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

Spatiotemporal distribution of chondroitin sulfate proteoglycans in the developing mouse retina and optic nerve

The aim of the present study was to determine the distribution of chondroitin sulfate proteoglycans in the mouse retina and optic nerve of the prenatal and postnatal mouse by immunohistochemistry. At embryonic day (E) 18, chondroitin-4-sulfate (C4S), chondroitin-6-sulfate (C6S) and biglycan were detected in the retina and optic nerve. However, aggrecan was seen in the retina but not in the optic nerve. At postnatal day (P) 7, aggrecan and biglycan were clearly observed in the optic nerve, inner nuclear layer and ganglion cell layer and diffuse in the outer retina. C4S diffusely distributed in the retina and optic nerve, but C6S was mainly confined to the photoreceptor layer and optic nerve sheath. At P42, biglycan showed diffuse distribution in the retina and optic nerve with intense staining in nerve-fiber rich layers. Aggrecan showed weak staining at the inner plexiform layer with higher density in the outer and inner nuclear layers, outer plexiform layer and ganglion cell layer. Both C4S and C6S were detected in the optic nerve and retina, but C6S showed strong immunostaining in the photoreceptor layer. The distributions of these proteoglycans with respect of time course during development of the retina and optic nerve suggest that they may have unique or overlapping roles in development and maintenance of the retina and optic nerve.

Proteoglycans in retina

In this article, we summarize the roles of proteoglycans in retinal tissue. Chondroitin sulfate and heparan sulfate proteoglycans are the major constituents in proteoglycans expressed in retinal tissue. Soluble heparan sulfate proteoglycans are found in the extracellular matrices of the basement membrane, such as the inner limiting membrane and Bruch's membrane, whereas heparan sulfate proteoglycans with their membrane-binding domain are localized primarily in the neurites of retinal neuronal cells, indicating their role as receptors for cytokines. The distribution of chondroitin sulfate proteoglycans is classified into two regions: nerve fiber-rich layers such as the optic nerve, inner plexiform layer and outer plexiform layer, and the interphotoreceptor matrix (IPM). The expression in the nerve fiber-rich layers of several chondroitin sulfate proteoglycans, such as neurocan and phosphacan, is restricted in the nervous tissues, and is upregulated as retinal development proceeds, then decreases after maturation of the retina. In vitro data suggest that these proteoglycans regulate axon guidance and synapse formation during the development of nervous tissue. In contrast, in adult vertebrate retina, the IPM is a rich source of chondroitin sulfate proteoglycans. Histologic data from animals with experimental retinitis pigmentosa, and the existence of the hyaluronan-binding domain in their core proteins, indicate that these proteoglycans contribute to the structural link between the neural retina and retinal pigment epithelium via the interaction with hyaluronan, which is also abundant in the IPM. Furthermore, several chondroitin sulfate proteoglycans in the nerve fiber-rich layers contain the hyaluronan-binding domain, so it is likely that the interaction of proteoglycans with hyaluronan plays an important role in neural network formation in the central nervous system.

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.

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.

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.

Retinal degeneration in the nervous mutant mouse. I. Light microscopic cytopathology and changes in the interphotoreceptor matrix

Nervous is an autosomal recessive mutation in mice (gene symbol, nr) that produces a progressive cerebellar and retinal degeneration. We have examined various cytopathological features of the photoreceptor degeneration by light microscopy. An increase in the number of pyknotic photoreceptor nuclei in the outer nuclear layer (ONL) is first seen at postnatal day (P) 11. Between P13 and P19 there is a rapid loss of photoreceptors, with the ONL about 60% the thickness of littermate controls at P19. Between P19 and 2.5 months of age, photoreceptor cell loss is minimal, and there is a relatively slow loss of these cells between 3 and 7.5 months of age. At 7.5 months, the ONL consists of single row of nuclei, most of which are lost over the ensuing months, although a few photoreceptor nuclei persist at 17 months of age and older. Both rods and cones are lost at comparable rates for the first 2 months of life, but rods are somewhat preferentially lost at later ages. A very slight central-to-peripheral gradient of photoreceptor degeneration exists in the nr/nr retina, but no superior-inferior hemispheric differences are evident. The rate, spatiotemporal gradient, and hemispheric similarity in photoreceptor degeneration are the same in albino nr/nr mice reared either in cyclic light or in the dark, and in pigmented nr/nr mice. Autoradiographic analysis of rod outer segment renewal shows that outer segment membranes are synthesized in nervous homozygotes. Rhythmic outer segment disc shedding and phagocytosis by the retinal pigment epithelium occur at approximately normal rates in nr/nr mice. Histochemical and immunocytochemical study of the interphotoreceptor matrix (IPM) reveals the exclusion of stainable IPM from the outer segment zone by lamellar whorls of outer segment membrane, accumulation of stainable IPM in the basal region of the outer segment zone, and the absence of an intense band of stainable IPM at the apical surface of the retinal pigment epithelium. These changes in the IPM are similar to those seen in the Royal College of Surgeons rat. However, comparison of cytopathological changes in these two mutants reveal that the IPM defect probably is not the primary cause of photoreceptor cell death in nr/nr mice, and that similar phenotypic appearance does not necessarily signify similar pathological processes.

Neurotrophic activity of interphotoreceptor matrix on human Y79 retinoblastoma cells

A neurotrophic activity of adult monkey and bovine interphotoreceptor matrix (IPM) was examined by using cultured human Y79 retinoblastoma cells as a model system. The cells were stimulated for 7 days in suspension culture with soluble IPM components and then attached to poly-D-lysine substratum. IPMs from both species induced greater than 80% neuronal differentiation of Y79 cell aggregates after 11 days of attachment as adjudged morphologically by the extension of lengthy, neurite-like processes. Immunocytochemical studies indicate that differentiated Y79 cells had an increased level of expression of neuron-specific enolase and a concomitant decreased expression of glial fibrillary acidic protein. This neurotrophic activity cannot be ascribed to nerve growth factor, platelet-derived growth factor, fibroblast growth factor, epidermal growth factor, or transforming growth factor beta. Although the nature of the factor and its cellular source have yet to be characterized, it may be related to a recently described neurotrophic protein secreted by human fetal retinal pigment epithelial cells in culture. Our findings provide evidence supporting the neuroblastic potential of the Y79 cell line and indicate that the IPM contains a potent neurotrophic activity. Such factors may be important to normal differentiation and maintenance of function of the neural 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.

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.

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.