Retinoid X receptor ? gene transcripts are expressed by a subset of early generated retinal cells and eventually restricted to photoreceptors

A framework to identify functional interactors that contribute to disrupted early retinal development in Vsx2 ocular retardation J mice

BACKGROUND

A goal of developmental genetics is to identify functional interactions that underlie phenotypes caused by mutations. We sought to identify functional interactors of Vsx2, which when mutated, disrupts early retinal development. We utilized the Vsx2 loss-of-function mouse, ocular retardation J (orJ), to assess interactions based on principles of positive and negative epistasis as applied to bulk transcriptome data. This was first tested in vivo with Mitf, a target of Vsx2 repression, and then to cultures of orJ retina treated with inhibitors of Retinoid-X Receptors (RXR) to target Rxrg, an up-regulated gene in the orJ retina, and gamma-Secretase, an enzyme required for Notch signaling, a key mediator of retinal proliferation and neurogenesis.

RESULTS

Whereas Mitf exhibited robust positive epistasis with Vsx2, it only partially accounts for the orJ phenotype, suggesting other functional interactors. RXR inhibition yielded minimal evidence for epistasis between Vsx2 and Rxrg. In contrast, gamma-Secretase inhibition caused hundreds of Vsx2-dependent genes associated with proliferation to deviate further from wild-type, providing evidence for convergent negative epistasis with Vsx2 in regulating tissue growth.

CONCLUSIONS

Combining in vivo and ex vivo testing with transcriptome analysis revealed quantitative and qualitative characteristics of functional interaction between Vsx2, Mitf, RXR, and gamma-Secretase activities.

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.

Lineage tracing analysis of cone photoreceptor associated cis-regulatory elements in the developing chicken retina

During vertebrate retinal development, transient populations of retinal progenitor cells with restricted cell fate choices are formed. One of these progenitor populations expresses the Thrb gene and can be identified by activity of the ThrbCRM1 cis-regulatory element. Short-term assays have concluded that these cells preferentially generate cone photoreceptors and horizontal cells, however developmental timing has precluded an extensive cell type characterization of their progeny. Here we describe the development and validation of a recombinase-based lineage tracing system for the chicken embryo to further characterize the lineage of these cells. The ThrbCRM1 element was found to preferentially form photoreceptors and horizontal cells, as well as a small number of retinal ganglion cells. The photoreceptor cell progeny are exclusively cone photoreceptors and not rod photoreceptors, confirming that ThrbCRM1 progenitor cells are restricted from the rod fate. In addition, specific subtypes of horizontal cells and retinal ganglion cells were overrepresented, suggesting that ThrbCRM1 progenitor cells are not only restricted for cell type, but for cell subtype as well.

Mechanisms of Photoreceptor Patterning in Vertebrates and Invertebrates

Across the animal kingdom, visual systems have evolved to be uniquely suited to the environments and behavioral patterns of different species. Visual acuity and color perception depend on the distribution of photoreceptor (PR) subtypes within the retina. Retinal mosaics can be organized into three broad categories: stochastic/regionalized, regionalized, and ordered. We describe here the retinal mosaics of flies, zebrafish, chickens, mice, and humans, and the gene regulatory networks controlling proper PR specification in each. By drawing parallels in eye development between these divergent species, we identify a set of conserved organizing principles and transcriptional networks that govern PR subtype differentiation.

Single cell transcriptome profiling of developing chick retinal cells

The vertebrate retina is a specialized photosensitive tissue comprised of six neuronal and one glial cell types, each of which develops in prescribed proportions at overlapping timepoints from a common progenitor pool. While each of these cells has a specific function contributing to proper vision in the mature animal, their differential representation in the retina as well as the presence of distinctive cellular subtypes makes identifying the transcriptomic signatures that lead to each retinal cell's fate determination and development challenging. We have analyzed transcriptomes from individual cells isolated from the chick retina throughout retinogenesis. While we focused our efforts on the retinal ganglion cells, our transcriptomes of developing chick cells also contained representation from multiple retinal cell types, including photoreceptors and interneurons at different stages of development. Most interesting was the identification of transcriptomes from individual mixed lineage progenitor cells in the chick as these cells offer a window into the cell fate decision-making process. Taken together, these data sets will enable us to uncover the most critical genes acting in the steps of cell fate determination and early differentiation of various retinal cell types.

Light, lipids and photoreceptor survival: live or let die?

Due to its constant exposure to light and its high oxygen consumption the retina is highly sensitive to oxidative damage, which is a common factor in inducing the death of photoreceptors after light damage or in inherited retinal degenerations. The high content of docosahexaenoic acid (DHA), the major polyunsaturated fatty acid in the retina, has been suggested to contribute to this sensitivity. DHA is crucial for developing and preserving normal visual function. However, further roles of DHA in the retina are still controversial. Current data support that it can tilt the scale either towards degeneration or survival of retinal cells. DHA peroxidation products can be deleterious to the retina and might lead to retinal degeneration. However, DHA has also been shown to act as, or to be the source of, a survival molecule that protects photoreceptors and retinal pigment epithelium cells from oxidative damage. We have established that DHA protects photoreceptors from oxidative stress-induced apoptosis and promotes their differentiation in vitro. DHA activates the retinoid X receptor (RXR) and the ERK/MAPK pathway, thus regulating the expression of anti and pro-apoptotic proteins. It also orchestrates a diversity of signaling pathways, modulating enzymatic pathways that control the sphingolipid metabolism and activate antioxidant defense mechanisms to promote photoreceptor survival and development. A deeper comprehension of DHA signaling pathways and context-dependent behavior is required to understand its dual functions in retinal physiology.

Transcriptome profiling of developing photoreceptor subtypes reveals candidate genes involved in avian photoreceptor diversification

Avian photoreceptors are a diverse class of neurons, comprised of four single cones, the two members of the double cone, and rods. The signaling events and transcriptional regulators driving the differentiation of these diverse photoreceptors are largely unknown. In addition, many distinctive features of photoreceptor subtypes, including spectral tuning, oil droplet size and pigmentation, synaptic targets, and spatial patterning, have been well characterized, but the molecular mechanisms underlying these attributes have not been explored. To identify genes specifically expressed in distinct chicken (Gallus gallus) photoreceptor subtypes, we developed fluorescent reporters that label photoreceptor subpopulations, isolated these subpopulations by using fluorescence-activated cell sorting, and subjected them to next-generation sequencing. By comparing the expression profiles of photoreceptors labeled with rhodopsin, red opsin, green opsin, and violet opsin reporters, we have identified hundreds of differentially expressed genes that may underlie the distinctive features of these photoreceptor subtypes. These genes are involved in a variety of processes, including phototransduction, transcriptional regulation, cell adhesion, maintenance of intra- and extracellular structure, and metabolism. Of particular note are a variety of differentially expressed transcription factors, which may drive and maintain photoreceptor diversity, and cell adhesion molecules, which may mediate spatial patterning of photoreceptors and act to establish retinal circuitry. These analyses provide a framework for future studies that will dissect the role of these various factors in the differentiation of avian photoreceptor subtypes.

Long-distance retinoid signaling in the zebra finch brain

All-trans retinoic acid (ATRA), the main active metabolite of vitamin A, is a powerful signaling molecule that regulates large-scale morphogenetic processes during vertebrate embryonic development, but is also involved post-natally in regulating neural plasticity and cognition. In songbirds, it plays an important role in the maturation of learned song. The distribution of the ATRA-synthesizing enzyme, zRalDH, and of ATRA receptors (RARs) have been described, but information on the distribution of other components of the retinoid signaling pathway is still lacking. To address this gap, we have determined the expression patterns of two obligatory RAR co-receptors, the retinoid X receptors (RXR) α and γ, and of the three ATRA-degrading cytochromes CYP26A1, CYP26B1, and CYP26C1. We have also studied the distribution of zRalDH protein using immunohistochemistry, and generated a refined map of ATRA localization, using a modified reporter cell assay to examine entire brain sections. Our results show that (1) ATRA is more broadly distributed in the brain than previously predicted by the spatially restricted distribution of zRalDH transcripts. This could be due to long-range transport of zRalDH enzyme between different nuclei of the song system: Experimental lesions of putative zRalDH peptide source regions diminish ATRA-induced transcription in target regions. (2) Four telencephalic song nuclei express different and specific subsets of retinoid-related receptors and could be targets of retinoid regulation; in the case of the lateral magnocellular nucleus of the anterior nidopallium (lMAN), receptor expression is dynamically regulated in a circadian and age-dependent manner. (3) High-order auditory areas exhibit a complex distribution of transcripts representing ATRA synthesizing and degrading enzymes and could also be a target of retinoid signaling. Together, our survey across multiple connected song nuclei and auditory brain regions underscores the prominent role of retinoid signaling in modulating the circuitry that underlies the acquisition and production of learned vocalizations.

Visual pigments and opsin expression in the juveniles of three species of fish (rainbow trout, zebrafish, and killifish) following prolonged exposure to thyroid hormone or retinoic acid

Thyroid hormone (TH) and retinoic acid (RA) are powerful modulators of photoreceptor differentiation during vertebrate retinal development. In the embryos and young juveniles of salmonid fishes and rodents, TH induces switches in opsin expression within individual cones, a phenomenon that also occurs in adult rodents following prolonged (12 week) hypothyroidism. Whether changes in TH levels also modulate opsin expression in the differentiated retina of fish is unknown. Like TH, RA is essential for retinal development, but its role in inducing opsin switches, if any, has not been studied. Here we investigate the action of TH and RA on single-cone opsin expression in juvenile rainbow trout, zebrafish, and killifish and on the absorbance of visual pigments in rainbow trout and zebrafish. Prolonged TH exposure increased the wavelength of maximum absorbance (λmax ) of the rod and the medium (M, green) and long (L, red) wavelength visual pigments in all fish species examined. However, unlike the opsin switch that occurred following TH exposure in the single cones of small juvenile rainbow trout (alevin), opsin expression in large juvenile rainbow trout (smolt), zebrafish, or killifish remained unchanged. RA did not induce any opsin switches or change the visual pigment absorbance of photoreceptors. Neither ligand altered cone photoreceptor densities. We conclude that RA has no effect on opsin expression or visual pigment properties in the differentiated retina of these fishes. In contrast, TH affected both single-cone opsin expression and visual pigment absorbance in the rainbow trout alevin but only visual pigment absorbance in the smolt and in zebrafish. The latter results could be explained by a combination of opsin switches and chromophore shifts from vitamin A1 to vitamin A2.

Counterphase modulation flicker photometry: phenotypic and genotypic associations

The OSCAR test, a clinical device that uses counterphase flicker photometry, is believed to be sensitive to the relative numbers of long-wavelength and middle-wavelength cones in the retina, as well as to individual variations in the spectral positions of the photopigments. As part of a population study of individual variations in perception, we obtained OSCAR settings from 1058 participants. We report the distribution characteristics for this cohort. A randomly selected subset of participants was tested twice at an interval of at least one week: the test-retest reliability (Spearman's rho) was 0.80. In a whole-genome association analysis we found a provisional association with a single nucleotide polymorphism (rs16844995). This marker is close to the gene RXRG, which encodes a nuclear receptor, retinoid X receptor γ. This nuclear receptor is already known to have a role in the differentiation of cones during the development of the eye, and we suggest that polymorphisms in or close to RXRG influence the relative probability with which long-wave and middle-wave opsin genes are expressed in human cones.

Tissue-specific and embryonic expression of the retinoid X receptors in Sebastiscus marmoratus

Retinoid X receptors (RXRs) are highly conserved members of the nuclear receptor family and mediate various physiological processes in vertebrates. Most studies on RXRs have concentrated on their structure and function in mammals and their characterization and developmental expression in Danio rerio. However, there is little information concerning the distribution of RXRs in teleost tissues. In the present study, we cloned partial sequences of three RXR subtypes (RXRa, -b, -g) from Sebastiscus marmoratus by RACE PCR and analyzed the phylogeny of the teleost and the tetrapod RXR genes, and identified some inconsistencies with previous studies. The tissue-specific and embryonic expression profiles of each RXR gene were explored using real time quantitative PCR. This analysis demonstrated that these RXRs were expressed in all test tissues indicating their participation in many physiological processes. However, we found a great difference in the distribution of RXRg between teleosts and mammals. Furthermore, we followed expression of the three subtypes through various embryo developmental stages and found that the RXRa orthologues of teleosts might be involved in the development of the anterior hindbrain, tailbud and neural crest and in the formation of the pharynx and fin, that RXRb played ubiquitous roles in fish early development, and that RXRg probably played a role in brain and nervous system development and function.

Transient inactivation of Notch signaling synchronizes differentiation of neural progenitor cells

In the developing nervous system, the balance between proliferation and differentiation is critical to generate the appropriate numbers and types of neurons and glia. Notch signaling maintains the progenitor pool throughout this process. While many components of the Notch pathway have been identified, the downstream molecular events leading to neural differentiation are not well understood. We have taken advantage of a small molecule inhibitor, DAPT, to block Notch activity in retinal progenitor cells, and analyzed the resulting molecular and cellular changes over time. DAPT treatment causes a massive, coordinated differentiation of progenitors that produces cell types appropriate for their developmental stage. Transient exposure of retina to DAPT for specific time periods allowed us to define the period of Notch inactivation that is required for a permanent commitment to differentiate. Inactivation of Notch signaling revealed a cascade of proneural bHLH transcription factor gene expression that correlates with stages in progenitor cell differentiation. Microarray/QPCR analysis confirms the changes in Notch signaling components, and reveals new molecular targets for investigating neuronal differentiation. Thus, transient inactivation of Notch signaling synchronizes progenitor cell differentiation, and allows for a systematic analysis of key steps in this process.

Making the gradient: thyroid hormone regulates cone opsin expression in the developing mouse retina

Most mammals have two types of cone photoreceptors, which contain either medium wavelength (M) or short wavelength (S) opsin. The number and spatial organization of cone types varies dramatically among species, presumably to fine-tune the retina for different visual environments. In the mouse, S- and M-opsin are expressed in an opposing dorsal-ventral gradient. We previously reported that cone opsin patterning requires thyroid hormone beta2, a nuclear hormone receptor that regulates transcription in conjunction with its ligand, thyroid hormone (TH). Here we show that exogenous TH inhibits S-opsin expression, but activates M-opsin expression. Binding of endogenous TH to TRbeta2 is required to inhibit S-opsin and to activate M-opsin. TH is symmetrically distributed in the retina at birth as S-opsin expression begins, but becomes elevated in the dorsal retina at the time of M-opsin onset (postnatal day 10). Our results show that TH is a critical regulator of both S-opsin and M-opsin, and suggest that a TH gradient may play a role in establishing the gradient of M-opsin. These results also suggest that the ratio and patterning of cone types may be determined by TH availability during retinal development.

Notch 1 inhibits photoreceptor production in the developing mammalian retina

The transmembrane receptor Notch1 plays a role in development and homeostasis in vertebrates and invertebrates. The mammalian retina is an excellent tissue in which to dissect the precise role of Notch signaling in regulating cell fate and proliferation. However, a systematic analysis has been limited by the early embryonic lethality of Notch1-null mice. Here, Notch1 was conditionally removed from the murine retina either early or late in development. Removal of Notch1 early led to a reduction in the size of the retina as well as aberrant morphology. A decrease in the number of progenitor cells and premature neurogenesis accounted for the reduction in size. Unexpectedly, ablation of Notch1 in early progenitor cells led to enhanced cone photoreceptor production, and ablation of Notch1 at later points led to an almost exclusive production of rod photoreceptor cells. These data suggest that Notch1 not only maintains the progenitor state, but is required to inhibit the photoreceptor fate. These cone enriched mutant mice should prove to be a valuable resource for the study of this relatively rare mammalian photoreceptor cell type.

Characterization of retinoid-X receptor genes rxra, rxrba, rxrbb and rxrg during zebrafish development

During development of vertebrate embryos, retinoic acid plays a variety of roles that are mediated by binding to retinoic acid receptors (Rars) and their heterodimerization partners, the retinoid receptors (Rxrs). Here, we characterize the expression patterns of four zebrafish rxr genes during development and provide an analysis of the phylogenetic relationships between zebrafish and tetrapod Rxr genes based on sequence similarities and conserved syntenies. This analysis prompted the renaming of several of the zebrafish rxr genes to match their tetrapod orthologs. Understanding phylogenetic relationships among Rxr genes and their expression patterns during development provides a foundation for future studies of Rxr functions.

Early expression of thyroid hormone receptor beta and retinoid X receptor gamma in the Xenopus embryo

The role of thyroid hormone in Xenopus metamorphosis is particularly well understood as it plays an essential role in that process. However, recent evidence suggests that thyroid hormone may play an earlier role in amphibian embryogenesis. We demonstrate that Xenopus thyroid hormone receptor beta (XTR beta) is expressed shortly after neural fold closure, and that its expression is localized to the developing retina. Retinoid X receptor gamma (RXR gamma), a potential dimerization partner for XTR beta, was also found to be expressed in the retina at early stages, and at later stages RXR gamma was also expressed in the liver diverticulum. Addition of either thyroid hormone or 9-cis retinoic acid, the ligands for XTR beta and RXR gamma, respectively, did not alter the expression of their receptors. However, the addition of thyroid hormone and 9-cis retinoic acid did alter rhodopsin mRNA expression. Addition of thyroid hormone generates a small expansion of the rhodopsin expression domain. When 9-cis retinoic acid or a combination of thyroid hormone and 9-cis retinoic acid was administered, there was a decrease in the expression domain of rhodopsin in the developing retina. These results provide evidence for an early role for XTR beta and RXR gamma in the developing Xenopus retina.

The chicken RaxL gene plays a role in the initiation of photoreceptor differentiation

The paired type homeodomain gene, Rax, was previously identified as a key molecule in early eye formation in mice and humans. We report the expression patterns of two Rax family members from chicken, Rax and RaxL, and on the function of RaxL in photoreceptor development. Both Rax and RaxL are expressed in early retinal progenitor cells, with Rax being expressed at a significantly higher level than RaxL. At the time that photoreceptors begin to form, RaxL appears at a relatively high level in a subset of cells within the zone of proliferating progenitor cells. Subsequently, it is expressed in cells migrating to the photoreceptor layer, where it is highly expressed during the initial, but not late, stages of photoreceptor differentiation. To test the function of RaxL, a putative dominant-negative allele of RaxL comprising a fusion of the engrailed repressor domain and a region of RaxL (EnRaxLDeltaC) was introduced in vivo into the early chick eye using a retroviral vector. EnRaxLDeltaC, but not the dominant negative Rax (EnRaxDeltaC), caused a significant reduction in expression of early markers of photoreceptor cells. Examination of the transactivation activity of RaxL on a reporter construct bearing a canonical photoreceptor-specific enhancer element showed that RaxL exhibited significant activation activity, and that this activity was severely diminished in the presence of EnRaxLDeltaC. The effect on photoreceptor gene expression in vivo was specific in that other cell types were unaffected, as was general proliferation in the retina. The reduction in numbers of cells expressing photoreceptor markers was probably due to decreased survival of developing photoreceptor cells, as there was increased apoptosis among cells of the retina expressing dominant-negative RaxL. We propose that RaxL plays a role in the initiation of differentiation, and also possibly commitment, of photoreceptor cells in the chicken retina.

Electrical muscle activity pattern and transcriptional and posttranscriptional mechanisms regulate PKA subunit expression in rat skeletal muscle

We have examined protein kinase A (PKA) subunit expression in adult rat skeletal muscles. Northern blots identified PKA catalytic alpha and regulatory (R) I alpha and RII alpha subunits as the major subunits expressed in slowly contracting soleus (SOL) and rapidly contracting extensor digitorum longus (EDL) muscles. In addition, the steady-state RNA levels of PKA subunit mRNAs and activities of RI alpha and RII alpha promoters are similar in SOL and EDL. These data indicate that posttranscriptional mechanisms account for the twofold differences in PKA subunit protein levels reported earlier. Electrical stimulation of denervated SOL with an EDL-like activity pattern (fast pattern) transformed SOL into an EDL-like muscle with regard to PKA protein levels. These experiments suggest that the posttranscriptional regulation is activity pattern-dependent. Denervation specifically increased RI alpha promoter activity and RI alpha mRNA levels in SOL and EDL. Further experiments indicated that the RI alpha 1a upstream sequences were activated following denervation. Direct electrical stimulation prevented the rise in RI alpha mRNA levels following denervation, demonstrating that electrical muscle activity regulates transcription.

A model of retinal cell differentiation in the chick embryo

This article presents an overview of retinal cell differentiation in the chick embryo, in the context of a hypothetical model based on information generated during the last several years. The model proposes that: (1) most (if not all) proliferating neuroepithelial cells have the potential to give rise to a progeny comprising two or more different cell types; (2) the time at which cells undergo their terminal mitosis does not determine their differentiated fate; (3) many postmitotic precursor cells remain plastic (i.e., uncommitted) for some time after terminal mitosis, during which they encounter position-dependent signals as they migrate toward their definitive laminar position within the retina; (4) as a consequence of these inductive stimuli, precursor cells that migrate to different retinal layers express different transcriptional regulators; (5) morphologically undifferentiated precursor cells are committed to cell type-specific, complex patterns of differentiation, which they can express even when isolated from their normal microenvironment, and (6) even after precursor cells become committed to a specific identity, additional inductive signals are necessary for the cells to complete the development of a fully mature phenotype. The article presents a summary of the supportive evidence, as well as a critical evaluation of the model, and concludes with an overview of unanswered questions regarding retinal cell differentiation and a brief evaluation of the prospects for further progress in this field.

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.

Development of the visual system of the chick. I. Cell differentiation and histogenesis

This review summarizes present knowledge on the embryonic development of the avian visual projections, based on the domestic chick as a model system. The reductionist goal to understand formation and function of complex neuroanatomical systems on a causal level requires a synthesis of classic developmental biology with recent advances on the molecular mechanisms of cell differentiation and histogenesis. It is the purpose of this article. We are discussing the processes underlying patterning of the anterior neural tube, when the retina and optic tectum are specified and their axial polarity is determined. Then the development of these structures is described from the molecular to the anatomical level. Following sections deal with the establishment of secondary visual connections, and the developmental interactions between compartments of the retinotectal system. Using this latter pathway, from the retina to the optic tectum, many investigations aimed at mechanisms of axonal pathfinding and connectivity have accumulated a vast body of research, which will be covered by a following review.

The ontogeny of ultraviolet sensitivity, cone disappearance and regeneration in the sockeye salmon Oncorhynchus nerka

This study examines the spectral sensitivity and cone topography of the sockeye salmon Oncorhynchus nerka throughout its life history with special emphasis on ultraviolet sensitivity. Electrophysiological recordings from the optic nerve show that ultraviolet sensitivity is greatly diminished at the smolt stage but reappears in adult fish weighing about 201 g. Concomitantly, light microscopy observations of the retina show that ultraviolet cones disappear from the dorsal and temporal retina at the smolt stage but reappear at the adult stage. These changes occur for sockeye salmon raised in fresh water or salt water after smoltification. In contrast to this ultraviolet cycle, the other cone mechanisms (short-, middle- and long-wavelength-sensitive) and the rod mechanism remain present throughout ontogeny. The natural appearance and disappearance of ultraviolet cones in salmonid retinas follows surges in blood thyroxine at critical developmental periods. Their presence coincides with times of prominent feeding on zooplankton and/or small fish that may be more visible under ultraviolet light. It is proposed that the primary function of ultraviolet cones in salmonids is to improve prey contrast.

Retina-specific nuclear receptor: A potential regulator of cellular retinaldehyde-binding protein expressed in retinal pigment epithelium and Müller glial cells

In an effort to identify nuclear receptors important in retinal disease, we screened a retina cDNA library for nuclear receptors. Here we describe the identification of a retina-specific nuclear receptor (RNR) from both human and mouse. Human RNR is a splice variant of the recently published photoreceptor cell-specific nuclear receptor [Kobayashi, M., Takezawa, S., Hara, K., Yu, R. T., Umesono, Y., Agata, K., Taniwaki, M., Yasuda, K. & Umesono, K. (1999) Proc. Natl. Acad. Sci. USA 96, 4814-4819] whereas the mouse RNR is a mouse ortholog. Northern blot and reverse transcription-PCR analyses of human mRNA samples demonstrate that RNR is expressed exclusively in the retina, with transcripts of approximately 7.5 kb, approximately 3.0 kb, and approximately 2.3 kb by Northern blot analysis. In situ hybridization with multiple probes on both primate and mouse eye sections demonstrates that RNR is expressed in the retinal pigment epithelium and in Müller glial cells. By using the Gal4 chimeric receptor/reporter cotransfection system, the ligand binding domain of RNR was found to repress transcriptional activity in the absence of exogenous ligand. Gel mobility shift assays revealed that RNR can interact with the promoter of the cellular retinaldehyde binding protein gene in the presence of retinoic acid receptor (RAR) and/or retinoid X receptor (RXR). These data raise the possibility that RNR acts to regulate the visual cycle through its interaction with cellular retinaldehyde binding protein and therefore may be a target for retinal diseases such as retinitis pigmentosa and age-related macular degeneration.