Intracranial injury acutely induces the expression of the secreted isoform of the CNS-specific hyaluronan-binding protein BEHAB/brevican

Hyaluronic Acid Interacting Molecules Mediated Crosstalk between Cancer Cells and Microenvironment from Primary Tumour to Distant Metastasis

Hyaluronic acid (HA) is a prominent component of the extracellular matrix, and its interactions with HA-interacting molecules (HAIMs) play a critical role in cancer development and disease progression. This review explores the multifaceted role of HAIMs in the context of cancer, focusing on their influence on disease progression by dissecting relevant cellular and molecular mechanisms in tumour cells and the tumour microenvironment. Cancer progression can be profoundly affected by the interactions between HA and HAIMs. They modulate critical processes such as cell adhesion, migration, invasion, and proliferation. The TME serves as a dynamic platform in which HAIMs contribute to the formation of a unique niche. The resulting changes in HA composition profoundly influence the biophysical properties of the TME. These modifications in the TME, in conjunction with HAIMs, impact angiogenesis, immune cell recruitment, and immune evasion. Therefore, understanding the intricate interplay between HAIMs and HA within the cancer context is essential for developing novel therapeutic strategies. Targeting these interactions offers promising avenues for cancer treatment, as they hold the potential to disrupt critical aspects of disease progression and the TME. Further research in this field is imperative for advancing our knowledge and the treatment of cancer.

The role of extracellular matrix alterations in mediating astrocyte damage and pericyte dysfunction in Alzheimer's disease: A comprehensive review

The brain is a highly vascularized tissue protected by the blood-brain barrier (BBB), a complex structure allowing only necessary substances to pass through into the brain while limiting the entrance of harmful toxins. The BBB comprises several components, and the most prominent features are tight junctions between endothelial cells (ECs), which are further wrapped in a layer of pericytes. Pericytes are multitasked cells embedded in a thick basement membrane (BM) that consists of a fibrous extracellular matrix (ECM) and are surrounded by astrocytic endfeet. The primary function of astrocytes and pericytes is to provide essential blood supply and vital nutrients to the brain. In Alzheimer's disease (AD), long-term neuroinflammatory cascades associated with infiltration of harmful neurotoxic proteins may lead to BBB dysfunction and altered ECM components resulting in brain homeostatic imbalance, synaptic damage, and declined cognitive functions. Moreover, BBB structure and functional integrity may be lost due to induced ECM alterations, astrocyte damage, and pericytes dysfunction, leading to amyloid-beta (Aβ) hallmarks deposition in different brain regions. Herein, we highlight how BBB, ECM, astrocytes, and pericytes dysfunction can play a leading role in AD's pathogenesis and discuss their impact on brain functions.

The Role of BEHAB/Brevican in the Tumor Microenvironment: Mediating Glioma Cell Invasion and Motility

Malignant gliomas are the most common tumors in the central nervous system (CNS) and, unfortunately, are also the most deadly. The lethal nature of malignant gliomas is due in large part to their unique and distinctive ability to invade the surrounding neural tissue. The invasive and dispersive nature of these tumors makes them particularly challenging to treat, and currently there are no effective therapies for malignant gliomas. The brain tumor microenvironment plays a particularly important role in mediating the invasiveness of gliomas, and, therefore, understanding its function is key to developing novel therapies to treat these deadly tumors. A defining aspect of the tumor microenvironment of gliomas is the unique composition of the extracellular matrix that enables tumors to overcome the typically inhibitory environment found in the CNS. One conspicuous component of the glioma tumor microenvironment is the neural-specific ECM molecule, brain-enriched hyaluronan binding (BEHAB)/brevican (B/b). B/b is highly overexpressed in gliomas, and its expression in these tumors contributes importantly to the tumor invasiveness and aggressiveness. However, B/b is a complicated protein with multiple splice variants, cleavage products, and glycoforms that contribute to its complex functions in these tumors and provide unique targets for tumor therapy. Here we review the role of B/b in glioma tumor microenvironment and explore targeting of this protein for glioma therapy.

Cerebrospinal fluid markers of extracellular matrix remodelling, synaptic plasticity and neuroinflammation before and after cranial radiotherapy

BACKGROUND

Advances in the treatment of brain tumours have increased the number of long-term survivors, but at the cost of side effects following cranial radiotherapy ranging from neurocognitive deficits to outright tissue necrosis. At present, there are no tools reflecting the molecular mechanisms underlying such side effects, and thus no means to evaluate interventional effects after cranial radiotherapy. Therefore, fluid biomarkers are of great clinical interest.

OBJECTIVE

Cerebrospinal fluid (CSF) levels of proteins involved in inflammatory signalling, synaptic plasticity and extracellular matrix (ECM) integrity were investigated following radiotherapy to the brain.

METHODS

Patients with small-cell lung cancer (SCLC) eligible for prophylactic cranial irradiation (PCI) were asked to participate in the study. PCI was prescribed either as 2 Gy/fraction to a total dose of 30 Gy (limited disease) or 4 Gy/fraction to 20 Gy (extensive disease). CSF was collected by lumbar puncture at baseline, 3 months and 1 year following PCI. Protein concentrations were measured using immunobased assays or mass spectrometry.

RESULTS

The inflammatory markers IL-15, IL-16 and MCP-1/CCL2 were elevated in CSF 3 months following PCI compared to baseline. The plasticity marker GAP-43 was elevated 3 months following PCI, and the same trend was seen for SNAP-25, but not for SYT1. The investigated ECM proteins, brevican and neurocan, showed a decline following PCI. There was a strong correlation between the progressive decline of soluble APPα and brevican levels.

CONCLUSION

To our knowledge, this is the first time ECM-related proteins have been shown to be affected by cranial radiotherapy in patients with cancer. These findings may help us to get a better understanding of the mechanisms behind side effects following radiotherapy.

Extracellular matrix and traumatic brain injury

The brain extracellular matrix (ECM) plays a crucial role in both the developing and adult brain by providing structural support and mediating cell-cell interactions. In this review, we focus on the major constituents of the ECM and how they function in both normal and injured brain, and summarize the changes in the composition of the ECM as well as how these changes either promote or inhibit recovery of function following traumatic brain injury (TBI). Modulation of ECM composition to facilitates neuronal survival, regeneration and axonal outgrowth is a potential therapeutic target for TBI treatment.

Müller glial microRNAs are required for the maintenance of glial homeostasis and retinal architecture

To better understand the roles of microRNAs in glial function, we used a conditional deletion of Dicer1 (Dicer-CKO_MG) in retinal Müller glia (MG). Dicer1 deletion from the MG leads to an abnormal migration of the cells as early as 1 month after the deletion. By 6 months after Dicer1 deletion, the MG form large aggregations and severely disrupt normal retinal architecture and function. The most highly upregulated gene in the Dicer-CKO_MG MG is the proteoglycan Brevican (Bcan) and overexpression of Bcan results in similar aggregations of the MG in wild-type retina. One potential microRNA that regulates Bcan is miR-9, and overexpression of miR-9 can partly rescue the effects of Dicer1 deletion on the MG phenotype. We also find that MG from retinitis pigmentosa patients display an increase in Brevican immunoreactivity at sites of MG aggregation, linking the retinal remodeling that occurs in chronic disease with microRNAs.

Tumor Necrosis Factor-stimulated Gene-6 (TSG-6) Is Constitutively Expressed in Adult Central Nervous System (CNS) and Associated with Astrocyte-mediated Glial Scar Formation following Spinal Cord Injury

Tumor necrosis factor (TNF)-stimulated gene-6 (TSG-6) binds to hyaluronan and can reorganize/stabilize its structure, also enhancing the binding of this glycosaminoglycan to its cell surface receptor, CD44. TSG-6 is rapidly up-regulated in response to inflammatory cytokines protecting tissues from the damaging effects of inflammation. Despite TSG-6 treatment having been shown to improve outcomes in an experimental model of traumatic brain injury, TSG-6 expression has not been extensively studied in the central nervous system (CNS). We hereby analyzed the expression profile of TSG-6 in the developing CNS and following injury. We show that TSG-6 is expressed in the rat CNS by GFAP(+) and CD44(+) astrocytes, solely in the mature brain and spinal cord, and is not present during the development of the CNS. TSG-6(-/-) mice present a reduced number of GFAP(+) astrocytes when compared with the littermate TSG-6(+/-) mice. TSG-6 expression is drastically up-regulated after injury, and the TSG-6 protein is present within the glial scar, potentially coordinating and stabilizing the formation of this hyaluronan-rich matrix. This study shows that TSG-6 is expressed in the CNS, suggesting a role for TSG-6 in astrocyte activation and tissue repair. We hypothesize that within this context TSG-6 could participate in the formation of the glial scar and confer anti-inflammatory properties. Further studies are required to elucidate the therapeutic potential of targeting TSG-6 after CNS injury to promote its protective effects while reducing the inhibitory properties of the glial scar in axon regeneration.

Hyaluronan as a therapeutic target in human diseases

Accumulation and turnover of extracellular matrix is a hallmark of tissue injury, repair and remodeling in human diseases. Hyaluronan is a major component of the extracellular matrix and plays an important role in regulating tissue injury and repair, and controlling disease outcomes. The function of hyaluronan depends on its size, location, and interactions with binding partners. While fragmented hyaluronan stimulates the expression of an array of genes by a variety of cell types regulating inflammatory responses and tissue repair, cell surface hyaluronan provides protection against tissue damage from the environment and promotes regeneration and repair. The interactions of hyaluronan and its binding proteins participate in the pathogenesis of many human diseases. Thus, targeting hyaluronan and its interactions with cells and proteins may provide new approaches to developing therapeutics for inflammatory and fibrosing diseases. This review focuses on the role of hyaluronan in biological and pathological processes, and as a potential therapeutic target in human diseases.

Reorganization of Synaptic Connections and Perineuronal Nets in the Deep Cerebellar Nuclei of Purkinje Cell Degeneration Mutant Mice

The perineuronal net (PN) is a subtype of extracellular matrix appearing as a net-like structure around distinct neurons throughout the whole CNS. PNs surround the soma, proximal dendrites, and the axonal initial segment embedding synaptic terminals on the neuronal surface. Different functions of the PNs are suggested which include support of synaptic stabilization, inhibition of axonal sprouting, and control of neuronal plasticity. A number of studies provide evidence that removing PNs or PN-components results in renewed neurite growth and synaptogenesis. In a mouse model for Purkinje cell degeneration, we examined the effect of deafferentation on synaptic remodeling and modulation of PNs in the deep cerebellar nuclei. We found reduced GABAergic, enhanced glutamatergic innervations at PN-associated neurons, and altered expression of the PN-components brevican and hapln4. These data refer to a direct interaction between ECM and synapses. The altered brevican expression induced by activated astrocytes could be required for an adequate regeneration by promoting neurite growth and synaptogenesis.

Expression of the Astrocyte Water Channel Aquaporin-4 in the Mouse Brain

Aquaporin-4 (AQP4) is a bidirectional water channel that is found on astrocytes throughout the central nervous system. Expression is particularly high around areas in contact with cerebrospinal fluid, suggesting that AQP4 plays a role in fluid exchange between the cerebrospinal fluid compartments and the brain. Despite its significant role in the brain, the overall spatial and region-specific distribution of AQP4 has yet to be fully characterized. In this study, we used Western blotting and immunohistochemical techniques to characterize AQP4 expression and localization throughout the mouse brain. We observed AQP4 expression throughout the forebrain, subcortical areas, and brainstem. AQP4 protein levels were highest in the cerebellum with lower expression in the cortex and hippocampus. We found that AQP4 immunoreactivity was profuse on glial cells bordering ventricles, blood vessels, and subarachnoid space. Throughout the brain, AQP4 was expressed on astrocytic end-feet surrounding blood vessels but was also heterogeneously expressed in brain tissue parenchyma and neuropil, often with striking laminar specificity. In the cerebellum, we showed that AQP4 colocalized with the proteoglycan brevican, which is synthesized by and expressed on cerebellar astrocytes. Despite the high abundance of AQP4 in the cerebellum, its functional significance has yet to be investigated. Given the known role of AQP4 in synaptic plasticity in the hippocampus, the widespread and region-specific expression pattern of AQP4 suggests involvement not only in fluid balance and ion homeostasis but also local synaptic plasticity and function in distinct brain circuits.

Sugar glues for broken neurons

Proteoglycans (PGs) regulate diverse functions in the central nervous system (CNS) by interacting with a number of growth factors, matrix proteins, and cell surface molecules. Heparan sulfate (HS) and chondroitin sulfate (CS) are two major glycosaminoglycans present in the PGs of the CNS. The functionality of these PGs is to a large extent dictated by the fine sulfation patterns present on their glycosaminoglycan (GAG) chains. In the past 15 years, there has been a significant expansion in our knowledge on the role of HS and CS chains in various neurological processes, such as neuronal growth, regeneration, plasticity, and pathfinding. However, defining the relation between distinct sulfation patterns of the GAGs and their functionality has thus far been difficult. With the emergence of novel tools for the synthesis of defined GAG structures, and techniques for their characterization, we are now in a better position to explore the structure-function relation of GAGs in the context of their sulfation patterns. In this review, we discuss the importance of GAGs on CNS development, injury, and disorders with an emphasis on their sulfation patterns. Finally, we outline several GAG-based therapeutic strategies to exploit GAG chains for ameliorating various CNS disorders.

Gliosis-specific transcription factor OASIS coincides with proteoglycan core protein genes in the glial scar and inhibits neurite outgrowth

OASIS gene, a member of the CREB/ATF transcription factor family, is upregulated in gliosis after CNS injury. However it remains to be determined how OASIS is implicated in gliotic reaction. In a glial scar, chondroitin sulfate proteoglycans (CSPGs) are also upregulated, which engenders the inhibition of axonal regeneration. We investigated the functional role of OASIS in gliosis in relation to CSPG core proteins that render lesions non-permissive for regenerating axons. We first examined the gene expression localization of OASIS using several markers in a cryo-injured mouse brain and compared the expression pattern of CSPG core protein genes with that of OASIS in a glial scar by double-labeling in situ hybridization. Our findings suggest that OASIS is induced in proximal reactive astrocytes that exhibit upregulated expression for CSPGs, including NG2 proteoglycan, versican, brevican, neurocan, and phosphacan core. Furthermore, the membrane fraction derived from OASIS-transfected C6 cells inhibits neurite outgrowth of NG108-15 cells, whereas its neurite outgrowth inhibitory effect is abrogated after chondroitinase ABC treatment. OASIS is likely to be involved in the regulatory mechanism of non-permissive environments for axonal outgrowth.

Unique features of extracellular matrix in the mouse medial nucleus of trapezoid body--implications for physiological functions

The medial nucleus of the trapezoid body (MNTB) is a vital structure of sound localization circuits in the auditory brainstem. Each principal cell of MNTB is contacted by a very large presynaptic glutamatergic terminal, the calyx of Held. The MNTB principal cells themselves are surrounded by extracellular matrix components forming prominent perineuronal nets (PNs). Throughout the CNS, PNs, which form lattice-like structures around the somata and proximal dendrites, are associated with distinct types of neurons. PNs are highly enriched in hyaluronan and chondroitin sulfate proteoglycans therefore providing a charged surface structure surrounding the cell body and proximal neurites of these neurons. The localization and composition of PNs have lead investigators to a number of hypotheses about their functions including: creating a specific extracellular ionic milieu around these neurons, stabilizing synapses, and influencing the outgrowth of axons. However, presently the precise functions of PNs are still quite unclear primarily due to the lack of an ideal experimental model system that is highly enriched in PNs and in which the synaptic transmission properties can be precisely measured. The MNTB principal cells could offer such a model, since they have been extensively characterized electrophysiologically. However, extracellular matrix (ECM) in these neurons has not yet been precisely detailed. The present study gives a detailed examination of the ECM organization and structural differences in PNs of the mouse MNTB. The different PN components and their distribution pattern are scrutinized throughout the MNTB. The data are complemented by electron microscopic investigations of the unique ultrastructural localization of PN-components and their interrelation with distinct pre- and postsynaptic MNTB cell structures. Therefore, we believe this work identifies the MNTB as an ideal system for studying PN function.

Kindlin-1 enhances axon growth on inhibitory chondroitin sulfate proteoglycans and promotes sensory axon regeneration

Growing and regenerating axons need to interact with the molecules in the extracellular matrix as they traverse through their environment. An important group of receptors that serve this function is the integrin superfamily of cell surface receptors, which are evolutionarily conserved αβ heterodimeric transmembrane proteins. The function of integrins is controlled by regulating the affinity for ligands (also called "integrin activation"). Previous results have shown that CNS inhibitory molecules inactivate axonal integrins, while enhancing integrin activation can promote axon growth from neurons cultured on inhibitory substrates. We tested two related molecules, kindlin-1 and kindlin-2 (Fermitin family members 1 and 2), that can activate β1, β2, and β3 integrins, for their effects on integrin signaling and integrin-mediated axon growth in rat sensory neurons. We determined that kindlin-2, but not kindlin-1, is endogenously expressed in the nervous system. Knocking down kindlin-2 levels in cultured sensory neurons impaired their ability to extend axons, but this was partially rescued by kindlin-1 expression. Overexpression of kindlin-1, but not kindlin-2, in cultured neurons increased axon growth on an inhibitory aggrecan substrate. This was found to be associated with enhanced integrin activation and signaling within the axons. Additionally, in an in vivo rat dorsal root injury model, transduction of dorsal root ganglion neurons to express kindlin-1 promoted axon regeneration across the dorsal root entry zone and into the spinal cord. These animals demonstrated improved recovery of thermal sensation following injury. Our results therefore suggest that kindlin-1 is a potential tool for improving axon regeneration after nervous system lesions.

Hyaluronan as an immune regulator in human diseases

Accumulation and turnover of extracellular matrix components are the hallmarks of tissue injury. Fragmented hyaluronan stimulates the expression of inflammatory genes by a variety of immune cells at the injury site. Hyaluronan binds to a number of cell surface proteins on various cell types. Hyaluronan fragments signal through both Toll-like receptor (TLR) 4 and TLR2 as well as CD44 to stimulate inflammatory genes in inflammatory cells. Hyaluronan is also present on the cell surface of epithelial cells and provides protection against tissue damage from the environment by interacting with TLR2 and TLR4. Hyaluronan and hyaluronan-binding proteins regulate inflammation, tissue injury, and repair through regulating inflammatory cell recruitment, release of inflammatory cytokines, and cell migration. This review focuses on the role of hyaluronan as an immune regulator in human diseases.

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.

Aggrecanase and aggrecan degradation in osteoarthritis: a review

Aggrecanase-mediated aggrecan degradation is a significant event in early-stage osteoarthritis (OA). Aggrecanases belonging to the 'A Disintegrin And Metalloproteinase with ThromboSpondin motifs' (ADAMTS) family of proteinases play a significant role in aggrecan depletion in osteoarthritic cartilage. There has been considerable interest in the possible role of these aggrecanases, especially ADAMTS-4 and ADAMTS-5, as therapeutic targets in OA. This article discusses recent data regarding ADAMTS-4 and ADAMTS-5 in OA, with emphasis on the relationship between aggrecanase and aggrecan degradation as well as the role of aggrecanase in OA.

The proteoglycan brevican binds to fibronectin after proteolytic cleavage and promotes glioma cell motility

The adult neural parenchyma contains a distinctive extracellular matrix that acts as a barrier to cell and neurite motility. Nonneural tumors that metastasize to the central nervous system almost never infiltrate it and instead displace the neural tissue as they grow. In contrast, invasive gliomas disrupt the extracellular matrix and disperse within the neural tissue. A major inhibitory component of the neural matrix is the lectican family of chondroitin sulfate proteoglycans, of which brevican is the most abundant member in the adult brain. Interestingly, brevican is also highly up-regulated in gliomas and promotes glioma dispersion by unknown mechanisms. Here we show that brevican secreted by glioma cells enhances cell adhesion and motility only after proteolytic cleavage. At the molecular level, brevican promotes epidermal growth factor receptor activation, increases the expression of cell adhesion molecules, and promotes the secretion of fibronectin and accumulation of fibronectin microfibrils on the cell surface. Moreover, the N-terminal cleavage product of brevican, but not the full-length protein, associates with fibronectin in cultured cells and in surgical samples of glioma. Taken together, our results provide the first evidence of the cellular and molecular mechanisms that may underlie the motility-promoting role of brevican in primary brain tumors. In addition, these results underscore the important functional implications of brevican processing in glioma progression.

Discordant localization of WFA reactivity and brevican/ADAMTS-derived fragment in rodent brain

Background

Proteoglycan (PG) in the extracellular matrix (ECM) of the central nervous system (CNS) may act as a barrier for neurite elongation in a growth tract, and regulate other characteristics collectively defined as structural neural plasticity. Proteolytic cleavage of PGs appears to alter the environment to one favoring plasticity and growth. Brevican belongs to the lectican family of aggregating, chondroitin sulfate (CS)-bearing PGs, and it modulates neurite outgrowth and synaptogenesis. Several ADAMTSs (a disintegrin and metalloproteinase with thrombospondin motifs) are glutamyl-endopeptidases that proteolytically cleave brevican. The purpose of this study was to localize regions of adult CNS that contain a proteolytic-derived fragment of brevican which bears the ADAMTS-cleaved neoepitope sequence. These regions were compared to areas of Wisteria floribunda agglutin (WFA) reactivity, a common reagent used to detect "perineuronal nets" (PNNs) of intact matrix and a marker which is thought to label regions of relative neural stability.

Results

WFA reactivity was found primarily as PNNs, whereas brevican and the ADAMTS-cleaved fragment of brevican were more broadly distributed in neuropil, and in particular regions localized to PNNs. One example is hippocampus where the ADAMTS-cleaved brevican fragment is found surrounding pyramidal neurons, in neuropil of stratum oriens/radiatum and the lacunosum moleculare. The fragment was less abundant in the molecular layer of the dentate gyrus. Mostly PNNs of scattered interneurons along the pyramidal layer were identified by WFA. In lateral thalamus, the reticular thalamic nucleus stained abundantly with WFA whereas ventral posterior nuclei were markedly immunopositive for ADAMTS-cleaved brevican. Using Western blotting techniques, no common species were reactive for brevican and WFA.

Conclusion

In general, a marked discordance was observed in the regional localization between WFA and brevican or the ADAMTS-derived N-terminal fragment of brevican. Functionally, this difference may correspond to regions with varied prevalence for neural stability/plasticity.

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.

Potential regulatory relationship between the nested gene DDC8 and its host gene tissue inhibitor of metalloproteinase-2

Nested genes are fairly common within the mammalian nervous system, yet few studies have examined whether the guest and host genes might be coordinately regulated. Tissue inhibitors of metalloproteinase (TIMPs) inhibit extracellular matrix proteolysis mediated by metzincin proteases. TIMP-2 is the only TIMP not nested within a synapsin gene. It does, however, serve as a host for differential display clone 8 (DDC8), a testis-specific gene whose expression is upregulated during spermatogenesis. Here, we demonstrate that DDC8 is not testis specific. Furthermore, DDC8 expression in nonneural and neural tissues mimics that of TIMP-2, including its upregulation in response to traumatic brain injury, suggesting a potential regulatory relationship. The most striking observation is that the TIMP-2 knockout mouse brain contains TIMP-2 mRNA encoding exons 2-5, which are downstream of DDC8, but not exon 1, which contains the signal sequence and cysteine residue required for MMP inhibition, indicating a functional knockout. That TIMP-2 transcripts in wild-type brain contain DDC8 sequence suggests alternative splicing between the two genes.

From barriers to bridges: chondroitin sulfate proteoglycans in neuropathology

Emerging studies have revealed new roles for the neural extracellular matrix in neuropathologies. The structure of this matrix is unusual and uniquely enriched in chondroitin sulfate proteoglycans, particularly those of the lectican family. Historically, lecticans have attracted considerable interest in the normal and injured brain for their prominent roles as inhibitors of cellular motility, neurite extension and synaptic plasticity. However, these molecules are structurally heterogeneous, have distinct expression patterns and mediate unique interactions, suggesting that they might have other functions in addition to their traditional role as chemorepulsants. Here, we review recent work demonstrating unique modifications and structural microheterogeneity of the lecticans in the diseased CNS, which might relate to novel roles of these molecules in neuropathologies.

Potential roles for hyaluronan and CD44 in kainic acid-induced mossy fiber sprouting in organotypic hippocampal slice cultures

The most well-documented synaptic rearrangement associated with temporal lobe epilepsy is mossy fiber sprouting (MFS). MFS is a pronounced expansion of granule cell mossy fiber axons into the inner dentate molecular layer. The recurrent excitatory network formed by MFS is hypothesized to play a critical role in epileptogenesis, which is the transformation of the normal brain into one that is prone to recurrent spontaneous seizures. While many studies have focused on the functional consequences of MFS, relatively few have investigated the molecular mechanisms underlying the increased propensity of mossy fibers to invade the inner molecular layer. We hypothesized that changes in two components of the extracellular matrix, hyaluronan and its primary receptor, CD44, contribute to MFS. Hyaluronan contributes to laminar-specificity in the hippocampus and increases in hyaluronan and CD44 are associated with temporal lobe epilepsy. We tested our hypothesis in an in vitro model of MFS using a combination of histological and biochemical approaches. Application of kainic acid (KA) to organotypic hippocampal slice cultures induced robust MFS into the inner dentate molecular layer compared with vehicle-treated controls. Degradation of hyaluronan with hyaluronidase significantly reduced but did not eliminate KA-induced MFS, suggesting that hyaluronan played a permissive role in MFS, but that loss of hyaluronan signaling alone was not sufficient to block mossy fiber reorganization. Comparison of CD44 expression with MFS revealed that when CD44 expression in the molecular layers was high, MFS was minimal and when CD44 expression/function was reduced following KA treatment or with function blocking antibodies, MFS was increased. The time course of KA-induced reductions in CD44 expression was identical to the temporal progression of KA-induced MFS reported previously in hippocampal slice cultures, suggesting that reduced CD44 expression may help promote MFS. Understanding the molecular mechanisms underlying MFS may lead to therapeutic interventions that limit epileptogenesis.

Localized gene expression of axon guidance molecules in neuronal co-cultures

Axonal growth cones are guided to their targets by contact-dependent mechanisms or by diffusible chemotropic factors. Axon guidance by these factors typically involves culturing neurons on an acellular substrate which may not represent the in vivo biological environment. We developed two novel in vitro methods to create patterned gene expression of guidance molecules in a physiologically-relevant cellular environment. In the Matrigel assay, a droplet of adenovirus-Matrigel suspension was placed on astrocytes grown in Matrigel. The adenovirus diffused through the gel and transduced underlying astrocytes, creating a radial infection gradient within a localized area. In the second model, recombinant adenovirus was bound to an anti-hexon antibody adsorbed onto stripe patterns of nitrocellulose. Once the cells were added, only those contacting the adenovirus were infected. The outgrowth pattern of chick DRG neurons on NGF, semaphorin 3A and brevican were studied. As expected, results showed robust axonal growth toward NGF as opposed to either secreted Sema 3A or membrane bound brevican, however subtle differences in axonal growth responses were observed in comparison to those obtained with less physiologically-relevant methods. Novel to this technology, the location and area of molecule expression can be controlled and manipulated in an intricate cellular environment.

Tissue inhibitor of metalloproteinase-2 promotes neuronal differentiation by acting as an anti-mitogenic signal

Although traditionally recognized for maintaining extracellular matrix integrity during morphogenesis, the function of matrix metallo-proteinases (MMPs) and their inhibitors, the tissue inhibitors of metalloproteinases (TIMPs), in the mature nervous system is essentially unknown. Here, we report that TIMP-2 induces pheochromocytoma PC12 cell-cycle arrest via regulation of cell-cycle regulatory proteins, resulting in differentiation and neurite outgrowth. TIMP-2 decreases cyclins B and D expression and increases p21Cip expression. Furthermore, TIMP-2 promotes cell differentiation via activation of the cAMP/Rap1/ERK (extracellular signal-regulated kinase) pathway. Expression of dominant-negative Rap1 blocks TIMP-2-mediated neurite outgrowth. Both the cell-cycle arrest and neurite outgrowth induced by TIMP-2 was independent of MMP inhibitory activity. Consistent with the PC12 cell data, primary cultures of TIMP-2 knock-out cerebral cortical neurons exhibit significantly reduced neurite length, which is rescued by TIMP-2. These in vitro results were corroborated in vivo. TIMP-2 deletion causes a delay in neuronal differentiation, as demonstrated by the persistence of nestin-positive progenitors in the neocortical ventricular zone. The interaction of TIMP-2 with alpha3beta1 integrin in the cerebral cortex suggests that TIMP-2 promotes neuronal differentiation and maintains mitotic quiescence in an MMP-independent manner through integrin activation. The identification of molecules responsible for neuronal quiescence has significant implications for the ability of the adult brain to generate new neurons in response to injury and neurological disorders, such as Alzheimer's and Parkinson's diseases.

Tissue inhibitor of metalloproteinase-2(TIMP-2)-deficient mice display motor deficits

The degradation of the extracellular matrix is regulated by matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs). Matrix components of the basement membrane play critical roles in the development and maintenance of the neuromuscular junction (NMJ), yet almost nothing is known about the regulation of MMP and TIMP expression in either the pre- or postsynaptic compartments. Here, we demonstrate that TIMP-2 is expressed by both spinal motor neurons and skeletal muscle. To determine whether motor function is altered in the absence of TIMP-2, motor behavior was assessed using a battery of tests (e.g., RotaRod, balance beam, hindlimb extension, grip strength, loaded grid, and gait analysis). TIMP-2(-/-) mice fall off the RotaRod significantly faster than wild-type littermates. In addition, hindlimb extension is reduced and gait is both splayed and lengthened in TIMP-2(-/-) mice. Motor dysfunction is more pronounced during early postnatal development. A preliminary analysis revealed NMJ alterations in TIMP-2(-/-) mice. Juvenile TIMP-2(-/-) mice have increased nerve branching and acetylcholine receptor expression. Adult TIMP-2(-/-) endplates are enlarged and more complex. This suggests a role for TIMP-2 in NMJ sculpting during development. In contrast to the increased NMJ nerve branching, cerebellar Purkinje cells have decreased neurite outgrowth. Thus, the TIMP-2(-/-) motor phenotype is likely due to both peripheral and central defects. The tissue specificity of the nerve branching phenotype suggests the involvement of different MMPs and/or extracellular matrix molecules underlying the TIMP-2(-/-) motor phenotype.

Prepulse inhibition and fear-potentiated startle are altered in tissue inhibitor of metalloproteinase-2 (TIMP-2) knockout mice

The ability to discriminate between potential dangers and recall those stimuli is essential for survival. This emotional learning requires the involvement of higher brain structures, including the amygdala, hippocampus and related cortical structures. Long-term changes in synaptic transmission and structure are important for the establishment and consolidation of fear memory. The structural changes associated with this synaptic plasticity likely require alterations in the composition of the extracellular matrix (ECM). ECM integrity is maintained by the opposing action of matrix metalloproteinases (MMPs) and their specific inhibitors, tissue inhibitors of metalloproteinases (TIMPs). To date, no studies have examined the role of MMPs or TIMPs in conditioned fear. Here, we show that neither male nor female mice deficient in TIMP-2 (knockout) exhibit prepulse inhibition of the startle reflex, suggesting deficits in pre-attentional sensorimotor gating. In addition, knockout mice and mice expressing a mutant truncated TIMP-2 (knock-down) show deficits in fear-potentiated startle. This is the first report of a phenotype for the TIMP-2(-/-) mice and suggests that TIMP-2 may play a role in the synaptic plasticity underlying learning and memory.

Lesion-induced differential expression and cell association of Neurocan, Brevican, Versican V1 and V2 in the mouse dorsal root entry zone

Lack of regeneration in the CNS has been attributed to many causes, including the presence of inhibitory molecules such as chondroitin sulfate proteoglycans (CSPGs). However, little is known about the contribution of CSPGs to regeneration failure in vivo, in particular at the dorsal root entry zone (DREZ), a unique CNS region that blocks regeneration of sensory fibers following dorsal root injury without glial scar formation. The goal of the present study was to evaluate the presence, regulation, and cellular identity of the proteoglycans Brevican, Neurocan, Versican V1 and Versican V2 in the DREZ using CSPG-specific antibodies and nucleic acid probes. Brevican and Versican V2 synthesized before the lesion were still present at high levels in the extracellular matrix of the DREZ several weeks after injury. In addition, Brevican was transiently expressed by reactive oligodendrocytes, and by a subset of astrocytes thereafter. Versican V2 mRNA appeared in NG2-positive cells with the morphology of oligodendrocyte precursor cells. Neurocan and Versican V1 levels were low before injury, and appeared in nestin-positive astrocytes and in NG2-positive cells, respectively, following lesion. Versican V1, but not V2, was also transiently increased in the peripheral dorsal root post-lesion. This is the first thorough description of the expression and cell association of individual proteoglycans following dorsal root lesion. It demonstrates that the proteoglycans Brevican, Neurocan, Versican V1, and Versican V2 are abundant in the DREZ at the time regenerating sensory fibers reach the PNS/CNS border and may therefore participate in growth-inhibition in this region.

Pathological and molecular progression of astrocytomas in a GFAP:12 V-Ha-Ras mouse astrocytoma model

We previously characterized a genetically engineered mouse astrocytoma model with embryonic astrocyte-specific, activated (12)V-Ha-RAS (GFAP-RAS) transgenesis. The GFAP-RAS line Ras-B8 appears normal at birth, but 50% of mice die by 4 months from low- and high-grade astrocytomas. We examined the development and progression of astrocytomas in the Ras-B8 genetically engineered mouse. At embryonic day 16.5 (E16.5), there were no pathological differences compared to control littermates, aside from transgene expression. Diffuse astroglial hyperplasia was the first distinguishing feature in the 1-week-old Ras-B8 mice; however, these astrocytes were not transformed in vitro or in vivo. From 3 to 8 weeks the incidence of low-grade astrocytomas progressively increased with 85% of 12-week-old mice harboring low- or high-grade astrocytomas, the latter characterized by increased proliferation, nuclear atypia, and angiogenesis. Tp 53 mutations were detected in both astrocytoma grades, with high-grade astrocytomas expressing elevated levels of epidermal growth factor receptor and vascular endothelial growth factor, plus decreased levels of PTEN and p16, similar to human astrocytomas. We postulate that expression of (12)V-Ha-RAS in astroglial precursors induces astroglial hyperplasia, but transformation and subsequent progression requires additional molecular alterations resulting from aberrant activated p21-RAS. Of interest, many of these acquired alterations occur in human astrocytomas, further validating GFAP-RAS as a useful model for studying astrocytoma development and progression.

Evidence for proteolytic cleavage of brevican by the ADAMTSs in the dentate gyrus after excitotoxic lesion of the mouse entorhinal cortex

BACKGROUND

Brevican is a member of the lectican family of aggregating extracellular matrix (ECM) proteoglycans that bear chondroitin sulfate (CS) chains. It is highly expressed in the central nervous system (CNS) and is thought to stabilize synapses and inhibit neural plasticity and as such, neuritic or synaptic remodeling would be less likely to occur in regions with intact and abundant, lectican-containing, ECM complexes. Neural plasticity may occur more readily when these ECM complexes are broken down by endogenous proteases, the ADAMTSs (adisintegrin and metalloproteinase with thrombospondin motifs), that selectively cleave the lecticans. The purpose of these experiments was to determine whether the production of brevican or the ADAMTS-cleaved fragments of brevican were altered after deafferentation and reinnervation of the dentate gyrus via entorhinal cortex lesion (ECL).

RESULTS

In the C57Bl6J mouse, synaptic density in the molecular layer of the dentate gyrus, as measured by synaptophysin levels in ELISA, was significantly attenuated 2 days (nearly 50% of contralateral) and 7 days after lesion and returned to levels not different from the contralateral region at 30 days. Immunoreactive brevican in immunoblot was elevated 2 days after lesion, whereas there was a significant increase in the proteolytic product at 7, but not 30 days post-lesion. ADAMTS activity, estimated using the ratio of the specific ADAMTS-derived brevican fragment and intact brevican levels was increased at 7 days, but was not different from the contralateral side at 2 or 30 days after deafferentation.

CONCLUSION

These findings indicate that ADAMTS activity in the dentate outer molecular layer (OML) is elevated during the initial synaptic reinnervation period (7 days after lesion). Therefore, proteolytic processing of brevican appears to be a significant extracellular event in the remodeling of the dentate after EC lesion, and may modulate the process of sprouting and/or synaptogenesis.

Altered production and proteolytic processing of brevican by transforming growth factor beta in cultured astrocytes

Brevican, a proteoglycan of the lectican family, inhibits neurite outgrowth and may also stabilize synapses. Little is known about its expression or function in vitro. This study seeks to determine whether a brevican-containing matrix is present in neural cultures, and if so, how the production of brevican may be modulated. To accomplish this, the content of brevican and its proteolytic fragments were measured in primary cultures of neurons, astrocytes and microglia after treatment with cytokines. These experiments revealed that astrocytes and neurons express several isoforms of brevican, whereas microglia do not produce this proteoglycan. Cleavage fragments of brevican were found primarily in neuronal and astrocyte culture medium. ADAMTS4 (a disintegrin and metalloproteinase with thrombospondin motifs), a protease that selectively cleaves lecticans, was detected in cultures of neurons, astrocytes and microglia. When astrocytes were challenged with various cytokines, it was found that treatment with transforming growth factor beta (TGFbeta) resulted in a marked increase in intact brevican in the culture medium that was accompanied by a trend for a decrease in ADAMTS-generated fragments of brevican and apparent ADAMTS activity. Thus, TGFbeta may play a role in neuronal plasticity through its regulation of brevican and the activity of the ADAMTSs.

The effect of hypoxic-ischemic brain injury in perinatal rats on the abundance and proteolysis of brevican and NG2

Oligodendrocyte (OL) progenitor cells are particularly susceptible to perinatal hypoxia/ischemia (H-I) resulting in decreased myelination and attenuated development of white matter fiber tracts. Brevican is an aggregating chondroitin sulfate proteoglycan (CSPG) secreted by OLs and their progenitors prior to and during active developmental myelination whereas neuron-glia antigen 2 (NG2) is a transmembrane CSPG produced by early OL progenitors. Although both proteoglycans are associated with maturation of OLs, it is not known if they are altered by H-I brain injury in the neonate. We have therefore examined the time course of changes in brevican and NG2 abundance and proteolysis in the neonatal rat hippocampus after H-I. In a standard H-I model of unilateral carotid artery ligation and exposure to hypoxia, a cavitary infarct involving the ipsilateral parietal and temporal regions of cerebral cortex, hippocampus, and striatum of most rat pups was clearly evident 4 days after H-I. The abundance of total extractable brevican was markedly reduced in the ipsilateral hippocampus at 1 and 14 days after H-I (relative to the contralateral side). At these times, the total G1 proteolytic fragment of brevican was lower in the ipsilateral hippocampus and the level of a protease-generated brevican fragment was significantly diminished in the OL-rich hippocampal fimbria. Hippocampal NG2 levels were also lower at 1 and 4 days after H-I, but were not different from the contralateral side at 14 days. Since brevican, brevican G1 fragment, and NG2 loss occur around the time of progressive cell death and the appearance of the infarct, it may be that H-I rapidly induces a cellular response that actively depletes these proteoglycans from the hippocampal matrix. While the mechanism of this loss is unclear, it would appear to be an early event in the process that could be involved in apoptotic cell death and/or tissue injury.

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

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

Neurogenesis in the adult brain: new strategies for central nervous system diseases

New cells are continuously generated from immature proliferating cells throughout adulthood in many organs, thereby contributing to the integrity of the tissue under physiological conditions and to repair following injury. In contrast, repair mechanisms in the adult central nervous system (CNS) have long been thought to be very limited. However, recent findings have clearly demonstrated that in restricted areas of the mammalian brain, new functional neurons are constantly generated from neural stem cells throughout life. Moreover, stem cells with the potential to give rise to new neurons reside in many different regions of the adult CNS. These findings raise the possibility that endogenous neural stem cells can be mobilized to replace dying neurons in neurodegenerative diseases. Indeed, recent reports have provided evidence that, in some injury models, limited neuronal replacement occurs in the CNS. Here, we summarize our current understanding of the mechanisms controlling adult neurogenesis and discuss their implications for the development of new strategies for the treatment of neurodegenerative diseases.

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.

Secreted brevican mRNA is expressed in the adult rat pituitary

Brevican is the most abundant chondroitin sulfate proteoglycan in the extracellular matrix (ECM) of the adult rat brain. It has been found only in the central nervous system (CNS). In this study, we found that secreted brevican transcript was detectable in the pituitary of both male and female adult rats by reverse transcriptase-polymerase chain reaction (RT-PCR) amplification. In posterior lobe of pituitary, pituicytes were heavily labelled. In anterior and intermediate lobes of pituitary, signals for brevican transcripts were observed in cells of various sizes. These data demonstrated that secreted brevican mRNA is expressed in the adult rat pituitary and brevican might not be a CNS-specific ECM.

A novel membrane-associated glycovariant of BEHAB/brevican is up-regulated during rat brain development and in a rat model of invasive glioma

BEHAB (brain-enriched hyaluronan-binding protein)/brevican is the most abundant chondroitin sulfate proteoglycan in the extracellular matrix of the adult rat brain. BEHAB/brevican expression is up-regulated coincident with glial cell proliferation and/or motility, including during early central nervous system development and in invasive glioma. An understanding of the molecular interactions that mediate BEHAB/brevican function is still in its infancy because of the existence of several BEHAB/brevican isoforms, each of which may mediate different functions. Here, we describe a novel BEHAB/brevican isoform, B/b130, and demonstrate that it is neither the glycosylphosphatidylinositol-linked splice variant of BEHAB/brevican nor a cleavage product of the full-length protein (B/b150). B/b130 is an underglycosylated isoform of BEHAB/brevican, lacking glycosaminoglycan chains as well as most of the sugars that invest B/b150. B/b130 localizes exclusively to the particulate fraction of rat brain and associates with the cell membrane by a previously undescribed calcium-independent mechanism. In addition, B/b130 is the major isoform of BEHAB/brevican that is up-regulated in a rat model of invasive glioma and may therefore contribute to the invasive ability of glioma cells. Further understanding of BEHAB/brevican isoforms will advance our knowledge of the function of this ECM component and may help identify new potential therapeutic targets for primary brain tumors.

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

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

TIGR is upregulated in the chronic glial scar in response to central nervous system injury and inhibits neurite outgrowth

Reactive astrocytes respond to central nervous system (CNS) injury and disease by elaborating a glial scar that is inhibitory to axonal regeneration. To identify genes that may be involved in the astrocytic response to injury, we used differential display polymerase chain reaction and an in vivo model of the CNS glial scar. Expression of the trabecular meshwork inducible glucocorticoid response (TIGR) gene was increased in gliotic tissue compared with the uninjured cerebral cortex. Increased TIGR expression by reactive astrocytes was confirmed by in situ hybridization, quantitative reverse transcriptase-polymerase chain reaction, immunoblot analysis, and immunohistochemistry. Although mutations of the TIGR gene have been implicated in glaucoma, a function for TIGR has not been reported. Since TIGR is secreted, we assessed a possible role in inhibition of neuronal regeneration with an in vitro bioassay and found that this protein is a potent inhibitor of neurite outgrowth. Thus, TIGR is a newly identified component of the CNS glial scar that is likely to contribute to neuronal regenerative failure characteristic of the mammalian CNS.

Slit and glypican-1 mRNAs are coexpressed in the reactive astrocytes of the injured adult brain

The slit family serves as a repellent for growing axons toward correct targets during neural development. A recent report describes slit mRNAs expressed in various brain regions in adult rats. However, their functions in the adult nervous system remain unknown. In the present study, we investigated whether slit mRNAs were expressed in the cryo-injured brain, using in situ hybridization. All slit family members were expressed at the lesion. Slit2 mRNA was the most intensely expressed in the cells surrounding the necrotic tissue. A double-labeling study showed that slit2 mRNA was expressed in the glial fibrillary acidic protein (GFAP)-positive reactive astrocytes. In addition, glypican-1, a heparan sulfate proteoglycan that serves as a high-affinity receptor for Slit protein, was coexpressed with slit2 mRNA in the reactive astrocytes. These findings suggested that slit2 might prevent regenerating axons from entering into the lesion in concert with glypican-1.

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

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

Association between protease-specific proteolytic cleavage of brevican and synaptic loss in the dentate gyrus of kainate-treated rats

Proteolytic fragments generated by ADAMTS (a disintegrin and metalloprotease with thrombospondin motifs)-mediated cleavage of the aggregating chondroitin sulfate proteoglycan, brevican, have been identified, but not localized in the CNS. The purpose of this study, using kainate-induced CNS lesion, was to examine the spatial and quantitative relationship between ADAMTS1 and 4 mRNA expression and ADAMTS-mediated cleavage of brevican (as determined by the abundance of the neo-epitope QEAVESE at the C-terminal of the cleaved brevican G1 domain). In untreated rats, in situ hybridization and reverse transcriptase polymerase chain reaction indicated that ADAMTS4 expression was higher than ADAMTS1 and was localized to hippocampus, temporal lobe and other areas of cortex, striatum and hypothalamus. ADAMTS4 mRNA expression in these regions correlated with the presence of the QEAVESE neo-epitope, which was concentrated in perineuronal nets and in neuropil. In rats that seized after kainate, there was a dramatic elevation in ADAMTS1 and ADAMTS4 transcript that correlated and co-localized with a robust elevation in an extractable, 55-kDa fragment of brevican in temporal lobe and hippocampus. This fragment consisted, at least in part, of the ADAMTS-cleaved epitope G1-QEAVESE. The kainate-induced elevation in this ADAMTS-cleaved fragment was localized to amygdaloid and thalamic nuclei, hippocampus, caudate-putamen, cingulate cortex, and the outer molecular layer of the dentate gyrus where it was accompanied by a robust elevation in ADAMTS1 and 4 mRNA and a 28% decline in synaptic density 5 days after kainate.Thus, complexes of extracellular matrix proteins that exist in perineuronal nets and in the neuropil are cleaved by specific matrix-degrading proteases at early time points during excitotoxic neurodegeneration. The observed ADAMTS-induced cleavage of brevican in the dentate outer molecular layer is closely associated with diminished synaptic density, and may, therefore, contribute to synaptic loss and/or reorganization in this region.

Limited growth of severed CNS axons after treatment of adult rat brain with hyaluronidase

Many chondroitin sulfate proteoglycans (CSPGs) have been shown to influence CNS axon growth in vitro and in vivo. These interactions can be mediated through the core protein or through the chondroitin sulfate (CS) glycosaminoglycan (GAG) side chains. We have shown previously that degrading CS GAG side chains using chondroitinase ABC enhances dopaminergic nigrostriatal axon regeneration in vivo. We test the hypothesis that interfering with complete CSPGs also limit axon growth in vivo. Neurocan, versican, aggrecan, and brevican CSPGs may be anchored within extracellular matrix through binding to hyaluronan glycosaminoglycan. We examine whether degradation of hyaluronan using hyaluronidase might release these inhibitory CSPGs from the extracellular matrix and thereby enhance regeneration of cut nigrostriatal axons. Anesthetized adult rats were given knife cut lesions of the right hemisphere nigrostriatal tract and cannulae were secured transcranially thereby allowing repeated perilesional infusion of saline or saline containing hyaluronidase once daily for 10 days post-axotomy. Eleven days post-transection brains from animals under terminal anesthesia were recovered for histological evaluation. Effective delivery of substance was inferred from the observed reduction in perilesional immunoreactivity for neurocan and versican after treatment with hyaluronidase (relative to saline). Immunolabeling using antibodies against tyrosine hydroxylase was used to examine the response of cut dopaminergic nigral neurons. After transection and treatment with saline, dopaminergic nigral neurons sprouted in a region lacking astrocytes, neurocan and versican. Axons did not regenerate into the lesion surround that contained astrocytes and abundant neurocan and versican. After transection and treatment with hyaluronidase, there was a significant increase in the number of cut dopaminergic nigral axons growing up to 800 microm anterior to the site of transection. However, cut dopaminergic nigral axons still did not regenerate into the lesion surround that contained reduced (albeit residual) neurocan and versican immunoreactivity. Thus, partial degradation of hyaluronan and chondroitin sulfate and depletion of hyaluronan-binding CSPGs enhances local sprouting of cut CNS axons, but long-distance regeneration fails in regions containing residual hyaluronan-binding CSPGs. Hyaluronan, chondroitin sulfate and hyaluronan-binding CSPGs therefore likely contribute toward the failure of spontaneous axon regeneration in the injured adult mammalian brain and spinal cord.

The extracellular matrix in axon regeneration

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

Chondroitin sulphate proteoglycans in the CNS injury response

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

Glycosaminoglycan levels and proteoglycan expression are altered in the hippocampus of patients with mesial temporal lobe epilepsy

Extracellular matrix proteoglycans (PGs) and glycosaminoglycans (GAGs) play a crucial role in cell differentiation and synaptogenesis by modulating neurite outgrowth. The chondroitin sulfate (CS)-rich PG, the receptor protein tyrosine phosphatase zeta/beta (RPTP zeta/beta), has been related to neural morphogenesis and axon guidance. Hippocampal sclerosis is the most frequent pathologic finding in patients with intractable mesial temporal lobe epilepsy (MTLE), which is associated with neuron loss, reactive gliosis, and mossy fiber sprouting. In the present study, we investigated the concentration of CS, heparan sulfate (HS) and hyaluronic acid (HA) in the hippocampus and temporal neocortex as well as RPTP zeta/beta expression in the hippocampus of patients with MTLE. Compared to autopsy control tissue, epileptic hippocampi showed a significantly increased concentration of CS (224%; p=0.0109) and HA (146%; p=0.039). HS was instead similar to control values. No differences were found in the concentration of CS, HS, or HA in the temporal neocortex of epileptic patients when compared to control values. In contrast, RPTP zeta/beta immunoreactivity was induced in astrocytes of the inner molecular layer of the dentate gyrus of the sclerotic hippocampus. Because matrix compounds have been associated with tissue injury and repair, the present findings suggest that changes in PGs and GAGs might be related to damage-induced gliosis and neuronal reorganization in the hippocampus of MTLE patients.

Increased syndecan expression by pleiotrophin and FGF receptor-expressing astrocytes in injured brain tissue

Syndecan-1, -2, -3, and -4 are heparan sulfate proteoglycans that are differentially expressed during development and wound repair. To determine whether syndecans are also involved in brain injury, we examined the expression of syndecan core proteins genes in cryo-injured mouse brain, using in situ hybridization. All syndecan mRNA transcripts were similarly expressed in the region surrounding the necrotic tissue, exhibiting peak levels at day 7 after injury. Comparison with cellular markers showed that reactive astrocytes were the primary source of syndecans. Syndecans serve as co-receptors for fibroblast growth factor (FGF) and as a reservoir for another heparin-binding growth factor, pleiotrophin (PTN, or heparin-binding growth-associated molecule. In our model, FGF receptor1 (FGFR1) and PTN mRNA levels were upregulated in reactive astrocytes. The distribution patterns of FGFR1 and PTN overlapped considerably with those of syndecan-1 and -3 mRNAs, respectively. These results suggest that syndecans are expressed primarily in reactive astrocytes, and may provide a supportive environment for regenerating axons in concert with heparin-binding growth factors (e.g., FGF and PTN) in the injured brain.