Retinal dystrophy resulting from ablation of RXR alpha in the mouse retinal pigment epithelium

Benefits and Caveats in the Use of Retinal Pigment Epithelium-Specific Cre Mice

The retinal pigment epithelium (RPE) is an important monolayer of cells present in the outer retina, forming a major part of the blood-retina barrier (BRB). It performs many tasks essential for the maintenance of retinal integrity and function. With increasing knowledge of the retina, it is becoming clear that both common retinal disorders, like age-related macular degeneration, and rare genetic disorders originate in the RPE. This calls for a better understanding of the functions of various proteins within the RPE. In this regard, mice enabling an RPE-specific gene deletion are a powerful tool to study the role of a particular protein within the RPE cells in their native environment, simultaneously negating any potential influences of systemic changes. Moreover, since RPE cells interact closely with adjacent photoreceptors, these mice also provide an excellent avenue to study the importance of a particular gene function within the RPE to the retina as a whole. In this review, we outline and compare the features of various Cre mice created for this purpose, which allow for inducible or non-inducible RPE-specific knockout of a gene of interest. We summarize the various benefits and caveats involved in the use of such mouse lines, allowing researchers to make a well-informed decision on the choice of Cre mouse to use in relation to their research needs.

NURR1 expression regulates retinal pigment epithelial-mesenchymal transition and age-related macular degeneration phenotypes

Phenotypic variations in the retinal pigment epithelial (RPE) layer are often a predecessor and driver of ocular degenerative diseases, such as age-related macular degeneration (AMD), the leading cause of vision loss in the elderly. We previously identified the orphan nuclear receptor-related 1 (NURR1), from a nuclear receptor atlas of human RPE cells, as a candidate transcription factor potentially involved in AMD development and progression. In the present study we characterized the expression of NURR1 as a function of age in RPE cells harvested from human donor eyes and in donor tissue from AMD patients. Mechanistically, we found an age-dependent shift in NURR1 dimerization from NURR1-RXRα heterodimers toward NURR1-NURR1 homodimers in primary human RPE cells. Additionally, overexpression and activation of NURR1 attenuated TNF-α-induced epithelial-to-mesenchymal transition (EMT) and migration, and modulated EMT-associated gene and protein expression in human RPE cells independent of age. In vivo, oral administration of IP7e, a potent NURR1 activator, ameliorated EMT in an experimental model of wet AMD and improved retinal function in a mouse model that presents with dry AMD features, impacting AMD phenotype, structure, and function of RPE cells, inhibiting accumulation of immune cells, and diminishing lipid accumulation. These results provide insight into the mechanisms of action of NURR1 in the aging eye, and demonstrate that the relative expression levels and activity of NURR1 is critical for both physiological and pathological functions of human RPE cells through RXRα-dependent regulation, and that targeting NURR1 may have therapeutic potential for AMD by modulating EMT, inflammation, and lipid homeostasis.

Molecular docking, dynamics, and pharmacology studies on bexarotene as an agonist of ligand-activated transcription factors, retinoid X receptors

Retinoid X receptors (RXRs) belong to the nuclear receptor superfamily, and upon ligand activation, these receptors control gene transcription via either homodimerization with themselves or heterodimerization with the partner-nuclear receptor. The protective effects of RXRs and RXR agonists have been reported in several neurodegenerative diseases, including in the retina. This study was aimed to prioritize compounds from natural and synthetic origin retinoids as potential RXR agonists by molecular docking and molecular dynamic simulation strategies. The docking studies indicated bexarotene as a lead compound that can activate various RXR receptor isoforms (α, β, and γ) and has a strong binding affinity to the receptor protein than retinoic acid, which is known as a natural endogenous RXR agonist. Dynamic simulation studies confirmed that the hydrogen bonding and hydrophobic interactions were highly stable in all the three isoforms of the RXR-bexarotene complex. To further validate the significance of the RXR receptor in neurons, in vitro pharmacological treatment of neuronal SH-SY5Y cells with bexarotene was performed. In vitro data from SH-SY5Y cells confirmed that bexarotene activated RXR-simulated neurite outgrowth significantly. We conclude that bexarotene could be potentially used as an exogenous activator of RXRs and emerge as a good drug target for several neurodegenerative disorders.

Role of Nuclear Receptors in Central Nervous System Development and Associated Diseases

The nuclear hormone receptor (NHR) superfamily is composed of a wide range of receptors involved in a myriad of important biological processes, including development, growth, metabolism, and maintenance. Regulation of such wide variety of functions requires a complex system of gene regulation that includes interaction with transcription factors, chromatin-modifying complex, and the proper recognition of ligands. NHRs are able to coordinate the expression of genes in numerous pathways simultaneously. This review focuses on the role of nuclear receptors in the central nervous system and, in particular, their role in regulating the proper development and function of the brain and the eye. In addition, the review highlights the impact of mutations in NHRs on a spectrum of human diseases from autism to retinal degeneration.

High glucose-induced proteome alterations in retinal pigmented epithelium cells and its possible relevance to diabetic retinopathy

Diabetic retinopathy can cause poor vision and blindness. Previous research has shown that high blood glucose weakens retinal capillaries and induces glycoxidation. However, the detailed molecular mechanisms underlying the effects of high blood glucose on development of diabetic retinopathy have yet to be elucidated. In this study, we cultured a retinal pigmented epithelium cell line (ARPE-19) in mannitol-balanced 5.5 mM, 25 mM, and 100 mM d-glucose media, and evaluated protein expression and redox-regulation. We identified 56 proteins that showed significant changes in protein expression, and 33 proteins showing significant changes in thiol reactivity, in response to high glucose concentration. Several proteins that are involved in signal transduction, gene regulation, and transport showed significant changes in expression, whereas proteins involved in metabolism, transport, and cell survival displayed changes in thiol reactivity. Further analyses of clinical plasma specimens confirmed that the proteins lamin B2, PUMA, WTAP, ASGR1, and prohibitin 2 showed type 2 diabetic retinopathy-dependent alterations. In summary, in this study, we used a comprehensive retinal cell-based proteomic approach for the identification of changes in protein expression and redox-associated retinal markers induced by high glucose concentration. Some of the identified proteins have been validated with clinical samples and provide potential targets for the prognosis and diagnosis of diabetic retinopathy.

Evidence for baseline retinal pigment epithelium pathology in the Trp1-Cre mouse

The increasing popularity of the Cre/loxP recombination system has led to the generation of numerous transgenic mouse lines in which Cre recombinase is expressed under the control of organ- or cell-specific promoters. Alterations in retinal pigment epithelium (RPE), a multifunctional cell monolayer that separates the retinal photoreceptors from the choroid, are prevalent in the pathogenesis of a number of ocular disorders, including age-related macular degeneration. To date, six transgenic mouse lines have been developed that target Cre to the RPE under the control of various gene promoters. However, multiple lines of evidence indicate that high levels of Cre expression can be toxic to mammalian cells. In this study, we report that in the Trp1-Cre mouse, a commonly used transgenic Cre strain for RPE gene function studies, Cre recombinase expression alone leads to RPE dysfunction and concomitant disorganization of RPE layer morphology, large areas of RPE atrophy, retinal photoreceptor dysfunction, and microglial cell activation in the affected areas. The phenotype described herein is similar to previously published reports of conditional gene knockouts that used the Trp1-Cre mouse, suggesting that Cre toxicity alone could account for some of the reported phenotypes and highlighting the importance of the inclusion of Cre-expressing mice as controls in conditional gene targeting studies.

The neuroprotective effect of pyrroloquinoline quinone on traumatic brain injury

Pyrroloquinoline quinone (PQQ) is a water-soluble, anionic, quinonoid substance that has been established as an essential nutrient in animals. Owing to the inherent properties of PQQ as an antioxidant and redox modulator in various systems, PQQ is expected to be used in pharmacological applications in the near future. Although many recent studies have investigated its neuroprotective effects, the effect of PQQ on traumatic brain injury (TBI) has not been examined. In this study we employed Morris water maze (MWM) training, the results of which showed that PQQ led to improved behavioral performance in post-TBI animals. Considering that many experiments have suggested that β-1,4-galactosyltransferase I (β-1,4-GalT-I) and -V play significant roles in inflammation and the nervous system, in the present study we used Western blot analysis to study the effect of PQQ on the expression of β-1,4-GalT-I and -V. We found apparent expression upregulation of β-1,4-GalT-I and -V after PQQ was systemically administered. Lectin-fluorescent staining with RCA-I also revealed that PQQ contributed to expression upregulation of the galactosidase β-1 (Gal β-1), 4-galactosyltransferase N-acylsphingosine (4-GlcNAc) group in microglia and neurons of the cortex and hippocampal CA2 region. In summary, our experiment established that PQQ may play an important role in recovery post-TBI.

Retinoic acid from retinal pigment epithelium induces T regulatory cells

Primary cultured retinal pigment epithelial (RPE) cells can convert T cells into T regulatory cells (Tregs) through inhibitory factor(s) including transforming growth factor β (TGFβ) in vitro. Retinoic acid (RA) enhances induction of CD4(+) Tregs in the presence of TGFβ. We investigated whether RA produced by RPE cells can promote generation of Tregs. We found that in vitro, RA-treated T cells expressed high levels of Foxp3 in the presence of recombinant TGFβ. In GeneChip analysis, cultured RPE cells constitutively expressed RA-associated molecules such as RA-binding proteins, enzymes, and receptors. RPE from normal mice, but not vitamin A-deficient mice, contained significant levels of TGFβ. RPE-induced Tregs from vitamin A-deficient mice failed to suppress activation of target T cells. Only a few Foxp3(+) T cells were found in intraocular cells from vitamin A-deficient experimental autoimmune uveitis (EAU) mice, whereas expression was higher in cells from normal EAU mice. RA receptor antagonist-pretreated or RA-binding protein-siRNA-transfected RPE cells failed to convert CD4(+) T cells into Tregs. Our data support the hypothesis that RPE cells produce RA, thereby enabling bystander T cells to be converted into Tregs through TGFβ promotion, which can then participate in the establishment of immune tolerance in the eye.

Eye morphogenesis and patterning of the optic vesicle

Organogenesis of the eye is a multistep process that starts with the formation of optic vesicles followed by invagination of the distal domain of the vesicles and the overlying lens placode resulting in morphogenesis of the optic cup. The late optic vesicle becomes patterned into distinct ocular tissues: the neural retina, retinal pigment epithelium (RPE), and optic stalk. Multiple congenital eye disorders, including anophthalmia or microphthalmia, aniridia, coloboma, and retinal dysplasia, stem from disruptions in embryonic eye development. Thus, it is critical to understand the mechanisms that lead to initial specification and differentiation of ocular tissues. An accumulating number of studies demonstrate that a complex interplay between inductive signals provided by tissue-tissue interactions and cell-intrinsic factors is critical to ensuring proper specification of ocular tissues as well as maintenance of RPE cell fate. While several of the extrinsic and intrinsic determinants have been identified, we are just at the beginning in understanding how these signals are integrated. In addition, we know very little about the actual output of these interactions. In this chapter, we provide an update of the mechanisms controlling the early steps of eye development in vertebrates, with emphasis on optic vesicle evagination, specification of neural retina and RPE at the optic vesicle stage, the process of invagination during morphogenesis of the optic cup, and maintenance of the RPE cell fate.

Neural progenitor potential in cultured Müller glia: effects of passaging and exogenous growth factor exposure

The Müller radial glial cell is the principal support cell of the adult mammalian retina. Recent reports suggest that these cells retain the capacity to proliferate, express phenotypes reminiscent of retinal progenitor cells (RPC) and generate neuron-like progeny. We isolated rodent Müller cells and generated cultures that could be passaged under conditions used in neural stem/progenitor cell colonies. We demonstrate that during the early period of primary culture, Müller glia proliferate into sphere colonies and express a select regimen of phenotypes normally seen in RPCs. This effect correlates temporally with the loss of retinal neurons post-dissection. When chronically maintained in vitro, Müller cells can be repeatedly passaged, and up-regulate early RPC phenotypes that are suggestive of cellular de-differentiation. Furthermore, exposure of Müller glial cultures to differentiating conditions containing growth factors stimulates Müller glia to up-regulate phenotypes associated with retinal neurons. These data provide further evidence that isolated, adult Müller glia retain functional and phenotypic features of RPCs.

Inducible expression of cre recombinase in the retinal pigmented epithelium

PURPOSE

The retinal pigmented epithelium (RPE) expresses many genes that play important roles in the support and maintenance of photoreceptors. The present study was conducted to develop a system amenable to the dissection of the temporal function of these genes, specifically within RPE cells. Transgenic mice were generated and characterized in which the expression of Cre recombinase could be specifically induced within the RPE.

METHODS

Transgenic mice carrying the human vitelliform macular dystrophy-2 (VMD2) promoter (P(VMD2))-directed reverse tetracycline-dependent transactivator (rtTA) and the tetracycline-responsive element (TRE)-directed cre were generated. Inducible Cre expression was achieved by feeding doxycycline to these mice and was characterized by using a Cre-activatable lacZ reporter mouse strain (R26R).

RESULTS

A beta-galactosidase assay of rtTA/Cre-R26R mice demonstrated that the basal level of Cre expression without doxycycline induction was negligible. Addition of doxycycline led to induction of RPE-specific Cre expression/function at least from embryonic day 9 to postnatal day 60. The highest induction occurred at approximately postnatal day 4. As measured by ERG and histology, retinal function and morphology were normal in 10-month-old rtTA/Cre mice that were treated with doxycycline at weaning age.

CONCLUSIONS

Transgenic mice were generated that express Cre recombinase in the RPE in an inducible fashion. These mice will be useful for studies of the RPE-specific role of genes that are expressed in the RPE as well as other cells, particularly for avoiding embryonic lethality and dissecting the function of genes that play dual roles in development and adulthood.

Retinal dysfunction and progressive retinal cell death in SOD1-deficient mice

The superoxide dismutase (SOD) family is a major antioxidant system, and deficiency of Cu,Zn-superoxide dismutase (SOD1) in mice leads to many different phenotypes that resemble accelerated aging. The purpose of this study was to examine the morphology and physiology of the sensory retina in Sod1(-/-) mice. The amplitudes of the a- and b-waves of electroretinograms elicited by stimuli of different intensity were reduced in senescent Sod1(-/-) mice, and this reduction in amplitude was more pronounced with increasing age. Retinal morphometric analyses showed a reduced number of nuclei in both the inner nuclear cell layer and outer nuclear cell layer. Electron microscopy revealed swollen cells and degenerated mitochondria in the inner nuclear cell and outer nuclear cell layer of senescent Sod1(-/-) mice indicating necrotic cell death. Terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling revealed no significant differences in the number of apoptotic cells between Sod1(-/-) and wild-type mice, and activated caspase-3 could not be detected in the retina of Sod1(-/-) mice. In addition to the age-related macular degeneration-like phenotypes previously reported, Sod1(-/-) mice also present progressive retinal degeneration. Our results indicate that Sod1(-/-) mice may be a good model system in which to study the mechanism of reactive oxygen species-mediated retinal degeneration.

Contribution of targeted conditional somatic mutagenesis to deciphering retinoid X receptor functions and to generating mouse models of human diseases

The last decade has witnessed an enormous rise in the interest for retinoid signalling and its cognate receptors, because of their central role in the coordination of development and homeostasis, through their ability to orchestrate the expression of numerous target genes. These receptors include six nuclear receptor (NR) family members, the retinoic acid receptor (RAR) alpha, beta and gamma, and the retinoid X receptor (RXR) alpha, beta and gamma, which are expressed in many cell types in mammals. Analysis of the development of mouse embryos bearing retinoid receptor null mutations demonstrated that these receptors transduce the effects of retinoic acid (RA, the active derivative of vitamin A) in vivo, and revealed impressive complexity. However, frequent redundancy in receptor functions and lethality of compound RAR-null mutants, as well as of RXRalpha-null mutants, precluded the characterisation of the functions of these receptors during late development and postnatally. We illustrate here how recent developments of conditional targeted somatic mutagenesis have opened new avenues in analysing the physiological functions of retinoid X receptor signalling in a variety of tissues and cell types, as well as in exploring the pathophysiological consequences of their alteration that led to novel mouse models of human diseases.

Dual roles for adenomatous polyposis coli in regulating retinoic acid biosynthesis and Wnt during ocular development

Congenital hypertrophy/hyperplasia of the retinal pigmented epithelium is an ocular lesion found in patients harboring mutations in the adenomatous polyposis coli (APC) tumor suppressor gene. We report that Apc-deficient zebrafish display developmental abnormalities of both the lens and retina. Injection of dominant-negative Lef reduced Wnt signaling in the lens but did not rescue retinal differentiation defects. In contrast, treatment of apc mutants with all-trans retinoic acid rescued retinal differentiation defects but had no apparent effect on the lens. We identified Rdh5 as a retina-specific retinol dehydrogenase controlled by APC. Morpholino knockdown of Rdh5 phenocopied the apc mutant retinal differentiation defects and was rescued by treatment with exogenous all-trans retinoic acid. Microarray analyses of apc mutants and Rdh5 morphants revealed a profound overlap in the transcriptional profile of these embryos. These findings support a model wherein Apc serves a dual role in regulating Wnt and retinoic acid signaling within the eye and suggest retinoic acid deficiency as an explanation for APC mutation-associated retinal defects such as congenital hypertrophy/hyperplasia of the retinal pigmented epithelium.

Spatial and temporal patterns of growth and differentiation of cone oil droplets in the chick retina

Avian cone photoreceptors have an oil droplet in the outer portion of their inner segment that acts as a long-pass cut-off filter between incident light and visual pigment. Chick cone droplets are mainly red, orange, yellow, green, and colorless, and the colors are due to three carotenoid pigments with characteristic absorption spectra. Little is known of the differentiation of this organelle, the natural marker of cones, and the little that is known is largely controversial. We used flat whole-mounts of fresh retinas to study the time and place of the appearance of droplets, their growth rates, the sequence of droplet color differentiation, and the spatial distribution of these colors. We show that droplet differentiation starts on embryonic Day 10 (E10) in a relatively small area above the optic nerve head. The differentiation spreads to the rest of the retina in a manner similar to that of photoreceptor neurogenesis, with three decreasing gradients of droplet size and color between E13-E20: from central to peripheral, dorsal to ventral, and temporal to nasal. The rate of growth of the droplets was not constant, but showed a maximum between E17 and postnatal Day 1 (P1) in most of the retinal zones. Color differentiation started at E16-E17, 5-6 days after their appearance, when the droplets were already of considerable size. Initially, all droplets were colorless, and then turned pale green or yellow to acquire progressively the mature colors. Differentiation ended in the whole retina by P15, with ventral droplets of larger diameter than dorsal ones, the peripheral ones generally larger than the central ones, and with the color distribution varying with the retinal area. Our results show that growth and color differentiation of the droplets is regulated temporally and spatially, and the cones complete differentiation at P15 rather than at prenatal stages, as is thought generally.