Immunocytochemical characterization of reactive optic nerve astrocytes and meningeal cells

Remodeling of the Lamina Cribrosa: Mechanisms and Potential Therapeutic Approaches for Glaucoma

Glaucomatous optic neuropathy is the leading cause of irreversible blindness in the world. The chronic disease is characterized by optic nerve degeneration and vision field loss. The reduction of intraocular pressure remains the only proven glaucoma treatment, but it does not prevent further neurodegeneration. There are three major classes of cells in the human optic nerve head (ONH): lamina cribrosa (LC) cells, glial cells, and scleral fibroblasts. These cells provide support for the LC which is essential to maintain healthy retinal ganglion cell (RGC) axons. All these cells demonstrate responses to glaucomatous conditions through extracellular matrix remodeling. Therefore, investigations into alternative therapies that alter the characteristic remodeling response of the ONH to enhance the survival of RGC axons are prevalent. Understanding major remodeling pathways in the ONH may be key to developing targeted therapies that reduce deleterious remodeling.

Biomaterial and Therapeutic Approaches for the Manipulation of Macrophage Phenotype in Peripheral and Central Nerve Repair

Injury to the peripheral or central nervous systems often results in extensive loss of motor and sensory function that can greatly diminish quality of life. In both cases, macrophage infiltration into the injury site plays an integral role in the host tissue inflammatory response. In particular, the temporally related transition of macrophage phenotype between the M1/M2 inflammatory/repair states is critical for successful tissue repair. In recent years, biomaterial implants have emerged as a novel approach to bridge lesion sites and provide a growth-inductive environment for regenerating axons. This has more recently seen these two areas of research increasingly intersecting in the creation of 'immune-modulatory' biomaterials. These synthetic or naturally derived materials are fabricated to drive macrophages towards a pro-repair phenotype. This review considers the macrophage-mediated inflammatory events that occur following nervous tissue injury and outlines the latest developments in biomaterial-based strategies to influence macrophage phenotype and enhance repair.

Oligodendrocyte Lineage Marker Expression in eGFP-GFAP Transgenic Mice

Oligodendrocytes, the myelinating cells of the central nervous system, orchestrate several key cellular functions in the brain and spinal cord, including axon insulation, energy transfer to neurons, and, eventually, modulation of immune responses. There is growing interest for obtaining reliable markers that can specifically label oligodendroglia and their progeny. In many studies, anti-CC1 antibodies, presumably recognizing the protein adenomatous polyposis coli (APC), are used to label mature, myelinating oligodendrocytes. However, it has been discussed whether anti-CC1 antibodies could recognize as well, under pathological conditions, other cell populations, particularly astrocytes. In this study, we used transgenic mice in which astrocytes are labeled by the enhanced green fluorescent protein (eGFP) under the control of the human glial fibrillary acidic protein (GFAP) promoter. By detailed co-localization studies we were able to demonstrate that a significant proportion of eGFP-expressing cells co-express markers of the oligodendrocyte lineage, such as the transcription factor Oligodendrocyte Transcription Factor 2 (OLIG2); the NG2 proteoglycan, also known as chrondroitin sulfate proteoglycan 4 (CSPG4); or APC. The current finding that the GFAP promoter drives transgene expression in cells of the oligodendrocyte lineage should be considered when interpreting results from co-localization studies.

Ischemic injury leads to extracellular matrix alterations in retina and optic nerve

Retinal ischemia occurs in a variety of eye diseases. Restrained blood flow induces retinal damage, which leads to progressive optic nerve degeneration and vision loss. Previous studies indicate that extracellular matrix (ECM) constituents play an important role in complex tissues, such as retina and optic nerve. They have great impact on de- and regeneration processes and represent major candidates of central nervous system glial scar formation. Nevertheless, the importance of the ECM during ischemic retina and optic nerve neurodegeneration is not fully understood yet. In this study, we analyzed remodeling of the extracellular glycoproteins fibronectin, laminin, tenascin-C and tenascin-R and the chondroitin sulfate proteoglycans (CSPGs) aggrecan, brevican and phosphacan/RPTPβ/ζ in retinae and optic nerves of an ischemia/reperfusion rat model via quantitative real-time PCR, immunohistochemistry and Western blot. A variety of ECM constituents were dysregulated in the retina and optic nerve after ischemia. Regarding fibronectin, significantly elevated mRNA and protein levels were observed in the retina following ischemia, while laminin and tenascin-C showed enhanced immunoreactivity in the optic nerve after ischemia. Interestingly, CSPGs displayed significantly increased expression levels in the optic nerve. Our study demonstrates a dynamic expression of ECM molecules following retinal ischemia, which strengthens their regulatory role during neurodegeneration.

Early remodelling of the extracellular matrix proteins tenascin-C and phosphacan in retina and optic nerve of an experimental autoimmune glaucoma model

Glaucoma is characterized by the loss of retinal ganglion cells (RGCs) and optic nerve fibres. Previous studies noted fewer RGCs after immunization with ocular antigens at 28 days. It is known that changes in extracellular matrix (ECM) components conduct retina and optic nerve degeneration. Here, we focused on the remodelling of tenascin‐C and phosphacan/receptor protein tyrosine phosphatase β/ζ in an autoimmune glaucoma model. Rats were immunized with optic nerve homogenate (ONA) or S100B protein (S100). Controls received sodium chloride (Co). After 14 days, no changes in RGC number were noted in all groups. An increase in GFAPmRNA expression was observed in the S100 group, whereas no alterations were noted via immunohistochemistry in both groups. Extracellular matrix remodelling was analyzed after 3, 7, 14 and 28 days. Tenascin‐C and 473HD immunoreactivity in retinae and optic nerves was unaltered in both immunized groups at 3 days. At 7 days, tenascin‐C staining increased in both tissues in the ONA group. Also, in the optic nerves of the S100 group, an intense tenascin‐C staining could be shown. In the retina, an increased tenascin‐C expression was also observed in ONA animals via Western blot. 473HD immunoreactivity was elevated in the ONA group in both tissues and in the S100 optic nerves at 7 days. At 14 days, tenascin‐C and 473HD immunoreactivity was up‐regulated in the ONA retinae, whereas phosphacan expression was up‐regulated in both groups. We conclude that remodelling of tenascin‐C and phosphacan occurred shortly after immunization, already before RGC loss. We assume that both ECM molecules represent early indicators of neurodegeneration.

Defects in neural guidepost structures and failure to remove leptomeningeal cells from the septal midline behind the interhemispheric fusion defects in Netrin1 deficient mice

Corpus callosum (CC) is the largest commissural tract in mammalian brain and it acts to coordinate information between the two cerebral hemispheres. During brain development CC forms at the boundary area between the cortex and the septum and special transient neural and glial guidepost structures in this area are thought to be critical for CC formation. In addition, it is thought that the fusion of the two hemispheres in the septum area is a prerequisite for CC formation. However, very little is known of the molecular mechanisms behind the fusion of the two hemispheres. Netrin1 (NTN1) acts as an axon guidance molecule in the developing central nervous system and Ntn1 deficiency leads to the agenesis of CC in mouse. Here we have analyzed Ntn1 deficient mice to better understand the reasons behind the observed lack of CC. We show that Ntn1 deficiency leads to defects in neural, but not in glial guidepost structures that may contribute to the agenesis of CC. In addition, Nnt1 was expressed by the leptomeningeal cells bordering the two septal walls prior to fusion. Normally these cells are removed when the septal fusion occurs. At the same time, the Laminin containing basal lamina produced by the leptomeningeal cells is disrupted in the midline area to allow the cells to mix and the callosal axons to cross. In Ntn1 deficient embryos however, the leptomeninges and the basal lamina were not removed properly from the midline area and the septal fusion did not occur. Thus, NTN1 contributes to the formation of the CC by promoting the preceding removal of the midline leptomeningeal cells and interhemispheric fusion.

Phenotype overlap in glial cell populations: astroglia, oligodendroglia and NG-2(+) cells

The extent to which NG-2(+) cells form a distinct population separate from astrocytes is central to understanding whether this important cell class is wholly an oligodendrocyte precursor cell (OPC) or has additional functions akin to those classically ascribed to astrocytes. Early immuno-staining studies indicate that NG-2(+) cells do not express the astrocyte marker GFAP, but orthogonal reconstructions of double-labeled confocal image stacks here reveal a significant degree of co-expression in individual cells within post-natal day 10 (P10) and adult rat optic nerve (RON) and rat cortex. Extensive scanning of various antibody/fixation/embedding approaches identified a protocol for selective post-embedded immuno-gold labeling. This first ultrastructural characterization of identified NG-2(+) cells revealed populations of both OPCs and astrocytes in P10 RON. NG-2(+) astrocytes had classic features including the presence of glial filaments but low levels of glial filament expression were also found in OPCs and myelinating oligodendrocytes. P0 RONs contained few OPCs but positively identified astrocytes were observed to ensheath pre-myelinated axons in a fashion previously described as a definitive marker of the oligodendrocyte lineage. Astrocyte ensheathment was also apparent in P10 RONs, was absent from developing nodes of Ranvier and was never associated with compact myelin. Astrocyte processes were also shown to encapsulate some oligodendrocyte somata. The data indicate that common criteria for delineating astrocytes and oligodendroglia are insufficiently robust and that astrocyte features ascribed to OPCs may arise from misidentification.

Efficient uptake and dissemination of scrapie prion protein by astrocytes and fibroblasts from adult hamster brain

Prion infections target neurons and lead to neuronal loss. However, the role of non-neuronal cells in the initiation and spread of infection throughout the brain remains unclear despite the fact these cells can also propagate prion infectivity. To evaluate how different brain cells process scrapie prion protein (PrPres) during acute infection, we exposed neuron-enriched and non-neuronal cell cultures from adult hamster brain to fluorescently-labeled purified PrPres and followed the cultures by live cell confocal imaging over time. Non-neuronal cells present in both types of cultures, specifically astrocytes and fibroblasts, internalized PrPres more efficiently than neurons. PrPres was trafficked to late endosomal/lysosomal compartments and rapidly transported throughout the cell bodies and processes of all cell types, including contacts between astrocytes and neurons. These observations suggest that astrocytes and meningeal fibroblasts play an as yet unappreciated role in prion infections via efficient uptake and dissemination of PrPres.

Glycogen synthase kinase 3 beta (GSK3β) at the tip of neuronal development and regeneration

Gaining a basic understanding of the inhibitory molecules and the intracellular signaling involved in axon development and repulsion after neural lesions is of clear biomedical interest. In recent years, numerous studies have described new molecules and intracellular mechanisms that impair axonal outgrowth after injury. In this scenario, the role of glycogen synthase kinase 3 beta (GSK3β) in the axonal responses that occur after central nervous system (CNS) lesions began to be elucidated. GSK3β function in the nervous tissue is associated with neural development, neuron polarization, and, more recently, neurodegeneration. In fact, GSK3β has been considered as a putative therapeutic target for promoting functional recovery in injured or degenerative CNS. In this review, we summarize current understanding of the role of GSK3β during neuronal development and regeneration. In particular, we discuss GSK3β activity levels and their possible impact on cytoskeleton dynamics during both processes.

Neurotrophic factors and the regeneration of adult retinal ganglion cell axons

The adult central nervous system (CNS) has only a limited capacity to regenerate axons after injury. This is due to a number of factors including the presence of extrinsic inhibitory factors that limit plasticity, lack of effective trophic support, and intrinsic changes in neuronal responsiveness. In this review, we describe the expression and role of neurotrophins in retinal ganglion cells (RGCs) during development and adulthood, and the receptors and miscellaneous signaling systems that influence axonal regeneration after injury. The impact of exogenous neurotrophic factors on adult RGCs injured at different sites in the visual pathway is described for several modes of delivery, including recombinant factors, viral vectors, cell transplantation, as well as combinatorial treatments involving other pharmacotherapeutic agents. Indirect, off-target effects of neurotrophic factors on RGC axonal regeneration are also considered. There remain unresolved issues relating to optimal delivery of neurotrophic factors, and we emphasize the need to develop safe, reliable methods for the regulation of exogenous supply of these factors to the injured CNS.

Analysis of combinatorial variability reveals selective accumulation of the fibronectin type III domains B and D of tenascin-C in injured brain

Tenascin-C (Tnc) is a multimodular extracellular matrix glycoprotein that is markedly upregulated in CNS injuries where it is primarily secreted by reactive astrocytes. Different Tnc isoforms can be generated by the insertion of variable combinations of one to seven (in rats) alternatively spliced distinct fibronectin type III (FnIII) domains to the smallest variant. Each spliced FnIII repeat mediates specific actions on neurite outgrowth, neuron migration or adhesion. Hence, different Tnc isoforms might differentially influence CNS repair. We explored the expression pattern of Tnc variants after cortical lesions and after treatment of astrocytes with various cytokines. Using RT-PCR, we observed a strong upregulation of Tnc transcripts containing the spliced FnIII domains B or D in injured tissue at 2-4 days post-lesion (dpl). Looking at specific combinations, we showed a dramatic increase of Tnc isoforms harboring the neurite outgrowth-promoting BD repeat with both the B and D domains being adjacent to each other. Isoforms containing only the axon growth-stimulating spliced domain D were also dramatically enhanced after injury. Injury-induced increase of Tnc proteins comprising the domain D was confirmed by Western Blotting and immunostaining of cortical lesions. In contrast, the FnIII modules C and AD1 were weakly modulated after injury. The growth cone repulsive A1A2A4 domains were poorly expressed in normal and injured tissue but the smallest isoform, which is also repellant, was highly expressed after injury. Expression of the shortest Tnc isoform and of variants containing B, D or BD, was strongly upregulated in cultured astrocytes after TGFbeta1 treatment, suggesting that TGFbeta1 could mediate, at least in part, the injury-induced upregulation of these isoforms. We identified complex injury-induced differential regulations of Tnc isoforms that may well influence axonal regeneration and repair processes in the damaged CNS.

Integrins mediate adhesion of medulloblastoma cells to tenascin and activate pathways associated with survival and proliferation

Medulloblastoma spreads by leptomeningeal dissemination rather than by infiltration that characterizes other CNS tumors, eg, gliomas. This study represents an initial attempt to identify both the molecules that mediate medulloblastoma adhesion to leptomeninges and the pathways that are key to survival and proliferation of tumor following adhesion. As a first step in molecule identification, we produced adhesion of D283 medulloblastoma cells to the extracellular matrix (ECM) of H4 glioma cells in vitro. Within this context, D283 cells preferentially expressed the alpha9 and beta1 integrin subunits; antibody and disintegrin blockade of alpha9 and beta1 binding eliminated the adhesion. The H4 ECM was enriched in tenascin, a binding partner for the alpha9beta1 integrin heterodimer. Purified tenascin-C supported D283 cell adhesion. The adhesion was blocked by antibodies to alpha9 and beta1 integrin. In vivo data were similar; immunohistochemistry of primary human medulloblastomas with leptomeningeal extension demonstrated increased expression of alpha9 and beta1 integrins as well as tenascin at the interface of brain and leptomeningeal tumor. These data suggest that tumor-cell expressions of alpha9 and beta1 integrins in combination with extracellular tenascin are necessary for medulloblastoma adhesion to the leptomeninges. As a first step in the identification of pathways that mediate survival and proliferation of tumor following adhesion, we demonstrated that adhesion to H4 ECM was associated with survival and proliferation of D283 cells as well as activation of the MAPK pathway in a growth factor deficient environment. Antibody blockade of alpha9 and beta1 integrin binding that eliminated adhesion also eliminated the in vitro survival benefit. These data suggest that adhesion of medulloblastoma to the meninges is necessary for the survival and proliferation of these tumor cells at the secondary site.

Evidence for distinct leptomeningeal cell-dependent paracrine and EGF-linked autocrine regulatory pathways for suppression of fibrillar collagens in astrocytes

A unique and unresolved property of the central nervous system is that its extracellular matrix lacks fibrillar elements. In the present report, we show that astrocytes secrete triple helices of fibrillar collagens type I, III and V in culture, while no astroglial collagen expression could be detected in vivo. We discovered two inhibitory mechanisms that could underlie this apparent discrepancy. Thus, we uncover a strong inhibitory effect of meningeal cells on astrocytic collagen expression in coculture assays. Furthermore, we present evidence that EGF-receptor activation downregulates collagen expression in astrocytes via an autocrine loop. These investigations provide a rational framework to explain why the brain is devoid of collagen fibers, which is a unique feature that characterizes the structure of the neural extracellular matrix. Moreover, fibrillar collagens were found transiently upregulated in a laser-induced cortical lesion, suggesting that these could contribute to the glial scar that inhibits axonal regeneration.

Semaphorins in axon regeneration: developmental guidance molecules gone wrong?

Semaphorins are developmental axon guidance cues that continue to be expressed during adulthood and are regulated by neural injury. During the formation of the nervous system, repulsive semaphorins guide axons to their targets by restricting and channelling their growth. They affect the growth cone cytoskeleton through interactions with receptor complexes that are linked to a complicated intracellular signal transduction network. Following injury, regenerating axons stop growing when they reach the border of the glial-fibrotic scar, in part because they encounter a potent molecular barrier that inhibits growth cone extension. A number of secreted semaphorins are expressed in the glial-fibrotic scar and at least one transmembrane semaphorin is upregulated in oligodendrocytes surrounding the lesion site. Semaphorin receptors, and many of the signal transduction components required for semaphorin signalling, are present in injured central nervous system neurons. Here, we review evidence that supports a critical role for semaphorin signalling in axon regeneration, and highlight a number of challenges that lie ahead with respect to advancing our understanding of semaphorin function in the normal and injured adult nervous system.

Chemorepellent axon guidance molecules in spinal cord injury

Regenerating axons stop growing when they reach the border of the glial-fibrotic scar, presumably because they encounter a potent molecular barrier inhibiting growth cone advance. Chemorepulsive axon guidance molecules provide a non-permissive environment restricting and channeling axon growth in the developing nervous system. These molecules could also act as growth-inhibitory molecules in the regenerating nervous system. The receptors for repulsive guidance cues are expressed in the mature nervous system, suggesting that adult neurons are sensitive to the activity of developmentally active repulsive proteins. In this review, we summarize recent observations on semaphorins, ephrins, and slits in the injured brain and spinal cord, providing evidence that these proteins are major players in inhibiting axonal regeneration and establishing the glial-fibrotic scar.

Inhibition of protease-resistant prion protein formation in a transformed deer cell line infected with chronic wasting disease

Chronic wasting disease (CWD) is an emerging transmissible spongiform encephalopathy (prion disease) of North American cervids, i.e., mule deer, white-tailed deer, and elk (wapiti). To facilitate in vitro studies of CWD, we have developed a transformed deer cell line that is persistently infected with CWD. Primary cultures derived from uninfected mule deer brain tissue were transformed by transfection with a plasmid containing the simian virus 40 genome. A transformed cell line (MDB) was exposed to microsomes prepared from the brainstem of a CWD-affected mule deer. CWD-associated, protease-resistant prion protein (PrP(CWD)) was used as an indicator of CWD infection. Although no PrP(CWD) was detected in any of these cultures after two passes, dilution cloning of cells yielded one PrP(CWD)-positive clone out of 51. This clone, designated MDB(CWD), has maintained stable PrP(CWD) production through 32 serial passes thus far. A second round of dilution cloning yielded 20 PrP(CWD)-positive subclones out of 30, one of which was designated MDB(CWD2). The MDB(CWD2) cell line was positive for fibronectin and negative for microtubule-associated protein 2 (a neuronal marker) and glial fibrillary acidic protein (an activated astrocyte marker), consistent with derivation from brain fibroblasts (e.g., meningeal fibroblasts). Two inhibitors of rodent scrapie protease-resistant PrP accumulation, pentosan polysulfate and a porphyrin compound, indium (III) meso-tetra(4-sulfonatophenyl)porphine chloride, potently blocked PrP(CWD) accumulation in MDB(CWD) cells. This demonstrates the utility of these cells in a rapid in vitro screening assay for PrP(CWD) inhibitors and suggests that these compounds have potential to be active against CWD in vivo.

MICAL flavoprotein monooxygenases: expression during neural development and following spinal cord injuries in the rat

MICALs comprise of a family of phylogenetically conserved, multidomain cytosolic flavoprotein monooxygenases. Drosophila (D-)MICAL binds the neuronal Sema1a receptor PlexA, and D-MICAL-PlexA interactions are required in vivo for Sema1a-induced axon repulsion. The biological functions of vertebrate MICAL proteins, however, remain unknown. Here, we describe three rodent MICAL genes and analyze their expression in the intact rat nervous system and in two models of spinal cord injury. MICAL-1, -2, and -3 expression patterns in the embryonic, postnatal, and adult nervous system support the idea that MICALs play roles in neural development and plasticity. In addition, MICAL expression is elevated in oligodendrocytes and in meningeal fibroblasts at sites of spinal cord injury but is unchanged in lesioned corticospinal tract neurons. Furthermore, we find that the selective monooxygenase inhibitor EGCG attenuates the repulsive effects of Sema3A and Sema3F in vitro, but not those of several other repulsive cues and substrates. These results implicate MICALs in neuronal regeneration and support the possibility of employing EGCG to attenuate Sema3-mediated axon repulsion in the injured spinal cord.

Expression of neurotrimin in the normal and injured adult human spinal cord

STUDY DESIGN

Neurotrimin (Ntm) is a member of the family of neural cell adhesion molecules. Its expression pattern suggests that Ntm promotes axonal fasciculation, guides nerve fibers to specific targets and stabilizes synapses as it accumulates coincident with synaptogenesis. Strong labeling of Ntm was observed in motor and sensory areas of the postnatal rat cortex. It is not known whether Ntm is present in adult human spinal cord (SC). In the present study, a monoclonal antibody specific for Ntm (1B1), is applied to the first study of the expression of Ntm in normal and injured adult human SC.

OBJECTIVE

(1) To investigate the expression pattern of Ntm in adult normal human SC, and (2) to observe the changes of Ntm expression after SC injury and compare the differences between normal and injured adult human SC.

METHODS

Human SC tissue was obtained from necropsies of patients with (n=5) and without (n=4) SC injury. The 1B1 Ntm monoclonal antibody was used for immunohistochemical staining on paraffin embedded sections with an ABC kit.

RESULTS

(1) In total, 12 slides were analyzed for each group from both cervical and thoracic levels. Motor neurons and Clarke's neurons and glial-like cells were mild to moderately positive in all uninjured SC specimens. (2) In injured SC, no staining was observed in the injury epicenter between two and three levels proximally and distally, but was detected five levels away. (3) In patients older than 67 years of age, Ntm-positive inclusions were present in the white matter of the SC with or without injury. (4) Some meningeal cells were strongly Ntm-positive, especially in the uninjured human SC.

CONCLUSION

Ntm is expressed by motor and Clarke's neurons and glial cells in uninjured human SC. The downregulation of Ntm in the injured SC suggests that its expression is regulated by afferent input.

Retinoic acid synthesis by a population of NG2-positive cells in the injured spinal cord

Retinoic acid (RA) promotes growth and differentiation in many developing tissues but less is known about its influence on CNS regeneration. We investigated the possible involvement of RA in rat spinal cord injury (SCI) using the New York University (NYU) impactor to induce mild or moderate spinal cord contusion injury. Changes in RA at the lesion site were determined by measuring the activity of the enzymes for its synthesis, the retinaldehyde dehydrogenases (RALDHs). A marked increase in enzyme activity occurred by day 4 and peaked at days 8-14 following the injuries. RALDH2 was the only detectable RALDH present in the control or injured spinal cord. The cellular localization of RALDH2 was identified by immunostaining. In the noninjured spinal cord, RALDH2 was detected in oligodendroglia positive for the markers RIP and CNPase. Expression was also intense in the arachnoid membrane surrounding the spinal cord. After SCI the increase in RALDH2 was independent of the RIP- and CNPase-positive cells, which were severely depleted. Instead, RALDH2 was present in a cell type not previously identified as capable of synthesizing RA, that expressed NG2 and that was negative for markers of astrocytes, oligodendroglia, microglia, neurons, Schwann cells and immature lymphocytes. We postulate that the RALDH2- and NG2-positive cells migrate into the injured sites from the adjacent arachnoid membrane, where the RALDH2-positive cells proliferate substantially following SCI. These findings indicate that close correlations exist between RA synthesis and SCI and that RA may play a role in the secondary events that follow acute SCI.

A novel primary culture technique for adult retina allows for evaluation of CNS axon regeneration in rodents

Unraveling the causes of regeneration failure in the adult injured CNS has remained a challenge in neurobiology. The notion that CNS neurons lose their regenerative potential during development has been challenged by the identification of several promoters of axon growth. Novel methods are required that allow to study and quantify interactions of molecular determinants, and to envisage future treatment applications. Here we report a novel, highly reproducible method for monitoring axonal regeneration of mature retinal ganglion cells (RGCs) in vitro. In contrast to earlier explantation methods, primary cultures derived from adult rodent retina are kept viable without growth factor supplements. Further, since intraretinal RGC axons remain unmyelinated, regeneration can be followed independently of non-permissive white matter compounds. Applying tracing techniques prior to retinal explantation, cell survival can be correlated to outgrowth activity on the single cell level. Following intervention with pharmacological, growth factor, or gene transfer treatments, retinal explants, and partially RGC neurites, can be processed for protein and gene expression analysis. This novel procedure will prove useful to get insight into complex cell survival and regeneration promoting cascades, and will complement in vivo strategies such as transgenic and knock out mouse models.

Myelin-, reactive glia-, and scar-derived CNS axon growth inhibitors: expression, receptor signaling, and correlation with axon regeneration

Axon regeneration is arrested in the injured central nervous system (CNS) by axon growth-inhibitory ligands expressed in oligodendrocytes/myelin, NG2-glia, and reactive astrocytes in the lesion and degenerating tracts, and by fibroblasts in scar tissue. Growth cone receptors (Rc) bind inhibitory ligands, activating a Rho-family GTPase intracellular signaling pathway that disrupts the actin cytoskeleton inducing growth cone collapse/repulsion. The known inhibitory ligands include the chondroitin sulfate proteoglycans (CSPG) Neurocan, Brevican, Phosphacan, Tenascin, and NG2, as either membrane-bound or secreted molecules; Ephrins expressed on astrocyte/fibroblast membranes; the myelin/oligodendrocyte-derived growth inhibitors Nogo, MAG, and OMgp; and membrane-bound semaphorins (Sema) produced by meningeal fibroblasts invading the scar. No definitive CSPG Rc have been identified, although intracellular signaling through the Rho family of G-proteins is probably common to all the inhibitory ligands. Ephrins bind to signalling Ephs. The ligand-binding Rc for all the myelin inhibitors is NgR and requires p75(NTR) for transmembrane signaling. The neuropilin (NP)/plexin (Plex) Rc complex binds Sema. Strategies for promoting axon growth after CNS injury are thwarted by the plethora of inhibitory ligands and the ligand promiscuity of some of their Rc. There is also paradoxical reciprocal expression of many of the inhibitory ligands/Rc in normal and damaged neurons, and NgR expression is restricted to a limited number of neuronal populations. All these factors, together with an incomplete understanding of the normal functions of many of these molecules in the intact CNS, presently confound interpretive acumen in regenerative studies.

Meningeal cell-derived semaphorin 3A inhibits neurite outgrowth

The neural scar that forms after injury to the mammalian central nervous system is a barrier to sprouting and regenerating axons. In addition to reactive astrocytes that are present throughout the lesion site, leptomeningeal fibroblasts invade the lesion core. When isolated in vitro, these cells form a very poor substrate for growing neurites, even more so than reactive astrocytes. Nevertheless the molecular mechanisms involved in this growth inhibition are not well understood. Semaphorins have been reported to be upregulated in meningeal cells (MCs) on mechanical injury to the brain and spinal cord. In the present study, we show that Sema3A mRNA and active protein are produced by cultured meningeal cells. A protein extract from these cells induces the collapse of embryonic dorsal root ganglion (DRG) growth cones. This collapsing activity is partially blocked by neuropilin-1 antibodies and is absent in meningeal cells derived from Sema3A-knockout mice. In addition to growth cone collapse, recombinant Sema3A but not Sema3C inhibits neurite outgrowth of embryonic DRGs. Consistent with this result we find that the inhibitory effect of meningeal cells on neurite outgrowth is partially overcome on Sema3A-deficient MCs. Furthermore we show that the inhibitory effect of MC-derived Sema3A on neurite outgrowth is modulated by nerve growth factor. Our results show that Sema3A, a chemorepellent during nervous system development, is a major neurite growth-inhibitory molecule in meningeal fibroblasts and is therefore likely to contribute to the inhibitory properties of the neural scar.

Remyelination in multiple sclerosis: a new role for neurotrophins?

Multiple sclerosis (MS) is a common neurological disease, which affects young adults. Its course is unpredictable and runs over decades. It is considered as an autoimmune disease, and is neuropathologically characterized by demyelination, variable loss of oligodendroglial cells, and axonal degeneration. Demyelination provides a permitting condition for axonal degeneration, which seems to be causative of permanent neurological deficits. Hence, the current treatment, which works preferentially immunmodulatory, should be complemented by therapeutics, which improves remyelination not only for restoring conduction velocity but also for preventing an irreversible axonal damage. One strategy to achieve this aim would be to promote remyelination by stimulating oligodendroglial cells remaining in MS lesions. While central nervous system neurons were already known to respond to neurotrophins (NT), interactions with glial cells became apparent more recently. In vitro and in vivo studies have shown that NT influence proliferation, differentiation, survival, and regeneration of mature oligodendrocytes and oligodendroglial precursors in favor of a myelin repair. Two in vivo models provided direct evidence that NT can improve remyelination. In addition, their neuroprotective and anti-inflammatory role would support a repair. Hence, a wealth of data point to NT as promising therapeutical candidates.

The astrocyte/meningeal cell interface is a barrier to neurite outgrowth which can be overcome by manipulation of inhibitory molecules or axonal signalling pathways

Invading meningeal cells form a barrier to axon regeneration after damage to the spinal cord and other parts of the CNS, axons stopping at the interface between meningeal cells and astrocytes. Axon behavior was examined using an in vitro model of astrocyte/meningeal cell interfaces, created by plating aggregates of astrocytes and meningeal cells onto coverslips. At these interfaces growth of dorsal root ganglion axons attempting to grow from astrocytes to meningeal cells was blocked, but axons grew rapidly from meningeal cells onto astrocytes. Meningeal cells were examined for expression of axon growth inhibitory molecules, and found to express NG2, versican, and semaphorins 3A and 3C. Astrocytes express growth promoting molecules, including N-Cadherin, laminin, fibronectin, and tenascin-C. We treated cultures in various ways to attempt to promote axon growth across the inhibitory boundaries. Blockade of NG2 with antibody and blockade of neuropilin 2 but not neuropilin 1 both promoted axon growth from astrocytes to meningeal cells. Blockade of permissive molecules on astrocytes with N-Cadherin blocking peptide or anti beta-1 integrin had no effect. Manipulation of axonal signalling pathways also increased axon growth from astrocytes to meningeal cells. Increasing cAMP levels and inactivation of rho were both effective when the cultures were fixed in paraformaldehyde, demonstrating that their effect is on axons and not via effects on the glial cells.

The chondroitin sulfate proteoglycans neurocan, brevican, phosphacan, and versican are differentially regulated following spinal cord injury

Chondroitin sulfate proteoglycans (CSPGs) are extracellular matrix (ECM) molecules that are widely expressed throughout the developing and adult CNS. In vitro studies demonstrate their potential to restrict neurite outgrowth, and it is believed that CSPGs also inhibit axonal regeneration after CNS injury in vivo. Previous studies demonstrated that CSPGs are generally upregulated after spinal cord injury, and more recent reports have begun to identify individual proteoglycans that may play dominant roles in limiting axonal regeneration. The current study systematically examined the extended deposition patterns after CNS injury of four putatively inhibitory CSPGs that have not been extensively investigated previously in vivo: neurocan, brevican, phosphacan, and versican. After spinal cord injury, neurocan, brevican, and versican immunolabeling increased within days in injured spinal cord parenchyma surrounding the lesion site and peaked at 2 weeks. Neurocan and versican were persistently elevated for 4 weeks postinjury, and brevican expression persisted for at least 2 months. On the other hand, phosphacan immunolabeling decreased in the same region immediately following injury but later recovered and then peaked after 2 months. Combined glial fibrillary acidic protein (GFAP) immunohistochemistry and in situ hybridization demonstrated that GFAP astrocytes constituted a source of neurocan production after spinal cord injury. Thus, the production of several CSPG family members is differentially affected by spinal cord injury, overall establishing a CSPG-rich matrix that persists for up to 2 months following injury. Optimization of strategies to reduce CSPG expression to enhance regeneration may need to target several different family members over an extended period following injury.

Changes in distribution, cell associations, and protein expression levels of NG2, neurocan, phosphacan, brevican, versican V2, and tenascin-C during acute to chronic maturation of spinal cord scar tissue

Previous studies have correlated the failure of axon regeneration after spinal cord injury with axons contacting scar tissue rich in chondroitin sulfate proteoglycans (CSPGs; Davies et al., 1999). In the present study, we have conducted immunohistochemical and quantitative Western blot analysis of five axon-growth-inhibitory CSPGs and tenascin-C within stab injuries of adult rat spinal cord at time points ranging from 24 hr to 6 months post injury. Quantitative Western blot analysis showed robust increases in neurocan, tenascin-C, and NG2 levels by 24 hr, suggesting that these molecules play a role in preventing axon regeneration across acutely forming scar tissue. Peak levels of 245/130 kD neurocan, NG2, and 250/200 kD tenascin-C were reached at 8 days, with maximum levels of phosphacan and 140/80 kD brevican attained later, at 1 month post injury. Versican V2 protein levels, however, displayed an opposite trend, dropping below unlesioned spinal cord values at all time points studied. Confocal microscopy at 8 days post injury revealed heightened immunoreactivity for phosphacan, NG2, and tenascin-C, particularly within fibronectin(+) scar tissue at lesion centers. In contrast, neurocan was displayed within lesion margins on the processes of stellate NG2(+) cells and, to a much lesser extent, by astrocytes. At 6 months post injury, 130 kD neurocan, brevican, and NG2 levels within chronic scar tissue remained significantly above control. Our results show novel expression patterns and cell associations of inhibitory CSPGs and tenascin-C that have important implications for axon regeneration across acute and chronic spinal cord scar tissue.

The extracellular matrix in axon regeneration

This review of ECM molecules shows quite clearly the function of the ECM in development but more importantly in the mature CNS after injury. Most of the proteoglycans, especially the large CS-PGs, are able to inhibit neurite outgrowth and in vivo experiments are now in progress to specifically inhibit these important molecules. The nature of growth promoter ECM molecules in the CNS after injury, either within or distant from the injury is now becoming better appreciated and we suggest that the laminin family should be important targets for exploration. Indeed, a better understanding of the interaction of laminin with those ECM components that are inhibitory is a clear goal for the future. Our ultimate aim must be to change the balance of factors at lesion sites to allow the more permissive environment after CNS injury to predominate.

Chondroitin sulphate proteoglycans in the CNS injury response

As the preceding discussion has demonstrated, experimental data now indicate that the expression of a number of different CSPGs is increased following CNS injury. The hyalectans neurocan, versican and [figure: see text] brevican, plus NG2 and phosphacan are upregulated following injury and all have been shown to exhibit inhibitory effects on neurite outgrowth in vitro. It is likely therefore that the increased expression of these molecules contributes to the non-permissive nature of the glial scar. The relative contributions of individual molecules remain, however, to be determined. It is important to remember also that not only does the glial scar contain many different inhibitory molecules, but that these are the products of a number of different cells, including not just astrocytes, but also oligodendrocyte progenitor and meningeal cells. It is arguable that the latter two cell types make a greater contribution than astrocytes to the inhibitory environment of the injured CNS. Recently, attempts have been made to alter the CSPG component of the glial scar in the hope that this will facilitate improved axonal regeneration. Three studies (Bradbury et al., 2002; Yick et al., 2000; Moon et al., 2001) have reported an improved regenerative response following treatment of the injured CNS with chondroitinase ABC. CSPGs represent a significant source of inhibition within the injured CNS; these studies indicate that successful CNS regeneration may be brought about by interventions which target these molecules and/or the cells which produce them.

Spinal cord injury elicits expression of keratan sulfate proteoglycans by macrophages, reactive microglia, and oligodendrocyte progenitors

Keratan sulfate proteoglycans (KSPGs) are extracellular matrix molecules that appear to establish boundaries for axonal growth in the developing brain and spinal cord. In vitro studies confirm that KSPGs define inhibitory boundaries to extending neurites. The aim of the current study was to investigate whether KSPGs are expressed after spinal cord injury (SCI) and thereby might act as potential inhibitors of axonal growth. Adult Fischer 344 rats were subjected to spinal cord lesions, and the temporal and spatial expression of KSPGs was examined using the 5D4 monoclonal anti-KSPG antibody. In the intact spinal cord, a subpopulation of microglia expressed 5D4-KSPG throughout the white and gray matter. Within 24 hr of injury, 5D4-KSPG immunoreactivity substantially increased and appeared on cellular profiles in close proximity to the spinal cord lesion site, peaking 3 d after injury. Double immunolabeling revealed that 5D4-KSPG expression arose from multiple cell types at the lesion site, including reactive microglia, macrophages, and oligodendrocyte progenitors. Astrocytes were not identified as a source of 5D4-KSPG. The robust and extensive production of 5D4-KSPG at sites of SCI precedes the expression of other putatively inhibitory proteoglycan molecules such as chondroitin sulfate proteoglycans. This is the first demonstration that KSPGs are expressed after SCI in a temporal and spatial relationship that could exert an early and important role in modulating axonal growth after SCI.

NG2 is a major chondroitin sulfate proteoglycan produced after spinal cord injury and is expressed by macrophages and oligodendrocyte progenitors

Several extracellular matrix (ECM) molecules have been identified as potent inhibitors of neurite outgrowth in vitro and are believed to limit axonal growth after CNS injury. Recent studies have shown that different members of the chondroitin sulfate proteoglycan (CSPG) class of putatively inhibitory ECM molecules are expressed after a number of CNS injuries. The purpose of this study was to evaluate the relative amounts of individual CSPGs expressed after spinal cord injury (SCI) and identify their cells of origin. Evaluation of total soluble CSPGs 2 weeks after dorsal column lesion in the rat demonstrated that NG2 is highly upregulated and is a major CSPG species. Immunocytochemical analysis further demonstrated that NG2 expression is upregulated within 24 hr of injury, peaks at 1 week, and remains elevated for at least an additional 7 weeks. NG2 expression results from a multicellular response to injury, including both reactive macrophages and oligodendrocyte progenitors; astrocytes were not identified as a major source of NG2. Immunocytochemical analysis of other CSPG family members 7 d after injury showed moderate upregulation of versican, brevican, and neurocan, and downregulation of phosphacan. Axonal tracing experiments demonstrated dense NG2 labeling adjacent to the forward processes of transected corticospinal tract axons in a spatial profile that could restrict axonal growth. Thus, NG2 is a major component of this putatively inhibitory class of ECM molecules expressed at sites of SCI and may restrict axonal regeneration.

Characterization of rat meningeal cultures on materials of differing surface chemistry

To better understand the interactions of cells derived from meningeal tissues with the surfaces of devices used for the treatment of central nervous system disorders, the behavior of primary postnatal day 1 rat meningeal cultures was evaluated on biomaterials of differing surface chemistry. Meningeal cultures in serum containing media were analyzed for attachment, spread cell area, proliferation, the production of extracellular matrix (ECM), and neuronal outgrowth. In general, both cell attachment as well as cell spread area decreased with increasing substrate hydrophobicity, whereas cell division as indicated by BrdU incorporation and time to confluence, was lower on the most hydrophobic materials. We suggest that such differences immediately after cell seeding were most likely mediated by differences in surface adsorption of proteins. In longer-term experiments, most of the materials were colonized by meningeal cultures irrespective of surface chemistry, and all cultures were equally inhibitory to neuronal outgrowth suggesting that over time, cells can modify the substrate perhaps by secretion of extracellular matrix molecule proteins. Our data suggests that cell type-specific differences in response to different biomaterials may play an important role in determining the ultimate nature and composition of the CNS at the host-biomaterial interface.

Expression of myocilin/TIGR in normal and glaucomatous primate optic nerves

Myocilin/TIGR was the first molecule discovered to be linked with primary open angle glaucoma (POAG), a blinding disease characterized by progressive loss of retinal ganglion cells. Mutations in myocilin/TIGR have been associated with age of disease onset and severity. The function of myocilin/TIGR and its role in glaucoma is unknown. Myocilin/TIGR has been studied in the trabecular meshwork to determine a role in regulation of intraocular pressure. The site of damage to the axons of the retinal ganglion cells is the optic nerve head (ONH). The myocilin/TIGR expression was examined in fetal through adult human optic nerve as well as in POAG. Myocilin/TIGR was expressed in the myelinated optic nerve of children and normal adults but not in the fetal optic nerve before myelination. Also examined was the expression in monkeys with experimental glaucoma. The results demonstrate that optic nerve head astrocytes constitutively express myocilin/TIGR in vivo in primates. Nevertheless, myocilin/TIGR is apparently reduced in glaucomatous ONH. The colocalization of myocilin/TIGR to the myelin suggests a role of myocilin/TIGR in the myelinated optic nerve.

Inactivation of the glial fibrillary acidic protein gene, but not that of vimentin, improves neuronal survival and neurite growth by modifying adhesion molecule expression

Intermediate filaments (IFs) are a major component of the cytoskeleton in astrocytes. Their role is far from being completely understood. Immature astrocytes play a major role in neuronal migration and neuritogenesis, and their IFs are mainly composed of vimentin. In mature differentiated astrocytes, vimentin is replaced by the IF protein glial fibrillary acidic protein (GFAP). In response to injury of the CNS in the adult, astrocytes become reactive, upregulate the expression of GFAP, and reexpress vimentin. These modifications contribute to the formation of a glial scar that is obstructive to axonal regeneration. Nevertheless, astrocytes in vitro are considered to be the ideal substratum for the growth of embryonic CNS axons. In the present study, we have examined the potential role of these two major IF proteins in both neuronal survival and neurite growth. For this purpose, we cocultured wild-type neurons on astrocytes from three types of knock-out (KO) mice for GFAP or/and vimentin in a neuron-astrocyte coculture model. We show that the double KO astrocytes present many features of immaturity and greatly improve survival and neurite growth of cocultured neurons by increasing cell-cell contact and secreting diffusible factors. Moreover, our data suggest that the absence of vimentin is not a key element in the permissivity of the mutant astrocytes. Finally, we show that only the absence of GFAP is associated with an increased expression of some extracellular matrix and adhesion molecules. To conclude, our results suggest that GFAP expression is able to modulate key biochemical properties of astrocytes that are implicated in their permissivity.

Retinal ganglion cell and nonneuronal cell responses to a microcrush lesion of adult rat optic nerve

Injury of the optic nerve has served as an important model for the study of cell death and axon regeneration in the CNS. Analysis of axon sprouting and regeneration after injury by anatomical tracing are aided by lesion models that produce a well-defined injury site. We report here the characterization of a microcrush lesion of the optic nerve made with 10-0 sutures to completely transect RGC axons. Following microcrush lesion, 62% of RGCs remained alive 1 week later, and 28% of RGCs, at 2 weeks. Optic nerve sections stained by hematoxylin-based methods showed a thin line of intensely stained cells that invaded the lesion site at 24 h after microcrush lesion. The lesion site became increasingly disorganized by 2 weeks after injury, and both macrophages and blood vessels invaded the lesion site. The microcrush lesion was immunoreactive for chondroitin sulfate proteoglycans (CSPG), and an adjacent GFAP-negative zone developed early after the lesion, disappearing by 1 week. Luxol fast blue staining showed a myelin-free zone at the lesion site, and myelin remained distal to the lesion at 8 weeks. To study the axonal response to microcrush lesion, anterograde tracing was used. Within 6 h after injury all RGC axons retracted back from the site of lesion. By 1 week after injury, axons regrew toward the lesion, but most stopped abruptly at the injury scar. The few axons that were able to cross the injury site did not extend further in the optic nerve white matter by 8 weeks postlesion. Our observations suggest that both the CSPG-positive scar and the myelin-derived growth inhibitory proteins contribute to the failure of RGC regeneration after injury.

BDNF increases the number of axotomized rat retinal ganglion cells expressing GAP-43, L1, and TAG-1 mRNA--a supportive role for nitric oxide?

The death of neurons and the limited ability to activate growth-associated genes prevent the restoration of lesioned fiber tracts in the adult mammalian CNS. Here, we characterized the effects of the survival-promoting neurotrophin brain-derived neurotrophic factor (BDNF) on mRNA expression of GAP-43, L1, TAG-1, and SC-1 in axotomized and regenerating rat retinal ganglion cells (RGCs). BDNF led to de novo upregulation of TAG-1 mRNA in axotomized RGCs and to a threefold increase in the number of GAP-43 and L1 mRNA-expressing RGCs. SC-1 expression remained unchanged. However, BDNF did not improve long-distance axon regeneration into a peripheral nerve graft. Surprisingly, potentiating BDNF-mediated neuroprotection by simultaneous administration of a spin trap or a NOS inhibitor counteracted the BDNF-induced growth-associated gene expression. This led us to hypothesize that the BDNF effects on GAP-43, L1, and TAG-1 mRNA expression are mediated by a NO-dependent mechanism. In summary, our data support the idea that survival and axon regeneration of lesioned CNS neurons can be regulated independently.

Peripheral nerve injury fails to induce growth of lesioned ascending dorsal column axons into spinal cord scar tissue expressing the axon repellent Semaphorin3A

We have investigated the hypothesis that the chemorepellent Semaphorin3A may be involved in the failure of axonal regeneration after injury to the ascending dorsal columns of adult rats. Following transection of the thoracic dorsal columns, fibroblasts in the dorsolateral parts of the lesion site showed robust expression of Semaphorin3A mRNA. In addition, dorsal root ganglion (DRG) neurons with projections through the dorsal columns to the injury site persistently expressed both Semaphorin3A receptor components, neuropilin-1 and plexin-A1. These ascending DRG collaterals failed to invade scar regions occupied by Semaphorin3A-positive fibroblasts, even in animals which had received conditioning lesions of the sciatic nerve to enhance regeneration. Other axon populations in the dorsal spinal cord were similarly unable to penetrate Semaphorin3A-positive scar tissue. These data suggest that Semaphorin3A may create an exclusion zone for regenerating dorsal column fibres and that enhancing the intrinsic regenerative response of DRG neurons has only limited effects on axonal regrowth. Tenascin-C and chondroitin sulphate proteoglycans were also detected at the injury site, which was largely devoid of central nervous system (CNS) myelin, showing that several classes of inhibitory factors, including semaphorins, with only partially overlapping spatial and temporal patterns of expression are in a position to participate in preventing regenerative axonal growth in the injured dorsal columns. Interestingly, conditioning nerve injuries enabled numerous ascending DRG axons to regrow across areas of strong tenascin-C and chondroitin sulphate proteoglycan expression, while areas containing Semaphorin3A and CNS myelin were selectively avoided by (pre)primed axonal sprouts.

Attempted endogenous tissue repair following experimental spinal cord injury in the rat: involvement of cell adhesion molecules L1 and NCAM?

It is widely accepted that the devastating consequences of spinal cord injury are due to the failure of lesioned CNS axons to regenerate. The current study of the spontaneous tissue repair processes following dorsal hemisection of the adult rat spinal cord demonstrates a phase of rapid and substantial nerve fibre in-growth into the lesion that was derived largely from both rostral and caudal spinal tissues. The response was characterized by increasing numbers of axons traversing the clearly defined interface between the lesion and the adjacent intact spinal cord, beginning by 5 days post operation (p.o.). Having penetrated the lesion, axons became associated with a framework of NGFr-positive non-neuronal cells (Schwann cells and leptomeningeal cells). Surprisingly few of these axons were derived from CGRP- or SP-immunoreactive dorsal root ganglion neurons. At the longest survival time (56 days p.o.), there was a marked shift in the overall orientation of fibres from a largely rostro-caudal to a dorso-ventral axis. Attempts to identify which recognition molecules may be important for these re-organizational processes during attempted tissue repair demonstrated the widespread and intense expression of the cell adhesion molecules (CAM) L1 and N-CAM. Double immunofluorescence suggested that both Schwann cells and leptomeningeal cells contributed to the pattern of CAM expression associated with the cellular framework within the lesion.

The optic nerve head in glaucoma: role of astrocytes in tissue remodeling

Primary open angle glaucoma is a common eye disease characterized by loss of the axons of the retinal ganglion cells leading to progressive loss of vision. The site of damage to the axons is at the level of the lamina cribrosa in the optic nerve head. The mechanism of axonal loss is unknown but elevated intraocular pressure and age are the most common factors associated with the disease. Previous studies in human glaucoma and in experimental glaucoma in monkeys have established a relationship between chronic elevation of intraocular pressure and remodeling of the optic nerve head tissues known clinically as cupping of the optic disc. This review focuses on the astrocytes, the major cell type in the optic nerve head. Astrocytes participate actively in the remodeling of neural tissues during development and in disease. In glaucomatous optic neuropathy, astrocytes play a major role in the remodeling of the extracellular matrix of the optic nerve head, synthesize growth factors and other cellular mediators that may affect directly, or indirectly, the axons of the retinal ganglion cells. Due to the architecture of the lamina cribrosa, formed by the cells and the fibroelastic extracellular matrix, astrocytes may respond to changes in intraocular pressure in glaucoma, leading to some of the detrimental events that underlie axonal loss and retinal ganglion cell degeneration.

Differential expression of neural cell adhesion molecule isoforms in normal and glaucomatous human optic nerve heads

Type 1B astrocytes of the human optic nerve head (ONH) constitutively express neural cell adhesion molecule (NCAM) in vivo and in vitro. Increased synthesis of NCAM has been detected in reactive astrocytes in the glaucomatous ONH of human donor eyes. Several NCAM isoforms are generated through alternate RNA splicing in tissue- and disease-specific patterns. In this study, we analyzed expression of NCAM isoforms in ONH of normal donors at different ages and in glaucoma. Total RNA was extracted from ONH of fetal, normal adult and glaucomatous eyes, and cultured human ONH astrocytes, fetal brain astrocytes and an astrocytoma cell line, for reverse transcriptase-polymerase chain reaction (RT-PCR) analysis. To distinguish between NCAM 180 and 140 isoforms, exon-specific primer sets covering exons 13-19 were used. Isoform-specific riboprobes were used for in situ hybridization (ISH) in glaucomatous and in age-matched ONH. By RT-PCR, NCAM 140 was the predominant isoform in adult ONH as well as in all cultured cells. NCAM 180 mRNA was strongly expressed in glaucoma, whereas in normal adult tissues it was not detectable. ISH confirmed expression of NCAM in normal adult ONH and localized NCAM 140 mRNA to astrocytes. ISH demonstrated expression of NCAM 180 mRNA in reactive astrocytes in glaucomatous ONH. Our results demonstrate that the NCAM 180 isoform is induced in glaucoma. NCAM 180 may play a role in astrocyte interaction with extracellular matrix (ECM), vessels, axons and other astrocytes and, through its expanded cytoplasmic domain, serve as a signaling molecule for reactive astrocytes during remodeling of the ONH in glaucoma.

Astrocytes prevent neuronal death induced by reactive oxygen and nitrogen species

Reactive oxygen and nitrogen species (RO/NS) such as nitric oxide (NO), hydroxyl radical (OH.), and superoxide anion (O(2)(-)) are generated in a variety of neuropathological processes and damage neurons. In the present study, we investigated the neuroprotective effects of rat astrocytes against RO/NS-induced damage using neuron-glia cocultures, and the effects were compared to those of microglial cells. Sodium nitroprusside (SNP), 3-morpholinosydnonimine (SIN-1), and FeSO(4) were used to generate NO, O(2)(-) and NO, and OH., respectively. Solely cultured neurons, which were transiently exposed to these agents, degenerated, possibly through apoptotic mechanisms as revealed by in situ detection of DNA fragmentation, whereas neurons cocultured with either astrocytes or microglial cells were viable even after exposure to RO/NS. In contrast, most neurons cocultured with meningeal fibroblasts degenerated. Astrocyte-conditioned medium partially attenuated RO/NS-induced neuronal damage. When neurons were cultured on astrocyte-derived extracellular matrix (AsECM), neuronal death induced by SNP and FeSO(4) was almost completely inhibited. AsECM contained significant amounts of laminin and fibronectin, and pure fibronectin and laminin also protected neurons against RO/NS-induced damage in the same manner as AsECM. These results suggest that astrocytes can protect neurons against RO/NS-induced damage by secreting soluble and insoluble factors.

Ultrastructural studies of dorsal root axons regenerating through adult frog optic and sciatic nerves

Optic nerves of adult fish and amphibia can successfully regenerate, in part because their glial cells, unlike those of mammals, provide an environment permissive to regrowth. We altered the environment of regenerating dorsal root axons in the frog, Rana pipiens, by grafting segments of optic nerve to test the permissiveness of CNS glial cells to other sensory neurons. We compared these preparations to grafts of segments of sciatic nerve. After allowing various times for survival, light and electron microscopy were used to evaluate the grafts. An agglomeration of astrocytes, tightly joined by desmosomes, initially formed in the center of the optic nerve grafts. Around this grew regenerating dorsal root axons, accompanied by Schwann cells. At early stages, some axons formed dilated terminal structures, which were not seen in peripheral nerve grafts. The appearance of blood vessels within the graft and the dispersion of cells allowed larger numbers of axons to grow through the graft. By eight weeks, 48% of dorsal root sensory axons had grown through optic nerve grafts, compared to 84% for sciatic nerve. These results suggest that astrocytes from optic nerve are not inhibitory to, and provide a suitable substrate for, regrowing sensory neurons.