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

Substrate specificity and kinetics for VP14, a carotenoid cleavage dioxygenase in the ABA biosynthetic pathway

The plant growth regulator, abscisic acid (ABA), is synthesized via the oxidative cleavage of an epoxy-carotenoid. Specifically, a double bond is cleaved by molecular oxygen and an aldehyde is formed at the site of cleavage in both products. The Vp14 gene from maize encodes an oxidative cleavage enzyme for ABA biosynthesis and the recombinant VP14 protein catalyzes the cleavage reaction in vitro. The enzyme has a strict requirement for a 9-cis double bond adjacent to the site of cleavage (the 11-12 bond), but shows some plasticity in other features of carotenoids that are cleaved. A kinetic analysis with the 9-cis isomer of five carotenoids displays several substrate activity relationships. One of the carotenoids was not readily cleaved, but inhibited the cleavage of another substrate in mixed assays. Of the remaining four carotenoids used in this study, three of the substrates have similar V(max) values. The V(max) for the cleavage of one carotenoid substrate was significantly higher. Molecular modeling and several three-dimensional quantitative substrate-activity relationship programs were used to analyze these results. In addition to a 9-cis double bond, the presence and orientation of the ring hydroxyl affects substrate binding or the subsequent cleavage. Additional variations that affect substrate cleavage are proposed.

Why study rod cell death in retinal degenerations and how?

Age-related macular degeneration (AMD) is a main causes of severe visual impairment in the elderly in industrialized countries. The pathogenesis of this complex diseases is largely unknown, even though clinical characteristics and histopathology are well described. Because several aging changes are identical to those observed in AMD, there appears to exist an unknown switch mechanism from normal ageing to disease. Recent anatomical studies using elegant innovative techniques reveal that there is a 30% rod loss in normal ageing, which is increased in early AMD. Those and other observations by Curcio and co-workers indicate that early rod loss is an important denominator of AMD (Curcio CA. Eye 2001; 15:376). As in retinitis pigmentosa (RP), rods appear to die by apoptosis. Thus it seems mandatory to study the regulation of rod cell death in animal models to unravel possible mechanisms of rod loss in AMD. Our laboratory investigates signal transduction pathways and gene regulation of rod death in our model of light-induced apoptosis. The transcription factor AP1 is essential, whereas other classical pro- and antiapoptotic genes appear to be less important in our model system. Caspase-1 gene expression is distinctly upregulated after light exposure and there are several factors which completely protect against light-induced cell death, such as the anesthetic halothane, dexamethasone and the absence of bleachable rhodopsin during light exposure. A fast rhodopsin regeneration rate increased damage susceptibility. Our data indicate that rhodopsin is essential for the initiation of light-induced rod loss. Following photon absorption, there may be the generation of photochemically active molecules wich then induce the apoptotic death cascade.

Prevention and therapy in hereditary retinal degenerations

A survey of the present state of developments concerning prevention and therapy of hereditary retinal diseases is given. In recent years the techniques of molecular genetics have led to a rapid identification of a great number of cloned or mapped genes responsible for such diseases. Moreover, further progress has been made in the understanding of disease mechanisms by describing the defective proteins and their altered metabolism. The most recent preventive and therapeutic strategies including gene therapy, dietary regimens, anti-apoptotic agents, transplantation and retinal prosthesis are discussed.

Retinal neuroprotection by growth factors: a mechanistic perspective

For more than a decade it has been known that certain growth factors inhibit apoptosis in genetically determined and experimental models of inner and outer retinal degeneration. The molecular mechanisms underlying these protective effects and the signaling that supports the survival of photoreceptors and retinal ganglion cells in these models have recently come under more in depth investigation. This paper reviews our current understanding of the balance of pro- and antiapoptotic signals that determine cell fate in the retina and how the activation of key signal transduction pathways by specific classes of neurotrophins protects retinal neurons.

Biochemical and physiological properties of rhodopsin regenerated with 11-cis-6-ring- and 7-ring-retinals

Phototransduction is initiated by the photoisomerization of rhodopsin (Rho) chromophore 11-cis-retinylidene to all-trans-retinylidene. Here, using Rho regenerated with retinal analogs with different ring sizes, which prevent isomerization around the C(11)=C(12) double bond, the activation mechanism of this G-protein-coupled receptor was investigated. We demonstrate that 11-cis-7-ring-Rho does not activate G-protein in vivo and in vitro, and that it does not isomerize along other double bonds, suggesting that it fits tightly into the binding site of opsin. In contrast, bleaching 11-cis-6-ring-Rho modestly activates phototransduction in vivo and at low pH in vitro. These results reveal that partial activation is caused by isomerization along other double bonds in more rigid 6-locked retinal isomers and protonation of key residues by lowering pH in 11-cis-6-ring-Rhos. Full activation is not achieved, because isomerization does not induce a complete set of conformational rearrangements of Rho. These results with 6- and 7-ring-constrained retinoids provide new insights into Rho activation and suggest a potential use of locked retinals, particularly 11-cis-7-ring-retinal, to inactivate opsin in some retinal degeneration diseases.

11-cis-retinal reduces constitutive opsin phosphorylation and improves quantum catch in retinoid-deficient mouse rod photoreceptors

Rpe65(-/-) mice produce minimal amounts of 11-cis-retinal, the ligand necessary for the formation of photosensitive visual pigments. Therefore, the apoprotein opsin in these animals has not been exposed to its normal ligand. The Rpe65(-/-) mice contain less than 0.1% of wild type levels of rhodopsin. Mass spectrometric analysis of opsin from Rpe65(-/-) mice revealed unusually high levels of phosphorylation in dark-adapted mice but no other structural alterations. Single flash and flicker electroretinograms (ERGs) from 1-month-old animals showed trace rod function but no cone response. B-wave kinetics of the single-flash ERG are comparable with those of dark-adapted wild type mice containing a full compliment of rhodopsin. Application (intraperitoneal injection) of 11-cis-retinal to Rpe65(-/-) mice increased the rod ERG signal, increased levels of rhodopsin, and decreased opsin phosphorylation. Therefore, exogenous 11-cis-retinal improves photoreceptor function by regenerating rhodopsin and removes constitutive opsin phosphorylation. Our results indicate that opsin, which has not been exposed to 11-cis-retinal, does not generate the activity generally associated with the bleached apoprotein.

Evidence for two apoptotic pathways in light-induced retinal degeneration

Excessive phototransduction signaling is thought to be involved in light-induced and inherited retinal degeneration. Using knockout mice with defects in rhodopsin shut-off and transducin signaling, we show that two different pathways of photoreceptor-cell apoptosis are induced by light. Bright light induces apoptosis that is independent of transducin and accompanied by induction of the transcription factor AP-1. By contrast, low light induces an apoptotic pathway that requires transducin. We also provide evidence that additional genetic factors regulate sensitivity to light-induced damage. Our use of defined mouse mutants resolves some of the complexity underlying the mechanisms that regulate susceptibility to retinal degeneration.

Genetic animal models for retinal degeneration

Inherited retinal degenerations are a common cause of blindness in Western countries. A mechanism for most retinal degenerations is still unknown; hence, a suitable treatment for most of these diseases has yet to be found. Before one can rationally design a treatment, it is necessary to understand the pathway from a gene mutation to the phenotype in patients. Animal models are crucial to understand this process and to develop a treatment. Some naturally occurring animal models are known. However, over the past few years, transgenic engineering has allowed the generation of a rapidly growing number of animal models. In this review, we give an overview of the broad variety of genetic animal models for retinal degeneration.

Prenatal human ocular degeneration occurs in Leber's congenital amaurosis (LCA2)

BACKGROUND

Leber's congenital amaurosis (LCA) encompasses the most precocious and severe forms of inherited retinal dystrophy, displaying very significant visual handicap at or soon after birth. Among the currently identified mutations, alterations in the gene coding for retinal pigment epithelium 65-kDa protein (RPE65) lead to LCA2. Existing animal models for LCA2 (RPE65(-/-) null mice and naturally occurring RPE65(-/-) Briard dogs) exhibit near normal retinal histology at birth, although no recordable photofunction can be detected. Structural degeneration in both cases occurs with delayed onset, cone death generally preceding that of rods.

METHODS

We obtained retinal tissue from a voluntarily aborted embryo of an LCA2 carrier in order to compare histopathology and immunohistochemistry with age-matched normal foetal retina.

RESULTS

Compared to normal retinas, affected retina displayed cell loss and thinning of the outer nuclear (photoreceptor) layer, decreased immunoreactivity for key phototransduction proteins, and aberrant synaptic and inner retinal organisation. The gene mutation abolished detectable expression of RPE65 within the retinal pigment epithelium (RPE) of affected eyes, and ultrastructural examination revealed the presence of lipid and vesicular inclusions not seen in normal RPE. In addition, mutant eyes demonstrated thickening, detachment and collagen fibril disorganisation in the underlying Bruch's membrane, and the choroid was distended and abnormally vascularised, in comparison with controls.

CONCLUSIONS

Such data contrast with the late-onset ocular changes observed in animal models, indicating caution should be exercised when inferring human retinal pathophysiology from information based on other species.

Gene therapy for retinal and choroidal diseases

The eye is a small compartment separated from the systemic circulation by the blood-ocular barriers, providing advantages for intraocular gene transfer - an approach which is being investigated for several types of retinal and choroidal diseases. A compelling application is gene replacement for homozygous loss-of-function mutations in genes differentially expressed in photoreceptors or retinal pigmented epithelial (RPE) cells that result in retinal degeneration. Considerable progress has been made in this area, including demonstration of return of visual function in RPE65 (-/-) dogs after subretinal injection of adeno-associated viral vectors encoding RPE65, providing groundwork for a clinical trial in patients with Leber's Congenital Amaurosis. Proof of principle has been provided for intraocular gene transfer of ribozymes for dominantly inherited retinal degenerations. Survival factor gene therapy shows promise for treatments that may be used in multiple retinal degenerations. Transduction of intraocular and/or periocular cells with constructs that encode antiangiogenic proteins provides a new approach for sustained local delivery treatment of retinal and choroidal neovascularisation. While considerable investigation remains to work out critical details, there is substantial evidence suggesting that in the near future, gene therapy-based treatments will be an important addition to what is currently offered to patients with retinal and/or choroidal diseases.

HEK293S cells have functional retinoid processing machinery

Rhodopsin activation is measured by the early receptor current (ERC), a conformation-associated charge motion, in human embryonic kidney cells (HEK293S) expressing opsins. After rhodopsin bleaching in cells loaded with 11-cis-retinal, ERC signals recover in minutes and recurrently over a period of hours by simple dark adaptation, with no added chromophore. The purpose of this study is to investigate the source of ERC signal recovery in these cells. Giant HEK293S cells expressing normal wild-type (WT)-human rod opsin (HEK293S) were regenerated by solubilized 11-cis-retinal, all-trans-retinal, or Vitamin A in darkness. ERCs were elicited by flash photolysis and measured by whole-cell recording. Visible flashes initially elicit bimodal (R(1), R(2)) ERC signals in WT-HEK293S cells loaded with 11-cis-retinal for 40 min or overnight. In contrast, cells regenerated for 40 min with all-trans-retinal or Vitamin A had negative ERCs (R(1)-like) or none at all. After these were placed in the dark overnight, ERCs with outward R(2) signals were recorded the following day. This indicates conversion of loaded Vitamin A or all-trans-retinal into cis-retinaldehyde that regenerated ground-state pigment. 4-butylaniline, an inhibitor of the mammalian retinoid cycle, reversibly suppressed recovery of the outward R(2) component from Vitamin A and 11-cis-retinal-loaded cells. These physiological findings are evidence for the presence of intrinsic retinoid processing machinery in WT-HEK293S cells similar to what occurs in the mammalian eye.

Photoreceptor degeneration and dysfunction in aging and age-related maculopathy

The relative rate of rod and cone degeneration is a fundamental characteristic of any disorder affecting photoreceptors, including aging and age-related maculopathy (ARM). The human macula consists of a small cone-dominated fovea surrounded by a rod-dominated parafovea. In aging and early ARM, rods degenerate before cones, a decline in scotopic (rod-mediated) sensitivity is more prominent than a decline in photopic (cone-mediated) sensitivity, and the time course of dark adaptation of rods slows dramatically. The topography of rod dysfunction and loss in early ARM matches the location of pathology in the retinal pigment epithelium (RPE)/Bruch's membrane complex visible in the ocular fundus. Rod dysfunction and loss in aging and ARM may be due to retinoid deficiency at the level of the photoreceptors cause by impaired retinoid translocation across the RPE/Bruch's membrane complex, a hypothesis deserving of further investigation.

Recovery of visual functions in a mouse model of Leber congenital amaurosis

The visual process is initiated by the photoisomerization of 11-cis-retinal to all-trans-retinal. For sustained vision the 11-cis-chromophore must be regenerated from all-trans-retinal. This requires RPE65, a dominant retinal pigment epithelium protein. Disruption of the RPE65 gene results in massive accumulation of all-trans-retinyl esters in the retinal pigment epithelium, lack of 11-cis-retinal and therefore rhodopsin, and ultimately blindness. We reported previously (Van Hooser, J. P., Aleman, T. S., He, Y. G., Cideciyan, A. V., Kuksa, V., Pittler, S. J., Stone, E. M., Jacobson, S. G., and Palczewski, K. (2000) Proc. Natl. Acad. Sci. U. S. A. 97, 8623-8628) that in Rpe65-/- mice, oral administration of 9-cis-retinal generated isorhodopsin, a rod photopigment, and restored light sensitivity to the electroretinogram. Here, we provide evidence that early intervention by 9-cis-retinal administration significantly attenuated retinal ester accumulation and supported rod retinal function for more than 6 months post-treatment. In single cell recordings rod light sensitivity was shown to be a function of the amount of regenerated isorhodopsin; high doses restored rod responses with normal sensitivity and kinetics. Highly attenuated residual rod function was observed in untreated Rpe65-/- mice. This rod function is likely a consequence of low efficiency production of 11-cis-retinal by photo-conversion of all-trans-retinal in the retina as demonstrated by retinoid analysis. These studies show that pharmacological intervention produces long lasting preservation of visual function in dark-reared Rpe65-/- mice and may be a useful therapeutic strategy in recovering vision in humans diagnosed with Leber congenital amaurosis caused by mutations in the RPE65 gene, an inherited group of early onset blinding and retinal degenerations.

What is lipofuscin? Defining characteristics and differentiation from other autofluorescent lysosomal storage bodies

Lipofuscins, also known as age-pigments, have three defining characteristics: (1) they consist of intracellular secondary lysosomes; (2) they have a yellow autofluorescent emission when excited by near ultraviolet or blue light; and (3) they accumulate during normal senescence. Lysosomal storage bodies with similar fluorescence properties accumulate in various cell types as a result of specific pathological conditions or experimental manipulations. As a class, the latter are often referred to as ceroid pigments. In general, the mechanisms involved in the formation of ceroid pigments cannot be assumed to be closely similar to those involved in lipofuscin formation. In fact, the mechanisms of formation almost certainly differ, not only between lipofuscins and ceroids, but also among different lipofuscins and different ceroids. Presently, the most detailed knowledge about the mechanisms involved in lipofuscin formation come from studies on the retinal pigment epithelium (RPE) of the eye. These studies indicate that at least the autofluorescent constituents of RPE lipofuscin are generated from derivatives of vitamin A that occur in the retina. Oxidative stress to the retina appears to promote the formation of these RPE fluorophores. Whether similar mechanisms are involved in the formation of the lipofuscins that occur in other tissues remains to be determined. The mechanisms involved in RPE lipofuscin fluorophore formation are closely related to metabolic pathways that are specific to the retina. Thus, it appears likely that the mechanisms by which lipofuscins form in other tissues differ fundamentally from those that underlie RPE lipofuscin formation.

Expression and promoter activation of the Rpe65 gene in retinal pigment epithelium cell lines

PURPOSE

To examine the expression and promoter activation of the retinal pigment epithelium (RPE)-preferentially expressed Rpe65 gene in the commonly available RPE cell lines.

METHODS

Reverse transcription coupled to polymerase chain reaction (RT-PCR) was performed after total RNA extraction from different RPE (ARPE-19, monkey, hTERT-RP1 and D407) and non-RPE (COS-7, HeLa, HepG2 and HS27) cell lines. Promoter activity was assayed by transient transfection of luciferase reporter constructs containing nested deletions of the 5' flanking region of the mouse Rpe65 gene. The involvement of a putative TATA box in the basal promoter expression was studied by site-directed mutagenesis in D407 cells and binding of TATA box-related transcription factors to that region was demonstrated by Electrophoretic Mobility Shift Assays (EMSA).

RESULTS

Expression of the human RPE65 cDNA was observed in all the RPE cell lines tested, and in COS-7 cells (monkey RPE65 cDNA). Transient transfections of the mouse Rpe65 promoter/luciferase transgene containing nested deletions of the Rpe65 5' flanking region showed that fragments containing bases -655 to +48 and -1240 to +48 generated specific promoter activity only in the D407 cell line. A promoter fragment from -49 to +48 directed basal promoter activity in all the cell lines tested. Part of this basal activity was due to a putative TATA box that specifically binds transcription factors contained in a D407 nuclear extract.

CONCLUSIONS

Although transcription of the Rpe65 gene occurs in all the tested cell lines, we find that the D407 cell line is the only one capable of directing specific mouse Rpe65 promoter activity. This limits the study of the transcriptional regulation of the mouse Rpe65 gene in vitro to this particular cell line.

Bernard Strehler-inspiration for basic research into the mechanisms of aging

Bernard Strehler, who passed away recently, has provided inspiration and intellectual guidance for a generation of scientists interested in the biology of aging. My own career in this field was launched in large part by the ideas and concepts discussed by Dr Strehler in his book, Time, Cells, and Aging. Much of my scientific career has been devoted to studying one aspect of the aging process-the intracellular accumulation of autofluorescent lysosomal storage bodies (lipofuscin) during senescence. Work in my laboratory has contributed to the elucidation of the molecular mechanisms that underlie formation of lipofuscin in the retinal pigment epithelium of the eye. The challenge for the work on lipofuscin, and for much of the current research on aging, is to determine whether specific age-related changes such as lipofuscin accumulation, are involved in determining maximum life span. Bernard Strehler's eloquent statement of this challenge will hopefully continue to inspire new research to further our understanding of the aging phenomenon.

RPE65 gene: multiplex PCR and mutation screening in patients from India with retinal degenerative diseases

We used multiplex PCR follwed by sequencing to screen for mutations in the 14 exons of the RPE65 gene in early-childhood-onset autosomal recessive retinitis pigmentosa (arRP) and Leber's congenital amaurosis (LCA) patients. The RPE65 protein is believed to play an important role in the metabolism of vitamin A in the visual cycle and mutations identified in the gene could have implications for vitamin A-based therapeutic intervention. We were able to identify a homozygous mutation (AAT --> AAG) in exon 9 in an arRP patient and a heterozygous missense transversion (AAT --> AAG) also in exon 9 of an LCA patient. We also identified a polymorphism in exon 10 (GAG --> GAA) in an arRP as well as an LCA patient. Mutation screening would be greatly facilitated by multiplex PCR which could cut down costs, labour and time involved. The nucleotide changes observed in this study could be de novo. Though a larger study has been undertaken, from the preliminary results it appears that in India the RPE65 gene seems to be less involved in causation of LCA.

The complexities of ocular genetics

The cliché 'a picture is worth a thousand words' is a testament to the power of the visual system in helping us deal with our physical environment. Rarely do perturbations to the visual system, even minor ones, go unnoticed. Major defects in eye development may occur in the absence of systemic problems which threaten health. Ocular anomalies offer a window into many developmental events which would otherwise be difficult to study.

Genetic analysis of photoreceptor cell development in the zebrafish retina

To gain insight into the genetic mechanisms of photoreceptor development, we analyzed a collection of zebrafish mutations characterized by early photoreceptor cell loss. The mutant defects impair outer segment formation and are accompanied by an abnormal distribution of visual pigments. Rods and different cone types display defects of similar severity suggesting that genetic pathways common to all photoreceptors are affected. To investigate whether these phenotypes involve cell-cell interaction defects, we analyzed genetically mosaic animals. Interaction of niezerka photoreceptors with wild-type tissues improves the survival of mutant cells and restores their elongated morphology. In contrast, cells carrying mutations in the loci brudas, elipsa, fleer, and oval retain their defective phenotypes in a wild-type environment indicating cell-autonomy. These experiments identify distinct phenotypic categories of photoreceptor mutants and indicate that zebrafish photoreceptor defects involve both cell-autonomous and cell-nonautonomous mechanisms.

Retinal dystrophy due to paternal isodisomy for chromosome 1 or chromosome 2, with homoallelism for mutations in RPE65 or MERTK, respectively

Uniparental disomy (UPD) is a rare condition in which a diploid offspring carries a chromosomal pair from a single parent. We now report the first two cases of UPD resulting in retinal degeneration. We identified an apparently homozygous loss-of-function mutation of RPE65 (1p31) in one retinal dystrophy patient and an apparently homozygous loss-of-function mutation of MERTK (2q14.1) in a second retinal dystrophy patient. In both families, the gene defect was present in the patient's heterozygous father but not in the patient's mother. Analysis of haplotypes in each nuclear kindred, by use of DNA polymorphisms distributed along both chromosomal arms, indicated the absence of the maternal allele for all informative markers tested on chromosome 1 in the first patient and on chromosome 2 in the second patient. Our results suggest that retinal degeneration in these individuals is due to apparently complete paternal isodisomy involving reduction to homoallelism for RPE65 or MERTK loss-of-function alleles. Our findings provide evidence for the first time, in the case of chromosome 2, and confirm previous observations, in the case of chromosome 1, that there are no paternally imprinted genes on chromosomes 1 and 2 that have a major effect on phenotype.

Isomerization of 11-cis-retinoids to all-trans-retinoids in vitro and in vivo

The regeneration of 11-cis-retinal, the universal chromophore of the vertebrate retina, is a complex process involving photoreceptors and adjacent retinal pigment epithelial cells (RPE). 11-cis-Retinal is coupled to opsins in both rod and cone photoreceptor cells and is photoisomerized to all-trans-retinal by light. Here, we show that RPE microsomes can catalyze the reverse isomerization of 11-cis-retinol to all-trans-retinol (and 13-cis-retinol), and membrane exposure to UV light further enhances the rate of this reaction. This conversion is inhibited when 11-cis-retinol is in a complex with cellular retinaldehyde-binding protein (CRALBP), providing a clear demonstration of the protective effect of retinoid-binding proteins in retinoid processes in the eye, a function that has been long suspected but never proven. The reverse isomerization is nonenzymatic and specific to alcohol forms of retinoids, and it displays stereospecific preference for 11-cis-retinol and 13-cis-retinol but is much less efficient for 9-cis-retinol. The mechanism of reverse isomerization was investigated using stable isotope-labeled retinoids and radioactive tracers to show that this reaction occurs with the retention of configuration of the C-15 carbon of retinol through a mechanism that does not eliminate the hydroxyl group, in contrast to the enzymatic all-trans-retinol to 11-cis-retinol reaction. The activation energy for the conversion of 11-cis-retinol to all-trans-retinol is 19.5 kcal/mol, and 20.1 kcal/mol for isomerization of 13-cis-retinol to all-trans-retinol. We also demonstrate that the reverse isomerization occurs in vivo using exogenous 11-cis-retinol injected into the intravitreal space of wild type and Rpe65-/- mice, which have defective forward isomerization. This study demonstrates an uncharacterized activity of RPE microsomes that could be important in the normal flow of retinoids in the eye in vivo during dark adaptation.

Adenoviral vector-mediated beta-glucuronidase cDNA transfer to treat MPS VII RPE in vitro

PURPOSE

To develop an effective therapy for treating glycosaminoglycan (GAG) storage in mucopolysaccharidosis VII (MPS VII) retinal pigment epithelium (RPE) in vitro using adenoviral vector mediated human beta-glucuronidase cDNA (Ad-GUSB) transfer.

METHODS

Ad-GUSB was used to infect RPE at confluency. The transduction condition was optimized varying time of infection and number of infectious particles. The beta-glucuronidase (GUSB) activity was measured in transduced cells and media using a fluorogenic substrate. The GAG profiles were examined by metabolically labeling RPE with (35)Na(2)SO(4).

RESULTS

Transduced RPE, irrespective of species or disease status, expressed a high level of beta-glucuronidase. The expressed enzyme restored normal levels of GAGs in the RPE cells of homozygous affected MPS VII dogs by metabolizing stored GAGs. The over-expressed enzyme (>10 000 nmoles/hr/mg) failed to restore normal level of GAGs. A high level of GUSB expression was maintained in vitro at least nine weeks.

CONCLUSIONS

Adenoviral vector could mediate transfer of GUSB in MPS VII affected RPE and RPE of various species, and the expression was observed to be stable in vitro. However, controlled expression of GUSB was essential for the metabolism of stored GAGs to achieve normal levels.

Identifying UV-cone responses in the murine superior colliculus

A microelectrode was used to detect local visually evoked potentials from clusters of neurons in the murine superior colliculus. Chromatic stimuli and selective chromatic adaptation were used to identify responses of UV and middle-wavelength-sensitive (M) cones and/or rods. Three types of evoked potentials were found: those driven by UV and M cones and/or rods and those driven only by UV cone or only by M cones and/or rods. UV cone responses were more frequent in the medial and those from M cones more common in the lateral part of the superior colliculus. All three responses were found in the same area. UV cones provide a significant input to the murine superior colliculus. The spatial distribution of these responses in the superior colliculus reflects the organization of UV cones in the retina. Although synergistic inputs from UV- and M-cone and/or rod inputs appear to mix in local evoked responses in the superior colliculus, some areas are found to transmit only UV- or only M-cone and/or rod responses, indicating that there cannot be a widespread mixing of UV- and M-cone opsins in all murine cones.

A mutation of early photoreceptor development, mikre oko, reveals cell-cell interactions involved in the survival and differentiation of zebrafish photoreceptors

To gain insight into mechanisms involved in photoreceptor development, we characterized a zebrafish mutation in the mikre oko locus that produces early loss of photoreceptor cells. mikre oko photoreceptors lose their elongated morphology at the time of wild-type outer segment formation and undergo cell death within a few days. To investigate whether this phenotype involves cell-cell interaction defects, we performed analysis of genetically mosaic animals. Interactions of mikre oko photoreceptors with wild-type cells rescue several aspects of the mutant phenotype. When placed in a wild-type environment, mikre oko photoreceptor cells retain elongated morphology and survive longer. Moreover, although mutant mikre oko photoreceptor outer segments develop only infrequently and are usually disorganized, mikre oko cone and rod cells in mosaic retinas develop robust outer segments that closely resemble the wild type. In contrast to the outer segments, the proximal regions of mikre oko photoreceptor cells, including their inner segments, the nuclear regions, and the synaptic termini, retain the mutant appearance. mikre oko outer segment rescue is not mediated by interactions with the retinal pigment epithelium. These studies demonstrate that the differentiation of outer segments is surprisingly independent from the more proximal photoreceptor cell features and that outer segment development includes retinal pigment epithelium-independent cell-cell interactions.

The sights along route 65

Regeneration of rod visual pigments after photobleaching requires a protein called Rpe65. Several studies clarify its role in visual physiology and pathology, including a new one that shows it is required pigment regeneration in cone cells.

Ultraviolet and middle wavelength sensitive cone responses in the electroretinogram (ERG) of normal and Rpe65 -/- mice

Ultra-violet (UV) and middle wavelength sensitive (M) cone responses were identified in the ERG of normal and Rpe65 -/- mice using chromatic flashes and selective chromatic adaptation. In normal mice, the UV-cone response was as large as, or larger, in the presence of a bright yellow adapting light than it is in the presence of a dim white light. The M-cone response became undetectable in the presence of the yellow adapting light. Yellow adapting light initially reduced the UV response, but it recovered in 8-10 min. The M-cone response did not recover. UV-cone responses were undetectable in Rpe65 -/- mice. The M-cone response of young Rpe65 -/- mice was almost as large as in normal mice. A yellow adapting light only diminished this M-cone response. With age, the M-cone response further decreased in Rpe -/- mice. We show a pronounced loss of UV-cone function in Rpe65 -/- mice, which may be related to a defect UV-cones share with rods. The M-cone function is also affected already in young Rpe65 -/- mice. The transient effect of a yellow adapting light on the UV-cone response of normal mice is suggested to be neural, because it disappears during maintained light adaptation.

New views on RPE65 deficiency: the rod system is the source of vision in a mouse model of Leber congenital amaurosis

Leber congenital amaurosis (LCA) is the most serious form of the autosomal recessive childhood-onset retinal dystrophies. Mutations in the gene encoding RPE65, a protein vital for regeneration of the visual pigment rhodopsin in the retinal pigment epithelium, account for 10-15% of LCA cases. Whereas previous studies of RPE65 deficiency in both animal models and patients attributed remaining visual function to cones, we show here that light-evoked retinal responses in fact originate from rods. For this purpose, we selectively impaired either rod or cone function in Rpe65-/- mice by generating double- mutant mice with models of pure cone function (rhodopsin-deficient mice; Rho-/-) and pure rod function (cyclic nucleotide-gated channel alpha3-deficient mice; Cnga3-/-). The electroretinograms (ERGs) of Rpe65-/- and Rpe65-/-Cnga3-/- mice were almost identical, whereas there was no assessable response in Rpe65-/-Rho-/- mice. Thus, we conclude that the rod system is the source of vision in RPE65 deficiency. Furthermore, we found that lack of RPE65 enables rods to mimic cone function by responding under normally cone-isolating lighting conditions. We propose as a mechanism decreased rod sensitivity due to a reduction in rhodopsin content to less than 1%. In general, the dissection of pathophysiological processes in animal models through the introduction of additional, selective mutations is a promising concept in functional genetics.

Update on the molecular genetics of retinitis pigmentosa

Retinitis pigmentosa (RP) is a heterogeneous group of retinal dystrophies characterized by photoreceptor cell degeneration. RP causes night blindness, a gradual loss of peripheral visual fields, and eventual loss of central vision. Advances in molecular genetics have provided new insights into the genes responsible and the pathogenic mechanisms of RP. The genetics of RP is complex, and the disease can be inherited in autosomal dominant, recessive, X-linked, or digenic modes. Twenty-six causative genes have been identified or cloned for RP, and an additional fourteen genes have been mapped, but not yet identified. Eight autosomal dominant forms are due to mutations in RHO on chromosome 3q21-24, RDS on 6p21.1-cen, RP1 on 8p11-21, RGR on 10q23, ROM1 on 11q13, NRL on 14q11.1-11.2, CRX on 19q13.3, and PRKCG on 19q13.4. Autosomal recessive genes include RPE65 on chromosome 1p31, ABCA4 on 1p21-13, CRB1 on 1q31-32.1, USH2A on 1q41, MERTK on 2q14.1, SAG on 2q37.1, RHO on 3q21-24, PDE6B on 4p16.3, CNGA1 on 4p14-q13, PDE6A on 5q31.2-34, TULP1 on 6p21.3, RGR on 10q, NR2E3 on 15q23, and RLBP1 on 15q26. For X-linked RP, two genes, RP2 and RP3 (RPGR), have been cloned. Moreover, heterozygous mutations in ROM1 on 11q13, in combination with heterozygous mutations in RDS on 6p21.1-cen, cause digenic RP (the two-locus mechanism). These exciting molecular discoveries have defined the genetic pathways underlying the pathogenesis of retinitis pigmentosa, and have raised the hope of genetic testing for RP and the development of new avenues for therapy.

Characterization of a dehydrogenase activity responsible for oxidation of 11-cis-retinol in the retinal pigment epithelium of mice with a disrupted RDH5 gene. A model for the human hereditary disease fundus albipunctatus

In the vertebrate retina, the final step of visual chromophore production is the oxidation of 11-cis-retinol to 11-cis-retinal. This reaction is catalyzed by 11-cis-retinol dehydrogenases (11-cis-RDHs), prior to the chromophore rejoining with the visual pigment apo-proteins. The RDH5 gene encodes a dehydrogenase that is responsible for the majority of RDH activity. In humans, mutations in this gene are associated with fundus albipunctatus, a disease expressed by delayed dark adaptation of both cones and rods. In this report, an animal model for this disease, 11-cis-rdh-/- mice, was used to investigate the flow of retinoids after a bleach, and microsomal membranes from the retinal pigment epithelium of these mice were employed to characterize remaining enzymatic activities oxidizing 11-cis-retinol. Lack of 11-cis-RDH leads to an accumulation of cis-retinoids, particularly 13-cis-isomers. The analysis of 11-cis-rdh-/- mice showed that the RDH(s) responsible for the production of 11-cis-retinal displays NADP-dependent specificity toward 9-cis- and 11-cis-retinal but not 13-cis-retinal. The lack of 13-cis-RDH activity could be a reason why 13-cis-isomers accumulate in the retinal pigment epithelium of 11-cis-rdh-/- mice. Furthermore, our results provide detailed characterization of a mouse model for the human disease fundus albipunctatus and emphasize the importance of 11-cis-RDH in keeping the balance between different components of the retinoid cycle.

Null mutation in the human 11-cis retinol dehydrogenase gene associated with fundus albipunctatus

PURPOSE

Recent studies show that mutations in the gene encoding 11-cis retinol dehydrogenase are associated with fundus albipunctatus. The authors wanted to investigate whether additional, more severe, mutations in the 11-cis retinol dehydrogenase gene might be responsible for more severe forms of hereditary retinal diseases.

DESIGN

Case-control molecular genetics study.

PARTICIPANTS AND CONTROLS

Two index patients, 7 relatives, and 50 control individuals.

METHODS

The authors screened two index patients diagnosed with fundus albipunctatus for mutations in exons 2 to 5 and exon/intron boundaries of the 11-cis retinol dehydrogenase gene by direct sequencing. Control individuals were screened for the presence of the mutations using allele-specific oligonucleotide hybridization.

MAIN OUTCOME MEASURES

Mutations in exons 2 to 5 and exon/intron boundaries of the 11-cis retinol dehydrogenase gene.

RESULTS

In a compound heterozygote, two novel mutations were found: a 4 bp insertion in exon 2 and a missense mutation Cys267Trp in exon 5. In a second pedigree, a homozygous frameshift mutation in codon 43 (Arg42ct[1-bpdel]) was detected. In both families, the mutations segregate with the disease. The mutations were not found in 50 control individuals.

CONCLUSIONS

On the basis of our observations, it is unlikely that mutations in the 11-cis retinol dehydrogenase gene are associated with other, possibly more severe, retinal pathologic conditions/dystrophies or syndromic diseases in which the retina is also affected.

A photic visual cycle of rhodopsin regeneration is dependent on Rgr

During visual excitation, rhodopsin undergoes photoactivation and bleaches to opsin and all-trans-retinal. To regenerate rhodopsin and maintain normal visual sensitivity, the all-trans isomer must be metabolized and reisomerized to produce the chromophore 11-cis-retinal in biochemical steps that constitute the visual cycle and involve the retinal pigment epithelium (RPE; refs. 3-8). A key step in the visual cycle is isomerization of an all-trans retinoid to 11-cis-retinol in the RPE (refs. 9-11). It could be that the retinochrome-like opsins, peropsin, or the retinal G protein-coupled receptor (RGR) opsin12-16 are isomerases in the RPE. In contrast to visual pigments, RGR is bound predominantly to endogenous all-trans-retinal, and irradiation of RGR in vitro results in stereospecific conversion of the bound all-trans isomer to 11-cis-retinal. Here we show that RGR is involved in the formation of 11-cis-retinal in mice and functions in a light-dependent pathway of the rod visual cycle. Mutations in the human gene encoding RGR are associated with retinitis pigmentosa.

Pathways to photoreceptor cell death in inherited retinal degenerations

The mutations that cause many forms of inherited retinal degenerations have been identified, yet the mechanisms by which these mutations lead to death of photoreceptor cells of the retina are not completely understood. Investigations of the pathways from mutation to retinal degeneration have focused on spontaneous and engineered animal models of disease. Based on the studies performed to date, four major categories of degeneration mechanism can be identified. These include disruption of photoreceptor outer segment morphogenesis, metabolic overload, dysfunction of retinal pigment epithelial cells, and chronic activation of phototransduction. Future investigations will likely identify additional mechanisms of photoreceptor damage. This review will summarize what has been learned from studying animal models of non-syndromic inherited retinal degenerations.

Confronting complexity: the interlink of phototransduction and retinoid metabolism in the vertebrate retina

Absorption of light by rhodopsin or cone pigments in photoreceptors triggers photoisomerization of their universal chromophore, 11-cis-retinal, to all-trans-retinal. This photoreaction is the initial step in phototransduction that ultimately leads to the sensation of vision. Currently, a great deal of effort is directed toward elucidating mechanisms that return photoreceptors to the dark-adapted state, and processes that restore rhodopsin and counterbalance the bleaching of rhodopsin. Most notably, enzymatic isomerization of all-trans-retinal to 11-cis-retinal, called the visual cycle (or more properly the retinoid cycle), is required for regeneration of these visual pigments. Regeneration begins in rods and cones when all-trans-retinal is reduced to all-trans-retinol. The process continues in adjacent retinal pigment epithelial cells (RPE), where a complex set of reactions converts all-trans-retinol to 11-cis-retinal. Although remarkable progress has been made over the past decade in understanding the phototransduction cascade, our understanding of the retinoid cycle remains rudimentary. The aim of this review is to summarize recent developments in our current understanding of the retinoid cycle at the molecular level, and to examine the relevance of these reactions to phototransduction.

[Molecular analysis of the RPE65 gene in 72 Spanish families with autosomal recessive retinitis pigmentosa]

BACKGROUND

Autosomal recessive retinitis pigmentosa (arRP) is the most common form of retinitis pigmentosa (RP). It is characterized by a high degree of allelic and non-allelic genetic heterogeneity. Previously,it has been demonstrated that the RPE65 gene is responsible for 2% recessive or isolated RP cases and 16% Leber's congenite amaurosis cases. Although the concrete function of RPE65 is unknown as yet, it has been found to be involved in vitamin A metabolism and rhodopsin regeneration.

PATIENTS AND METHOD

We studied the involvement of the RPE65 gene in 72 arRP Spanish families by means of indirect molecular and mutation analysis.

RESULTS

The results obtained using the intragenic microsatellite marker D1S2803 allowed us to exclude RPE65 as the causative gene of the disease in 80.5% of the families studied. Three new variants of the RPE65 gene have been identified: IVS6-43delA, IVS6-42delT and IVS6-33CG. We found that IVS6-33C-->G was a common polymorphism. The other variants, namely IVS6-43delA and IVS6-42delT, were not identified in 150 control chromosomes studied. The segregation analysis of IVS6-42delT variant seemed to exclude it as being involved in the disease.

CONCLUSIONS

Our results argue against the involvement of RPE65 gene in the families studied, indicating that the prevalence of RPE65 abnormalities in arRP Spanish families may be lower than that observed in other populations.

Molecular analysis of vitamin A formation: cloning and characterization of beta-carotene 15,15'-dioxygenases

Beta-carotene 15,15'-dioxygenase cleaves beta-carotene into two molecules of retinal and is the key enzyme in the metabolism of carotene to vitamin A. Although the enzyme has been known for more than 40 years, all attempts to purify the protein to homogeneity or to clone its gene have failed until recently, when the successful cloning and sequencing of cDNAs encoding enzymes with beta-carotene 15,15'-dioxygenase activity from Drosophila (J. von Lintig and K. Vogt, 2000, J. Biol. Chem. 275, 11915-11920) and chicken (A. Wyss et al., 2000, Biochem. Biophys. Res. Commun. 271, 334-336) were reported. Very soon it became clear, that we have cloned two members of a new family of carotenoid cleaving enzymes. Overall homologies are very high, certain amino acid stretches almost identical. Thus, beta-carotene 15,15'-dioxygenase can be considered as evolutionarily well conserved. These findings open up wide perspectives for further analysis of this important biosynthetic pathway, concerning basic and medical research as well as biotechnological aspects related to vitamin A supply, which are discussed here.

Molecular cloning of a novel lipocalin-1 interacting human cell membrane receptor using phage display

Human lipocalin-1 (Lcn-1, also called tear lipocalin), a member of the lipocalin structural superfamily, is produced by a number of glands and tissues and is known to bind an unusually large array of hydrophobic ligands. Apart from its specific function in stabilizing the lipid film of human tear fluid, it is suggested to act as a physiological scavenger of potentially harmful lipophilic compounds, in general. To characterize proteins involved in the reception, detoxification, or degradation of these ligands, a cDNA phage-display library from human pituitary gland was constructed and screened for proteins interacting with Lcn-1. Using this method an Lcn-1 interacting phage was isolated that expressed a novel human protein. Molecular cloning and analysis of the entire cDNA indicated that it encodes a 55-kDa protein, lipocalin-1 interacting membrane receptor (LIMR), with nine putative transmembrane domains. The cell membrane location of this protein was confirmed by immunocytochemistry and Western blot analysis of membrane fractions of human NT2 cells. Independent biochemical investigations using a recombinant N-terminal fragment of LIMR also demonstrated a specific interaction with Lcn-1 in vitro. Based on these data, we suggest LIMR to be a receptor of Lcn-1 ligands. These findings constitute the first report of cloning of a lipocalin interacting, plasma membrane-located receptor, in general. In addition, a sequence comparison supports the biological relevance of this novel membrane protein, because genes with significant nucleotide sequence similarity are present in Takifugu rubripes, Drosophila melanogaster, Caenorhabditis elegans, Mus musculus, Bos taurus, and Sus scrofa. According to data derived from the human genome sequencing project, the LIMR-encoding gene has to be mapped on human chromosome 12, and its intron/exon organization could be established. The entire LIMR-encoding gene consists of about 13.7 kilobases in length and contains 16 introns with a length between 91 and 3438 base pairs.

Photoreceptor topography in ageing and age-related maculopathy

The relative rate of rod and cone degeneration is a fundamental characteristic of any disorder affecting photoreceptors, including ageing and age-related maculopathy (ARM). The macula consists of a small cone-dominated fovea surrounded by a rod-dominated parafovea. In donor eyes with grossly normal maculas, the number of foveal cones is stable and the number of parafoveal rods decreases by 30% over adulthood. These trends continue in early ARM. In exudative ARM, the photoreceptors that survive over disciform scars are largely cones, and rods decline precipitously in relation to thick subretinal pigment epithelium deposits. The preferential vulnerability of rods over cones has been confirmed by recent functional studies showing that the loss of scotopic sensitivity is greater than the loss of photopic sensitivity throughout adulthood and in patients with early ARM. A hypothesis that these effecfs are due to to retinoid deficiency at the level of the photoreceptors is proposed. The topography of rod loss in ageing and ARM is consistent with the location of early ARM lesions described in population-based studies and is not consistent with the location of fundus autofluorescence due to lipofuscin.

Mutations in the gene encoding lecithin retinol acyltransferase are associated with early-onset severe retinal dystrophy

The chromophore of the visual pigments, 11-cis retinal, is derived from vitamin A (all-trans retinol) through a series of reactions that take place in retinal pigment epithelium (RPE); (ref. 1). The first of these reactions is catalyzed by lecithin retinol acyltransferase (LRAT); (ref. 2). We screened 267 retinal dystrophy patients for mutations in LRAT and identified disease-associated mutations (S175R and 396delAA) in three individuals with severe, early-onset disease. We showed that the S175R mutant has no acyltransferase activity in transfected COS-7 cells. Our findings highlight the importance of genetic defects in vitamin A metabolism as causes of retinal dystrophies and extend prospects for retinoid replacement therapy in this group of diseases.

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)

Gene therapy restores vision in a canine model of childhood blindness

The relationship between the neurosensory photoreceptors and the adjacent retinal pigment epithelium (RPE) controls not only normal retinal function, but also the pathogenesis of hereditary retinal degenerations. The molecular bases for both primary photoreceptor and RPE diseases that cause blindness have been identified. Gene therapy has been used successfully to slow degeneration in rodent models of primary photoreceptor diseases, but efficacy of gene therapy directed at photoreceptors and RPE in a large-animal model of human disease has not been reported. Here we study one of the most clinically severe retinal degenerations, Leber congenital amaurosis (LCA). LCA causes near total blindness in infancy and can result from mutations in RPE65 (LCA, type II; MIM 180069 and 204100). A naturally occurring animal model, the RPE65-/- dog, suffers from early and severe visual impairment similar to that seen in human LCA. We used a recombinant adeno-associated virus (AAV) carrying wild-type RPE65 (AAV-RPE65) to test the efficacy of gene therapy in this model. Our results indicate that visual function was restored in this large animal model of childhood blindness.