Two histidine residues are essential for catalysis by lecithin retinol acyl transferase

Lecithin retinol acyl transferase (LRAT) is a novel membrane bound enzyme that catalyzes the formation of retinyl esters from vitamin A and lecithin. The enzyme is both essential for vision and for the general mobilization of vitamin A. The sequence of LRAT defines it as a novel enzyme unrelated to any other protein of known function. LRAT possesses a catalytically essential active site cysteine residue. The enzyme also contains six histidine residues. It is shown here that two of these residues (H57 and H163) are essential for catalysis. A mechanistic hypothesis is presented to account for these observations.

Glycosaminoglycan synthesis and secretion by the retinal pigment epithelium: polarized delivery of hyaluronan from the apical surface

Hyaluronan and chondroitin sulfate glycosaminoglycan secretion from retinal pigment epithelial cells was established in confluent cultures with high transepithelial resistance. Cell cultures were maintained on Millicell-PCF culture plates, which allow separation of culture medium exposed to apical and basal epithelial surfaces. Following various times in culture, apical and basal culture media were sampled at three day intervals and the glycosaminoglycan content was quantified. Samples were digested with proteinase K to free the glycosaminoglycans from their core proteins, the glycosaminoglycans were ethanol precipitated, and subjected to hyaluronidase SD and chondroitinase ABC digestion to release hyaluronan and chondroitin sulfate disaccharides. Disaccharides were fluorotagged with 2-aminoacridone, separated on polyacrylamide gels and the molar fluorescence in each disaccharide band quantitated. Hyaluronan in the apical medium was significantly higher than in the basal medium (5-12 times) at all recovery intervals (P<0.0001). In contrast, the distribution of unsulfated chondroitin, 4-sulfated chondroitin and 6-sulfated chondroitin disaccharides in apical and basal media was non-polar. Confocal microscopy of cultures probed with a hyaluronan-specific fluorotag established that the HA evident in these cultures is restricted to the apical border of the RPE cultures. Collectively, these data indicate that hyaluronan synthesized by the retinal pigment epithelium is secreted preferentially from the apical surface, suggesting that this tissue is an important source of hyaluronan present in the interphotoreceptor matrix.

Genomic organization and mutation analysis of the gene encoding lecithin retinol acyltransferase in human retinal pigment epithelium

PURPOSE

To determine the structure of the human lecithin retinol acyltransferase (LRAT) gene, map its chromosomal localization, and screen for mutations in humans with various hereditary retinal degenerations.

METHODS

Using DNA probes specific for LRAT, a bacterial artificial chromosome (BAC) clone containing the LRAT gene was isolated, subcloned into DNA fragments and relevant subclones characterized by sequencing. Exon-intron junctions were determined by comparison with the cDNA sequence previously published. Southern blot analysis was performed on human genomic DNA samples digested with several restriction enzymes. Fluorescence in situ hybridization (FISH) analysis of normal metaphase chromosomes derived from phytohemagglutinin (PHA) stimulated peripheral blood lymphocytes and radiation hybrid mapping were used for localization of the LRAT gene. Single-strand conformation polymorphism analysis (SSCP) was used to screen for potential mutations in patients with age-related macular degeneration, Leber congenital amaurosis, retinitis pigmentosa, and cone-rod dystrophy.

RESULTS

The human LRAT gene is organized into three exons of 219, 541, and 2058 bp and two introns of 103 and 4117 bp. Southern blot analysis of digested genomic DNA revealed a single band, suggesting a single copy of the LRAT gene. The human LRAT gene was localized to chromosome 4q31.2, a locus having no previous association with human eye disease. Additionally, the bovine LRAT homologue sequence was deduced and a general LRAT protein topology is suggested. No polymorphisms that segregated with retinal disease phenotypes were identified in 374 unrelated probands.

CONCLUSIONS

The organization of the LRAT gene, based on cDNA clones derived from the retinal pigment epithelium (RPE) has been determined. Its structure is less complex than other acyltransferases such as lecithin cholesterol acyltransferase (LCAT) and acyl CoA acyltransferase (ACAT). The absence of polymorphisms in the probands examined suggests a very low mutation level in the LRAT gene from the diseases analyzed.

Tetracycline-inducible system for photoreceptor-specific gene expression

PURPOSE

To develop a system for inducible photoreceptor-specific gene expression in transgenic mice. The tetracycline regulatory system was chosen because it possesses the useful property of direct control of gene expression through use of an exogenous agent, doxycycline, a tetracycline derivative.

METHODS

Transgenic mice were generated that carried the reverse tetracycline-controlled transactivator under the control of the photoreceptor-specific promoters for rhodopsin and interphotoreceptor retinoid-binding protein. These animals were crossed with transgenic mice carrying the lacZ reporter gene under control of the tetracycline operator cassette, creating doubly transgenic mice. Doxycycline was administered to induce expression of the reporter gene. Reporter assays were then performed to evaluate lacZ expression.

RESULTS

Doxycycline administration led to photoreceptor-specific expression of the lacZ reporter gene in the doubly transgenic mice. X-gal staining was restricted to photoreceptor inner segments and synaptic termini. Induction could be achieved by addition of the drug to the animals' drinking water or by intravitreal injection. Induction was noted within 24 hours of doxcycline administration. Because of variability among animals, there was an approximate correlation, but not a clean dose-response curve relating drug dose to level of reporter expression.

CONCLUSIONS

A transgenic system for inducible photoreceptor-specific gene expression has been developed. This system is currently being exploited to study the effects of regulated expression of genes of biological interest.

Esterification of all-trans-retinol in normal human epithelial cell strains and carcinoma lines from oral cavity, skin and breast: reduced expression of lecithin:retinol acyltransferase in carcinoma lines

When exogenous [(3)H]retinol (vitamin A) was added to culture medium, normal human epithelial cells from the oral cavity, skin, lung and breast took up and esterified essentially all of the [(3)H]retinol within a few hours. As shown by [(3)H]retinol pulse-chase experiments, normal epithelial cells then slowly hydrolyzed the [(3)H]retinyl esters to [(3)H]retinol, some of which was then oxidized to [(3)H]retinoic acid (RA) over a period of several days. In contrast, cultured normal human fibroblasts and human umbilical vein endothelial cells (HUVEC) did not esterify significant amounts of [(3)H]retinol; this lack of [(3)H]retinol esterification was correlated with a lack of expression of lecithin:retinol acyltransferase (LRAT) transcripts in normal fibroblast and HUVEC strains. These results indicate that normal, differentiated cell types differ in their ability to esterify retinol. Human carcinoma cells (neoplastically transformed epithelial cells) of the oral cavity, skin and breast did not esterify much [(3)H]retinol and showed greatly reduced LRAT expression. Transcripts of the neutral, bile salt-independent retinyl ester hydrolase and the bile salt-dependent retinyl ester hydrolase were undetectable in all of the normal cell types, including the epithelial cells. These experiments suggest that retinoid-deficiency in the tumor cells could develop because of the lack of retinyl esters, a storage form of retinol.

Analysis and quantitation of mRNAs encoding the alpha- and beta-subunits of rod photoreceptor cGMP phosphodiesterase in neonatal retinal degeneration (rd) mouse retinas

The retinal degeneration(rd) mouse is a commonly-studied animal model of the family of human-inherited retinal blindness known as retinitis pigmentosa, and is a likely model in which therapies for these conditions will continue to be developed and tested. Mutation of the beta-subunit of the rod photoreceptor cell-specific cyclic GMP phosphodiesterase is known to cause photoreceptor apoptosis in these mice. However, the molecular phenotype of this mutation in terms of quantitative levels of the phosphodiesterase alpha- and beta-subunit messenger RNAs remains unknown. In this study, the expression of the alpha- and beta-phosphodiesterase subunits is compared in C57BL/6J +/+, rd /+, and rd / rd mouse retinas. Using the techniques of quantitative reverse transcription polymerase chain reaction and quantitative in situ hybridization, the expression of the subunit mRNAs was measured in retinas of postnatal mice 0-14 days of age. Additionally, full length coding sequences were amplified for both subunits, and the beta-phosphodiesterase subunit mRNA was further evaluated for evidence of alternative splicing. Lastly, a relative decrease in expression of the mutant beta-phosphodiesterase allele in rd /+ mice was observed.

Osteonectin/SPARC secreted by RPE and localized to the outer plexiform layer of the monkey retina

PURPOSE

Osteonectin/SPARC is a secreted protein that has been implicated in ocular disease. Deletion of osteonectin/SPARC causes age-onset cataract in mice and the cataractous human lens has increased expression of osteonectin/SPARC. In this study, the expression and localization of osteonectin/SPARC in the monkey retina were determined as was secretion by cultured human retinal pigment epithelial (RPE) cells.

METHODS

Adult Rhesus monkey eyes (Macaca mulatta) were dissected, and 5-mm macula and peripheral retina punches were obtained. Supernatants were collected from cultured human RPE cells. Subcellular fractionation of whole monkey retina was also performed. Osteonectin/SPARC expression and/or secretion was monitored by Northern and Western blot analyses, and localization was determined by immunocytochemistry.

RESULTS

Outside of the retina osteonectin/SPARC mRNA is broadly expressed in many human tissues. Northern blot analysis shows that in the retina osteonectin/SPARC is expressed almost exclusively by the macular RPE/choroid. Western blot analysis revealed osteonectin/SPARC in both the macula and the peripheral neural retina but only in trace amounts in the RPE/choroid. In subcellular fractions of the whole retina, osteonectin/SPARC was detected, mainly in the soluble fraction but also in the membrane and nuclear fractions. Immunohistochemical analysis localized osteonectin/SPARC specifically to the outer plexiform layer. Western blot analysis of conditioned medium from human RPE cells cultured on porous substrates indicated that osteonectin/SPARC is secreted in large amounts from both the apical and basal sides of the RPE.

CONCLUSIONS

Collectively these data provide evidence that osteonectin/SPARC is synthesized in the macular RPE, secreted, and subsequently transported to the outer plexiform layer. The expression pattern of osteonectin/SPARC in the subcellular retinal fractions is consistent with a soluble protein that is transported and internalized.

A brief review of retinitis pigmentosa and the identified retinitis pigmentosa genes

The family of inherited ocular diseases that is collectively known as retinitis pigmentosa is a major cause of progressive retinal disease worldwide. As such, this family of diseases has been the object of much scientific scrutiny, both clinical and basic. The recent application of molecular genetic analyses has heralded the rapid elucidation of the underlying gene defects in many cases. In this article, the fundamental clinical and electroretinographic characteristics of retinitis pigmentosa will be recalled. Additionally, the current understanding of the genetic causes of retinitis pigmentosa will be reviewed, and the identified causative genes will be classified into groups related by function.

Lecithin retinol acyltransferase contains cysteine residues essential for catalysis

Lecithin retinol acyltransferase (LRAT) is an essential enzyme in vitamin A metabolism and mobilization. The membrane-bound enzyme catalyzes the transfer of an acyl group from the sn-1 position of lecithin to vitamin A to generate retinyl esters. The sequence of LRAT is novel and hence does not suggest a mechanistic class to which the enzyme belongs. However, the activity of the enzyme is exceedingly sensitive to affinity labeling and group-specific reagents directed toward thiol groups. LRAT from human retinal pigment epithelium has cysteine residues at positions 161, 168, 182, and 208. Site-specific mutagenic studies show that C182 and C208 can be converted to alanines with little affect on activity. The activities of the C161A and C168A mutants are virtually nil. Moreover, while C168S is substantially active, C161S possesses only a few percent of the activity of wild-type (WT) LRAT. Also, pH-rate profiles show that C168S has virtually the same profile as WT LRAT, while C161S shows an aberrant profile quite unlike that of WT LRAT. Therefore, LRAT is a thiol acyltransferase and C161 may be the essential nucleophilic residue critical for catalysis.

Is the Duchenne muscular dystrophy gene also an X-linked retinitis pigmentosa locus?

Deletion mutations and linkage mapping have localized an X-linked retinitis pigmentosa locus to Xp21, and a disease gene (RPGR) has been characterized. However, mutations have not been identified in most families expected to segregate the disease at this locus. Here, a retina-specific mRNA transcript from the Duchenne muscular dystrophy gene is identified. Based on these data, it is hypothesized that the Duchenne muscular dystrophy gene may represent a second Xp21 site at which retinitis pigmentosa mutations occur.

Molecular and biochemical characterization of lecithin retinol acyltransferase

The enzyme responsible for conversion of all-trans-retinol into retinyl esters, the lecithin retinol acyltransferase (LRAT) has been characterized at the molecular level. The cDNA coding for this protein was cloned and its amino acid sequence deduced. LRAT is composed of a polypeptide of 230 amino acid residues with a calculated mass of 25.3 kDa. Tissue distribution analysis by Northern blot showed expression of a 5.0-kilobase transcript in the human retinal pigment epithelium as well as in other tissues that are known for their high LRAT activity and vitamin A processing. Affinity labeling experiments using specific compounds with high affinity for LRAT and monospecific polyclonal antibodies raised in rabbits against two peptide sequences for LRAT confirmed the molecular mass of LRAT as a 25-kDa protein. High performance liquid chromatography analysis of the reaction product formed by HEK-293 cells transfected with LRAT cDNA confirmed the ability of the transfected cells to convert [3H]all-trans-retinol into authentic [3H]all-trans-retinyl palmitate as chemically determined.

Polarity of 11-cis retinal release from cultured retinal pigment epithelium

PURPOSE

Fetal bovine retinal pigment epithelium (RPE) was grown on porous supports to investigate the polarity of 11-cis retinal (RAL) release from these cells and the influence that the interphotoreceptor retinoid-binding protein (IRBP) has on this process.

METHODS

[3H]all-trans retinol (ROL) was delivered to the basal surface of the cultured RPE by serum retinol-binding protein (RBP). Apo IRBP was added to either the apical or basal medium, or was absent from the incubation entirely.

RESULTS

The greatest level of [3H]11-cis RAL was detected in the apical medium but only when apo IRBP was present there. When apo IRBP was present only in the basal medium, or was absent from the incubation entirely, low levels of [3H]11-cis RAL were released apically and basally.

CONCLUSIONS

If 11-cis RAL release were constitutive, one would expect to find elevated levels of this retinoid in the apical and basal media in the absence of apo IRBP. Instead, the enhancement of [3H]11-cis RAL release into the apical, but not the basal, medium in the presence of apo IRBP suggests that [3H]11-cis RAL release is polarized and dependent on the presence of apo IRBP. It is postulated, therefore, that a mechanism such as an IRBP membrane receptor in the apical plasma membrane may be responsible for this polarity.

A report card on diversity: lessons for business from higher education

Institutions of higher learning in the United States have long played a disproportionate role in supplying leadership talent to the world's business and professional organizations. For 30 years, the most selective schools have been working to increase diversity in their student bodies. New research by the former presidents of Princeton and Harvard suggests that the experiences and initiatives of these academic institutions can provide business leaders with insight into how to create diverse organizations that succeed. The first insight has to do with clarity of mission. It is not enough to pursue diversity because it is "the right thing to do." In an insert, Raymond Gilmartin, the CEO of Merck, echoes that view, discussing the relationship between diversity and Merck's competitiveness. The second insight concerns recruiting. The authors challenge what they call "the myth of pure merit," the notion that recruiting is a precise science based only on grades and test scores. Instead, they argue, merit is about assembling a team by deciding which applicants, considered individually and collectively, will contribute most to achieving the company's goals. The third insight concerns how organizations help employees perform to their potential. Of the factors contributing to high graduation rates at the most selective schools, higher expectations and the efforts of mentors stand out as most important. Finally, the fourth insight is about how to achieve accountability in a corporate setting. Boards must ask: Are our recruiting policies working? and How are recruited employees doing?

Transgenic analysis of rds/peripherin N-glycosylation: effect on dimerization, interaction with rom1, and rescue of the rds null phenotype

Rds/peripherin is an integral membrane glycoprotein that is present in the rims of photoreceptor outer segment disks. In mammals, it is thought to stabilize the disk rim through heterophilic interactions with the related nonglycosylated protein roml. Glycosylation of rds/peripherin at asparagine 229 is widely conserved in vertebrates. In this study, we investigated the role of rds/peripherin N-glycosylation. We generated transgenic mice that expressed only S231A-substituted rds/peripherin in their retinas. This protein was not glycosylated but formed covalent dimers with itself and with glycosylated rds/peripherin. Nonglycosylated rds/peripherin also interacted noncovalently with rom1 homodimers to form a heterooligomeric complex. The glycosylated rds/peripherin..rom1 complex bound to concanavalin A-Sepharose, suggesting that the glycan is not directly involved in the interaction between these proteins. In double transgenic mice expressing normal and S231A-substituted rds/peripherin, the mRNA-to-protein ratios were similar for both transgenes, indicating no effect of N-glycosylation on rds/peripherin stability. Finally, expression of nonglycosylated rds/peripherin in transgenic mice rescued the phenotype of outer segment nondevelopment in retinal degeneration slow (rds-/-) null mutants. These observations indicate that N-glycosylation of rds/peripherin is not required for its normal processing, stability, or in vivo function.

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.

Non-cell-autonomous photoreceptor degeneration in rds mutant mice mosaic for expression of a rescue transgene

The inherited retinal dystrophies represent a large and heterogenous group of hereditary neurodegenerations, for many of which, the molecular defect has been defined. However, the mechanism of cell death has not been determined for any form of retinal degeneration. The retinal degeneration slow (rds-/-) mutation of mice is associated with nondevelopment of photoreceptor outer segments and gradual death of photoreceptor cell bodies, attributed to the absence of the outer segment protein rds/peripherin. Here, we examined the effects of a transgene encoding normal rds/peripherin that had integrated into the X-chromosome in male and female rds-/- mutant retinas. In 2-month-old transgenic males and homozygous-transgenic females on rds-/-, we observed virtually complete rescue of both the outer segment nondevelopment and photoreceptor degeneration. In contrast, hemizygous-transgenic rds-/- female littermates showed patchy distributions of the transgene mRNA, by in situ hybridization analysis, and of photoreceptor cells that contain outer segments. This pattern is consistent with random inactivation of the X-chromosome and mosaic expression of the transgene. Surprisingly, we observed significant photoreceptor cell loss in both transgene-expressing and nonexpressing patches in hemizygous female retinas. These observations were supported by nuclease protection analysis, which showed notably lower than predicted levels of transgene mRNA in retinas from hemizygous females compared with male and homozygous female littermates. This phenotype suggests an important component of non-cell-autonomous photoreceptor death in rds-/- mutant mice. These results have significance to both the etiology and potential treatment of human inherited retinal degenerations.

Transplantation of retinal pigment epithelial cells and immune response in the subretinal space

PURPOSE

To evaluate the fate of retinal pigment epithelial cell (RPE) allografts in the subretinal space.

METHODS

Transplantation was performed in Royal College of Surgeons (RCS) rats. Two rat strains, BD IX and LEJ, which have incompatible major histocompatibility complex (MHC) haplotypes, were selected as healthy RPE donors. Transplantation was performed when recipients were 19 to 21 days old. Host systemic immunity was enhanced by challenge with donor spleen cells 2 weeks after RPE transplantation. Control subjects were administered injections of saline or host spleen cells. The animals were killed at 3 or 5 months of age. The extent of photoreceptor rescue was determined by counting the maximum layers of surviving photoreceptor nuclei in histologic sections. Reverse transcription-polymerase chain reaction analysis of the grafts was performed with host- and donor-specific primers.

RESULTS

Despite the absence of acute immune rejection, chronic rejection occurred and was evidenced by an increased loss of photoreceptor cells in immunologically challenged RCS rats. Grafts with disparity at MHC class I and class II lost their ability to rescue photoreceptor cells more readily than did grafts with disparity at MHC class II alone. Furthermore, the donor RPE cells that were normally MHC class II-negative expressed MHC class II mRNA in the subretinal space after transplantation.

CONCLUSIONS

Systemic immunity appeared to exert a slow but significant influence in the subretinal space. Therefore, in planning future trials involving human subjects, the immunologically privileged status of the subretinal space should be regarded as imperfect.

Bovine epidermal fatty acid-binding protein: determination of ligand specificity and cellular localization in retina and testis

The fatty acid-binding protein (FABP) family consists of small, cytosolic proteins believed to be involved in the uptake, transport, and solubilization of their hydrophobic ligands. Members of this family have highly conserved sequences and tertiary structures. Using an antibody against testis lipid-binding protein, a member of the FABP family, a protein was identified from bovine retina and testis that coeluted with exogenously added docosahexaenoic acid during purification. Amino acid sequencing and subsequent isolation of its cDNA revealed it to be nearly identical to a bovine protein expressed in the differentiating lens and to be the likely bovine homologue of the human epidermal fatty acid-binding protein (E-FABP). From quantitative Western blot analysis, it was estimated that bovine E-FABP comprised 0.9%, 0.1%, and 2.4% of retina, testis, and lens cytosolic proteins, respectively. Binding studies using the fluorescent probe ADIFAB indicated that this protein bound fatty acids of differing levels of saturation with relatively high affinities. Kd values ranged from 27 to 97 nM. In addition, the protein was immunolocalized to the Müller cells in the retina as well as to Sertoli cells in the testis. The location of bovine E-FABP in cells known to be supportive to other cell types in their tissues and the ability of E-FABP to bind a variety of fatty acids with similar affinities indicate that it may be involved in the uptake and transport of fatty acids essential for the nourishment of the surrounding cell types.

Structural and functional characterization of recombinant human cellular retinaldehyde-binding protein

Cellular retinaldehyde-binding protein (CRALBP) is abundant in the retinal pigment epithelium (RPE) and Müller cells of the retina where it is thought to function in retinoid metabolism and visual pigment regeneration. The protein carries 11-cis-retinal and/or 11-cis-retinol as endogenous ligands in the RPE and retina and mutations in human CRALBP that destroy retinoid binding functionality have been linked to autosomal recessive retinitis pigmentosa. CRALBP is also present in brain without endogenous retinoids, suggesting other ligands and physiological roles exist for the protein. Human recombinant cellular retinaldehyde-binding protein (rCRALBP) has been over expressed as non-fusion and fusion proteins in Escherichia coli from pET3a and pET19b vectors, respectively. The recombinant proteins typically constitute 15-20% of the soluble bacterial lysate protein and after purification, yield about 3-8 mg per liter of bacterial culture. Liquid chromatography electrospray mass spectrometry, amino acid analysis, and Edman degradation were used to demonstrate that rCRALBP exhibits the correct primary structure and mass. Circular dichroism, retinoid HPLC, UV-visible absorption spectroscopy, and solution state 19F-NMR were used to characterize the secondary structure and retinoid binding properties of rCRALBP. Human rCRALBP appears virtually identical to bovine retinal CRALBP in terms of secondary structure, thermal stability, and stereoselective retinoid-binding properties. Ligand-dependent conformational changes appear to influence a newly detected difference in the bathochromic shift exhibited by bovine and human CRALBP when complexed with 9-cis-retinal. These recombinant preparations provide valid models for human CRALBP structure-function studies.

Generation and analysis of transgenic mice expressing P216L-substituted rds/peripherin in rod photoreceptors

PURPOSE

In this study, the authors present the biochemical, morphologic, and physiological analyses of a transgenic mouse model for retinal degeneration slow (RDS)-mediated retinitis pigmentosa caused by a proline 216 to leucine (P216L) amino acid substitution in rds/peripherin.

METHODS

The authors assembled a mutant rds transgene that encodes rds/peripherin with a P216L substitution. Transgenic mice were generated on wild-type (+/+), heterozygous (rds-/+), and homozygous (rds-/rds-) null genetic backgrounds. These mice were analyzed biochemically, by light and electron microscopy, and by electroretinography.

RESULTS

In P216L-transgenic mice on a +/- background, the authors observed expression-level-dependent photoreceptor degeneration and outer-segment shortening. Expression of the P216L transgene on an rds-/+ background resulted in more severe photoreceptor degeneration and outer-segment dysplasia than seen in nontransgenic rds-/+ mutants. Severely dysplastic outer segments were detectable in P216L transgenics on an rds-/rds-null background. The reduction in b-wave amplitudes by electroretinography were well correlated with the degree of photoreceptor degeneration, but not outer-segment dysplasia in these different rds mutants.

CONCLUSIONS

The phenotype in P216L-transgenic mice on an rds-/+ genetic background probably is caused by a combination of two genetic mechanisms: a direct dominant effect of the P216L substituted protein, and a reduction in the level of normal rds/peripherin. The expression pattern of the normal and mutant genes in these animals is similar to that predicted for humans with RDS-mediated autosomal-dominant retinitis pigmentosa. These mice may thus be considered an animal model for this disease.

Expression and synthesis of the Na,K-ATPase beta 2 subunit in human retinal pigment epithelium

Na,K-ATPase in the retinal pigment epithelium (RPE) is apically localized, whereas in most other tissues this pump is found predominantly in the basolateral membrane domain. As part of our investigations into the molecular aspects of this pump in the RPE, we have cloned the cDNA and characterized the expression of the gene encoding the beta 2 subunit isoform of Na,K-ATPase in human, rat and bovine RPE and in the bovine choroid plexus. We have also detected the beta 2 isoform polypeptide in the human RPE (hRPE). Comparison of complete coding sequences derived from cloned cDNAs revealed that all beta 2 sequences from RPE, and the choroid plexus, differed uniformly at positions: P51/L, M121/I, and L148/R from the published sequences for human retina and liver. However, analysis of 10 RT-PCR clones derived from 5 fetal and 2 adult human retinas sequenced in our laboratory, revealed that only the P51/L residue was different with the hRPE beta 2 subunit sequence. Northern blot analysis indicated a 3.4-kb RNA transcript for the beta 2 subunit, a 4.5-kb RNA for the alpha 1 subunit and a doublet of 2.3 and 2.6 kb for the beta 1 subunit, respectively. alpha 1 (100 kDa), beta 1 (45 kDa) and beta 2 (65 kDa) isoforms were detected in hRPE extracts by immunoblotting. No alpha 2 and alpha 3 RNA transcripts were found in the hRPE. Quantification of beta 2 mRNA by RT-PCR revealed 2.7 x 10(5) molecules per ng of poly A+ RNA. This is similar to the beta 1 isoform levels reported previously from our laboratory. These data demonstrate the coexistence of significant amounts of alpha 1, beta 1 and beta 2 Na,K-ATPase subunits in the RPE. It is therefore reasonable to suggest that both alpha 1 beta 1 and alpha 1 beta 2 heterodimers are present in these cells.