Retinal ganglion cell survival in vitro maintained by a chondroitin sulfate proteoglycan from the superior colliculus carrying the HNK-1 epitope

Strategies for Oligodendrocyte and Myelin Repair in Traumatic CNS Injury

A major consequence of traumatic brain and spinal cord injury is the loss of the myelin sheath, a cholesterol-rich layer of insulation that wraps around axons of the nervous system. In the central nervous system (CNS), myelin is produced and maintained by oligodendrocytes. Damage to the CNS may result in oligodendrocyte cell death and subsequent loss of myelin, which can have serious consequences for functional recovery. Demyelination impairs neuronal function by decelerating signal transmission along the axon and has been implicated in many neurodegenerative diseases. After a traumatic injury, mechanisms of endogenous remyelination in the CNS are limited and often fail, for reasons that remain poorly understood. One area of research focuses on enhancing this endogenous response. Existing techniques include the use of small molecules, RNA interference (RNAi), and monoclonal antibodies that target specific signaling components of myelination for recovery. Cell-based replacement strategies geared towards replenishing oligodendrocytes and their progenitors have been utilized by several groups in the last decade as well. In this review article, we discuss the effects of traumatic injury on oligodendrocytes in the CNS, the lack of endogenous remyelination, translational studies in rodent models promoting remyelination, and finally human clinical studies on remyelination in the CNS after injury.

The bright side of the glial scar in CNS repair

Following CNS injury, in an apparently counterintuitive response, scar tissue formation inhibits axonal growth, imposing a major barrier to regeneration. Accordingly, scar-modulating treatments have become a leading therapeutic goal in the field of spinal cord injury. However, increasing evidence suggests a beneficial role for this scar tissue as part of the endogenous local immune regulation and repair process. How can these opposing effects be reconciled? Perhaps it is all a matter of timing.

Excitotoxic protection by polyanionic polysaccharide: evidence of a cell survival pathway involving AMPA receptor-MAPK Interactions

Growing numbers of studies indicate that polysaccharides influence signaling events important for brain function. It has been speculated that such polysaccharide modulation of neuronal signals can promote synaptogenesis and cell maintenance. Here, we tested whether dextran sulfate, a polyanion that mimics natural mucopolysaccharides, protects hippocampal neurons against excitotoxic insults. An excitotoxin was applied to primary hippocampal cultures in the absence or presence of a large 500-kDa dextran sulfate (DS-L), a smaller 5-8-kDa species (DS-S), or sulfate-free dextran of 500 kDa. Only DS-L prevented neuronal damage as determined by a membrane permeability assay and phase contrast morphology. The sulfate and size dependence is also characteristic of DS-L's modulatory action on the channel activity of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors. The extent of neuroprotection correlates with the level of modulation of AMPA responses, and DS-L exhibits comparable EC(50) values for the two effects (3-7 nM). DS-L also modulates the link between AMPA receptors and mitogen-activated protein kinase (MAPK) involving extracellular signal-regulated protein kinase (ERK), well known for its involvement in cell survival and repair. Correspondingly, protection against N-methyl-D-aspartate (NMDA) excitotoxicity was evident in hippocampal slice cultures when DS-L was applied 30 min postinsult. These findings suggest that polysaccharides elicit neuroprotection in the brain, including enhanced repair responses through the AMPA receptor-MAPK axis.

Carbohydrate mimics promote functional recovery after peripheral nerve repair

OBJECTIVE

The outcome of peripheral nerve repair is often unsatisfactory, and efficient therapies are not available. We tested the therapeutic potential of functional mimics of the human natural killer cell glycan (3-sulfoglucuronyl beta1-3 galactoside) (HNK-1) epitope, a carbohydrate indicated to favor specificity of motor reinnervation in mice.

METHODS

We applied a linear HNK-1 mimic peptide, scrambled peptide, or vehicle substances in polyethylene cuffs used to reconstruct the severed femoral nerves of adult mice. We used video-based motion analysis and morphological and tracing techniques to monitor the outcome of nerve repair.

RESULTS

After glycomimetic application, quadriceps muscle function recovered to 93% of normal within 3 months. Restoration of function was less complete (71-76%) in control groups. Better functional recovery was associated with larger motoneuron somata, better axonal myelination in the quadriceps nerve, and enhanced precision of target reinnervation. Lesion-induced death of motoneurons was reduced by 20 to 25%. The glycomimetic enhanced survival and neurite outgrowth of both mouse and human motoneurons in vitro by 30 to 75%. Application of a novel cyclic glycomimetic also enhanced functional recovery in vivo.

INTERPRETATION

The improved outcome of nerve repair after glycomimetic application may be attributed to neurotrophic effects. Our results hold promise for therapeutic use in humans.

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.

Roles of the telencephalic cells and their chondroitin sulfate proteoglycans in delimiting an anterior border of the retinal pathway

The axons of the retinal ganglion cells run on the diencephalotelencephalic boundary on their way to the tectum; however, they do not invade the telencephalon anteriorly. To investigate the mechanisms that prevent the retinal axons from entering the telencephalic territory, the effects of the telencephalic cells were examined on the outgrowth of the retinal axons in vitro; the retinal outgrowth was selectively inhibited by the cellular substrate derived from the telencephalon. The responsible factor for the selective inhibition was, furthermore, found in the telencephalic membranes and the fraction of peripheral membrane molecules from the telencephalon. Because the inhibitory effect was destroyed by chondroitinase ABC but not by heat, this inhibition was attributable to the carbohydrate chains of chondroitin sulfate proteoglycans (CSPGs) adhering to the membranes of the telencephalic cells. To understand the function of the telencephalic CSPGs on the retinal pathfinding in vivo, their carbohydrate chains [chondroitin sulfate glycosaminoglycan (CS-GAG)] were removed from the embryonic brains by intraventricular injection of chondroitinase ABC; the removal of CS-GAG resulted in an anterior enlargement of the optic tract. The results indicate that the telencephalic cells delimit the anterior border of the optic tract with their CSPGs and prevent the retinal axons from aberrantly entering the anterior territory.

Neuronal cell death, nerve growth factor and neurotrophic models: 50 years on

Viktor Hamburger has just died at the age of 100. It is 50 years since he and Rita Levi-Montalcini laid the foundations for the study of naturally occurring cell death and of neurotrophic factors in the nervous system. In a period of less than 10 years, from 1949 to 1958, Hamburger and Levi-Montalcini made the following seminal discoveries: that neuron cell death occurs in dorsal root ganglia, sympathetic ganglia and the cervical column of motoneurons; that the predictions arising from this observation, namely that survival is dependent on the supply of a trophic factor, could be substantiated by studying the effects of a sarcoma on the proliferation of ganglionic processes both in vivo and in vitro; and that the proliferation of these processes could be used as an assay system to isolate the factor. This work provides a short review mostly of the early history of this subject in the context of the Hamburger/Levi-Montalcini paradigm. This acts as an introduction to a consideration of models that have been proposed to account for how the different sources of growth factors provide for the survival of neurons during development. It is suggested that what has been called the 'social-control' model provides the most parsimonious quantitative description of the contribution of trophic factors to neuronal survival, a concept for which we are in debt to Viktor Hamburger and Rita Levi-Montalcini.

Axonal regeneration of retinal ganglion cells: effect of trophic factors

A variety of neurotrophic factors can influence the cell functions of the developing, mature and injured retinal ganglion cells. The discovery that retinal ganglion cell loss can be alleviated by neurotrophic factors has generated a great deal of interest in the therapeutic potential of these molecules. Recently, evidence has provided valuable information on the receptors that mediate these events and the intracellular signaling cascades after the binding of these ligands. Signaling by neurotrophic factors does not seem to restrict to retrograde messenger from the target but also includes local interactions with neighbouring cells along the axonal pathways, anterograde signaling from the afferents and autocrine signaling. More insight into the mechanisms of action of neurotrophic factors and the signal transduction pathway leading to the protection and regeneration of retinal ganglion cells may allow the design of new therapeutic strategies.

Patterns of chondroitin sulfate immunoreactivity in the developing tectum reflect regional differences in glycosaminoglycan biosynthesis

The glycosaminoglycan chondroitin sulfate (CS) is expressed in many parts of the developing brain, both in regions where axons preferentially grow and in areas that axons distinctly avoid. Some in vitro studies suggest that CS and proteoglycans (PGs) that carry CS enhance axon growth, whereas others suggest that CS and CSPGs inhibit it. In the developing hamster, there is evidence that midbrain raphe cells act as a barrier to prevent growth of optic axons across the tectal midline. Here we show that in the newborn hamster, CS immunoreactivity is substantially higher in midline than in lateral tectum, raising the possibility that CSPGs play a role in the unilateral containment of optic axons. However, analysis of tectal PGs by anion exchange chromatography and denaturing gel electrophoresis failed to detect substantial differences between midline and lateral tectum in either the types or relative amounts of CSPG and heparan sulfate PG protein cores. In contrast, metabolic labeling of tectal slices in vitro documented that incorporation of 35S-sulfate into macromolecules is significantly increased at the tectal midline, in a pattern resembling chondroitin sulfate immunoreactivity. This difference was evident whether slices were labeled for 1 hr or overnight and was not paralleled by a difference in overall protein synthesis, suggesting that the rate of synthesis of sulfated macromolecules is specifically elevated in midline tectum. We propose that the concentration of CS at the midline of the developing tectum is a reflection of a higher rate of synthesis or sulfation of glycosaminoglycans by midline cells, rather than a higher level of production of any particular CSPG. These results suggest that the distribution of some axon guidance signals in development may be controlled by differential regulation of glycosaminoglycan biosynthetic enzymes.

Double staining of proteoglycans and the HNK-1 carbohydrate epitope in pseudoexfoliation material

PURPOSE

We combined the techniques of both immunogold and cupromeronic-blue staining with electron microscopy to investigate the possible relationship between proteoglycans and the HNK-1 carbohydrate epitope in pseudoexfoliation material.

METHODS

The anterior segment was obtained from an enucleated eye with pseudoexfoliation syndrome. After cupromeronic-blue staining, following the method of Scott, specimens were embedded in L. R. White resin. Ultrathin sections were incubated successively in primary monoclonal antibodies, and 10 nm gold-conjugated secondary antibody.

RESULTS

The present study showed simultaneous presence of the HNK-1 epitope and proteoglycans associated with zonular fibrils and pseudoexfoliation material on the posterior surface of the iris. The HNK-1 epitope was immunolocalized to the periphery of the pseudoexfoliation fibers, while cupromeronic blue-positive filaments bridged adjacent pseudoexfoliation fibers.

CONCLUSIONS

Proteoglycans and the HNK-1 epitope might play a substantial role in the formation of pseudoexfoliation material via their adhesive function. Proteoglycans probably do not directly bear the HNK-1 epitope in pseudoexfoliation material.

Immunohistochemical localization of neurocan in the lower auditory nuclei of the dog

Chondroitin sulfate proteoglycans are present at high levels in the lower auditory system of mammals. Axon terminals on the principal neurons in the superior olivary nuclei contain chondroitin 4- and 6-sulfate, while the broad extracellular matrix around axon terminals contains chondroitin sulfate D, a highly sulfated chondroitin sulfate rich in the disaccharide unit of GlcA(2S)beta1 --> 3GalNAc(6S), in the dog. In the present study, we investigated the immunohistochemical staining of neurocan, a brain-specific proteoglycan, in the lower auditory tract of the dog, including an analysis by immunoelectron microscopy. Immunolocalization of neurocan was conspicuous in the medial and lateral superior olivary nuclei and much less intense immunostaining was seen in the cochlear nucleus and posterior colliculus. No immunoreactivity were found in other nuclei. The immunostaining in the medial and lateral superior olivary nuclei was observed as perineuronal nets around large principal neurons at the light-microscopic level, while no immunostaining was observed in the upper segment of the medial superior olivary nucleus and the medial segment of the lateral superior olivary nucleus, in which medium-sized and small neurons were located. Immunoelectron microscopy revealed the reaction products of immunostaining on cell membranes of the perikarya of principal neurons and on cell membranes of presynaptic terminals which made axo-somatic synapses on the principal cells. No immunoreactivity was detected at synaptic junctions, in the extracellular matrix or within axon terminals. In the cochlear nucleus, immunoreactive perineuronal nets were found around a small number of neurons and immunoreactive nerve fibers were scattered in the anterior ventral cochlear nucleus. In the posterior colliculus, perineuronal nets, which were weakly immunostained, were sparsely distributed in the central nucleus. These results suggest that different locations of chondroitin sulfate proteoglycans, including neurocan, may be associated with focal sites composed of neuronal surface, terminal boutons and extracellular matrix in the lower auditory tract of the adult dog.

Selective early innervation of a subset of epidermal cells in Xenopus may be mediated by chondroitin sulfate proteoglycans

The epidermis of early Xenopus embryos is innervated by the Rohon-Beard (RB) neurons lying within the spinal cord and by extramedullary (EM) neurons lying outside of the cord. We have examined the innervation patterns of the three epidermal cell types using wholemount preparations of skin double-labelled with the HNK-1 antibody as a marker for neurons and with antibodies to chondroitin sulfate proteoglycan (CSPG). Cells of one of the three epidermal cell types, here termed conical cells, are innervated well before the other two. In wholemounts of embryonic skin incubated with antibodies to chondroitin-6-sulfate (C6S), all epidermal cells except conical cells show CSPG immunoreactivity in their basal lamina. Double-labelling of skin preparations with HNK-1 and anti-C6S confirmed that these conical cells which lack C6S immunoreactivity are the first to be innervated by RB axons. It is proposed that C6S-bearing proteoglycan initially inhibits innervation of cells whose basal lamina contain the proteoglycan, thus favoring innervation of the conical cells which lack it.

Developmentally regulated release of intraretinal neurotrophic factors in vitro

The effects of conditioned media either from aggregates or from explants of embryonic chick retinae and of recombinant neurotrophins were tested upon the survival in vitro of ganglion cells in dissociated cell cultures from the retina of newborn rats. Ganglion cells were identified by the detection of retrogradely transported horseradish peroxidase injected bilaterally into the superior colliculus. Conditioned media increased significantly the survival of ganglion cells after 2 days in culture, at a wide range of plating densities, and had no effect upon adhesion of rat retinal cells. Media conditioned by cell ensembles from chick retinae from embryonic day 8 (E8) to E16 had neurotrophic effects. Release of neurotrophic activity peaked at E10 E12, irrespective of the numbers of cells or total concentration of protein in the conditioned media. The active molecules were non-dialyzable and were released either in the presence or in the absence of fetal calf serum. The neurotrophic activity was abolished by trypsinization, and recovered by salting-out with 25 75% ammonium sulfate. NT-4, BDNF and, to a lesser extent, NT-3, increased the survival of ganglion cells in our assay, while NGF had no effect. The data show that chick retinal cells release soluble trophic proteins according to a developmentally regulated pattern. These neurotrophic factors may be involved in local competitive interactions that help control naturally occurring neuron death among ganglion cells of the vertebrate retina.

Immunoelectron microscopic localization of the HNK-1 carbohydrate epitope in the anterior segment of pseudoexfoliation and normal eyes

PURPOSE

To study the presence of the cell-adhesion related HNK-1 carbohydrate epitope in the anterior segment of pseudoexfoliation and normal eyes by immunoelectron microscopy.

METHODS

Anterior segment tissue of 6 autopsy eyes with pseudoexfoliation (PEX) syndrome (5 eyes without glaucoma and 1 with glaucoma), and 6 normal autopsy eyes without PEX syndrome were studied by an electron microscopic immunogold technique using a monoclonal antibody to the HNK-1 epitope.

RESULTS

In both normal and PEX eyes, the HNK-1 epitope could be immunolocalized to the basement membranes of both ciliary epithelia and posterior iris pigmented epithelium, to the lens capsule and zonular lamella, and to the pigmented epithelial cells of iris and ciliary body. Within the inner connective tissue layer of the ciliary body, the gold label was mainly associated with the periphery of elastic fibers and microfibrillar bundles. PEX material on the surfaces of posterior iris, ciliary body, anterior lens capsule, zonular fibers, and uveal part of the trabecular meshwork reacted strongly with the HNK-1 antibody. In contrast, PEX material accumulations within the iris stroma or the juxtacanalicular tissue of the trabecular meshwork showed only weak immunoreactivity, while PEX material in the conjunctiva was totally negative.

CONCLUSIONS

The wide distribution of the HNK-1 epitope in anterior segment tissues and its association with a variety of extracellular and cellular structures was ultrastructurally demonstrated. In PEX syndrome, the varying labelling density of PEX fibers indicates a deviating carbohydrate composition in different locations of the eye. The HNK-1 epitope might be involved in the adhesiveness of PEX deposits on intraocular surfaces.

Chondroitin/dermatan sulphate promotes the survival of neurons from rat embryonic neocortex

Recently we have shown that biglycan, a small chondroitin sulphate proteoglycan of the extracellular matrix, supports the survival of cultured neurons from the developing neocortex of embryonic day 15 rats. Here we investigate the structure-function relationship of this neurotrophic proteoglycan and show that chondroitin/dermatan sulphate chains are the active moieties supporting survival. Heparin, a highly sulphated glucosaminoglycan, is less active than the galactosaminoglycans (chondroitin-4-sulphate, chondroitin-6-sulphate and dermatan sulphate), whereas hyaluronic acid, an unsulphated glucosaminoglycan, does not support neuron survival. Galactosaminoglycans must be in direct contact with neurons to cause survival. Experiments with elevated potassium concentrations and antagonists of voltage-gated calcium channels exclude the involvement of membrane depolarization. However, genistein and an erbstatin analogue, which are inhibitors of tyrosine kinases with low specificity, abolished neuron survival in the presence of chondroitin/dermatan sulphate, whereas a selective inhibitor of neurotrophin receptor kinases (K252a) had no suppressive effect. Thus, yet unidentified tyrosine kinases are involved in the chondroitin/dermatan sulphate-dependent survival of neocortical neurons. In the embryonic stages of rat neocortical development chondroitin sulphate is mainly located in layers I, V and VI and the subplate. Chondroitin sulphate expression is maintained after birth, extends up to cortical layer IV on postnatal day 7, and is down-regulated until postnatal day 21 concomitant with the period of naturally occurring cell death. The latter observation is consistent with a putative role of chondroitin sulphate in the control of neuron survival during cortical histogenesis.

Cultured astrocytes express biglycan, a chondroitin/dermatan sulfate proteoglycan supporting the survival of neocortical neurons

Astrocyte-conditioned medium (ACM) supports the survival of rat E15 neocortical neurons. Using a microtiter assay for neuronal survival, we demonstrated that part of the survival activity is associated with a proteoglycan fraction obtained after two chromatographic steps: (1) preparative Q-Sepharose anion-exchange chromatography under non-denaturating conditions and (2) MonoQ chromatography in the presence of 8 M urea. Analytical SDS-polyacrylamide gradient gel electrophoresis of pooled active MonoQ-fractions (MQ-pool) revealed a broad proteoglycan band migrating with an apparent M(r) in the range of 150-400 kDa. Digestion of the MQ-pool with chondroitin-ABC-lyase yielded a major core protein of 50 kDa. In Western blots the high molecular weight (150-400 kDa) material as well as the 50 kDa core protein band were immunoreactive to chicken polyclonal antibodies raised against purified biglycan from rat meningeal fibroblasts. Northern blot analysis of total RNA prepared from highly enriched astrocyte cultures revealed a single 2.9 kb biglycan transcript. By using in situ hybridization we demonstrated that essentially all cells in these cultures expressed biglycan mRNA. Furthermore, highly purified biglycan from bovine cartilage was shown to markedly enhance survival of rat neocortical neurons. In conclusion, we have shown that astrocytes synthesize and release the small chondroitin/dermatan sulfate proteoglycan (CS/DSPG) biglycan, a molecule that was found to support survival of neocortical neurons in vitro.

Neuronal cell death in the mammalian nervous system: the calmortin hypothesis

1. This review is concerned with the calcium-dependent mechanisms involved in neuronal cell death. To this end, it provides definitions of the major types of cell death and then describes what is known of their occurrence during development and degeneration of the mammalian nervous system. 2. An analysis is presented of the different sources and compartments of calcium in neurons and of how these are related to the known calcium-dependent enzymes whose excess activation will lead to cell death. 3. The review uses the relatively large amount of pertinent information now available for other cell types, especially thymocytes, to reveal our limited knowledge of how calcium controls neuronal cell death. 4. In the final section, consideration is given to the identification of those factors that may mitigate against the calcium-dependent pathways leading to neuronal degeneration.

Persistent correlation between expression of a sulfated carbohydrate antigen and adrenergic differentiation in cultures of quail trunk neural crest cells

The carbohydrate antigen recognized by monoclonal antibodies such as HNK-1 (first characterized as recognizing human natural killer cells) and NC-1 (raised against quail neural-crest-derived cells) is found on a number of molecules associated with cell differentiation in vertebrates [42]. Previous work has shown that subpopulations of cultured quail trunk neural crest cells can be separated by fluorescence-activated cell sorting (FACS) on the basis of expression of this carbohydrate antigen. When neural crest cells are separated after 2 days in culture, adrenergic cells develop preferentially within the HNK-1-reactive subpopulation [27]. We wished to investigate whether the capacity for adrenergic differentiation remained associated with the HNK-1-positive cell population at later times in vitro, when the percentage of HNK-1-reactive cells has declined. The present study found that neural crest cells separated according to HNK-1-reactivity after 4 days in culture also showed preferential development of adrenergic cells in HNK-1-positive-enriched cultures, indicating that the HNK-1 epitope is persistently expressed in vitro on cells with adrenergic potential after 4 days of culture. To investigate the possible function of this epitope in development of the adrenergic phenotype, HNK-1 was added to unsorted neural crest cell cultures. The presence of antibody resulted in a decrease in the percentage of HNK-1-reactive cells during the initial 24 h after replating, but had no effect on the number of catecholamine-positive cells which developed after 7 days. We conclude that the epitope recognized by the HNK-1 antibody does not appear to function in the induction of the adrenergic phenotype. However, this antigenic determinant is useful as a predictive early marker which defines a subset of neural crest cells that includes those with the ability to undergo adrenergic differentiation.

NADPH diaphorase expression in the rat retina after axotomy--a supportive role for nitric oxide

The large majority of mammalian retinal ganglion cells degenerate following section of their axons in the optic nerve. It has been suggested that some axotomized retina ganglion cells die because of toxic agents produced within their immediate environment. Our hypothesis was that nitric oxide might be one of the toxic factors implicated in the death of adult retinal ganglion cells post-axotomy. In the first instance, we determined whether there were any changes in the retinal expression of NADPH diaphorase both 3 and 14 days following intraorbital section of the optic nerve in adult rats. Secondly, if nitric oxide was indeed implicated in the death of ganglion cells, then trophic factors which rescue these neurons might do so by decreasing the expression of nitric oxide synthase. Recently, we found that a collicular proteoglycan purified from the major target of retinal ganglion cells, the superior colliculus, rescued a greater proportion of adult ganglion cells from axotomy-induced death than most other known trophic factors. We thus injected this proteoglycan intraocularly after section of the optic nerve and examined its effect on the expression of NADPH diaphorase in the retina. Thirdly, an inhibitor of nitric oxide synthetase was repeatedly injected into the eye following the section of the optic nerve in order to determine if such a treatment might improve the survival of retinal ganglion cells. The present results indicate that section of the optic nerve does not alter the overall levels of NADPH diaphorase within the adult rat retina. Intraocular injections of the collicular proteoglycan actually increased the number of neurons expressing NADPH diaphorase, particularly in the ganglion cell layer. Finally, inhibition of nitric oxide synthetase following axotomy resulted in increased loss of retinal ganglion cells over a 2 week period when compared with controls. Our findings indicate that, rather than being toxic, small amounts of nitric oxide may be important for the survival of a proportion of injured retina ganglion cells.

Nitric oxide-mediated death of cultured neonatal retinal ganglion cells: neuroprotective properties of glutamate and chondroitin sulfate proteoglycan

The release of nitric oxide and stimulation of glutamate receptors by excitatory amino acids has been linked to neuronal degeneration and toxicity. In the rat retina approximately 60% of retinal ganglion cells (RGCs) die during the first postnatal week. In this study we examined the effects of nitric oxide synthase blockers and glutamate on the survival of neonatal RGCs in vitro over a 16 h assay period. Less than 10% of P1 RGCs survived in serum free defined media alone (control), however survival was increased, in a dose-dependent manner, when L-glutamate (10 microM-10 mM) was added to the media; a maximum of 70% of RGCs could be maintained with the addition of 5 mM glutamate. This effect was blocked by the NMDA and non-NMDA receptor blockers APV and DNQX and was age dependent; the survival of RGCs from P5 but not P7 rats was enhanced by the addition of glutamate even in high calcium concentrations (10 mM). When the nitric oxide synthase blockers L-NAME (5 mM) or haemoglobin (25 microM) were added to the culture media, up to 61% of P1 RGCs survived. The addition of the 480 kDa chondroitin sulfate proteoglycan (SCCP) previously shown to enhance RGC survival in vivo and in vitro, potentiated the action of glutamate and L-NAME and increased RGC survival to over 90% with almost all RGCs expressing a profusion of processes. These results suggest that the release of nitric oxide and glutamate by cells within the retina may contribute to the regulation of RGC numbers in vivo during development.

Boundaries and inhibitory molecules in developing neural tissues

Numerous studies of the past decade have illuminated the importance of intercellular adhesion events for neural pattern formation. It has been documented that members of the Ig and cadherin gene superfamilies, that glycoproteins and, probably to some extent, proteoglycans of the extracellular matrix play a role in this context. Recent observations suggest that, in addition to adhesive interactions, repulsive and/or inhibitory phenoma are also of importance in regulating neural pattern formation. Several molecules are under study which are considered possible mediators of inhibitory interactions in the nervous system. The hypothesis has been advanced that some of these might be partially responsible for restrictive, boundary-like properties ascribed to glial cells in developing and regenerating tissues. The current review summarizes these studies and focusses on molecular aspects of boundary and compartmentation phenomena.

Trophic effect of collicular proteoglycan on neonatal rat retinal ganglion cells in situ

Naturally occurring neuronal death is widespread in the central nervous system of mammals. To date, the causes and mechanisms of such death are poorly understood. A major hypothesis is that developing neurons compete for limited amounts of trophic factor(s) released from their target centres as in the case of the peripheral nervous system and nerve growth factor. The present study aims to test this 'trophic hypothesis' in the mammalian central nervous system. In the rat, more than 50% of retinal ganglion cells die in the early post-natal period. Schulz and coworkers [57] purified a potential trophic agent from their major target, the superior colliculus, which was identified as a 480 kDa chondroitin sulfate proteoglycan. This proteoglycan or control solutions were injected into the eyes of rat pups during the post-natal part of the period of naturally occurring ganglion cell death. It was found that the collicular proteoglycan prevented the death of a significant number of the ganglion cells that would normally have been lost over a post-injection period of one or two days. The effect of the proteoglycan was dose- and time-dependent. These results support the notion that trophic interactions are a determining factor in the survival of retinal ganglion cells during the period of naturally occurring cell death. It is also the first time that a proteoglycan has been shown to possess neurotrophic properties in situ.

Effect of collicular proteoglycan on the survival of adult rat retinal ganglion cells following axotomy

Consistent with numerous previous studies, we have found that in adult rats 29% of cells retrogradely prelabelled by injections into retino-recipient nuclei are lost 1 week after intraorbital section of the optic nerve. This figure increases to 76% 2 weeks after axotomy. Intraocular injections of 150 ng of 480 kDa chondroitin sulphate proteoglycan purified from the superior colliculi of neonatal rats were performed every third day after axotomy. This procedure resulted in the loss of only 3 and 28% of the axotomized retinal ganglion cells 7 and 14 days respectively after optic nerve section. Intraocular injections of chondroitin sulphate type C, one of the sugar types present on the collicular proteoglycan, also resulted in a significant saving of axotomized ganglion cells (with the loss of only 48% 14 days after optic nerve lesion). These findings suggest that the collicular proteoglycan, and to a lesser extent its sugar moieties, substantially slows down the degeneration of adult retinal ganglion cells following axotomy.

The pattern of expression of a 13-kDa protein (CS-32 antigen) in the retina and its target nuclei in developing and mature rats

A polyclonal antibody (designated CS-32) immunopurifies a 13-kDa protein from the superior colliculus (SC) of neonatal rat. There is a strikingly inverse temporal relationship in expression of the 13-kDa protein in the developing retina and its principal target nuclei, SC and the dorsal lateral geniculate nucleus. The 13-kDa is a cell-surface protein expressed exclusively by neurones and is clearly associated with synapses.