Chondroitin 6-sulfate glycosaminoglycan is a major constituent of primate cone photoreceptor matrix sheaths

Loss of Chondroitin Sulfate Modification Causes Inflammation and Neurodegeneration in skt Mice

One major aspect of the aging process is the onset of chronic, low-grade inflammation that is highly associated with age-related diseases. The molecular mechanisms that regulate these processes have not been fully elucidated. We have identified a spontaneous mutant mouse line, small with kinky tail (skt), that exhibits accelerated aging and age-related disease phenotypes including increased inflammation in the brain and retina, enhanced age-dependent retinal abnormalities including photoreceptor cell degeneration, neurodegeneration in the hippocampus, and reduced lifespan. By positional cloning, we identified a deletion in chondroitin sulfate synthase 1 (Chsy1) that is responsible for these phenotypes in skt mice. CHSY1 is a member of the chondroitin N-acetylgalactosaminyltransferase family that plays critical roles in the biosynthesis of chondroitin sulfate, a glycosaminoglycan (GAG) that is attached to the core protein to form the chondroitin sulfate proteoglycan (CSPG). Consistent with this function, the Chsy1 mutation dramatically decreases chondroitin sulfate GAGs in the retina and hippocampus. In addition, macrophage and neutrophil populations appear significantly altered in the bone marrow and spleen of skt mice, suggesting an important role for CHSY1 in the functioning of these immune cell types. Thus, our study reveals a previously unidentified impact of CHSY1 in the retina and hippocampus. Specifically, chondroitin sulfate (CS) modification of proteins by CHSY1 appears critical for proper regulation of immune cells of the myeloid lineage and for maintaining the integrity of neuronal tissues, since a defect in this gene results in increased inflammation and abnormal phenotypes associated with age-related diseases.

Pericellular interphotoreceptor matrix dictates outer retina critical surface tension

Retinal detachments create two pathological surfaces, the surface of the outer neural retinal, and an apical retinal-pigmented epithelium (RPE) surface. The physicochemical properties of these two new surfaces are poorly understood. At a molecular level little is known how detachments form, how to optimize reattachment, or prevent extension of the detachment. A major limitation is lack of information about the biophysical consequences of the retina-RPE separation. The primary challenge is determining the molecular properties of the pathological interface surfaces. Here, using detached bovine retina, we show that this hurdle can be overcome through a combination of biophysical and ultrastructural approaches. The outer surface of freshly detached bovine neural retina, and isolated molecular components of the outer retina were subjected to: 1) Contact angle goniometry to determine the critical surface tension of the outer retinal surface, isolated insoluble interphotoreceptor matrix (IPM) and purified interphotoreceptor retinoid binding protein (IRBP); 2) Multiple attenuated internal reflectance infrared (MAIR-IR) spectroscopy was used to characterize the molecular composition of the retinal surface. MAIR-IR depth penetration was established through ellipsometric measurement of barium-stearate films. Light microscopy, immunohistochemistry and electron microscopy defined the structures probed spectroscopically. Furthermore, the data were correlated to IR spectra of docosahexaenoic acid, hyaluronan, chondroitin-6-sulfate and IRBP, and imaging by IR-microscopy. We found that the retinal critical surface tension is 24 mN/m, similar to isolated insoluble IPM and lower than IRBP. Barium-stearate calibration studies established that the MAIR-IR spectroscopy penetration depth was 0.2 μm. Ultrastructural observations and MAIR-IR studies of isolated outer retina components determined that the pericellular IPM coating the outer retinal surface is primarily responsible for these surface properties. The critical surface tension of detached bovine retina is dictated not by the outer segments, but by a pericellular IPM covering the outer segment tips.

Cone outer segment extracellular matrix as binding domain for interphotoreceptor retinoid-binding protein

Cones are critically dependent on interphotoreceptor retinoid-binding protein (IRBP) for retinoid delivery in the visual cycle. Cone-dominant vertebrates offer an opportunity to uncover the molecular basis of IRBP's role in this process. Here, we explore the association of IRBP with the interphotoreceptor matrix (IPM) of cones vs. rods in cone dominant retinas from chicken (Gallus domesticus), turkey (Meleagris gallopavo), and pig (Sus scrofa). Retinas were detached and fixed directly or washed in saline prior to fixation. Disassociated photoreceptors with adherent matrix were also prepared. Under 2 mM CaCl(2) , insoluble matrix was delaminated from saline washed retinas. The distribution of IRBP, as well as glycans binding peanut agglutinin (cone matrix) and wheat germ agglutinin (rod/cone matrix), was defined by confocal microscopy. Retina flat mounts showed IRBP diffusely distributed in an interconnecting, lattice-like pattern throughout the entire matrix. Saline wash replaced this pattern with fluorescent annuli surrounding individual cone outer segments. In isolated cones and matrix sheets, IRBP colocalized with the peanut agglutinin binding matrix glycans. Our results reveal a wash-resistant association of IRBP with a matrix domain immediately surrounding cone outer segments. The cone matrix sheath may be responsible for IRBP-mediated cone targeting of 11-cis retinoids.

Can ultrasound solve the transport barrier of the neural retina?

PURPOSE

Intravitreal injection of nonviral gene complexes may be promising in the treatment of retinal diseases. This study investigates the permeation of lipoplexes and polystyrene nanospheres through the neural retina and their uptake by the retinal pigment epithelium (RPE) either with or without ultrasound application.

MATERIALS AND METHODS

Anterior parts and vitreous of bovine eyes were removed. The neural retina was left intact or peeled away from the RPE. (Non)pegylated lipoplexes and pegylated nanospheres were applied. After 2 h incubation, the RPE cells were detached and analyzed for particle uptake by flow cytometry and confocal microscopy.

RESULTS

The neural retina is a significant transport barrier for pegylated nanospheres and (non)pegylated lipoplexes. Applying ultrasound improved the permeation of the nanoparticles up to 130 nm.

CONCLUSIONS

Delivery of liposomal DNA complexes to the RPE cells is strongly limited by the neural retina. Ultrasound energy may be a useful tool to improve the neural retina permeability, given the nucleic acid carriers are small enough. Our results underline the importance to design and develop very small carriers for the delivery of nucleic acids to the neural retina and the RPE after intravitreal injection.

Embryonic stem cells and retinal repair

In this review we examine the potential of embryonic stem cells (ESCs) for use in the treatment of retinal diseases involving photoreceptors and retinal pigment epithelium (RPE). We outline the ontogenesis of target retinal cell types (RPE, rods and cones) and discuss how an understanding of developmental processes can inform our manipulation of ESCs in vitro. Due to their potential for cellular therapy, special emphasis is placed upon the derivation and culture of human embryonic stem cells (HESCs) and their differentiation towards a retinal phenotype. In terms of achieving this goal, we suggest that much of the success to date reflects permissive in vitro environments provided by established protocols for HESC derivation, propagation and neural differentiation. In addition, we summarise key factors that may be important for enhancing efficiency of retinal cell-type derivation from HESCs. The retina is an amenable component of the central nervous system (CNS) and as such, diseases of this structure provide a realistic target for the application of HESC-derived cellular therapy to the CNS. In order to further this goal, the second component of our review focuses on the cellular and molecular cues within retinal environments that may influence the survival and behaviour of transplanted cells. Our analysis considers both the potential barriers to transplant integration in the retina itself together with the remodelling in host visual centres that is known to accompany retinal dystrophy.

Purinergic signalling in the subretinal space: a role in the communication between the retina and the RPE

The retinal pigment epithelium (RPE) is separated from the photoreceptor outer segments by the subretinal space. While the actual volume of this space is minimal, the communication that occurs across this microenvironment is important to the visual process, and accumulating evidence suggests the purines ATP and adenosine contribute to this communication. P1 and P2 receptors are localized to membranes on both the photoreceptor outer segments and on the apical membrane of the RPE which border subretinal space. ATP is released across the apical membrane of the RPE into this space in response to various triggers including glutamate and chemical ischemia. This ATP is dephosphorylated into adenosine by a series of ectoenzymes on the RPE apical membrane. Regulation of release and ectoenzyme activity in response to light-sensitive signals can alter the balance of purines in subretinal space, and thus coordinate communication across subretinal space with the visual process.

Differential binding to glycotopes among the layers of three mammalian retinal neurons by man-containing N-linked glycan, T(alpha) (Galbeta1-3GalNAcalpha1-), Tn (GalNAcalpha1-Ser/Thr) and I (beta)/II (beta) (Galbeta1-3/4GlcNAcbeta-) reactive lectins

Carbohydrate structures between retinal neurons and retinal pigment epithelium (RPE) play an important role in maintaining the integrity of retinal adhesion to underlying RPE, and in retinal detachment pathogenesis. Since relevant knowledge is still in the primary stage, glycotopes on the adult retina of mongrel canines (dog), micropigs and Sprague-Dawley rats were examined by lectino-histochemistry, using a panel of 16 different lectins. Paraffin sections of eyes were stained with biotinylated lectins, and visualized by streptavidin-peroxidase and diaminobenzidine staining. Mapping the affinity profiles, it is concluded that: (i) all sections of the retina reacted well with Morniga M, suggesting that N-linked glycans are present in all layers of the retina; (ii) no detectable human blood group ABH active glycotopes were found among retinal layers; (iii) outer and inner segments contained glycoconjugates rich in ligands reacting with T (alpha) (Galbeta1-3GalNAcalpha1-Ser/Thr) and Tn (GalNAcalpha1-Ser/Thr) specific lectins; (iv) cone cells of retina specifically bound peanut agglutinin (PNA), which recognizes T (alpha) residues and could be used as a specific marker for these photoreceptors; (v) the retinas of rat, dog and pig, had a similar binding profile but with different intensity; (vi) each retinal layer had its own binding characteristic. This information may provide useful background knowledge for normal retinal physiology and miscellaneous retinal diseases, including retinal detachment (RD) and age-related macular degeneration (ARMD).

Neural retina limits the nonviral gene transfer to retinal pigment epithelium in an in vitro bovine eye model

We investigated the permeation of liposomal and polymeric gene delivery systems through neural retina into retinal pigment epithelium (RPE) and determined the roles of various factors in permeation and subsequent uptake of the delivery systems by RPE. Anterior parts and vitreous of fresh bovine eyes were removed. Retina was left intact or peeled away. Complexes of ethidium monoazide (EMA)-labeled plasmid DNA and cationic carriers (polyethyleneimine, poly-L-lysine, DOTAP liposomes) were pipetted on the retina or RPE. Two hours later the neural retina was removed, if present, and the RPE cells were detached. Contaminants were removed by sucrose centrifugation, and the RPE cells were analyzed for DNA uptake by flow cytometry. Cellular uptake of FITC-dextrans (molecular weight [mw] 20,000, 500,000 and 2,000,000), FITC-poly-L-lysine (mw 20,000), FITC-labeled oligonucleotide (15-mer), and naked EMA-labeled plasmid DNA was determined after pipetting the solutions on the RPE or neural retina. Location of the fluorescent materials in the retina was visualized with fluorescence microscopy. Neural retina decreased the cellular uptake of DNA complexes by an order of magnitude, the uptake of FITC-dextrans slightly, whereas delivery of polycationic FITC-poly-L-lysine to RPE was almost completely inhibited. Neural retina decreased the cellular uptake of FITC-oligonucleotides, while the uptake of uncomplexed plasmid was always negligible. Conclusions from FACS and fluorescence microscopy were similar: delivery of polymeric and liposomal DNA complexes into RPE are limited by the neural retina. This is due to the size and positive charge of the complexes.

Major glycosaminoglycan species in the developing retina: synthesis, tissue distribution and effects upon cell death

Retinal explants maintained in culture medium retain their histotypic structure and develop similarly to the in vivo condition. Extracellular matrix components, particularly the glycosaminoglycans which are not routinely present in dissociated cell cultures are involved in various cellular events. In this work we characterized and determined the localization of sulfated glycosaminoglycans in the extracellular matrix of rat retinal explants at various stages of normal postnatal development and tested whether disruption of the tissue glycosaminoglycan composition may impose either trophic or toxic effects upon distinct retinal cell populations. Our data show that chondroitin sulfate and heparan sulfate glycosaminoglycan chains are synthesized in different proportions during postnatal retinal development. A peak of synthesis of chondroitin sulfates is evident at around P14. Immunohistochemistry showed chondroitin 6-sulfate in the plexiform layers during the earlier stages while later, intense immunoreactivity was found in the outer retina. Heparan sulfate was found in the neuroblastic layer (NBL) at P1, in both nuclear layers from P5 onwards and in the ganglion cell layer (GCL) at all stages. In contrast to chondroitin 6-sulfate, immunoreactivity to heparan sulfate was absent from the outer retina at both P14 and P21. Treatment with heparitinase modulated the rates of cell death in both the GCL and the NBL in P1 retinal explants. Taken together our data show that among the major sulfated glycosaminoglycans, the developing rat retina synthesizes only heparan sulfate and chondroitin sulfates in a spatiotemporally regulated manner, with a peak of chondroitin sulfates at P14, possibly related to photoreceptor differentiation. In addition, our data suggest a role for heparan sulfate as a modulator of sensitivity to cell death in the retina.

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.

Expression and characterization of the IPM 150 gene (IMPG1) product, a novel human photoreceptor cell-associated chondroitin-sulfate proteoglycan

The interphotoreceptor matrix (IPM) occupies the extracellular space between the apical surface of the retinal pigmented epithelium and the external limiting membrane of the neural retina. This space contains two chondroitin sulfate proteoglycans, designated IPM 150 and IPM 200, which are likely to effect retinal adhesion and photoreceptor survival. In an effort to characterize human IPM 150, several cDNA clones encoding its core protein have been isolated from a human retinal cDNA library. Translation of overlapping cDNA sequences yields a novel core protein with a predicted molecular mass of 89.3 kDa. Northern and dot-blot analyses as well as the isolation of expressed sequence tags demonstrate that IPM 150 mRNA is expressed not only in the neural retina but also in several other non-ocular tissues. In situ hybridization analyses indicate that, in the eye, IPM 150 mRNA is expressed specifically by cone and rod photoreceptor cells. Characterization of IPM 150 proteoglycan core protein and identification of its site of synthesis are important steps towards understanding the architecture and biology of the IPM.

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.

Molecular characterization and genomic mapping of human IPM 200, a second member of a novel family of proteoglycans

We report herein the characterization of the cDNA for a novel human chondroitin sulfate proteoglycan, designated IPM 200, and the chromosomal location of its gene, designated IMPG2. IPM 200 was isolated from the retinal interphotoreceptor matrix, a unique extracellular matrix that occupies the subretinal space between the apices of the retinal pigment epithelium and the neural retina. The cDNA contains an open reading frame of 3,726 bp that codes for a core protein with a deduced molecular weight of 138.5 kDa. The deduced IPM 200 core protein contains a putative transmembrane domain, two EGF-like repeats, numerous N- and O-linked glycosylation consensus sequences and one consensus sequence for glycosaminoglycan attachment. IMPG2 maps to human chromosome 3q12.2-12.3. Based on homologies within their amino acid sequences we propose that IPM 200 and a previously described human proteoglycan, IPM 150, form a new family of extracellular matrix glycoconjugates.

Neurotrophic growth factors stimulate glycosaminoglycan synthesis in identified retinal cell populations in vitro

Glycosaminoglycans (GAG) are known to participate in central nervous system processes such as development, cell migration, and neurite outgrowth, but little is known with respect to their regulation through soluble neurotrophic factors. In the present study, we have addressed this issue using cell culture models of three distinct cell populations derived from young rat retinas, namely, purified M uller glia, pigmented epithelium, and neurons respectively. Cultures were maintained in chemically defined media in the presence or absence of either basic fibroblast or epidermal growth factor. In control glial and epithelial cultures, hyaluronic acid dominated the soluble GAG pool, with lesser contributions from dermatan sulfate, chondroitin sulfate, and heparan sulfate (in decreasing order). Retinal neuronal GAG were almost exclusively chondroitin sulfate (approximately 90%). Treatment of glial and epithelial cultures with either factor led to dose-dependent increases in especially hyaluronic acid synthesis (a maximum 6-fold increase relative to control levels), with smaller but consistent changes in chondroitin sulfate. Similar treatment of retinal neurons did not lead to any changes in GAG synthesis. These data indicate that glia and pigment epithelia are the principal sources of GAG components in retina at least in vitro, and that endogenous neurotrophic growth factors can greatly modify GAG synthesis in these two retinal cell populations. Such data suggest that a delicate balance may exist between growth factor availability and glycoconjugate metabolism in vivo, participating in normal or pathological states of the retina.

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.

Immunolocalization of CD44 in the dystrophic rat retina

The distribution of the cell surface adhesion/receptor molecule CD44 was studied in retinas of the Royal College of Surgeons (RCS) rat which exhibits an inherited retinal dystrophy. In this animal model, the retinal pigment epithelium fails to phagocytize shed photoreceptor outer segment material, a membranous debris layer accumulates in the subretinal space and the photoreceptor cells degenerate. Using immunoperoxidase and immunogold labeling, CD44 was localized to Müller cell apical microvilli in normal rat retinas, as noted in other species. For the RCS rat, immunoperoxidase labeling of 18 day and 1 month retinas showed the typical microvillar labeling pattern. At 2 months postnatal, following degeneration of most of the photoreceptors, a more condensed band of microvillar label was observed. At 3 months, when photoreceptor degeneration was virtually complete, only distinct regions of dense label remained between the neural retina and debris zone. Upon ultrastructural and immunogold analysis, these regions were found to contain closely packed Müller cell microvilli. At all ages studied, labeling for CD44 in the inner retina did not increase, as it does in other forms of retinal degeneration which lack a debris zone. However, by 3 months the debris zone was labeled for CD44 indicating that CD44 molecules remain on Müller cell microvilli and processes which have extended into and become part of the debris zone. This may be caused by an altered distribution of still undetermined ligands for CD44 which are present within the interphotoreceptor matrix of the RCS rat retina.

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.

Alternative splicing of the unique "PLUS" domain of chicken PG-M/versican is developmentally regulated

We investigated the occurrence of alternatively spliced forms (V0, V1, V2, and V3) of PG-M/versican, a large chondroitin sulfate proteoglycan in developing chicken retinas, using the reverse transcription-polymerase chain reaction. We characterized the PLUS domain, which is apparently unique to the chicken molecule and is regulated by alternative splicing. PG-M in chicken retinas consisted of four forms with (V0, V1, V2, and V3) and two forms without (V1 and V3) the PLUS domain (PG-M+ and PG-M-, respectively). The four forms of PG-M+ were found in all samples examined, but the occurrence of the two PG-M- forms was regulated developmentally. Genomic analysis has revealed that the PLUS and CS-alpha domains are encoded by a single exon, and this exon has an internal alternative 5'-splice donor site, allowing alternative spliced forms that do not include the 3'-end of the exon. Sequences corresponding to the chicken PLUS domain (plus) were not found in mouse and human and may have disappeared during evolution. Sequence similarity suggests that the PLUS domain corresponds to the keratan sulfate attachment domain of aggrecan and that it has a distinct function in the chicken eye.

Distribution of CD44 in the retina during development and the rds degeneration

It was previously demonstrated that the 90 kDa cell adhesion molecule CD44 is localized to Müller cell apical microvilli which project into the interphotoreceptor matrix in the adult retina. For the current report, the distribution of CD44 was studied in developing normal BALB/c mouse retinas and in rds mutant mouse retinas of various ages. Using immunoperoxidase techniques, positive label for CD44 was first detected on Müller cell microvilli after 7 days postnatal in BALB/c retinas and the band of label widened as the microvilli elongated and the retina reached maturity. Labeling of the radiating Müller cell bodies within the retina was also observed during development, and this distribution of CD44 is most likely related to retinal cell differentiation and maturation which are taking place at this time. This label was not observed in mature retinas. In the rds mouse retina, a normal labeling pattern for CD44 was observed on Müller cell microvilli at early ages, prior to photoreceptor degeneration. During the time course of the degeneration, microvillar labeling was still seen, however labeling of the radiating Müller cell bodies within the retina increased dramatically. Following degeneration, the Müller cell microvilli appeared to have retracted or collapsed and the inner retinal labeling was not observed. Only a residual label remained at the level of the outer limiting membrane. The increased presence of CD44 within the rds retina during photoreceptor degeneration is likely due to an upregulation of CD44 synthesis by the Müller cells and/or to the transfer of CD44 from retracting microvilli into lower portions of the Müller cells.

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.

Structure and composition of the rodent lamina cribrosa

To define the architecture and extracellular matrix composition of the lamina cribrosa in rodents, normal, adult pigmented rat and guinea pig eyes were frozen and sectioned for light microscopic immunohistochemistry. Antibodies specific for collagens I, III, IV and VI, laminin, elastin, and chondroitin and dermatan sulfate proteoglycans were exposed to longitudinal and cross-sections of optic nerve heads and their binding distributions observed with the avidin-biotin-peroxidase complex technique. Cross-sections of the intraocular portion of the rat optic nerve head revealed a horizontally oval shape with distinct, vertically oriented, laminar beams. The guinea pig optic nervehead cross-section was circular, with randomly oriented beams. In both animals, collagens I, III and VI were found throughout the laminar beams, along with elastin fibrils. Collagen IV and laminin antibodies deposited along laminar beam margins and within the beams, representing astrocytic and vascular endothelial cell basement membranes. Both animals showed evidence for dermatan and chondroitin sulfate-containing proteoglycans in all connective tissue structures of the nerve head. In the rat, chondroitin-4 sulfate proteoglycans appeared localized to the sclera and laminar beams. The rat and the guinea pig optic nerve head possess an identifiable lamina cribrosa with structural proteins nearly identical to that of the primate. Both animals may provide affordable alternative animal models for in vivo studies on the role of the lamina cribrosa in glaucomatous optic nerve damage.

Immunohistochemical localization of chondroitin sulfate proteoglycan and tenascin in the human eye compared with the HNK-1 epitope

BACKGROUND

A previous study revealed the HNK-1 epitope in the human ciliary body beneath the ciliary epithelium. The molecules bearing this 3-sulphoglucuronic acid-containing oligosaccharide epitope in the eye remain unknown. As chondroitin sulphate proteoglycan (CSPG) and tenascin are potential candidates as bearers of the HNK-1 epitope, their distribution in the human eye was compared with that of the HNK-1 epitope.

METHODS

Fifty-five formalin-fixed, paraffin-embedded human eyes, including 20 normal eyes and 35 eyes with exfoliation syndrome or glaucoma, were studied immunohistochemically with monoclonal antibody (MAb) CS-56 to CSPG, MAb TN2 to tenascin, and MAbs HNK-1 and VC1.1 to the HNK-1 epitope. Additionally, four frozen lens capsules with exfoliation material were studied by indirect immunofluorescence.

RESULTS

A population of dendritic cells in the inner connective tissue layer of the ciliary body and exfoliation material were immunoreactive with antibodies to the HNK-1 epitope, but no labelling for CSPG and tenascin was seen in them, including frozen sections. The inner surface of the nonpigmented ciliary epithelium was reactive for the HNK-1 epitope, and at the ora serrata also for CSPG. In some eyes with glaucoma, immunoreaction for CSPG and tenascin was seen beneath the epithelium and endothelium of the cornea. The nerve fibre layer of the retina was labelled for tenascin. In the sclera, all antibodies labelled the ground substance, and in some large blood vessels immunoreaction for CSPG and tenascin was seen subendothelially.

CONCLUSION

Apart from the sclera, the distribution of CSPG and tenascin was different form that of the HNK-1 epitope, suggesting that this carbohydrate epitope may not be borne by these molecules in the human ciliary body.

Distribution of heparan sulfate proteoglycans in embryonic chicken neural retina and isolated inner limiting membrane

Quantitative distribution of proteoglycans was studied in retinal neural epithelium and its basement membrane (inner limiting membrane). Heparan sulfate proteoglycans (HSPGs) were primarily associated with both inner and outer plexiform (synaptic) layers, and inner limiting membrane (ILM), as determined by autoradiographs of lyase-digested cryosections. Based on distribution of 35S-sulfate-labeled proteoglycans, the isolated ILM contained on average approximately three fourths of its proteoglycans as HSPGs and one fourth as chondroitin sulfate/dermatan sulfate proteoglycans (CS/DSPGs), whereas the remaining retina contained approximately equal amounts of the two proteoglycans (PGs). Immunohistochemical staining indicates that the core proteins of the HSPGs in the ILM are distinct from those of the plexiform layers. The photoreceptor layer, which other studies have shown to contain much of the extracellular CS/DSPGs, was not examined. Enrichment of distinct HSPGs in the ILM and plexiform layers support the conclusion that the HSPGs may be intimately involved in the different developmental events characterizing the two regions: development and extension of ganglion cell axons in the former, synaptogenesis and neuronal function in the latter.

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)

Proteoglycan synthesis in cultures of murine retinal neurons and photoreceptors

1. In recent years, a number of histochemical and immunocytochemical studies have suggested that proteoglycans, particularly those in the interphotoreceptor matrix, exhibit altered distributions in several murine models for retinal degenerations. We are using a cell culture system to characterize the proteoglycans synthesized by neurons and photoreceptors derived from mouse retina, with the long-term goal of analyzing their role in retinal degenerations. 2. In this study we describe initial studies using cells derived from the retinas of normal mice. Cultures of retinal neurons and photoreceptors, which were free of glial, epithelia, or endothelial cells, were labeled with 3H-glucosamine and 35SO4. Proteoglycans isolated from the medium and cell layer were analyzed on the basis of charge, relative hydrodynamic size, and glycosaminoglycan content. 3. The studies indicate that the cultures actively synthesize proteoglycans. The medium contained predominantly chondroitin sulfate/dermatan sulfate, while the cell layer had a higher proportion of heparan sulfate, indicating a differential distribution between the two compartments.

Monkey photoreceptor calycal processes and interphotoreceptor matrix as observed by scanning electron microscopy

Photoreceptors and the interphotoreceptor matrix of monkey retina were observed by scanning electron microscopy. Cone photoreceptors are easily distinguished from rod photoreceptors by their wide conical inner segments. The calycal processes of 50 rods and 50 cones were counted and measured. The calycal processes of cones were distinct, short, and uniform in diameter (0.1 micron). They were arranged equidistantly and, in most cases, were not continuous with longitudinal inner segment ridges, as previously suggested. In contrast to cones, rod calycal processes were fewer in number, were about one-fourth the number of the cones, were of variable length (0.7 micron to 3.0 micron), and tapered to a fine point at their distal ends. The interphotoreceptor matrix appeared spongelike, made up of anastomosing plates and strands filling the interphotoreceptor space. Other than an increased amount of matrix around cones, no structural difference between rod- and cone-associated interphotoreceptor matrix was observed.

Sequestration of basic fibroblast growth factor in the primate retinal interphotoreceptor matrix

The interphotoreceptor matrix (IPM) occupies the extracellular space between the photoreceptors of the retina and the apical surface of the retinal pigmented epithelium. A large proportion of the IPM is composed of aqueous-insoluble glycoconjugates, including chondroitin sulfate-containing proteoglycans, the distribution of which exhibits both apical-basal and photoreceptor cell type-specific heterogeneities. The precise function of most insoluble IPM constituents is unknown, although the available evidence suggests some may contribute to retinal adhesion or photoreceptor survival. We have now identified basic fibroblast growth factor (bFGF), or an immunologically related protein from the FGF family, within the IPM. The IPM is labeled on sections of primate retinas by a battery of polyclonal antibodies (Abs) directed against various peptide sequences of bFGF and by an Ab to bovine brain bFGF. bFGF Abs also bind to purified preparations of aqueous-insoluble IPM. All bFGF Abs utilized cross-react with equivalent low molecular mass components of 16.5-17.5 kDa on Western blots of insoluble IPM proteins, purified bFGF, and recombinant bFGF. The Abs do not bind any aqueous-soluble IPM components, suggesting that the bFGF is normally bound to an insoluble IPM constituent(s) in situ. The fact that bFGF is sequestered in the IPM and is located in such close proximity to the photoreceptors, the retinal pigmented epithelium, and Mueller's glia raises the strong possibility that it is synthesized by and regulates the activities of one or more of these three cell types in vivo.

The cone matrix sheath in the normal and diseased retina: cytochemical and biochemical studies of peanut agglutinin-binding proteins in cone and rod-cone degeneration

The fate of the cone-associated extracellular domain, or cone matrix sheath (CMS), was examined in two canine models of hereditary retinal degeneration. The diseases, which affect cones selectively (cd = cone degeneration), or rods and cones temporally (prcd = progressive rod-cone degeneration), were examined biochemically (SDS-PAGE/lectin blots) and cytochemically (light microscopy) using peanut agglutinin lectin (PNA) to selectively label this domain and associated structures. Most of the cones had disappeared in the adult cd retina. In the remaining cones, PNA labeled the ectopically located somata and the CMSs that were present around severely diseased ones. Loss of cones resulted in background label in the IPM and the loss of the pedicle-associated label in the OPL. SDS-PAGE of retinal extracts showed that all the major classes of the lower molecular weight PNA-binding proteins were present, but only the 40- and 60-kD bands remained prominent. Because of the selectivity of the cd mutation, this suggests considerable heterogeneity within the various size classifications of the retinal PNA-binding glycoproteins. In prcd, CMSs were normal at a time when cones were structurally normal and disease was limited to the rod outer segments. The CMSs remained intact during the degenerative phase of the disease, and only became compressed in association with the collapse and narrowing of the photoreceptor layer; CMS labeling was lost with disappearance of the cone inner segment. The lectin biochemical results were normal until 1.7 years of age; thereafter, there was a decreased prominence of all major bands. Because of spatial heterogeneity in disease severity, it was not possible to correlate the lectin biochemical and cytochemical results in prcd.

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.

Comparison of photoreceptor-specific matrix domains in the cat and monkey retinas

Two lectins, wheat germ agglutinin (WGA) and peanut agglutinin (PNA), were used to compare domains within the interphotoreceptor matrices (IPM) of the cat and monkey, two species where the morphological relationship between the retinal pigment epithelium (RPE) and photoreceptors is distinctly different. In the monkey, PNA labeling was heaviest over the cone outer segments and a discrete region of the interphotoreceptor matrix bordering the cone inner and outer segments--a region which has been termed the cone matrix sheath. Near the apical border of the RPE, the outer margin of the PNA-labeled matrix is surrounded by a circular array of apical microvilli. In the cat retina, PNA labeling was highest over a region of the IPM lying between the outer margin of the cone sheath processes and surrounding rod matrix. In contrast, intracellular labeling of cone inner and outer segments was sparse. The RPE apical processes forming the cone sheath were not labeled. In the monkey retina, WGA preferentially labeled rod outer segments and the region of the IPM around rod inner and outer segments. The cone matrix sheath was not preferentially labeled using this lectin. Rod inner segments and cone inner and outer segments were labeled moderately. In the cat retina, WGA labeling was dense over both rod outer segments and cone outer segments as well as the cone sheath. Rod and cone inner segments, as well as the IPM around both rods and cones, were moderately labeled. These observations suggest that the specialized processes arising from the apical surface of retinal pigment epithelial cells, together with photoreceptor-specific extracellular matrix domains, contribute to the formation of specific microenvironments around rod and cone photoreceptor cells.

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.

Ontogeny of basic fibroblast growth factor binding sites in mouse ocular tissues

Basic fibroblast growth factor (bFGF) binding to ocular tissues has been studied by autoradiographical and biochemical approaches directly performed on sections during mouse embryonic and postnatal development. Frozen sections of embryos (9 to 18 days), newborns, and adults (1 day to 6 months) were incubated with iodinated bFGF. One specific FGF binding site (KD = 2.5 nM) is colocalized with heparan sulfate proteoglycans of the basement membranes and is heparitinase sensitive. It first appears at Day 9 around the neural tube, the optic vesicles, and below the head ectoderm and by Day 14 of embryonic development is found in all basement membranes of the eye. At Day 16, very intensely labeled patches appear, corresponding to mast cells which have been characterized by metachromatic staining of their heparin-rich granulations with toluidine blue. In addition to the latter binding, we have also observed a general diffuse distribution of silver grains on all tissues and preferentially in the ecto- and neuroectodermic tissues. From Days 17-18, there is heterogeneous labeling inside the retina, localized in the pigmented epithelium and in three different layers colocalized with the inner and outer plexiform layers and with the inner segments of the photoreceptors. This binding is heparitinase resistant but N-glycanase sensitive and may represent a second specific binding site corresponding to cellular FGF receptors (KD = 280 pM). Both types of binding patterns observed suggest a significant role for bFGF in eye development and physiology.

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.

Effect of glycoconjugates on rod outer segment phagocytosis by retinal pigment epithelial explants in vitro assessed by a specific double radioimmunoassay procedure

Rod outer segment (ROS) phagocytosis by explanted bovine retinal pigment epithelium (RPE) was evaluated by a procedure using an indirect double radioimmunoassay which distinguished between ROS attached to the RPE cell surface and those which had been ingested. This approach has been used to investigate the effect of a variety of glycoconjugates on the phagocytic process. Inclusion of the glycosaminoglycans (GAGs) chondroitin sulphate type-A (CS-A) and type-C (CS-C), hyaluronic acid (HA) or dermatan sulphate (DS) in the incubation medium significantly inhibited the ingestion phase of ROS phagocytosis, whereas the binding phase was inhibited to a lesser extent. The interphotoreceptor matrix (IPM), containing these GAGs as part of proteoglycans, also had an inhibitory effect on phagocytosis. The free monosaccharides mannose, fucose and galactose all stimulated the ingestion of ROS by RPE cells. These findings support the suggestion that glycoconjugates may have a physiological role in the photoreceptor renewal process.

Differential distribution of interphotoreceptor retinoid-binding protein (IRBP) around retinal rod and cone photoreceptors

Light microscopic immunogold cytochemistry was used to examine the distribution of interphotoreceptor retinoid-binding protein (IRBP) in the interphotoreceptor matrix (IPM) surrounding rod and cone photoreceptors. Silver enhancement of retinas reacted with anti-IRBP antibodies using the two stage labeling procedure showed dense staining of the IPM around rod photoreceptor outer segments and the apical region of the RPE. However, the IPM around cone photoreceptors was lightly labeled for IRBP. This region of light labeling extended from the RPE to the distal one-fourth to one-third of the cone inner segment. Although most of the area surrounding cone outer segments was lightly labeled, a dense band of label was seen along the margins of cone outer segments. This study confirms that heterogeneity exists in the distribution of IRBP in the IPM and provides new evidence that IRBP is not homogeneously distributed around rod and cone photoreceptors.

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.

Microscopic and biochemical characterization of lectin binding sites in the cephalopod retina

Using light and electron microscope cytochemistry and lectin blotting techniques, we have shown that the lectins concanavalin A (Con A), Ricinus communis agglutinin (RCA), and peanut agglutinin (PNA) bind to specific glycoconjugants in the adult cephalopod retina. For light microscope lectin cytochemistry, aldehyde-fixed, frozen, or Araldite-embedded, etched sections of cephalopod retinas were incubated with FITC- or TRITC-conjugated lectins and examined by using epifluorescence microscopy. Con A labeled structures in the entire retina including the inner limiting membrane (ILM), rhabdomeric membranes, interphotoreceptor matrix (IPM), and structures in the photoreceptor inner segments. RCA labeling was similar to that of Con A except that there was a decrease in the staining of the rhabdom tips near the ILM. PNA labeled only the interphotoreceptor matrix between apposing rhabdomeres. The intensity of staining of the IPM by PNA also decreased or was absent toward the rhabdom tips. None of the lectins labeled the myeloid bodies located in the photoreceptor inner segments. Electron microscope (EM) lectin cytochemistry was performed on aldehyde-fixed, LR White-embedded tissue or on Araldite-embedded, periodate-etched sections by using gold-conjugated lectins. EM results confirmed the observations made by light microscopy. Lectin blots with a retinal extract or light-sensitive membrane fraction revealed a variety of protein bands labeled by all three lectins. Con A and RCA labeled opsin and its aggregates whereas PNA did not. None of the lectins labeled retinochrome. The labeling of the cephalopod IPM by PNA suggests a structural similarity between the IPM of vertebrates and invertebrates. In other studies, we have demonstrated the presence of a retinoid binding protein in the IPM of cephalopods.(ABSTRACT TRUNCATED AT 250 WORDS)

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.