Domains of neuronal heparan sulphate proteoglycans involved in neurite growth on laminin

Hippo cell signaling and HS-proteoglycans regulate tissue form and function, age-dependent maturation, extracellular matrix remodeling, and repair

This review examined how Hippo cell signaling and heparan sulfate (HS)-proteoglycans (HSPGs) regulate tissue form and function. Despite being a nonweight-bearing tissue, the brain is regulated by Hippo mechanoresponsive cell signaling pathways during embryonic development. HS-proteoglycans interact with growth factors, morphogens, and extracellular matrix components to regulate development and pathology. Pikachurin and Eyes shut (Eys) interact with dystroglycan to stabilize the photoreceptor axoneme primary cilium and ribbon synapse facilitating phototransduction and neurotransduction with bipolar retinal neuronal networks in ocular vision, the primary human sense. Another HSPG, Neurexin interacts with structural and adaptor proteins to stabilize synapses and ensure specificity of neural interactions, and aids in synaptic potentiation and plasticity in neurotransduction. HSPGs also stabilize the blood-brain barrier and motor neuron basal structures in the neuromuscular junction. Agrin and perlecan localize acetylcholinesterase and its receptors in the neuromuscular junction essential for neuromuscular control. The primary cilium is a mechanosensory hub on neurons, utilized by YES associated protein (YAP)-transcriptional coactivator with PDZ-binding motif (TAZ) Hippo, Hh, Wnt, transforming growth factor (TGF)-β/bone matrix protein (BMP) receptor tyrosine kinase cell signaling. Members of the glypican HSPG proteoglycan family interact with Smoothened and Patched G-protein coupled receptors on the cilium to regulate Hh and Wnt signaling during neuronal development. Control of glycosyl sulfotransferases and endogenous protease expression by Hippo TAZ YAP represents a mechanism whereby the fine structure of HS-proteoglycans can be potentially modulated spatiotemporally to regulate tissue morphogenesis in a similar manner to how Hippo signaling controls sialyltransferase expression and mediation of cell-cell recognition, dysfunctional sialic acid expression is a feature of many tumors.

HS, an Ancient Molecular Recognition and Information Storage Glycosaminoglycan, Equips HS-Proteoglycans with Diverse Matrix and Cell-Interactive Properties Operative in Tissue Development and Tissue Function in Health and Disease

Heparan sulfate is a ubiquitous, variably sulfated interactive glycosaminoglycan that consists of repeating disaccharides of glucuronic acid and glucosamine that are subject to a number of modifications (acetylation, de-acetylation, epimerization, sulfation). Variable heparan sulfate chain lengths and sequences within the heparan sulfate chains provide structural diversity generating interactive oligosaccharide binding motifs with a diverse range of extracellular ligands and cellular receptors providing instructional cues over cellular behaviour and tissue homeostasis through the regulation of essential physiological processes in development, health, and disease. heparan sulfate and heparan sulfate-PGs are integral components of the specialized glycocalyx surrounding cells. Heparan sulfate is the most heterogeneous glycosaminoglycan, in terms of its sequence and biosynthetic modifications making it a difficult molecule to fully characterize, multiple ligands also make an elucidation of heparan sulfate functional properties complicated. Spatio-temporal presentation of heparan sulfate sulfate groups is an important functional determinant in tissue development and in cellular control of wound healing and extracellular remodelling in pathological tissues. The regulatory properties of heparan sulfate are mediated via interactions with chemokines, chemokine receptors, growth factors and morphogens in cell proliferation, differentiation, development, tissue remodelling, wound healing, immune regulation, inflammation, and tumour development. A greater understanding of these HS interactive processes will improve therapeutic procedures and prognoses. Advances in glycosaminoglycan synthesis and sequencing, computational analytical carbohydrate algorithms and advanced software for the evaluation of molecular docking of heparan sulfate with its molecular partners are now available. These advanced analytic techniques and artificial intelligence offer predictive capability in the elucidation of heparan sulfate conformational effects on heparan sulfate-ligand interactions significantly aiding heparan sulfate therapeutics development.

Flexible Roles for Proteoglycan Sulfation and Receptor Signaling

Proteoglycans (PGs) in the extracellular matrix (ECM) play vital roles in axon growth and navigation, plasticity, and regeneration of injured neurons. Different classes of PGs may support or inhibit cell growth, and their functions are determined in part by highly specific structural features. Among these, the pattern of sulfation on the PG sugar chains is a paramount determinant of a diverse and flexible set of outcomes. Recent studies of PG sulfation illustrate the challenges of attributing biological actions to specific sulfation patterns, and suggest ways in which highly similar molecules may exert opposing effects on neurons. The receptors for PGs, which have yet to be fully characterized, display a similarly nuanced spectrum of effects. Different classes of PG function via overlapping families of receptors and signaling pathways. This enables them to control axon growth and guidance with remarkable specificity, but it poses challenges for determining the precise binding interactions and downstream effects of different PGs and their assorted sulfated epitopes. This review examines existing and emerging evidence for the roles of PG sulfation and receptor interactions in determining how these complex molecules influence neuronal development, growth, and function.

A novel nanoparticle delivery system for in vivo targeting of the sciatic nerve: impact on regeneration

Aim: Innovative solutions in the development of drug delivery systems targeting the nerve tissue are awaited. In this regard, a novel system for the delivery of drugs to the sciatic nerve was created using nanomedical principles. Materials & methods: Chitosan was the vehicle material used in the experiment. Heparin bound to growth factors has been administered to enhance peripheral nerve regeneration, and since heparin possesses the appropriate charge to be able to form nanoparticles with chitosan, it appears to be a good candidate to base this new delivery system on. Results: Maximal absorption took place throughout the extracellular matrix at day 15. No major inflammatory response was observed, indicating that this is a safe and biocompatible system for drug delivery to nerves. Sensorimotor performance and nerve regeneration of mice receiving these nanoparticles were superior as compared with controls. Conclusion: Our work demonstrates a versatile nanoparticle delivery system that successfully targets drugs ‘in vivo’ to the sciatic nerve, opening novel avenues in the field of nanomedicine to the design of therapeutic strategies that enhance axonal regeneration.

Original submitted 22 July 2011; Revised submitted 8 December 2011; Published online 4 April 2012

Matrix metalloproteinases and proteoglycans in axonal regeneration

After an injury to the adult mammalian central nervous system (CNS), a variety of growth-inhibitory molecules are upregulated. A glial scar forms at the site of injury and is composed of numerous molecular substances, including chondroitin sulfate proteoglycans (CSPGs). These proteoglycans inhibit axonal growth in vitro and in vivo. Matrix metalloproteinases (MMPs) can degrade the core protein of some CSPGs as well as other growth-inhibitory molecules such as Nogo and tenascin-C. MMPs have been shown to facilitate axonal regeneration in the adult mammalian peripheral nervous system (PNS). This review will focus on the various roles of proteoglycans and MMPs within the injured nervous system. First, we will present a general background on the injured central nervous system and explore the roles that proteoglycans play in the injured PNS and CNS. Second, we will discuss the various functions of MMPs within the injured PNS and CNS. Special attention will be paid to the possibility of how MMPs might modify the growth-inhibitory extracellular environment of the injured adult mammalian spinal cord and facilitate axonal regeneration in the CNS.

Muscle reinnervation and IGF-I synthesis are affected by exposure to heparin: an effect partially antagonized by anti-growth hormone-releasing hormone

Sciatic nerve crush was performed in 2-day-old rats, then reinnervation of the extensor digitorum longus muscle, motor neuron survival, and muscle IGF-I production were monitored. In saline-treated rats, the extent of reinnervation was around 50% and the number of EDL reinnervating motor neurons was significantly reduced. In heparin-treated rats the extent of muscle reinnervation, the recovery of nerve-evoked muscle twitch tension, and the number of motor neurons reinnervating the extensor digitorum longus muscle were greatly enhanced compared to saline-treated rats. In addition, treatment with heparin increased markedly insulin-like growth factor-I levels in denervated muscles. The concomitant exposure to anti-growth hormone releasing hormone partially abolished the stimulatory action of heparin on muscle reinnervation and prevented the increase of insulin-like growth factor-I muscle levels.

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.

Glycosaminoglycan-promoted muscle reinnervation and insulin-like growth factor-I levels are affected by anti-growth hormone-releasing hormone exposure

The present study shows that exposure to antibodies to growth hormone-releasing hormone (GHRH) partially counteracted the promoting effects of treatment with glycosaminoglycans (GAGs) on muscle reinnervation. Sciatic nerve crush was performed in 2-day-old rats, and reinnervation of the extensor digitorum longus muscle was monitored. The extent of reinnervation was rather poor in saline-treated rats, whereas in GAG-treated rats the extent of muscle reinnervation, the recovery of nerve-evoked muscle twitch tension, and the number of motor neurons reinnervating the extensor digitorum longus muscle were greatly enhanced. In addition, treatment with glycosaminoglycans increased markedly insulin-like growth factor-I (IGF-I) levels in denervated muscles. Both types of stimulatory action exerted by GAGs were affected by concomitant exposure to anti-GHRH, with abolition of IGF-I muscle increase and a smaller enhancement in muscle reinnervation.

Heparan sulfate proteoglycan expression in the optic chiasm of mouse embryos

Previous studies have demonstrated that heparan sulfate (HS) proteoglycans (PGs) regulate neurite outgrowth through binding to a variety of cell surface molecules, extracellular matrix proteins, and growth factors. The present study investigated the possible involvement of HS-PGs in retinal axon growth by examining its expression in the retinofugal pathway of mouse embryos by using a monoclonal antibody against the HS epitope. Immunoreactive HS was first detected in all regions of the retina at embryonic day (E) 11. The staining was gradually lost in the central regions and restricted to the retinal periphery at later developmental stages (E12--E16). Prominent staining for HS was consistently found in the retinal fiber layer and at the optic disk, indicating a possible supportive role of HS-PGs in axon growth in the retina. At the ventral diencephalon, immunostaining for HS was first detected at E12, before arrival of any retinal axons. The staining matched closely the neurons that are immunopositive for the stage-specific embryonic antigen 1 (SSEA-1). At E13 to E16, when axons are actively exploring their paths across the chiasm, immunoreactivity for HS was particularly intense at the midline. This characteristic expression pattern suggests a role for HS-PGs in defining the path of early axons in the chiasm and in regulating development of axon divergence at the midline. Furthermore, HS immunoreactivity is substantially reduced at regions flanking both sides of the midline, which coincides spatially to the position of actin-rich growth cones from subpial surface to the deep regions of the optic axon layer at the chiasm. Moreover, at the threshold of the optic tract, immunoreactive HS was localized to deep parts of the fiber layer. These findings indicate that changes in age-related fiber order in the optic chiasm and optic tract of mouse embryos are possibly regulated by a spatially restricted expression of HS-PGs.

Chondroitin sulfate proteoglycan immunoreactivity increases following spinal cord injury and transplantation

Extrinsic factors appear to contribute to the lack of regeneration in the injured adult spinal cord. It is likely that these extrinsic factors include a group of putative growth inhibitory molecules known as chondroitin sulfate proteoglycans (CSPGs). The aims of this study were to determine: (1) the consequences of spinal cord contusion injury on CSPG expression, (2) if CSPGs can be degraded in vivo by exogenous enzyme application, and (3) the effects of intraspinal transplantation on the expression of CSPGs. Chondroitin 6-sulfate proteoglycan immunoreactivity (CSPG-IR) dramatically increased following spinal cord contusion injury both at and adjacent to the injury site compared to normal controls (no surgical procedure) and laminectomy-only controls by 4 days postinjury. The dramatic increase in CSPG-IR persisted around the lesion and in the dorsal one-half to two-thirds of the spinal cord for at least 40 days postinjury. Glial fibrillary acidic protein (GFAP)-IR patterns were similarly intensified and spatially restricted as CSPG-IR patterns. These results suggest that: (1) CSPGs may contribute to the lack of regeneration following spinal cord injury and (2) astrocytes may contribute to the production of CSPGs. In addition, our results show that CSPGs could be cleaved in vivo with exogenous chondroitinase ABC application. This demonstration of cleavage may the basis for a model to directly assess CSPGs' role in growth inhibition in vivo (studies in progress) and hold potential as a therapeutic approach to enhance growth. Interestingly, the robust, injury-induced CSPG-IR patterns were not altered by intraspinal grafts of fetal spinal cord. The CSPG expression profile in the host spinal cord was similar to time-matched contusion-only animals. This was also true of GFAP-IR patterns. Furthermore, the fetal spinal cord tissue, which was generally CSPG negative at the time of transplantation, developed robust CSPG expression by 30 days posttransplantation. This increase in CSPG expression in the graft was paired with a moderate increase in GFAP-IR. CSPG-IR patterns suggest that these molecules may contribute to the limited regeneration seen following intraspinal transplantation. In addition, it suggests that the growth permissiveness of the graft may change overtime as CSPG expression develops within the graft. These correlations in the injured and transplanted spinal cord support CSPGs' putative growth inhibitory effect in the adult spinal cord.

The glial scar and central nervous system repair

Damage to the central nervous system (CNS) results in a glial reaction, leading eventually to the formation of a glial scar. In this environment, axon regeneration fails, and remyelination may also be unsuccessful. The glial reaction to injury recruits microglia, oligodendrocyte precursors, meningeal cells, astrocytes and stem cells. Damaged CNS also contains oligodendrocytes and myelin debris. Most of these cell types produce molecules that have been shown to be inhibitory to axon regeneration. Oligodendrocytes produce NI250, myelin-associated glycoprotein (MAG), and tenascin-R, oligodendrocyte precursors produce NG2 DSD-1/phosphacan and versican, astrocytes produce tenascin, brevican, and neurocan, and can be stimulated to produce NG2, meningeal cells produce NG2 and other proteoglycans, and activated microglia produce free radicals, nitric oxide, and arachidonic acid derivatives. Many of these molecules must participate in rendering the damaged CNS inhibitory for axon regeneration. Demyelinated plaques in multiple sclerosis consists mostly of scar-type astrocytes and naked axons. The extent to which the astrocytosis is responsible for blocking remyelination is not established, but astrocytes inhibit the migration of both oligodendrocyte precursors and Schwann cells which must restrict their access to demyelinated axons.

Glycosaminoglycans treatment increases IGF-I muscle levels and counteracts motor neuron death: A novel nonanticoagulant action

The present study shows that sciatic nerve crush in 2-day-old rats causes extensor digitorum longus (EDL) muscle atrophy and motor neuron loss and that treatment with glycosaminoglycans (GAGs) promotes muscle reinnervation, motor neuron survival, and markedly increases insulin-like growth factor-I (IGF-I) content in the denervated muscles. EDL muscle denervation-induced atrophy in saline-treated rats is progressive and reaches the greatest extent at 42 days after birth, which correlates with reduced EDL weight growth. There is also a partial reinnervation as shown by the number of reinnervated EDL muscle fibers (65.4% of control) and by the poor restoration of the indirect isometric twitch tension (62% of control) that is further reduced under tetanic stimulation (34% of control). The number of surviving motor neurons that innervate EDL muscle drops from 55 +/- 3 to 29 +/- 8. In GAGs-treated 42-day-old rats, the effects of neonatal nerve lesioning on EDL muscle atrophy and denervation are successfully reversed, and the isometric twitch tension and the capacity to hold tetanic stimulation are restored to almost control levels. The number of surviving EDL motor neurons is also increased to 43 +/- 4. Treatment with GAGs selectively affects IGF-I content in denervated hindlimb muscles, which is augmented from 7.02 +/- 0.71 ng/mg tissue to 25.72 +/- 0.7 in the EDL and from 3.2 +/- 0.18 to a robust 211 +/- 9.6 in the soleus.

Muscle reinnervation following neonatal nerve crush. Interactive effects of glycosaminoglycans and insulin-like growth factor-I

This study shows that glycosaminoglycans promote muscle reinnervation following neonatal sciatic nerve injury. Such an effect appears to be mediated by insulin-like growth factor-1. The glycosaminoglycan moiety of proteoglycans is a constituent of the basal lamina active on nerve regeneration by means of the interaction with laminin and with several growth factors. We have previously shown that supplementation of glycosaminoglycans affects neuronal degeneration and regeneration. In this study we report that following neonatal lesion of the rat sciatic nerve glycosaminoglycan treatment promoted extensor digitorum longus muscle reinnervation with consequent improvement of muscle morphology. In saline-treated rats, reinnervation was only partial and there was a marked muscle fibre atrophy. In addition glycosaminoglycan treatment of lesioned rats increased insulin-like growth factor-I messenger RNA and protein in the reinnervated muscle, and insulin-like growth factor-I and insulin-like growth factor binding protein-3 plasma levels. Similarly, treatment of nerve lesioned rats with insulin-like growth factor-I promoted muscle reinnervation and prevention of muscle fibre atrophy, higher levels of insulin-like growth factor-I in the reinnervated muscle and of insulin-like growth factor-I and insulin-like growth factor binding proteins in plasma. These data suggest that glycosaminoglycans are potent stimulants of muscle reinnervation and that their effects may be mediated by increased levels of insulin-like growth factor-I.

Glycosaminoglycan supplementation promotes nerve regeneration and muscle reinnervation

This study shows that treatment of rats with exogenous glycosaminoglycans stimulates peripheral nerve regeneration, increases the abundance of mRNAs for myelin proteins and promotes muscle reinnervation. After the sciatic nerve had been crushed the number of regenerating axons in the distal stump was markedly and highly significantly increased by glycosaminoglycan treatment throughout the experimental period. The increased number of axons was correlated with increased axon and fibre (axon+myelin) diameter. The abundance of mRNAs for P0 protein and myelin basic protein of regenerating nerves was also affected by treatment with glycosaminoglycans. The increase in mRNA was also observed in the contralateral unlesioned nerve. Such a phenomenon did not occur in saline-treated rats. Glycosaminoglycan treatment markedly increased the number of muscle fibres reinnervated and accelerated the restoration of muscle twitch tension elicited by nerve stimulation. The effect was particularly evident during the early stages (16 and 21 days after nerve crush) of muscle reinnervation.

Essential role of heparan sulfates in axon navigation and targeting in the developing visual system

Heparan sulfate (HS) is abundant in the developing brain and is a required co-factor for many types of fibroblast growth factor (FGF) signaling in vitro. We report that some HSs, when added exogenously to the developing Xenopus optic pathway, severely disrupt target recognition causing axons from the retina to bypass their primary target, the optic tectum. Significantly, HS sidechains from a neuroepithelial perlecan variant that preferentially bind FGF-2, HS(FGF-2), cause aberrant targeting, whereas those that preferentially bind FGF-1 do not. Charge-matched fragments of HS(FGF-2) show that the mistargeting activity associates with the FGF-binding fragments. Heparitinase removal of native HSs at the beginning of optic tract formation retards retinal axon elongation; addition of FGF-2 restores axon extension but axons lose directionality. Late HS removal, after axons have extended through the tract, elicits a tectal bypass phenotype indicating a growth promoting and guidance function for native HSs. Our results demonstrate that different HS sidechains from the same core protein differentially affect axon growth in vivo, possibly due to their distinct FGF-binding preferences, and suggest that growth factors and HSs are important partners in regulating axon growth and guidance in the developing visual system.

Developmental expression of keratan sulfate-like immunoreactivity distinguishes thalamic nuclei and cortical domains

Proteoglycans influence axonal outgrowth in several experimental paradigms, and their distribution during development suggests a role in axon guidance. We have used a monoclonal antibody, 5D4, that recognizes an epitope on sulfated keratans (KS), to define the distribution of keratan sulfate proteoglycans (KSPGs) in the developing thalamus and cortex of the rat. During development, 5D4 immunolabeling is present on thalamic axons as they grow through the internal capsule and subplate but is not present in the adjacent pathway for cortical efferent axons. Individual thalamic nuclei differ markedly in their expression of KSPGs; these distinctions persist throughout the period of developmentally regulated expression. Major cortical domains also differ in their expression of KSPGs, which are expressed throughout medial (cingulate and retrosplenial) cortex well before neocortex. Immunolabeling for KSPGs diminishes 2 weeks after birth; in the adult it is associated with small glia. The 5D4 epitope is present on several KSPGs (320, 220, and 160 kD) on Western blots during development but only in a broad 200-kD band in adult brain. Immunolabeling is degraded on sections and Western blots by keratanase II but not by keratanase I or chondroitinase ABC, confirming that the antibody recognizes KS. Bands identified by 5D4 on Western blots differ from those identified by antibodies to known KSPGs (aggrecan, claustrin, SV2, ABAKAN, phosphacan-KS), indicating that 5D4 is labeling KSPGs not previously described in the brain. The selective expression of KSPGs during development suggests that they may be a part of the molecular identity of thalamic nuclei and cortical domains that defines their connectivity.

Differential regulation of neuronal proteoglycans by activation of excitatory amino acid receptors

Cell surface proteoglycans (PGs) have been implicated in neuronal growth, migration and differentiation and can play pivotal roles both in cell-cell and cell-substrate interactions during the development of the nervous system. We have previously shown that glutamate activation of excitatory amino acid receptors induces the synthesis and release of PGs with neurite-promoting activity from hippocampal neurones. In this study, we have investigated the activity-dependent regulation of mRNA expression of two PGs in fetal hippocampal neurones using a competitive reverse transcriptase-polymerase chain reaction and correlated this expression with neuronal growth. Both cerebroglycan (CBG), a glycosylphatidylinositol-anchored heparan sulphate PG, and neurocan, a developmentally regulated chondroitin sulphate PG, are expressed in hippocampal neurones. Exposure of hippocampal neurones to 100 microM glutamate for 5 min resulted in an increase in CBG mRNA levels and an increase in axonal and dendritic length. The increase in CBG mRNA levels following glutamate exposure was mediated via both N-methyl-D-aspartate and metabotropic receptor activation.

Glycosaminoglycans in nerve injury: II. Effects on transganglionic degeneration and on the expression of neurotrophic factors

Injury to the sciatic nerve leads to the transganglionic degeneration of sensory axons and to the induction of neurotrophins and p75 nerve growth factor receptor synthesis by the denervated Schwann cells. Sciatic nerve axotomy caused a marked loss of substance P and of met-enkephalin in the lumbar cord. Substance P immunostaining and pre-proenkephalin mRNA expression were reduced in the dorsal horn layers I and II ipsilaterally to the lesion. Treating rats with low doses (0.25 mg/kg) of heparin or COS 8, a natural glycosaminoglycan mixture with low anticoagulant activity, the peptide loss was prevented and the content increased of about 50% above control values. The effects of COS 8 treatment were also evident on Schwann cells. COS 8 augmented the increase of nerve growth factor, brain-derived neurotrophic factor, and NT-3 mRNA expression in the distal stump of the axotomized sciatic nerve. Therefore, it can be concluded that glycosaminoglycans neuroprotective effects on lesioned sensory axons might have been mediated by the dramatic promotion of neurotrophin synthesis. Although the in vitro studies (Lesma et al.: J Neurosci Res, 1996) suggested also a likely direct effect as extracellular matrix components that is not mediated by trophic factors.

Glycosaminoglycans in nerve injury: 1. Low doses of glycosaminoglycans promote neurite formation

This study has shown that glycosaminoglycans added to the culture medium may affect neurite formation in SH-SY5Y neuroblastoma cells. The most effective glycosaminoglycans are heparin and COS 8, a preparation with low anticoagulant activity. Promotion of neuritogenesis was remarkable at concentrations as low as 10(-8) and 10(-10). When added at 10(-4) M both agents are inhibitory. Chondroitin-4 sulfate, dermatan sulfate, and heparan sulfate were also effective, the doses required were, however, as high as 10(-4) M for promoting and 10(-4) M for inhibiting neuritogenesis. Thereby low doses of glycosaminoglycans promote, while higher doses inhibit neurite formation. The effects were observed when neuritogenesis was promoted in neuroblastoma cultures either by deprivation of serum or by addition of retinoic acid, in the former case neuritogenesis occurred within 48 hr; in the latter, in 14 days. PC12 pheochromocytoma cells neuritogenesis was triggered by adding NGF to the culture medium. We have also observed that glycosaminoglycan supplementation to the culture medium lowered the quantity of NGF required to form neurites by PC12 cells. Glycosaminoglycans at the dose of 10(-8) M allow the formation of PC12 neurites even in presence of 1 ng/ml NGF, a dose that normally is ineffective.

Glioma invasion in the central nervous system

Invading glioma cells seem to follow distinct anatomic structures within the central nervous system. Tumor cell dissemination may occur along structures, such as the basement membranes of blood vessels or the glial limitans externa, that contain extracellular matrix (ECM) proteins. Frequently, invasive glioma cells are also found to migrate along myelinated fiber tracts of white matter. This behavior is most likely a consequence of using constitutive extracellular ligands expressed along the pathways of preferred dissemination. The extracellular space in anatomic structures, such as blood vessel basement membranes or between myelinated axons, is profoundly different, thus suggesting that glioma cells may be able to use a multiplicity of matrix ligands, possibly activating separate mechanisms for invasion. In addition, enzymatic modification of the extracellular space or deposition of ECM by the tumor cells may also create a more permissive environment for tumor spread into the adjacent brain. Tumor cell invasion is defined as translocation of neoplastic cells through host cellular and ECM barriers. This process has been studied in other cancers, in which a cascade of events has been described that involves receptor-mediated matrix adhesion, degradation of matrix by tumor-secreted metalloproteinases, and, subsequently, active cell locomotion into the newly created space. Although some of these mechanisms may play an important role in glioma invasion, there are some significant differences that are mainly the result of the profoundly different composition of the extracellular environment within the brain. This review focuses on the composition of central nervous system ECM and the recent evidence for the use by glioma cells of multiple invasion mechanisms in response to this unique environment.

A potent inhibitor of neurite outgrowth that predominates in the extracellular matrix of reactive astrocytes

In a model of astrogliosis in vitro, cultured cortical astrocytes were triggered into a functionally reactive state by an immobilized fragment of the beta-amyloid peptide. Induced astrocytes produced an extracellular matrix that inhibited the outgrowth of embryonic CNS axons. Within the extracellular matrix deposited by reactive astrocytes, we found an overall increase in the deposition of chondroitin sulphate that accounted for the inhibition. Specifically, we have detected an increased biosynthesis of a small chondroitin/heparan sulphate proteoglycan that is a potent inhibitor of axon outgrowth. We further suggest that this proteoglycan, or related molecules yet to be discovered, may play a role in gliosis-mediated regenerative failure of CNS axons.

Growth cone behavior in the presence of soluble chondroitin sulfate proteoglycan (CSPG), compared to behavior on CSPG bound to laminin or fibronectin

Proteoglycans (PGs) are complex macromolecules of the extracellular matrix (ECM) that have a wide variety of effects on developing and regenerating neurons in vivo and in vitro. One hypothesis regarding the mechanisms of PG regulation of neuronal behavior states that the conformation of PGs may be critical, and thus that ECM- or cell surface-bound PGs may operate differently than secreted (soluble) PGs. Therefore, this study examined differences between the effects of soluble chondroitin sulfate proteoglycan (CSPG) and substratum-bound CSPG on neuronal growth cone behavior. Dissociated chicken dorsal root ganglion (DRG) neurons were cultured on either laminin (LN) or fibronectin (FN), both sensory neurite outgrowth-promotin glycoproteins. CSPG (or chondroitin sulfate alone) was either bound to FN or LN, or was added to the culture media. Subsequently, using time lapse video microscopy and image analysis, this study measured: (1) neuronal attachment, (2) neurite outgrowth, (3) rate of neurite elongation, and (4) filopodial length and lifespan. To determine the site of CSPG action, DRG neurons were grown on either: CS-1, a FN peptide [Humphries M. J. et al. (1987) J. biol. Chem. 262, 6886-6892], or a recombinant FN protein, RFNIIIcs (Maejne, submitted), both of which permit DRG attachment and outgrowth but do not have recognized CSPG binding sites, and the resulting neuronal behavior was compared to that of DRG neurons grown on intact FN. The results of these studies confirm that the effect of CSPG on DRG neurons is concentration-, conformation- and substratum-dependent. On I.N, soluble CSPG had little to no effect on neurite initiation or outgrowth, while substratum-bound CSPG inhibited neurite outgrowth. In contrast, on FN, soluble CSPG inhibited neurite outgrowth and decreased the rate of neurite elongation. Soluble CSPG did not affect the length of sensory growth cone filopodia or filopodial lifespan on either LN or FN. From the FN fragment experiments, we found that: (1) soluble CSPG reduces neurite outgrowth on FN or FN fragments, but not on LN, up to 80%, and reduces elongation rate on FN up to 50%, and (2) soluble CSPG regulates neuronal behavior by binding directly to growth cones elongating on FN. Given that substratum-bound CSPG from a variety of sources is inhibitory to neurite outgrowth and to the rate of neurite elongation, while soluble CSPG often has different effects on growth cone behavior, the regulation of growth cone behavior by CSPGs may be dependent upon CSPG conformation. Further, CSPG may affect growth cone behavior by either binding to the substratum or by binding directly to growth cones.

Differential heparin inhibition of skeletal muscle alpha-dystroglycan binding to laminins

The laminin binding properties of alpha-dystroglycan purified from rabbit skeletal muscle membranes were examined. In a solid phase microtiter assay, 125I-laminin (laminin-1) bound to purified alpha-dystroglycan in a specific and saturable manner with a half-maximal concentration of 8 nM. The binding of 125I- alpha-dystroglycan to native laminin and merosin (a mixture of laminin-2 and -4) was also compared using the solid phase assay. The absolute binding of 125I- alpha-dystroglycan to laminin (6955 +/- 250 cpm/well) was similar to that measured for merosin (7440 +/- 970 cpm/well). However, inclusion of 1 mg/ml heparin in the incubation medium inhibited 125I-alpha-dystroglycan binding to laminin by 84 +/- 4.3% but inhibited 125I-alpha-dystroglycan binding to merosin by only 17 +/- 5.2%. Similar results were obtained with heparan sulfate, while de-N-sulfated heparin, hyaluronic acid, and chondroitin sulfate had no differential effect. These results were confirmed by iodinated laminin and merosin overlay of electrophoretically separated and blotted dystrophin-glycoprotein complex. In contrast to the results obtained with skeletal muscle alpha-dystroglycan, both laminin and merosin binding to purified brain alpha-dystroglycan was significantly inhibited by heparin. Our data support the possibility that one or more heparan sulfate proteoglycans may specifically modulate the interaction of alpha-dystroglycan with different extracellular matrix proteins in skeletal muscle.

An analysis of astrocytic cell lines with different abilities to promote axon growth

The adult mammalian central nervous system (CNS) lacks the capacity to support axonal regeneration. There is increasing evidence to suggest that astrocytes, the major glial population in the CNS, may possess both axon-growth promoting and axon-growth inhibitory properties and the latter may contribute to the poor regenerative capacity of the CNS. In order to examine the molecular differences between axon-growth permissive and axon-growth inhibitory astrocytes, a panel of astrocyte cell lines exhibiting a range of axon-growth promoting properties was generated and analysed. No clear correlation was found between the axon-growth promoting properties of these astrocyte cell lines with: (i) the expression of known neurite-outgrowth promoting molecules such as laminin, fibronectin and N-cadherin; (ii) the expression of known inhibitory molecules such tenascin and chondroitin sulphate proteoglycan; (iii) plasminogen activator and plasminogen activator inhibitor activity; and (iv) growth cone collapsing activity. EM studies on aggregates formed from astrocyte cell lines, however, revealed the presence of an abundance of extracellular matrix material associated with the more inhibitory astrocyte cell lines. When matrix deposited by astrocyte cell lines was assessed for axon-growth promoting activity, matrix from permissive lines was found to be a good substrate, whereas matrix from the inhibitory astrocyte lines was a poor substrate for neuritic growth. Our findings, taken together, suggest that the functional differences between the permissive and the inhibitory astrocyte cell lines reside largely with the ECM.

Structural analysis of glycosaminoglycans derived from axonally transported proteoglycans in regenerating goldfish optic nerve

Structural characteristics of glycosaminoglycans (GAGs) derived from axonally transported proteoglycans (PGs) were compared in 21 days regenerating and intact goldfish optic tracts. Twenty one days following unilateral optic nerve crushes, fish received intraocular injections of 35SO4. Eight hours post injection, tracts were removed and the 35SO4-labeled GAGs, chondroitin sulfate (CS) and heparan sulfate (HS), isolated. The HS from regenerating optic tracts had a DEAE elution profile indicative of decreased charge density, while heparitinase treatment of HS followed by Sephadex G50 analysis of the resulting fragments showed a change in the elution pattern, suggesting reduced overall sulfation. HPLC analysis of HS disaccharides revealed a difference in the sulfation pattern of regenerating tract HS, characterized by the reduced presence of tri-sulfated disaccharides. Other structural features, such as the sizes of CS and HS, and the sulfation of CS, showed no changes during regeneration. These results indicate that changes in the structure of axonally transported HS accompany regeneration of goldfish optic axons.

Characterization of proteoglycans synthesized by murine embryonal carcinoma cells (P19) reveals increased expression of perlecan (heparan sulfate proteoglycan) during neuronal differentiation

Proteoglycans (PGs) incorporated into cell layer and secreted into media were characterized during retinoic acid-induced neuronal differentiation of cultured P19 murine embryonal carcinoma cells. Heparan sulfate significantly increased (P < 0.01) in cell layer following neuronal differentiation of P19 cells by 3.9-fold. CL-4B gel chromatography revealed the major PGs present in cell layer of stem cells eluted as a broad peak with a Kav = 0.65, and was susceptible to chondroitin ABC lyase. The chondroitin ABC lyase resistant material eluted as a broad peak between Kav = 0.40 and Kav = 0.60, and was only partially digested with heparitinase/heparinase (with resistant material eluting at Kav = 0.70). Therefore, the cell layer of stem cells contained primarily chondroitin sulfate/dermatan sulfate (CS/DS) PGs, with lesser amounts of heparan sulfate proteoglycans (HSPGs). This was confirmed by SDS-PAGE. The CS/DS PGs in the cell layer of stem cells had an apparent M(r) of approximately > 200 kDa, and the HSPGs had an apparent M(r) of approximately 140-230 kDa. In contrast, the major PGs in the cell layer of neurons consisted primarily of HSPGs, with only a minor proportion of CS/DS PGs. Furthermore, both gel filtration chromatography and SDS-PAGE analysis revealed a larger HSPG in the cell layer of neurons (Kav = 0.3-0.6 on CL-4B following chondroitin ABC lyase digestion; M(r) 170 kDa- > 400 kDa on SDS-PAGE) in comparison to stem cells (Kav = 0.4-0.6 on CL-4B following chondroitin ABC lyase digestion; M(r) 140-230 kDa on SDS-PAGE). Likewise, the major PGs secreted into media of stem cells consisted almost exclusively of CS/DS PGs, with lesser amounts of HSPGs, whereas an increase in HSPGs in the media of neurons was apparent. Western, Northern, and immunocytochemical analysis demonstrated that mRNA transcript and protein levels for a specific HSPG (i.e., perlecan) markedly increased in cell layer following P19 neuronal differentiation. Perlecan core protein was identified by Western blot analysis using specific monoclonal and polyclonal antibodies, as a large HSPG with a core protein of apparent M(r) approximately 370-400 kDa, and was observed primarily in extracts from neurons. Northern blot analysis with a cDNA to perlecan revealed a significant (P < 0.01) 12.7-fold increase in expression of perlecan in neurons (day 9) in comparison to stem cells. The increase in perlecan message during P19 neuronal differentiation was concomitant with a significant (P < 0.01) 26.3-fold increase in message for beta-amyloid precursor protein (beta PP).(ABSTRACT TRUNCATED AT 400 WORDS)

Isolation and partial characterization of a cell-surface heparan sulfate proteoglycan from embryonic rat spinal cord

Cell-surface heparan sulfate proteoglycans (HSPGs) are potential mediators of neuronal cell adhesion, spreading, and neurite outgrowth on various extra-cellular matrix molecules. One possible site of HSPG attachment is a heparin binding domain of fibronectin, which is present in the synthetic peptide FN-C/H II. In this study, HSPGs extracted from embryonic rat spinal cord by detergent were purified by ion-exchange chromatography, gel filtration, and affinity chromatography on an agarose column coupled with FN-C/H II conjugated to ovalbumin (OA). Heparitinase treatment of the iodinated HSPG fraction led to the appearance of a major protein core with a molecular size of 72 kDa, as determined by reducing SDS-PAGE. The intact proteoglycan has a molecular size of approximately 150-165 kDa, containing heparan sulfate glycosaminoglycan chains of about 10-15 kDa. Anti-HSPG antibodies recognized the 72 kDa core protein by immunoblotting, and stained the surface of spinal cord neurons, oligodendrocytes, and a subset of astrocytes. These results identify a cell-surface HSPG that may mediate neuron-substratum or neuron-glia interactions in embryonic central nervous system.

On the regional distribution of heparan sulfate proteoglycan immunoreactivity in the rat brain

By means of two monoclonal antibodies specific for heparan sulfate (HS)-related epitopes, one (10E4) against native HS chains and one (3G10) against desaturated uronates, a highly regional and differential distribution of these two epitopes have been observed in the adult rat brain. The 10E4 epitope immunoreactivity (IR) is mainly found in the substantia nigra, the red nucleus and the subgranular zone of the dentate gyrus, while the 3G10 epitope IR is mainly found in the CA2 area of the hippocampal formation and the pyramdial cells in the layer V of the frontoparietal cortex. The codistribution of both types of IRs with basic fibroblast growth factor (bFGF, FGF-2) in neurons and astroglia supports the notion that heparan sulfate proteoglycans (HSPG) in the extracellular matrix may serve as a site for storage of bFGF and assist in the bFGF-induced activation of the high-affinity FGF receptors linked to astroglia and neurons in these discrete areas.

Nervous tissue proteoglycans

The structure, biosynthesis, localization, and possible functional roles of nervous tissue glycosaminoglycans and proteoglycans were last reviewed several years ago. Since that time, there has been an exponential increase in publications on the neurobiology of proteoglycans. This review will therefore focus on reports which have appeared in the period after 1988, and especially on those concerning the properties of individual characterized nervous tissue proteoglycans. Related areas such as the regulation of glycosaminoglycan biosynthesis and the roles of cell surface proteoglycans in adhesion and growth control are covered in other contributions to this special topic issue.

Nervous tissue proteoglycans

The structure, biosynthesis, localization, and possible functional roles of nervous tissue glycosaminoglycans and proteoglycans were last reviewed several years ago. Since that time, there has been an exponential increase in publications on the neurobiology of proteoglycans. This review will therefore focus on reports which have appeared in the period after 1988, and especially on those concerning the properties of individual characterized nervous tissue proteoglycans. Related areas such as the regulation of glycosaminoglycan biosynthesis and the roles of cell surface proteoglycans in adhesion and growth control are covered in other contributions to this special topic issue.

Binding of secreted human neuroblastoma proteoglycans to the Alzheimer's amyloid A4 peptide

Proteoglycans (PGs) may play a fundamental role in all forms of amyloidosis. In Alzheimer's disease, proteoglycans are found deposited in senile plaques and in neurofibrillary tangles. However, the cellular source of these deposited PGs and their role in amyloidosis in Alzheimer's disease is unknown. Proteoglycans were purified from conditioned medium of human neuroblastoma cells (SKNSH-SY 5Y). Two species of proteoglycans were identified by enzyme susceptibility including a heparan sulfate proteoglycan and a dermatan sulfate proteoglycan. A monoclonal antibody to the protein core of a vascular basement membrane heparan sulfate proteoglycan found in senile plaques in Alzheimer's disease cross-reacted with the proteoglycans secreted by human neuroblastoma cells. Binding between 35SO4-labelled neuroblastoma proteoglycans and the Alzheimer amyloid (A4) peptide was demonstrated by affinity chromatography. Specificity studies demonstrated that binding of human neuroblastoma proteoglycans to the amyloid peptide was specific for a heparan sulfate glycosaminoglycan, with some binding to a dermatan sulfate proteoglycan. Binding to A4 was also demonstrated by a chemically deglycosylated protein core preparation. No significant binding of neuroblastoma proteoglycans was found to two other basic peptides derived from the extracellular domain of the beta-amyloid precursor, demonstrating the specificity of proteoglycan binding to the A4 peptide. Human neuroblastoma proteoglycans may bind to the-Alzheimer amyloid A4 peptide in a region with a heparin binding consensus sequence [VHHQKL] which also contains the cleavage site of the beta-amyloid precursor protein. Neuronal proteoglycans may either regulate the secretion of the amyloid protein precursor or modify the binding of the amyloid protein precursor to other cellular adhesion molecules. Alterations in this binding may be related to the pathogenesis of amyloid deposition in Alzheimer's disease.

Laminin in neural development

This short and selective review of the role of laminin in neural development discusses emerging concepts about the way that elements of the extracellular matrix control the differentiation of embryonic neurons. New laminin isoforms have recently been discovered, discoveries which now reveal the very great heterology of basement membranes in different regions of the nervous system, at different stages of development. The problems of identifying true, neuronal-specific laminin receptors are also discussed, particularly with reference to neuronal pathway formation.

Extracellular matrix and neurite outgrowth

The complex relationship between neuronal cells and the extracellular matrix molecules with which they interact both positively and negatively is currently being investigated on many fronts. Major areas of experimental emphasis include the characterization of an increasing number of extracellular matrix and cell surface associated molecules, the identification of receptors for these molecules, and the analysis of the function of extracellular matrix molecules with respect to neuronal process outgrowth.

Influence of N-linked oligosaccharides on the processing and neurite-promoting activity of proteoglycans released by neurons in vitro

Inhibitors of enzymes involved in processing of N-linked oligosaccharides were used to examine biosynthesis and the neurite-promoting activity of proteoglycans produced by and released from dissociated chick embryo spinal cord neurons in vitro. In the cell compartment and in conditioned medium both castanospermine and swainsonine inhibited 3H-glucosamine incorporation into glycoprotein but only castanospermine reduced 3H-glucosamine incorporation into heparan sulphate and chondroitin sulphate proteoglycans. All of the neurite-promoting activity of neuron-conditioned medium that complexed to laminin was associated with heparan sulphate proteoglycans as determined by heparitinase digestion. Neuron-conditioned medium prepared in the presence of castanospermine displayed a 38 +/- 6% (mean +/- SD) reduction in 3H-glucosamine incorporation into heparan sulphate proteoglycans and a 30 +/- 5% reduction in substrate-attached neurite-promoting activity compared to control conditioned medium and to conditioned medium prepared in the presence of swainsonine. When neurons were coincubated with castanospermine, neurite growth on a laminin substrate was 50 +/- 10% of control growth or growth in the presence of swainsonine. However, when neuron-conditioned medium was used to pretreat the laminin substrate the inhibitory effect on neurite growth produced by castanospermine coincubation was reversed. Influences on neuronal processing of N-linked oligosaccharides alter neurite growth directly and also alter the neurite-promoting activity of neuron-conditioned medium by inhibiting the synthesis of heparan sulphate proteoglycans. These studies provide further evidence for an autocrine role for heparan sulphate proteoglycans in neurite growth.

A role for proteoglycans in the guidance of a subset of pioneer axons in cultured embryos of the cockroach

Several molecules involved in the development of the nervous system have specific binding sites for the glycosaminoglycan (GAG) side chains of proteoglycans. Exogenous GAGs should bind to these sites, competitively inhibit interactions with proteoglycans, and perturb development. GAGs added to the culture medium perturb the in situ growth of pioneer axons in cultured cockroach embryos by producing axon defasciculation and growth in incorrect directions. The specificity of this phenomenon is evident from the following observations: Of all the GAGs tested only heparin and heparan sulfate produced perturbation; of the six axon tracts being pioneered during the culture period only two of them are perturbed by the GAGs; and similar perturbations are produced when embryos are cultured in the presence of heparinase II and heparitinase.