The retinal pigment epithelium in health and disease

Retinal pigment epithelial cells (RPE) constitute a simple layer of cuboidal cells that are strategically situated behind the photoreceptor (PR) cells. The inconspicuousness of this monolayer contrasts sharply with its importance [1]. The relationship between the RPE and PR cells is crucial to sight; this is evident from basic and clinical studies demonstrating that primary dysfunctioning of the RPE can result in visual cell death and blindness. RPE cells carry out many functions including the conversion and storage of retinoid, the phagocytosis of shed PR outer segment membrane, the absorption of scattered light, ion and fluid transport and RPE-PR apposition. The magnitude of the demands imposed on this single layer of cells in order to execute these tasks, will become apparent to the reader of this review as will the number of clinical disorders that take origin from these cells.

Mutation spectrum and splicing variants in the OPA1 gene

Optic atrophy type 1 (OPA1, MIM 165500) is a dominantly inherited optic neuropathy that features low visual acuity leading in many cases to legal blindness. We have recently shown, with others, that mutations in the OPA1 gene encoding a dynamin-related mitochondrial protein, underlie the dominant form of optic atrophy. Here we report that OPA1 has eight mRNA isoforms as a result of the alternative splicing of exon 4 and two novel exons named 4b and 5b. In addition, we screened a cohort of 19 unrelated patients with dominant optic atrophy by direct sequencing of the 30 OPA1 exons (including exons 4b and 5b) and found mutations in 17 (89%) of them of which 8 were novel. A majority of these mutations were truncative (65%) and located in exons 8 to 28, but a number of them were amino acid changes predominantly found in the GTPase domain (exons 8 to 15). We hypothesize that at least two modifications of OPA1 may lead to dominant optic atrophy, that is alteration in GTPase activity and loss of the last seven C-terminal amino acids that putatively interact with other proteins.

Retinal dystrophies caused by mutations in RPE65: assessment of visual functions

AIMS

To characterise the disease in patients with mutations in RPE65.

METHODS

Individuals from two families were studied clinically.

RESULTS

13 and 20 year old compound heterozygote individuals from one family with R234X and 1121delA mutations showed nystagmus, macular dystrophy and low contrasted spots in the fundus. Some heterozygotes had macular drusen. A 40 year old compound heterozygote individual from another family with L22P and H68Y mutations had few bone spicule pigment deposits and macular atrophy.

CONCLUSION

Compound heterozygote individuals had severe rod-cone dystrophies featuring few pigment deposits in the fundus, pigment epithelium atrophy, and early involvement of the macula, with variations in severity leading to the diagnosis of Leber's congenital amaurosis or retinitis pigmentosa. Macular drusen in heterozygotes carrying a null allele may reflect the decreased capacity in the RPE65 function.

Segregation of a mutation in CNGB1 encoding the beta-subunit of the rod cGMP-gated channel in a family with autosomal recessive retinitis pigmentosa

Retinitis pigmentosa (RP) is a clinically and genetically heterogeneous group of retinal diseases leading to blindness. By performing full genome linkage analysis in a consanguineous French family affected with severe autosomal recessive RP, we have excluded linkage to known loci involved in RP and mapped a novel locus to chromosome 16q13-q21 (Zmax=2.83 at theta=0 at the D16S3089 locus). Two candidate genes KIFC3 and CNGB1 mapping to this critical interval have been screened for mutations. The CNGB1 gene, which encodes the beta-subunit of the rod cGMP-gated channel, is mutated in the family presented in this study.

[Molecular genetics of pigmentary retinopathies: identification of mutations in CHM, RDS, RHO, RPE65, USH2A and XLRS1 genes]

PURPOSE

To evaluate the occurrence and inheritance of various types of pigmentary retinopathy in patients followed at the outpatient clinic in the university hospital, Montpellier, France. To characterize genes and mutations causing these conditions.

METHODS

Ophthalmic examination and various visual tests were performed. Mutations were sought from genomic DNA by PCR amplification of exons associated with single-strand conformation analysis and/or direct sequencing.

RESULTS

Among 315 patients over an 8-year period, cases of retinitis pigmentosa (63.2%), Usher's syndrome (10.2%), Stargardt's disease (5.4%), choroideremia (3.2%), Leber's congenital amaurosis (3.2%), congenital stationary night blindness (2.9%), cone dystrophy (2.5%), dominant optic atrophy (1.9%), X-linked juvenile retinoschisis (1.6%), Best's disease (1.6%), and others (4.3%) were diagnosed. In retinitis pigmentosa, inheritance could be determined in 54.2% of the cases including dominant autosomic (26.6%), recessive autosomic (22.6%), and X-linked cases (5%) while it could not be confirmed in 45.7% of the cases (simplex cases in the majority). For the 6 examined genes, mutations were found in 22 out of 182 propositus (12.1%). Analysis of phenotype-genotype correlations indicates that in retinitis pigmentosa, RDS is more frequently associated with macular involvement and retinal flecks, RHO with regional disease, and RPE65 with the great severity of the disease with some cases of Leber's congenital amaurosis.

CONCLUSIONS

Identification of genes may help in diagnosis and in genetic counseling, especially in simplex cases with retinitis pigmentosa. In this latter condition, molecular diagnosis will be necessary to rationalize future treatments.

Nuclear gene OPA1, encoding a mitochondrial dynamin-related protein, is mutated in dominant optic atrophy

Optic atrophy type 1 (OPA1, MIM 165500) is a dominantly inherited optic neuropathy occurring in 1 in 50,000 individuals that features progressive loss in visual acuity leading, in many cases, to legal blindness. Phenotypic variations and loss of retinal ganglion cells, as found in Leber hereditary optic neuropathy (LHON), have suggested possible mitochondrial impairment. The OPA1 gene has been localized to 3q28-q29 (refs 13-19). We describe here a nuclear gene, OPA1, that maps within the candidate region and encodes a dynamin-related protein localized to mitochondria. We found four different OPA1 mutations, including frameshift and missense mutations, to segregate with the disease, demonstrating a role for mitochondria in retinal ganglion cell pathophysiology.

Expression pattern of mammalian cochlea outer hair cell (OHC) mRNA: screening of a rat OHC cDNA library

The aim of this study was to characterize the mRNA content of mammalian cochlear outer hair cells (OHCs) and to search for specific genes possibly involved in their unique properties. Indeed, OHCs, which feature high-frequency electromotility, are responsible for the exquisite sensitivity and frequency selectivity of the cochlea. Damage to these cells, which occurs in various conditions, causes a reduction in the cochlear sensitivity by about 50 dB and the alteration of frequency discrimination. Total RNA was extracted from about 2000 mechanically dissociated OHCs, and a polymerase chain reaction (PCR) amplified cDNA library was constructed. The presence of the alpha-9 acetylcholine receptor subunit, preferentially expressed in OHCs, was found by direct PCR amplification of the library. A systematic sequencing of 218 clones showed 78% known genes, 11% EST-related sequences, and 11% unknown genes. The known-gene group was characterized by two main features: a large proportion (55%) of mitochondrial transcripts and an abundance in calcium-binding proteins, such as calmodulin and calbindin, for which expression has already been demonstrated in OHCs. Another protein, the oncomodulin recently shown to be OHC specific, was also found, and its mRNA expression was confirmed by in situ hybridization. Among the 24 unknown genes, 7 were expressed in a restricted pattern, including one expressed in cochlea and spleen and, to a lesser extent, in lungs.

Autosomal recessive retinal dystrophy associated with two novel mutations in the RPE65 gene

Retinal dystrophies are a complex set of hereditary diseases of the retina that result in the degeneration of photoreceptors. Recent studies have shown that mutations in RPE65, a gene that codes for a retinal pigment epithelium (RPE)-specific protein thought to be involved in the 11-cis-retinoid metabolism, a key process in vision, cause severe, early onset retinal dystrophy. We describe two novel missense RPE65 mutations, L22P and H68Y, in a compound heterozygote with autosomal recessive retinal dystrophy. The relatively mild phenotype associated with these mutations suggests a possible link between the severity of the disease and the type of mutations in the RPE65 gene.

[Clinical features and genetic analysis in a family with X-linked incomplete congenital stationary night blindness (CSNBi)]

PURPOSE

We describe particular clinical features in a three-generation family with X-linked CSNBi and present the genetic analysis.

METHOD

The diagnosis of CSNBi was established on clinical and electrophysiological criteria. Polymorphic DNA markers of the Xp region were analyzed by fluorescent polymerase chain reaction.

RESULTS

Clinical findings evidenced an atypical association of both myopia and hyperopia in the same brotherhood. The most interesting feature in this family was the observation of major worsening of the clinical shape between the first and the third generation of affected individuals. DNA analysis did not show significant linkage between the disease and markers of the Xp11-p21 region. Southern analysis did not show expansion of trinucleotide repeat CAG/CTG and CCG/CGG over the three generation.

CONCLUSION

Haplotypic analysis together with clinical observations allow to exclude the existence of a myopia gene closely linked to the CSNB2 locus. The clinical anticipation observed in this family does not seem to be linked with trinucleotide repeat expansion CAG/CTG or CCG/CGG.

Rat messenger RNA for the retinal pigment epithelium-specific protein RPE65 gradually accumulates in two weeks from late embryonic days

The RPE65 protein appears late during the retinal development. To study the basis for this regulation, the rat RPE65 cDNA was sequenced and the mRNA subsequently quantitated at various stages by competitive RT-PCR. RPE65 mRNA was detected as early as E18 (36 copies/ng of whole eye total RNA). It gradually accumulates up to P12 (27000 copies/ng) at which point it reaches a steady state level. This increase is interrupted for 3 days (P2-P4) during which the levels of mRNA remain stable. This timing and rate of accumulation parallels that of rat and mouse opsin mRNA and suggests that common factors may control the activation of genes in photoreceptors and retinal pigment epithelium cells.

RPE65, the major retinal pigment epithelium microsomal membrane protein, associates with phospholipid liposomes

The retinal pigment epithelium (RPE)-specific protein RPE65 is the major protein of the RPE microsomal membrane fraction. Though RPE65 lacks transmembrane domains or signal peptide, detergents are required for its maximally effective solubilization in isotonic buffers. However, in 0.75-1.0 M KCl, RPE65 is as soluble without detergent, indicating a peripheral membrane association. We wished to understand why this non-membrane-inserted protein was so closely associated with RPE microsomal membranes. To explore the possible involvement of interactions with phospholipids, an isotonic salt-soluble extract of RPE was incubated with phosphatidylcholine (PC)/phosphatidylserine (PS)/phosphatidylinositol liposomes and centrifuged to sediment the liposomes. RPE65 cosedimented with the liposome pellet. RPE65 also cosedimented with synthetic dipalmitoyl-, 1-palmitoyl, 2-docosahexaenoyl-PC or dipalmitoyl-PS liposomes. Incubation with 1 mM Ca2+ or 1 mM EGTA had no effect, indicating a Ca2+-independent association. A spectrophotometric assay showed that this interaction of RPE65 with phospholipid vesicles resulted in increased light scattering, consistent with phospholipid vesicle aggregation. Resonance energy transfer experiments showed that any putative aggregation occurred without subsequent vesicle fusion. This PC affinity was further confirmed by incubation of RPE extract with dimyristoyl-PC-immobilized artificial membrane (IAM.PC) matrix. The RPE65 selectively bound and was elutable with 2% detergent. This RPE65-phospholipid liposome association may explain the solubilization characteristics of RPE65 and may be related to the function of RPE65 and to its physical association with the RPE smooth endoplasmic reticulum.

Identification of preferentially expressed cochlear genes by systematic sequencing of a rat cochlea cDNA library

107 expressed sequence tags (ESTs) from a rat cochlea cDNA library were identified by systematic sequencing coupled to database selection and RT-PCR analysis of novel sequences. This approach led us to select a clone, pCO8, showing no significant homology with any database sequence, that corresponds to a mRNA whose expression is restricted to the cochlea, except for traces detected in brain. Additional clones with novel sequences enriched in the cochlea were also found. ESTs bearing significant homologies with database sequences (63 out of 107) were classified according to the putatively encoded protein. They include tissue-specific genes not previously described in the cochlea as well as known genes from other species. We performed in situ hybridization in cochlear tissues to localize the pCO8 mRNA and that of clone pCO6 which is 100% homologous to the delayed rectifier potassium channel drk1. We found that both mRNAs were exclusively expressed in the cellular body of the primary auditory neurons from the spiral ganglion of the cochlea. These results indicate that this approach is an efficient way to identify novel genes that could be of importance in cochlear function.

Molecular cloning and expression of alpha parvalbumin in the guinea pig cochlea

We have cloned and sequenced an alpha parvalbumin cDNA from the guinea pig cochlea. The deduced amino acid sequence shows greater identity with the rabbit sequence (86.3%) than with other mammalian sequences (< 82%). Using in situ hybridization and immunohistochemistry, alpha parvalbumin mRNA and protein were found in primary auditory neurons and inner hair cells, in agreement with RT-PCR data showing alpha parvalbumin mRNA expression in the spiral ganglion and the organ or Corti.

The gene for the retinal pigment epithelium-specific protein RPE65 is localized to human 1p31 and mouse 3

The human and murine chromosomal localization for the gene for the retinal pigment epithelium-specific protein RPE65 was determined. Using interspecific backcross analysis, we mapped Rpe65 to the distal end of mouse chromosome 3. In the human, using a human-hamster hybrid panel, RPE65 was mapped to chromosome 1. By the use of fluorescence in situ hybridization, this localization was refined to 1p31. The mouse and human loci for this potential candidate gene for hereditary retinal disease do not match those of any known disease in mouse or man.

Molecular cloning and expression of RPE65, a novel retinal pigment epithelium-specific microsomal protein that is post-transcriptionally regulated in vitro

Studies reported previously from this laboratory have shown that microsomal membranes of the vertebrate retinal pigment epithelium (RPE) contain an RPE-specific 65-kDa protein, RPE65, which bears the determinant recognized by the strictly tissue-specific monoclonal antibody RPE9, and which is developmentally regulated (Hamel, C. P., Tsilou, E., Harris, E., Pfeffer, B. A., Hooks, J. J., Detrick, B., and Redmond, T. M. (1993) J. Neurosci. Res. 34, 414-425). Microsequencing of 17 tryptic and chymotryptic peptides obtained from the in situ digestion of the RPE65 blotted on nitrocellulose yielded primary sequences that were used to generate oligonucleotide probes. An 84-nucleotide guessmer was used to isolate two clones from a bovine RPE lambda Zap II cDNA library. Rapid amplification of cDNA ends was used to complete the 5' and 3' ends, resulting in a 3,115-base pair composite cDNA. The open reading frame encodes a novel protein of 533 amino acid residues with a computed molecular weight of 60,940. This protein does not match any other sequence in the data bases. The 231 amino acids obtained from peptide sequencing match 43% of the amino acid sequence deduced from the cDNA. The protein has a calculated pI of 6.41 and is not predicted to have any transmembrane segments. The open reading frame expressed in Escherichia coli has an apparent molecular weight identical to that of the native protein and is recognized by the monoclonal antibody RPE9, further corroborating its validity. Northern blot analysis detected a major mRNA species of 3.15 kilobases for RPE65, as well as shorter species, only in RPE and not in other tissues (including other ocular tissues). Cultured RPE cells (7 weeks in primary culture) contained RPE65 mRNA in amounts equivalent to fresh RPE. Such cells, however, contained no immunodetectable RPE65. The possible structure of this RPE-specific protein and hypotheses for the absence of translation in vitro are discussed.

A developmentally regulated microsomal protein specific for the pigment epithelium of the vertebrate retina

In the vertebrate retina, the retinal pigment epithelium (RPE) performs specific functions critical to the normal process of vision. Although some of these functions are well documented, molecular data are still scarce. Using the RPE-specific monoclonal antibody RPE9, raised against human RPE cells, we have identified a novel 65 kD protein, conserved in mammals, birds, and frogs. This RPE-specific protein was found to be nonglycosylated. It was most effectively solubilized in the presence of detergent suggesting that it is associated with the RPE cell membranes. Its partitioning in the detergent phase of Triton X-114 and its solubilization in 0.75 M and 1.0 M KCl suggest that it interacts with the membrane either through a polypeptide anchor or charged amino acids. Cell fractionation by differential solubilization and differential centrifugation demonstrated that the protein was preferentially associated with the microsomal membrane fraction, where it is the major protein. Developmental expression of this 65 kD protein was examined in neonatal rats. Morphologically well-differentiated RPE cells did not express the 65 kD protein at birth. However, expression was detectable at postnatal day 4, that is, one to two days before the photoreceptors develop their outer segments, suggesting that the expression of the 65 kD protein may be coordinated with other developmental events in the intact retina. This is further supported by the fact that RPE cells in confluent culture lose the expression of this protein within two weeks, while they maintain their characteristic epithelial morphology. Because of its specificity, its evolutionary conservation, and its timing of expression, it is possible that this protein may be involved in one of the key roles of RPE and as such is an important molecular marker for RPE differentiation.

Murine coronavirus induces an acute and long-lasting disease of the retina

The ability of the coronavirus mouse hepatitis virus, strain JHM, to grow in the retinas of BALB/c mice was examined. Inoculation into the vitreous chamber produced significant changes. Immunoperoxidase staining of frozen sections with either monoclonal or polyclonal antiserum revealed coronaviral antigens in the iris, ciliary body, and a few ganglion cells on day 1. The retinal pigment epithelial cells began expressing viral antigen on day 2 and large amounts of antigen were present in these cells on day 3. Viral antigens were detected in all layers of the neural retina by day 6 and were absent after day 7. Infectious virus was recovered from retinas harvested at 5 days. The drop in viral antigen expression was correlated with an elevation in virus-specific antibody; the latter began to rise on day 5 and plateaued after day 8. In hematoxylin and eosin- or periodic acid -Schiff-stained sections of virus-inoculated left eyes, but not mock-inoculated right eyes, lesions spanning all layers of the neural retina were detected by day 3. Subsequently abnormalities in retinal pigment epithelial cells appeared, sometimes around the entire circumference of the retina. Significant retinal abnormalities, notably photoreceptor degeneration, persisted through 6 weeks. These results demonstrate that coronaviruses can cause acute infection of the posterior pole of the eye, resulting in only a mild inflammatory response and long-lasting disease. This murine disease may be considered a model for degenerative diseases of the pigment epithelium and photoreceptors in man.

Evaluation of Ia expression in rat ocular tissues following inoculation with interferon-gamma

It is becoming increasingly clear that IFN-gamma is a potent immunoregulatory protein which influences MHC class II (Ia) antigen expression and cellular functions of B cells, T cells, NK cells and macrophages. During the past 5 yr our laboratory has provided evidence that IFN-gamma modulates class II antigens on retinal cells (retinal pigment epithelial cells, endothelial cells) and is localized within the eye during human inflammatory conditions. In this study we evaluate the direct effect of IFN-gamma on ocular tissue. Lewis rats were inoculated intravitreally or under the retina with either recombinant IFN-gamma (20,000 U) or saline. At 2 hr, 1, 2 and 6 days postinoculation, the eyes were removed and frozen sections were evaluated by immunocytochemical staining with monoclonal anti-Ia antibodies and an irrelevant monoclonal anti-T cell antibody. Saline treated tissue and tissue removed 2 hr after IFN-gamma inoculation showed no significant staining for Ia antigens. However, eyes evaluated 24 hr after IFN-gamma inoculation revealed Ia expression on a variety of ocular cells localized in the conjunctiva and anterior segment, such as conjunctival epithelium, keratocytes, iris epithelium, ciliary epithelium and choroidal cells. In the retina, retinal pigment epithelial (RPE) cells were Ia positive only when IFN-gamma was injected directly under the retina. In conjunction with Ia expression, two striking changes were noted. An iritis was seen and infiltrating cells were detected in the inner retinal layers. Both of these phenomena have been observed in certain inflammatory eye diseases. These studies clearly substantiate the concept that IFN-gamma can regulate class II antigens in the eye and thus may perpetuate immune reactivity in this site.

Nonspecific immunoglobulin E in aqueous humor: evaluation in uveitis

The levels of immunoglobulin E (IgE) in aqueous humor (AH) and serum were determined in 74 uveitis patients and 13 control patients with senile cataract. Uveitis patients' levels ranged from less than 0.06 to 460 IU/ml according to the paper radioimmunosorbent test (PRIST; limit of sensitivity, 0.06 IU/ml). Only one control patient had an AH level higher than the sensitivity limit (0.08 IU/ml). Of the five patients with the highest levels in the AH, four had toxocaral retinitis. The ratio of AH/serum to IgE normalized to the albumin concentration was calculated in 25 uveitis patients and was greater than 0.60 in 13 cases, indicating a possible intraocular IgE synthesis.