Retinoic acid receptors and retinoid X receptors in the mature retina: subtype determination and cellular distribution

Activation of retinoid X receptors protects retinal neurons and pigment epithelial cells from BMAA-induced death

Exposure to the non-protein amino acid cyanotoxin β-N-methylamino-L-alanine (BMAA), released by cyanobacteria found in many water reservoirs has been associated with neurodegenerative diseases. We previously demonstrated that BMAA induced cell death in both retina photoreceptors (PHRs) and amacrine neurons by triggering different molecular pathways, as activation of NMDA receptors and formation of carbamate-adducts was only observed in amacrine cell death. We established that activation of Retinoid X Receptors (RXR) protects retinal cells, including retina pigment epithelial (RPE) cells from oxidative stress-induced apoptosis. We now investigated the mechanisms underlying BMAA toxicity in these cells and those involved in RXR protection. BMAA addition to rat retinal neurons during early development in vitro increased reactive oxygen species (ROS) generation and polyADP ribose polymers (PAR) formation, while pre-treatment with serine (Ser) before BMAA addition decreased PHR death. Notably, RXR activation with the HX630 agonist prevented BMAA-induced death in both neuronal types, reducing ROS generation, preserving mitochondrial potential, and decreasing TUNEL-positive cells and PAR formation. This suggests that BMAA promoted PHR death by substituting Ser in polypeptide chains and by inducing polyADP ribose polymerase activation. BMAA induced cell death in ARPE-19 cells, a human epithelial cell line; RXR activation prevented this death, decreasing ROS generation and caspase 3/7 activity. These findings suggest that RXR activation prevents BMAA harmful effects on retinal neurons and RPE cells, supporting this activation as a broad-spectrum strategy for treating retina degenerations.

Association of Plasma Vitamins and Carotenoids, DNA Methylation of LCAT, and Risk of Age-Related Macular Degeneration

Dysregulation of lipid metabolism has been implicated in age-related macular degeneration (AMD), the leading cause of blindness among the elderly. Lecithin cholesterol acyltransferase (LCAT) is an important enzyme responsible for lipid metabolism, which could be regulated by DNA methylation during the development of various age-related diseases. This study aimed to assess the association between LCAT DNA methylation and the risk of AMD, and to examine whether plasma vitamin and carotenoid concentrations modified this association. A total of 126 cases of AMD and 174 controls were included in the present analysis. LCAT DNA methylation was detected by quantitative real-time methylation-1specific PCR (qMSP). Circulating vitamins and carotenoids were measured using reversed-phase high-performance liquid chromatography (RP-HPLC). DNA methylation of LCAT was significantly higher in patients with AMD than those in the control subjects. After multivariable adjustment, participants in the highest tertile of LCAT DNA methylation had a 5.37-fold higher risk (95% CI: 2.56, 11.28) of AMD compared with those in the lowest tertile. Each standard deviation (SD) increment of LCAT DNA methylation was associated with a 2.23-fold (95% CI: 1.58, 3.13) increased risk of AMD. There was a J-shaped association between LCAT DNA methylation and AMD risk (P_{non-linearity} = 0.03). Higher concentrations of plasma retinol and β-cryptoxanthin were significantly associated with decreased levels of LCAT DNA methylation, with the multivariate-adjusted β coefficient being -0.05 (95% CI: -0.08, -0.01) and -0.25 (95% CI: -0.42, -0.08), respectively. In joint analyses of LCAT DNA methylation and plasma vitamin and carotenoid concentrations, the inverse association between increased LCAT DNA methylation and AMD risk was more pronounced among participants who had a lower concentration of plasma retinol and β-cryptoxanthin. These findings highlight the importance of comprehensively assessing LCAT DNA methylation and increasing vitamin and carotenoid status for the prevention of AMD.

Disease-causing mutations in genes encoding transcription factors critical for photoreceptor development

Photoreceptor development of the vertebrate visual system is controlled by a complex transcription regulatory network. OTX2 is expressed in the mitotic retinal progenitor cells (RPCs) and controls photoreceptor genesis. CRX that is activated by OTX2 is expressed in photoreceptor precursors after cell cycle exit. NEUROD1 is also present in photoreceptor precursors that are ready to specify into rod and cone photoreceptor subtypes. NRL is required for the rod fate and regulates downstream rod-specific genes including the orphan nuclear receptor NR2E3 which further activates rod-specific genes and simultaneously represses cone-specific genes. Cone subtype specification is also regulated by the interplay of several transcription factors such as THRB and RXRG. Mutations in these key transcription factors are responsible for ocular defects at birth such as microphthalmia and inherited photoreceptor diseases such as Leber congenital amaurosis (LCA), retinitis pigmentosa (RP) and allied dystrophies. In particular, many mutations are inherited in an autosomal dominant fashion, including the majority of missense mutations in CRX and NRL. In this review, we describe the spectrum of photoreceptor defects that are associated with mutations in the above-mentioned transcription factors, and summarize the current knowledge of molecular mechanisms underlying the pathogenic mutations. At last, we deliberate the outstanding gaps in our understanding of the genotype-phenotype correlations and outline avenues for future research of the treatment strategies.

Retinoic acid inhibitors mitigate vision loss in a mouse model of retinal degeneration

Rod and cone photoreceptors degenerate in retinitis pigmentosa (RP). While downstream neurons survive, they undergo physiological changes, including accelerated spontaneous firing in retinal ganglion cells (RGCs). Retinoic acid (RA) is the molecular trigger of RGC hyperactivity, but whether this interferes with visual perception is unknown. Here, we show that inhibiting RA synthesis with disulfiram, a deterrent of human alcohol abuse, improves behavioral image detection in vision-impaired mice. In vivo Ca2+ imaging shows that disulfiram sharpens orientation tuning of visual cortical neurons and strengthens fidelity of responses to natural scenes. An RA receptor inhibitor also reduces RGC hyperactivity, sharpens cortical representations, and improves image detection. These findings suggest that photoreceptor degeneration is not the only cause of vision loss in RP. RA-induced corruption of retinal information processing also degrades vision, pointing to RA synthesis and signaling inhibitors as potential therapeutic tools for improving sight in RP and other retinal degenerative disorders.

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.

Identification and Integrated Analysis of MicroRNA and mRNA Expression Profiles During Agonistic Behavior in Chinese Mitten Crab (Eriocheir sinensis) Using a Deep Sequencing Approach

As a commercially important species, the Chinese mitten crab (Eriocheir sinensis) has been cultured for a long time in China. Agonistic behavior often causes limb disability and requires much energy, which is harmful to the growth and survival of crabs. In this paper, we divided crabs into a control group (control, no treatment) and an experimental group (fight, agonistic behavior after 1 h) and then collected the thoracic ganglia (TG) to extract RNA. Subsequently, we first used a deep sequencing approach to examine the transcripts of microRNAs (miRNAs) and messenger RNAs (mRNAs) in E. sinensis displaying agonistic behavior. According to the results, we found 29 significant differentially expressed miRNAs (DEMs) and 116 significant differentially expressed unigenes (DEGs). The DEMs esi-miR-199a-5p, esi-let-7d, esi-miR-200a, and esi-miR-200b might participate in the regulation of agonistic behavior by mediating neuroregulation and energy metabolism. Focusing on the transcripts of the mRNAs, the renin–angiotensin system (RAS) Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway might be involved in the regulation of agonistic behavior through glucose metabolism as this pathway was significantly enriched with DEGs. Besides, an integrated analysis of the miRNA and mRNA profiles revealed that the retinoid X receptor (RXR) was also involved in visual signal transduction, which was important for agonistic behavior. In addition, four vital agonistic behavior-related metabolic pathways, including the cAMP signaling, MAPK, protein digestion and absorption, and fatty acid metabolism pathways, were significantly enriched with the predicted target unigenes. In conclusion, the findings of this study might provide important insight enhancing our understanding of the underlying molecular mechanisms of agonistic behavior in E. sinensis.

Retinoic Acid Induces Hyperactivity, and Blocking Its Receptor Unmasks Light Responses and Augments Vision in Retinal Degeneration

Light responses are initiated in photoreceptors, processed by interneurons, and synaptically transmitted to retinal ganglion cells (RGCs), which send information to the brain. Retinitis pigmentosa (RP) is a blinding disease caused by photoreceptor degeneration, depriving downstream neurons of light-sensitive input. Photoreceptor degeneration also triggers hyperactive firing of RGCs, obscuring light responses initiated by surviving photoreceptors. Here we show that retinoic acid (RA), signaling through its receptor (RAR), is the trigger for hyperactivity. A genetically encoded reporter shows elevated RAR signaling in degenerated retinas from murine RP models. Enhancing RAR signaling in healthy retinas mimics the pathophysiology of degenerating retinas. Drug inhibition of RAR reduces hyperactivity in degenerating retinas and unmasks light responses in RGCs. Gene therapy inhibition of RAR increases innate and learned light-elicited behaviors in vision-impaired mice. Identification of RAR as the trigger for hyperactivity presents a degeneration-dependent therapeutic target for enhancing low vision in RP and other blinding disorders.

Toxicity of triphenyltin on the development of retinal axons in zebrafish at low dose

The impacts of triphenyltin (TPT) on ecological health are of particular concern due to the unexpectedly high levels found in wild fish around the world. Here, zebrafish embryos were exposed to TPT via in ovo nano-injection to study its toxicity on the development of retinal axons in fish. Lipophilic dye labeling revealed obvious defects in retinal axon development in larvae with normally shaped eyes, with incidences of 0, 1.08%, 2.66%, 4.26%, and 6.85% observed in the control, 0.8, 4.0, 20.0, and 100ng TPT-Cl/g wet weight (ww) exposure groups, respectively, showing a dose-dependent increase. Since the lowest observable effective concentration of TPT to induce retinal axon development defects was 0.8ng TPT-Cl/g ww, which is lower than the concentrations in wild fish eggs, this defect would occur in wild fish larvae. Alterations in the expressions of pax6 and ephrinBs, which regulate the establishment of retinal polarity, were correlated with defect incidence. Expression levels of the CYP26A1 gene and protein were significantly up-regulated in all exposure groups compared with the control, which may lead to significant decreases in concentrations of all-trans retinoic acid (atRA). Such a disruption of RA metabolism would, at least partly, contribute to the incidence of developmental defects in retinal axons. This study is the first to report that TPT can interfere with development of retinal axons in fish at low dose.

Isolated and Syndromic Retinal Dystrophy Caused by Biallelic Mutations in RCBTB1, a Gene Implicated in Ubiquitination

Inherited retinal dystrophies (iRDs) are a group of genetically and clinically heterogeneous conditions resulting from mutations in over 250 genes. Here, homozygosity mapping and whole-exome sequencing (WES) in a consanguineous family revealed a homozygous missense mutation, c.973C>T (p.His325Tyr), in RCBTB1. In affected individuals, it was found to segregate with retinitis pigmentosa (RP), goiter, primary ovarian insufficiency, and mild intellectual disability. Subsequent analysis of WES data in different cohorts uncovered four additional homozygous missense mutations in five unrelated families in whom iRD segregates with or without syndromic features. Ocular phenotypes ranged from typical RP starting in the second decade to chorioretinal dystrophy with a later age of onset. The five missense mutations affect highly conserved residues either in the sixth repeat of the RCC1 domain or in the BTB1 domain. A founder haplotype was identified for mutation c.919G>A (p.Val307Met), occurring in two families of Mediterranean origin. We showed ubiquitous mRNA expression of RCBTB1 and demonstrated predominant RCBTB1 localization in human inner retina. RCBTB1 was very recently shown to be involved in ubiquitination, more specifically as a CUL3 substrate adaptor. Therefore, the effect on different components of the CUL3 and NFE2L2 (NRF2) pathway was assessed in affected individuals’ lymphocytes, revealing decreased mRNA expression of NFE2L2 and several NFE2L2 target genes. In conclusion, our study puts forward mutations in RCBTB1 as a cause of autosomal-recessive non-syndromic and syndromic iRD. Finally, our data support a role for impaired ubiquitination in the pathogenetic mechanism of RCBTB1 mutations.

Self-formation of optic cups and storable stratified neural retina from human ESCs

In this report, we demonstrate that an optic cup structure can form by self-organization in human ESC culture. The human ESC-derived optic cup is much larger than the mouse ESC-derived one, presumably reflecting the species differences. The neural retina in human ESC culture is thick and spontaneously curves in an apically convex manner, which is not seen in mouse ESC culture. In addition, human ESC-derived neural retina grows into multilayered tissue containing both rods and cones, whereas cone differentiation is rare in mouse ESC culture. The accumulation of photoreceptors in human ESC culture can be greatly accelerated by Notch inhibition. In addition, we show that an optimized vitrification method enables en bloc cryopreservation of stratified neural retina of human origin. This storage method at an intermediate step during the time-consuming differentiation process provides a versatile solution for quality control in large-scale preparation of clinical-grade retinal tissues.

Thyroid hormone controls cone opsin expression in the retina of adult rodents

Mammalian retinas display an astonishing diversity in the spatial arrangement of their spectral cone photoreceptors, probably in adaptation to different visual environments. Opsin expression patterns like the dorsoventral gradients of short-wave-sensitive (S) and middle- to long-wave-sensitive (M) cone opsin found in many species are established early in development and thought to be stable thereafter throughout life. In mouse early development, thyroid hormone (TH), through its receptor TRβ2, is an important regulator of cone spectral identity. However, the role of TH in the maintenance of the mature cone photoreceptor pattern is unclear. We here show that TH also controls adult cone opsin expression. Methimazole-induced suppression of serum TH in adult mice and rats yielded no changes in cone numbers but reversibly altered cone patterns by activating the expression of S-cone opsin and repressing the expression of M-cone opsin. Furthermore, treatment of athyroid Pax8(-/-) mice with TH restored a wild-type pattern of cone opsin expression that reverted back to the mutant S-opsin-dominated pattern after termination of treatment. No evidence for cone death or the generation of new cones from retinal progenitors was found in retinas that shifted opsin expression patterns. Together, this suggests that opsin expression in terminally differentiated mammalian cones remains subject to control by TH, a finding that is in contradiction to previous work and challenges the current view that opsin identity in mature mammalian cones is fixed by permanent gene silencing.

Retinal light damage: mechanisms and protection

By its action on rhodopsin, light triggers the well-known visual transduction cascade, but can also induce cell damage and death through phototoxic mechanisms - a comprehensive understanding of which is still elusive despite more than 40 years of research. Herein, we integrate recent experimental findings to address several hypotheses of retinal light damage, premised in part on the close anatomical and metabolic relationships between the photoreceptors and the retinal pigment epithelium. We begin by reviewing the salient features of light damage, recently joined by evidence for retinal remodeling which has implications for the prognosis of recovery of function in retinal degenerations. We then consider select factors that influence the progression of the damage process and the extent of visual cell loss. Traditional, genetically modified, and emerging animal models are discussed, with particular emphasis on cone visual cells. Exogenous and endogenous retinal protective factors are explored, with implications for light damage mechanisms and some suggested avenues for future research. Synergies are known to exist between our long term light environment and photoreceptor cell death in retinal disease. Understanding the molecular mechanisms of light damage in a variety of animal models can provide valuable insights into the effects of light in clinical disorders and may form the basis of future therapies to prevent or delay visual cell loss.

An RPE cell line as a useful in vitro model for studying retinoic acid receptor beta: expression and affinity

Retinoids mediate their biological effect by interacting with specific nuclear receptors. Of the several known RAR (retinoic acid receptor) subtypes, RAR-beta is of particular interest, since its expression is silenced in many cancers and it is believed to be a tumour suppressor. Specific ligands of RAR-beta can potentially be used in anti-cancer therapy. In the present study, we have investigated the feasibility of using HRPE cells (human retinal pigment epithelial cells) as an experimental model for characterizing RAR-beta-ligand interaction. RT-PCR (reverse transcription-PCR) and Western blot analyses show that HRPE cells specifically express only RAR-beta and none of the other receptor subtypes. In addition, we show that the expression of RAR-beta increases with increasing passage number of the cells. Interestingly, the increase in RAR-beta expression is not associated with telomere shortening, a typical biomarker of cellular senescence. In the present study, we also describe a protocol for characterizing RAR-beta-ligand interactions using nuclear extract from late passage HRPE cells as a source of endogenous RAR-beta. Using [(3)H]CD367 as the ligand, RAR-beta in HRPE cells showed an affinity of 9.6 +/- 0.6 nM and a B(max) of 780 +/- 14 fmol/mg of protein. We have confirmed the feasibility of using this assay to detect the interaction of ligands with RAR-beta by investigating the ability of certain flavonoids to inhibit the binding of [(3)H]CD367 to nuclear extracts from HRPE cells. The inhibition constant of the flavonoids for RAR-beta was between approx. 1-30 microM, showing that the flavonoids interact with RAR-beta with low affinity.

Regulation of photoreceptor gene expression by Crx-associated transcription factor network

Rod and cone photoreceptors in the mammalian retina are special types of neurons that are responsible for phototransduction, the first step of vision. Development and maintenance of photoreceptors require precisely regulated gene expression. This regulation is mediated by a network of photoreceptor transcription factors centered on Crx, an Otx-like homeodomain transcription factor. The cell type (subtype) specificity of this network is governed by factors that are preferentially expressed by rods or cones or both, including the rod-determining factors neural retina leucine zipper protein (Nrl) and the orphan nuclear receptor Nr2e3; and cone-determining factors, mostly nuclear receptor family members. The best-documented of these include thyroid hormone receptor beta2 (Tr beta2), retinoid related orphan receptor Ror beta, and retinoid X receptor Rxr gamma. The appropriate function of this network also depends on general transcription factors and cofactors that are ubiquitously expressed, such as the Sp zinc finger transcription factors and STAGA co-activator complexes. These cell type-specific and general transcription regulators form complex interactomes; mutations that interfere with any of the interactions can cause photoreceptor development defects or degeneration. In this manuscript, we review recent progress on the roles of various photoreceptor transcription factors and interactions in photoreceptor subtype development. We also provide evidence of auto-, para-, and feedback regulation among these factors at the transcriptional level. These protein-protein and protein-promoter interactions provide precision and specificity in controlling photoreceptor subtype-specific gene expression, development, and survival. Understanding these interactions may provide insights to more effective therapeutic interventions for photoreceptor diseases.

Role of all-trans retinoic acid in neurite outgrowth and axonal elongation

The vitamin A metabolite, all-trans retinoic acid (atRA) plays essential roles in nervous system development, including neuronal patterning, survival, and neurite outgrowth. Our understanding of how the vitamin A acid functions in neurite outgrowth comes largely from cultured embryonic neurons and model neuronal cell systems including human neuroblastoma cells. Specifically, atRA has been shown to increase neurite outgrowth from embryonic DRG, sympathetic, spinal cord, and olfactory receptor neurons, as well as dissociated cerebra and retina explants. A role for atRA in axonal elongation is also supported by a limited number of studies in vivo, in which a deficiency in retinoid signaling produced either by dietary or genetic means has been shown to alter neurite outgrowth from the spinal cord and hindbrain regions. Human neuroblastoma cells also show enhanced numbers of neurites and longer processes in response to atRA. The mechanism whereby retinoids regulate neurite outgrowth includes, but is not limited to, the regulation of the transcription of neurotrophin receptors. More recent evidence supports a role for atRA in regulating components of other signaling pathways or candidate neurite-regulating factors. Some of these effects, such as that on neuron navigator 2 (NAV2), may be direct, whereas others may be secondary to other atRA-induced changes in the cell. This review focuses on what is currently known about neurite initiation and growth, with emphasis on the manner in which atRA may influence these events.

Retinoic acid regulates the expression of photoreceptor transcription factor NRL

NRL (neural retina leucine zipper) is a key basic motif-leucine zipper (bZIP) transcription factor, which orchestrates rod photoreceptor differentiation by activating the expression of rod-specific genes. The deletion of Nrl in mice results in functional cones that are derived from rod precursors. However, signaling pathways modulating the expression or activity of NRL have not been elucidated. Here, we show that retinoic acid (RA), a diffusible factor implicated in rod development, activates the expression of NRL in serum-deprived Y79 human retinoblastoma cells and in primary cultures of rat and porcine photoreceptors. The effect of RA is mimicked by TTNPB, a RA receptor agonist, and requires new protein synthesis. DNaseI footprinting and electrophoretic mobility shift assays (EMSA) using bovine retinal nuclear extract demonstrate that RA response elements (RAREs) identified within the Nrl promoter bind to RA receptors. Furthermore, in transiently transfected Y79 and HEK293 cells the activity of Nrl-promoter driving a luciferase reporter gene is induced by RA, and this activation is mediated by RAREs. Our data suggest that signaling by RA via RA receptors regulates the expression of NRL, providing a framework for delineating early steps in photoreceptor cell fate determination.

Retinoid X receptor regulates Nur77/TR3-dependent apoptosis [corrected] by modulating its nuclear export and mitochondrial targeting

Retinoid X receptor (RXR) plays a central role in the regulation of intracellular receptor signaling pathways by acting as a ubiquitous heterodimerization partner of many nuclear receptors, including the orphan receptor Nur77 (also known as TR3 [corrected] or NGFI-B), which translocates from the nucleus to mitochondria, where it interacts with Bcl-2 to induce apoptosis. Here, we report that RXRalpha is required for nuclear export and mitochondrial targeting of Nur77 through their unique heterodimerization that is mediated by dimerization interfaces located in their DNA-binding domain. The effects of RXRalpha are attributed to a putative nuclear export sequence (NES) present in its carboxyl-terminal region. RXRalpha ligands suppress NES activity by inducing RXRalpha homodimerization or altering RXRalpha/Nur77 heterodimerization. The RXRalpha NES is also silenced by RXRalpha heterodimerization with retinoic acid receptor or vitamin D receptor. Consistently, we were able to show that the mitochondrial targeting of the RXRalpha/Nur77 heterodimer and its induction of apoptosis are potently inhibited by RXR ligands. Together, our results reveal a novel nongenotropic function of RXRalpha and its involvement in the regulation of the Nur77-dependent apoptotic pathway [corrected]

Characterization of retinoic acid neuromodulation in the carp retina

Visual sensation in vertebrates starts with the isomerization of 11-cis retinaldehyde into all-trans retinaldehyde. Aldehyde dehydrogenases, present in the pigment epithelium and some retinal cells, convert all-trans retinaldehyde into all-trans retinoic acid (at-RA). Evidence in the retina and the hippocampus has accumulated, showing that at-RA, besides being a morphogenetic factor, also acts as a neuromodulator. In mature retina, at-RA affects visual processing by acting on gap junctional conductances and the synaptic transfer between photoreceptors and horizontal cells. We present evidence supporting a neuromodulatory role of at-RA in the carp retina. High performance liquid chromatography (HPLC) measurements and an RA bioassay indicate a light dependency of at-RA formation, which can explain the observed effects of at-RA on spinule formation at horizontal cell dendrites in this retina. Furthermore, inhibiting endogenous metabolism and catabolism of at-RA affects formation and persistence of spinules in a way, supporting a direct involvement of at-RA in this light-dependent mechanism of synaptic plasticity. The action of at-RA, however, seems independent of the dopaminergic system, known for its light-signaling role in the retina, because at-RA effects on spinule formation persisted in retina depleted of dopaminergic neurons or in the presence of haloperidol. Together, these data indicate that at-RA acts effectively as a direct neuromodulator in carp retina, transmitting information about ambient light conditions to the neuronal retina.

Vitamin D receptor and retinoid X receptor interactions in motion

Vitamin D receptor (VDR) and retinoid X receptor (RXR) are members of the nuclear receptor superfamily and they bind target DNA sequences as heterodimers to regulate transcription. This article surveys the latest findings regarding the roles of dimerizing RXR in VDR function and emphasizes potential areas for future developments. We first highlight the importance of dimerization with RXR for both the ligand-independent (hair growth) and ligand-dependent functions of VDR (calcium homeostasis, bone development and mineralization, control of cell growth and differentiation). Emerging information regarding the regulatory control of dimerization based on biochemical, structural, and genetic studies is then presented. Finally, the main focus of this article is a new dynamic perspective of dimerization functions, based on recent research with fluorescent protein chimeras in living cells by microscopy. These studies revealed that both VDR and RXR constantly shuttle between the cytoplasm and the nucleus and between subnuclear compartments, and showed the transient nature of receptor--DNA and receptor--coregulator interactions. Because RXR dimerizes with most of the nuclear receptors, regulation of receptor dynamics by RXR has a broad significance.

Thyroid-beta2 and the retinoid RAR-alpha, RXR-gamma and ROR-beta2 receptor mRNAs; expression profiles in mouse retina, retinal explants and neocortex

In neonatal retinal explants cultured long-term green cones are missing. Recently it was reported that thyroid hormone beta2 receptors (TR-beta2) are essential for these green cones to differentiate. Therefore transcript level of these receptors was investigated in our mouse retinal explants. However, thyroid receptors function as heterodimers with retinoid receptors (RR); so the fate of selected RRs was similarly analyzed using semi-quantitative RT-PCR. Loss of TR-beta2 and RR (RXR-gamma and ROR-beta2) mRNAs was observed after culturing the neonatal retina for 12 days. This indicates that these proteins are involved in determination of green cone identity. In addition, levels of the selected RR transcripts are differentially affected by short- or long-term culture. In the latter case an attached retinal pigment epithelium seems to play a protective role. Furthermore, divergent diurnal peaks of RR mRNAs are present in young as well as aged mouse retina and neocortex. This data might be relevant in the context of human ageing disorders.

Dimerization with retinoid X receptors promotes nuclear localization and subnuclear targeting of vitamin D receptors

The vitamin D receptor (VDR) acts as heterodimer with the retinoid X receptor alpha (RXR) to control transcriptional activity of target genes. To explore the influence of heterodimerization on the subcellular distribution of these receptors in living cells, we developed a series of fluorescent-protein chimeras. The steady-state distribution of the yellow fluorescent protein-RXR was more nuclear than the unliganded green fluorescent protein (GFP)-VDR. Coexpression of RXR-blue fluorescent protein (BFP) promoted nuclear accumulation of GFP-VDR by influencing both nuclear import and retention. Fluorescence resonance energy transfer microscopy (FRET) demonstrated that the unliganded GFP-VDR and RXR-BFP form heterodimers. The increase in nuclear heterodimer content correlated with an increase in basal transcriptional activity. FRET also revealed that calcitriol induces formation of multiple nuclear foci of heterodimers. Mutational analysis showed a correlation between hormone-dependent nuclear VDR foci formation and DNA binding. RXR-BFP also promoted hormone-dependent nuclear accumulation and intranuclear foci formation of a nuclear localization signal mutant receptor (nlsGFP-VDR) and rescued its transcriptional activity. Heterodimerization mutant RXR failed to alter GFP-VDR and nlsGFP-VDR distribution or activity. These experiments suggest that RXR has a profound effect on VDR distribution. This effect of RXR to promote nuclear accumulation and intranuclear targeting contributes to the regulation of VDR activity and probably the activity of other heterodimerization partners.

Direct gating by retinoic acid of retinal electrical synapses

Retinoic acid (RA), a signaling molecule derived from vitamin A, controls growth and differentiation of a variety of cell types through regulation of gene transcription. In the vertebrate retina, RA also regulates gap junction-mediated physiological coupling of retinal neurons through a nontranscriptional mechanism. Here we report that RA rapidly and specifically modulates synaptic transmission at electrical synapses of cultured retinal horizontal cells through an external RAR(beta)(/gamma)-like binding site, the action of which is independent of second messenger cascades. External application of all-trans retinoic acid (at-RA) reversibly reduced the amplitude of gap junctional conductance in a dose-dependent manner, but failed to affect non-gap-junctional channels, including glutamate receptors. In contrast, internal dialysis with at-RA was ineffective, indicating an external site of action. Selective RAR(beta)(/gamma) ligands, but not an RAR(alpha)-selective agonist, mimicked the action of at-RA, suggesting that gating of gap junctional channels is mediated through an RAR(beta)(/gamma)-like binding site. At-RA did not act on gap junctional conductance by lowering [pH](i) or by increasing [Ca(2+)](i). A G protein inhibitor and protein kinase inhibitors did not block at-RA uncoupling effects indicating no second messenger systems were involved. Direct action of at-RA on gap junction channels was further supported by its equivalent action on whole-cell hemi-gap-junctional currents and on cell-free excised patch hemichannel currents. At-RA significantly reduced single-channel open probability but did not change unitary conductance. Overall, the results indicate that RA modulates horizontal cell electrical synapses by activation of novel nonnuclear RAR(beta)(/gamma)-like sites either directly on, or intimately associated with, gap junction channels.

Disruption of the 11-cis-retinol dehydrogenase gene leads to accumulation of cis-retinols and cis-retinyl esters

To elucidate the possible role of 11-cis-retinol dehydrogenase in the visual cycle and/or 9-cis-retinoic acid biosynthesis, we generated mice carrying a targeted disruption of the 11-cis-retinol dehydrogenase gene. Homozygous 11-cis-retinol dehydrogenase mutants developed normally, including their retinas. There was no appreciable loss of photoreceptors. Recently, mutations in the 11-cis-retinol dehydrogenase gene in humans have been associated with fundus albipunctatus. In 11-cis-retinol dehydrogenase knockout mice, the appearance of the fundus was normal and punctata typical of this human hereditary ocular disease were not present. A second typical symptom associated with this disease is delayed dark adaptation. Homozygous 11-cis-retinol dehydrogenase mutants showed normal rod and cone responses. 11-cis-Retinol dehydrogenase knockout mice were capable of dark adaptation. At bleaching levels under which patients suffering from fundus albipunctatus could be detected unequivocally, 11-cis-retinol dehydrogenase knockout animals displayed normal dark adaptation kinetics. However, at high bleaching levels, delayed dark adaptation in 11-cis-retinol dehydrogenase knockout mice was noticed. Reduced 11-cis-retinol oxidation capacity resulted in 11-cis-retinol/13-cis-retinol and 11-cis-retinyl/13-cis-retinyl ester accumulation. Compared with wild-type mice, a large increase in the 11-cis-retinyl ester concentration was noticed in 11-cis-retinol dehydrogenase knockout mice. In the murine retinal pigment epithelium, there has to be an additional mechanism for the biosynthesis of 11-cis-retinal which partially compensates for the loss of the 11-cis-retinol dehydrogenase activity. 11-cis-Retinyl ester formation is an important part of this adaptation process. Functional consequences of the loss of 11-cis-retinol dehydrogenase activity illustrate important differences in the compensation mechanisms between mice and humans. We furthermore demonstrate that upon 11-cis-retinol accumulation, the 13-cis-retinol concentration also increases. This retinoid is inapplicable to the visual processes, and we therefore speculate that it could be an important catabolic metabolite and its biosynthesis could be part of a process involved in regulating 11-cis-retinol concentrations within the retinal pigment epithelium of 11-cis-retinol dehydrogenase knockout mice.

Characterization of human retinal fascin gene (FSCN2) at 17q25: close physical linkage of fascin and cytoplasmic actin genes

Retinal fascin is a newly identified photoreceptor-specific paralog of the actin-bundling protein fascin. Fascins crosslink f-actin into highly ordered bundles within dynamic cell extensions such as neuronal growth cone filopodia. We have isolated cDNA and genomic clones of human retinal fascin and characterized the structure of the human retinal fascin gene (FSCN2). The cDNA predicts a protein of 492 amino acids and molecular mass 55,057 that shows 94% identity to bovine retinal fascin and 56% identity to human fascin. Promoter analysis reveals a consensus retinoic acid response element and several potential binding sites for transcription factors Crx and Nrl, which correlates with the retina-specific expression of FSCN2 mRNA. Fluorescence in situ hybridization analysis and genomic clone sequencing indicate that the FSCN2 gene lies within 200 kb of the actin gene ACTG1 at 17q25. Database searches revealed that the human fascin gene FSCN1 and actin gene ACTB at 7p22 also coexist within a 200-kb genomic clone. The close physical linkage of these fascin/actin gene pairs suggests that they derive from a common gene duplication event and allows comparison of fascin and actin phylogenetic analyses. Finally, a possible link to the retinitis pigmentosa 17 allele (RP17) at distal 17q was excluded by demonstration of multiple independent segregation events in two RP17 kindreds. Informative FSCN2 polymorphisms were identified and will serve as useful markers in future linkage studies. The likely function of retinal fascin, in light of known fascin roles in other cell types, is to assemble actin microfilaments in support of photoreceptor disk morphogenesis.