Histopathology of the Retina from a Three Year-Old Suspected to Have Joubert Syndrome

PURPOSE

To define the retinal pathology in a 3 year-old eye donor who died from complications of an undiagnosed genetic syndrome.

METHODS

Eyes were fixed and analyzed using macroscopic fundus photography (MF), confocal scanning laser ophthalmoscopy (cSLO) and spectral-domain optical coherence tomography (SD-OCT). Small areas from the perifovea and periphery were processed for histology and indirect immunofluorescence, using antibodies specific to retinal proteins such as rhodopsin, cone arrestin, RPE65 and others. Available medical records were also reviewed.

RESULTS

With all three imaging modalities, the affected donor's eyes lacked the distinct morphological detail typically observed with these techniques in postmortem control eyes. MF images showed a "photonegative effect" due to a hypopigmented macula relative to a hyperpigmented retinal background. cSLO imaging demonstrated a weak autofuorescence signal that was largely devoid of the usual retinal structures compared to the control. SD-OCT suggested disorganization of the affected retina, absence of a photoreceptor layer, and degeneration of the choroid in the macular area. Histologic findings indicated a highly disorganized photoreceptor layer in the macula and periphery. The RPE layer displayed thinning in some regions of the periphery and decreased pigmentation in most areas. Rods and cones were significantly reduced in the affected retina but a few cones were detected in the perifovea. Centrin-2 labeling was mostly absent from the connecting cilium of the photoreceptor cells. Medical record review pointed to a possible clinical diagnosis of Joubert syndrome.

CONCLUSIONS

The retinal degenerative findings, and absence of centrin-2 labeling are compatible with the expected retinal phenotype in patients with Joubert syndrome.

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

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

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.

Interphotoreceptor matrix in the fovea and peripheral retina of the primate Macaca mulatta: distribution and glycoforms of SPACR and SPACRCAN

SPACR and SPACRCAN localization in the interphotoreceptor matrix (IPM) of the fovea and peripheral retina of Macaca mulatta was established with antibodies to these core proteins and the chondroitin sulfate epitopes and lectin binding properties of these molecules were defined. The IPM of both rods and cones labeled with anti-SPACR, anti-SPACRCAN, anti-Delta Di6S antibodies and wheat germ agglutinin (WGA). Whereas anti-SPACR and anti-SPACRCAN antibodies labeled rod and cone matrix compartments with similar intensity, the Delta Di6S chondroitin antibody labeling was more intense around cones than rods. Peanut lectin (PNA) labeling was present only around cones. No IPM labeling was observed with Delta Di0S-chondroitin or Delta Di4S-chondroitin antibodies. Western blots of undigested IPM extracts showed anti-SPACR immunoreactivity at 150 kDa, colocalizing with the position of WGA and PNA binding. In Western blots of the chondroitinase ABC digested sample and samples double digested with chondroitinase ABC and AC II, anti-SPACR immunoreactivity, WGA and PNA labeling intensity were virtually identical to that in the undigested sample, with prominent staining of the 150 kDa SPACR band. In contrast, anti-SPACRCAN immunoreactivity was not present in the undigested sample, but was evident in both the chondroitinase ABC and double digested samples as a broad band at approximately 230 kDa. Delta Di6S, Delta Di4S, WGA and PNA labeling colocalized with the anti-SPACRCAN immunoreactivity in the chondroitinase ABC digested sample. These findings indicate that SPACR and SPACRCAN are present around cones in the fovea and both rods and cones in the peripheral retina, but that the specific glycoforms of these molecules are different depending on whether present in the cone or rod associated IPM.

Bestrophin, the product of the Best vitelliform macular dystrophy gene (VMD2), localizes to the basolateral plasma membrane of the retinal pigment epithelium

Best vitelliform macular dystrophy is a dominantly inherited, early onset, macular degenerative disease that exhibits some histopathologic similarities to age-related macular degeneration. Although the vitelliform lesion is common in the fundus of individuals with Best disease, diagnosis is based on a reduced ratio of the light peak to dark trough in the electrooculogram. Recently, the VMD2 gene on chromosome 11q13, encoding the protein bestrophin, was identified. The function of bestrophin is unknown. To facilitate studies of bestrophin, we produced both rabbit polyclonal and mouse monoclonal antibodies that proved useful for Western blotting, immunoprecipitation, and immunocytochemistry. To characterize bestrophin, we initially probed the retinal pigment epithelium (RPE)-derived cell lines ARPE-19, D407, and RPE-J. All of the cell lines expressed bestrophin mRNA by reverse transcription-PCR, but not on Western blots. Bestrophin in human RPE partitioned in the detergent phase during Triton X-114 extraction and could be modified by biotin in intact cells, indicative of a plasma membrane localization. Immunocytochemical staining of macaque and porcine eyes indicated that bestrophin is localized at the basolateral plasma membrane of RPE cells. When expressed in RPE-J cells by adenovirus-mediated gene transfer, bestrophin again was determined by confocal microscopy and cell surface biotinylation to be a basolateral plasma membrane protein. The basolateral plasma membrane localization of bestrophin suggests the possibility that bestrophin plays a role in generating the altered electrooculogram of individuals with Best disease.

SPACR in the interphotoreceptor matrix of the mouse retina: molecular, biochemical and immunohistochemical characterization

Mouse SPACR cDNA was cloned by screening a mouse retina cDNA library using a PCR probe derived from human SPACR cDNA. Mouse SPACR cDNA comprises 3675 bp containing an open reading frame coding for 742 amino acids. Multitissue Northern blot analysis and in situ hybridization studies indicate that SPACR expression is restricted to retinal photoreceptors. The SPACR core protein was identified with Western blotting following SDS-PAGE with a SPACR C-terminal peptide polyclonal antibody and a chondroitin-6-sulfate Deltadisaccharide monoclonal antibody. The 150 kD immunopositive band was isolated, digested with trypsin and the peptides analysed by MALDI mass spectroscopy. Peptide mass mapping confirmed the identity of the 150 kD immunopositive band to be mouse SPACR core protein. Alignment comparisons of the deduced amino acid sequence of mouse and human SPACR show 64% homology. Like SPACR in the human interphotoreceptor matrix, the mouse orthologue contains a large central mucin-like domain flanked by consensus sites for N-linked oligosaccharide attachment, one EGF-like domain and four hyaluronan-binding motifs. Unlike human SPACR, which contains no conventional consensus sites for glycosaminoglycan attachment, mouse SPACR contains three. Recent biochemical studies of human and mouse SPACR protein indicate that this novel interphotoreceptor matrix molecule is a glycoprotein in human and a proteoglycan in the mouse. The presence of consensus sites for glycosaminoglycan attachment in the deduced sequence of mouse SPACR and the absence of these sites in human SPACR provide molecular verification of our biochemical results, suggesting that differences in post-translational modifications of SPACR may be important in SPACR function in foveate and non-foveate retinas.

SPACRCAN, a novel human interphotoreceptor matrix hyaluronan-binding proteoglycan synthesized by photoreceptors and pinealocytes

The interphotoreceptor matrix is a unique extracellular complex occupying the interface between photoreceptors and the retinal pigment epithelium in the fundus of the eye. Because of the putative supportive role in photoreceptor maintenance, it is likely that constituent molecules play key roles in photoreceptor function and may be targets for inherited retinal disease. In this study we identify and characterize SPACRCAN, a novel chondroitin proteoglycan in this matrix. SPACRCAN was cloned from a human retinal cDNA library and the gene localized to chromosome 3q11.2. Analysis of SPACRCAN mRNA and protein revealed that SPACRCAN is expressed exclusively by photoreceptors and pinealocytes. SPACRCAN synthesized by photoreceptors is localized to the interphotoreceptor matrix where it surrounds both rods and cones. The functional protein contains 1160 amino acids with a large central mucin domain, three consensus sites for glycosaminoglycan attachment, two epidermal growth factor-like repeats, a putative hyaluronan-binding motif, and a potential transmembrane domain near the C-terminal. Lectin and Western blotting indicate an M(r) around 400,000 before and 230,000 after chondroitinase ABC digestion. Removal of N- and O-linked oligosaccharides reduces the M(r) to approximately 160,000, suggesting that approximately 60% of the mass of SPACRCAN is carbohydrate. Finally, we demonstrate that SPACRCAN binds hyaluronan and propose that associations between SPACRCAN and hyaluronan may be involved in organization of the insoluble interphotoreceptor matrix, particularly as SPACRCAN is the major proteoglycan present in this matrix.

Chondroitin sulfate proteoglycan core proteins in the interphotoreceptor matrix: a comparative study using biochemical and immunohistochemical analysis

This study characterizes the core proteins of chondroitin sulfate-type glycosaminoglycans located in the interphotoreceptor matrix and establishes the tissue distribution of chondroitin immunoreactivity in human, bovine, mouse and rat retinas. Monoclonal antibodies specific to unsulfated (DeltaDiOS), 4-sulfated (DeltaDi4S) and 6-sulfated (DeltaDi6S) chondroitin were employed. Retinal sections and IPM samples were either (a) digested with chondroitinase ABC to expose antibody specific epitopes, (b) double digested with chondroitinase ABC and chondroitinase AC II to remove specific epitopes, or (c) left undigested to evaluate mimotope labeling. In tissue sections from each species studied, positive immunoreactivity to the DeltaDi6S antibody was present in the IPM surrounding both rods and cones. In human and bovine, DeltaDi6S labeling of the cone matrix compartments was more intense than labeling of the matrix surrounding rods. Intense DeltaDi6S immunoreactivity was present surrounding the foveal cones. In mouse and rat, no differences in labeling intensity of IPM surrounding rod and cone photoreceptors were evident, although labeling of the IPM near the apical surface of the retinal pigment epithelium and around the photoreceptor inner segments was more pronounced than that surrounding the outer segments. All DeltaDi6S antibody labeling was eliminated with chondroitinase AC II digestion. No IPM immunoreactivity in tissue sections was observed when the DeltaDi0S or DeltaDi4S antibodies were used. In Western blots of IPM extracts treated with chondroitinase ABC, prominent DeltaDi6S immunoreactive bands were present at approximately 230 kD and 150 kD in each species studied, with the exception of the human, where the 150 kD component is not a chondroitin proteoglycan. Each of the prominent DeltaDi6S immunoreactive bands showed minor immunoreactivity to the DeltaDi4S antibody. No DeltaDi0S immunoreactivity was noted in Western blots of IPM samples from any species. All immunoreactivity was lost following chondroitinase AC II digestion. These observations document similarities in the electrophoretic mobility of IPM proteoglycan core proteins released following chondroitinase ABC digestion in the four species studied, but reveal pronounced differences in the tissue distribution. Bovine and human IPM show greater concentrations of DeltaDi6S immunoreactivity surrounding cones than rods, whereas rodent tissues show higher concentrations near the retinal pigment epithelium and around the photoreceptor inner segments than around the outer segments. The pattern of distribution of these proteoglycan molecules is highly conserved in these species, suggesting a common role in IPM structure and function.

TIMP-3 in Bruch's membrane: changes during aging and in age-related macular degeneration

PURPOSE

To assess the distribution, content, and function of tissue inhibitor of metalloproteinases (TIMP)-3 during aging in normal eyes for comparison with the levels observed in eyes with age-related macular degeneration (AMD).

METHODS

Donor tissues analyzed included 36 normal eyes (14-96 years old) and 15 AMD eyes (74 -98 years old). A tissue strip including the fovea was used for immunohistochemistry. Western blot analysis was performed on extracts of the retinal pigment epithelium (RPE)- choroid complex from the posterior part of each eye. Immunoreactivity of TIMP-3 bands in each western blot was densitometrically quantitated. The inhibitory function of TIMP-3 was evaluated with reverse zymography.

RESULTS

TIMP-3 was present uniformly across Bruch's membrane in the normal samples. In samples from donors more than 50 years of age, immunostaining was intense. TIMP-3 content ranged from 92 to 1061 ng/cm2 and increased with age (r = 0.66). In AMD eyes, TIMP-3 distribution in Bruch's membrane was abundant in areas of continuous soft drusen but absent in areas below RPE atrophy. TIMP-3 levels in AMD eyes were significantly higher than in age-matched normal eyes (577 versus 877 ng/cm2; P = 0.009). Inhibitory activity correlated well with TIMP-3 content (r = 0.82) and was also significantly higher in AMD eyes than in age-matched normal eyes (P < 0.001).

CONCLUSIONS

During normal aging, TIMP-3 content in Bruch's membrane of the macula shows a significant increase. TIMP-3 content in AMD eyes was elevated relative to that of age-matched normal eyes. Higher levels of TIMP-3 may contribute to the thickening of Bruch's membrane observed in AMD.

SPACR, a novel interphotoreceptor matrix glycoprotein in human retina that interacts with hyaluronan

SPACR (sialoprotein associated with cones and rods), is the major 147-150-kDa glycoprotein present in the insoluble interphotoreceptor matrix of the human retina. Immunocytochemistry localizes SPACR to the matrix surrounding rods and cones (Acharya, S., Rayborn, M. E., and Hollyfield, J. G. (1998) Glycobiology 8, 997-1006). From affinity-purified SPACR, we obtained seven peptide sequences showing 100% identity to the deduced sequence of IMPG1, a purported chondroitin 6-sulfate proteoglycan core protein, which binds peanut agglutinin and is localized to the interphotoreceptor matrix. We show here that SPACR is the most prominent 147-150-kDa band present in the interphotoreceptor matrix and is the gene product of IMPG1. SPACR is not a chondroitin sulfate proteoglycan, since it is not a product of chondroitinase ABC digestion and does not react to a specific antibody for chondroitin 6-sulfate proteoglycan. Moreover, the deduced amino acid sequence reveals no established glycosaminoglycan attachment site. One hyaluronan binding motif is present in the predicted sequence of SPACR. We present evidence that SPACR has a functional hyaluronan binding domain, suggesting that interactions between SPACR and hyaluronan may serve to form the basic macromolecular scaffold, which comprises the insoluble interphotoreceptor matrix.

Characterization of SPACR, a sialoprotein associated with cones and rods present in the interphotoreceptor matrix of the human retina: immunological and lectin binding analysis

Rod and cone photoreceptors project from the outer retinal surface into a carbohydrate-rich interphotoreceptor matrix (IPM). Unique IPM glycoconjugates are distributed around rods and cones. Wheat germ agglutinin (WGA) strongly decorates the rod matrix domains and weakly decorates the cone matrix domains. This study characterizes the major WGA-binding glycoprotein in the human IPM, which we refer to as SPACR (sialoprotein associated with cones and rods). SPACR, which has a molecular weight of 147 kDa, was isolated and purified from the IPM by lectin affinity chromatography. A polyclonal antibody to SPACR was prepared that colocalizes in tissue preparations with WGA-binding domains in the IPM. Sequential digestion of SPACR with N- and O-glycosidases results in a systematic increase in electrophorectic mobility, indicating the presence of both N- and O-linked glycoconjugates. Complete deglycosylation results in a reduction in the relative molecular mass of SPACR by about 30%. Analysis of lectin binding allowed us to identify some of the structural characteristics of SPACR glycoconjugates. Treatment with neuraminidase exposes Galbeta1-3GalNAc disaccharide as indicated by positive peanut agglutinin (PNA) staining, accompanied by the loss of WGA staining. Maackia amurensis agglutinins (MAA-1 and MAA-2), specific for sialic acid in alpha2-3 linkage to Gal, bind SPACR, while Sambucus nigra agglutinin (SNA), specific for alpha2-6 linked sialic acid, does not, indicating that the dominant glycoconjugate determinant on SPACR is the O-linked carbohydrate, NeuAcalpha2-3Galbeta1-3GalNAc. The abundance of sialic acid in SPACR suggests that this glycoprotein may contribute substantially to the polyanionic nature of the IPM. The carbohydrate chains present on SPACR could also provide sites for extensive crosslinking and participate in the formation of the ordered IPM lattice that surrounds the elongate photoreceptors projecting from the outer retinal surface.

Hyaluronan in the interphotoreceptor matrix of the eye: species differences in content, distribution, ligand binding and degradation

Hyaluronan (HA) distribution in the posterior eye wall from the vitreous through the sclera, with special consideration to localization in the retina and interphotoreceptor matrix (IPM), was evaluated in human, bovine, guinea pig, dog, rat and mouse tissues using a specific probe for HA (bHABC, biotinylated hyaluronan binding complex). The sclera, some regions of the choroid and vitreous body was positive for HA, as was the basal lamina of the retina (inner limiting membrane). bHABC binding was detected in the IPM of all species studied except the mouse. Predigestion with Streptomyces hyaluronidase for 3 hr before bHABC application eliminated binding in the vitreous, choroid, sclera and basal lamina of the retina, but did not eliminate bHABC binding in the IPM. In tissues from all species studied, incubation for 6 hr with hyaluronidase eliminated bHABC binding in the IPM, except for two human samples. In these two human samples, HA specific binding in the IPM persisted even after 24 hr enzyme treatment. bHABC failed to bind to any tissue layer when bHABC was preincubated with hyaluronan oligosaccharides before application. The resistance of the IPM HA to hyaluronidase digestion may reflect extensive coverage of HA binding sites by ligands present in this compartment which hinder enzyme access. The absence of bHABC binding to the IPM when the probe is preincubated with HA oligosaccharides indicates that the binding reflects specific interaction with HA. We conclude that, with the exception of the mouse, HA is a prominent constituent of the IPM, where it may serve to organize the matrix by functioning as a basic scaffold to which other macromolecules in the insoluble IPM are attached.

Hyaluronan localization in tissues of the mouse posterior eye wall: absence in the interphotoreceptor matrix

The distribution of hyaluronan (HA) in the posterior eye wall from the vitreous through the sclera, with special consideration to localization in the retina and interphotoreceptor matrix (IPM), was evaluated in mouse tissues using an HA specific probe (bHABC, biotinylated hyaluronan binding complex). The vitreous body was positive for HA, as was Bruch's membrane, expansive areas within the choroid, sclera and perimysial connective tissue of extraocular muscle. No HA-staining was detected in the IPM or in any other retina layer except for the basal lamina (inner limiting membrane of the retina) which abuts the vitreous. Predigestion of sections with trypsin or chondroitinase ABC before bHABC application did not produce additional HA-staining in the retina or IPM.

Elevated mRNA expression of brain-derived neurotrophic factor in retinal ganglion cell layer after optic nerve injury

PURPOSE

Recent studies show that exogenous brain-derived neurotrophic factor (BDNF) can promote retinal ganglion cell survival in vivo and in vitro. BDNF is expressed by a subpopulation of cells in the ganglion cell layer (GCL). To investigate whether endogenous BDNF may play a role in neuronal protection after ganglion cell trauma, BDNF expression in the retina was examined after optic nerve (ON) injury.

METHODS

The optic nerve in Sprague-Dawley rats was crushed intraorbitally posterior to the optic disc. For controls, the optic nerve on the opposite side in each animal was similarly exposed but was not crushed. After intervals of 6 hours to 6 weeks, eye tissues were processed for in situ hybridization, Northern blot, and RNase protection assay using radiolabeled rat riboprobes.

RESULTS

After ON injury, BDNF expression was significantly elevated in cells restricted to the GCL, and more cells demonstrated expression of BDNF than were observed in the controls. Elevated BDNF expression was first observed at 24 hours, peaked at 48 hours, and declined to the basal level 2 weeks after ON injury. Quantitative analysis showed a fivefold to sixfold increase in the number of BDNF-positive cells and a 54% increase in BDNF signal intensity in individual cells in the GCL 48 hours after ON injury. In control retinas without ON injury, BDNF expression was localized to some cells in the GCL, as was observed in normal eyes without surgery. Northern blot and RNase protection assay demonstrated a 38% elevation in BDNF expression above control levels 48 hours after ON injury.

CONCLUSIONS

These results indicate that cells in the GCL can upregulate gene expression of BDNF in response to ganglion cell axonal injury and suggest that endogenous BDNF may contribute to a natural neuroprotective process after ON injury.

A 221-bp fragment of the mouse opsin promoter directs expression specifically to the rod photoreceptors of transgenic mice

Mutations in the human rod opsin gene have been shown to segregate with autosomal dominant retinitis pigmentosa (ADRP) and photoreceptor degeneration in transgenic mice. While these degenerations are characterized by the primary degeneration of rods, cones eventually die as well. To determine whether this subsequent cone degeneration is the result of expression of mutant rod opsin in the cones, the retinal cell-type specificity of a 221-bp fragment of the mouse rod opsin promoter was evaluated. Two transgenic mouse lines generated by injecting a fusion gene comprised of a 221-bp fragment of the mouse rod opsin promoter and the simian virus 40 large tumor antigen gene (Tag) were examined. The expression of Tag causes photoreceptor cell degeneration in members of both transgenic lines. However, the two lines differed with respect to the level of Tag expression and the rate and extent of photoreceptor cell degeneration. Immunocytochemical localization of opsin and Tag in surviving photoreceptor cells was determined and the results were confirmed by reverse transcriptase polymerase chain reaction (RT-PCR). Rod- and cone-mediated function was evaluated by electroretinography (ERG). In the higher Tag-expressing transgenic line only one row of nuclei remained in the outer nuclear layer at postnatal day (P) 150. While these nuclei showed no antigenicity for rod opsin or Tag, they did stain with an antibody that reacts with both rod and cone S-antigens (arrestins), indicating that these cells were surviving photoreceptor nuclei. Positive staining with peanut agglutinin, which uniquely decorates matrix domains surrounding cones in the normal retina, confirmed that the surviving photoreceptor nuclei were of cone origin. RT-PCR substantiated the results from immunostaining; amplification product was obtained using blue cone opsin transcripts but not from either Tag or rod opsin transcripts. The second transgenic mouse line exhibited a much slower photoreceptor cell death that was associated with low levels of Tag transgene transcript. At P120, approximately 50% of photoreceptors remained and an approximately 45% reduction in the rod ERG a-wave was observed. Cone-mediated ERGs, however, were normal. The results demonstrate the rod-specific expression of Tag as directed by the 221-bp fragment of the mouse rod opsin promoter and suggest that the cone degeneration in ADRP or transgenic mice associated with mutations in the rod opsin gene is a secondary effect of rod degeneration.

Unscheduled DNA replication precedes apoptosis of photoreceptors expressing SV40 T antigen

Expression of simian virus 40 T antigen (Tag) in the rod photoreceptors of transgenic mice leads to cell death that is completed by the end of the third week of postnatal development. To understand the mechanistic link between Tag expression and the death of the expressing photoreceptors, cell cycle activity was followed in a transgenic mouse family that expresses Tag directed by the mouse opsin promoter. Tag-expressing photoreceptors also expressed rhodopsin suggesting that these cells were differentiated. The presence of Tag in the photoreceptors induced the expression of both proliferating cell nuclear antigen (PCNA) and thymidine kinase (TK). The abnormally high levels of PCNA and TK continued until the complete disappearance of the cells expressing Tag. Photoreceptor cell death was also associated with continued DNA synthesis that ceased shortly after postnatal day 16. The specific loss of the rod photoreceptors that re-entered the cell cycle accounted entirely for the loss of photoreceptors from the outer nuclear layer. The antiproliferative nature of the mature retina is directly involved in the apoptotic death of photoreceptors expressing Tag.

Basic fibroblast growth factor: increased gene expression in inherited and light-induced photoreceptor degeneration

In a recent study, we reported that bFGF content and mRNA expression levels are elevated in degenerating photoreceptors in the rd mouse retina as compared to the levels in photoreceptors in age matched normal retinas (Gao and Hollyfield, 1995). To determine whether bFGF up-regulation is a general feature of degenerating photoreceptors, bFGF expression was examined with in situ hybridization in several rodent models with retinal degeneration: (a) the normal mouse retina in which cGMP metabolism had been disrupted pharmacologically using an inhibitor of phosphodiesterase or an analog of cGMP, (b) the rds mouse with an inherited photoreceptor degeneration that does not involve alterations in cGMP metabolism, (c) light-damaged BALB/c mice subjected to bright constant light, and (d) light-stressed albino rats subjected to bright cyclic light. In each of these models, we observed dramatic up-regulation of bFGF gene expression in photoreceptors, although the relative levels varied among the different models and followed different temporal patterns. We conclude that increased expression of bFGF in photoreceptors during their degeneration is a general phenomenon occurring in rd, rds, light-damaging and light-stressing conditions. These observations indicate that bFGF can be up-regulated by a variety of photoreceptor insults. We speculate that endogenous bFGF may function as a photoreceptor protection/rescue factor that is activated in response to photoreceptor stress.

Basic fibroblast growth factor in retinal development: differential levels of bFGF expression and content in normal and retinal degeneration (rd) mutant mice

The spatial and temporal patterns of expression and content of bFGF during postnatal development of the retina were established in C57BL/6J mice. Western blot analysis, using an anti-rodent bFGF antibody, shows multiple molecular weights of 18, 20.5, and 22 kDa of bFGF protein isolated from the adult retina. A bioassay indicates that this putative basic fibroblast growth factor (bFGF) stimulates proliferation of BALB/c 3T3 fibroblasts in a dose-dependent manner identical to an authentic bFGF standard. Immunocytochemistry reveals that bFGF immunoreactivity is located primarily in the immature photoreceptors during postnatal development and is associated with the photoreceptor outer segment/interphotoreceptor matrix complex in the adult retina. bFGF mRNA expression pattern and levels were evaluated using mouse bFGF riboprobes with in situ hybridization and quantitative ribonuclease protection assay. bFGF mRNA expression is not detectable in the retina until Postnatal Day 10 (P10), although high levels of bFGF mRNA signals were consistently observed in astrocytes in the optic disc at all postnatal ages examined. From P10 to the adult stage, bFGF mRNA was localized mainly to the photoreceptor inner segments, and the bFGF mRNA levels were approximately the same at P10 and in the adult retina. The patterns of retinal bFGF expression and content during normal development established above were compared to these parameters in the retina of rd (C57BL/6J rd/rd), a spontaneous mouse mutant in which photoreceptors degenerate shortly after birth. More bFGF immunoreactivity was detected in the outer retina during photoreceptor degeneration than was present in normal photoreceptors at equivalent ages. Densitometry measurements indicate that the level of immunoreactivity is 56% to 1.8-fold higher in rd than in the normal retina between P6 and P10, respectively. This is at least partially due to elevated bFGF mRNA expression in rd retinas during photoreceptor degeneration. In situ hybridization showed more intense bFGF mRNA hybridization signals in rd photoreceptors from P10 to P15, and RNase protection assay demonstrated much higher hybridization signals in rd retinas from P6 to P10 than in the normal retinas at these stages. More bFGF mRNA hybridization signals were also present in some cells in the inner nuclear layer following photoreceptor cell death in the rd retina but were only weakly evident in the inner nuclear layer in the normal adult retina. These results provide the first evidence that a naturally occurring neuronal degeneration is accompanied by elevated expression of bFGF in degenerating neurons prior to cell death.(ABSTRACT TRUNCATED AT 400 WORDS)

Development of the interphotoreceptor matrix in Xenopus laevis

Xenopus laevis interphotoreceptor matrix (IPM) contains a relatively aqueous insoluble wheat germ agglutinin (WGA)-binding component containing unidentified sialoglycoconjugates (Wood et al [1984] J. Comp. Neurol. 228:299-307). The appearance of WGA-binding macromolecules in the IPM was assessed during late embryonic stages (32-45) and in retinal rudiment cultures, using lectin cytochemistry and Western blotting techniques. Metabolic labeling of the neural retina versus retinal pigment epithelium (RPE)-choroid of juvenile Xenopus with 35S-MET was also evaluated in vivo and in vitro. Lectin cytochemistry of eyes from developmental stages 32-42 demonstrated distinct WGA-ferritin-binding sites on the developing outer segment membranes and in the IPM compartment. At stages 44-46 extensive WGA-binding domains were present as an extracellular network with other randomly scattered domains near the retinal pigment epithelium. Retinal rudiments from stage 32-33 were isolated and allowed to differentiate in hanging drop culture (Hollyfield and Witkowsky [1974] J. Exp. Zool. 189:357-377) with or without an investing pigment epithelium. Cultures developing with RPE exhibited an elaborate IPM with an anastomosing meshwork of WGA-ferritin binding sites. In the absence of RPE only limited amounts of binding restricted to the immediate vicinity of the developing photoreceptor outer segment membranes was observed. When Western blots were probed with WGA-HRP, stage 32-45 retinas demonstrated a major WGA-binding band of 126 kD. Similar amounts of WGA-binding macromolecules were synthesized in preparations cultured in the presence or absence of the investing RPE. During development the major WGA-binding component is a 126-kD protein. Equivalent synthesis of this protein in the presence and absence of RPE suggests that the PE is not required for synthesis of this 126-kD component. These results suggest that the retina is the primary site of synthesis of the WGA-binding components of the Xenopus IPM, whereas the PE plays a principal role in their assembly and organization.

Cell birthdays in Xenopus laevis retina

Using an in vitro differentiation system, we reevaluated the stages of development during which retinal neurons become postmitotic in Xenopus laevis. We also examined whether retinal detachment and removal of the pigment epithelium stimulated proliferation of previously postmitotic retinal cells. Retinas with and without an adherent pigment epithelium (stages 24-40 and stage 33/34, respectively) were removed from X. laevis embryos, placed in culture and allowed to differentiate in the presence of 3H-thymidine. After 2 days, eyes were fixed and processed for autoradiography. The proportion of labeled to unlabeled nuclei in the posterior pole of the retina was determined for each of the three cell layers. Early in development, most unlabeled cells were found in the ganglion cell layer; at stage 24, 53% of the cells showed no labeling, but by stage 32-33/34 all of the cells present were unlabeled. Within the outer nuclear layer, 17% of the cells failed to incorporate the 3H-thymidine at stage 24 and by stage 33/34, 100% of the cells were unlabeled. Within the inner nuclear layer, 13.5% of the cells failed to show labeling at stage 24, whereas at stage 40, none of the cells were labeled. There was no difference in the proportion of labeled to unlabeled nuclei in any of the cell layers when retinas were allowed to differentiate either with or without an adherent pigment epithelium. These results indicate that as early as stage 24, some cells that will become positioned in each of the nuclear strata fail to incorporate 3H-thymidine, suggesting that these cells become postmitotic very early in neurogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased retinal synthesis of heparan sulfate proteoglycan and HNK-1 glycoproteins following photoreceptor degeneration

In an earlier analysis of the retinal biosynthesis of proteoglycan, we noted that, following photoreceptor degeneration in the rd (retinal degeneration) mouse, the remaining inner retina exhibited a marked elevation in synthesis of heparan sulfate proteoglycan (HSPG), well above the level observed in the normal (nondegenerate) retina, as well as a pronounced increase in sulfation of protein substrates. Biochemical and autoradiographic results of 35S-amino acid utilization reported here confirm that the 35SO4(2-) differences seen previously are accompanied by increased protein synthesis in the rd retina. An intact photoreceptor cell layer is neither a barrier to nor a sink for the amino acid precursor. Further, we have examined sulfate utilization in four other rodent strains with photoreceptor degenerations. In each of the models examined, an increase in retinal synthesis of 35SO4(2-)-labeled HSPG and glycoproteins occurs following photoreceptor degeneration. We have metabolically labeled with Na2(35)SO4 isolated retinal cultures from the following: (a) mice with light-induced photoreceptor degeneration; (b) rd mice; (c) transgenic mice with photoreceptor degeneration; (d) RCS rats; and (e) rats with light-induced photoreceptor degeneration. Comparisons were made with concurrent cultures of control nondegenerate retinal tissues. Protein and proteoglycan-enriched fractions were prepared from the incubation media and guanidine HCl/detergent extracts of the retinas by ion-exchange chromatography. The 35SO4(2-)-proteoglycans were identified by chondroitinase ABC and nitrous acid treatments. Retinas lacking photoreceptors produced at least five times the amount of 35SO4(2-)-HSPG found in control incubations. The RCS and light-damaged rats also showed increased synthesis of 35SO4(2-)-chondroitin sulfate proteoglycan relative to the control, through the increase was of lesser magnitude than the HSPG effect. 35SO4(2-)-protein in degenerate and light-damaged retinas always contained at least twice the radioactivity found in comparable control preparations. The bulk of the increased radiolabeling was found in N-linked oligosaccharides, including several recognized by the HNK-1 antibody. These data suggest that a sustained increase in HSPG and HNK-1 glycoprotein synthesis is a consistent response of inner retinal cells following loss of photoreceptors and is independent of the cause of photoreceptor degeneration.

Outer segment disc membrane assembly in the absence of the pigment epithelium: the effect of exogenous sugars

When retinal rudiments of Xenopus laevis embryos are allowed to differentiate in vitro, membrane precursors destined for the outer segment do not form well-ordered stacks of membrane leaflets if the pigment epithelium is removed from the outer retinal surface. However, addition of certain saccharides permits the organization of outer segment membranes into stacked membranous saccules, much like that demonstrated during in vivo or in vitro development in the presence of an investing pigment epithelium. Sugars that support the formation of well-organized photoreceptor outer segments include lactose, galactose and their derivatives, while numerous other carbohydrates failed to support outer segment organization.

Photoreceptor outer segment development in Xenopus laevis: influence of the pigment epithelium

Opsin gene expression, synthesis, and photoreceptor outer segment morphology were evaluated during retinal development in Xenopus laevis. Retinal rudiments were harvested during in vivo development from embryonic stages 31 through 46 or were allowed to develop in vitro after removal from stage 33/34 embryos for 1, 2, or 3 days either with or without an investing pigment epithelium. Opsin mRNA was detected at stage 33/34 and the transcript level increased until stage 40 and remained at this level through stage 46. Opsin was first detected at stage 37/38 and progressively increased through stage 46. Rudimentary photoreceptor outer segment membranes occasionally appeared as early as stage 33/34 and they gradually increased in length, forming well-defined stacks of collapsed membranous saccules (discs) during in vivo development. The maturation of eye rudiments in culture was followed to determine how closely in vivo and in vitro development compare and to examine the ability of photoreceptors to differentiate when maintained in the absence of an overlying pigment epithelium (PE) layer. With the PE present, opsin mRNA as well as opsin content steadily increased over the entire culture period. After 1 day of culture, short cilia with minimal amounts of outer segment membranous material were present. By Day 3, the degree of outer segment differentiation corresponded morphologically to approximately stage 43 of in vivo development. When cultured in the absence of an investing PE, the opsin mRNA level increased minimally during the 3 days in culture. Opsin content increased, yet the relative amount was approximately 50% less than that present in retinas developing in the presence of the PE. Membranous material was elaborated; however, the outer segments appeared to be highly disorganized and formed whorl-like structures rather than the normal stacked disc morphology. These results suggest that the PE may be involved in regulating opsin at the transcriptional and/or translational levels and also participates in the organization of rod outer segment membranes.

Simulation of human autosomal dominant retinitis pigmentosa in transgenic mice expressing a mutated murine opsin gene

Autosomal dominant retinitis pigmentosa (ADRP), slowly progressing over decades, leads to severe visual impairment and in some cases to complete blindness. More than 40 mutations in the human opsin gene have been linked to some forms of this genetically heterogeneous disease. In photoreceptor cells of ADRP patients with mutations in the opsin gene, normal rhodopsin is thought to be synthesized concomitantly with mutated rhodopsin, which, by an unknown mechanism, causes the slow degeneration of the photoreceptor cells. To establish a transgenic mouse line that carries a mutated mouse opsin gene in addition to the endogenous opsin gene, we introduced a mouse opsin gene containing mutations in exon 1 into the germ line of a normal mouse. The alterations consisted of three amino acid substitutions near the N terminus of rhodopsin, Val-20-->Gly (V20G), Pro-23-->His (P23H), and Pro-27-->Leu (P27L). The P23H mutation is the most prevalent mutation in human ADRP patients. During early postnatal development, mice heterozygous for the mutated opsin gene appear to develop normal photoreceptors, but their light-sensitive outer segments never reach normal length. With advancing age, both rod and cone photoreceptors are reduced progressively in number. The slow degeneration of the transgenic retina is associated with a gradual decrease of light-evoked electroretinogram responses. Our results show that simultaneous expression of mutated and normal opsin genes induces a slow degeneration of both rod and cone photoreceptors and that the course of the retinal degeneration of the mutant mouse retina mimics the course of human ADRP.

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

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

Characterization of the interphotoreceptor matrix surrounding rod photoreceptors in the human retina

Previous studies have documented the presence of specific lectin-binding domains in the insoluble interphotoreceptor matrix (IPM) isolated from human retina. Peanut agglutinin (PNA) selectively binds to cone matrix domains whereas wheat germ agglutinin (WGA) binds to matrix domains surrounding rods. In the present study, the rod-associated WGA-binding domains are further characterized using lectin-based cytochemistry and polyacrylamide gel electrophoresis in combination with neuraminidase digestion. The lectin-binding patterns of non-neuraminidase-treated samples are similar to those described in previous reports. After neuraminidase treatment, both rod and cone matrix domains demonstrate PNA binding while the binding of WGA to rod matrix domains is reduced. However, the binding of WGA to photoreceptor outer segments is not affected by neuraminidase. Blots of IPM proteins probed with lectins indicate that the WGA-binding macromolecules are represented as a group of high molecular weight glycoproteins, whereas the PNA-binding components are represented as a group of lower molecular weight glycoproteins. The major WGA-binding glycoprotein (147 kDa) shows reduced binding affinity to WGA and increased binding affinity to PNA following neuraminidase treatment. Further, this 147-kDa glycoprotein, although similar in molecular weight to IRBP (interphotoreceptor retinol-binding protein) (141 kDa), is not recognized by the lectin, concanavalin A (Con A), or by an anti-IRBP antibody. Our data suggest that: (1) the major component of the WGA-binding domain demonstrated by polyacrylamide gel analysis is a glycoprotein with a molecular weight of 147 kDa containing galactose residues that are masked by terminal sialic acid residues; and (2) the binding of WGA to photoreceptor outer segments is resistant to neuraminidase, consistent with the earlier reports that WGA-binding domains of photoreceptor outer segments may not be sialyl-containing glycoconjugates.

Proteoglycans in the mouse interphotoreceptor matrix. VI. Evidence for photoreceptor synthesis of chondroitin sulfate proteoglycan using genetically fractionated retinas

To determine the role of photoreceptors in the synthesis of chondroitin sulfate proteoglycan (CS-PG) present in the interphotoreceptor matrix (IPM), 35SO4(2-) was used as a tracer for comparison of proteoglycans synthesized in vitro in the absence of the pigment epithelium by normal retinas and retinas from retinal degeneration (rd) mice at stages before and after photoreceptor degeneration. Isolated retinas from 10 day post-partum (P-10) pups, adult normal mice (C57BL/6J ++/++) and retinal degeneration mice (C57BL/6J rdle/rdle) were incubated for 7 hr with 35SO4(2-) to label newly synthesized sulfated proteoglycans. At P-10, rd retinas have not undergone extensive photoreceptor degeneration, whereas in the adult retinas from this strain, only a few cone photoreceptors remain. At the termination of the labeling period, proteoglycans in the incubation medium and those remaining in guanidine hydrochloride (GuHCl) extracts of the retina were analysed separately and identified by their susceptibility to enzymatic or nitrous acid depolymerization. At P-10, no significant differences were observed in the types or sizes of newly synthesized proteoglycan in normal and rd retinas. Medium samples from P-10 retinas contained near equal amounts of 35S-labeled CS-PG and heparan sulfate proteoglycan (HS-PG), while in GuHCl extracts, approximately 90% of the 35SO4(2-) was incorporated into HS-PG, with the remainder found in CS-PG. Comparisons of adult tissue revealed a divergence of proteoglycan synthesis profiles. Retinas from normal adults label predominantly CS-PG. [35S]proteoglycan from normal retina incubation medium was approximately 96% CS-PG, and GuHCl extracts were about 73% CS-PG. From adult rd retinas these values were 18 and 10%, respectively. Per retina, this shows the rd retinas labeling less than 4% of the medium CS-PG, and about 50% of the GuHCl extractable CS-PG compared to normal retinas. Labeled HS-PG comprised about 28% of the normal retina GuHCl extracts, but was not detected in the incubation medium. In contrast, HS-PG synthesis accounted for about 76% of the medium proteoglycan label, and about 85% of the extracted proteoglycan in the adult rd retina. In fact, 35SO4(2-) labeling of HS-PG in the rd retina GuHCl extracts exceeded by 1000% the level observed in normal retina extracts on a per retina basis. Retinas from both strains incorporate significant amounts of 35SO4(2-) into proteins with rd achieving higher specific activity. IRBP was identified as a 35SO4(2-) labeled protein by immunoadsorption from aliquots of the incubation medium.(ABSTRACT TRUNCATED AT 400 WORDS)

Basic fibroblast growth factor (bFGF) immunolocalization in the rodent outer retina demonstrated with an anti-rodent bFGF antibody

The distribution of bFGF in mouse and rat retinas was established using an antibody generated against a rat specific bFGF sequence. The patterns established with this antibody were distinctly different in comparison to the immunostaining patterns using antibodies prepared against similar regions of bovine and human bFGF sequences. Using the anti-rat antibody, at postpartum day 5, both species demonstrate bFGF-like labeling in the outer retina, primarily associated with developing photoreceptors. The anti-rat bFGF antibody in the adult mouse produces intense labeling at the level of the photoreceptor outer segment, whereas in the adult rat, Müller's cells and RPE were intensely labeled but the photoreceptors appeared to be unlabeled. In contrast, the bovine and human antibodies consistently label ganglion cells in both mouse and rat retinas, cells in the inner aspect of the inner nuclear layer and developing horizontal cells, whereas no photoreceptor labeling was observed. These results suggest that minor species differences in a short segment of the bFGF molecule used to generate these antibodies may result in major differences in apparent bFGF-like immunolocalization.

Interphotoreceptor matrix in the human retina: cone-like domains surround a small population of rod photoreceptors

The interphotoreceptor matrix (IPM) in mammalian retinas is subdivided into rod and cone specific compartments: peanut agglutinin (PNA) binding glycoconjugates are associated with cones, whereas wheat germ agglutinin (WGA) binding glycoconjugates are associated with rods. To establish the identity of a photoreceptor cell type in the human retina with rod dimensions but with a matrix domain which stains with PNA, double label studies, using PNA-ferritin to decorate the extracellular domains and immunocytochemical techniques using a rod specific anti-opsin antibody were conducted. The PNA-binding domains were observed in the cone-associated IPM as well as in the IPM surrounding a small population of rod-shaped photoreceptors. The outer segments of these rod-shaped photoreceptors showed intense labeling with a rod specific anti-opsin antibody as did all other rods which were free of PNA-labeling. A quantitative analysis of all retinal quadrants indicates that this novel rod represents approximately 0.3% of the total rod population in the human retina.

Proteoglycans in the mouse interphotoreceptor matrix. V. Distribution at the apical surface of the pigment epithelium before and after retinal separation

The distribution of chondroitin sulfate proteoglycans in the mouse interphotoreceptor matrix (IPM) proximal to the retinal pigment epithelium (RPE) was evaluated with EM histochemical techniques using Cupromeronic Blue (CmB) before and after retinal separation. Densely packed, sheet-like processes surrounding vesicle-like compartments containing CmB staining were normal constituents of the IPM at the apical surface of the RPE. Most of the vesicle-like compartments, which appeared to be isolated from the IPM in single section profiles, were found to be continuous with the IPM when three-dimensional reconstructions of serial thin sections were evaluated. Analyses of stereo image pairs of semithin sections visualized by electron spectroscopic imaging (ESI) also demonstrated that the CmB stained proteoglycans in the lumen of these pseudovesicles were in continuity with the CmB stained components present in the IPM. Moreover, ESI demonstrated that the CmB stained profiles formed an elaborate interconnecting network extending from the apical border of the RPE to the level of the outer limiting membrane of the retina. After removal of the retina, rinsing of the RPE with Ringer's solution prior to fixation eliminated proteoglycan staining near the base of the apical processes, whereas staining near the tips of these processes remained. The CmB stained filaments remaining after rinsing were thicker and shorter, and made fewer interconnections than those in the non-separated preparations. These results suggest that two types of chondroitin sulfate proteoglycans are present in the IPM which differ in distribution and in the degree of aqueous solubility. Additionally, a closely associated retina appears to be required for maintenance of the normal structure of proteoglycans associated with the RPE surface. The elaborate topography at the RPE apical surface may play a role in the delivery and/or recovery of components of the IPM.

Photoreceptor degeneration induced by the expression of simian virus 40 large tumor antigen in the retina of transgenic mice

Expression of the viral oncogene encoding the simian virus 40 (SV40) large tumor antigen (T antigen) typically promotes tumorigenesis in mammalian cells. To generate transgenic mice that express T antigen in rod photoreceptors, a chimeric construct consisting of a mouse opsin promoter fragment fused to the coding region of SV40 T antigen was generated. Expression of T antigen in the transgenic retina began at early stages of postnatal development concomitant with expression of endogenous opsin. Instead of inducing hyperplasia or tumor formation, T-antigen expression caused a rapidly progressing photoreceptor degeneration. The degeneration was accompanied by sustained DNA synthesis in photoreceptor cells, as evidenced by incorporation of [3H]thymidine and by the appearance of mitotic figures at postnatal day 10, a stage when nontransgenic photoreceptor cells are postmitotic and quiescent. Although transgenic photoreceptor cells undergo S phase and enter mitosis, the consequences of T-antigen expression are not proliferation and tumorigenesis but proliferation and cell death.

Aging of the human retina. Differential loss of neurons and retinal pigment epithelial cells

The impact of aging on cell loss in the human retina was examined in foveal and temporal equatorial regions in eyes from 35 donors with ages spanning a 78-yr period from the second to the ninth decade of life. Equatorial cones and retinal pigment epithelial cells (RPE) decreased at uniform rates from the second to the ninth decade, 16 and 14 cells/mm2/yr, respectively. Equatorial rods and cells in the ganglion cell layer (GCL) showed nonuniform rate decreases with age. The rates of rod and GCL cell loss were faster between the second and fourth decades (970 and 9 cells/mm2/yr, respectively) than between the fourth and ninth decades (570-330 and 6-3 cells/mm2/yr). The rod and GCL cell densities at the temporal equator maintained a constant ratio (rods-GCL cell ratio = 103 +/- 0.4, mean +/- standard deviation) and the same reduction slope ratio at different times during aging. Thus, the equatorial rod and GCL cell losses were correlated statistically. The ratio of equatorial photoreceptors to RPE cells showed no significant change with age, suggesting parallel loss of these closely apposed cells. At the foveal center, the variability of cone density between individuals in each decade grouping was large (1.7- to threefold). No significant differences were found in cone or RPE cell densities at the foveal center from the second to ninth decade, suggesting that the densities of foveal cones and RPE cells were stable throughout this period. Foveal RPE density was significantly higher than equatorial RPE density in each age group. No significant difference was found between the equatorial photoreceptor-RPE ratio and foveal cone-RPE ratio in any age group. Cells in the GCL in the fovea decreased by approximately 16% from the second to the sixth decade. These results indicated that (1) rod photoreceptors and cells in the GCL were more vulnerable to loss during aging than cones; (2) photoreceptors and RPE cells showed parallel changes during aging; and (3) the photoreceptor loss accompanying aging was less pronounced in the fovea than in the peripheral retina.

Idiopathic retinal degeneration in the dog: differential patterns of [3H]uridine incorporation and HIOMT-like immunoreactivity in surviving photoreceptors

Photoreceptor cell pathology was investigated in an 8-yr-old mixed-breed dog which had displayed visual symptoms of 1 month duration. An electroretinogram detected no light-evoked responses. Light and electron microscopic features showed marked thinning and atrophy of the outer both the tapetal and non-tapetal retina appeared to be involved. In the non-tapetal region, a majority of the rod inner segments were missing, while scattered mitochondria-filled stubby inner segments of cones were readily identified. Inner segments of both rods and cones were observed in the tapetal region. Photoreceptor outer segments were completely absent from the affected retina, and no outer segment debris was observed between the photoreceptor layer and the retinal pigmented epithelium (RPE). Autoradiographic analysis of 3-mm retinal disks from the degenerate retina following incubation with [3H]uridine indicated that only 61% +/- 13 S.D. of the remaining nuclei of rod photoreceptors were undergoing RNA synthesis, whereas more than 99% of cone nuclei incorporated the label. Normal and degenerate retina were also analysed for localization of hydroxyindole-O-methyltransferase (HIOMT)-like immunoreactivity. While the normal retina showed immunoreactivity in both rod and cone photoreceptors with more intense immunoreactivity present in cones, the degenerate retina showed HIOMT-like immunoreactivity only in the remaining cone photoreceptors. The results of this study of idiopathic photoreceptor degeneration of the canine retina suggest that although both photoreceptor types are involved, rods are more severely affected than cones.

Proteoglycans in the mouse interphotoreceptor matrix. IV. Retinal synthesis of chondroitin sulfate proteoglycan

To determine whether the mouse retina contributes chondroitin sulfate proteoglycan (CS-PG) to the interphotoreceptor matrix (IPM), 35SO4(2-) was used as a tracer for newly synthesized proteoglycan by retinas maintained in vitro in the absence of pigment epithelium. Following incubation with the tracer for 3 hr, the 35S-labeled proteoglycans present in the incubation medium and associated with isolated photoreceptor outer segments were analyzed separately. Proteoglycan was extracted with 4 M guanidine, and then separated on a G-25 column followed by DEAE ion exchange chromatography in the presence of 8 M urea. The proteoglycan fraction was eluted with a linear NaCl gradient of 0.15-1.0 M. Eluted 35S-labeled macromolecules were susceptible to chondroitinase AC and ABC degradation, indicating that virtually all the 35S-labeled proteoglycan synthesized by the mouse retina and secreted into the incubation media is of the chondroitin sulfate type. In parallel autoradiographic analysis of retinas following 35SO4(2-) incubation, silver grains were present over all retinal compartments, with 41-48% associated with the photoreceptor layer. Quantitative autoradiography of retinas following chondroitinase AC digestion of fixed retinas revealed significant (P less than 0.025) reduction in silver grains associated with the photoreceptor outer segment layer as compared to controls. These combined biochemical and autoradiographic studies indicate that the retina, possibly the photoreceptors, synthesize at least a portion of the CS-PG present in the IPM of the mouse.

Sulfated proteoglycans in Bruch's membrane of the human eye: localization and characterization using cupromeronic blue

The distribution of sulfated proteoglycans in Bruch's membrane of the human eye was evaluated histochemically using Cupromeronic Blue in combination with specific enzyme digestions and nitrous acid treatment. Five distinct categories of filament-shaped profiles were present following staining with this dye. Type 1 (90 +/- 13 nm long and 7 +/- 1 nm in diameter) (mean +/- S.D.) and type 2 (43 +/- 7 nm long and 5 +/- 1 nm in diameter) filaments were associated with collagen fibrils in the inner and outer collagenous zones. Type 3 profiles (70 +/- 18 nm long and 8 +/- 1 nm in diameter) were present in two locations--along the cortical border of the central elastic zone and within the basal infoldings of the pigment epithelium. Type 4 (60 +/- 11 nm long and 6 +/- 1 nm in diameter) and type 5 (200 +/- 100 nm long and 100 +/- 50 nm in diameter) filaments were associated with the basal laminae of the retinal pigment epithelium and choriocapillaris. Chondroitinase AC treatment eliminated the staining of type 1 filaments. Chondroitinase ABC treatment eliminated the staining of both type 1 and type 2 filaments. Nitrous acid eliminated the staining of type 4 and type 5 filaments. Incubations with keratanase or hyaluronidase did not alter the staining of any filament type. Type 3 filaments were resistant to all enzyme digestions and nitrous acid treatment. These results are consistent with an interpretation that Bruch's membrane contains chondroitin sulfate, dermatan sulfate and heparan sulfate-type proteoglycans. Proteoglycans containing chondroitin sulfate (type 1) and dermatan sulfate (type 2) are associated uniquely with collagen fibrils. Heparan sulfate type proteoglycans (types 4 and 5) are associated with the basal lamina of the pigment epithelium and choriocapillaris. The identity of type 3 profiles, which were resistant to all enzyme and nitrous acid digestions employed, could not be established at this time.

Proteoglycans in the mouse interphotoreceptor matrix. III. Changes during photoreceptor development and degeneration in the rds mutant

The distribution of sulfated proteoglycans in the interphotoreceptor matrix (IPM) was examined during development and degeneration of photoreceptors in the rds mouse with electron microscopy after staining with the cationic dye Cupromeronic Blue (CmB). Three distinct CmB-positive filaments types were observed: type A (45-55 nm long and around 5 nm in diameter), type B (up to 0.5 micron long and 5-10 nm in diameter), and type C filaments (up to 1 micron long and 15-25 nm in diameter. During early postnatal development, before degenerative changes occur in photoreceptors, CmB-positive filaments were virtually identical in morphology and pattern of development as those recently reported for the normal mouse IPM (Tawara, Varner and Hollyfield, 1989, Exp. Eye Res. 48, 815-39). From 10 days to 1 year of age, during the period of progressive degeneration and loss of photoreceptor cells, numerous type B and type C filaments were present in the IPM. Type B filaments were distributed throughout the IPM, whereas type C were predominantly located around the apical termination of photoreceptor inner segments and between the pigment epithelial microvilli. Type A filaments were located principally in the apical cytoplasm of the pigment epithelial cells and in the proximal IPM. In the 20-month-old rds mouse, a time when virtually no photoreceptor cells remain, only minimal CmB staining was evident at the interface between the pigment epithelium and retina. Pretreatment with chondroitinase AC eliminated most CmB-positive filaments from the 18-day-old and 20-month-old mouse IPM. These findings suggest that there are no major differences in structural type or early postnatal development of chondroitin sulfate-type proteoglycans in the IPM between rds and normal mice. Any differences in distribution of chondroitin sulfate-type proteoglycans between rds and normal mice can be accounted for by the absence of photoreceptor outer segments and progressive loss of photoreceptor cells in this mutant. The disappearance of these IPM proteoglycans following photoreceptor degeneration suggests that photoreceptors may be critically involved in the maintenance of these matrix components.

Regional variation within the interphotoreceptor matrix from fovea to the retinal periphery

The interphotoreceptor matrix in the human retina comprises a highly organised extracellular compartment. Using fluorescent labelled peanut agglutinin and the cationic dyes Cupromeronic and Cuprolinic Blue, unique cylindrical domains surrounding cone photoreceptors can be demonstrated. These cone specific domains are relatively insoluble and are closely adherent to cone photoreceptors and to the pigment epithelium, suggesting that these structures may play a role in retinal attachment.

HIOMT-like immunoreactivity in the vertebrate retina: a species comparison

Localization of the melatonin-synthesizing enzyme, hydroxyindole-O-methyltransferase (HIOMT)-like immunoreactivity was examined by light and electron microscopic immunocytochemistry, in the retinas of several species that have been used as animal models to study the retinal melatonin system. HIOMT-like immunoreactivity was observed in the retinal photoreceptors of rabbit, pigmented rat, guinea-pig, chick, goldfish. African clawed toad, and leopard frog. Additionally, most species displayed HIOMT immunoreactivity in a population of bipolar cells in the inner nuclear layer. At the ultrastructural level, HIOMT-like immunoreactivity was localized to the cytoplasm of rod and cone photoreceptors, and a population of cone bipolar cells. These observations are identical to earlier observations in the human retina. The similar pattern of HIOMT-like immunoreactivity among species suggests a phylogenetic conservation of the melatonin-synthesizing capability of retinal photoreceptors and some bipolar cells.

Spontaneous transfer of retinoic acid, retinyl acetate, and retinyl palmitate between single unilamellar vesicles

The transfer of retinoic acid, retinyl acetate, and retinyl palmitate between single unilamellar vesicles was studied by resonance energy transfer. The retinoic acid transfers spontaneously between single unilamellar vesicles with a first order rate constant of 9.6 s-1 at 15 degrees C and pH 7.4. At 30 degrees C, the transfer rate was 3.5 times faster than that at 10 degrees C. At pH 7.4, the transfer rate was almost 2 orders of magnitude faster than that observed at pH 1.6. Increasing the concentration of NaCl decreased the retinoic acid transfer rate significantly. Retinyl acetate transfers with a rate constant of 0.15 s-1, but no spontaneous transfer of retinyl palmitate was observed over 60 min. The evidence supports the proposal that retinoic acid and retinyl acetate transfer between single unilamellar vesicles occur via the aqueous phase. In contrast, no spontaneous transfer of retinyl palmitate was observed. However, linear free energy relationships and the thermodynamic parameters for retinyl acetate transfer permit the calculation of rate constant for retinyl palmitate transfer.

Distribution and characterization of sulfated proteoglycans in the human trabecular tissue

The distribution of proteoglycans in the trabecular tissue of human eyes was evaluated histochemically, using Cupromeronic Blue in combination with a series of enzyme digestions and nitrous acid treatment. Within the extracellular matrix of the trabecular meshwork, many Cupromeronic Blue-positive filaments were observed in association with collagen fibrils, basal lamina, a basal lamina-like material and a fine fibrillar-like material. Pretreatment with chondroitinase AC reduced the staining associated with the collagen fibrils, whereas filament staining in this location was completely eliminated by pretreatment with chondroitinase ABC. Nitrous acid treatment eliminated almost all the filament staining associated with the basal lamina and basal lamina-like material. When the tissue was treated with chondroitinase AC, chondroitinase ABC or nitrous acid, filament staining associated with the fine fibrillar-like material was reduced. After a combined treatment with nitrous acid followed by incubation with chondroitinase ABC, all filament staining in the trabecular meshwork was eliminated. These results are consistent with an interpretation that human trabecular tissue contains three distinct types of sulfated proteoglycans, namely chondroitin sulfate, dermatan sulfate and heparan sulfate-type proteoglycans. Proteoglycans containing chondroitin sulfate and dermatan sulfate are present in association with collagen fibrils. Proteoglycans of heparan sulfate-type are associated with the basal lamina and the basal lamina-like material. Chondroitin sulfate, dermatan sulfate and heparan sulfate-type proteoglycans are present in association with the fine fibrillar-like component. The basal lamina, basal lamina-like material and fine fibrillar-like components associated with these negatively charged proteoglycans may be important contributors to aqueous outflow resistance in the juxtacanalicular connective tissue.

Proteoglycans in the mouse interphotoreceptor matrix. II. Origin and development of proteoglycans

The development and cellular origin of chondroitin sulfate-type proteoglycans in the interphotoreceptor matrix (IPM) of the mouse retina were examined with electron microscopy after staining with the cationic dye, Cupromeronic Blue. In the IPM of the developing retina, three structural types of Cupromeronic Blue-positive filaments were observed. Type A filaments measured 45-55 nm in length and around 5 nm in diameter. At birth, a few type A filaments were present in the IPM and they increased in number during subsequent days postpartum. Type A filaments were present in high density in the cytoplasm of the pigment epithelial cells at all stages examined where they were associated with the Golgi apparatus and cytoplasmic vesicles. Type B filaments, measuring 60-80 nm in length and 5-10 nm in diameter, were occasionally observed in the IPM of the newborn mouse. Type B filaments increased in number during subsequent development and by 14 days postpartum, their location was restricted predominantly to the IPM around photoreceptor inner segments and the connecting cilia. Type C filaments, measuring 15-25 nm diameter with extremely long profiles (up to several microns), were first consistently observed in the IPM at 6 days postpartum, the time when photoreceptor outer segments begin to develop. During subsequent days postpartum, type C filaments increased in number. By 18 days postpartum, when photoreceptors have achieved their adult length, type C filaments were interconnected in a net-like meshwork investing the photoreceptor outer segments. All three Cupromeronic Blue-positive filament types were sensitive to chondroitinase AC, but not to Streptomyces hyaluronidase. Cupromeronic Blue-stained filaments were rarely observed in the cytoplasm of the photoreceptor or Müller cells at any of the stages examined. These findings suggest that proteoglycans of the chondroitin sulfate-type present between the retina and pigment epithelium of the adult mouse are secreted principally by the retinal pigment epithelial cells, and that these components begin to be elaborated into the IPM a few days prior to the stage when photoreceptor outer segment development is initiated. Organization of the proteoglycans to form an extracellular meshwork surrounding the photoreceptors takes place during the period when photoreceptor outer segments are elongating to achieve their adult length.

Serotoninergic neurons in the retina of Xenopus laevis: selective staining, identification, development, and content

Uptake of 3H-serotonin followed by autoradiography, and uptake of the serotonin analog 5,7-dihydroxytryptamine (5,7-DHT), with subsequent staining, were each used to define a unique set of neurons in the retina of the African clawed frog, Xenopus laevis. Both techniques demonstrated the same population of neurons, on the basis of perikaryal size, shape, and position within the retina. Two classes of amacrine cells accumulated 5,7-DHT at the proximal (vitread) margin of the inner nuclear layer; the two classes were distinguished by the size of their perikarya. Two similar populations of cells, observed in the ganglion cell layer with lower frequency, may represent "displaced" counterparts of these two amacrine cell types. A class of bipolar cells whose perikarya were located in middle-to-distal regions of the inner nuclear layer also accumulated 5,7-DHT and 3H-serotonin. Processes of these cells contributed to a dense plexus of fine fibers that appeared evenly distributed throughout the inner plexiform layer. 3H-Serotonin-accumulating cells first appeared in the developing retina at stage 35/36, a time immediately after retinal stratification but before elaboration of either plexiform layer. Electron microscopic analysis permitted an identification of 3H-serotonin-accumulating terminals in the inner plexiform layer. Serotonin-labeled terminals containing conventional contacts, suggestive of amacrine cells, were presynaptic to unidentified processes and postsynaptic to bipolar cells. Labeled terminals containing ribbon contacts, indicative of bipolar cells, were postsynaptic to amacrine cells. The amount of serotonin contained in isolated retinas was 15 pmol/mg protein as measured by HPLC with electrochemical detection. We attempted to stimulate the release of accumulated 3H-serotonin from mature retinas by increasing the K+-concentration in the bathing medium. Although preloaded glycine is readily released from 14C-glycine-accumulating neurons, from the same retinas there was no calcium-dependent, K+-stimulated release of 3H-serotonin. This finding suggests that serotonin and glycine are processed differently by retinal neurons, the consequence of which results in differing responses to 40 mM K+.

Mechanism of vitamin A movement between rod outer segments, interphotoreceptor retinoid-binding protein, and liposomes

Vitamin A movement between rod outer segment (ROS) membranes, interphotoreceptor retinoid-binding protein (IRBP), and liposomes was examined by two different methods. Equilibrium exchange of all-trans-retinol was followed by assessing the transfer of [3H]retinol from liposomes to ROS membranes as compared to a nontransferable marker, [14C]triolein. In the absence of IRBP, a rapid, spontaneous transfer of [3H] retinol to the ROS membranes occurred. In the presence of 2 microM IRBP, retinol transfer decreased by approximately one-half, whereas a similar concentration of bovine serum albumin had no effect on this spontaneous transfer. Kinetics of retinol transfer between single unilamellar vesicles were determined by the method of fluorescence energy transfer. The first order rate constant for this transfer was 0.85 s-1 at 22 degrees C at either pH 7.4 or pH 2.8. This rate was not affected by varying the concentration of acceptor vesicles 50-fold or by varying their concentration 10-fold at a constant ratio of donor-to-acceptor vesicles. The presence of IRBP as an additional acceptor did not change the rate. The transfer was temperature-dependent with an activation energy of 7.8 kcal/mol. The transfer rate appeared to be an increasing exponential function of ionic strength since high concentrations of NaCl decreased the transfer rate significantly. The transfer rate of retinol from IRBP to single unilamellar vesicles also followed first order kinetics with a rate constant of 0.11 s-1 at 22 degrees C, which was approximately 8 times slower than that of transfer between vesicles. We conclude that the transfer of all-trans-retinol between liposomes and membranes can be accomplished rapidly via the aqueous phase, and that IRBP retards rather than facilitates this transfer process.

Melatonin enhances horizontal cell sensitivity in salamander retina

Intracellular electrophysiological recording techniques were utilized to investigate the possible function of retinal melatonin in the larval tiger salamander. Endogenous retinal melatonin was present and appeared to bind a membrane-enriched fraction of the salamander retina, as determined by radioimmunoassay and receptor binding studies. Melatonin added through the perfusion bath to flat-mounted retinas resulted in a horizontal cell (HC) hyperpolarization of 10-20 mV. Additionally, the amplitude of HC responses to short test flashes increased in the presence of melatonin. Voltage-intensity plots revealed that application of 500 microM of melatonin caused an increase of the HC light sensitivity and this effect was reversible. These results suggest that melatonin synthesized and released during the dark period of the diurnal cycle may alter the sensitivity of second-order neurons at a time of day when photopic input is at its lowest level.

Proteoglycans in the mouse interphotoreceptor matrix. I. Histochemical studies using cuprolinic blue

Cuprolinic Blue, when applied at a critical electrolyte concentration, can be utilized for assessing the localization and structural characteristics of proteoglycans with electron microscopy. We have used this cytochemical procedure to evaluate the distribution of proteoglycan in the interphotoreceptor matrix of the mouse retina. Cuprolinic Blue-positive filaments of two distinct morphological types were present surrounding both rod and cone photoreceptors. Large filaments, 115-135 nm long and 15-25 nm in diameter, were distributed in the interphotoreceptor matrix around the outer segment and outer portion of the inner segment. These filaments appeared linked to each other to form a complex meshwork. Smaller filaments, 60-70 nm long and 5-10 nm in diameter, were principally observed around the photoreceptor inner segments. Incubation of retinas with chondroitinase AC and chondroitinase ABC eliminated Cuprolinic Blue staining of both large and small filaments, whereas hyaluronidase treatment reduced the size of the filaments but did not eliminate their staining. When retinas were washed extensively prior to fixation and staining, Cuprolinic Blue-positive filaments remained associated with the photoreceptor cell surface. These results suggest that the interphotoreceptor matrix of the mouse retina contains at least two structural types of proteoglycan, of the chondroitin sulfate-type, which are differentially distributed in this compartment. One of the proteoglycans forms a complex meshwork which surrounds the photoreceptors. Both are insoluble and appear to be firmly attached to the photoreceptor plasma membrane.