6B4 proteoglycan/phosphacan is a repulsive substratum but promotes morphological differentiation of cortical neurons

Branched Chondroitin Sulfate Oligosaccharides Derived from the Sea Cucumber Acaudina molpadioides Stimulate Neurite Outgrowth

Fucosylated chondroitin sulfate (FCS) from the sea cucumber Acaudina molpadioides (FCS_Am) is the first one that was reported to be branched by disaccharide GalNAc-(α1,2)-Fuc_3S4S (15%) and sulfated Fuc (85%). Here, four size-homogenous fractions, and seven oligosaccharides, were separated from its β-eliminative depolymerized products. Detailed NMR spectroscopic and MS analyses revealed the oligomers as hexa-, hepta-, octa-, and nonasaccharide, which further confirmed the precise structure of native FCS_Am: it was composed of the CS-E-like backbone with a full content of sulfation at O-4 and O-6 of GalNAc in the disaccharide repeating unit, and the branches consisting of sulfated fucose (Fuc_4S and Fuc_2S4S) and heterodisaccharide [GalNAc-(α1,2)-Fuc_3S4S]. Pharmacologically, FCS_Am and its depolymerized derivatives, including fractions and oligosaccharides, showed potent neurite outgrowth-promoting activity in a chain length-dependent manner. A comparison of analyses among oligosaccharides revealed that the sulfate pattern of the Fuc branches, instead of the heterodisaccharide, could affect the promotion intensity. Fuc_2S4S and the saccharide length endowed the neurite outgrowth stimulation activity most.

Sensory deafferentation modulates and redistributes neurocan in the rat auditory brainstem

INTRODUCTION

Cochlear ablation causing sensory deafferentation (SD) of the cochlear nucleus triggers complex re-arrangements in the cellular and molecular communication networks of the adult mammalian central auditory system. Participation of the extracellular matrix (ECM) in these processes is not well understood.

METHODS

We investigated consequences of unilateral SD for the expression and distribution of the chondroitin sulfate proteoglycans, neurocan (Ncan) and aggrecan (Agg), alongside various plasticity markers in the auditory brainstem of the adult rat using immunohistochemical techniques.

RESULTS

In the deafferented ventral cochlear nucleus (VCN), Ncan expression increased massively within 3 postoperative days (POD), but rapidly decreased thereafter. Agg showed a similar but less pronounced progression. Decrease in Ncan was spatially and temporally related to the re-innervation of VCN documented by the emergence of growth-associated protein Gap43 contained in nerve fibers and presynaptic boutons. Concurrently, astrocytes grew and expressed matrix metalloproteinase-2 (MMP2), an enzyme known to emerge only under re-innervation of VCN. MMP2 is capable of cleaving both Ncan and Agg when released. A transient modulation of the ECM in the central inferior colliculus on the side opposite to SD occurred by POD1. Modulations of glutamatergic synapses and Gap43 expression were detected, reflecting state changes of the surrounding tissue induced by transsynaptic effects of SD.

CONCLUSIONS

The ECM variously participates in adaptive responses to sudden deafness by SD on several levels along the central auditory pathway, with a striking spatial and temporal relationship of Ncan modulation to astrocytic activation and to synaptogenesis.

Expression of aggrecan components in perineuronal nets in the mouse cerebral cortex

Specific regions of the cerebral cortex are highly plastic in an organism's lifetime. It is thought that perineuronal nets (PNNs) regulate plasticity, but labeling for Wisteria floribunda agglutinin (WFA), which is widely used to detect PNNs, is observed throughout the cortex. The aggrecan molecule-a PNN component-may regulate plasticity, and may also be involved in determining region-specific vulnerability to stress. To clarify cortical region-specific plasticity and vulnerability, we qualitatively analyzed aggrecan-positive and glycosylated aggrecan-positive PNNs in the mature mouse cerebral cortex. Our findings revealed the selective expression of both aggrecan-positive and glycosylated aggrecan-positive PNNs in the cortex. WFA-positive PNNs expressed aggrecan in a region-specific manner in the cortex. Furthermore, we observed variable distributions of PNNs containing WFA- and aggrecan-positive molecules. Together, our findings suggest that PNN components and their function differ depending on the cortical region, and that aggrecan molecules may be involved in determining region-specific plasticity and vulnerability in the cortex.

Effect of chondroitin sulfate proteoglycans on neuronal cell adhesion, spreading and neurite growth in culture

As one major component of extracellular matrix (ECM) in the central nervous system, chondroitin sulfate proteoglycans (CSPGs) have long been known as inhibitors enriched in the glial scar that prevent axon regeneration after injury. Although many studies have shown that CSPGs inhibited neurite outgrowth in vitro using different types of neurons, the mechanism by which CSPGs inhibit axonal growth remains poorly understood. Using cerebellar granule neuron (CGN) culture, in this study, we evaluated the effects of different concentrations of both immobilized and soluble CSPGs on neuronal growth, including cell adhesion, spreading and neurite growth. Neurite length decreased while CSPGs concentration arised, meanwhile, a decrease in cell density accompanied by an increase in cell aggregates formation was observed. Soluble CSPGs also showed an inhibition on neurite outgrowth, but it required a higher concentration to induce cell aggregates formation than coated CSPGs. We also found that growth cone size was significantly reduced on CSPGs and neuronal cell spreading was restrained by CSPGs, attributing to an inhibition on lamellipodial extension. The effect of CSPGs on neuron adhesion was further evidenced by interference reflection microscopy (IRM) which directly demonstrated that both CGNs and cerebral cortical neurons were more loosely adherent to a CSPG substrate. These data demonstrate that CSPGs have an effect on cell adhesion and spreading in addition to neurite outgrowth.

Putative Inhibitory Extracellular Matrix Molecules Do Not Prevent Dorsal Root Regeneration into Fetal Spinal Cord Transplants

We examined the distribution of several extracellular matrix molecules (ECM) and their relationship to regenerating axons in embryonic day 14 spinal cord transplants 1 to 12 weeks after transplantation into adult rats. We used immunocytochemical tech niques to label chondroitin sulfate proteoglycans (CSPGs) and tenascin-C in adjacent sections. Synthesis of these molecules by astrocytes is thought to be one mechanism by which astrocytes inhibit regeneration in the central nervous system (CNS); glial fibrillary acidic protein antibody was used to label astrocytes and examine their rela tionship to both the ECM molecules and regenerating calcitonin gene-related pep tide (CORP)-contammg dorsal roots. We also compared the expression and distribu tion of these five markers in transplants with normal spinal cord development.

Does CNS Myelin Inhibit Axon Regeneration?

Membrane proteins on oligodendrocytes and CNS myelin (NI35/250) have been shown to block axon out-growth in culture, and this is thought to be one of the major reasons for severely limited regeneration of severed axons in the CNS of higher vertebrates. In a recent study, adult dorsal root ganglion (DRG) neurons, which are sensitive to these inhibitory proteins, regenerated successfully after transplantation into two white matter tracts of the rat brain without any intervention to suppress the inhibitory activity of CNS myelin. The results and implications of these two studies are considered.

Extracellular matrix protein expression is brain region dependent

In the brain, extracellular matrix (ECM) components form networks that contribute to structural and functional diversity. Maladaptive remodeling of ECM networks has been reported in neurodegenerative and psychiatric disorders, suggesting that the brain microenvironment is a dynamic structure. A lack of quantitative information about ECM distribution in the brain hinders an understanding of region-specific ECM functions and the role of ECM in health and disease. We hypothesized that each ECM protein as well as specific ECM structures, such as perineuronal nets (PNNs) and interstitial matrix, are differentially distributed throughout the brain, contributing to the unique structure and function in the various regions of the brain. To test our hypothesis, we quantitatively analyzed the distribution, colocalization, and protein expression of aggrecan, brevican, and tenascin-R throughout the rat brain utilizing immunohistochemistry and mass spectrometry analysis and assessed the effect of aggrecan, brevican, and/or tenascin-R on neurite outgrowth in vitro. We focused on aggrecan, brevican, and tenascin-R as they are especially expressed in the mature brain, and have established roles in brain development, plasticity, and neurite outgrowth. The results revealed a differentiated distribution of all three proteins throughout the brain and indicated that their presence significantly reduces neurite outgrowth in a 3D in vitro environment. These results underline the importance of a unique and complex ECM distribution for brain physiology and suggest that encoding the distribution of distinct ECM proteins throughout the brain will aid in understanding their function in physiology and in turn assist in identifying their role in disease. J. Comp. Neurol. 524:1309-1336, 2016. © 2016 Wiley Periodicals, Inc.

Sugar-dependent modulation of neuronal development, regeneration, and plasticity by chondroitin sulfate proteoglycans

Chondroitin sulfate proteoglycans (CSPGs) play important roles in the developing and mature nervous system, where they guide axons, maintain stable connections, restrict synaptic plasticity, and prevent axon regeneration following CNS injury. The chondroitin sulfate glycosaminoglycan (CS GAG) chains that decorate CSPGs are essential for their functions. Through these sugar chains, CSPGs are able to bind and regulate the activity of a diverse range of proteins. CSPGs have been found both to promote and inhibit neuronal growth. They can promote neurite outgrowth by binding to various growth factors such as midkine (MK), pleiotrophin (PTN), brain-derived neurotrophic factor (BDNF) and other neurotrophin family members. CSPGs can also inhibit neuronal growth and limit plasticity by interacting with transmembrane receptors such as protein tyrosine phosphatase σ (PTPσ), leukocyte common antigen-related (LAR) receptor protein tyrosine phosphatase, and the Nogo receptors 1 and 3 (NgR1 and NgR3). These CS-protein interactions depend on specific sulfation patterns within the CS GAG chains, and accordingly, particular CS sulfation motifs are upregulated during development, in the mature nervous system, and in response to CNS injury. Thus, spatiotemporal regulation of CS GAG biosynthesis may provide an important mechanism to control the functions of CSPGs and to modulate intracellular signaling pathways. Here, we will discuss these sulfation-dependent processes and highlight how the CS sugars on CSPGs contribute to neuronal growth, axon guidance, and plasticity in the nervous system.

Proteoglycans: road signs for neurite outgrowth

Proteoglycans in the central nervous system play integral roles as "traffic signals" for the direction of neurite outgrowth. This attribute of proteoglycans is a major factor in regeneration of the injured central nervous system. In this review, the structures of proteoglycans and the evidence suggesting their involvement in the response following spinal cord injury are presented. The review further describes the methods routinely used to determine the effect proteoglycans have on neurite outgrowth. The effects of proteoglycans on neurite outgrowth are not completely understood as there is disagreement on what component of the molecule is interacting with growing neurites and this ambiguity is chronicled in an historical context. Finally, the most recent findings suggesting possible receptors, interactions, and sulfation patterns that may be important in eliciting the effect of proteoglycans on neurite outgrowth are discussed. A greater understanding of the proteoglycan-neurite interaction is necessary for successfully promoting regeneration in the injured central nervous system.

Oxygen-glucose deprivation induced glial scar-like change in astrocytes

BACKGROUND

It has been demonstrated that cerebral ischemia induces astrocyte reactivity, and subsequent glial scar formation inhibits axonal regeneration during the recovery phase. Investigating the mechanism of glial scar formation will facilitate the development of strategies to improve axonal regeneration. However, an in vitro model of ischemia-induced glial scar has not yet been systematically established.

METHODOLOGY AND PRINCIPAL FINDINGS

In the present study, we at the first time found that oxygen-glucose deprivation (OGD) in vitro can induce rat cortical astrocytes to present characteristics of glial scar. After OGD for 6 h, astrocytes showed a remarkable proliferation following 24 h reperfusion, evaluated by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay and BrdU immunocytochemistry. Meanwhile, the expression of glial fibrillary acidic protein significantly increased, so did the expression of neurocan, which is a hallmark of the glial scar. In further experiments, neurons were co-cultured with astrocytes, which had been exposed to OGD, and then the immunostaining of class III β-tubulin was carried out to assess the neurite growth. When the co-culture was performed at 48 h reperfusion of astrocytes, the neurite growth was obviously inhibited, and this inhibition could be reversed by chondroitinase ABC, which digests glycosaminoglycan chains on CSPGs, including neurocan. However, the processes of neurons were elongated, when the co-culture was performed immediately after OGD.

CONCLUSIONS AND SIGNIFICANCE

Our results indicated that after conditioned OGD the astrocytes presented the characteristics of the glial scar, which are also comparable to the astrocytes in acute and chronic phases after cerebral ischemia in vivo. Therefore, the present system may be used as an in vitro model to explore the mechanisms underlying glial scar formation and the treatments to improve axonal regeneration after cerebral ischemia.

Novel contact-dependent bone morphogenetic protein (BMP) signaling mediated by heparan sulfate proteoglycans

We previously proposed a model that DALLY, a Drosophila glypican, acts as a trans co-receptor to regulate BMP signaling in the germ line stem cell niche. To investigate the molecular mechanisms of contact-dependent BMP signaling, we developed novel in vitro assay systems to monitor trans signaling using Drosophila S2 cells. Using immunoblot-based as well as single-cell assay systems, we present evidence that Drosophila glypicans indeed enhance BMP signaling in trans in a contact-dependent manner in vitro. Our analysis showed that heparan sulfate modification is required for the trans co-receptor activity of DALLY. Two BMP-like molecules, Decapentaplegic (DPP) and Glass bottom boat, can mediate trans signaling through a heparan sulfate proteoglycan co-receptor in S2 cells. The in vitro systems reflect the molecular characteristics of heparan sulfate proteoglycan functions observed previously in vivo, such as ligand specificity and biphasic activity dependent on the ligand dosage. In addition, experiments using a DALLY-coated surface suggested that DALLY regulates DPP signaling in trans by its effect on the stability of DPP protein on the surface of the contacting cells. Our findings provide the molecular foundation for novel contact-dependent signaling, which defines the physical space of the stem cell niche in vivo.

Activity-dependent remodeling of chondroitin sulfate proteoglycans extracellular matrix in the hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS) consisting of arginine vasopressin (AVP) and oxytocin (OXT) magnocellular neurons shows the structural plasticity including the rearrangement of synapses, dendrites, and neurovascular contacts during chronic physiological stimulation. In this study, we examined the remodeling of chondroitin sulfate proteoglycans (CSPGs), main extracellular matrix (ECM), in the HNS after salt loading known as a chronic stimulation to cause the structural plasticity. In the supraoptic nucleus (SON), confocal microscopic observation revealed that the immunoreactivity of 6B4 proteoglycans (PG) was observed mainly at AVP-positive magnocellular neurons but that of neurocan was seen chiefly at OXT-positive magnocellular neurons. The immunoreactivity of phosphacan and aggrecan was seen at both AVP- and OXT-positive magnocellular neurons. Electron microscopic observation further showed that the immunoreactivity of phosphacan and neurocan was observed at astrocytic processes to surround somata, dendrites, and terminals, but not synaptic junctions. In the neurohypophysis (NH), the immunoreactivity of phosphacan, 6B4 PGs, and neurocan was observed at AVP-positive magnocellular terminals, but the reactivity of Wisteria floribunda agglutinin lectin was seen at OXT-positive ones. The immunoreactivity of versican was found at microvessel and that of aggrecan was not detected in the NH. Quantitative morphometrical analysis showed that the chronic physiological stimulation by 7-day salt loading decreased the level of 6B4 PGs in the SON and the level of phosphacan, 6B4 PGs, and neurocan in the NH. These results suggest that the extracellular microenvironment of CSPGs is different between AVP and OXT magnocellular neurons and activity-dependent remodeling of CSPGs could be involved in the structural plasticity of the HNS.

Traumatic brain injury results in disparate regions of chondroitin sulfate proteoglycan expression that are temporally limited

Axonal injury is a major hallmark of traumatic brain injury (TBI), and it seems likely that therapies directed toward enhancing axon repair could potentially improve functional outcomes. One potential target is chondroitin sulfate proteoglycans (CSPGs), which are major axon growth inhibitory molecules that are generally, but not always, up-regulated after central nervous system injury. The current study was designed to determine temporal changes in cerebral cortical mRNA or protein expression levels of CSPGs and to determine their regional localization and cellular association by using immunohistochemistry in a controlled cortical impact model of TBI. The results showed significant increases in versican mRNA at 4 and 14 days after TBI but no change in neurocan, aggrecan, or phosphacan. Semiquantitative Western blot (WB) analysis of cortical CSPG protein expression revealed a significant ipsilateral decrease of all CSPGs at 1 day after TBI. Lower CSPG protein levels were sustained until at least 14 days, after which the levels began to normalize. Immunohistochemistry data confirm previous reports of regional increases in CSPG proteins after CNS injury, seen primarily within the developing glial scar after TBI, but also corroborate the WB data by revealing wide areas of pericontusional tissue that are deficient in both extracellular and perineuronal net-associated CSPGs. Given the evidence that CSPGs are largely inhibitory to axonal growth, we interpret these data to indicate a potential for regional spontaneous plasticity after TBI. If this were the case, the gradual normalization of CSPG proteins over time postinjury would suggest that this may be temporally as well as regionally limited.

NG2 and phosphacan are present in the astroglial scar after human traumatic spinal cord injury

BACKGROUND

A major class of axon growth-repulsive molecules associated with CNS scar tissue is the family of chondroitin sulphate proteoglycans (CSPGs). Experimental spinal cord injury (SCI) has demonstrated rapid re-expression of CSPGs at and around the lesion site. The pharmacological digestion of CSPGs in such lesion models results in substantially enhanced axonal regeneration and a significant functional recovery. The potential therapeutic relevance of interfering with CSPG expression or function following experimental injuries seems clear, however, the spatio-temporal pattern of expression of individual members of the CSPG family following human spinal cord injury is only poorly defined. In the present correlative investigation, the expression pattern of CSPG family members NG2, neurocan, versican and phosphacan was studied in the human spinal cord.

METHODS

An immunohistochemical investigation in post mortem samples of control and lesioned human spinal cords was performed. All patients with traumatic SCI had been clinically diagnosed as having "complete" injuries and presented lesions of the maceration type.

RESULTS

In sections from control spinal cord, NG2 immunoreactivity was restricted to stellate-shaped cells corresponding to oligodendrocyte precursor cells. The distribution patterns of phosphacan, neurocan and versican in control human spinal cord parenchyma were similar, with a fine reticular pattern being observed in white matter (but also located in gray matter for phosphacan). Neurocan staining was also associated with blood vessel walls. Furthermore, phosphacan, neurocan and versican were present in the myelin sheaths of ventral and dorsal nerve roots axons. After human SCI, NG2 and phosphacan were both detected in the evolving astroglial scar. Neurocan and versican were detected exclusively in the lesion epicentre, being associated with infiltrating Schwann cells in the myelin sheaths of invading peripheral nerve fibres from lesioned dorsal roots.

CONCLUSION

NG2 and phosphacan were both present in the evolving astroglial scar and, therefore, might play an important role in the blockade of successful CNS regeneration. Neurocan and versican, however, were located at the lesion epicentre, associated with Schwann cell myelin on regenerating peripheral nerve fibres, a distribution that was unlikely to contribute to failed CNS axon regeneration. The present data points to the importance of such correlative investigations for demonstrating the clinical relevance of experimental data.

Deoxyribozyme-mediated knockdown of xylosyltransferase-1 mRNA promotes axon growth in the adult rat spinal cord

In the injured spinal cord, proteoglycans (PGs) within scar tissue obstruct axon growth through their glycosaminoglycan (GAG)-side chains. The formation of GAG-side chains (glycosylation) is catalysed by xylosyltransferase-1 (XT-1). Here, we knocked down XT-1 mRNA using a tailored deoxyribozyme (DNAXTas) and hypothesized that this would decrease the amount of glycosylated PGs and, consequently, promote axon growth in the adult rat spinal cord. A continuous 2-week delivery of DNAXTas near the rostral border of a peripheral nerve graft bridging the transected dorsal columns in the thoracic spinal cord resulted in an 81% decrease in XT-1 mRNA, an average of 1.4-fold reduction in GAG-side chains of chondroitin sulphate or heparan sulphate-PGs and 2.2-fold reduction in neurocan and brevican core proteins in scar tissue. Additionally, compared to control deoxyribozyme, the DNAXTas treatment resulted in a 9-fold increase in length and a 4-fold increase in density of ascending axons growing through the nerve graft and scar tissue present at the rostral spinal cord. Together our data showed that treatment with a deoxyribozyme against XT-1 mRNA decreased the amount of glycosylated PGs and promoted axon growth through scar tissue in the injured spinal cord. The deoxyribozyme approach may become a contributing factor in spinal cord repair strategies.

Extracellular matrix of the central nervous system: from neglect to challenge

The basic concept, that specialized extracellular matrices rich in hyaluronan, chondroitin sulfate proteoglycans (aggrecan, versican, neurocan, brevican, phosphacan), link proteins and tenascins (Tn-R, Tn-C) can regulate cellular migration and axonal growth and thus, actively participate in the development and maturation of the nervous system, has in recent years gained rapidly expanding experimental support. The swift assembly and remodeling of these matrices have been associated with axonal guidance functions in the periphery and with the structural stabilization of myelinated fiber tracts and synaptic contacts in the maturating central nervous system. Particular interest has been focused on the putative role of chondroitin sulfate proteoglycans in suppressing central nervous system regeneration after lesions. The axon growth inhibitory properties of several of these chondroitin sulfate proteoglycans in vitro, and the partial recovery of structural plasticity in lesioned animals treated with chondroitin sulfate degrading enzymes in vivo have significantly contributed to the increased awareness of this long time neglected structure.

F3/contactin-related proteins in Helix pomatia nervous tissue (HCRPs): distribution and function in neurite growth and neurotransmitter release

By using antibodies against mouse F3/contactin, we found immunologically related glycoproteins expressed in the nervous tissue of the snail Helix pomatia. Helix contactin-related proteins (HCRPs) include different molecules ranging in size from 90 to 240 kD. Clones isolated from a cDNA expression library allowed us to demonstrate that these proteins are translated from a unique 6.3-kb mRNA, suggesting that their heterogeneity depends on posttranslational processing. This is supported by the results of endoglycosidase F treatment, which indicate that the high-molecular-weight components are glycosylation variants of the 90-kD chain. In vivo and in cultures, HCRPs antibodies label neuronal soma and neurite extensions, giving the appearance of both cytoplasmic and cell surface immunostaining. On the other hand, no expression is found on nonneural tissues. Functionally, HCRPs are involved in neurite growth control and appear to modulate neurotransmitter release, as indicated by the inhibiting effects of specific antibodies on both functions. These data allow the definition of HCRPs glycoproteins as growth-promoting molecules, suggesting that they play a role in neurite development and presynaptic terminal maturation in the invertebrate nervous system.

DACS, novel matrix structure composed of chondroitin sulfate proteoglycan in the brain

Chondroitin sulfate proteoglycans (CSPGs) are major components of the extracellular matrix (ECM) in the brain. In the adult cerebral cortex, there are special CSPG-containing structures known as perineuronal nets (PNNs), which are highly condensed ECM structures. Here, we report a novel CSPG-containing structure distinct from PNNs in the adult mouse cerebral cortex. An anti-chondroitin sulfate antibody CS56 delineated a structure with a unique morphology like a dandelion clock. Accordingly, we named it DAndelion Clock-like Structure (DACS). Immunohistochemical evidence showed that DACSs surrounded a group of NeuN-positive/GABA-negative neurons. At ultrastructural level, CS56-immunoreactivities were localized in the cytoplasm and on the membrane of astrocytes. As the postnatal cerebral cortex matured, DACSs became visible around the end of the critical period. This is the first report demonstrating the presence of an ECM structure DACS composed of CSPGs around a group of cortical neurons in the adult cerebral cortex.

DSD-1-Proteoglycan/Phosphacan and Receptor Protein Tyrosine Phosphatase-Beta Isoforms during Development and Regeneration of Neural Tissues

Interactions between neurons and glial cells play important roles in regulating key events of development and regeneration of the CNS. Thus, migrating neurons are partly guided by radial glia to their target, and glial scaffolds direct the growth and directional choice of advancing axons, e.g., at the midline. In the adult, reactive astrocytes and myelin components play a pivotal role in the inhibition of regeneration. The past years have shown that astrocytic functions are mediated on the molecular level by extracellular matrix components, which include various glycoproteins and proteoglycans. One important, developmentally regulated chondroitin sulfate proteoglycan is DSD-1-PG/phosphacan, a glial derived proteoglycan which represents a splice variant of the receptor protein tyrosine phosphatase (RPTP)-beta (also known as PTP-zeta). Current evidence suggests that this proteoglycan influences axon growth in development and regeneration, displaying inhibitory or stimulatory effects dependent on the mode of presentation, and the neuronal lineage. These effects seem to be mediated by neuronal receptors of the Ig-CAM superfamily.

Rho GTPases regulate PTPmu-mediated nasal neurite outgrowth and temporal repulsion of retinal ganglion cell neurons

Members of the receptor protein tyrosine phosphatase (RPTP) subfamily of cell adhesion molecules (CAMs) mediate neurite outgrowth and growth cone repulsion. PTPmu is a growth permissive substrate for nasal retinal ganglion cell (RGC) neurites and a growth inhibitory substrate for temporal RGCs. In this manuscript, we demonstrate that the distinct PTPmu-dependent phenotypes of nasal outgrowth and temporal repulsion are regulated by Rho GTPases. The role of Rho GTPases in the regulation of nasal outgrowth and temporal repulsion was tested by utilizing dominant negative and constitutively active forms of Rac1, RhoA and Cdc42 in Bonhoeffer stripe assays. Nasal neurite outgrowth on PTPmu was blocked by Cdc42-DN. Temporal repulsion to a PTPmu substrate was substantially reduced by addition of Cdc42-DN. The molecule that regulates the switch between permissive versus repulsive responses to PTPmu is Rac1 for temporal neurons. Inhibition of Rac1 is required for repulsion of temporal neurons. Interestingly, adding Rac1-CA to temporal RGC neurons converted PTPmu-dependent repulsion to a permissive response. In addition, adding exogenous Rac1-DN to nasal neurons induced a phenotype switch from a permissive to repulsive response to PTPmu. Together these data suggest that Cdc42 activity is required for both permissive and repulsive responses to PTPmu. However, the key to PTPmu-dependent repulsion is inhibition of Rac1 activity in temporal RGC neurons.

Brain keratan sulfate and glial scar formation

In response to injury to the central nervous system (CNS), reactive astrocytes appear and accumulate in the wounded area, leading to glial scar formation. Glial scar is the physical barrier to axonal regeneration of injured neurons. Chondroitin sulfate proteoglycans are inhibitory to axon outgrowth and are upregulated in reactive astrocytes upon CNS injury. It is known that keratan sulfate proteoglycans (KSPGs) are also augmented after CNS injury and act as inhibitory cues. We give a brief overview of CNS injury and cover our recent data regarding the relationship between glial scar formation and KS. KS expression in the developing brain is detectable with 5D4, a KS-specific monoclonal antibody. These 5D4 immunoreactivities are eliminated in mice deficient in N-acetylglucosamine 6-O-sulfotransferase-1. In adult mice, brain injury apparently upregulates mRNA expression of N-acetylglucosamine 6-O-sulfotransferase-1 as well as 5D4-reactive KS in the wounded area. Intriguingly, the expression of 5D4-reactive KS and reactive astrocyte accumulation in the wounded area are dramatically diminished in the sulfotransferase-deficient mice. Consequently, the deficient mice exhibit a marked reduction in scar formation and enhancement of neuronal regeneration after brain injury. Thus, N-acetylglucosamine 6-O-sulfotransferase-1 plays indispensable roles in brain KS biosynthesis and glial scar formation after brain injury.

Protein tyrosine phosphatase hPTPN20a is targeted to sites of actin polymerization

The human genome encodes 38 classical tyrosine-specific PTPs (protein tyrosine phosphatases). Many PTPs have been shown to regulate fundamental cellular processes and several are mutated in human diseases. We report that the product of the PTPN20 gene at the chromosome locus 10q11.2 is alternatively spliced to generate 16 possible variants of the classical human non-transmembrane PTP 20 (hPTPN20). One of these variants, hPTPN20a, was expressed in a wide range of both normal and transformed cell lines. The catalytic domain of hPTPN20 exhibited catalytic activity towards tyrosyl phosphorylated substrates, confirming that it is a bona fide PTP. In serum-starved COS1 cells, hPTPN20a was targeted to the nucleus and the microtubule network, colocalizing with the microtubule-organizing centre and intracellular membrane compartments, including the endoplasmic reticulum and the Golgi apparatus. Stimulation of cells with epidermal growth factor, osmotic shock, pervanadate, or integrin ligation targeted hPTPN20a to actin-rich structures that included membrane ruffles. The present study identifies hPTPN20a as a novel and widely expressed phosphatase with a dynamic subcellular distribution that is targeted to sites of actin polymerization.

Chondroitin sulfate proteoglycan phosphacan associates with parallel fibers and modulates axonal extension and fasciculation of cerebellar granule cells

Phosphacan is a nervous system-specific chondroitin sulfate proteoglycan and one of the major components of extracellular matrix in the brain. In the present study, we examined its spatiotemporal expression, ultrastructural localization, binding manner, and in vitro analysis on cell adhesion, axonal extension, and fasciculation in rat cerebellum. The present light microscopic immunohistochemistry showed that phosphacan immunoreactivity was localized mainly at the molecular layer in the cerebellum, but not at the external granular layer. Further double labeling immunohistochemical and immunoelectron microscopic studies revealed that phosphacan was localized around parallel fibers, but not at synapses. The binding of phosphacan to membrane and/or extracellular matrix partly required Ca2+ and was mediated through its core glycoprotein. Phosphacan inhibited adhesion and axonal extension of cerebellar granule cells in dissociated culture, while it promoted axonal fasciculation of their aggregated culture. These results indicate that phosphacan around parallel fibers may be the repulsive substratum for adhesion and extension of granule cells and promote the fasciculation of parallel fibers.

Cortical neurons express PSI, a novel isoform of phosphacan/RPTPbeta

Phosphacan is a chondroitin sulfate proteoglycan representing the secreted extracellular part of a transmembrane receptor protein tyrosine phosphatase (RPTP-beta). These isoforms have been implicated in cell-extracellular matrix signaling events associated with myelination, axon growth, and cell migration in the developing central nervous system and may play critical roles in the context of brain pathologies. Recently, we have reported the identification of a new isoform of phosphacan, the phosphacan short isoform (PSI), the expression of which peaks in the second postnatal week. PSI interacts with the neuronal receptors L1 and F3/contactin and can promote neurite growth of cortical neurons. In this study, we have assessed, by in situ hybridization, the expression profile of PSI in the rat brain at postnatal day 7. PSI is largely expressed in the gray matter of the developing cerebral cortex in which it colocalizes with phosphacan, whereas the expression of RPTPbeta receptor forms is restricted to the ventricular area in which PSI has not been observed. Neurons from all layers of the cortex express PSI. In the cerebellum, on the other hand, no expression of PSI has been detected, although the other phosphacan/RPTP-beta isoforms show strong PSI expression here. Overall, our study suggests that PSI is expressed during the postnatal period in differentiated neurons of the cortex but is absent from structures in which proliferation and migration occur. The significance of these observations is discussed in the context of previous models of phosphacan/RPTP-beta functions.

PTPσ promotes retinal neurite outgrowth non-cell-autonomously

The receptor-like protein tyrosine phosphatase (RPTP) PTPsigma controls the growth and targeting of retinal axons, both in culture and in ovo. Although the principal actions of PTPsigma have been thought to be cell-autonomous, the possibility that RPTPs related to PTPsigma also have non-cell-autonomous signaling functions during axon development has also been supported genetically. Here we report that a cell culture substrate made from purified PTPsigma ectodomains supports retinal neurite outgrowth in cell culture. We show that a receptor for PTPsigma must exist on retinal axons and that binding of PTPsigma to this receptor does not require the known, heparin binding properties of PTPsigma. The neurite-promoting potential of PTPsigma ectodomains requires a basic amino acid domain, previously demonstrated in vitro as being necessary for ligand binding by PTPsigma. Furthermore, we demonstrate that heparin and oligosaccharide derivatives as short as 8mers, can specifically block neurite outgrowth on the PTPsigma substrate, by competing for binding to this same domain. This is the first direct evidence of a non-cell-autonomous, neurite-promoting function of PTPsigma and of a potential role for heparin-related oligosaccharides in modulating neurite promotion by an RPTP.

Growth factor and cytokine regulation of chondroitin sulfate proteoglycans by astrocytes

After injury to the adult central nervous system (CNS), numerous cytokines and growth factors are released that contribute to reactive gliosis and extracellular matrix production. In vitro examination of these cytokines revealed that the presence of transforming growth factor-beta1 (TGF-beta1) and epidermal growth factor (EGF) greatly increased the production of several chondroitin sulfate proteoglycans (CSPG) by astrocytes. Treatment of astrocytes with other EGF-receptor (ErbB1) ligands, such as TGF-alpha and HB-EGF, produced increases in CSPG production similar to those observed with EGF. Treatment of astrocytes, however, with heregulin, which signals through other members of the EGF-receptor family (ErbB2, ErbB3, ErbB4), did not induce CSPG upregulation. The specificity of activation through the ErbB1 receptor was further verified by using a selective antagonist (AG1478) to this tyrosine kinase receptor. Western blot analysis of astrocyte supernatant pre-digested with chondroitinase ABC indicated the presence of multiple core proteins containing 4-sulfated or 6-sulfated chondroitin. To identify some of these CSPGs, Western blots were screened using antibodies to several known CSPG core proteins. These analyses showed that treatment of astrocytes with EGF increased phosphacan expression, whereas treatment with TGF-beta1 increased neurocan expression. Reverse transcription-polymerase chain reaction (RT-PCR) was used to examine the expression of these molecules in vivo, which result in increased expression of TGF-beta1, EGF-receptor, neurocan, and phosphacan after injury to the brain. These data begin to elucidate some of the injury-induced growth factors that regulate the expression of CSPGs which could be targeted in the future to modulate CSPG production after injury to the central nervous system.

Activity-dependent regulation of a chondroitin sulfate proteoglycan 6B4 phosphacan/RPTPbeta in the hypothalamic supraoptic nucleus

The hypothalamic magnocellular neurons, synthesizing arginine vasopressin (AVP) and oxytocin, are well known to show structural plasticity during chronic physiological stimulation. We have previously reported that 6B4 phosphacan/receptor-type protein-tyrosine phosphatasebeta (RPTPbeta), a chondroitin sulfate proteoglycan is highly expressed in the supraoptic nucleus (SON) of adult hypothalamus. Here, we undertook to study the activity-dependent regulation of 6B4 phosphacan/RPTPbeta in this system. Double labeling confocal microscopy demonstrated in the SON that 6B4 phosphacan/RPTPbeta-immunoreactive perineuronal nets were seen around AVP-containing somata and dendrites and its distribution pattern was well coincided with that of TAG-1. Quantitative immunohistochemical and Western analyses showed that 1-week salt loading, known as the chronic physiological stimulation for inducing the structural changes such as synaptic remodeling and direct neuronal membrane apposition, decreased 6B4 phosphacan/RPTPbeta levels in the SON, but did not alter TAG-1 levels. The 6B4 phosphacan/RPTPbeta levels were returned to control basal values within 3 weeks after the cessation of the chronic stimulation. Activity-dependent decreases in 6B4 phosphacan/RPTPbeta levels of the SON were confirmed when Western and immunohistochemical samples were digested with chondroitinase ABC, indicating that the decrease in 6B4 phosphacan/RPTPbeta levels was due to disappearance of 6B4 phosphacan/RPTPbeta core protein rather than increase in chondroitin sulfate glycosaminoglycans. With electron microscopy, the electron-dense immunoproducts for 6B4 phosphacan/RPTPbeta were found on the membrane surface of axons and glial processes, but not at synaptic junctions in control SON, and its immunoreactivity was eliminated with the chronic salt loading. The present results indicate that the levels of 6B4 phosphacan/RPTPbeta are regulated with activity-dependent manner and may be concerned with the structural plasticity seen in the SON.

Regulation of RPTPbeta/phosphacan expression and glycosaminoglycan epitopes in injured brain and cytokine-treated glia

Several chondroitin sulfate proteoglycans (CSPGs) are upregulated after CNS injury and are thought to limit axonal regeneration in the adult mammalian CNS. Therefore, we examined the expression of the CSPG, receptor protein tyrosine phosphatase beta (RPTPbeta)/phosphacan, after a knife lesion to the cerebral cortex and after treatment of glial cultures with regulatory factors. The three splice variants of this CSPG gene, the secreted isoform, phosphacan, and the two transmembrane isoforms, the long and short RPTPbeta, were examined. Western blot and immunostaining analysis of injured and uninjured tissue revealed a transient decrease of phosphacan protein levels, but not of short RPTPbeta, in the injured tissue from 1 to 7 days postlesion (dpl). By real time RT-PCR, we show that phosphacan and long RPTPbeta mRNA levels are transiently down-regulated at 2 dpl, unlike those of short RPTPbeta which increased after 4 dpl. In contrast to the core glycoprotein, the phosphacan chondroitin sulfate (CS) glycosaminoglycan epitope DSD-1 was up-regulated after 7 dpl. Phosphacan was expressed by cultivated astrocytes and oligodendrocyte precursors but was more glycanated in oligodendrocyte precursors, which produce more of DSD-1 epitope than astrocytes. Epidermal growth factor/transforming growth factor alpha strongly increased the astrocytic expression of long RPTPbeta and phosphacan and slightly the short RPTPbeta protein levels, while interferon gamma and tumor necrosis factor alpha reduced astrocytic levels of phosphacan, but not of the receptor forms. Examining the effects of phosphacan on axon growth from rat E17 cortical neurons, we found that phosphacan stimulates outgrowth in a largely CS dependent manner, while it blocks the outgrowth-promoting effects of laminin through an interaction that is not affected by removal of the CS chains. These results demonstrate complex injury-induced modifications in phosphacan expression and glycanation that may well influence axonal regeneration and repair processes in the damaged CNS.

Receptor-like protein tyrosine phosphatase zeta/RPTP beta is expressed on tangentially aligned neurons in early mouse neocortex

Protein tyrosine phosphatase zeta (PTPzeta)/RPTPbeta is a chondroitin sulfate proteoglycan predominantly expressed in the brain. In this study, we examined immunohistochemical localisation of PTPzeta in the mouse telencephalon from embryonic day 9.5 (E9.5) to E15.5. During E10.5-E12.5, immunoreactivities for PTPzeta are specifically observed on the tangentially aligned neurons at the preplate (PP) of the neocortex, as well as on the neurons at the mantle layer (ML) of the ganglionic eminences (GEs). Likewise, neurons immunoreactive for CR50, a marker for Cajal-Retzius neurons, are aligned from the ML of the ganglionic eminences to the PP of the neocortex and co-express PTPzeta. During E13.5-E15.5, PTPzeta-positive neurons are present at the subplate (SP) as well as at the marginal zone (MZ) of the neocortex. These results indicate that PTPzeta is a useful marker for early-generated neocortical neurons in mice: Cajal-Retzius neurons as well as the subplate neurons.

Kainic acid-induced convulsions cause prolonged changes in the chondroitin sulfate proteoglycans neurocan and phosphacan in the limbic structures

Systemic administration of kainic acid induces repeated convulsive seizures (KA convulsions) that result in neuropathological changes similar to temporal lobe epilepsy and the appearance of spontaneous recurrent seizures (SRS). The appearance of SRS is considered a result of the remodeling of neuronal networks following neuronal degeneration. We investigated the changes in chondroitin sulfate proteoglycans (CSPGs) in the limbic structures after KA convulsions in the rat using monoclonal antibodies 1G2, which recognizes full-length neurocan and the C-terminal half of neurocan, neurocan C, and 6B4, which recognize phosphacan and protein tyrosine phosphatase zeta. After KA convulsions, full-length neurocan appeared by 24 h and reached a peak by 48 to 72 h, whereas phosphacan decreased within 24 h in the hippocampus. In immunohistochemistry, neurocan increased in the limbic structures coincident with the appearance of reactive astrocytes. Phosphacan decreased coincident with pyramidal cell loss in the hippocampus, and the number of phosphacan-positive perineuronal nets around parvalbumin neurons decreased, whereas parvalbumin neurons were relatively conserved. In contrast, phosphacan increased in the entorhinal and piriform cortices in correlation with the severity of neuronal loss. Both neurocan and phosphacan recovered to the control level by 8 weeks after KA convulsions in some rats, but the changes in neurocan and phosphacan described above still persisted in more than half the rats. The results indicate that KA convulsions induce prolonged changes in neurocan and phosphacan similar to those in the developing rat brain and suggest a role of these CSPGs in the remodeling of neuronal networks related to the establishment or enhancement of epileptogenesis.

The role of ECM molecules in activity-dependent synaptic development and plasticity

Growth and guidance of neurites (axons and dendrites) during development is the prerequisite for the establishment of functional neural networks in the adult organism. In the adult, mechanisms similar to those used during development may regulate plastic changes that underlie important nervous system functions, such as memory and learning. There is now ever-increasing evidence that extracellular matrix (ECM)-associated factors are critically involved in the formation of neuronal connections during development, and their plastic changes in the adult. Here, we review the current literature on the role of ECM components in activity-dependent synaptic development and plasticity, with the major focus on the thrombospondin type I repeat (TSR) domain-containing proteins. We propose that ECM components may modulate neuronal development and plasticity by: 1) regulating cellular motility and morphology, thus contributing to structural alterations that are associated with the expression of synaptic plasticity, 2) coordinating transsynaptic signaling during plasticity via their cell surface receptors, and 3) defining the physical parameters of the extracellular space, thereby regulating diffusion of soluble signaling molecules in the extracellular space (ECS).

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.

Expression of midkine and receptor-like protein tyrosine phosphatase (RPTP)-beta genes in the rat stomach and the influence of rebamipide

BACKGROUND

Midkine has been reported to bind to receptor-like protein tyrosine phosphatase (RPTP)-beta and to play important roles in growth and differentiation of various cells. Midkine is expressed in rat stomach during experimental ulcer healing, suggesting that the midkine-RPTP-beta system has some physiological functions in the stomach. Rebamipide is a mucoprotective drug used for the treatment of gastric ulcers. We have tested the hypothesis that the ulcer healing mechanism stimulated by rebamipide is linked physiologically to the gastric midkine-RPTP-beta system.

MATERIALS AND METHODS

Seven-week-old-male Wistar rats were used. Midkine and RPTP-beta gene expression in rat stomach was investigated by laser capture microdissection coupled with the reverse transcription-polymerase chain reaction (RT-PCR). The effects of rebamipide on midkine and RPTP-beta expression in rat stomach and the gastric epithelial cell line RGM1 were evaluated by RT-PCR and Northern blot analyses.

RESULTS

Midkine and RPTP-beta expression was detected in the gastric mucosal, submucosal and muscle layers. Rebamipide stimulated both midkine and RPTP-beta expression in rat stomach and RGM1 cells.

CONCLUSION

Rebamipide may protect the gastric mucosa by regulating midkine and RPTP-beta expression.

Phosphacan short isoform, a novel non-proteoglycan variant of phosphacan/receptor protein tyrosine phosphatase-beta, interacts with neuronal receptors and promotes neurite outgrowth

Phosphacan, one of the principal proteoglycans in the extracellular matrix of the central nervous system, is implicated in neuron-glia interactions associated with neuronal differentiation and myelination. We report here the identification of a novel truncated form of phosphacan, phosphacan short isoform (PSI), that corresponds to the N-terminal carbonic anhydrase- and fibronectin type III-like domains and half of the spacer region. The novel cDNA transcript was isolated by screening of a neonatal brain cDNA expression library using a polyclonal antibody raised against phosphacan. Expression of this transcript in vivo was confirmed by Northern blot hybridization. Analysis of brain protein extracts reveals the presence of a 90-kDa glycosylated protein in the phosphate-buffered saline-insoluble 100000 x g fraction that reacts with antisera against both phosphacan and a recombinant PSI protein and that has the predicted N-terminal sequence. This protein is post-translationally modified with oligosaccharides, including the HNK-1 epitope, but, unlike phosphacan, it is not a proteoglycan. The expression of the PSI protein varies during central nervous system development in a fashion similar to that observed for phosphacan, being first detected around embryonic day 16 and then showing a dramatic increase in expression to plateau around the second week post-natal. Both the native and recombinant PSI protein can interact with the Ig cell adhesion molecules, F3/contactin and L1, and in neurite outgrowth assays, the PSI protein can promote outgrowth of cortical neurons when used as a coated substrate. Hence, the identification of this novel isoform of phosphacan/receptor protein tyrosine phosphatase-beta provides a new component in cell-cell and cell-extracellular matrix signaling events in which these proteins have been implicated.

Concentration-dependent actions of glial chondroitin sulfate on the neuritic growth of midbrain neurons

Astrocytes located in two distinct regions of midbrain differ in their neuritic growth support abilities. Midbrain neurons cultured onto astrocyte monolayers from the lateral (L) region develop long and branched neurites while neurons cultured onto astrocyte monolayers from the medial (M) region develop short or no neurites. The extracellular matrix of these astrocytes has an important role in promoting or inhibiting the growth of these neurons. Differences on the compartmental distribution, as well as on the concentration of GAGs of L and M astrocytes, may be related to their differential capacity of supporting neuritic growth. Indeed, enzymatic digestion of heparan sulfate (HS) and chondroitin sulfate (CS) chains also pointed to an important function for GAGs on axon navigation. In order to better characterize the role of CS on the growth of midbrain neurites, we treated L and M astrocyte monolayers with 1 mM of beta-D-xyloside. Under these conditions, astrocytes oversynthesized and secreted CS protein-free chains to the culture medium. M astrocytes had a significant reduction in their neuritic growth-inhibiting ability after xyloside treatment, suggesting a promoting role for soluble CS in neuritic growth. Chondroitin 4-sulfate (CS-4) added in different concentrations to M astrocyte cultures turned this glia into a permissive substrate, acting in a linear way as far as the largest neurite was concerned. However, a U-shaped dose-effect curve on neurite growth resulted from the similar treatment of L astrocytes. These results suggest that glial CS-4 could be involved in the neurite growth modulating properties of midbrain neurons in a complex concentration-dependent way.

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.

Chondroitin sulfate disrupts axon pathfinding in the optic tract and alters growth cone dynamics

Little is known about the cues that guide retinal axons across the diencephalon en route to their midbrain target, the optic tectum. Here we show that chondroitin sulfate proteoglycans are differentially expressed within the diencephalon at a time when retinal axons are growing within the optic tract. Using exposed brain preparations, we show that the addition of exogenous chondroitin sulfate results in retinal pathfinding errors. Retinal axons disperse widely from their normal trajectory within the optic tract and extend aberrantly into inappropriate regions of the forebrain. Time-lapse analysis of retinal growth cone dynamics in vivo shows that addition of exogenous chondroitin sulfate causes intermittent stalling and increases growth cone complexity. These results suggest that chondroitin sulfate may modulate the guidance of retinal axons as they grow through the diencephalon towards the optic tectum.

Chondroitin sulfate proteoglycan phosphacan/RPTPbeta in the hypothalamic magnocellular nuclei

The hypothalamo-neurohypophysial system synthesizes and releases arginine vasopressin (AVP) and oxytocin (OXT) with physiological stimulation. In the present study, we investigated localization of a chondroitin sulfate proteoglycan (CSPG), phosphacan/RPTPbeta, in the supraoptic nucleus (SON) and paraventricular nucleus (PVN) of adult rats at both the light and electron microscopic levels. Immunohistochemical analyses demonstrated stronger phosphacan/RPTPbeta immunoreactivity within the SON and PVN compared with adjacent hypothalamic areas. Double labeling experiments showed phosphacan/RPTPbeta immunoreactivity constituting punctate networks to surround the somata and dendrites of AVP- and OXT-secreting magnocellular neurons. Electron microscopic examination further revealed strong phosphacan/RPTPbeta immunoreactivity at extracellular membrane surface of some axons, somata, and dendrites of the SON, but not of synaptic junctions. Interestingly, phosphacan/RPTPbeta immunoreactivity was also observed at extracellular surface membrane between astrocytic processes and neurons rather than between magnocellular neurons. The present results indicate the high expression of the CSPG, phosphacan/RPTPbeta at the extracellular space in the hypothalamic AVP- and OXT-secreting magnocellular neurons.

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.

Role of heparin-binding growth-associated molecule (HB-GAM) in hippocampal LTP and spatial learning revealed by studies on overexpressing and knockout mice

Heparin-binding growth-associated molecule (HB-GAM) is an extracellular matrix-associated protein with neurite outgrowth-promoting activity and which is suggested to be implicated in hippocampal synaptic plasticity. To study the functions of HB-GAM in adult brain we have produced HB-GAM overexpressing mice and compared phenotypic changes in the transgenic mice to those in the HB-GAM null mice. Both mutants were viable and displayed no gross morphological abnormalities. The basal synaptic transmission was normal in the area CA1 of hippocampal slices from the genetically modified mice. However, long-term potentiation (LTP) was attenuated in the mice overexpressing HB-GAM, whereas enhanced LTP was detected in the HB-GAM-deficient mice. Changes in LTP seen in vitro were paralleled by behavioral alterations in vivo. The animals overexpressing HB-GAM displayed faster learning in water maze and decreased anxiety in elevated plus-maze, while the HB-GAM knockouts demonstrated an opposite behavioral phenotype. These results show that HB-GAM suppresses LTP in hippocampus and plays a role in regulation of learning-related behavior.

Relationship between sprouting axons, proteoglycans and glial cells following unilateral nigrostriatal axotomy in the adult rat

Proteoglycans may modulate axon growth in the intact and injured adult mammalian CNS. Here we investigate the distribution and time course of deposition of a range of proteoglycans between 4 and 14 days following unilateral axotomy of the nigrostriatal tract in anaesthetised adult rats. Immunolabelling using a variety of antibodies was used to examine the response of heparan sulphate proteoglycans, chondroitin sulphate proteoglycans and keratan sulphate proteoglycans. We observed that many proteoglycans became abundant between 1 and 2 weeks post-axotomy. Heparan sulphate proteoglycans were predominantly found within the lesion core (populated by blood vessels, amoeboid macrophages and meningeal fibroblasts) whereas chondroitin sulphate proteoglycans and keratan sulphate proteoglycans were predominantly found in the lesion surround (populated by reactive astrocytes, activated microglia and adult precursor cells). Immunolabelling indicated that cut dopaminergic nigral axons sprouted prolifically within the lesion core but rarely grew into the lesion surround. We conclude that sprouting of cut dopaminergic nigral axons may be supported by heparan sulphate proteoglycans but restricted by chondroitin sulphate proteoglycans and keratan sulphate proteoglycans.

Cellular and molecular basis for the formation of lamina-specific thalamocortical projections

The neocortex is composed of a characteristic layered structure, which is a basis of extrinsic and intrinsic cortical connections. In recent years the cellular and molecular mechanisms, which are responsible for the formation of lamina-specific connections, have been explored by extensive molecular and in vitro studies. This article attempts to address what cell-cell interactions are required for axonal targeting and what molecules regulate cellular events, focusing upon the development of the thalamocortical projection.

CRYP-2/cPTPRO is a neurite inhibitory repulsive guidance cue for retinal neurons in vitro

Receptor protein tyrosine phosphatases (RPTPs) are implicated as regulators of axon growth and guidance. Genetic deletions in the fly have shown that type III RPTPs are important in axon pathfinding, but nothing is known about their function on a cellular level. Previous experiments in our lab have identified a type III RPTP, CRYP-2/cPTPRO, specifically expressed during the period of axon outgrowth in the chick brain; cPTPRO is expressed in the axons and growth cones of retinal and tectal projection neurons. We constructed a fusion protein containing the extracellular domain of cPTPRO fused to the Fc portion of mouse immunoglobulin G-1, and used it to perform in vitro functional assays. We found that the extracellular domain of cPTPRO is an antiadhesive, neurite inhibitory molecule for retinal neurons. In addition, cPTPRO had potent growth cone collapsing activity in vitro, and locally applied gradients of cPTPRO repelled growing retinal axons. This chemorepulsive effect could be regulated by the level of cGMP in the growth cone. Immunohistochemical examination of the retina indicated that cPTPRO has at least one heterophilic binding partner in the retina. Taken together, our results indicate that cPTPRO may act as a guidance cue for retinal ganglion cells during vertebrate development.

Soluble forms of NCAM and F3 neuronal cell adhesion molecules promote Schwann cell migration: identification of protein tyrosine phosphatases zeta/beta as the putative F3 receptors on Schwann cells

Neural cell adhesion molecule (NCAM) and F3 are both axonal adhesion molecules which display homophilic (NCAM) or heterophilic (NCAM, F3) binding activities and participate in bidirectional exchange of information between neurones and glial cells. Engineered Fc chimeric molecules are fusion proteins that contain the extracellular part of NCAM or F3 and the Fc region of human IgG1. Here, we investigated the effect of NCAM-Fc and F3-Fc chimeras on Schwann cell (SC) migration. Binding sites were identified at the surface of cultured SCs by chimera coated fluorospheres. The functional effect of NCAM-Fc and F3-Fc binding was studied in two different SC migration models. In the first, migration is monitored at specific time intervals inside a 1-mm gap produced in a monolayer culture of SCs. In the second, SCs from a dorsal root ganglion explant migrate on a sciatic nerve cryosection. In both systems addition of the chimeras significantly increased the extent of SC migration and this effect could be prevented by the corresponding anti-NCAM or anti-F3 blocking antibodies. Furthermore, antiproteoglycan-type protein tyrosine phosphatase zeta/beta (RPTPzeta/beta) antibodies identified the presence of RPTPzeta/beta on SCs and prevented the enhancing effect of soluble F3 on SC motility by 95%. The F3-Fc coated Sepharose beads precipitated RPTPzeta/beta from SC lysates. Altogether these data point to RPTPzeta/beta is the putative F3 receptor on SCs. These results identify F3 and NCAM receptors on SC as potential mediators of signalling occurring between axons and glial cells during peripheral nerve development and regeneration.

Identification of GIT1/Cat-1 as a substrate molecule of protein tyrosine phosphatase zeta /beta by the yeast substrate-trapping system

We used a genetic method, the yeast substrate-trapping system, to identify substrates for protein tyrosine phosphatases zeta (PTPzeta/RPTPbeta). This method is based on the yeast two-hybrid system, with two essential modifications: conditional expression of protein tyrosine kinase v-src (active src) to tyrosine-phosphorylate the prey proteins and screening by using a substrate-trap mutant of PTPzeta (PTPzeta-D1902A) as bait. By using this system, several substrate candidates for PTPzeta were isolated. Among them, GIT1/Cat-1 (G protein-coupled receptor kinase-interactor 1/Cool-associated, tyrosine-phosphorylated 1) was examined further. GIT1/Cat-1 bound to PTPzeta-D1902A dependent on the substrate tyrosine phosphorylation. Tyrosine-phosphorylated GIT1/Cat-1 was dephosphorylated by PTPzeta in vitro. Immunoprecipitation experiments indicated that PTPzeta-D1902A and GIT1/Cat-1 form a stable complex also in mammalian cells. Immunohistochemical analyses revealed that PTPzeta and GIT1/Cat-1 were colocalized in the processes of pyramidal cells in the hippocampus and neocortex in rat brain. Subcellular colocalization was further verified in the growth cones of mossy fibers from pontine explants and in the ruffling membranes and processes of B103 neuroblastoma cells. Moreover, pleiotrophin, a ligand for PTPzeta, increased tyrosine phosphorylation of GIT1/Cat-1 in B103 cells. All these results indicate that GIT1/Cat-1 is a substrate molecule of PTPzeta.