Proteoglycans in retina

Neuron-specific TGF-beta signaling deficiency results in retinal detachment and cataracts in mice

We generated a mouse model (cKO) with a conditional deletion of TGF-beta signaling in the retinal neurons by crossing TGF-beta receptor I (TGF-beta RI) floxed mice with nestin-Cre mice. Almost all of the newborn cKO mice had retinal detachment at the retinal pigment epithelium (RPE)/photoreceptor layer junction of the neurosensory retina (NSR). The immunostaining for chondroitin-6-sulfate showed a very weak reaction in cKO mice in contrast to intense staining in the photoreceptor layer in wild-type mice. Macroscopic cataracts, in one or both eyes, were observed in 50% of the mice by 6 months of age, starting as early as the first month after birth. The cKO mouse model demonstrates that the TGF-beta signaling deficiency in retinal cells leads to decreased levels of chondroitin sulfate proteoglycan in the retinal interphotoreceptor matrix. This in turn causes retinal detachment due to the loss of adhesion of the NSR to RPE.

Biological characterization of gene response in Rpe65-/- mouse model of Leber's congenital amaurosis during progression of the disease

RPE65 is the retinal isomerase essential for conversion of all-trans-retinyl ester to 11-cis-retinol in the visual cycle. Leber's congenital amaurosis (LCA), an autosomal recessive form of RP resulting in blindness, is commonly caused by mutations in the Rpe65 gene. Whereas the molecular mechanisms by which these mutations contribute to retinal disease remain largely unresolved, affected patients show marked RPE damage and photoreceptor degeneration. We evaluated gene expression in Rpe65-/- mouse model of LCA before and at the onset of photoreceptor cell death in 2, 4, and 6 month old animals. Microarray analysis demonstrates altered expression of genes involved in phototransduction, apoptosis regulation, cytoskeleton organization, and extracellular matrix (ECM) constituents. Cone-specific phototransduction genes are strongly decreased, reflecting early loss of cones. In addition, remaining rods show modified expression of genes encoding components of the cytoskeleton and ECM. This may affect rod physiology and interaction with the adjacent RPE and lead to loss of survival signals, as reflected by the alteration of apoptosis-related genes Together, these results suggest that RPE65 defect triggers an overall remodeling of the neurosensitive retina that may, in turn, disrupt photoreceptor homeostasis and induce apoptosis signaling cascade toward retinal cell death.

GLYCOMICS APPROACH TO STRUCTURE-FUNCTION RELATIONSHIPS OF GLYCOSAMINOGLYCANS

Extracellular modulation of phenotype is an emerging paradigm in this current postgenomics age of molecular and cell biology. Glycosaminoglycans (GAGs) are primary components of the cell surface and the cell-extracellular matrix (ECM) interface. Advances in the technology to analyze GAGs and in whole-organism genetics have led to a dramatic increase in the known important biological role of these complex polysaccharides. Owing to their ubiquitous distribution at the cell-ECM interface, GAGs interact with numerous proteins and modulate their activity, thus impinging on fundamental biological processes such as cell growth and development. Many recent reviews have captured important aspects of GAG structure and biosynthesis, GAG-protein interactions, and GAG biology. GAG research is currently at a stage where there is a need for an integrated systems or glycomics approach, which involves an integration of all of the above concepts to define their structure-function relationships. Focusing on heparin/heparan (HSGAGs) and chondroitin/dermatan sulfate (CSGAGs), this review highlights the important aspects of GAGs and summarizes these aspects in the context of taking a glycomics approach that integrates the different technologies to define structure-function relationships of GAGs.

Restricted distribution of D-unit-rich chondroitin sulfate carbohydrate chains in the neuropil encircling the optic tract and on a subset of retinal axons in chick embryos

To obtain basic information about the structural diversity and functional specificity of chondroitin sulfates (CSs) in the formation of the retinotectal pathway in chick embryos, the distribution of CSs around the optic tract was investigated by using anti-CS monoclonal antibodies with different specificities. The CSs are unbranched polymers composed of repeating disaccharide units of glucuronic acid (GlcA) and N-acetylgalactosamine (GalNAc). The disaccharide units are classified into O-, A-, C-, D-, and E-units based on the position(s) of the added sulfate group(s). The MO-225 monoclonal antibody recognizes CSs that are rich in the D-unit [GlcA(2S)beta1-3GalNAc(6S)]; the MO-225 epitopes were distributed in the diencephalotelencephalic boundary and the neuropil encircling the optic tract. In addition, they were distributed on membrane surfaces of the retinal axons running in an interface layer in contact with the neuropil encircling the optic tract. The results suggest that D-unit-rich CSs are involved in delimiting the border of the optic tract and in the chronological sorting of the retinal axons.

Identification and characterization of photomedins: novel olfactomedin-domain-containing proteins with chondroitin sulphate-E-binding activity

We screened more than 60000 RIKEN mouse cDNAs for novel ECM (extracellular matrix) proteins by extensive computational screening followed by recombinant expression and immunohistochemical characterization. We identified two novel olfactomedin-family proteins characterized by the presence of tandem CXCXCX9C motifs in the N-terminal region, a coiled-coil domain and an olfactomedin domain in the C-terminal region. These proteins, named photomedin-1 and photomedin-2, were secreted as disulphide-bonded dimers (photomedin-1) or oligomers/multimers (photomedin-2) with O-linked carbohydrate chains, although photomedin-1 was proteolytically processed in the middle of the molecule after secretion. In the retina, photomedin-1 was selectively expressed in the outer segment of photoreceptor cells and photomedin-2 was expressed in all retinal neurons. Among a panel of ECM components, including glycosaminoglycans, photomedins preferentially bound to chondroitin sulphate-E and heparin. These results, together, indicate that photomedins are novel olfactomedin-domain-containing extracellular proteins capable of binding to proteoglycans containing these glycosaminoglycan chains.

Ectopic norrin induces growth of ocular capillaries and restores normal retinal angiogenesis in Norrie disease mutant mice

Norrie disease is an X-linked retinal dysplasia that presents with congenital blindness, sensorineural deafness, and mental retardation. Norrin, the protein product of the Norrie disease gene (NDP), is a secreted protein of unknown biochemical function. Norrie disease (Ndp(y/-)) mutant mice that are deficient in norrin develop blindness, show a distinct failure in retinal angiogenesis, and completely lack the deep capillary layers of the retina. We show here that the transgenic expression of ectopic norrin under control of a lens-specific promoter restores the formation of a normal retinal vascular network in Ndp(y/-) mutant mice. The improvement in structure correlates with restoration of neuronal function in the retina. In addition, lenses of transgenic mice with ectopic expression of norrin show significantly more capillaries in the hyaloid vasculature that surrounds the lens during development. In vitro, lenses of transgenic mice in coculture with microvascular endothelial cells induce proliferation of the cells. Transgenic mice with ectopic expression of norrin show more bromodeoxyuridine-labeled retinal progenitor cells at embryonic day 14.5 and thicker retinas at postnatal life than wild-type littermates, indicating a putative direct neurotrophic effect of norrin. These data provide direct evidence that norrin induces growth of ocular capillaries and that pharmacologic modulation of norrin might be used for treatment of the vascular abnormalities associated with Norrie disease or other vascular disorders of the retina.

Molecular mechanisms of optic axon guidance

Axon guidance is one of the critical processes during vertebrate central nervous system (CNS) development. The optic nerve, which contains the axons of retinal ganglion cells, has been used as a powerful model to elucidate some of the mechanisms underlying axon guidance because it is easily manipulated experimentally, and its function is well understood. Recent molecular biology studies have revealed that numerous guidance molecules control the development of the visual pathway. This review introduces the molecular mechanisms involved in each critical step during optic axon guidance. Axonal projections to the optic disc are thought to depend on adhesion molecules and inhibitory extracellular matrices such as chondroitin sulfate. The formation of the head of the optic nerve and the optic chiasm require ligand-receptor interactions between netrin-1 and the deleted in colorectal cancer receptor, and Slit proteins and Robo receptors, respectively. The gradient distributions of ephrin ligands and Eph receptors are essential for correct ipsilateral projections at the optic chiasm and the topographic mapping of axons in the superior colliculus/optic tectum. The precise gradient is regulated by transcription factors determining the retinal dorso-ventral and nasal-temporal polarities. Moreover, the axon guidance activities by Slit and semaphorin 5A require the existence of heparan sulfate, which binds to numerous guidance molecules. Recent discoveries about the molecular mechanisms underlying optic nerve guidance will facilitate progress in CNS developmental biology and axon-regeneration therapy.

HISTOPATHOLOGIC EVALUATION OF THE INTERNAL LIMITING MEMBRANE SURGICALLY EXCISED FROM EYES WITH DIABETIC MACULOPATHY

PURPOSE

To histopathologically evaluate the internal limiting membrane (ILM) in diabetic eyes with macular edema as compared to nondiabetic controls.

METHODS

The authors ultrastructurally and immunohistochemically studied ILM specimens that were intentionally peeled from five eyes with diabetic maculopathy, comprising four with diffuse diabetic macular edema and one with macular hole accompanying diabetic retinopathy (DM group), and five with nondiabetic idiopathic macular hole (MH group). They compared ultrastructural and immunohistochemical findings between the two groups.

RESULTS

A larger amount of cellular elements was observed on the vitreous side of the ILM in the DM group. The thickness of the ILM in the DM group was significantly increased (mean 4.8 +/- 1.6 microm) compared with that in the MH group (1.8 +/- 0.6 microm) (P < 0.0001). Immunoreactions for heparan sulfate proteoglycan in the ILM were more abundant in the DM group than in the MH group.

CONCLUSION

The ILM thickening and cell abundance on the vitreous surface might contribute to the course and the pathogenesis of diabetic maculopathy.

Proteoglycans as cues for axonal guidance in formation of retinotectal or retinocollicular projections

Understanding the formation of neuronal circuits has long been one of the basic problems in developmental neurobiology. Projections from the retina to their higher center, the optic tectum in nonmammalian vertebrates and the superior colliculus in mammals, are most amenable to experimental approaches; thus, much information has been accumulated about the mechanisms of axonal guidance. The retinal axons navigate along the appropriate pathway with the help of a series of guidance cues. Although much of the work has focused on proteinaceous factors, proteoglycans have been identified as playing important roles in retinal axon guidance. Chondroitin sulfate proteoglycans and heparan sulfate proteoglycans are involved in essential decisions of axon steering along the pathway. However, it has not been determined whether diversity of the carbohydrate chains results in differential effects and how their diversity is recognized by growth cones, which represent an important area of future research.

Fibroblast growth factor-2 gene delivery stimulates axon growth by adult retinal ganglion cells after acute optic nerve injury

Basic fibroblast growth factor (or FGF-2) has been shown to be a potent stimulator of retinal ganglion cell (RGC) axonal growth during development. Here we investigated if FGF-2 upregulation in adult RGCs promoted axon regrowth in vivo after acute optic nerve injury. Recombinant adeno-associated virus (AAV) was used to deliver the FGF-2 gene to adult RGCs providing a sustained source of this neurotrophic factor. FGF-2 gene transfer led to a 10-fold increase in the number of axons that extended past 0.5 mm from the lesion site compared to control nerves. Detection of AAV-mediated FGF-2 protein in injured RGC axons correlated with growth into the distal optic nerve. The response to FGF-2 upregulation was supported by our finding that FGF receptor-1 (FGFR-1) and heparan sulfate (HS), known to be essential for FGF-2 signaling, were expressed by adult rat RGCs. FGF-2 transgene expression led to only transient protection of injured RGCs. Thus the effect of this neurotrophic factor on axon extension could not be solely attributed to an increase in neuronal survival. Our data indicate that selective upregulation of FGF-2 in adult RGCs stimulates axon regrowth within the optic nerve, an environment that is highly inhibitory for regeneration. These results support the hypothesis that key factors involved in axon outgrowth during neural development may promote regeneration of adult injured neurons.

Spacrcan binding to hyaluronan and other glycosaminoglycans. Molecular and biochemical studies

Photoreceptors project from the outer retinal surface into a specialized glycocalyx, the interphotoreceptor matrix (IPM), which contains hyaluronan (HA) and two novel proteoglycans, Spacr and Spacrcan. This matrix must be stable enough to function in the attachment of the retina to the outer eye wall yet porous enough to allow movement of metabolites between these tissues. How this matrix is organized is not known. HA is a potential candidate in IPM organization since biochemical studies show that these proteoglycans bind HA. RHAMM (receptor for HA-mediated motility)-type HA binding motifs (HABMs) are present in their deduced amino acid sequence and may be the sites of this HA interaction. To test this hypothesis, we subcloned three fragments of mouse Spacrcan that contain the putative HABMs. We found that each recombinant fragment binds HA. Binding decreased when residues in the HABMs were mutated. This provides direct evidence that the RHAMM-type HABMs in Spacrcan are involved in hyaluronan binding. Since chondroitin sulfate and heparan sulfate proteoglycans are important for retinal development and function, we also evaluated the binding of these recombinant proteins to heparin and chondroitin sulfates, the glycosaminoglycan side chain of these proteoglycans. We found that each recombinant protein bound to both heparin and chondroitin sulfates. Binding to chondroitin sulfates involved these HABMs, because mutagenesis reduced binding. Binding to heparin was probably not mediated through these HABMs since heparin binding persisted following their mutagenesis. These studies provide the first evidence defining the sites of protein-carbohydrate interaction of molecules present in the IPM.

Regulated expression of apolipoprotein E by human retinal pigment epithelial cells

In early age-related macular degeneration (AMD), lipid-containing deposits (drusen) accumulate in Bruch's membrane underlying the retinal pigment epithelium (RPE). Recent studies indicate that apolipoprotein E (apoE) may play a role in lipid trafficking in AMD. Compared with the apoE3 allele, the apoE4 and apoE2 alleles are associated with decreased and increased risk for AMD, respectively; drusen contain high levels of apoE, and apoE null mice develop lipid deposits in Bruch's membrane similar to those observed in AMD. Primary cultures of human RPE cells expressing the apoE3 allele were grown on Transwell culture plates. Western blotting, ELISA assay, and mass spectrometry confirmed that apoE3 was secreted into the apical and basal chambers and that secretion was upregulated by thyroid hormone, 9-cis-retinoic acid, and 22(R)-hydroxycholesterol. In addition, basally secreted apoE associated with exogenously added HDL. These results indicate that apoE secretion can be regulated by specific hormones and that apoE associates with HDL. The findings are consistent with a role for apoE in lipid trafficking through Bruch's membrane and may be relevant to AMD.

Expression of glycosaminoglycans during development of the rat retina

PURPOSE

To investigate the spatiotemporal expression of glycosaminoglycans during development of the rat retina.

METHODS

Hyaluronan and sulfated glycosaminoglycans, including chondroitin sulfate, heparan sulfate and keratan sulfate were detected using biotinylated hyaluronan binding protein, immunohistochemical analysis, respectively, in the rat retina at various stages of development.

RESULTS

Hyaluronan was expressed in the nerve fiber layer, inner plexiform layer and outer plexiform layer during early postnatal stages (postnatal day 1-14; P1-P14) and was undetectable after P21. In contrast, hyaluronan was faintly observed in the photoreceptor layer on P7, and gradually increased up to P49. The spatiotemporal expression pattern of chondroitin sulfate was similar to that of hyaluronan. Heparan sulfate was also detected in the nerve fiber layer, inner plexiform layer and outer plexiform layer during early postnatal stages (P1-P14). In addition, heparan sulfate was expressed in the inner limiting membrane during all stages of development. Keratan sulfate was not detected in the retina at any stage of development.

CONCLUSIONS

Hyaluronan, chondroitin sulfate and heparan sulfate are expressed in nerve fiber-rich layers during early postnatal stages and may regulate neurite outgrowth. In adulthood, both hyaluronan and chondroitin sulfate are expressed in the photoreceptor layer and may consist of the interphotoreceptor matrix. In addition, heparan sulfate is expressed in the inner limiting membrane throughout the various stages of development and may be associated with the structure of the inner limiting membrane.

Evidence of protein transduction but not intercellular transport by proteins fused to HIV tat in retinal cell culture and in vivo

The human immunodeficiency virus type-1 Tat protein is known to exit virally infected cells and enter the nucleus of adjacent uninfected cells. This property has been mapped to an 11-amino-acid protein transduction domain (PTD). When the PTD of Tat is fused to heterologous proteins and added exogenously to cells, the fusion peptide is able to demonstrate protein transduction across plasma membranes. Recent reports indicate that endogenously expressed Tat fusion peptides can demonstrate intercellular transport and improve biodistribution of therapeutic protein in the context of adenovirus vectors. Intercellular transport and protein transduction have not been observed in some studies and in the former have been attributed to an artifact of fixation. We have attempted to resolve these studies using an approach that unambiguously distinguishes cells that express Tat fusion protein from those that receive it from their environment. We find no evidence of intercellular transport in the context of an adenovirus vector in cell culture or in vivo. Instead, we find that Tat fusion peptides are down regulated in terms of expression not only in the context of adenovirus vectors, but also when expressed from transfected plasmid DNA. However, when Tat fusion peptides are released from cells by degradation of the plasma membrane, the fusion peptides demonstrate protein transduction without the need for cell fixation, indicating a unidirectional transport of Tat fusion proteins across the plasma membrane. Our data are consistent with previously reported studies and help to explain the apparently different results obtained from several different laboratories.