapter provides an overview of retinal development, and includes a summary of the process of eye organogenesis, a discussion of major principles of retinal neurogenesis, and describes some of the key molecular factors critical for retinal development. Defects in many of these factors underlie diseases of the eye, and an understanding of the process of retinal development will be critical for successful future applications of regenerative therapies for eye disease.

Mutations in TUBGCP4 alter microtubule organization via the γ-tubulin ring complex in autosomal-recessive microcephaly with chorioretinopathy

We have identified TUBGCP4 variants in individuals with autosomal-recessive microcephaly and chorioretinopathy. Whole-exome sequencing performed on one family with two affected siblings and independently on another family with one affected child revealed compound-heterozygous mutations in TUBGCP4. Subsequent Sanger sequencing was performed on a panel of individuals from 12 French families affected by microcephaly and ophthalmic manifestations, and one other individual was identified with compound-heterozygous mutations in TUBGCP4. One synonymous variant was common to all three families and was shown to induce exon skipping; the other mutations were frameshift mutations and a deletion. TUBGCP4 encodes γ-tubulin complex protein 4, a component belonging to the γ-tubulin ring complex (γ-TuRC) and known to regulate the nucleation and organization of microtubules. Functional analysis of individual fibroblasts disclosed reduced levels of the γ-TuRC, altered nucleation and organization of microtubules, abnormal nuclear shape, and aneuploidy. Moreover, zebrafish treated with morpholinos against tubgcp4 were found to have reduced head volume and eye developmental anomalies with chorioretinal dysplasia. In summary, the identification of TUBGCP4 mutations in individuals with microcephaly and a spectrum of anomalies in eye development, particularly photoreceptor anomalies, provides evidence of an important role for the γ-TuRC in brain and eye development.

ALDH1A3 loss of function causes bilateral anophthalmia/microphthalmia and hypoplasia of the optic nerve and optic chiasm

The major active retinoid, all-trans retinoic acid, has long been recognized as critical for the development of several organs, including the eye. Mutations in STRA6, the gene encoding the cellular receptor for vitamin A, in patients with Matthew-Wood syndrome and anophthalmia/microphthalmia (A/M), have previously demonstrated the importance of retinol metabolism in human eye disease. We used homozygosity mapping combined with next-generation sequencing to interrogate patients with anophthalmia and microphthalmia for new causative genes. We used whole-exome and whole-genome sequencing to study a family with two affected brothers with bilateral A/M and a simplex case with bilateral anophthalmia and hypoplasia of the optic nerve and optic chiasm. Analysis of novel sequence variants revealed homozygosity for two nonsense mutations in ALDH1A3, c.568A>G, predicting p.Lys190*, in the familial cases, and c.1165A>T, predicting p.Lys389*, in the simplex case. Both mutations predict nonsense-mediated decay and complete loss of function. We performed antisense morpholino (MO) studies in Danio rerio to characterize the developmental effects of loss of Aldh1a3 function. MO-injected larvae showed a significant reduction in eye size, and aberrant axonal projections to the tectum were noted. We conclude that ALDH1A3 loss of function causes anophthalmia and aberrant eye development in humans and in animal model systems.

Early retinoic acid deprivation in developing zebrafish results in microphthalmia

Vitamin A deficiency causes impaired vision and blindness in millions of children around the world. Previous studies in zebrafish have demonstrated that retinoic acid (RA), the acid form of vitamin A, plays a vital role in early eye development. The objective of this study was to describe the effects of early RA deficiency by treating zebrafish with diethylaminobenzaldehyde (DEAB), a potent inhibitor of the enzyme retinaldehyde dehydrogenase (RALDH) that converts retinal to RA. Zebrafish embryos were treated for 2 h beginning at 9 h postfertilization. Gross morphology and retinal development were examined at regular intervals for 5 days after treatment. The optokinetic reflex (OKR) test, visual background adaptation (VBA) test, and the electroretinogram (ERG) were performed to assess visual function and behavior. Early treatment of zebrafish embryos with 100 μM DEAB (9 h) resulted in reduced eye size, and this microphthalmia persisted through larval development. Retinal histology revealed that DEAB eyes had significant developmental abnormalities but had relatively normal retinal lamination by 5.5 days postfertilization. However, the fish showed neither an OKR nor a VBA response. Further, the retina did not respond to light as measured by the ERG. We conclude that early deficiency of RA during eye development causes microphthalmia as well as other visual defects, and that timing of the RA deficiency is critical to the developmental outcome.

Plasticity of photoreceptor-generating retinal progenitors revealed by prolonged retinoic acid exposure

Background

Retinoic acid (RA) is important for vertebrate eye morphogenesis and is a regulator of photoreceptor development in the retina. In the zebrafish, RA treatment of postmitotic photoreceptor precursors has been shown to promote the differentiation of rods and red-sensitive cones while inhibiting the differentiation of blue- and UV-sensitive cones. The roles played by RA and its receptors in modifying photoreceptor fate remain to be determined.

Results

Treatment of zebrafish embryos with RA, beginning at the time of retinal progenitor cell proliferation and prior to photoreceptor terminal mitosis, resulted in a significant alteration of rod and cone mosaic patterns, suggesting an increase in the production of rods at the expense of red cones. Quantitative pattern analyses documented increased density of rod photoreceptors and reduced local spacing between rod cells, suggesting rods were appearing in locations normally occupied by cone photoreceptors. Cone densities were correspondingly reduced and cone photoreceptor mosaics displayed expanded and less regular spacing. These results were consistent with replacement of approximately 25% of positions normally occupied by red-sensitive cones, with additional rods. Analysis of embryos from a RA-signaling reporter line determined that multiple retinal cell types, including mitotic cells and differentiating rods and cones, are capable of directly responding to RA. The RA receptors RXRγ and RARαb are expressed in patterns consistent with mediating the effects of RA on photoreceptors. Selective knockdown of RARαb expression resulted in a reduction in endogenous RA signaling in the retina. Knockdown of RARαb also caused a reduced production of rods that was not restored by simultaneous treatments with RA.

Conclusions

These data suggest that developing retinal cells have a dynamic sensitivity to RA during retinal neurogenesis. In zebrafish RA may influence the rod vs. cone cell fate decision. The RARαb receptor mediates the effects of endogenous, as well as exogenous RA, on rod development.

The search for non-chordate retinoic acid signaling: lessons from chordates

Signaling by retinoic acid (RA) is an important pathway in the development and homeostasis of vertebrate and invertebrate chordates, with a critical role in mesoderm patterning. Classical studies on the distribution of nuclear receptors of animals suggested that the family of RA receptors (RARs/NR1B) was restricted to chordates, while the family of RA X receptors (RXR/NR2B) was distributed from cnidarians to chordates. However, the accumulation of data from genome projects and studies in non-model species is questioning this traditional view. Here we discuss the evidence for non-chordate RA signaling systems in the light of recent advances in our understanding of carotene (pro-Vitamin A) metabolism and of the identification of potential RARs and members of the NR1 family in echinoderms and lophotrochozoan trematodes, respectively. We conclude, as have others before (Bertrand et al., 2004. Mol Biol Evol 21(10):1923-1937), that signaling by RA is more likely an ancestral feature of bilaterians than a chordate innovation.

RBP4 disrupts vitamin A uptake homeostasis in a STRA6-deficient animal model for Matthew-Wood syndrome

The cellular uptake of vitamin A from its RBP4-bound circulating form (holo-RBP4) is a homeostatic process that evidently depends on the multidomain membrane protein STRA6. In humans, mutations in STRA6 are associated with Matthew-Wood syndrome, manifested by multisystem developmental malformations. Here we addressed the metabolic basis of this inherited disease. STRA6-dependent transfer of retinol from RBP4 into cultured NIH 3T3 fibroblasts was enhanced by lecithin:retinol acyltransferase (LRAT). The retinol transfer was bidirectional, strongly suggesting that STRA6 acts as a retinol channel/transporter. Loss-of-function analysis in zebrafish embryos revealed that Stra6 deficiency caused vitamin A deprivation of the developing eyes. We provide evidence that, in the absence of Stra6, holo-Rbp4 provokes nonspecific vitamin A excess in several embryonic tissues, impairing retinoic acid receptor signaling and gene regulation. These fatal consequences of Stra6 deficiency, including craniofacial and cardiac defects and microphthalmia, were largely alleviated by reducing embryonic Rbp4 levels by morpholino oligonucleotide or pharmacological treatments.

Neurogenesis in the fish retina

The retinas of teleost fish have long been of interest to developmental neurobiologists for their persistent plasticity during growth, life history changes, and response to injury. Because the vertebrate retina is a highly conserved tissue, the study of persistent plasticity in teleosts has provided insights into mechanisms for postembryonic retinal neurogenesis in mammals. In addition, in the past 10 years there has been an explosion in the use of teleost fish-zebrafish (Danio rerio) in particular-to understand the mechanisms of embryonic retinal neurogenesis in a model vertebrate with genetic resources. This review summarizes the key features of teleost retinal neurogenesis that make it a productive and interesting experimental system, and focuses on the contributions to our knowledge of retinal neurogenesis that uniquely required or significantly benefited from the use of a fish model system.

Production and characterization of a continuous embryonic cell line from sea bass (Dicentrarchus labrax L.)

Continuous cell lines represent an important tool both for biological studies and for their applications in marine biotechnology. In this article we describe the production and characterization of a continuous adherent cell line, named DLEC, derived from early embryos of the European sea bass Dicentrarchus labrax L. (Actinopterygii, Moronidae). Cells were obtained by disrupting 2- to 12-hour-old embryos and culturing resulting cells at 18 degrees C in RPMI medium containing 5% fetal calf serum (FCS) and 10% supernatant fraction of the embryo homogenate. After 8 weeks culture medium was replaced with Liebovitz's L15 medium containing 10% FCS and DLEC cells started proliferation. Subsequently, they were continuously cultured until the 50th passage without evident changes in their morphology. DLEC cells show a fibroblast-like shape and a modal chromosome number of 48, as do the wild-type cells; conversely the constant presence of six to nine meta-submetacentric elements in the karyotype (vs. zero to two in the wild-type) indicates the occurrence of chromosomal rearrangements during stabilization. DLEC cells are sensitive to substances known to induce differentiation of mammalian cells such as retinoic acid and phorbol esters. They have been transfected using liposomes with a commercial plasmid vector containing a reporter gene, thus suggesting a possible importance as an alternative expression system of recombinant vertebrate proteins in teleost cells.