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Proteoglycans and Brain Repair

Francesca Properzi and James W. Fawcett

Brain Repair Centre, Cambridge University, Cambridge CB2 2PY, United Kingdom

	Abstract

 
Proteoglycans are complex molecules composed of long, unbranched sugar chains attached to a protein core. In the mammalian central nervous system, they are a major component of the extracellular matrix and of the cellular surface. After a central nervous system injury, their expression in the lesion area changes strongly and contributes to the inhibition of axon regrowth and brain repair.

	Introduction

Top Introduction Intrinsic neuronal influences on... Signaling pathways in the... Neuronal extrinsic environment:... Neuronal extrinsic environment:... Proteoglycans: diverse roles in... CSPGs HSPGs Conclusions References

 
Why does regeneration fail after a central nervous system (CNS) injury? Brain repair after injury is a complex process, which among other events requires the regeneration of the damaged axons and the reestablishment of the correct neuronal connections. In the adult mammalian CNS, axon regeneration fails. Both intrinsic neuronal mechanisms and influences from the environment surrounding neurons are involved in the failure of axonal regrowth.

	Intrinsic neuronal influences on regeneration

Top Introduction Intrinsic neuronal influences on... Signaling pathways in the... Neuronal extrinsic environment:... Neuronal extrinsic environment:... Proteoglycans: diverse roles in... CSPGs HSPGs Conclusions References

 
In the CNS, some neurons can regenerate their axons better than others when grown in the same environment. For instance, neurons from the cerebellar deep nuclei can regrow their axons through grafts of Schwann cells or embryonic brain tissue after axotomy, whereas Purkinje cells, also a cerebellar cell type, do not extend their axons through exactly the same tissue. There have been many studies aimed at finding the mechanisms behind these differences, and various correlations have been found. Molecules, such as the neuronal growth-associated protein GAP-43 and the adhesion molecules leukocyte protein 1 (L1) and its close homologue CHL1, are upregulated after injury to axons with good regenerative potential but not after injury to axons that will not regenerate (10). These findings suggest that a precise signaling mechanism needs to be activated after axonal injury to trigger neurite regrowth. However, to date manipulation of these various molecules has not been sufficient to make Purkinje cell axons regenerate. There is a suggestion that the neuronal intrinsic regenerative potential can be changed. Dorsal root ganglia (DRG) bipolar neurons have two axons, one extending into the CNS, which does not regenerate, and the other extending into the peripheral nervous system (PNS), which does. Lesions of the peripheral axon lead to a conditioning effect, which means that if the central axon is transected after the peripheral one, the regenerative ability of the central axon is much greater.

	Signaling pathways in the axon

Top Introduction Intrinsic neuronal influences on... Signaling pathways in the... Neuronal extrinsic environment:... Neuronal extrinsic environment:... Proteoglycans: diverse roles in... CSPGs HSPGs Conclusions References

 
There are many inhibitory molecules in the environment of the damaged CNS (see below). However, it is unknown through which signaling mechanism they block axon regeneration. Recent data suggest that many of these molecules block regeneration via activation of the small GTPase RhoA. Its inactivation promotes axonal regeneration over inhibitory substrates in vitro and both regeneration of neurons and functional recovery of the operated animals after spinal cord injury (12). Regeneration may also be boosted by high cyclic nucleotide cAMP levels. For example, enhancing cAMP in CNS neurons, such as primary cerebellar cells, by using a neurotrophin treatment can promote axon regrowth on an inhibitory substrate, and similar results have been obtained in retinal ganglion cells and in the spinal cord. Cai and colleagues (8) suggested that environmental signals promote or inhibit axon growth depending on the neuronal levels of cAMP. One possible action of cAMP could be to act as an endogenous inhibitor of RhoA by activating protein kinase A, which in turn inactivates RhoA through a phosphorylation step.

Axotomy activates many neuronal signaling mechanisms, and an obvious question is which ones are important to the regenerative process. One way of looking at this is to compare signaling in axons with good and poor regenerative potential. Some CNS neurons of poor regenerative potential, like cerebellar Purkinjie cells and retinal ganglion cells, show impairment in the c-Jun-mediated signaling mechanism after injury. This transcription factor, similarly to cAMP, is highly upregulated in the PNS after injury and during regeneration and can promote neurite extension in PC12 neuronal cultures. However, its overexpression in central neurons does not increase the rate of axon growth after injury (9). Similarly, GAP-43, a molecule intimately involved in the control of axon growth cone signaling, is greatly upregulated in successfully growing axons. Yet increasing its levels in the CNS does not promote axon regeneration, although increasing both GAP-43 and its sister molecule the cytoskeleton-associated protein CAP-23 leads to some increase in regenerative potential.

	Neuronal extrinsic environment: the inhibition by myelin

Top Introduction Intrinsic neuronal influences on... Signaling pathways in the... Neuronal extrinsic environment:... Neuronal extrinsic environment:... Proteoglycans: diverse roles in... CSPGs HSPGs Conclusions References

 
Many of the inhibitory molecules that block axon regeneration in the CNS are associated with myelin. The three identified to date are Nogo-A, myelin-associated glycoprotein and oligodendrocyte myelin glycoprotein. Recently, they have all been shown to bind the same neuronal receptor NgR. This is a glycosyl phosphatidylinositol-linked molecule, which must therefore signal via a partner. This has been identified as the p75 low-affinity neurotrophin receptor, through which it can activate the Rho GTPase pathway (19). The inhibitory effect of myelin-associated glycoprotein/myelin on DRG central axon regrowth can be completely prevented by injecting cAMP in the DRG neuronal cell bodies or by prelesioning the peripheral DRG axons, which has also been shown to increase the levels of cAMP (16). The inactivation of Nogo-A or its receptor by using an antagonist peptide or blocking antibodies can promote axonal regeneration and functional recovery from spinal cord injuries (14). Axonal regeneration following spinal cord injury is promoted also in a line of Nogo-A knockout mice. The inhibitory action of myelin impedes neuronal growth not only after a CNS injury but also in physiological conditions. Inactivating Nogo-A by using a specific monoclonal antibody called IN-1 induces the rearrangement of intact nerve fibers as well as the sprouting of uninjured Purkinjie cell axons. Not all neurons in the brain, however, express Nogo receptor, suggesting that there must be additional inhibitory mechanisms in the CNS.

"Not all neurons in the brain, however, express Nogo receptor, suggesting that there must be additional inhibitory mechanisms in the CNS."

	Neuronal extrinsic environment: the role of the glial scar

Top Introduction Intrinsic neuronal influences on... Signaling pathways in the... Neuronal extrinsic environment:... Neuronal extrinsic environment:... Proteoglycans: diverse roles in... CSPGs HSPGs Conclusions References

 
The glial scar forms during the inflammatory process as consequence of a lesion. Its formation involves astrocytes, meningeal cells, microglia, and oligodendrocyte precursors (OPCs) activated at different times and with diverse roles in the inflammatory process. The scar final structure is composed mainly of a meshwork of astrocytic processes bound together by tight and gap junctions. This reactive tissue restores the structural integrity of the lesion area, but in doing so it creates an environment in which neurons are unable to regrow their processes (2).

There are many observations that indicate that the glial scar tissue is inhibitory to axon regeneration. Perhaps the clearest demonstration comes from the experiments performed by Davies and colleagues (11). They demonstrated that in CNS tracts where the glial scar is absent axons can regenerate even in the presence of myelin, but where the axons encountered the glial scar the growth was blocked. The upregulation of three groups of molecules correlates with the nonpermissive properties of the CNS scar, tenascins, chondroitin sulfate proteoglycans (CSPGs, which will be described later), and class III semaphorins (2).

Tenascins are extracellular matrix (ECM)-related glycoproteins encoded by four different genes, two of which, tenascin-C and tenascin-R, are expressed in the CNS. They have multiple binding sites for cell surface integrins and other receptors, and they can also be alternatively spliced, leading to cell type-specific effects on adhesion, migration, and the formation and extension of cellular processes, including neurites and axons. Tenascin C and R are highly upregulated after CNS lesions in the glial scar. Their influence on axon regrowth is complex and probably involves binding to neuronal adhesion molecules such as integrins and neuronal cell adhesion molecule (NCAM) as well as other ECM inhibitory components such as CSPGs. Tenascin-C has been shown to have an inhibitory effect on axon regeneration; however, it may promote the growth of embryonic axons through the alternatively spliced fibronectin type III domain D. Overall, tenascins can modulate the permissive conditions of the CNS extracellular environment for axon elongation, but the molecular mechanisms involved are still unclear.

Semaphorins are involved in axon guidance during development. In the embryo they display a chemorepulsive activity on growth cones, preventing them from extending toward the wrong target. In transected spinal cord, and in brain injuries that penetrate the meningeal surface, class III semaphorins are upregulated in meningeal cells that invade from the surface. Spinal cord neurons of the descending motor pathways express semaphorin receptor components and are unable to grow in the semaphorin-rich area.

	Proteoglycans: diverse roles in the CNS response to injury

Top Introduction Intrinsic neuronal influences on... Signaling pathways in the... Neuronal extrinsic environment:... Neuronal extrinsic environment:... Proteoglycans: diverse roles in... CSPGs HSPGs Conclusions References

 
Proteoglycans consist of a protein core and long, sulfated polysaccharides (glycosaminoglycans; GAGs) made of disaccharide unit repeats. These have a high negative charge, owing to the presence of acidic sugar residues and/or modification by sulfate groups.

The disaccharide unit forming the GAG chain is different for each group of proteoglycans. They are classified into four groups: heparan sulfate proteoglycans (HSPGs), CSPGs, dermatan sulfate proteoglycans (DSPGs), and keratan sulfate proteoglycans (KSPGs). However, different types of GAG chains can coexist in the same core protein. The core proteins carrying GAGs can be transmembrane or ECM proteins (4).

Synthesis of GAG chains takes place mainly in the Golgi, and there is increasing evidence that the cell- and tissue-specific expression of the modifying enzymes and the formation of specialized enzyme complexes inside this cellular compartment controls the generation of microstructural domains within the GAG chains. These particular microstructures can form high-affinity binding sites for growth factors, protease inhibitors, and ECM molecules and may greatly influence the overall function of these molecules (13).

In brain injuries, CSPGs have been shown to have a role in the inhibition of axon growth in association with their upregulation in the glial scar (2). Also HSPGs, which have a primary role in the development of the CNS, specifically in axon guidance, seem to be involved in the CNS response to injury (6). However, their role in this process is still not clear.

	CSPGs

Top Introduction Intrinsic neuronal influences on... Signaling pathways in the... Neuronal extrinsic environment:... Neuronal extrinsic environment:... Proteoglycans: diverse roles in... CSPGs HSPGs Conclusions References

 
CSPGs are expressed on the surface of most cells and in the ECM of most tissues. They are important regulators of many biological processes, such as cell migration, cell recognition, and bone development. In the CNS, they take part in the development of the brain, modulating cell migration, neurite outgrowth, and axonal path finding (6). They are greatly upregulated in the glial scar after CNS lesions (2).

Structure. CSPGs are characterized by the presence of chondroitin sulfate GAGs forming the sugar chain, composed of alternating units of N-acetyl galactosamine and glucoronic acid. Four different subtypes of disaccharide units have been identified, carrying sulfate groups in different positions, known as CS-A, CS-C, CS-D, and CS-E, each with a characteristic sulfation pattern (Fig. 1). The ratio between these subgroups in several tissues is regulated during development and in some pathological conditions and can modulate binding with ligands.

View larger version (13K): [in this window] [in a new window]   FIGURE 1. Chondroitin sulfate proteoglycans (CSPGs) are composed of long, unbranched chondroitin sulfate sugar chains attached to a protein core. Each chain is formed by chondroitin sulfate disaccharide unit repeats. Four different chondroitin sulfate disaccharide unit subtypes have been described bearing one or two negatively charged sulfate groups in different positions. They are the CS-A and CS-C, sulfated respectively in positions 4 and 6, and the disulfated CS-D and CS-E bearing the sulfate groups in positions 2 and 6 (CS-D) or in positions 4 and 6 (CS-E). Chondroitinase treatment removes chondroitin sulfate sugar chains from the protein core, disrupting the structure of CSPGs.

The core proteins known to carry mainly chondroitin sulfate GAGs are hyalectans (brevican, neurocan, versican, aggrecan), NG2, phosphocan, appican, decorin, biglycan, and neuroglycan C (4). Hyalectans are mainly secreted molecules that take part in the formation of the ECM in several tissues. NG2 is a transmembrane glycoprotein and in the CNS is selectively expressed on the OPC surface. Phosphacan is an extracellular variant of receptor-type protein-tyrosine phosphatase ß. This CSPG together with appican is a CNS-specific ECM component secreted mainly by astrocytes. Another CNS-specific CSPG is neuroglycan C, a transmembrane protein expressed by neurons that carries several chondroitin sulfate side chains in the NH₂-terminal half of its core protein. Decorin and biglycan are small proteins ubiquitously expressed carrying both chondroitin sulfate and dermatan sulfate GAGs (2).

Ligands. A wide range of molecules binds to CSPGs (both core protein and chondroitin sulfate chains) with high selectivity. One of the most well-characterized ligands of hyalectans is hyaluronan, which binds with high affinity to the NH₂-terminal globular domain (6). The hyaluronan receptor CD44 also can directly bind to versican and aggrecan through the interaction with the GAGs CS-A and CS-C. These interactions probably give the ECM structural integrity. Other known high-affinity ligands for CNS CSPGs are the NCAM-like adhesion molecules Ng-CAM/L1, NCAM, Nr-CAM, TAG-1/axonin-1, and contactin and the ECM glycoproteins tenascin-C and tenascin-R (6). Also, growth factors like FGF-2, midkine, chemokines, and L- and P- selectins bind to CSPGs. The large transmembrane CSPG NG2 has been shown to bind to collagens V and VI through the central domain of its core protein and to interact with platelet-derived growth factor receptor. The binding between CSPGs and their ligands require specific CSPG structural motifs and can involve both the protein core and the chondroitin sulfate GAGs.

CSPG upregulation in the glial scar. In response to a CNS injury, CSPG expression increases abruptly in the area surrounding the lesion where the glial scar is forming. Most CSPG core proteins are upregulated soon after the injury both in the brain and in the spinal cord, some of them on the reactive glial cell surface and others in the ECM. The CSPGs are mainly produced by three cell types: astrocytes, oligodendrocyte lineage cells, and meningeal cells (2).

Both astrocytes and oligodendrocyte lineage cells produce neurocan. Its expression begins to increase immediately after injury, reaching a peak between 4 and 7 days, both in brain and spinal cord lesions (Fig. 2). Levels have been reported to still be high 6 mo after injury in the spinal cord (17). The other CSPGs produced by astrocytes are phosphacan and brevican. Phosphacan mRNA and protein levels drop rapidly immediately after a CNS lesion, then start to increase 7 days to 1 mo after injury. Similarly to phosphacan, brevican protein levels decrease immediately after spinal cord injury, reaching a negative peak 48 h later, after which they rise again (17). Moreover, production of neurocan by astrocytes is increased in vitro after treatment with the cytokines TGF- and TGF-ß, both of which are released after brain injury (1).

View larger version (78K): [in this window] [in a new window]   FIGURE 2. Immunolocalization of neurocan in a brain cortical stab lesion. B shows the strong upregulation of this CSPG around the lesioned area, which is absent in the uninjured controlateral side (A). Original magnification, x20; Scale bar, 100 µm. Reprinted from Ref. 1, with permission.

As well as upregulation of the CSPG protein cores, the overall level of CSPG GAGs strongly increases after CNS injuries (11). It is not known which of the core proteins carry a greater number of GAGs in the lesion, but the low levels of versican glycanation in the lesioned CNS suggest that this process is specific to the different CSPGs (3).

The type of chondroitin sulfate GAG motifs upregulated in the scar has not yet been characterized, but so far most data indicate that the glycanation increase only involves specific chondroitin sulfate subtypes. One of the first antibodies used to identify CSPG upregulation in the scar, CS-56, binds to a subtype of chondroitin sulfate GAGs, which includes double -sulfated CS-D. A number of studies show an upregulation of its epitope in the CNS glial scar (11).

CSPGs and the inhibition of axon growth. CSPG upregulation in the scar forms a barrier to the regenerating nerve fibers. The molecular mechanism of this inhibition is not fully understood. Many CSPGs are able to block the axon growth-promoting properties of laminin and other ECM proteins. NG2 has a specific cell surface receptor that could mediate its inhibitory properties on axon growth. However, the actions of these molecules are probably several and remain to be worked out. Much of the inhibition due to CSPGs must be due to actions of the GAG chains, since removal of the GAGs with chondroitinase (Fig. 1) or preventing sulfation of the GAGs with chlorate can promote axon regeneration in vitro through astrocytic cell lines and primary astrocytes. Chondroitinase can facilitate the regeneration of nerve fibers in vivo in the brain after nigrostriatal tract lesions and in the rat spinal cord after dorsal column transections (7, 15). Moreover, in the spinal cord chondroitinase treatment promoted the functional recovery of the operated animals. These findings suggest that CSPG sugar chains are partly responsible for the nonpermissive activity of CSPGs on axon growth; however, it is still not known if this inhibitory process requires a subtype of chondroitin sulfate GAG bearing a specific sulfation pattern. Interestingly, neurite outgrowth of CNS embryonic neurons is promoted by CS-E and also by CS-D in vitro but not by CS-A, CS-C, and unsulfated chondroitin sulfate, suggesting a critical role of chondroitin sulfate sulfation pattern in modulating neuronal outgrowth.

Some inhibition of axon growth by CSPGs is a direct effect of the core proteins and does not require GAG presence. As shown by a membrane stripe assay, NG2 expressed on OPCs is strongly nonpermissive to growing neurites, and digestion with chondroitinase does not promote axon elongation in this system. Similar results have been obtained by using phosphacan and neurocan purified from postnatal brain, which both have an inhibitory effect on neurite outgrowth of retinal ganglion cells in cultures, which is independent of GAG presence. Versican V2 isolated from adult brain inhibits neurite outgrowth of central and peripheral neurons when growing on a permissive substrate. Also, in this case versican’s effect was not affected by chondroitinase treatment (4).

In conclusion, both CSPG core proteins and GAG chains have an inhibitory activity on axon growth, which seems to act independently. Disrupting their inhibitory activity after injury could improve axon regeneration and therefore promote brain repair. At present, several studies are been carried out with this aim, and the preliminary results obtained by using chondroitinase to promote functional recovery after spinal cord lesions are very promising.

CSPGs and CNS plasticity. CNS neuronal circuits are able to reshape themselves in response to external stimuli and experience. Brain plasticity is a characteristic of CNS during early postnatal development and is greatly reduced after a critical period that coincides with the formation of CSPG perineuronal nets surrounding neuronal soma and dendrites.

In the adult visual cortex before the critical period, monocular deprivation leads to a shift in ocular dominance related to the plasticity of the activated neuronal circuits. Ocular dominance plasticity is lost after the critical period, and interestingly it can be restored by treatment with chondroitinase. The developmental change in the ECM surrounding neurons is therefore an important factor in the control of synaptic plasticity. Enhancing brain plasticity could be useful in promoting brain repair by stimulating the sprouting and the formation of new functional connections by surrounding uninjured fibers.

	HSPGs

Top Introduction Intrinsic neuronal influences on... Signaling pathways in the... Neuronal extrinsic environment:... Neuronal extrinsic environment:... Proteoglycans: diverse roles in... CSPGs HSPGs Conclusions References

 
HSPGs are expressed in the surface of most animal cells and in the ECM of the majority of tissues. Several ECM proteins, growth factors, degradative enzymes, and protease inhibitors are known to interact with them; thus they modulate a great number of cellular functions. Bernfield et al. (5) described the following functions for the HSPGs: 1) coreceptor for insoluble ligands like ECM components or cells; 2) coreceptors for soluble ligands like growth factor chemokines and cytokines; 3) internalization receptors for soluble ligands; and 4) soluble paracrine effectors produced by the shedding of the ectodomain from the cell surface.

Structure. Heparan sulfate GAGs are made by disaccharide units alternating a glucoronic acid or its epimer iduronic acid and a N-acetylglucosamine. After heparan sulfate GAG synthesis, several structural changes take place in the Golgi due to the activity of epimerases and heparan sulfate sulfotransferases. This results in a very high variability of heparan sulfate GAG sulfation patterns. Hypervariable sulfated regions, however, are separated by extended sequences of N-acetylated saccharides of very low sulfation. This pattern distinguishes heparan sulfate GAGs from heparin, which is essentially highly sulfated along its entire length and in contrast to the ubiquitously distributed heparan sulfate has so far been found only in a subgroup of mast cells in the connective tissue (13).

The core proteins carrying heparan sulfate GAGs are syndecans, glypicans, perlecan, and agrins. Glypicans are cell membrane-bound HSPGs that are widely expressed in the CNS. So far, six different mammalian glypicans and one Drosophila homologue have been identified. Syndecans are transmembrane proteins that carry predominantly heparan sulfate side chains. All four mammalian syndecans are expressed in developing and adult brain tissues. These are syndecan-1, syndecan-2 (fibroglycan), syndecan-3 (N-syndecan), and syndecan-4 (ryudocan or amphiglycan). There is little homology between the extracellular domains of the different syndecans, whereas they share very similar transmembrane domains, and all have short cytoplasmic domains. The release of the syndecan ectodomains by membrane shedding may play an important role in the function of these proteoglycans. Perlecan is an ECM proteoglycan with a wide tissue distribution associated with the basement membranes. Agrins are a large basement membrane-associated heparan sulfate proteoglycans, which were originally detected at the neuromuscular junction, and have also been located in various nonmuscular tissues, including the brain (4).

Ligands. Heparan sulfates bind to a wide range of molecules. The increasing list of HSPG ligands includes several cytokines, chemokines, growth factors, and ECM molecules. It is known that the different ligands bind to selective heparan sulfate oligosaccharide sequences in which the degree and pattern of sulfation is critical. The type of heparan sulfate GAGs required to mediate the binding has been identified only for few ligands like FGF-2, antithrombin, and herpes simplex virus glycoproteins (13). For several growth factors, prolonged signaling through the growth factor receptor requires the formation of a trimolecular complex between the growth factor, its receptor, and an HSPG. The binding to heparan sulfate GAGs requires the presence of at least two basic, positively charged aminoacidic residues in the ligand-binding sequence. On this basis, putative binding sites for heparan sulfate GAGs have been identified for the majority of the chemokines. It is not known if the identified sequences mediate the binding in vivo.

HSPGs in neural development. The formation of neural circuits during development includes three separate stages: neurogenesis, axon growth toward the proper target, and the formation of synapses. HSPGs are involved in each of these steps (20).

During neurogenesis, they modulate neuronal stem cell differentiation mediating the binding of these cells with FGF-2 through a process that probably involves glypican-4. This HSPG is selectively expressed on stem cells and is absent in postmitotic neuronal cells.

HSPGs have a primary role in determining the correct neuronal pathway during nervous system development. Adding exogenous heparan sulfate GAGs to growing axons causes aberrant targeting in several organisms both in the CNS and PNS. In rodents, heparitinase treatment, which disrupts heparan sulfate oligosaccharide chains, stops retinal ganglion cell axon growth during development by interfering with the binding of FGF-2 with its receptor. Moreover in mammalian CNS, heparin-binding growth-associated molecule (HB-GAM) is an ECM protein associated with early axonal tracts during development. This molecule interacts with neuronal syndecan-3 and in vitro is a strong positive substrate for axon growth and can promote the elongation of different types of central axons. Consistent with the role of HSPGs in mediating axon growth and guidance during brain development, glypican-2 is also selectively expressed on the growth cone of elongating axons. Interestingly, its expression disappears once the neurites have reached the target. During CNS development, syndecan-2 is involved in the formation of synapses, and in the adult glypican 2 has a role in dendritic spine rearrangement during brain plasticity processes.

"HSPGs have a primary role in determining the correct neuronal pathway during nervous system development."

HSPGs in the CNS injury response. Very little is known about the regulation or function of these proteoglycans in CNS injuries. In peripheral nerve lesions where neurons easily regenerate and restore functional connections, HSPGs are upregulated. The introduction of exogenous heparan sulfate into transacted sciatic nerves of guinea pigs promotes axon regrowth and remyelination (20).

There is evidence that even in CNS nigrostriatal lesions, heparan sulfate GAGs are upregulated in the glial scar in association with fibroblasts and macrophages. It has also beenshown that cultured OPCs can secrete a glial form of heparin, suggesting the possibility of the presence of this molecule in the glial scar. However, this has not been demonstrated so far. Moreover, perlecan has been shown to be upregulated and secreted by injured neurons and reactive astrocytes in the scar following intracerebroventricular kainate injections in the mouse hippocampus. The role of HSPG upregulation in the CNS scar is not clear, but the possibility that their presence in the ECM could interfere with the binding of axon growth-promoting factors (like HB-GAM) with the neuronal surface HSPGs cannot be excluded.

In situ hybridization data have shown that reactive astrocytes in the surroundings of the glial scar upregulate syndecan mRNAs after a cortical lesion. Their expression pattern overlaps consistently with the FGF receptor 1 mRNA, suggesting that they may be involved in mediating the FGF-2 binding to its receptor. FGF-2 is known to promote astrocyte proliferation and reactivity both during development and after injury.

A very recent study has also shown that transgenic mice lacking thrombin proteinase-activated receptor PAR-1 do not show any astroglial reactivity after CNS injury. Indeed, thrombin, which is a well-described HSPG ligand, is known to induce the proliferation of astrocytes through the binding with PAR-1 and the subsequent activation of extracellular signal-regulated protein kinases ERK1 and ERK2 (18).

	Conclusions

Top Introduction Intrinsic neuronal influences on... Signaling pathways in the... Neuronal extrinsic environment:... Neuronal extrinsic environment:... Proteoglycans: diverse roles in... CSPGs HSPGs Conclusions References

 
In conclusion, an important mechanism behind the failure of axon regrowth in the CNS is the inhibitory glial scar that develops after an injury. Proteoglycans are the major molecular components of this negative environment where they can directly inhibit the regeneration of axons. A full understanding of their structure and function in the glial scar would be a great help to develop an appropriate strategy to promote axon regeneration and brain repair.

	Acknowledgments

 
Because of editorial restrictions, the reference list is incomplete. We apologize to the authors who could not be cited whose work is mentioned in this review.

	References

Top Introduction Intrinsic neuronal influences on... Signaling pathways in the... Neuronal extrinsic environment:... Neuronal extrinsic environment:... Proteoglycans: diverse roles in... CSPGs HSPGs Conclusions References

 

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