Proteoglycans in the developing brain: new conceptual insights for old proteins

Chondroitin sulfate synthase-2. Molecular cloning and characterization of a novel human glycosyltransferase homologous to chondroitin sulfate glucuronyltransferase, which has dual enzymatic activities

Chondroitin sulfate is found in a variety of tissues as proteoglycans and consists of repeating disaccharide units of N-acetylgalactosamine and glucuronic acid residues with sulfate residues at various places. We found a novel human gene (GenBank accession number AB086063) that possesses a sequence homologous with the human chondroitin sulfate glucuronyltransferase gene which we recently cloned and characterized. The full-length open reading frame encodes a typical type II membrane protein comprising 775 amino acids. The protein had a domain containing beta 3-glycosyltransferase motif but lacked a typical beta 4-glycosyltransferase motif, which is the same as chondroitin sulfate glucuronyltransferase, whereas chondroitin synthase had both domains. The putative catalytic domain was expressed in COS-7 cells as a soluble enzyme. Surprisingly, both glucuronyltransferase and N-acetylgalactosaminyltransferase activities were observed when chondroitin, chondroitin sulfate, and their oligosaccharides were used as the acceptor substrates. The reaction products were identified to have the linkage of GlcUA beta 1-3GalNAc and GalNAc beta 1-4GlcUA at the non-reducing terminus of chondroitin for glucuronyltransferase activity and N-acetylgalactosaminyltransferase activity, respectively. Quantitative real time PCR analysis revealed that the transcripts were ubiquitously expressed in various human tissues but highly expressed in the pancreas, ovary, placenta, small intestine, and stomach. These results indicate that this enzyme could synthesize chondroitin sulfate chains as a chondroitin sulfate synthase that has both glucuronyltransferase and N-acetylgalactosaminyltransferase activities. Sequence analysis based on three-dimensional structure revealed the presence of not typical but significant beta 4-glycosyltransferase architecture.

Sulfated proteoglycans as modulators of neuronal migration and axonal decussation in the developing midbrain

Proteoglycans are abundant in the developing brain and there is much circumstantial evidence for their roles in directional neuronal movements such as cell body migration and axonal growth. We have developed an in vitro model of astrocyte cultures of the lateral and medial sectors of the embryonic mouse midbrain, that differ in their ability to support neuritic growth of young midbrain neurons, and we have searched for the role of interactive proteins and proteoglycans in this model. Neurite production in co-cultures reveals that, irrespective of the previous location of neurons in the midbrain, medial astrocytes exert an inhibitory or nonpermissive effect on neuritic growth that is correlated to a higher content of both heparan and chondroitin sulfates (HS and CS). Treatment of astrocytes with chondroitinase ABC revealed a growth-promoting effect of CS on lateral glia but treatment with exogenous CS-4 indicated a U-shaped dose-response curve for CS. In contrast, the growth-inhibitory action of medial astrocytes was reversed by exogenous CS-4. Treatment of astrocytes with heparitinase indicated that the growth-inhibitory action of medial astrocytes may depend heavily on HS by an as yet unknown mechanism. The results are discussed in terms of available knowledge on the binding of HS proteoglycans to interactive proteins, with emphasis on the importance of unraveling the physiological functions of glial glycoconjugates for a better understanding of neuron-glial interactions.

Region and lamina-specific distribution of extracellular matrix proteoglycans, hyaluronan and tenascin-R in the mouse hippocampal formation

The extracellular matrix is known to show region-specific characteristics in the adult brain. Our comparative cytochemical study is focused on the laminar organisation of major extracellular matrix constituents in the murine hippocampal formation, including the regions CA1, CA2 and CA3 of the hippocampus proper, the dentate gyrus, the subiculum and the presubiculum. Components related to chondroitin sulphate proteoglycans were detected by N-acetylgalactosamine-binding Wisteria floribunda agglutinin, colloidal iron staining, and antibodies to different proteoglycan domains, including the Cat-301 and Cat-315 epitopes of aggrecan, as well as neurocan, brevican and phosphacan. The distribution patterns of these components were correlated with the patterns revealed for hyaluronan and the brain-specific extracellular matrix glycoprotein, tenascin-R, known to be ligands of extracellular matrix proteoglycans. Lectin binding clearly labelled perineuronal nets of the extracellular matrix around interneurons, which were preferentially located within or near the principal cell layers in all regions. In the hippocampus proper, the CA2 subfield showed an intense labelling of the neuropil around pyramidal cell bodies and the neuropil zones in the strata oriens and radiatum. These patterns were also seen after immunoreaction for chondroitin proteoglycan domains, brevican and phosphacan, as well as after detection of hyaluronan and tenascin-R. Characteristic laminar and intralaminar patterns were additionally expressed in the neuropil in all regions. In the dentate gyrus, the staining intensity for brevican, phosphacan and tenascin-R was predominant in the middle molecular layer, and for Cat-315 in the inner molecular layer, whereas immunoreactivity for neurocan increased within the outer molecular layer towards the hippocampal fissure. Our findings indicate that proteoglycans, hyaluronan and tenascin-R show differential patterns of co-expression in the individual regions and laminae of the hippocampal formation. The inhomogeneous composition of these major components suggests that the extracellular matrix is specifically adapted to the functional domains of intrahippocampal connections and afferent fibre systems.

Heparan sulphate proteoglycans in Alzheimer's disease and amyloid-related disorders

Proteoglycans are associated with all kinds of amyloid deposits in the human body. These complex macromolecules, in particular heparan sulphate proteoglycans, have also been implicated in several features of the pathogenesis of Alzheimer's disease (AD), including the genesis of senile plaques, cerebrovascular amyloid, and neurofibrillary tangles. In this review we focus on the role of proteoglycans and glycosaminoglycans in amyloidogenesis in general and in AD in particular. Heparan sulphate proteoglycans may promote amyloid-beta peptide (Abeta) or tau fibrillisation on the one hand, and provide resistance against proteolytic breakdown on the other. Knowledge about the role of proteoglycans in AD pathology may eventually be of therapeutic use, because small polysulphated compounds, which can interfere with the interaction between proteoglycan and Abeta, have been shown to stop or even prevent amyloidogenesis.

Characterization of a heparan sulfate 3-O-sulfotransferase-5, an enzyme synthesizing a tetrasulfated disaccharide

Heparan sulfate d-glucosaminyl 3-O-sulfotransferases (3-OSTs) catalyze the transfer of sulfate from 3'-phosphoadenosine 5'-phosphosulfate (PAPS) to position 3 of the glucosamine residue of heparan sulfate and heparin. A sixth member of the human 3-OST family, named 3-OST-5, was recently reported (Xia, G., Chen, J., Tiwari, V., Ju, W., Li, J.-P., Malmstrom, A., Shukla, D., and Liu, J. (2002) J. Biol. Chem. 277, 37912-37919). In the present study, we cloned putative catalytic domain of the human 3-OST-5 and expressed it in insect cells as a soluble enzyme. Recombinant 3-OST-5 only exhibited sulfotransferase activity toward heparan sulfate and heparin. When incubated heparan sulfate with [35S]PAPS, the highest incorporation of35S was observed, and digestion of the product with a mixture of heparin lyases yielded two major35S-labeled disaccharides, which were determined as DeltaHexA-GlcN(NS,3S,6S) and DeltaHexA(2S)-GlcN(NS,3S) by further digestion with 2-sulfatase and degradation with mercuric acetate. However, when used heparin as acceptor, we identified a highly sulfated disaccharide unit as a major product. This had a structure of DeltaHexA(2S)-GlcN(NS,3S,6S). Quantitative real-time PCR analysis revealed that 3-OST-5 was highly expressed in fetal brain, followed by adult brain and spinal cord, and at very low or undetectable levels in the other tissues. Finally, we detected a tetrasulfated disaccharide unit in bovine intestinal heparan sulfate. To our knowledge, this is the first report to describe not only the natural occurrence of tetrasulfated disaccharide unit but also the enzymatic formation of this novel structure.

Phosphacan and neurocan are repulsive substrata for adhesion and neurite extension of adult rat dorsal root ganglion neurons in vitro

Phosphacan (PC) and neurocan (NC) are major chondroitin sulfate proteoglycans (CS-PGs) in nervous tissue and are involved in the modulation of cell adhesion and neurite outgrowth during neural development and regeneration. In the present study, we examined the effects of PC and NC on the attachment and neurite extension of adult rat dorsal root ganglion (DRG) neurons in vitro. Treatment with PC and NC on poly-L-lysine (PL) significantly impaired both neuronal attachment and neurite extension in a concentration-dependent manner (10 microg/ml > 1 microg/ml >> 0.1 microg/ml), and they were partially suppressed by chondroitinase ABC (ChABC) digestion. The CS-PGs applied to culture medium (1 microg/ml) also displayed inhibitory effects on neurite extension, which were not altered by ChABC treatment. These results show that PC and NC are repulsive substrata for adhesion and neurite regeneration of adult DRG neurons in vitro and suggest that both chondroitin sulfate moieties and core proteins are responsible for the inhibitory actions of the CS-PGs. We also conducted immunohistochemical analyses with the monoclonal antibodies to core proteins of PC (mAb 6B4) and NC (mAb 1G2), which revealed that only a few neurons in the DRG section were stained with these antibodies. In contrast, most DRG neurons at different stages (12 h, 1 day, 2 days, and 4 days) in culture were immunoreactive to mAb 6B4 and mAb 1G2. Taking these findings together, it is plausible that both CS-PGs expressed in the cultured neurons may play a role in the modulation of attachment, survival, and neurite regeneration.

Heparan sulfate proteoglycans modulate monocyte migration across cerebral endothelium

Heparan sulfate proteoglycans (HSPGs) are known to participate in a wide range of biological events, including cellular trafficking. In this study we report that in situ cerebral blood vessels highly express HSPGs. Of the syndecan family, syndecan-2 is highly expressed on virtually all brain vessels and syndecan-1 and -3 are only present on larger blood vessels. These endothelial HSPGs have a functional role in monocyte diapedesis across brain endothelium, as assessed in our in vitro adhesion and migration assays. Our data indicate that heparin prevents monocyte adhesion to brain endothelium by interacting solely with the monocyte. Transendothelial migration of monocytes can be prevented by preincubation of brain endothelium with heparin by enzymatic removal of heparan sulphate side chains or by inhibition of cellular sulfation. Blocking of G-protein-dependent signaling in the monocytes prevented monocyte adhesion and migration to similar extent, suggesting that G-dependent signaling may be involved in HSPG-mediated monocyte adhesion and transendothelial migration. Our data demonstrate that brain endothelial HSPGs have a modulatory role in the transendothelial migration of monocytes in a direct and indirect fashion and may therefore contribute to the formation of neuroinflammatory lesions.

A chondroitin sulfate proteoglycan PTPzeta /RPTPbeta regulates the morphogenesis of Purkinje cell dendrites in the developing cerebellum

PTPzeta/RPTPbeta, a receptor-type protein tyrosine phosphatase synthesized as a chondroitin sulfate (CS) proteoglycan, uses a heparin-binding growth factor pleiotrophin (PTN) as a ligand, in which the CS portion plays an essential role in ligand binding. Using an organotypic slice culture system, we tested the hypothesis that PTN-PTPzeta signaling is involved in the morphogenesis of Purkinje cell dendrites. An aberrant morphology of Purkinje cell dendrites such as multiple and disoriented primary dendrites was induced in slice cultures by (1) addition of a polyclonal antibody against the extracellular domain of PTPzeta, (2) inhibition of protein tyrosine phosphatase activity, (3) enzymatic removal of the CS chains, (4) addition of exogenous CS chains, and (5) addition of exogenous PTN, all of which disturb PTN-PTPzeta signaling. These treatments also reduced the immunoreactivity to GLAST, a glial glutamate transporter, on Bergmann glial processes. Furthermore, a glutamate transporter inhibitor also induced the abnormal morphogenesis of Purkinje cell dendrites. Altogether, these findings suggest that PTN-PTPzeta signaling regulates the morphogenesis of Purkinje cell dendrites and that the mechanisms underlying that regulation involve the GLAST activity in Bergmann glial processes.

Testican-1 inhibits attachment of Neuro-2a cells

Testican-1 is a highly conserved, multidomain, chondroitin sulfate proteoglycan that is most abundantly transcribed in the brain by neurons. This testican messenger RNA is not detected in normal quiescent astrocytes, but is up regulated when these cells are activated in response to injury such as cerebral stroke. Other chondroitin sulfate proteoglycans found in glial scars, including neurocan, have been shown to inhibit neural cell attachment and neurite extensions and may thus impede axonal regeneration. Here we report the expression and purification of a proteoglycan form of recombinant testican and its effects on neuron-derived cells in culture. We demonstrate that testican inhibits attachment of Neuro-2a cells and their ability to form neurite extensions. Both testican proteoglycan and the core glycoprotein that has been depleted of chondroitin sulfate inhibit cell attachment. Pre-treatment of the culture substratum with testican inhibits Neuro-2a attachment, but pre-treatment of the cells with testican does not inhibit their attachment. Testican, therefore, blocks attachment sites on cultureware and may also block attachment sites in the extracellular matrix of the brain.

Localization of aggrecan and versican in the developing rat central nervous system

The localization of aggrecan and mRNA splice variants of versican in the developing rat central nervous system has been examined by using specific polyclonal antibodies to the nonhomologous glycosaminoglycan attachment regions of these hyaluronan-binding chondroitin sulfate proteoglycans. At embryonic day 16 (E16), aggrecan and versican splice variants containing either or both the alpha-and beta-domains are present in the marginal zone and subplate of the cerebral cortex and in the amygdala, internal capsule, and the optic and lateral olfactory tracts. There is strong staining of versican but not of aggrecan in the hippocampus and dentate gyrus by E19, whereas both aggrecan and alpha-versican are present in the fimbria. At E19, aggrecan is seen throughout the cerebral cortex, whereas the distribution of versican is considerably more limited, being confined essentially to the marginal zone and subplate. At 1 week postnatal, both aggrecan and versican are present in the prospective white matter and in the molecular and granule cell layers of the cerebellum, but neither proteoglycan is seen in the external granule cell layer. alpha- but not beta-versican staining is seen in Purkinje cells, and aggrecan staining of Purkinje cells is also rather minimal. In the spinal cord at E13, aggrecan is present in the dorsal root entry zone, ventral funiculus, mantle layer, and floor plate, as well as in the dorsal root ganglia and ventral roots. However, alpha-versican is confined to the dorsal root entry zone and the ependyma surrounding the spinal canal, and beta-versican is not present in spinal cord parenchyma at this developmental stage, being limited to the surrounding connective tissue. By E19, there are significant amounts of all three proteoglycans in the spinal cord. Aggrecan staining is most intense in the lateral funiculus and the fasciculi gracilis and cuneatus, where alpha-versican staining is also strong. In contrast, beta-versican is seen predominantly in the motor columns. Differences in the localization and temporal expression patterns of these chondroitin sulfate proteoglycans suggest that, like neurocan and phosphacan, they have partially complementary roles during central nervous system development.

Gene expression of EXT1 and EXT2 during mouse brain development

Heparan sulfate (HS) and heparan sulfate proteoglycans (HSPGs) play significant roles in various biological processes. There is a wealth of circumstantial and experimental evidence suggesting the roles of HS in mammalian neural development. HS synthesis is governed by a series of enzymes. Among them, two enzymes, EXT1 and EXT2, catalyze polymerization of glucuronic acid and N-acetylglucosamine, the crucial step of HS synthesis. To obtain insight into the roles of HS in neural development, we examined the spatiotemporal expression patterns of EXT1 and EXT2 during mice brain development. RT-PCR analyses showed that expression of EXT1 and EXT2 peaks during early postnatal period in the cerebrum and around birth in the cerebellum. In situ hybridization revealed that in the embryonic brain, EXT1 and EXT2 were localized primarily in the neuroepithelial cells surrounding the lateral ventricles, the mesencephalic vesicle, and the fourth ventricle. In the early postnatal stage, intense expression of EXT1 and EXT2 was observed in the cerebral cortex and the hippocampus formation. In the postnatal cerebellum, expression of EXT1 and EXT2 was mainly observed in external and internal granular layers. Our results demonstrate that EXT1 and EXT2 are highly expressed in the developing brain, and that their expression is developmentally regulated, suggesting that HS is involved in various neurodevelopmental processes.

Development and maintenance of neuronal architecture at the ventral midline of C. elegans

Work in flies, nematodes and vertebrates has shown that genes involved in axon patterning at the ventral midline are functionally conserved across phylogeny. Recent studies in Caenorhabditis elegans have implicated several new extracellular molecules, such as nidogen and heparan sulfate proteoglycans, in axonal tract formation at the midline. Furthermore, a conceptually new mechanism that regulates the maintenance of axon positioning at the midline has been described in C. elegans.

Collapsin response mediator proteins of neonatal rat brain interact with chondroitin sulfate

Chondroitin sulfate proteoglycans are structurally and functionally important components of the extracellular matrix of the central nervous system. Their expression in the developing mammalian brain is precisely regulated, and cell culture experiments implicate these proteoglycans in the control of cell adhesion, neuron migration, neurite formation, neuronal polarization, and neuron survival. Here, we report that a monoclonal antibody against chondroitin sulfate-binding proteins from neonatal rat brain recognizes collapsin response mediator protein-4 (CRMP-4), which belongs to a family of proteins involved in collapsin/semaphorin 3A signaling. Soluble CRMPs from neonatal rat brain bound to chondroitin sulfate affinity columns, and CRMP-specific antisera co-precipitated chondroitin sulfate. Moreover, chondroitin sulfate and CRMP-4 were found to be localized immuno-histochemically in overlapping distributions in the marginal zone and the subplate of the cerebral cortex. CRMPs are released to culture supernatants of NTera-2 precursor cells and of neocortical neurons after cell death, and CRMP-4 is strongly expressed in the upper cortical plate of neonatal rat where cell death is abundant. Therefore, naturally occurring cell death is a plausible mechanism that targets CRMPs to the extracellular matrix at certain stages of development. In summary, our data indicate that CRMPs, in addition to their role as cytosolic signal transduction molecules, may subserve as yet unknown functions in the developing brain as ligands of the extracellular matrix.

Multiple regions of the NG2 proteoglycan inhibit neurite growth and induce growth cone collapse

The NG2 chondroitin sulfate proteoglycan, an integral membrane proteoglycan, inhibits axon growth from cerebellar granule neurons and dorsal root ganglia (DRG) neurons in vitro. The extracellular domain of the NG2 core protein contains three subdomains: an N-terminal globular domain (domain 1), a central extended domain that has the sites for glycosaminoglycan (GAG) attachment (domain 2), and a juxtamembrane domain (domain 3). Here, we used domain-specific fusion proteins and antibodies to map the inhibitory activity within the NG2 core protein. Fusion proteins encoding domain 1 (D1-Fc) or domain 3 (D3-Fc) of NG2 inhibited axon growth from cerebellar granule neurons when the proteins were substrate-bound. These proteins also induced growth cone collapse from newborn DRG neurons when added to the culture medium. Domain 2 only inhibited axon growth when the GAG chains were present. Neutralizing antibodies directed against domain 1 or 3 blocked completely the inhibition from substrates coated with D1-Fc or D3-Fc. When the entire extracellular domain of NG2 was used as a substrate, however, both neutralizing antibodies were needed to reverse completely the inhibition. When NG2 was expressed on the surface of HEK293 cells, the neutralizing anti-D1 antibody was sufficient to block the inhibition, whereas the anti-D3 antibody had no effect. These results suggest that domains 1 and 3 of NG2 can inhibit neurite growth independently. These inhibitory domains may be differentially exposed depending on whether NG2 is presented as an integral membrane protein or as a secreted protein associated with the extracellular matrix.

Modulators of axonal growth and guidance at the brain midline with special reference to glial heparan sulfate proteoglycans

Bilaterally symmetric organisms need to exchange information between the left and right sides of their bodies to integrate sensory input and to coordinate motor control. Thus, an important choice point for developing axons is the Central Nervous System (CNS) midline. Crossing of this choice point is influenced by highly conserved, soluble or membrane-bound molecules such as the L1 subfamily, laminin, netrins, slits, semaphorins, Eph-receptors and ephrins, etc. Furthermore, there is much circumstantial evidence for a role of proteoglycans (PGs) or their glycosaminoglycan (GAG) moieties on axonal growth and guidance, most of which was derived from simplified models. A model of intermediate complexity is that of cocultures of young neurons and astroglial carpets (confluent cultures) obtained from medial and lateral sectors of the embryonic rodent midbrain soon after formation of its commissures. Neurite production in these cocultures reveals that, irrespective of the previous location of neurons in the midbrain, medial astrocytes exerted an inhibitory or non-permissive effect on neuritic growth that was correlated to a higher content of both heparan and chondroitin sulfates (HS and CS). Treatment with GAG lyases shows minor effects of CS and discloses a major inhibitory or non-permissive role for HS. The results are discussed in terms of available knowledge on the binding of HSPGs to interative proteins and underscore the importance of understanding glial polysaccharide arrays in addition to its protein complement for a better understanding of neuron-glial interactions.

Making the connection: retinal axon guidance in the zebrafish

Genetic screens in zebrafish have identified a large number of mutations that affect neural connectivity in the developing visual system. These mutants define genes essential for accurate retinal axon guidance in the eye and brain and the characterization of these mutants is helping to define the cellular and molecular mechanisms that guide axons in the vertebrate embryo. The combination of zebrafish genetic and embryological approaches promises to greatly increase our understanding of how multiple guidance mechanisms establish the complex neural interconnectivity of the vertebrate brain.

Signalling by glial cell line-derived neurotrophic factor (GDNF) requires heparan sulphate glycosaminoglycan

Glial cell line-derived neurotrophic factor, GDNF, is vital to the development and maintenance of neural tissues; it promotes survival of sympathetic, parasympathetic and spinal motor neurons during development, protects midbrain dopaminergic neurons from apoptosis well enough to be a promising treatment for Parkinson's disease, and controls renal and testicular development. Understanding how GDNF interacts with its target cells is therefore a priority in several fields. Here we show that GDNF requires glycosaminoglycans as well as the already-known components of its receptor complex, c-Ret and GFRalpha-1. Without glycosaminoglcyans, specifically heparan sulphate, c-Ret phosphorylation fails and GDNF cannot induce axonogenesis in neurons, in PC-12 cells, or scatter of epithelial cells. Furthermore, exogenous heparan sulphate inhibits rather than assists GDNF signalling. The involvement of heparan sulphates in GDNF signalling raises the possibility that modulation of heparan expression may modulate signalling by GDNF in vivo.

Order out of chaos: assembly of ligand binding sites in heparan sulfate

Virtually every cell type in metazoan organisms produces heparan sulfate. These complex polysaccharides provide docking sites for numerous protein ligands and receptors involved in diverse biological processes, including growth control, signal transduction, cell adhesion, hemostasis, and lipid metabolism. The binding sites consist of relatively small tracts of variably sulfated glucosamine and uronic acid residues in specific arrangements. Their formation occurs in a tissue-specific fashion, generated by the action of a large family of enzymes involved in nucleotide sugar metabolism, polymer formation (glycosyltransferases), and chain processing (sulfotransferases and an epimerase). New insights into the specificity and organization of the biosynthetic apparatus have emerged from genetic studies of cultured cells, nematodes, fruit flies, zebrafish, rodents, and humans. This review covers recent developments in the field and provides a resource for investigators interested in the incredible diversity and specificity of this process.

Mapping sites responsible for interactions of agrin with neurons

The multidomain proteoglycan agrin is a critical organizer of postsynaptic differentiation at the skeletal neuromuscular junction. Agrin is also abundant in the brain, but its roles there are unknown. As a step toward understanding these roles, we mapped sites responsible for interactions of neurons with agrin. First, we used a series of recombinant agrin fragments to show that at least four sites on agrin interact with chick ciliary neurons. Use of blocking antibodies and peptides indicated that neurons adhere to a site in the second of three G domains by means of alphaVbeta1 integrin, and to a site in the last of four epidermal growth factor (EGF) repeats via a distinct beta1 integrin. A third, integrin-independent adhesion site is near to but distinct from the site that induces postsynaptic differentiation in muscles. These domains are insufficient, however, to account for neurite outgrowth-inhibiting properties of full-length agrin, which are mediated by the N-terminal half of the molecule. We then used a second set of agrin mutants to demonstrate and map a transmembrane domain in the amino-terminus of the SN-isoform of agrin. The extracellular matrix-bound form of agrin, called LN, bears an amino-terminus required for secretion and binding to laminin. The SN form, which is selectively expressed by neurons, bears a variant amino terminus that converts agrin from a secreted, matrix-associated protein to a type-II transmembrane protein, providing a mechanism for presenting agrin in central, as opposed to neuromuscular, synaptic clefts. The SN-amino terminus can mediate externalization and membrane anchoring of heterologous proteins, but is insufficient to target them to the synapse. Together, these studies define sites that contribute to the subcellular localization of and signaling by neuronal agrin.

Growth factor treatment promotes mobilization of young but not aged adult subventricular zone precursors in response to demyelination

Precursor cells of the adult mouse subventricular zone (SVZ) are mobilized and recruited by a lysolecithin (LPC)-induced demyelination of the corpus callosum. Because age decreases the proliferation of the SVZ neural precursors as well as the potential for myelin repair of the adult central nervous system, we have compared the ability of young and aged adult neural precursors to respond to LPC-induced demyelination. With age, the SVZ cells lost their capacity to proliferate and to be recruited by the lesion. Whereas a single injection of fibroblast growth factor-2 or transforming growth factor-alpha stimulated the proliferation of SVZ and rostral migratory stream precursors in both groups of animals after demyelination, they favored recruitment at the lesion in young mice but not in aged ones. In vitro experiments using neurospheres derived from young and aged animals indicated that both populations have the same migratory performances. Our in vivo data thus suggest that aged neural precursors may loose their intrinsic capacities to respond to demyelination-induced signals. Alternatively, their function may be altered by modification of the aged extracellular environment.

Transient expression of juvenile-type neurocan by reactive astrocytes in adult rat brains injured by kainate-induced seizures as well as surgical incision

Neurocan is one of the major chondroitin sulfate proteoglycans expressed in nervous tissues. The expression of neurocan is developmentally regulated, and full-length neurocan is detected in juvenile brains but not in adult brains. In the present study, we demonstrated by western blot analysis that full-length neurocan transiently appeared in adult rat hippocampus when it was lesioned by kainate-induced seizures. Immunohistochemical studies showed that neurocan was detected mainly around the CA1 region although the seizure resulted in neuronal cell degeneration in both the CA1 and CA3 regions of the hippocampus. Double-labeling for neurocan mRNA and glial fibrillary acidic protein demonstrated that many reactive astrocytes expressed neurocan mRNA. The re-expression of full-length neurocan was also observed in the surgically injured adult rat brain. In contrast, the expression of other nervous tissue chondroitin sulfate proteoglycans, such as phosphacan and neuroglycan C, was not intensified but rather was either reduced in the kainate-lesioned hippocampus or in the surgically injured cerebral cortex. These observations indicate that induction of neurocan expression by reactive astrocytes is a common phenomenon in the repair process of adult brain injury, and therefore, it can be postulated that juvenile-type neurocan plays some roles in brain repair.

Proteoglycans in retina

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

Increased nerve and blood vessel ingrowth associated with proteoglycan depletion in an ovine anular lesion model of experimental disc degeneration

STUDY DESIGN

Nerves and blood vessel distribution in discs were localized immunohistochemically and correlated with the proteoglycan contents of normal and degenerate disc tissues.

OBJECTIVE

The aim of the present study was to systematically evaluate whether nerve and blood vessel ingrowth was associated with depletion of disc proteoglycans and degenerative changes in an established experimental model of disc degeneration.

SUMMARY OF BACKGROUND DATA

Animal models of disc degeneration, allowing longitudinal study of pathogenic mechanisms, are limited. The ovine model enables systematic monitoring of blood vessel and nerve ingrowth during the development of disc degeneration after injury to the anulus fibrosus.

METHODS

Merino sheep received a controlled left anterolateral surgical defect in the outer anulus fibrosus of the L1-L2 and L3-L4 discs (lesion group); sham-operated controls received the retroperitoneal anterolateral approach only. Animals were killed 3, 6, 12, and 26 months postoperation, and the discs were collected for histology and compositional and morphologic analyses. Sagittal tissue sections were stained with toluidine blue and hematoxylin and eosin; Type IV collagen immunolocalization visualized blood vessel ingrowth, and nerves were immunolocalized using monoclonal antibodies to growth-associated protein (GAP-43), protein gene product 9.5, and glial fibrillary acidic protein.

RESULTS

Compositional and histologic results demonstrated early focal depletion 3-12 months postoperation of glycosaminoglycan associated with lesion development, increased blood vessel and nerve ingrowth, and infiltration of cells from the outer anulus fibrosus along the plane of the original defect. Blood vessel numbers in the outer to mid third of the anulus fibrosus were elevated in the lesion discs 3-6 months postoperation reaching a maximum at 12 months postoperation; nerves immunoreactive with protein gene product 9.5 (also maximal at 12 months postoperation) were often found associated (but not exclusively) with blood vessels, and some nerves were also reactive with GAP-43 and glial fibrillary acidic protein, but only at 12 months postoperation.

CONCLUSIONS

Nerve and blood vessel ingrowth into the anulus fibrosis were strongly associated with proteoglycan depletion. The ovine anular lesion model of disc degeneration is a useful experimental model for the systematic evaluation of nerve and blood vessel development after anular injury.

Vaccinia virus complement control protein enhances functional recovery after traumatic brain injury

Inflammation is a major contributor to the neuropathological consequences of traumatic brain injury (TBI). Previous studies have shown that proinflammatory complement activation fragments are present in the injured brain within the first 24 h after trauma. To investigate whether complement activation within the injured brain leads to the neuropathology and subsequent functional impairment associated with TBI, we examined what effect administration of a complement inhibitor, the vaccinia virus complement control protein (VCP), would have on spatial learning and memory in brain injured rats, as measured using the Morris Water Maze (MWM) procedure. Animals were subjected to a lateral fluid percussion brain injury of moderate severity and, 15 min later, received a 10-microL injection of either full-length VCP, a truncated version of VCP (VCPt), which lacks the complement inhibitory activity but retains the heparin binding activity of VCP, or saline directly into the cortex. Results of such intervention indicated that, at 2 weeks postinjury, both VCP and VCPt treatment attenuated impairments in spatial memory, but not neuropathological damage, as compared to the saline treated controls. These results were surprising and suggest that the neuroprotective effects following administration of VCP after acute brain injury are mediated by mechanisms other than complement inhibition. Potential mechanisms are discussed.

Caspr3 and caspr4, two novel members of the caspr family are expressed in the nervous system and interact with PDZ domains

The NCP family of cell-recognition molecules represents a distinct subgroup of the neurexins that includes Caspr and Caspr2, as well as Drosophila Neurexin-IV and axotactin. Here, we report the identification of Caspr3 and Caspr4, two new NCPs expressed in nervous system. Caspr3 was detected along axons in the corpus callosum, spinal cord, basket cells in the cerebellum and in peripheral nerves, as well as in oligodendrocytes. In contrast, expression of Caspr4 was more restricted to specific neuronal subpopulations in the olfactory bulb, hippocampus, deep cerebellar nuclei, and the substantia nigra. Similar to the neurexins, the cytoplasmic tails of Caspr3 and Caspr4 interacted differentially with PDZ domain-containing proteins of the CASK/Lin2-Veli/Lin7-Mint1/Lin10 complex. The structural organization and distinct cellular distribution of Caspr3 and Caspr4 suggest a potential role of these proteins in cell recognition within the nervous system.

Heparan sulfate proteoglycan-dependent induction of axon branching and axon misrouting by the Kallmann syndrome gene kal-1

Kallmann syndrome is a neurological disorder characterized by various behavioral and neuroanatomical defects. The X-linked form of this disease is caused by mutations in the KAL-1 gene, which codes for a secreted molecule that is expressed in restricted regions of the brain. Its molecular mechanism of action has thus far remained largely elusive. We show here that expression of the Caenorhabditis elegans homolog of KAL-1 in selected sensory and interneuron classes causes a highly penetrant, dosage-dependent, and cell autonomous axon-branching phenotype. In a different cellular context, heterologous C. elegans kal-1 expression causes a highly penetrant axon-misrouting phenotype. The axon-branching and -misrouting activities require different domains of the KAL-1 protein. In a genetic modifier screen we isolated several loci that either suppress or enhance the kal-1-induced axonal defects, one of which codes for an enzyme that modifies specific residues in heparan sulfate proteoglycans, namely heparan-6O-sulfotransferase. We hypothesize that KAL-1 binds by means of a heparan sulfate proteoglycan to its cognate receptor or other extracellular cues to induce axonal branching and axon misrouting.

Sex-related differences in gene expression in neonatal rat hypothalamus assessed by cDNA microarray analysis

Sexual differentiation of the rodent brain is recognized to involve transcriptional activation of multiple genes induced by gonadal steroids at developmental stages. To identify the genes differing in expression level between sexes, we analyzed gene expression in male and female rat hypothalami at postnatal day 5 by means of a cDNA microarray consisting of 2352 genes. By comparing the expression pattern between sexes, we identified 12 male-enriched genes and 20 female-enriched genes. Among them, the expression pattern of 1 male-enriched gene, jagged homolog 1, and those of 2 female-enriched genes, p27Kip1 and p130, were confirmed to be consistent with microarray data by RT-PCR. Investigation of these genes should help to elucidate the molecular and cellular mechanisms underlying sexual differentiation of the rodent central nervous system.

Chondrodysplasias due to proteoglycan defects

The proteoglycans, especially the large chondroitin sulfate proteoglycan aggrecan, have long been viewed as important components of the extracellular matrix of cartilage. The drastic change in expression during differentiation from mesenchyme to cartilage, the loss of tissue integrity associated with proteoglycan degradation in several disease processes and, most important, the demonstration of abnormalities in proteoglycan production concomitant with the aberrant growth patterns exhibited by the brachymorphic mouse, the cartilage matrix deficient mouse, and the nanomelic chick provide the strongest evidence that the proteoglycan aggrecan is essential during differentiation and for maintenance of the skeletal elements. More recently, mutations associated with proteoglycans other than aggrecan, especially the heparan sulfate proteoglycans, glypican and perlecan, suggest an important role for these molecules in skeletal development as well. This review focuses on the molecular bases of the hereditary proteoglycan defects in animal models, as well as of some human chondrodysplasias, that collectively are providing a better understanding of the role of proteoglycans in the development and maintenance of the skeletal elements.

Extracellular matrix molecules, long-term potentiation, memory consolidation and the brain angiotensin system

Considerable evidence now suggests an interrelationship among long-term potentiation (LTP), extracellular matrix (ECM) reconfiguration, synaptogenesis, and memory consolidation within the mammalian central nervous system. Extracellular matrix molecules provide the scaffolding necessary to permit synaptic remodeling and contribute to the regulation of ionic and nutritional homeostasis of surrounding cells. These molecules also facilitate cellular proliferation, movement, differentiation, and apoptosis. The present review initially focuses on characterizing the ECM and the roles of cell adhesion molecules (CAMs), matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs), in the maintenance and degradation of the ECM. The induction and maintenance of LTP is described. Debate continues over whether LTP results in some form of synaptic strengthening and in turn promotes memory consolidation. Next, the contribution of CAMs and TIMPs to the facilitation of LTP and memory consolidation is discussed. Finally, possible roles for angiotensins, MMPs, and tissue plasminogen activators in the facilitation of LTP and memory consolidation are described. These enzymatic pathways appear to be very important to an understanding of dysfunctional memory diseases such as Alzheimer's disease, multiple sclerosis, brain tumors, and infections.

Cell surface heparan sulfate and its roles in assisting viral infections

Heparan sulfate, a highly sulfated polysaccharide, is present on the surface of mammalian cells and in the extracellular matrix in large quantities. The sulfated monosaccharide sequences within heparan sulfate determine the protein binding specificity and regulate biological functions. Numerous viruses and parasites utilize cell surface heparan sulfate as receptors to infect target cells. Due to the structural complexity of heparan sulfate, it was considered a nonspecific cell surface receptor by interacting with the positive motifs of viral proteins. However, recent studies reveal that heparan sulfate plays multiple roles in assisting viral infection, and the activities in promoting viral infections require unique monosaccharide sequences, suggesting that heparan sulfate could serve as a specific receptor for viral infection. The currently available techniques for the structural analysis of heparan sulfate provide essential information about the specific roles of heparan sulfate in assisting viral infections. The knowledge accumulated in this fast growing field will permit us to have a better understanding of the mechanism of viral infection and will lead to the development of new antiviral agents.

Specific heparan sulfate structures involved in retinal axon targeting

Heparan sulfate (HS), a structurally diverse molecule comprising distinct sequences of sulfated disaccharide units, is abundant in the developing brain and binds to axon guidance molecules. Addition of HS to the developing Xenopus optic pathway causes severe targeting errors yet it is not known how the structural diversity of this molecule relates to its role in axon guidance. We have used an in vivo brain assay to identify the structural characteristics of HS that induce aberrant axon targeting. Inhibiting sulfation of endogenous HS with chlorate causes axons to bypass their target, the tectum, and treatment with chemically modified heparins reveals that 2-O- and 6-O-sulfate groups have potent bypass-inducing activity. Experiments with purified heparin saccharides show that bypass-inducing activity correlates with distinct structures, particularly those containing a combination of 2-O- and 6-O-sulfate groups. Taken together the results indicate that specific sequences, rather than gross structural composition, are critical for activity. In situ hybridisation revealed that HS 6-O-sulfotransferase is regionally expressed along the border of the dorsal optic tract whereas 2-O-sulfotransferase is expressed broadly. Our results demonstrate that specific HS sequences are essential for regulating retinotectal axon targeting and suggest that regionalised biosynthesis of specific HS structures is important for guiding axons into the tectum.

Proteoglycans of the extracellular matrix and growth control

Regulated cell growth results from the biological balance between soluble growth-regulating factors, their receptors and the elicited signal cascade on the one hand side and from extracellular macromolecular components and their interplay with membrane receptors on the other side. Proteoglycans have recently been recognized not only to play a part in providing shape and biomechanical strength of organs and tissues, but also to exhibit direct and indirect cell signalling properties. In this review, we discuss the direct growth-regulating role of proteoglycans with special emphasis on the lectican family and on the family of small proteoglycans with leucine-rich repeats (SLRPs). Indirect actions of proteoglycans by modulation of growth factor activities and growth factor distribution are exemplified by discussing the TGF-beta-binding properties of SLRPs and the interactions of core proteins of matrix proteoglycans with other growth factors. It is emphasized that the modulatory role of proteoglycans on cell proliferation cannot be separated from their participation in tissue organization in general, thereby explaining the diverse and sometimes contradictory reports on the effects of proteoglycans on cell proliferation and differentiation.

White matter extracellular matrix chondroitin sulfate/dermatan sulfate proteoglycans in multiple sclerosis

Extracellular matrix (ECM) alterations in the central nervous system (CNS) of multiple sclerosis (MS) patients result from blood-brain barrier breakdown, release and activation of proteases, and synthesis of ECM components. To elucidate their potential pathophysiologic roles, we analyzed expression of major CNS ECM proteoglycans (PGs) in MS and control CNS tissues. In active MS plaque edges, 3 CNS lecticans (versican, aggrecan, and neurocan) and dermatan sulfate PG were increased in association with astrocytosis; in active plaque centers they were decreased in the ECM and accumulated in foamy macrophages, suggesting that these ECM PGs are injured and phagocytosed along with myelin. In inactive lesions they were diminished and in normal-appearing white matter they showed heretofore-unappreciated abnormal heterogeneous aggregation. Phosphacan, an ECM PG abundant in both gray and white matter, was less markedly altered. Since in development the spaciotemporal expression of ECM PGs influences neurite outgrowth, cell migration, axon guidance, and myelination, these data suggest that 1) enhanced white matter lectican and dermatan sulfate PG expression in the pro-inflammatory milieu of expanding lesion edges contributes to their sharp boundaries and the failure of neuronal ingrowth; 2) decreases in plaque centers may preclude regeneration and repair; and 3) diffuse ECM PG damage relates to axon degeneration outside of overt lesions. Thus, ECM PG alterations are specific, temporally dynamic, and widespread in MS patients and may play critical roles in lesion pathogenesis and CNS dysfunction.

Expression pattern and secretion of human and chicken heparanase are determined by their signal peptide sequence

Cleavage of heparan sulfate (HS) proteoglycans affects the integrity and function of tissues and thereby fundamental phenomena, involving cell migration and response to changes in the extracellular microenvironment. The role of HS-degrading enzymes, commonly referred to as heparanases, in normal development has not been identified. The present study focuses on cloning, expression, and properties of a chicken heparanase and its distribution in the developing chicken embryo. We have identified a chicken EST, homologous to the recently cloned human heparanase, to clone and express a functional chicken heparanase, 60% homologous to the human enzyme. The full-length chicken heparanase cDNA encodes a 60-kDa proenzyme that is processed at the N terminus into a 45-kDa highly active enzyme. The most prominent difference between the chicken and human enzymes resides in the predicted signal peptide sequence, apparently accounting for the chicken heparanase being readily secreted and localized in close proximity to the cell surface. In contrast, the human enzyme is mostly intracellular, localized in perinuclear granules. Cells transfected with a chimeric construct composed of the chicken signal peptide preceding the human heparanase exhibited cell surface localization and secretion of heparanase, similar to cells transfected with the full-length chicken enzyme. We examined the distribution pattern of the heparanase enzyme in the developing chicken embryo. Both the chicken heparanase mRNA and protein were expressed, as early as 12 h post fertilization, in cells migrating from the epiblast and forming the hypoblast layer. Later on (72 h), the enzyme is preferentially expressed in cells of the developing vascular and nervous systems. Cloning and characterization of heparanase, the first and single functional vertebrate HS-degrading enzyme, may lead to identification of other glycosaminoglycan degrading enzymes, toward elucidation of their significance in normal and pathological processes.

Heparan sulfate proteoglycans in the nervous system: their diverse roles in neurogenesis, axon guidance, and synaptogenesis

Development of the mammalian nervous system involves generation of neurons from neural stem cells, migration of generated neurons toward genetically determined locations, extension of axons and dendrites, and establishment of neuronal connectivity. Recent progresses revealed diverse role of heparan sulfate proteoglycans in these processes. This article reviews our current knowledge about the functional roles of heparan sulfate proteoglycans in three critical events in mammalian neural development, namely neurogenesis, axon guidance, and synapse development.