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

Differential effects of the cell cycle inhibitor, olomoucine, on functional recovery and on responses of peri-infarct microglia and astrocytes following photothrombotic stroke in rats

Background

Following stroke, changes in neuronal connectivity in tissue surrounding the infarct play an important role in both spontaneous recovery of neurological function and in treatment-induced improvements in function. Microglia and astrocytes influence this process through direct interactions with the neurons and as major determinants of the local tissue environment. Subpopulations of peri-infarct glia proliferate early after stroke providing a possible target to modify recovery. Treatment with cell cycle inhibitors can reduce infarct volume and improve functional recovery. However, it is not known whether these inhibitors can influence neurological function or alter the responses of peri-infarct glia without reducing infarction. The present study aimed to address these issues by testing the effects of the cell cycle inhibitor, olomoucine, on recovery and peri-infarct changes following photothrombotic stroke.

Methods

Stroke was induced by photothrombosis in the forelimb sensorimotor cortex in Sprague-Dawley rats. Olomoucine was administered at 1 h and 24 h after stroke induction. Forelimb function was monitored up to 29 days. The effects of olomoucine on glial cell responses in peri-infarct tissue were evaluated using immunohistochemistry and Western blotting.

Results

Olomoucine treatment did not significantly affect maximal infarct volume. Recovery of the affected forelimb on a placing test was impaired in olomoucine-treated rats, whereas recovery in a skilled reaching test was substantially improved. Olomoucine treatment produced small changes in aspects of Iba1 immunolabelling and in the number of CD68-positive cells in cerebral cortex but did not selectively modify responses in peri-infarct tissue. The content of the astrocytic protein, vimentin, was reduced by 30% in the region of the lesion in olomoucine-treated rats.

Conclusions

Olomoucine treatment modified functional recovery in the absence of significant changes in infarct volume. The effects on recovery were markedly test dependent, adding to evidence that skilled tasks requiring specific training and general measures of motor function can be differentially modified by some interventions. The altered recovery was not associated with specific changes in key responses of peri-infarct microglia, even though these cells were considered a likely target for early olomoucine treatment. Changes detected in peri-infarct reactive astrogliosis could contribute to the altered patterns of functional recovery.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-021-02208-w.

Sensory deafferentation modulates and redistributes neurocan in the rat auditory brainstem

INTRODUCTION

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Early treatment with minocycline following stroke in rats improves functional recovery and differentially modifies responses of peri-infarct microglia and astrocytes

BACKGROUND

Altered neuronal connectivity in peri-infarct tissue is an important contributor to both the spontaneous recovery of neurological function that commonly develops after stroke and improvements in recovery that have been induced by experimental treatments in animal models. Microglia and astrocytes are primary determinants of the environment in peri-infarct tissue and hence strongly influence the potential for neuronal plasticity. However, the specific roles of these cells and the timing of critical changes in their function are not well understood. Minocycline can protect against ischemic damage and promote recovery. These effects are usually attributed, at least partially, to the ability of this drug to suppress microglial activation. This study tested the ability of minocycline treatment early after stroke to modify reactive responses in microglia and astrocytes and improve recovery.

METHODS

Stroke was induced by photothrombosis in the forelimb sensorimotor cortex of Sprague-Dawley rats. Minocycline was administered for 2 days after stroke induction and the effects on forelimb function assessed up to 28 days. The responses of peri-infarct Iba1-positive cells and astrocytes were evaluated using immunohistochemistry and Western blots.

RESULTS

Initial characterization showed that the numbers of Iba1-positive microglia and macrophages decreased in peri-infarct tissue at 24 h then increased markedly over the next few days. Morphological changes characteristic of activation were readily apparent by 3 h and increased by 24 h. Minocycline treatment improved the rate of recovery of motor function as measured by a forelimb placing test but did not alter infarct volume. At 3 days, there were only minor effects on core features of peri-infarct microglial reactivity including the morphological changes and increased density of Iba1-positive cells. The treatment caused a decrease of 57% in the small subpopulation of cells that expressed CD68, a marker of phagocytosis. At 7 days, the expression of glial fibrillary acidic protein and vimentin was markedly increased by minocycline treatment, indicating enhanced reactive astrogliosis.

CONCLUSIONS

Early post-stroke treatment with minocycline improved recovery but had little effect on key features of microglial activation. Both the decrease in CD68-positive cells and the increased activation of astrogliosis could influence neuronal plasticity and contribute to the improved recovery.

Proteolytic Remodeling of Perineuronal Nets: Effects on Synaptic Plasticity and Neuronal Population Dynamics

The perineuronal net (PNN) represents a lattice-like structure that is prominently expressed along the soma and proximal dendrites of parvalbumin- (PV-) positive interneurons in varied brain regions including the cortex and hippocampus. It is thus apposed to sites at which PV neurons receive synaptic input. Emerging evidence suggests that changes in PNN integrity may affect glutamatergic input to PV interneurons, a population that is critical for the expression of synchronous neuronal population discharges that occur with gamma oscillations and sharp-wave ripples. The present review is focused on the composition of PNNs, posttranslation modulation of PNN components by sulfation and proteolysis, PNN alterations in disease, and potential effects of PNN remodeling on neuronal plasticity at the single-cell and population level.

Juvenile Hippocampal CA2 Region Expresses Aggrecan

Perineuronal nets (PNNs) are distributed primarily around inhibitory interneurons in the hippocampus, such as parvalbumin-positive interneurons. PNNs are also present around excitatory neurons in some brain regions and prevent plasticity in these neurons. A recent study demonstrated that PNNs also exist around mouse hippocampal pyramidal cells, which are the principle type of excitatory neurons, in the CA2 subregion and modulate the excitability and plasticity of these neurons. However, the development of PNNs in the CA2 region during postnatal maturation was not fully investigated. This study found that a main component of PNNs, aggrecan, existed in the pyramidal cell layer of the putative CA2 subarea prior to the appearance of the CA2 region, which was defined by the CA2 marker protein regulator of G protein signaling 14 (RGS14). We also found that aggrecan immunoreactivity was more evident in the anterior sections of the CA2 area than the posterior sections, which suggests that the function of CA2 PNNs varies along the anterior-posterior axis.

Crosstalk between glia, extracellular matrix and neurons

Extracellular matrix (ECM) molecules in the central nervous system form highly organized ECM structures around cell somata, axon initial segments, and synapses and play prominent roles in early development by guiding cell migration, neurite outgrowth and synaptogenesis, and by regulating closure of the critical period of development, synaptic plasticity and stability, cognitive flexibility, and axonal regeneration in adults. Major components of neural ECM, including chondroitin sulfate proteoglycans (CSPGs), tenascin-R and hyaluronic acid, are synthesized by both neurons and glial cells. The expression of these molecules is dynamically regulated during brain development in physiological conditions, shaping both neuronal and glial functions through multitude of molecular mechanisms. Upregulation of particular CSPGs and other ECM molecules, in particular by reactive astrocytes, after CNS injuries, during aging, neuroinflammation, and neurodegeneration on the one hand results in formation of growth-impermissive environment and impaired synaptic plasticity. On the other hand, ECM appeared to have a neuroprotective effect, at least in the form of perineuronal nets. CSPGs-degrading matrix metalloproteinases (MMPs) and several members of the disintegrin and metalloproteinase with thrombospondin motifs (ADAMTS) family of proteases are secreted by neurons and glia and may drive neural ECM remodeling in physiological conditions as well as after brain injury and other brain disorders. Thus, targeting expression of specific ECM molecules, associated glycans and degrading enzymes may lead to development of new therapeutic strategies promoting regeneration and synaptic plasticity.

Accumulation of Extracellular Matrix in Advanced Lesions of Canine Distemper Demyelinating Encephalitis

In demyelinating diseases, changes in the quality and quantity of the extracellular matrix (ECM) may contribute to demyelination and failure of myelin repair and axonal sprouting, especially in chronic lesions. To characterize changes in the ECM in canine distemper demyelinating leukoencephalitis (DL), histochemical and immunohistochemical investigations of formalin-fixed paraffin-embedded cerebella using azan, picrosirius red and Gomori`s silver stain as well as antibodies directed against aggrecan, type I and IV collagen, fibronectin, laminin and phosphacan showed alterations of the ECM in CDV-infected dogs. A significantly increased amount of aggrecan was detected in early and late white matter lesions. In addition, the positive signal for collagens I and IV as well as fibronectin was significantly increased in late lesions. Conversely, the expression of phosphacan was significantly decreased in early and more pronounced in late lesions compared to controls. Furthermore, a set of genes involved in ECM was extracted from a publically available microarray data set and was analyzed for differential gene expression. Gene expression of ECM molecules, their biosynthesis pathways, and pro-fibrotic factors was mildly up-regulated whereas expression of matrix remodeling enzymes was up-regulated to a relatively higher extent. Summarized, the observed findings indicate that changes in the quality and content of ECM molecules represent important, mainly post-transcriptional features in advanced canine distemper lesions. Considering the insufficiency of morphological regeneration in chronic distemper lesions, the accumulated ECM seems to play a crucial role upon regenerative processes and may explain the relatively small regenerative potential in late stages of this disease.

In Sickness and in Health: Perineuronal Nets and Synaptic Plasticity in Psychiatric Disorders

Rapidly emerging evidence implicates perineuronal nets (PNNs) and extracellular matrix (ECM) molecules that compose or interact with PNNs, in the pathophysiology of several psychiatric disorders. Studies on schizophrenia, autism spectrum disorders, mood disorders, Alzheimer's disease, and epilepsy point to the involvement of ECM molecules such as chondroitin sulfate proteoglycans, Reelin, and matrix metalloproteases, as well as their cell surface receptors. In many of these disorders, PNN abnormalities have also been reported. In the context of the “quadripartite” synapse concept, that is, the functional unit composed of the pre- and postsynaptic terminals, glial processes, and ECM, and of the role that PNNs and ECM molecules play in regulating synaptic functions and plasticity, these findings resonate with one of the most well-replicated aspects of the pathology of psychiatric disorders, that is, synaptic abnormalities. Here we review the evidence for PNN/ECM-related pathology in these disorders, with particular emphasis on schizophrenia, and discuss the hypothesis that such pathology may significantly contribute to synaptic dysfunction.

Involvement of chondroitin 6-sulfation in temporal lobe epilepsy

Chondroitin sulfate proteoglycans (CSPGs) are predominant components of the extracellular matrix in the central nervous system (CNS). Previously, we found an increase in the 4-sulfation/6-sulfation (4S/6S) ratio of CSPGs is required for perineuronal net (PNN) formation and results in functional maturation of parvalbumin-expressing interneurons (PV cells) and termination of the critical period in the visual cortex. Here, we report that chondroitin 6-sulfation and chondroitin 6-sulfation-enriched PNNs increased in the mouse cerebral cortex and hippocampus after kainic acid (KA) treatment; simultaneously, chondroitin 4-sulfation-enriched PNNs and the 4S/6S ratio decreased. Furthermore, chondroitin 6-O-sulfotransferase-1 (C6ST-1) transgenic (TG) mice, which overexpress chondroitin 6-sulfated chains and have a decreased 4S/6S ratio, were more susceptible to KA-induced seizures than wild-type mice. These results suggested that chondroitin 6-sulfation is relevant to epilepsy most probably because of dysregulated PNN formation and PV cell maturation.

Chondroitin sulfate proteoglycans: Key modulators in the developing and pathologic central nervous system

Chondroitin Sulfate Proteoglycans (CSPGs) are a major component of the extracellular matrix in the central nervous system (CNS) and play critical role in the development and pathophysiology of the brain and spinal cord. Developmentally, CSPGs provide guidance cues for growth cones and contribute to the formation of neuronal boundaries in the developing CNS. Their presence in perineuronal nets plays a crucial role in the maturation of synapses and closure of critical periods by limiting synaptic plasticity. Following injury to the CNS, CSPGs are dramatically upregulated by reactive glia which form a glial scar around the lesion site. Increased level of CSPGs is a hallmark of all CNS injuries and has been shown to limit axonal plasticity, regeneration, remyelination, and conduction after injury. Additionally, CSPGs create a non-permissive milieu for cell replacement activities by limiting cell migration, survival and differentiation. Mounting evidence is currently shedding light on the potential benefits of manipulating CSPGs in combination with other therapeutic strategies to promote spinal cord repair and regeneration. Moreover, the recent discovery of multiple receptors for CSPGs provides new therapeutic targets for targeted interventions in blocking the inhibitory properties of CSPGs following injury. Here, we will provide an in depth discussion on the impact of CSPGs in normal and pathological CNS. We will also review the recent preclinical therapies that have been developed to target CSPGs in the injured CNS.

Suppression of neurocan and enhancement of axonal density in rats after treatment of traumatic brain injury with scaffolds impregnated with bone marrow stromal cells

OBJECT

Neurocan is a major form of growth-inhibitory molecule (growth-IM) that suppresses axonal regeneration after neural injury. Bone marrow stromal cells (MSCs) have been shown to inhibit neurocan expression in vitro and in animal models of cerebral ischemia. Therefore, the present study was designed to investigate the effects of treatment of MSCs impregnated with collagen scaffolds on neurocan expression after traumatic brain injury (TBI).

METHODS

Adult male Wistar rats were injured with controlled cortical impact and treated with saline, human MSCs (hMSCs) (3 × 10(6)) alone, or hMSCs (3 × 10(6)) impregnated into collagen scaffolds (scaffold + hMSCs) transplanted into the lesion cavity 7 days after TBI (20 rats per group). Rats were sacrificed 14 days after TBI, and brain tissues were harvested for immunohistochemical studies, Western blot analyses, laser capture microdissections, and quantitative real-time reverse transcriptase polymerase chain reaction (qRT-PCR) to evaluate neurocan protein and gene expressions after various treatments.

RESULTS

Animals treated with scaffold + hMSCs after TBI showed increased axonal and synaptic densities compared with the other groups. Scaffold + hMSC treatment was associated with reduced TBI-induced neurocan protein expression and upregulated growth-associated protein 43 (GAP-43) and synaptophysin expression in the lesion boundary zone. In addition, animals in the scaffold + hMSC group had decreased neurocan transcription in reactive astrocytes after TBI. Reduction of neurocan expression was significantly greater in the scaffold + hMSC group than in the group treated with hMSCs alone.

CONCLUSIONS

The results of this study show that transplanting hMSCs with scaffolds enhances the effect of hMSCs on axonal plasticity in TBI rats. This enhanced axonal plasticity may partially be attributed to the downregulation of neurocan expression by hMSC treatment after injury.

Persistent decrease in multiple components of the perineuronal net following status epilepticus

In the rodent model of temporal lobe epilepsy, there is extensive synaptic reorganization within the hippocampus following a single prolonged seizure event, after which animals eventually develop epilepsy. The perineuronal net (PN), a component of the neural extracellular matrix (ECM), primarily surrounds inhibitory interneurons and, under normal conditions, restricts synaptic reorganization. The objective of the current study was to explore the effects of status epilepticus (SE) on PNs in the adult hippocampus. The aggrecan component of the PN was studied, acutely (48 h post-SE), sub-acutely (1 week post-SE) and during the chronic period (2 months post-SE). Aggrecan expressing PNs decreased by 1 week, likely contributing to a permissive environment for neuronal reorganization, and remained attenuated at 2 months. The SE-exposed hippocampus showed many PNs with poor structural integrity, a condition rarely seen in controls. Additionally, the decrease in the aggrecan component of the PN was preceded by a decrease in hyaluronan and proteoglycan link protein 1 (HAPLN1) and hyaluronan synthase 3 (HAS3), which are components of the PN known to stabilize the connection between aggrecan and hyaluronan, a major constituent of the ECM. These results were replicated in vitro with the addition of excess KCl to hippocampal cultures. Enhanced neuronal activity caused a decrease in aggrecan, HAPLN1 and HAS3 around hippocampal cells in vivo and in vitro, leaving inhibitory interneurons susceptible to increased synaptic reorganization. These studies are the foundation for future experiments to explore how loss of the PN following SE contributes to the development of epilepsy.

The perineuronal net component of the extracellular matrix in plasticity and epilepsy

During development the extracellular matrix (ECM) of the central nervous system (CNS) facilitates proliferation, migration, and synaptogenesis. In the mature nervous system due to changes in the ECM it provides structural stability and impedes proliferation, migration, and synaptogensis. The perineuronal net (PN) is a specialized ECM structure found primarily surrounding inhibitory interneurons where it forms a mesh-like structure around points of synaptic contact. The PN organizes the extracellular space by binding multiple components of the ECM and bringing them into close proximity to the cell membrane, forming dense aggregates surrounding synapses. The PN is expressed late in postnatal development when the nervous system is in the final stages of maturation and the critical periods are closing. Once fully expressed the PN envelopes synapses and leads to decreased plasticity and increases synaptic stability in the CNS. Disruptions in the PN have been studied in a number of disease states including epilepsy. Epilepsy is one of the most common neurologic disorders characterized by excessive neuronal activity which results in recurrent spontaneous seizures. A shift in the delicate balance between excitation and inhibition is believed to be one of the underlying mechanisms in the development of epilepsy. During epileptogenesis, the brain undergoes numerous changes including synaptic rearrangement and axonal sprouting, which require structural plasticity. Because of the PNs location around inhibitory cells and its role in limiting plasticity, the PN is an important candidate for altering the progression of epilepsy. In this review, an overview of the ECM and PN in the CNS will be presented with special emphasis on potential roles in epileptogenesis.

Microglial inhibitory factor (MIF/TKP) mitigates secondary damage following spinal cord injury

Spinal cord injury (SCI) induces an immune response during which microglia, the resident immunocompetent cells of the central nervous system, become activated and migrate to the site of damage. Depending on their state of activation, microglia secrete neurotoxic or neurotrophic factors that influence the surrounding environment and have a detrimental or restorative effect following SCI, including causing or protecting bystander damage to nearby undamaged tissue. Subsequent infiltration of macrophages contributes to the SCI outcome. We show here that suppressing microglia/macrophage activation using the tripeptide macrophage/microglia inhibitory factor (MIF/TKP) reduced secondary injury around the lesion epicenter in the murine dorsal hemisection model of SCI; it decreased the hypertrophic change of astrocytes and caused an increase in the number of axons present within the lesion epicenter. Moreover, timely inhibition of microglial/macrophage activation prevented demyelination and axonal dieback by modulating oligodendrocyte survival and oligodendrocyte precursor maturation. Microglia/macrophages located within or proximal to the lesion produced neurotoxic factors, such as tumor necrosis factor alpha (TNF-α). These results suggest that microglia/macrophages within the epicenter at early time points post injury are neurotoxic, contributing to demyelination and axonal degeneration and that MIF/TKP could be used in combination with other therapies to promote functional recovery.

Phosphacan and receptor protein tyrosine phosphatase β expression mediates deafferentation-induced synaptogenesis

This study documents the spatial and temporal expression of three structurally related chondroitin sulfated proteoglycans (CSPGs) during synaptic regeneration induced by brain injury. Using the unilateral entorhinal cortex (EC) lesion model of adaptive synaptogenesis, we documented mRNA and protein profiles of phosphacan and its two splice variants, full length receptor protein tyrosine phosphatase β (RPTPβ) and the short transmembrane receptor form (sRPTPβ), at 2, 7, and 15 days postlesion. We report that whole hippocampal sRPTPβ protein and mRNA are persistently elevated over the first two weeks after UEC. As predicted, this transmembrane family member was localized adjacent to synaptic sites in the deafferented neuropil and showed increased distribution over that zone following lesion. By contrast, whole hippocampal phosphacan protein was not elevated with deafferentation; however, its mRNA was increased during the period of sprouting and synapse formation (7d). When the zone of synaptic reorganization was sampled using molecular layer/granule cell (ML/GCL) enriched dissections, we observed an increase in phosphacan protein at 7d, concurrent with the observed hippocampal mRNA elevation. Immunohistochemistry also showed a shift in phosphacan distribution from granule cell bodies to the deafferented ML at 2 and 7d postlesion. Phosphacan and sRPTPβ were not colocalized with glial fibrillary acid protein (GFAP), suggesting that reactive astrocytes were not a major source of either proteoglycan. While transcript for the developmentally prominent full length RPTPβ was also increased at 2 and 15d, its protein was not detected in our adult samples. These results indicate that phosphacan and RPTPβ splice variants participate in both the acute degenerative and long-term regenerative phases of reactive synaptogenesis. These results suggest that increase in the transmembrane sRPTPβ tyrosine phosphatase activity is critical to this plasticity, and that local elevation of extracellular phosphacan influences dendritic organization during synaptogenesis.

Aggrecan expression, a component of the inhibitory interneuron perineuronal net, is altered following an early-life seizure

The perineuronal net (PN), a component of the neural extracellular matrix (ECM), is a dynamic structure whose expression decreases following diminished physiological activity. Here, we analyzed the effects of increased neuronal activity on the development of aggrecan, a component of the PN, in the hippocampus. We show aggrecan expression to be prominent around parvalbumin (PV) interneurons in the postnatal hippocampus. Moreover, after seizure induction in early life there was a significant increase in aggrecan expression in a region specific manner during the course of development. We conclude that increased neuronal activity leads to accelerated expression of PNs in the hippocampus that attenuates in the adult hippocampus. This study shows the dynamic nature of the PN component of the ECM and the role neuronal activity has in molding the extracellular milieu of inhibitory interneurons.

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.

Down-regulation of neurocan expression in reactive astrocytes promotes axonal regeneration and facilitates the neurorestorative effects of bone marrow stromal cells in the ischemic rat brain

The glial scar, a primarily astrocytic structure bordering the infarct tissue inhibits axonal regeneration after stroke. Neurocan, an axonal extension inhibitory molecule, is up-regulated in the scar region after stroke. Bone marrow stromal cells (BMSCs) reduce the thickness of glial scar wall and facilitate axonal remodeling in the ischemic boundary zone. To further clarify the role of BMSCs in axonal regeneration and its underlying mechanism, the current study focused on the effect of BMSCs on neurocan expression in the ischemic brain. Thirty-one adult male Wistar rats were subjected to 2 h of middle cerebral artery occlusion followed by an injection of 3 x 10(6) rat BMSCs (n = 16) or phosphate-buffered saline (n = 15) into the tail vein 24 h later. Animals were sacrificed at 8 days after stroke. Immunostaining analysis showed that reactive astrocytes were the primary source of neurocan, and BMSC-treated animals had significantly lower neurocan and higher growth associated protein 43 expression in the penumbral region compared with control rats, which was confirmed by Western blot analysis of the brain tissue. To further investigate the effects of BMSCs on astrocyte neurocan expression, single reactive astrocytes were collected from the ischemic boundary zone using laser capture microdissection. Neurocan gene expression was significantly down-regulated in rats receiving BMSC transplantation (n = 4/group). Primary cultured astrocytes showed similar alterations; BMSC coculture during reoxygenation abolished the up-regulation of neurocan gene in astrocytes undergoing oxygen-glucose deprivation (n = 3/group). Our data suggest that BMSCs promote axonal regeneration by reducing neurocan expression in peri-infarct astrocytes.

Accumulation of chondroitin sulfate proteoglycans in the microenvironment of spinal motor neurons in amyotrophic lateral sclerosis transgenic rats

Chondroitin sulfate proteoglycans (CSPGs) are the major components of extracellular matrix in the central nervous system. In the spinal cord under various types of injury, reactive gliosis emerges in the lesion accompanied by CSPG up-regulation. Several types of CSPG core proteins and their side chains have been shown to inhibit axonal regeneration in vitro and in vivo. In the present study, we examined spatiotemporal expression of CSPGs in the spinal cord of transgenic (Tg) rats with His46Arg mutation in the Cu/Zn superoxide dismutase gene, a model of amyotrophic lateral sclerosis (ALS). Immunofluorescence disclosed a significant up-regulation of neurocan, versican, and phosphacan in the ventral spinal cord of Tg rats compared with age-matched controls. Notably, Tg rats showed progressive and prominent accumulation of neurocan even at the presymptomatic stage. Immunoblotting confirmed the distinct increase in the levels of both the full-length neurocan and their fragment isoforms. On the other hand, the up-regulation of versican and phosphacan peaked at the early symptomatic stage, followed by diminishment at the late symptomatic stage. In addition, double immunofluorescence revealed a colocalization between reactive astrocytes and immunoreactivities for neurocan and phosphacan, especially around residual large ventral horn neurons. Thus, reactive astrocytes are suggested to be participants in the CSPG accumulation. Although the possible neuroprotective involvement of CSPG remains to be investigated, the present results suggest that both the reactive astrocytes and the differential accumulation of CSPGs may create a nonpermissive microenvironment for neural regeneration in neurodegenerative diseases such as ALS.

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

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

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.

Neuronal hyperactivity induces astrocytic expression of neurocan in the adult rat hippocampus

Extracellular matrix molecules are involved in the cellular functions of proliferation, migration, morphological differentiation, and synaptic plasticity. One candidate molecule of the extracellular matrix is the chondroitin sulfate proteoglycan neurocan. To determine whether neurocan expression is regulated by neuronal activity in the adult rat brain, we studied changes in hippocampal neurocan mRNA and protein expression following electrical stimulation of the perforant pathway in urethane-anesthetized rats. After 24 h of intermittent, unilateral 20 Hz stimulation, in situ hybridization revealed increased neurocan mRNA in glial fibrillary acidic protein (GFAP)-positive astrocytes bilaterally in all hippocampal subfields. These changes were quantified in the dentate molecular layer, the termination zone of the perforant pathway, using laser microdissection in combination with quantitative reverse transcription-polymerase chain reaction (RT-PCR). Immediately after 24 h stimulation, a six-fold upregulation was detected, which returned to control levels by 3 days post-stimulation. Neurocan immunoreactivity was similarly upregulated bilaterally. Immunostaining intensity reached a maximum by 4 days and returned to control levels by 14 days. The pattern of neurocan expression in the hippocampus depended on the intensity and duration of electrical stimulation. Under conditions of less intense afferent stimulation (4-24 h of 2.0 Hz paired-pulse stimulation, interpulse interval 40 ms), increases in neurocan mRNA and immunoreactivity were restricted to the ipsilateral termination zone of the stimulated perforant pathway. This layer-specific neurocan upregulation was not affected by intraperitoneal application of the NMDA-receptor antagonist MK-801. In conclusion, our data indicate that synaptic activity regulates the astrocytic expression of neurocan in a graded manner.

Two separate metalloproteinase activities are responsible for the shedding and processing of the NG2 proteoglycan in vitro

A high proportion of NG2 in the adult rat spinal cord is saline-soluble and migrates slightly faster than intact NG2 on SDS-PAGE, suggesting that it represents the shed ectodomain of NG2. In the injured cerebral cortex, much of the overall increase in NG2 is due to the saline-soluble (shed), rather than the detergent-soluble (intact), form. Hydroxamic acid metalloproteinase inhibitors, but not TIMPs, were able to prevent NG2 shedding in oligodendrocyte precursor cells (OPCs) in vitro. The generation of another truncated form of NG2 was, however, sensitive to TIMP-2 and TIMP-3. Two observations suggest that NG2 is involved in PDGF signaling in OPCs: the rate of NG2 shedding increased with cell density and NG2 expression was increased in the absence of PDGF. Ectodomain shedding converts NG2 into a diffusible entity able to interact with the growth cone, and we suggest that this release is likely to enhance its axon growth-inhibitory activity.

Expression of neurocan after transient middle cerebral artery occlusion in adult rat brain

Neurocan is one of the major chondroitin sulfate proteoglycans in the nervous tissues. The expression and proteolytic cleavage of neurocan are developmentally regulated in the normal rat brain, and the full-length neurocan is detected in juvenile brains but not in normal adult brains. Recently, some studies showed that the full-length neurocan was detectable even in the adult brain when it was exposed to mechanical incision or epileptic stimulation. In the present study, we demonstrated by Western blot analysis that the full-length neurocan transiently appeared in the peri-ischemic region of transient middle cerebral artery occlusion (tMCAO) in adult rat with a peak level at 4 days after tMCAO. Immunohistochemical analysis showed that a clear positive signal of neurocan was observed 4 days after tMCAO in the peri-ischemic region of cerebral cortex and caudate, where cells strongly positive in GFAP expression were also distributed. These results indicate that accumulation of the full-length neurocan produced by reactive astrocytes may be one of the processes for tissue repair and reconstruction of neural networks after focal brain ischemia as well.

Differential upregulation of extracellular matrix molecules associated with the appearance of granule cell dispersion and mossy fiber sprouting during epileptogenesis in a murine model of temporal lobe epilepsy

We have investigated changes in the extracellular matrix of the hippocampus associated with the early progression of epileptogenesis in a murine model of temporal lobe epilepsy using immunohistochemistry. In the first week following intrahippocampal injection of the glutamate agonist, domoate, there is a latent period at the end of which begins a sequential upregulation of extracellular matrix (ECM) molecules in the granule cell layer of the dentate gyrus, beginning with neurocan and tenascin-C. This expression precedes the characteristic dispersion of the granule cell layer which is evident at 14 days post-injection when the first recurrent seizures can be recorded. At this stage, an upregulation of the chondroitin sulfate proteoglycan, phosphacan, the DSD-1 chondroitin sulfate motif, and the HNK-1 oligosaccharide are also observed. The expression of these molecules is localized differentially in the epileptogenic dentate gyrus, especially in the sprouting molecular layer, where a strong upregulation of phosphacan, tenascin-C, and HNK-1 is observed but there is no expression of the proteoglycan, neurocan, nor of the DSD-1 chondroitin sulfate motif. Hence, it appears that granule cell layer dispersion is accompanied by a general increase in the ECM, while mossy fiber sprouting in the molecular layer is associated with a more restricted repertoire. In contrast to these changes, the expression of the ECM glycoproteins, laminin and fibronectin, both of which are frequently implicated in tissue remodelling events, showed no changes associated with either granule cell dispersion or mossy fiber sprouting, indicating that the epileptogenic plasticity of the hippocampus is accompanied by ECM interactions that are characteristic of the CNS.

Changes in the amounts of chondroitin sulfate proteoglycans in rat brain after neonatal hypoxia-ischemia

Chondroitin sulfate proteoglycans have been shown to participate in the pathogenesis of neuronal damages in the injured adult central nervous system (CNS). Upregulated expression of chondroitin sulfate proteoglycans has been reported around the injured sites and depletion of these chondroitin sulfate proteoglycans brings about increased axonal regeneration in the injured adult CNS. To examine if chondroitin sulfate proteoglycans are also involved in the pathologic process of hypoxia-ischemia in the neonatal brain, expressions of three chondroitin sulfate proteoglycans, neurocan, phosphacan, and neuroglycan C, were examined in rat brains after neonatal hypoxia-ischemia. Hypoxic-ischemic rats were produced by ligating the right carotid artery of 7-day-old rats, followed by 8% oxygen exposure. Western blot analysis revealed that in contrast to injured adult CNS, the amount of neurocan was reduced 24 hr after hypoxia in the neonatal hypoxic-ischemic cerebral hemisphere. The amounts of phosphacan and neuroglycan C were also reduced significantly 24 hr after hypoxia at the right injured cortex compared to those at the left cortex. Surprisingly, the immunohistologic staining for phosphacan was conversely intensified both at 24 hr and 8 days after hypoxia at the infarcted area. In addition, the habenula and fascicules retroflexus in the right cerebral hemisphere degenerated and became intensely immunostained with the anti-phosphacan antibody shortly after hypoxia. Hypoxic-ischemic insult may unmask phosphacan epitopes at the injured sites, resulting in intensified immunostaining. Because intensified immunostaining for neurocan and neuroglycan C was not observed, unmasking seems to be specific to phosphacan among these three chondroitin sulfate proteoglycans.

A disaccharide derived from chondroitin sulphate proteoglycan promotes central nervous system repair in rats and mice

Chondroitin sulphate proteoglycan (CSPG) inhibits axonal regeneration in the central nervous system (CNS) and its local degradation promotes repair. We postulated that the enzymatic degradation of CSPG generates reparative products. Here we show that an enzymatic degradation product of CSPG, a specific disaccharide (CSPG-DS), promoted CNS recovery by modulating both neuronal and microglial behaviour. In neurons, acting via a mechanism that involves the PKCalpha and PYK2 intracellular signalling pathways, CSPG-DS induced neurite outgrowth and protected against neuronal toxicity and axonal collapse in vitro. In microglia, via a mechanism that involves ERK1/2 and PYK2, CSPG-DS evoked a response that allowed these cells to manifest a neuroprotective phenotype ex vivo. In vivo, systemically or locally injected CSPG-DS protected neurons in mice subjected to glutamate or aggregated beta-amyloid intoxication. Our results suggest that treatment with CSPG-DS might provide a way to promote post-traumatic recovery, via multiple cellular targets.

Proteoglycans and injury of the central nervous system

Proteoglycan is a family of glycoproteins which carry covalently-linked glycosaminoglycan chains, such as chondroitin sulfate and heparan sulfate. Proteoglycans are believed to play important roles in morphogenesis and maintenance of various tissues including the central nervous system (CNS) through interactions with cell adhesion molecules and growth factors. In the CNS, a significant amount of evidence has been accumulated to show that proteoglycans function as modulators in various cellular events not only in the development, but also in the pathogenesis of neuronal diseases and lesions. When the CNS is injured, several chondroitin sulfate proteoglycans (CSPG) are up-regulated in glial scars formed around the lesion site. The glial scar also contains some molecules inhibitory to axonal growth, such as myelin-associated glycoprotein, Nogo, and Semaphorin. In vitro studies revealed that CSPG largely exert a repulsive effect on axonal regeneration, and a signal from CSPG modulates the actin cytoskeleton of outgrowing neurites through the Rho/ROCK pathway. These findings suggest that CSPG are responsible for unsuccessful axonal regeneration in glial scars. Various attempts to overcome the inhibitory effect of CSPG have been pursued in vivo. Digestion of chondroitin sulfate chains by chondroitinase ABC, suppression of CSPG core protein synthesis by decorin, suppression of glycosaminoglycan chain synthesis by a DNA enzyme, and inhibition of the Rho/ROCK pathway with specific inhibitors were all successful for increasing axonal regeneration. For a clinical application, the most effective combination of these treatments needs to be examined in the future.

Attenuation of glial scar formation in the injured rat brain by heparin oligosaccharides

Injury to the central nervous system causes glial reactions, which eventually lead to the formation of a glial scar and inhibit axonal regeneration. The present study aimed to reduce the extent of glial scar formation in injured cerebral cortex using heparin hexasaccharide (6-mer) and octasaccharide (8-mer). A single injection of 20 microl of heparin 6-mer or heparin 8-mer (10mg/ml), native heparin (10mg/ml), or saline vehicle was given into the wound cavity just after cryo-injury in the cerebral cortex. In saline-injected control rats, strong chondroitin sulfate-A (CS-A) immunoreactivity using 2H6 antibody was observed around the injured site. Double labeling using an antibody against glial fibrillary acidic protein, a glial marker, further demonstrated that CS-A immunoreactivity was mainly expressed on the reactive astrocytes at the glial scar, indicating that CS-A immunohistochemistry is useful for evaluating glial scar formation. Quantitative morphometrical analysis revealed that the area of CS-A immunoreactivity was significantly decreased by 53% in heparin-6-mer-injected animals and 44% in heparin-8-mer-injected ones 6 days after the injury, but native heparin had no effect on CS-A-immunoreactive areas. Both heparin oligosaccharides also attenuated the intensity of CS-A immunoreactivity in the reactive astrocytes and caused astrocytic cellular processes to be less branched. These results demonstrate that a single injection of heparin oligosaccharides attenuates glial scar formation, indicating that heparin oligosaccharides may be applicable to many fibrotic diseases and restore functional integrity.