Rhodopsin, 11-cis vitamin A, and interstitial retinol-binding protein (IRBP) during retinal development in normal and rd mutant mice

Aggregation of rhodopsin mutants in mouse models of autosomal dominant retinitis pigmentosa

Mutations in rhodopsin can cause it to misfold and lead to retinal degeneration. A distinguishing feature of these mutants in vitro is that they mislocalize and aggregate. It is unclear whether or not these features contribute to retinal degeneration observed in vivo. The effect of P23H and G188R misfolding mutations were examined in a heterologous expression system and knockin mouse models, including a mouse model generated here expressing the G188R rhodopsin mutant. In vitro characterizations demonstrate that both mutants aggregate, with the G188R mutant exhibiting a more severe aggregation profile compared to the P23H mutant. The potential for rhodopsin mutants to aggregate in vivo was assessed by PROTEOSTAT, a dye that labels aggregated proteins. Both mutants mislocalize in photoreceptor cells and PROTEOSTAT staining was detected surrounding the nuclei of photoreceptor cells. The G188R mutant promotes a more severe retinal degeneration phenotype and greater PROTEOSTAT staining compared to that promoted by the P23H mutant. Here, we show that the level of PROTEOSTAT positive cells mirrors the progression and level of photoreceptor cell death, which suggests a potential role for rhodopsin aggregation in retinal degeneration.

Glucose uptake by GLUT1 in photoreceptors is essential for outer segment renewal and rod photoreceptor survival

Photoreceptors consume glucose supplied by the choriocapillaris to support phototransduction and outer segment (OS) renewal. Reduced glucose supply underlies photoreceptor cell death in inherited retinal degeneration and age-related retinal disease. We have previously shown that restricting glucose transport into the outer retina by conditional deletion of Slc2a1 encoding GLUT1 resulted in photoreceptor loss and impaired OS renewal. However, retinal neurons, glia, and the retinal pigment epithelium play specialized, synergistic roles in metabolite supply and exchange, and the cell-specific map of glucose uptake and utilization in the retina is incomplete. In these studies, we conditionally deleted Slc2a1 in a pan-retinal or rod-specific manner to better understand how glucose is utilized in the retina. Using non-invasive ocular imaging, electroretinography, and histochemical and biochemical analyses we show that genetic deletion of Slc2a1 from retinal neurons and Müller glia results in reduced OS growth and progressive rod but not cone photoreceptor cell death. Rhodopsin levels were severely decreased even at postnatal day 20 when OS length was relatively normal. Arrestin levels were not changed suggesting that glucose uptake is required to synthesize membrane glycoproteins. Rod-specific deletion of Slc2a1 resulted in similar changes in OS length and rod photoreceptor cell death. These studies demonstrate that glucose is an essential carbon source for rod photoreceptor cell OS maintenance and viability.

Interphotoreceptor Retinol-Binding Protein Ameliorates Diabetes-Induced Retinal Dysfunction and Neurodegeneration Through Rhodopsin

Patients with diabetes often experience visual defects before any retinal pathologies are detected. The molecular mechanism for the visual defects in early diabetes has not been elucidated. Our previous study reported that in early diabetic retinopathy (DR), rhodopsin levels were reduced due to impaired 11-cis-retinal regeneration. Interphotoreceptor retinol-binding protein (IRBP) is a visual cycle protein and important for 11-cis-retinal generation. IRBP levels are decreased in the vitreous and retina of DR patients and animal models. To determine the role of IRBP downregulation in the visual defects in early DR, we induced diabetes in transgenic mice overexpressing IRBP in the retina. IRBP overexpression prevented diabetes-induced decline of retinal function. Furthermore, IRBP overexpression also prevented decreases of rhodopsin levels and 11-cis-retinal generation in diabetic mice. Diabetic IRBP transgenic mice also showed ameliorated retinal oxidative stress, inflammation, apoptosis, and retinal degeneration compared with diabetic wild-type mice. These findings suggest that diabetes-induced IRBP downregulation impairs the regeneration of 11-cis-retinal and rhodopsin, leading to retinal dysfunction in early DR. Furthermore, increased 11-cis-retinal-free opsin constitutively activates the phototransduction pathway, leading to increased oxidative stress and retinal neurodegeneration. Therefore, restored IRBP expression in the diabetic retina may confer a protective effect against retinal degeneration in DR.

Differential adaptations in rod outer segment disc membranes in different models of congenital stationary night blindness

Rod photoreceptor cells initiate scotopic vision when the light receptor rhodopsin absorbs a photon of light to initiate phototransduction. These photoreceptor cells are exquisitely sensitive and have adaptive mechanisms in place to maintain optimal function and to overcome dysfunctional states. One adaptation rod photoreceptor cells exhibit is in the packing properties of rhodopsin within the membrane. The mechanism underlying these adaptations is unclear. Mouse models of congenital stationary night blindness with different molecular causes were investigated to determine which signals are important for adaptations in rod photoreceptor cells. Night blindness in these mice is caused by dysfunction in either rod photoreceptor cell signaling or bipolar cell signaling. Changes in the packing of rhodopsin within photoreceptor cell membranes were examined by atomic force microscopy. Mice expressing constitutively active rhodopsin did not exhibit any adaptations, even under constant dark conditions. Mice with disrupted bipolar cell signaling exhibited adaptations, however, they were distinct from those in mice with disrupted phototransduction. These differential adaptations demonstrate that although multiple molecular defects can lead to a similar primary defect causing disease (i.e., night blindness), they can cause different secondary effects (i.e., adaptations). The lighting environment or signaling defects present from birth and during early rearing can condition mice and affect the adaptations occurring in more mature animals. A comparison of effects in wild-type mice, mice with defective phototransduction, and mice with defective bipolar cell signaling, indicated that bipolar cell signaling plays a role in this conditioning but is not required for adaptations in more mature animals.

Experimental Approaches for Defining the Role of the Ca2+-Modulated ROS-GC System in Retinal Rods of Mouse

Our ability to see is based on the activity of retinal rod and cone photoreceptors. Rods function when there is very little light, while cones operate at higher light levels. Photon absorption by rhodopsin activates a biochemical cascade that converts photic energy into a change in the membrane potential of the cell by decreasing the levels of a second messenger, cGMP, that control the gating of cation channels. But just as important as the activation of the cascade are the shut-off and recovery processes. The timing of shutoff and recovery ultimately affects sensitivity, temporal resolution and even the capacity for counting single photons. An important part of the recovery is restoration of cGMP through the action of rod outer segment membrane guanylate cyclases (ROS-GCs) and guanylate cyclase-activating proteins (GCAPs). In darkness, ROS-GCs catalyze the conversion of GTP to cGMP at a low rate, due to inhibition of cyclase activity by GCAPs. In the light, GCAP enhances ROS-GC activity. Mutations in the ROS-GC system can cause problems in vision, and even result in blindness due to photoreceptor death. The mouse has emerged as a particularly useful subject to study the role of ROS-GC because the technology for the manipulation of their genetics is advanced, making production of mice with targeted mutations much easier. Here we describe some experimental procedures for studying the retinal rods of wild-type and genetically engineered mice: biochemical assays of ROS-GC activity, immunohistochemistry, and single cell recording.

Identification and characterization of early photoreceptor cis-regulatory elements and their relation to Onecut1

Background

Cone and rod photoreceptors are two of the primary cell types affected in human retinal disease. Potential strategies to combat these diseases are the use of gene therapy to rescue compromised photoreceptors or to generate new functional photoreceptors to replace those lost in the diseased retina. Cis-regulatory elements specific to cones, rods, or both types of photoreceptors are critical components of successful implementation of these two strategies. The purpose of this study was to identify and characterize the cell type specificity and activity of cis-regulatory elements active in developing photoreceptors.

Methods

Cis-regulatory elements were introduced into the developing chicken and mouse retina by electroporation. Characterization of reporter activity in relation with cell type markers was determined using confocal microscopy. In addition, two high-throughput flow cytometry assay were developed to assess whether these elements were downstream of Onecut1 in the photoreceptor specification network.

Results

The majority of cis-regulatory elements were active in both cone and rod photoreceptors and were largely uninfluenced by a Onecut1 dominant-negative construct. Elements associated with the Thrb, Nr2e3, and Rhodopsin genes showed highly enriched activity in cones or rods, and were affected by interference in Onecut1 signaling. Rhodopsin promoter activity was the most highly influenced by Onecut1 activity and its induction could be modulated by the Maf family transcription factor L-Maf. Nr2e3 elements were observed to have activity in cone photoreceptors and Nr2e3 protein was expressed in developing cone photoreceptors, suggesting a role for this predominant rod gene in cone photoreceptor development.

Conclusions

The analysis presented here provides an experimental framework to determine the specificity and strength of photoreceptor elements within specific genetic networks during development. The Onecut1 transcription factor is one such factor that influences the gene regulatory networks specific to cones and rods, but not those that are common to both.

Electronic supplementary material

The online version of this article (10.1186/s13064-018-0121-x) contains supplementary material, which is available to authorized users.

Adaptations in rod outer segment disc membranes in response to environmental lighting conditions

The light-sensing rod photoreceptor cell exhibits several adaptations in response to the lighting environment. While adaptations to short-term changes in lighting conditions have been examined in depth, adaptations to long-term changes in lighting conditions are less understood. Atomic force microscopy was used to characterize the structure of rod outer segment disc membranes, the site of photon absorption by the pigment rhodopsin, to better understand how photoreceptor cells respond to long-term lighting changes. Structural properties of the disc membrane changed in response to housing mice in constant dark or light conditions and these adaptive changes required output from the phototransduction cascade initiated by rhodopsin. Among these were changes in the packing density of rhodopsin in the membrane, which was independent of rhodopsin synthesis and specifically affected scotopic visual function as assessed by electroretinography. Studies here support the concept of photostasis, which maintains optimal photoreceptor cell function with implications in retinal degenerations.

MRI of Retinal Free Radical Production With Laminar Resolution In Vivo

Purpose

Recent studies have suggested the hypothesis that quench-assisted 1/T1 magnetic resonance imaging (MRI) measures free radical production with laminar resolution in vivo without the need of a contrast agent. Here, we test this hypothesis further by examining the spatial and detection sensitivity of quench-assisted 1/T1 MRI to strain, age, or retinal cell layer-specific genetic manipulations.

Methods

We studied: adult wild-type mice; mice at postnatal day 7 (P7); cre dependent retinal pigment epithelium (RPE)-specific MnSOD knockout mice; doxycycline-treated Sod2^flox/flox mice lacking the cre transgene; and α-transducin knockout (Gnat1^−/−) mice on a C57Bl/6 background. Transretinal 1/T1 profiles were mapped in vivo in the dark without or with antioxidant treatment, or followed by light exposure. We calibrated profiles spatially using optical coherence tomography.

Results

Dark-adapted RPE-specific MnSOD knockout mice had greater than normal 1/T1 in the RPE and outer nuclear layers that was corrected to wild-type levels by antioxidant treatment. Dark and light Gnat1^−/− mice also had greater than normal outer retinal 1/T1 values. In adult wild-type mice, dark values of 1/T1 in the ellipsoid region and in the outer segment were suppressed by 13 minutes of light. By 29 minutes of light, 1/T1 reduction extended to the outer nuclear layer. Gnat1^−/− mice demonstrated a faster light-evoked suppression of 1/T1 values in the outer retina. In P7 mice, transretinal 1/T1 profiles were the same in dark and light.

Conclusions

Quench-assisted MRI has the laminar resolution and detection sensitivity to evaluate normal and pathologic production of free radicals in vivo.

Impact of reduced rhodopsin expression on the structure of rod outer segment disc membranes

Rhodopsin is the light receptor embedded in rod outer segment (ROS) disc membranes of photoreceptor cells that initiates vision via phototransduction. The relationship between rhodopsin expression and the formation of membrane structures in the ROS is unclear but important to better understand both normal function and pathological conditions. To determine the impact of reduced rhodopsin expression on the structure of ROS discs and the supramolecular organization of rhodopsin, ROS disc membrane samples from heterozygous rhodopsin knockout mice were examined by atomic force microscopy. Similar to rhodopsin in wild-type mice, rhodopsin formed nanodomains in ROS disc membranes of heterozygous knockout mice. The reduced rhodopsin expression in heterozygous knockout mice resulted in ROS disc membranes that were smaller compared to those in wild-type mice at all ages tested. Changes in ROS disc membrane properties were observed between 4 and 6 weeks of age in heterozygous knockout mice that were not present in age-matched wild-type mice. In 4 week old mice, the number and density of rhodopsin in ROS disc membranes was lower than that in age-matched wild-type mice. In contrast, 6 and 8 week old mice had more rhodopsin molecules present in disc membranes compared to 4 week old mice, which resulted in rhodopsin densities similar to those found in age-matched wild-type mice. Thus, mechanisms appear to be present that maintain a constant density of rhodopsin within ROS disc membranes even when reducing the expression of the light receptor by about half. These adaptive mechanisms, however, only occur after 4 weeks of age.

Receptor interacting protein kinase-mediated necrosis contributes to cone and rod photoreceptor degeneration in the retina lacking interphotoreceptor retinoid-binding protein

Interphotoreceptor retinoid-binding protein (IRBP) secreted by photoreceptors plays a pivotal role in photoreceptor survival with an unknown mechanism. A mutation in the human IRBP has been linked to retinitis pigmentosa, a progressive retinal degenerative disease. Mice lacking IRBP display severe early and progressive photoreceptor degeneration. However, the signaling pathway(s) leading to photoreceptor death in IRBP-deficient mice remains poorly understood. Here, we show that amounts of tumor necrosis factor-α (TNF-α) in the interphotoreceptor matrix and retinas of Irbp(-/-) mice were increased more than 10-fold and fivefold, respectively, compared with those in wild-type mice. Moreover, TNF-α receptor 1, an important membrane death receptor that mediates both programmed apoptosis and necrosis, was also significantly increased in Irbp(-/-) retina, and was colocalized with peanut agglutinin to the Irbp(-/-) cone outer segments. Although these death signaling proteins were increased, the caspase-dependent and independent apoptotic pathways were mildly activated in the Irbp(-/-) retinas, suggesting that other cell death mechanism(s) also contributes to the extensive photoreceptor degeneration in Irbp(-/-) retina. We found that receptor interacting protein 1 and 3 (RIP1 and RIP3) kinases, the intracellular key mediators of TNF-induced cellular necrosis, were elevated at least threefold in the Irbp(-/-) retinas. Moreover, pharmacological inhibition of RIP1 kinase significantly prevented cone and rod photoreceptor degeneration in Irbp(-/-) mice. These results reveal that RIP kinase-mediated necrosis strongly contributes to cone and rod degeneration in Irbp(-/-) mice, implicating the TNF-RIP pathway as a potential therapeutic target to prevent or delay photoreceptor degeneration in patients with retinitis pigmentosa caused by IRBP mutation.

Development of circadian rhythms: role of postnatal light environment

Mammals are born with an immature circadian system, which completes its development postnatally. Evidence suggests that the environment experienced by a newborn will impact and shape its development, which will have future consequences at the levels of circadian system function, circadian behaviour and physiology, and potentially, the animal's long-term health and welfare. Here we review the various stages in postnatal development of the circadian system, and discuss the data available on the long-term effects of early environment, in particular light environment, on the animal's brain, physiology and behaviour.

Exaggerated eye growth in IRBP-deficient mice in early development

PURPOSE

Because interphotoreceptor retinoid-binding protein (IRBP) is expressed before being needed in its presumptive role in the visual cycle, we tested whether it controls eye growth during development.

METHODS

The eyes of congenic IRBP knockout (KO) and C57BL/6J wild-type (WT) mice ranging in age from postnatal day (P)2 to P440 were compared by histology, laser micrometry, cycloplegic photorefractions, and partial coherence interferometry.

RESULTS

The size and weight of IRBP KO mouse eyes were greater than those of the WT mouse, even before eye-opening. Excessive ocular enlargement started between P7 and P10, with KO retinal arc lengths becoming greater compared with WT from P10 through P30 (18%; P < 0.01). The outer nuclear layer (ONL) of KO retinas became 20% thinner between P12 to P25, and progressed to 38% thinner at P30. At P30, there were 30% fewer cones per vertical section in KO than in WT retinas. Bromodeoxyuridine (BrdU) labeling indicated the same number of retinal cells were born in KO and WT mice. A spike in apoptosis was observed in KO outer nuclear layer at P25. These changes in size were accompanied by a large decrease in hyperopic refractive error, which reached -4.56 ± 0.70 diopters (D) versus +9.98 ± 0.993 D (mean ± SD) in WT, by postnatal day 60 (P60). CONCLUSIONS; In addition to its role in the visual cycle, IRBP is needed for normal eye development. How IRBP mediates ocular development is unknown.

The role of interphotoreceptor retinoid-binding protein on the translocation of visual retinoids and function of cone photoreceptors

The first event in light perception is absorption of a photon by the retinaldehyde chromophore of an opsin pigment in a rod or cone photoreceptor cell. This induces isomerization of the chromophore, rendering the bleached pigment insensitive to light. Restoration of light sensitivity requires chemical reisomerization of retinaldehyde via a multistep enzyme pathway, called the visual cycle, in cells of the retinal pigment epithelium (RPE). Interphotoreceptor retinoid-binding protein (IRBP) is present in the extracellular space between photoreceptors and the RPE. IRBP is known to bind visual retinoids. Previous studies on irbp(-/-) mice suggested that IRBP plays an insignificant role in opsin-pigment regeneration. However, the mice in these studies were uncontrolled for a severe mutation in the rpe65 gene. Rpe65 catalyzes the rate-limiting step in the visual cycle. Here, we examined the phenotype in irbp(-/-) mice homozygous for the wild-type (Leu450) rpe65 gene. We show that lack of IRBP causes delayed transfer of newly synthesized chromophore from RPE to photoreceptors. Removal of bleached chromophore from photoreceptors is also delayed in irbp(-/-) retinas after light exposure. It was previously shown that rods degenerate in irbp(-/-) mice. Here, we show that cones and rods degenerate at similar rates. However, cones are more affected functionally and show greater reductions in outer segment length than rods in irbp(-/-) mice. The disproportionate reductions in cone function and outer-segment length appear to result from mistrafficking of cone opsins due to impaired delivery of retinaldehyde chromophore, which functions as a chaperone for cone opsins but not rhodopsin.

Light-evoked FOS induction within the suprachiasmatic nuclei (SCN) of melanopsin knockout (Opn4-/-) mice: a developmental study

The aims of this study were to address three related questions: (1) Do the photosensitive ganglion cells of the mouse convey light information to the suprachiasmatic nuclei (SCN) at P0? (2) Do the differentiating rods and cones contribute to light-evoked FOS induction within the murine SCN at P4? (3) How does light-evoked FOS induction within the SCN of melanopsin knockout (Opn4-/-) mice differ at P4 and P14? Our approaches took advantage of the published descriptions of murine ocular development, melanopsin knockout (Opn4-/-) mouse, and light-induced expression of FOS (the phosphoprotein product of immediate early gene c-fos) within the SCN as a marker of retinohypothalamic tract competence. Collectively, our results show that photosensitive melanopsin-dependent retinal ganglion cells provide light information to the murine SCN on the day of birth, and possibly beforehand, and that developing rods and cones fail to provide light information to the SCN during early postnatal life. On the basis of previous publications and data presented here, we suggest that at ages around P14 the rods and cones might be capable of fully compensating for the loss of melanopsin-photosensitive ganglion cells if exposure to light is of sufficiently long duration. These results are related to the broader context of recent findings and the potential role(s) of a neonatal photoreceptor.

The retinal pigment epithelium in visual function

Located between vessels of the choriocapillaris and light-sensitive outer segments of the photoreceptors, the retinal pigment epithelium (RPE) closely interacts with photoreceptors in the maintenance of visual function. Increasing knowledge of the multiple functions performed by the RPE improved the understanding of many diseases leading to blindness. This review summarizes the current knowledge of RPE functions and describes how failure of these functions causes loss of visual function. Mutations in genes that are expressed in the RPE can lead to photoreceptor degeneration. On the other hand, mutations in genes expressed in photoreceptors can lead to degenerations of the RPE. Thus both tissues can be regarded as a functional unit where both interacting partners depend on each other.

Melanopsin-dependent photoreception provides earliest light detection in the mammalian retina

BACKGROUND

The visual system is now known to be composed of image-forming and non-image-forming pathways. Photoreception for the image-forming pathway begins at the rods and cones, whereas that for the non-image-forming pathway also involves intrinsically photosensitive retinal ganglion cells (ipRGCs), which express the photopigment melanopsin. In the mouse retina, the rod and cone photoreceptors become light responsive from postnatal day 10 (P10); however, the development of photosensitivity of the ipRGCs remains largely unexplored.

RESULTS

Here, we provide direct physiological evidence that the ipRGCs are light responsive from birth (P0) and that this photosensitivity requires melanopsin expression. Interestingly, the number of ipRGCs at P0 is over five times that in the adult retina, reflecting an initial overproduction of melanopsin-expressing cells during development. Even at P0, the ipRGCs form functional connections with the suprachiasmatic nucleus, as assessed by light-induced Fos expression.

CONCLUSIONS

The findings suggest that the non-image-forming pathway is functional long before the mainstream image-forming pathway during development.

Zinc-finger domains of the transcriptional repressor KLF15 bind multiple sites in rhodopsin and IRBP promoters including the CRS-1 and G-rich repressor elements

BACKGROUND

In the retina, many of the genes that encode components of the visual transduction cascade and retinoid recycling are exclusively expressed in photoreceptor cells and show highly stereotyped temporal and spatial expression patterns. Multiple transcriptional activators of photoreceptor-specific genes have been identified, but little is known about negative regulation of gene expression in the retina. We recently identified KLF15, a member of the Sp/Krüppel-like Factor family of zinc-finger containing transcription factors, as an in vitro repressor of the promoters of the photoreceptor-specific genes rhodopsin and IRBP/Rbp3. To gain further insight into the mechanism of KLF15-mediated regulation of gene expression, we have characterized the binding characteristics and specificity of KLF15's DNA binding domains and defined the KLF15 binding sites in the rhodopsin and IRBP promoters.

RESULTS

In EMSA and DNAseI footprinting assays, a KLF15-GST fusion protein containing the C-terminal zinc-finger domains (123 amino acids) showed zinc-dependent and sequence-specific binding to a 9 bp consensus sequence containing a core CG/TCCCC. Both the bovine rhodopsin and IRBP promoters contained multiple KLF15 binding sites that included the previously identified CRS-1 and G-rich repressor elements. KLF15 binding sites were highly conserved between the bovine, human, chimp and dog rhodopsin promoters, but less conserved in rodents. KLF15 reduced luciferase expression by bRho130-luc (containing 4 KLF15 sites) and repressed promoter activation by CRX (cone rod homeobox) and/or NRL (neural retina leucine zipper), although the magnitude of the reduction was smaller than previously reported for a longer bRho225-luc (containing 6 KFL15 sites).

CONCLUSION

KLF15 binds to multiple 9 bp consensus sites in the Rhodospin and IRBP promoters including the CRS-1 and G-rich repressor elements. Based on the known expression pattern of KLF15 in non-photoreceptor cells, we hypothesize an in vivo role for KLF15 in repressing photoreceptor-specific gene expression in the inner retina.

Interphotoreceptor retinoid-binding protein--an old gene for new eyes

Evolving 40 times independently, eyes are striking examples of convergent evolution in that 11-cis retinaldehyde is always used for photon capture, yet the mechanism for its regeneration may be dramatically different in between systems. In particular, insects, cephalopods and vertebrates show varying physical separation of the cis-->trans photoisomerization and chromphore reisomerization. In the vertebrate retina, these two processes are actually distributed between different cells. This compartmentalization is made possible by the phylogenetic innovation of the two-layered optic cup of the vertebrate retina. This unprecedented design created the subretinal space as a novel anatomical compartment allowing photoreceptors access to the retinal pigment epithelium (RPE) and Müller cells, the two cell types which share the burden of 11-cis retinoid regeneration. To take advantage of this arrangement, early vertebrates appear to have recruited for retinoid binding, the betabetaalpha-spiral fold proven useful in enoyl-CoA isomerase/hydratases, and the carboxy-terminal proteases for stabilizing hydrophobic ligands. Quadruplication of this functional domain within a single polypeptide lead to the emergence of interphotoreceptor retinoid-binding protein (IRBP). IRBP is the main soluble component of the IPM, and is prevented from diffusing out of the subretinal space because its large size excludes it from the photoreceptor/Müller cell zonulae adheretes. Despite this physical entrapment, IRBP is rapidly turned over within the IPM through a process that coordinates secretion of the protein by the photoreceptors, and its removal from the matrix by RPE and photoreceptor endocytosis. The present review will summarize what is known about the structure and function of IRBP to anticipate future avenues of research.

Spontaneous activity of opsin apoprotein is a cause of Leber congenital amaurosis

Mutations in Rpe65 disrupt synthesis of the opsin chromophore ligand 11-cis-retinal and cause Leber congenital amaurosis (LCA), a severe, early-onset retinal dystrophy. To test whether light-independent signaling by unliganded opsin causes the degeneration, we used Rpe65-null mice, a model of LCA. Dark-adapted Rpe65-/- mice behaved as if light adapted, exhibiting reduced circulating current, accelerated response turn-off, and diminished intracellular calcium. A genetic block of transducin signaling completely rescued degeneration irrespective of an elevated level of retinyl ester. These studies clearly show that activation of sensory transduction by unliganded opsin, and not the accumulation of retinyl esters, causes light-independent retinal degeneration in LCA. A similar mechanism may also be responsible for degeneration induced by vitamin A deprivation.

Rod sensitivity during Xenopus development

We have measured the sensitivity of rod photoreceptors from overnight-dark-adapted Xenopus laevis through developmental stages 46-66 into adulthood by using suction-pipette recording. The dark current increased gradually from approximately 5 pA at stage 46 to approximately 20 pA at stage 57, compared with an adult (metamorphosed) current of approximately 35 pA. This increase in dark current largely paralleled the progressive increase in length and diameter of the rod outer segment (ROS). Throughout stages 46-66, the dark current increased approximately linearly with ROS surface area. At stage 53, there was a steep (approximately 10-fold) increase in the rod flash sensitivity, accompanied by a steep increase in the time-to-peak of the half-saturated flash response. This covariance of sensitivity and time-to-peak suggested a change in the state of adaptation of rods at stage 53 and thereafter. When the isolated retina was preincubated with 11-cis-retinal, the flash sensitivity and the response time-to-peak of rods before stage 53 became similar to those at or after stage 53, suggesting that the presence of free opsin (i.e., visual pigment without chromophore) in rods before stage 53 was responsible for the adapted state (low sensitivity and short time-to-peak). By comparing the response sensitivity before stage 53 to the sensitivity at/after stage 53 measured from rods that had been subjected to various known bleaches, we estimated that 22-28% of rod opsin in stage 50-52 tadpoles (i.e., before stage 53) was devoid of chromophore despite overnight dark-adaptation. When continuously dark adapted for 7 d or longer, however, even tadpoles before stage 53 yielded rods with similar flash sensitivity and response time-to-peak as those of later-stage animals. In conclusion, it appears that chromophore regeneration is very slow in tadpoles before stage 53, but this regeneration becomes much more efficient at stage 53. A similar delay in the maturity of chromophore regeneration may partially underlie the low sensitivity of rods observed in newborn mammals, including human infants.

SPACRCAN in the developing retina and pineal gland of the rat: spatial and temporal pattern of gene expression and protein synthesis

SPACRCAN is a hyaluronan-binding proteoglycan that is present in the pineal gland and interphotoreceptor matrix of the retina. Here, we evaluate the pattern of SPACRCAN gene expression and protein appearance during retinal and pineal gland development in the rat. In situ hybridization histochemistry with SPACRCAN riboprobes indicates that hybridization signals are first evident in the retina over developing photoreceptor cells at embryonic day 16 (E16) and in the pineal gland at E21. Immunocytochemistry using a SPACRCAN antibody shows localization of SPACRCAN protein in the developing interphotoreceptor matrix by Postnatal day 5 (P5) and in the pineal gland by P6. These studies suggest that SPACRCAN mRNA expression may occur substantially earlier than the time when SPACRCAN protein is detectable in both the retina and the pineal gland. The period of retinal histogenesis when SPACRCAN is detected first is coincident with the time photoreceptors begin to extend from the outer retinal surface, suggesting that SPACRCAN may participate in the maturation and maintenance of the light-sensitive photoreceptor outer segment.

Studies of vitamin A metabolism in mouse model systems

Over the past several years, discoveries from mouse genetics have had direct impact on our understanding of vitamin A metabolism. Although the metabolism of vitamin A in the mouse does have some special features (for example very large stores of liver and pulmonary retinyl esters), the ability to construct knockout and transgenic mouse models has yielded an impressive amount of information directly relevant to understanding the general principles of vitamin A transport, storage and degradation. We discuss below the metabolism of vitamin A through a number of genetically engineered mouse strains with alterations in genes that affect this metabolism. The novelty of this experimental approach is evidenced by the fact that the oldest of these strains was first reported only eight years ago.1)

The zinc finger transcription factor, MOK2, negatively modulates expression of the interphotoreceptor retinoid-binding protein gene, IRBP

The human and murine MOK2 orthologue genes encode Krüppel/TFIIIA-related zinc finger proteins, which are factors able to recognize both DNA and RNA through their zinc finger motifs. MOK2 proteins have been shown to bind to the same 18-base pair (bp)-specific sequence in duplex DNA. This MOK2-binding site was found within introns 7 and 2 of human PAX3 and interphotoreceptor retinoid-binding protein (IRBP) genes, respectively. As these two genes are expressed in the brain as MOK2, we have suggested that PAX3 and IRBP genes are two potentially important target genes for the MOK2 protein. In this study, we focused our attention on IRBP as a potential MOK2 target gene. Sequence comparison and binding studies of the 18-bp MOK2-binding sites present in intron 2 of human, bovine, and mouse IRBP genes show that the 3'-half sequence is the essential core element for MOK2 binding. Very interestingly, 8-bp of this core sequence are found in a reverse orientation, in the IRBP promoter. We demonstrate that MOK2 can bind to the 8-bp sequence present in the IRBP promoter and repress its transcription when transiently overexpressed in retinoblastoma Weri-RB1 cells. In the IRBP promoter, it appears that the TAAAGGCT MOK2-binding site overlaps with the photoreceptor-specific CRX-binding element. We suggest that MOK2 represses transcription by competing with the cone-rod homeobox protein (CRX) for DNA binding, thereby decreasing transcriptional activation by CRX. Furthermore, we show that Mok2 expression in the developing mouse and in the adult retina seems to be concordant with IRBP expression.

Site-specific DNA hypomethylation permits expression of the IRBP gene

Interphotoreceptor retinoid binding protein (IRBP), a putative component of the visual cycle, is expressed selectively in the retina and pineal gland. This study examined whether site-specific DNA hypomethylation plays a role in this expression regulation. Southern blotting of HpaII and MspI digests of DNA from various bovine and murine tissues (whole brain, retina, pineal gland, superior colliculus, cortex, thymus, habenular nucleus, cornea, liver, tail, and kidney) revealed that specific CpG dinucleotides in the IRBP gene promoter are hypomethylated in DNA from retinal photoreceptor cells and pineal gland compared to DNA from other tissues. These sites are methylated in DNA from non-photoreceptor retinal cells. Exogenous methylation of these sites diminished DNA:protein binding in electrophoretic mobility shift assays. HpaII methylation of chloramphenicol acetyltransferase reporter constructs suppressed IRBP but not SV40 promoter activity in transiently transfected primary cultures of embryonic chick retinal cells. These data indicate that specific cytosines in the bovine and murine IRBP promoters are unmethylated in photoreceptive cells but methylated in other tissues. This differential DNA methylation may modulate IRBP gene expression since exogenous methylation of the murine sites suppresses reporter gene transcription, apparently by inhibiting DNA:protein binding events.

Gene structure, expression analysis, and membrane topology of RDH4

The murine retinol dehydrogenase RDH4 oxidizes several cis-isomers of retinol into their corresponding aldehydes. We have determined the structure of the murine gene, investigated the temporal and spatial expression of the enzyme, and analyzed the membrane topology of the enzyme. The gene has four translated exons, and several alternatively spliced exons in the 5'-untranslated region were identified. Immunohistochemical analysis showed expression of RDH4 in developing and adult mouse eye, particularly in the retinal pigment epithelium. In nonocular adult tissues, including liver, kidney, lung, and skin, RDH4 expression was widespread. The results suggest that RDH4 may have a dual and tissue-specific role in oxidation of 9-cis- and 11-cis-isomers of retinol into 9-cis-retinal and 11-cis-retinal, respectively. Furthermore, the lumenal orientation of the enzyme domain in the ER suggests that oxidation of both cis-isomers of retinol occurs in the ER.

Role of neurotrophin receptor TrkB in the maturation of rod photoreceptors and establishment of synaptic transmission to the inner retina

Brain-derived neurotrophic factor (BDNF) acts through TrkB, a receptor with kinase activity, and mitigates light-induced apoptosis in adult mouse rod photoreceptors. To determine whether TrkB signaling is necessary for rod development and function, we examined the retinas of mice lacking all isoforms of the TrkB receptor. Rod migration and differentiation occur in the mutant retina, but proceed at slower rates than in wild-type mice. In postnatal day 16 (P16) mutants, rod outer segment dimensions and rhodopsin content are comparable with those of photoreceptors in P12 wild type (WT). Quantitative analyses of the photoreceptor component in the electroretinogram (ERG) indicate that the gain and kinetics of the rod phototransduction signal in dark-adapted P16 mutant and P12 WT retinas are similar. In contrast to P12 WT, however, the ERG in mutant mice entirely lacks a b-wave, indicating a failure of signal transmission in the retinal rod pathway. In the inner retina of mutant mice, although cells appear anatomically and immunohistochemically normal, they fail to respond to prolonged stroboscopic illumination with the normal expression of c-fos. Absence of the b-wave and failure of c-fos expression, in view of anatomically normal inner retinal cells, suggest that lack of TrkB signaling causes a defect in synaptic signaling between rods and inner retinal cells. Retinal pigment epithelial cells and cells in the inner retina, including Müller, amacrine, and retinal ganglion cells, express the TrkB receptor, but rod photoreceptors do not. Moreover, inner retinal cells respond to exogenous BDNF with c-fos expression and extracellular signal-regulated kinase phosphorylation. Thus, interactions of rods with TrkB-expressing cells must be required for normal rod development.

Effect of targeted expression of clusterin in photoreceptor cells on retinal development and differentiation

Clusterin expression is increased in tissues undergoing apoptosis, including neurodegenerative retina, but the causal relationships remain to be clarified. To test the hypothesis that overexpression of clusterin could induce apoptosis in neurons, transgenic mice were generated in which rat clusterin transgene was expressed in photoreceptor cells under the transcriptional control of the human interphotoreceptor retinoid-binding protein (IRBP) promoter. Photoreceptor cell death in the resulting transgenic mice was examined by histology and TUNEL techniques. The expression of the clusterin transgene was confirmed by in situ hybridization in the photoreceptor cells, and results in a complex pattern of clusterin protein distribution in the retina. A reduction in apoptotic staining in the transgenic retinas was observed from birth to postnatal day 15. These results suggest that clusterin is not causally involved in apoptotic mechanisms of photoreceptor cell death, but may relate to cytoprotective functions.

Intraocular gene transfer of ciliary neurotrophic factor prevents death and increases responsiveness of rod photoreceptors in the retinal degeneration slow mouse

Several mutations causing both photoreceptor degeneration and malfunction have been identified in humans and animals. Although intraocular injection of trophic factors has been shown to reduce photoreceptor death in a few conditions of rapid photoreceptor loss, it is unclear whether long-term beneficial changes in functional properties of affected photoreceptors can be obtained by treatment with these factors. The rds/rds mouse is a spontaneous mutant bearing a null mutation in the rds/peripherin gene, which is linked to many forms of dominant retinal degenerations in humans. Here, we report that intraocular adenovirus-mediated gene transfer of ciliary neurotrophic factor (CNTF) in this mutant reduces photoreceptor loss, causes a significant increase in the length of photoreceptor segments, and results in a redistribution and an increase in the retinal content of the photopigment rhodopsin. These effects are accompanied by a significant increase in the amplitude of the a- and b-waves of the scotopic electroretinogram. These results suggest that continuous administration of CNTF could potentially be useful for the treatment of some forms of retinal degeneration.

Rpe65 is necessary for production of 11-cis-vitamin A in the retinal visual cycle

Mutation of RPE65 can cause severe blindness from birth or early childhood, and RPE65 protein is associated with retinal pigment epithelium (RPE) vitamin A metabolism. Here, we show that Rpe65-deficient mice exhibit changes in retinal physiology and biochemistry. Outer segment discs of rod photoreceptors in Rpe65-/- mice are disorganized compared with those of Rpe65+/+ and Rpe65+/- mice. Rod function, as measured by electroretinography, is abolished in Rpe65-/- mice, although cone function remains. Rpe65-/- mice lack rhodopsin, but not opsin apoprotein. Furthermore, all-trans-retinyl esters over-accumulate in the RPE of Rpe65-/- mice, whereas 11-cis-retinyl esters are absent. Disruption of the RPE-based metabolism of all-trans-retinyl esters to 11-cis-retinal thus appears to underlie the Rpe65-/- phenotype, although cone pigment regeneration may be dependent on a separate pathway.

Early onset photoreceptor abnormalities induced by targeted disruption of the interphotoreceptor retinoid-binding protein gene

Vision in all vertebrates is dependent on an exchange of retinoids between the retinal pigment epithelium and the visual photoreceptors. It has been proposed that the interphotoreceptor retinoid-binding protein (IRBP) is essential for this intercellular exchange, and that it serves to prevent the potentially cytotoxic effects of retinoids. Although its precise function in vivo has yet to be defined, the early expression of IRBP suggests that it may also be required for normal photoreceptor development. To further assess the biological role of IRBP, we generated transgenic mice with targeted disruption of the IRBP gene (IRBP-/- mice). Specifically, homologous recombination was used to replace the first exon and promoter region of the IRBP gene with a phosphoglycerate kinase-promoted neomycin-resistant gene. Immunocytochemical and Western blot analyses demonstrated the absence of IRBP expression in the IRBP-/- mice. As early as postnatal day 11, histological examination of the retinas of IRBP-/- mice revealed a loss of photoreceptor nuclei and changes in the structural integrity of the receptor outer segments. At 30 d of age, the photoreceptor abnormalities in IRBP-/- mice were more severe, and electroretinographic recordings revealed a marked loss in photic sensitivity. In contrast, no morphological or electrophysiological changes were detected in age-matched heterozygotes. These observations indicate that normal photoreceptor development and function are highly dependent on the early expression of IRBP, and that in the absence of IRBP there is a slowly progressive degeneration of retinal photoreceptors.

p107 is a suppressor of retinoblastoma development in pRb-deficient mice

Hemizygosity for the retinoblastoma gene RB in man strongly predisposes to retinoblastoma. In the mouse, however, Rb hemizygosity leaves the retina normal, whereas in Rb-/- chimeras pRb-deficient retinoblasts undergo apoptosis. To test whether concomitant inactivation of the Rb-related gene p107 is required to unleash the oncogenic potential of pRb deficiency in the mouse retina, we inactivated both Rb and p107 by homologous recombination in embryonic stem cells and generated chimeric mice. Retinoblastomas were found in five out of seven adult pRb/p107-deficient chimeras. The retinal tumors showed amacrine cell differentiation, and therefore originated from cells committed to the inner but not the outer nuclear layer. Retinal lesions were already observed at embryonic day 17.5. At this stage, the primitive nuclear layer exhibited severe dysplasia, including rosette-like arrangements, and apoptosis. These findings provide formal proof for the role of loss of Rb in retinoblastoma development in the mouse and the first in vivo evidence that p107 can exert a tumor suppressor function.

Reduction of all-trans-retinal limits regeneration of visual pigment in mice

Absorption of photons by pigments in photoreceptor cells results in photoisomerization of the chromophore, 11-cis-retinal, to all-trans-retinal and activation of opsin. Photolysed chromophore is converted back to the 11-cis-configuration via several enzymatic steps in photoreceptor and retinal pigment epithelial cells. We investigated the levels of retinoids in mouse retina during constant illumination and regeneration in the dark as a means of obtaining more information about the rate-limiting step of the visual cycle and about cycle intermediates that could be responsible for desensitization of the visual system. All-trans-retinal accumulated in the retinas during constant illumination and following flash illumination. Decay of all-trans-retinal in the dark following constant illumination occurred without substantial accumulation of all-trans-retinal, generated by constant approximately equal to visual pigment regeneration (t1/2 approximately 5 and t1/2 approximately 7 min, respectively). All-trans-retinal, generated by constant illumination, decayed approximately 3 times more rapidly than that generated by a flash and, as shown previously, the rate of rhodopsin regeneration following a flash was approximately 4 times slower than after constant illumination. The retinyl ester pool (> 95% all-trans-retinyl ester) did not show a statistically significant change in size or composition during illumination. In addition, constant illumination increased the amount of photoreceptor membrane-associated arrestin. The results suggest that the rate-limiting step of the visual cycle is the reduction of all-trans-retinal to all-trans-retinol by all-trans-retinol dehydrogenase. The accumulation of all-trans-retinal during illumination may be responsible, in part, for the reduction in sensitivity of the visual system that accompanies photobleaching and may contribute to the development of retinal pathology associated with light damage and aging.

Visual sensitivity and interphotoreceptor retinoid binding protein in the mouse: regulation by vitamin A

Interphotoreceptor retinoid binding protein (IRBP) is a retinoid and fatty acid binding glycoprotein secreted by rod and cone photoreceptors in all vertebrates. IRBP is believed to serve as a carrier for retinoids in the bleaching and regeneration cycle of rhodopsin. IRBP protein has been found to be decreased in vitamin A-deprived rats; it is rapidly recovered after retinol repletion. To understand the mechanism for this recovery, we determined whether vitamin A affects transcription and translation of the IRBP gene. Wild-type and transgenic mice harboring the IRBP promoter-CAT reporter fusion gene were maintained on a retinol-deficient diet supplemented with retinoic acid (-A) or on a control diet (+A) for up to 60 wk postweaning. Some of the -A mice were given retinol repletion for 7 days (-A+A). Electroretinography analysis revealed alterations in waveform and a 2 log unit decrease in b-wave sensitivity in the -A mice over a broad range of stimulus wavelengths. Retinol repletion effected a full recovery. Immunochemistry showed a significant decrease in the immunogold-labeled IRBP between the retinal pigment epithelium and the outer segments of the -A mice compared with +A and -A+A mice. Northern blots showed no differences in the amounts of IRBP or CAT mRNA between these three treatment groups. These results suggest that the regulation of IRBP by retinol is not transcriptional.

Spatiotemporal coordination of rod and cone photoreceptor differentiation in goldfish retina

In this study, we have compared spatial and temporal aspects of development of new rods and cones in the adult goldfish by using a combination of bromodeoxyuridine immunocytochemistry and opsin in situ hybridization to determine the intervals between terminal mitosis (cell "birth") and expression of opsin mRNA for each photoreceptor cell type. The goldfish opsins include rod opsin and four different cone opsins: red, green, blue, and ultraviolet. In a cohort of photoreceptors born at the same time, rods expressed opsin mRNA within 3 days of cell birth, while expression of cone opsin mRNA required at least 7 days. This temporal discrepancy in differentiation, coupled with a discordance in the site of cell genesis of rods and cones, allowed opsin expression to commence in both cell types in approximately the same retinal location. Commitment to the generic cone phenotype occurred within approximately 6 days throughout the cone cohort, as indicated by expression of interphotoreceptor retinoid-binding protein (IRBP) mRNA, but expression of a specific spectral phenotype was delayed until rods differentiated nearby. Onset of expression of cone opsin mRNA followed a phenotype-specific sequence: red, then green, then blue, and finally ultraviolet; in situ hybridization with two opsin probes confirmed that individual photoreceptors expressed only one type of opsin as they differentiated. This stepwise process of cone differentiation is consistent with the hypothesis that cell-cell interactions among developing photoreceptors may coordinate selection of specific photoreceptor phenotypes.

Interphotoreceptor retinoid-binding protein (IRBP): expression in the adult and developing Xenopus retina

Apposition of the neural retina and pigment epithelium is critical to photoreceptor development and function. Interphotoreceptor retinoid-binding protein (IRBP) is a major component of the extracellular matrix separating these epithelia in the African clawed frog Xenopus laevis (Gonzalez-Fernandez et al., [1993], J. Cell Sci. 105:7-21). In the adult retina, IRBP appears to mediate the transport of hydrophobic molecules, particularly retinoids and fatty acids, within the hydrophilic extracellular domain. In this paper, we compare the distribution of IRBP and its mRNA in adult and embryonic Xenopus retina. Xenopus IRBP antisense RNA, labeled with tritium or digoxigenin, was used for in situ hybridizaton studies. For immunohistochemistry, we used an antiserum against Xenopus IRBP expressed in Escherichia coli. In the adult, we found that IRBP is synthesized at similar levels by both rods and cones. The protein is restricted to the interphotoreceptor matrix, with lesser amounts in the pigment epithelial cytoplasm. In the embryo, expression of the mRNA for IRBP is restricted to the central retina, where photoreceptor differentiation has taken place. By contrast, the protein is distributed throughout the embryonic subretinal space. Therefore, the presence of IRBP precedes photoreceptor differentiation. In summary, IRBP is synthesized by both rods and cones and may be internalized by the pigment epithelium. In the embryo, IRBP is synthesized by the central retina and diffuses through the matrix, reaching the undifferentiated peripheral retina. In view of its ligand-binding properties, diffusion of IRBP may provide the peripheral neural retina with a vehicle to transport retinoids and docosahexaenoic acid (molecules critical to normal retinal development) from the pigment epithelium.

Circadian rhythms in mice can be regulated by photoreceptors with cone-like characteristics

In this report we have characterized the photopigments mediating circadian phase shifts in retinal degenerate (rd) mice. In aged rd/rd mice, which lack detectable opsin, high performance liquid chromatography (HPLC) was used to quantify the photopigment chromophore 11-cis-retinaldehyde. This chromophore was photoisomerized in whole eyes, suggesting the presence of a functional opsin-based photopigment system. We also analyzed the spectral sensitivity of phase shifting circadian locomotor rhythms. Our data implicate a photopigment that is consistent with the involvement of the middle wavelength-sensitive cone photoreceptors (M-cones; lambda(max) = 511 nm) found in the mouse retina. In addition, discrete near-ultraviolet (UV-A) pulses were capable of eliciting large phase shifts in circadian locomotor activity rhythms. This result is consistent with the involvement of the short wavelength-sensitive cone photoreceptors (UV-cones; lambda(max) = 359 nm) in photoentrainment. Collectively, these data suggest that both cone classes of the mouse may mediate the photic regulation of circadian rhythms. If this is the case, circadian sensitivity can be maintained by very few degenerate cones. Alternatively, an unknown class of ocular photoreceptor may fulfill this function.

Apoptosis or retinoblastoma: alternative fates of photoreceptors expressing the HPV-16 E7 gene in the presence or absence of p53

A transgenic mouse model for retinoblastoma was produced previously by directing SV40 T antigen expression to retinal photoreceptor cells using the promoter of the interstitial retinol-binding protein (IRBP) gene. This gene becomes active prior to the terminal differentiation of photoreceptors. Because T antigen-transforming activity is attributable, at least in part, to the inactivation of the retinoblastoma (pRb) and p53 tumor suppressor proteins, we addressed the role of p53 in the development of retinoblastoma in mice. Transgenic mice expressing HPV-16 E7 under the control of the IRBP promoter were generated to inactivate pRb in photoreceptors while leaving p53 intact. Rather than developing retinoblastomas, the retinas of these mice degenerate due to photoreceptor cell death at a time in development when photoreceptors are normally undergoing terminal differentiation. The dying cells exhibit the histological and ultrastructural features of apoptosis and contain fragmented DNA. p53 is required for the induction of apoptosis in this model, because mice expressing E7 in a p53 nullizygous background develop retinal tumors instead of undergoing retinal degeneration.

Retinoid dynamics in chicken eye during pre- and postnatal development

Changes in the steady state level of retinols, retinaldehydes and retinyl esters in the trans and 11-cis forms and trans retinoic acid were measured in whole chicken eye during development from day 6 in ovo to day 3 post-hatch. These retinoids, quantified by different HPLC systems, were detected in this time sequence: trans-retinol and trans-retinyl esters in the first week in ovo, 11-cis-retinol in the second week. The highest level of 11-cis-retinaldehyde and 11-cis-retinyl esters was reached at the end of development in ovo; however, their levels increased further after hatching. The retinoic acid level decreased at the end of the first week, rising again at the end of the second week. The enzyme activities involved in the metabolism of these retinoids-acyl-CoA: retinol acyltransferase, trans-retinol dehydrogenase, 11-cis-retinol dehydrogenase, trans-retinyl ester hydrolase and trans: 11-cis-retinol isomerase were also estimated and they were detectable already in the first week of development in ovo. At day 6 of the biosynthesis of retinoic acid by the retinaldehyde dehydrogenase activity from retina cytosol was also shown.

Retinoblastoma. Interphotoreceptor retinoid binding protein mRNA analysis by polymerase chain reaction

The authors used the polymerase chain reaction (PCR) to detect the mRNA for interphotoreceptor retinoid-binding protein (IRBP/RBP3), a photoreceptor specific protein, in small samples. They carried out these experiments to assess the feasibility of applying this technique to small tumor samples. Surgically excised tumor samples from four enucleations were analyzed. Messenger RNA isolation by acid guanidinium thiocyanate-phenol-chloroform extraction was followed by phenol-chloroform purification, reverse-transcription and amplification. The primers used were 5' TGATGACTCTGTCAGTG 3' in exon 3 (sense) and 5' TTGTGCTGGAGCATCTC 3' in exon 4 (antisense). Controls included an IRBP cDNA pIRBP 20-700 and RNA from normal human retina. All samples amplified the same size band if detected. Three tumor samples contained IRBP mRNA as indicated by amplified 234 bp band. These three samples showed a high IRBP protein level by slot blot and RNA for IRBP detected by northern blot. Hematoxylin-eosin staining of one of these samples revealed a well differentiated tumor with numerous Flexner-Wintersteiner rosettes. In the fourth tumor, a poorly differentiated neoplasm, no IRBP mRNA was detected. The authors' results showed a qualitative variation of IRBP mRNA levels, usually related to the histologic differentiation, with IRBP expressed in well differentiated tumors as well as in the normal human retina in contrast to a poorly differentiated tumor with no detectable IRBP. The feasibility of the reverse transcriptase-PCR (RT-PCR) technique to detect IRBP mRNA in small retinoblastoma tumors, was demonstrated.

Distribution of three retinal proteins in developing octopus photoreceptors

The expression of proteins unique to plasma membrane domains of developing photoreceptors is used as a marker for retinal differentiation in vertebrates. Invertebrate photoreceptors are also compartmentalized, but little information is available on the development of these compartments or the expression of retinal proteins specific to these cellular regions. Using routine electron microscopy techniques, we have made observations on the formation of photoreceptor organelles, including myeloid bodies and rhabdomeres, in embryonic octopus eyes from an early stage in development through hatching. Immunocytochemical experiments on the embryos demonstrate a timed expression of three retinal proteins during development, and the early separation of the octopus photoreceptor plasma membrane into distinct domains. Using polyclonal antibodies for opsin, retinochrome and retinal binding protein we have shown that opsin appears first and is confined to the distal end of the photoreceptor that will eventually differentiate into rhabdomeres. This membrane domain is separated from the proximal/inner segment plasma membrane by a septate junction. Retinochrome is expressed later when the myeloid bodies appear in the inner segments, and retinal binding protein is apparently not synthesized until sometime after hatching. These results suggest that, in the cephalopod retina, protein components of the retinoid cycling apparatus appear in a specific developmental sequence during the differentiation of this tissue.

Interphotoreceptor retinoid-binding protein (IRBP), a major 124 kDa glycoprotein in the interphotoreceptor matrix of Xenopus laevis. Characterization, molecular cloning and biosynthesis

We have demonstrated that the neural retina of Xenopus laevis secretes into the extracellular matrix surrounding the inner and outer segments of its photoreceptors a glycoprotein containing hydrophobic domains conserved in mammalian interphotoreceptor retinoid-binding proteins (IRBPs). The soluble extract of the interphotoreceptor matrix contains a 124 kDa protein that cross-reacts with anti-bovine IRBP immunoglobulins. In vitro [3H]fucose incorporation studies combined with in vivo light and electron microscopic autoradiographic analysis, showed that the IRBP-like glycoprotein is synthesized by the neural retina and secreted into the interphotoreceptor matrix. A 1.2 kb Xenopus IRBP cDNA was isolated by screening a stage 42 (swimming tadpole) lambda Zap II library with a human IRBP cDNA under low-stringency conditions. The cDNA hybridizes with a 4.2 kb mRNA in adult Xenopus neural retina, tadpole heads as well as a less-abundant mRNA of the same size in brain. During development, IRBP and opsin mRNA expression correlates with photoreceptor differentiation. The translated amino acid sequence of the Xenopus IRBP clone has an overall 70% identity with the fourth repeat of the human protein. Sequence alignment with the four repeats of human IRBP showed three highly conserved regions, rich in hydrophobic residues. This focal conservation predicts domains important to the protein's function, which presumably is to facilitate the exchange of 11-cis retinal and all-trans retinol between the pigment epithelium and photoreceptors, and to the transport of fatty acids through the hydrophilic interphotoreceptor matrix.

Appearance of cGMP-phosphodiesterase immunoreactivity parallels the morphological differentiation of photoreceptor outer segments in the rat retina

We have investigated by immunofluorescence the appearance of immunoreactive guanosine 3'-5' cyclic monophosphate phosphodiesterase (cGMP-PDE) during the postnatal development of the retina of the pigmented rat. We show that a sudden increase in immunoreactivity takes place during postnatal day five (P5), when rod outer segments begin to form; immunoreactivity develops rapidly in the following days. Labeling is restricted to the developing photoreceptor outer segments, sparing other retinal cells, as confirmed by electron microscopy immunocytochemistry. In addition, cGMP-PDE immunoreactivity follows a center-to-periphery gradient paralleling photoreceptor differentiation. It appears that cGMP-PDE is expressed when the photoreceptor subcellular compartments are already formed, and represents a specific marker of late photoreceptor differentiation. The appearance of cGMP-PDE during development is temporally correlated with the appearance of other proteins of the phototransduction machinery.

Early opsin expression in Xenopus embryos precedes photoreceptor differentiation

The visual pigment which serves as the first step in the phototransduction cycle in vertebrate rod cells consists of a retinal chromophore which is linked to the transmembrane protein, opsin. Opsin genes have been isolated from a number of different organisms and studies have shown opsin to be developmentally regulated with both mRNA and protein expression associated with the morphological differentiation of photoreceptor cells. Due to its potential utility as a marker for rod photoreceptor determination in studies of retinal tissue interactions, and because no amphibian opsin genes have as yet been cloned, we isolated cDNA clones of the Xenopus laevis opsin gene. Sequence analysis shows that within the coding region Xenopus opsin shares a high degree of identity with other rod opsin genes, except at the C-terminal where it more closely resembles the mammalian color opsins. A developmental analysis, on the other hand, reveals that Xenopus opsin transcripts are detectable in a retina-specific fashion early in retinal development. Using in situ hybridization we find that Xenopus opsin mRNA is initially restricted to a few isolated cells in the presumptive photoreceptor layer which express the gene at relatively high levels. This suggests that rod photoreceptor determination occurs in single cells, and that the mechanisms controlling opsin expression in Xenopus are initiated well before any evidence of morphological differentiation.