Introduction

The extracellular matrix (ECM) represents a collection of extracellular molecules secreted by cells, that provide structural and biochemical support to the surrounding cells. The ECM includes both the interstitial matrix and the basement membrane. Polysaccharides and fibrous proteins are the main constituents of the interstitial space. The basement membrane is a sheet-like deposition of ECM on which various epithelial cells rest. The ECM is composed of two main classes of macromolecules: proteoglycans and fibrous proteins (e.g., collagens, elastins, fibronectins and laminins) that are required for tissue morphogenesis, differentiation and homeostasis [1]. Many components of the extracellular matrix have been identified in brain (e.g., neural cell adhesion molecules, integrins, laminins, fibronectin, collagens, among others) and all have been implicated in the pathogenesis of several neurological disorders [2–5].

A number of extracellular matrix glycoproteins are expressed in the developing and adult nervous system. Neural stem cells, neurons, and glia express receptors that mediate interactions with specific extracellular matrix molecules. In contrast to other tissues, the ECM of the CNS lacks fibrous proteins under normal conditions. Instead, the neural ECM is rich in glycoproteins and proteoglycans. It has been estimated that the neural ECM makes up approximately 20% of the CNS parenchyma [6]. In vivo genetic and in vitro functional studies have provided evidence that the extracellular matrix critically affects virtually all aspects of nervous system development and function [7].

Matricellular proteins (MCPs) are dynamically expressed non-structural proteins that are present in the ECM [8, 9]. These proteins are rapidly turned-over and possess regulatory roles, rather than serving as stable structural elements of the ECM. They characteristically contain binding sites for ECM structural proteins and cell surface receptors, and are involved in sequestration and modulatory activities of specific growth factors [10]. Examples of matricellular proteins include the CCN family of proteins (also known as CCN intercellular signaling proteins), fibulins, osteopontin, periostin, SPARC family members, tenascin(s), and thrombospondins. Many of these proteins additionally have critical functions in wound healing and tissue repair.

MCPs in brain play a major role in developmental processes, as they are critically involved in the maintenance of neuronal integrity. Changes (increased or decreased) in levels of thrombospondins (TSPs), secreted protein acidic and rich in cysteine family (SPARC), including Hevin/SC1 (SPARC-like 1), Tenascin C (TNC), CCN 2 and 3, glypicans 4 and 6, galectins, plasminogen activator inhibitor-1 (PAI-1), autotaxin, fibulin and perisostin have also been strongly implicated in the pathogenesis of various neurological disorders. SPARC, tenascin, TSP-1 and CCN are known to be upregulated during axonal regeneration, glial scar formation, angiogenesis, and in the “rewiring” of neural circuitry following various neurological conditions. Conversely, decreased levels of MCPs have been strongly implicated the pathogenesis of various neurological conditions, including Alzheimer’s disease, amyotrophic lateral sclerosis, ischemia, traumatic inquiry, hepatic encephalopathy, Down’s syndrome, autism, multiple sclerosis, brain neoplasms, Parkinson’s disease and epilepsy. This review discusses the potential role of MCPs in various neurological disorders, as well as the possibility of targeting MCP’s for promising therapy of these conditions.

Thrombospondin-1/2 (TSP-1/2)

Thrombospondins are a family of multifunctional secreted glycoproteins with antiangiogenic properties [11]. The family consists of thrombospondins 1–5, of which TSP-1 and TSP-2 are produced by astrocytes, that promotes the development of new synapses, as well as maintain neuronal integrity.

TSP-1, also known as THBS1 or THP1 protein, is a member of the thrombospondin family that in humans is encoded by the THBS1 gene [12, 13]. It was first isolated from platelets that had been stimulated with thrombin, and was therefore referred to as the thrombin-sensitive protein, thrombospondin. TSP-1 binds to fibrinogen, fibronectin, laminin, type V collagen, and integrins alpha-V/beta-1, which initiate cell–cell, and cell–matrix interactions [14]. Its major function is the inhibition of the cell proliferation and migration of endothelial cells by interactions with CD36 expressed on the surface of these cells.

While TSP-1 was first recognized for its involvement in angiogenesis, other functions for TSP-1 were subsequently recognized, including apoptosis, activation of TGF-β and immune regulation in peripheral tissues. TSP-1 expression was also identified in the central nervous system (CNS). Post-natal and young adult animal brains, as well as normal human cortical astrocytes, were shown to express TSP-1 [15–19] (see Fig. 1). Cultured astrocytes were also shown to synthesize and secrete TSP-1 [16, 18–21]. Under normal conditions, TSP-1 causes an increase in the total number of synapses [20, 22, 23]. Astrocyte-derived TSP-1 was also shown to mediate the development of pre-synaptic plasticity in vitro [24]. Conversely, defective astrocytic TSP-1 release is known to be associated with neuronal dysfunction (see “Role of TSP-1/2 in Neurological Disorders”).

Fig. 1
figure 1

This figure illustrates the presence of TSP-1 in normal adult rat brain cortical astrocytes ([16], pp 333–347, and cover page). Glial fibrillary acidic protein (GFAP, red, astrocytes), TSP-1 (green), and DAPI (blue, nuclei). The co-localization (merged) image of TSP-1 and GFAP illustrates the enrichment of TSP-1 in astrocytes. Scale bar 20 µm

Full size image

TSP-2, also known as THBS2, is a disulfide-linked homotrimeric glycoprotein derived from a member of the thrombospondin family that mediates cell–cell, and cell-matrix interactions, and which has a high structural similarity to TSP-1. Similar to TSP-1, TSP-2 binds to extracellular matrix ligands, including transforming growth factor-beta-1, histidine-rich glycoprotein, TSG6, heparin, matrix metalloproteinase-2, and heparan sulfate proteoglycans [25]. TSP-2 also binds to cell surface receptors, including CD36, CD47, LDL receptor-related protein-1 (via calreticulin) and the integrins alpha-V/beta-3, alpha-4/beta-1, and alpha-6/beta-1 [26, 27]. Such binding results in the regulation of proliferation, adhesion, and migration of a number of normal and transformed cell types [28]. TSP-2 is characterized by a distinct regulation of gene expression by various growth factors and hormones. Similar to TSP-1, TSP-2 is highly expressed in developing blood vessels, suggesting a potential role in the regulation of angiogenesis [29, 30]. In contrast to TSP1, TSP2 does not activate latent TGFβ-1, but similar to TSP1 [31], TSP2 contains EGF-like segments that bind calcium in a cooperative manner [32, 33]. Targeted disruption of the TSP-2 gene in mice was shown to increase the number of small- and medium-sized blood vessels in various tissues [34]. TSP-2 expression has been identified in the CNS, particularly in astrocytes under normal conditions [20, 22, 35], and shares a similar sequence homology with TSP-1.

Role of TSP-1/2 in Neurological Disorders

Alzheimer’s Disease (AD)

The level of TSP-1 expression was shown to be decreased in brains of patients with AD [36]. Following treatment of cultured human astrocytoma cells (U373MG), as well as primary cultures of rat brain astrocytes with amyloid beta (Aβ), levels of secreted astrocytic TSP-1 were found to be decreased [18, 36]. In addition, treatment of cultured neurons with Aβ was shown to induce synaptic pathology, including decreased dendritic spine density and reduced synaptic activity [36]. These events were prevented by co-incubation of Aβ with recombinant TSP-1, the latter acting through the alpha-2-delta-1 TSP-1 receptor present on neurons [36]. Further, injection of TSP-1 into the subicula of the hippocampus of an AD model in mice, mitigated the Aβ-mediated reduction of synaptic proteins and related signaling pathways, suggesting that TSP-1 may be a useful therapeutic agent for AD [36]. Additionally, Rama Rao et al. [18] showed that exposure of astrocyte cultures to Aβ1-42 caused a significant (1-3-fold) increase in intracellular levels of TSP-1 that was associated with a reduction in its release. These authors reported that the addition of conditioned media derived from Aβ-treated cultured astrocytes, containing reduced levels of TSP-1 to cultured neurons, resulted in a significant loss of synaptophysin and the postsynaptic density protein 95 (PSD95). These findings suggest that an Aβ-mediated reduction in astrocytic TSP-1 release, contributes to the loss of synaptic integrity in AD, and that strategies aimed at enhancing the astrocytic release of TSP-1 may have therapeutic efficacy in AD.

While TSP-2 is also present in astrocytes and shares a similar sequence homology with TSP-1, its levels vary in different neurological conditions (e.g., decreased TSP-1 release was identified in experimental models of AD, whereas increased TSP-2 expression was detected in brains of patients with AD) [37].

Hepatic Encephalopathy (HE)

HE is the neurological disorder associated with severe liver disease, which presents in acute and chronic forms. Acute HE (AHE) occurs following massive liver necrosis due to viral hepatitis, acetaminophen toxicity, or exposure to various hepatotoxins [38]. It presents with severe brain edema, increase in intracranial pressure, and brain herniation resulting in a high mortality rate [39]. On the other hand, chronic HE (CHE), is usually the consequence of cirrhosis of the liver, generally associated with alcoholism, and presents predominantly with neuropsychiatric symptoms [40]. Patients with CHE exhibit a wide range of neurological symptoms, including altered mood, increased irritability, alterations in sleep/wake cycles, changes in muscle tone and cognitive impairments [40]. A major etiological factor in CHE is the presence of elevated blood and brain levels of ammonia, due to the inability of the diseased liver to significantly detoxify this substance [41].

The principal histopathological finding in CHE is the presence of an astrocytic abnormality referred to as Alzheimer type II astrocytes [42, 43]. The means by which astrocyte dysfunction contributes to the neurological abnormalities in CHE is poorly undertsood. As astrocytes are involved in synapse formation during development, as well as in their maintenance in adults [44], a dysfunction in these cells has been postulated to significantly contribute to the neurobehavioral abnormalities associated with CHE [45, 46]. It has been recently reported that the synaptic dysfunction induced by defective astrocytes may largely occur through a reduction in their levels of MCPs [16, 47].

It was previously shown that chronic ammonia exposure in cultured astrocytes results in decreased intra- and extracellular levels of TSP-1 [16]. A reduction in synaptophysin and PSD95 levels where identified in cultured neurons after exposure to conditioned media (CM) derived from ammonia-treated astrocytes, and such reduction was diminished when recombinant TSP-1 (rTSP-1) was added to the CM of ammonia-treated astrocytes [16]. Further, increased c-Myc content (which is known to inhibit TSP-1 synthesis) was identified in ammonia-treated cultured astrocytes, while silencing c-Myc or pharmacologically inhibiting c-Myc significantly enhanced intra- and extracellular levels of TSP-1 after ammonia treatment of these cultures [16]. Additionally, exposure of cultured neurons to CM from ammonia-treated astrocytes, in which c-Myc was silenced or inhibited by 10058-F4, caused a reversal of the synaptophysin loss. These findings suggest that an ammonia-induced reduction in astrocytic TSP-1 results in a decline in synaptic protein levels, which ultimately contributes to the pathogenesis of CHE [16].

Ischemia

Lin et al. [48] first demonstrated the presence of TSP-1 follwing the middle cerebral artery occlusion (MCA) model of ischemia in rats. These authors identified a biphasic expression of TSP-1 after ischemia (peaking at 1 and 72 h), particularly in meningeal endothelial cells, whereas glial, neuronal, and macrophage cells only expressed TSP-1 at 6 weeks after ischemia. In a similar model of ischemia in mice, Hayashi et al. [49], however, found that TSP-1 level increased within one hour after MCA occlusion, whereas its level decreased from 24 h to 21 days. While these authors used an identical animal model of ischemia, the reason for the differences in TSP-1 expression in these studies is unclear. Further, oxygen-glucose deprivation of murine cerebral endothelial cells in vitro was shown to increase the extent of methylation of the promoter region of TSP-1 with a concomitant decrease in TSP-1 mRNA and protein expression [50]. Similar findings were also reported by Hayashi et al. [49].

Following permanent MCA occlusion in mice, TSP-1 levels were shown to be increased in astrocytes [51]. Further, TSP-1 knock-out (KO) mice displayed a significant deficit in their ability to recover motor function as compared to wild-type (WT) mice [51]. Moreover, TSP-1 KO mice exhibited significant deficits in synaptic density and axonal sprouting, suggesting that a deficiency of TSP-1 results in impaired recovery after stroke [51]. Additionally, plasma TSP-1 level was shown to be increased in the cerebral ischemia associated with sickle cell anaemia [52]. Increased TSP-1 levels were also identified in copper-induced cerebral ischemic injury in mice [53], and in the plasma of patients with ischemic stroke [54], suggesting a potential role of TSP-1 in diminishing the extent of ischemic brain injury.

While the regulatory mechanism of TSP-1 in brain has not been fully investigated, Cekanaviciute et al. [55] reported that the expression of TSP-1 is tightly controlled by TGF-β as “Ast-Tbr2DN” mice, in which TGFβ signaling was specifically inhibited in astrocytes, were shown to inhibit TSP-1 expression, following middle cerebral artery occlusion. Additionally, Xing et al. [56] reported that lipocalin-2 enhances TSP-1 levels in cultured astrocytes.

Although TSP-1 expression was identified in almost all neural cells following ischemia, TSP-2 mRNA and protein levels were observed in endothelial cells, neurons, and macrophages, but not in glial cells [48]. Noteworthy, increasedTSP-1 expression was found in the early stage of ischemia (1–24 h), while TSP-2 overexpression occurred only several weeks after ischemia. While the role of TSP-1 has been extensively examined in ischemia, the involvement of TSP-2 in ischemia remains poorly understood.

Traumatic Brain Injury (TBI)

Increased levels of TSP-1 were observed in rat brain following TBI [57], as well as in cultured astrocytes after a scratch-induced injury [21]. We recently reported that exposure of cultured rat astrocytes to trauma (fluid-percussion injury) increased the levels of intra- and extracellular TSP-1 at early stages, whereas a reduction in such levels was observed at later stages. Additionally, a reduction in NMDA-nr1 and PSD95 levels were identified in cultured neurons post-trauma, while exposure of these traumatized neurons to CM from traumatized astrocytes derived from early stages of trauma, blocked the trauma-induced reduction in NMDA-nr1 and PSD95 levels. These findings suggest that TSP-1 prevents the loss of neuronal integrity following TBI.

Down’s Syndrome (DS) and Autism

Using cultured human astrocytes derived from DS fetal tissue, as well as cultured rat hippocampal astrocytes, Garcia et al. [58] reported that DS astrocytes are directly involved in the development of spine malformations and in a reduction in synaptic density. The authors further documented a deficit in astrocytic TSP-1 in brains from patients with DS, and that such depletion of TSP-1 resulted in abnormalities in spine morphology, whereas the addition of recombinant TSP-1 to neurons prevented such abnormalities. Furthermore, the addition of media from astrocyte cultures derived from TSP-1 KO mice to cultured neurons exhibited similar deficits in spine formation and structure [58].

Autism is a severe neurodevelopmental disorder associated with impaired social interactions and language communication. The mechanisms involved in the pathogenesis of autism are still unclear. Since the TSP-1 gene has been shown to play a critical role in synaptogenesis in the developing brain, Lu et al. [59] investigated the role of the TSP-1 gene in autism by analyzing the coding exons and the 5′-untranslated region (UTR) of the TSP-1 gene. The authors found that both common and rare variants of the TSP-1 gene were indeed associated with autism.

Parkinson’s Disease

Astrocytes derived from embryonic glial-restricted precursor cells by exposure to bone morphogenetic protein (GDAsBMP), which are known to synthesize and release greater levels of TSP-1 than wild-type astrocytes, when transplanted into the 6-hydroxydopamine lesioned rat striatum restored tyrosine hydroxylase expression, as well as promoted behavioral recovery [60]. A similar transplantation also reversed pathological changes, including the loss of parvalbumin-positive GABAergic interneurons, likely by the increased synthesis and release of TSP-1 [60].

Neoplasms

TSP-1 secretion was shown to be reduced in neoplasms derived from breast, bladder, fibroblasts and other organs, resulting in the induction of neovascularization required for tumor growth and metastasis. Full-length and short TSP-1-derived peptides were shown to inhibit angiogenesis by inducing endothelial cell apoptosis and thereby disrupt the vasculature of the growing tumor [61]. However, studies have shown that malignant glioma cells secrete high levels of TSP-1, which may be involved in the migration of glioma cells via interactions with alphavbeta3 and alpha3beta1 integrins [62], and that treatment of rats bearing either C6 or 9L tumors with TSP1-derived peptide D-reverse amKRFKQDGGWSHWSPWSSac was shown to retard brain tumor growth, likely as a result of slower de novo blood vessel formation and synergistic antiproliferative effects on tumor cells [63]. Further, loss of tumor suppressors on chromosome 10 was shown to contribute to the aggressive malignancy of glioblastomas, in part by the inhibition of angiogenesis by TSP-1 in lower grade tumors [64]. TSP-1 over-expression was slso shown to occur in a differentiation-specific manner in neuroblastic tumors [65]. Lastly, mutation of the p53 gene provides gliomas with an angiogenic phenotype by reducing thrombospondin-1 production, as well as by enhancing angiogenesis inducers in the early phase of malignant progression [66].

Other Conditions

TSP-1 expression was identified before the peak of angiogenesis, whereas a robust TSP-2 expression occurred at the peak of angiogenesis which continued until angiogenesis was completed. Using purified rat retinal ganglion cells (RGCs), Christopherson et al. [20] found that TSP-2 mimicked the ability of astrocyte conditioned media to increase the number of excitatory synapses in cultured RGCs. Additionally, immunodepletion of TSP-2 from astrocyte conditioned media prevented the astrocyte-induced synaptogenesis, suggesting that TSP-2 is a necessary astrocytic signal for the generation of excitatory synapses in RGCs. Collectively, these findings suggest that astrocytic TSP-1/2 are necessary signals involved in the maintenance of neuronal integrity.

Therapeutic Implications

Since TSP-1 is a large protein (~120 kDa), its potential clinical use is limited. However, agents that enhance or decrease their level in astrocytes have recently been targeted for the treatment of hepatic encephalopathy, cerebral ischemia, TBI, AD and neoplasms [16, 36, 51, 57, 67]. Additionally, small-molecules based on a CD36-binding peptide sequence derived from TSP1 are currently being tested for the treatment of various neurological disorders, including ischemia and neoplasms [67, 68]. One analog of TSP-1, ABT-510, exhibited potent pro-apoptotic activity in glioma cell lines [67], and was subsequently shown to display therapeutic benefit in several CNS malignancies [67].

Tenascin-C

Tenascin C (TNC) is a large hexameric extracellular glycoprotein that is expressed in the extracellular matrix. TNC is classified as an adhesion-modulating protein, as it was found to inhibit the cellular adhesion to fibronectin [69]. TNC is expressed in restricted perivascular areas of the CNS during development, disease or injury. In the developing CNS, TNC is first expressed by radial glia and later primarily by astrocytes, where it appears to exert autocrine effects involved in the regulation of astrocyte progenitor cell proliferation [70]. Its expression has also been identified in postnatal stages, as well as in the adult CNS [71, 72]. TNC contributes to the maturation of neural progenitor cells and to the proliferation and maintenance of oligodendrocyte precursors [73–75]. The TNC-regulated target gene, Sam68, is involved in the control of glial cell proliferation [76]. In vivo and in vitro studies further shows that TNC encodes latent, as well as inhibitory signals that mediate neuron migration and axon growth and guidance in the context of neuron-glia interactions [77–83]. TNC has been implicated in inflammation and tumor development in the CNS [84, 85] (see below for its expression status and specific functions in various neurological conditions).

Role in Neurological Disorders

Traumatic Brain Injury

Using in situ hybridization and immunocytochemistry, Laywell et al. [86] showed that stab wounds of the adult mouse cerebellar and cerebral cortices result in enhanced expression of TNC around the lesion site which was associated with a subset of astrocytes. The authors speculated that TNC up-regulation in the lesioned adult brain may be directly involved in failed regeneration, or indirectly involved through interactions with other glycoconjugates that either inhibit or facilitate neurite outgrowth. Hausmann and Betz [87] investigated the time-dependent vascular response during the first 30 weeks after TBI in humans and found a significantly increased astrocytic TNC immunoreactivity at 1.6 days following injury.

In a scratch model of trauma in cultured astrocytes, Nishio et al. and Wanner et al. [88, 89] demonstrated increased synthesis and release of TNC, as well as increased expression of TNC mRNA. Further, Nishio et al. [88] showed that the exogenous application of TNC to cultured astrocytes enhanced cell proliferation and migration, while its functional blocking with anti-TNC or anti-integrin beta-1 antibodies reduced such events. These findings suggest that mechanical injury induces the astrocytic production and release of TNC leading to astrocyte proliferation and migration. Increased TNC level has also been identified following cortical penetrating stab wounds in mice [90].

While astrocytes were shown to highly express TNC, Yu et al. [91] documented the upregulation of TNC in regenerating neurons of the nucleus of the medial longitudinal fasciculus (NMLF) in brainstem and spinal motoneurons. Further, inhibition of TNC expression by an antisense oligonucleotide resulted in impaired locomotor recovery, reduced regrowth of axons and diminished synapse formation by the regrowing spinal motoneurons, all vital indicators of regeneration. These findings suggest a promising role for TNC in promoting spinal cord regeneration following injury. For a comprehensive review on the role of TNC following injury to CNS [92].

Although the precise role of TNC in response to injury remains unclear, Zhang et al. [93] and Deller et al. [94] reported that regenerating spinal cord and entorhinal cortical neurons were closely associated with TNC expression, suggesting that TNC may be involved in axonal regrowth. Additionally, Ikeshima-Kataoka et al. [95] showed that in tenascin-deficient mice, the expression of interleukin (IL)-1β, tumor necrosis factor-α and IL-6 were higher, whereas levels of IL-4 and granulocyte colony-stimulating factor were lower in wild-type mice, suggesting that TNC also aids in the production of inflammatory cytokines in the injured brain. However, Gates et al. [96] reported that TNC inhibits neurite regeneration following injury, and that blocking TNC with antibodies enhanced neurite growth, suggesting that TNC release from astrocytes may detrimental for neurite outgrowth.

Alzheimer’s Disease (AD)

Increased plasma levels of TNC [97], as well as TNC plaques in reactive astrocytes and microglia along with phosphorylated tau-containing dystrophic neurites of amyloid cores were observed in patients with AD, suggest a role for TNC in Aβ plaque formation [98]. Further, Xie et al. [99] observed that TNC gene transcription was upregulated in cultured microglia after Aβ challenge, as well as in brains of transgenic mice (Tg) overexpressing mutated amyloid precursor protein (APP) in neural cells. Additionally, these authors showed that TNC deficiency induced by cross-breeding APP-Tg mice and TNC-deficient mice, reduced pro-inflammatory events and enhanced anti-inflammatory activation in the mutated APP-Tg mice brain. These changes were associated with a reduced cerebral Aβ content and higher levels of the PSD95. These findings suggest that a “functional” inhibition of TNC may exert beneficial effects in AD.

Multiple Sclerosis (MS)

Gutowski et al. [100] reported a major loss of TNC at the edge of acute MS plaques. In subacute lesions, however, TNC-immunopositive reactive astrocytes were prominent, reflecting the synthesis of TNC in subacute lesions. Increased TNC level was also observed in the sera of patients with MS [101]. TNC was also expressed in chronic MS plaques at levels similar to those seen in adjacent white matter. The authors suggested that the loss of TNC in acute plaques was indicative of an enzyme-mediated breakdown of the extracellular matrix, likely secondary to a disruption of the blood–brain barrier and associated leukocyte extravasation. The subsequent production of TNC may result in glial scar formation and thereby obstruct remyelination and axonal repair. It is also, however, possible that reduced astrogliosis resulting from reduced TNC levels in MS may be beneficial for remyelination due to the lesser degree of scar formation [102].

Zendedel et al. [103] examined the means by which astrocytes promote remyelination in chronically demyelinated lesions using the cuprizone mouse model of MS. Treatment of mice with triiodothyronin (which is thought to enhance remyelination by the induction of oligodendrocyte maturation) that had been fed with cuprizone, reduced the astrogliosis, possibly by diminishing TNC levels. These findings suggest that triiodothyronin promotes remyelination in chronic demyelinative lesions by enhancing oligodendrocyte maturation and by attenuating astrogliosis, likely as a consequence of reduced levels of TNC.

Epilepsy

An increase in TNC immunoreactivity (IR) was observed in the hippocampus of epileptic rats that displayed neuronal cell loss and associated gliosis [104]. TNC levels were increased both in reactive astrocytes and in axonal plasma membranes of the hippocampus in these rats. However, TNC-IR in the molecular layer of the hippocampus remained unchanged in kindled or kainate-treated rats. Additionally, the authors observed increased TNC-IR in zones in which axonal regeneration did not occur (the hippocampal CA3 region), whereas zones in which reactive synaptogenesis occurred (the molecular layer of kindled or kainate-treated rats) were devoid of TNC-IR. These findings suggest that TNC may obstruct the terminal sprouting of mossy fibers associated with neuronal loss and gliosis in the hippocampal CA3 region of kainate-treated rats, and that such loss may contribute to epileptogenesis.

TNC expression was also shown to be upregulated in rat hippocampal neurons in the early stages (4–5 h) following kainic acid (KA) injection, which was associated with the activation of granule cells and the sprouting of axon terminals [105]. TNC overexpression was also detected at later stages following KA injection in pyramidal cells in the CA1 and CA3 regions, and such overexpression was associated with reactive gliosis [105]. It is also possible, however, that the stimulation of TNC expression in KA-treated rats may be due to the induction of seizures through the activation of basic fibroblast growth factor [106–109]. A strong upregulation of TNC was also identified in the dentate gyrus, especially in the sprouting molecular layer of a murine model of temporal lobe epilepsy [110], as well as in seizures following pilocarpine administration [111].

In addition to TNC, tenascin-R (TNR) is also expressed in the CNS and has also been implicated in the pathogenesis of epilepsy. Mice deficient in TNR showed increased hippocampal excitability without displaying an increase in susceptibility to seizures in the pilocarpine model of epilepsy [112, 113]. Further, Hoffmann et al. [114] reported that kindling progression was retarded in TNR-/- mice, in which an increase in the number of S100-β expressing astrocytes in the dentate gyrus was enhanced by the deficiency in TNR. These findings suggest that in addition to TNC, TNR may also play a role in epileptogenesis.

Neoplasms

The involvement of tenascin in tumor suppression was first reported by Lee et al. [115, 116], who showed that nude mice given injections of Mab 81C6, an IgG2b immunoglobulin (which defines an epitope of the glioma-associated tenascin) displayed a significant delay in tumor growth. Subsequently, Higuchi et al. [117] reported that the degree of histological malignancy and the level of cell dedifferentiation of human gliomas correlated well with the expression of TNC. The authors suggested that that measurement of TNC in the CSF might be useful in the diagnosis of brain tumors, as well as in the monitoring tumor growth [118]. Additionally, a correlation between tenascin expression and the extent of tumor growth have been reported [119–124]. For references on the distribution of TNC in brain tumors, see [125–133].

Recent studies have established that both increased and decreased levels of TNC expression are associated with higher-grade gliomas [134], but not with low-grade gliomas [135], suggesting that TNC may not be suitable for the diagnosis/treatment of low-grade gliomas [134]. TNC loss-of-function promoted glioblastoma multiforme (GBM) neurosphere cell adhesion and actin cytoskeleton organization without any effect on cell growth in vitro, suggesting that tumor cells with decreased TNC expression may be sensitive to anti-proliferative treatment [136]. TNC also acts as a promoter of invasiveness of brain tumor-initiating cells (stem cells) through the ADAM-9 signaling pathway [122]. These findings suggest that altered levels of TNC in GBM may be a useful prognostic marker for this neoplasm [121].

Based on the above observations, various studies have used an anti-tenascin antibody to prevent malignant glioma growth [137–142]. Some studies instead have shown promising results using an anti-tenascin antibody [143–151]. However, the use of an anti-tenascin antibody for the treatment of gliomas was only rarely effective as relapses occurred within months after treatment [131, 143, 145, 152].

SPARC

The matricellular protein, secreted protein acidic and rich in cysteine (SPARC), also known as osteonectin and BM-40, is a calcium and collagen-binding glycoprotein. The SPARC family consists of four groups, (i) SPARC and hevin; (ii) SMOC 1 and 2; (iii) testicans 1–3; and (iv) and follistatin-like protein (Fstl-1). SPARC is predominantly expressed in astrocytes and microglia of the cerebellum [153], retinal Müller cells, and cerebellar Bergmann glia.

SPARC is expressed in the adult central nervous system. In the cerebellum, SPARC is present in Bergmann glia, with a strong expression along their radial fibres [154]. In the hippocampus, SPARC was observed in astrocytes [154]. The expression of SPARC in a subset of astrocytes, particularly in synaptically enriched areas, suggests a role for SPARC in the maintenance of extracellular matrix integrity in the adult brain [154–156].

SPARC regulates growth factor signaling, extracellular matrix assembly, cell-matrix interactions, as well as cell adhesion, migration, and cell proliferation [157, 158]. SPARC modulates other growth factors, including transforming growth factor-beta (TGF-β), extracellular matrix proteins, including collagen Types I–V, VIII and vitronectin [159]. SPARC has also been shown to regulate integrin-mediated adhesion [159]. The upregulation of SPARC has been associated with ischemia, neoplasms, and other neurological conditions (see below).

Chronic morphine administration frequently leads to altered behavioral responses (referred to as sensitization), which are thought to result from neuronal hyperactivity and accompanying changes to neuronal activity-dependent gene expression [160]. In particular, the repeated administration of morphine to mice was shown to increase locomotor activity, and that morphine-induced increase in SPARC levels in the basolateral amygdala contributed to the establishment of locomotor sensitization [160]. Upregulation of SPARC was also shown to inhibit DNA synthesis in human microvascular endothelial cells that were stimulated by vascular endothelial growth factor (VEGF) [161], suggesting that SPARC modulates the mitogenic activity of VEGF, that may decrease the association of VEGF with its cell-surface receptors.

Alzheimer’s Disease and Ischemic Conditions

Using mass spectroscopy, SPARC-like protein was identified in samples of cerebrospinal fluid from patients with AD, as compared to age-matched controls [162, 163]. Following a focal, noninvasive photothrombotic lesion (a model of ischemic stroke), reactive microglia rapidly downregulate SPARC expression at the lesion site, concomitant with reactive, perilesional astrocytes upregulating SPARC [164]. Additionally, induction of ischemic injury in SPARC null-mice showed enhanced microgliosis in and around the lesion site of the cerebral cortex, which was accompanied by a significantly enhanced functional recovery after lesion. Microglial cultures from adult SPARC null-mice were also shown to proliferate at a greater rate, which was reversed by the addition of SPARC, suggesting that SPARC is an important regulator of microglial proliferation [164]. Further, Baumann et al. [165] reported that transient global brain ischemia resulted in a significant reduction in SPARC levels in brain blood vessels, and that post-ischemic hypothermia stabilized brain vessels, as well as reduced BBB disruption, in part, by preventing the proteolytic degradation of SPARC.

Following a localized injury to the adult rat forebrain, a biphasic induction of a SPARC-related protein mRNA was detected in cortical neurons bordering the lesion site, followed by a more prolonged induction of SPARC in astrocytes proximal to the lesion site [166]. A similar SPARC-related protein induction pattern was observed in the hippocampus in response to a surgical lesion [166].

Neoplasms

SPARC was recently recognized as a prominent marker of gliomas [167]. SPARC is also known to delay tumor growth [168] by the activation of PTEN-mediated Ser78 HSP27 phosphorylation, as well as by the stimulation of the ERK/AKT and FAK/ILK signaling pathways [169–173], or by reduced VEGF secretion [174]. Further, RNA interference against SPARC was shown to promote the growth of human malignant glioma cells [175]. Some reports also indicate that SPARC promotes glioma invasion [168, 176, 177]. The precise role of SPARC in neoplastic progression, however, remains to be better defined.

Other Neurological Conditions

Increased SPARC in the hippocampus was shown to be associated with depression-related behavior [178]. SPARC secretion was also found to control excitatory CNS synaptogenesis [178], as well as glutamate receptor levels at developing synapses through beta-integrin interactions [179]. Additionally, SPARC was shown to stimulate Schwann cells, resulting in the promotion of dorsal root ganglion outgrowth in vitro and in vivo [180]. Retinal glial conditioned media, containing high levels of SPARC, was shown to significantly enhance neurite growth and branching of adult rat dorsal root ganglion (DRG) neurons in culture. Additionally, the transplantation of retinal glia significantly enhanced regeneration of DRG axons after a crush injury in adult rats through a process mediated by SPARC [181].

Glypicans

Glypicans (Gpc’s) belong to the family of heparin sulphate binding glycoproteins, which includes glypicans 1–6. Gpc’s are critically involved in developmental morphogenesis, and have been implicated as regulators of cell-signaling pathways, including Wnt and Hedgehog, fibroblast growth factors and bone morphogenic proteins [182, 183]. Like other MCPs, they are mostly expressed in brain during development. However, Gpc4 and 6 are expressed predominantly in adult brain astrocytes, with Gpc4 expression elevated in the hippocampus and Gpc6 enriched in the cerebellum.

Glypicans were first cloned, isolated and characterized by David et al. [184], Karthikeyan et al. [185]. It was subsequently reported that glypicans are predominantly neuronal membrane components that are present in late embryonic and postnatal CNS [186, 187]. Additionally, Veugelers et al. [188] reported Gpc-5 enrichment in adult human brain tissue, while Saunders et al. [189] showed that Gpc-5 was highly expressed in brain at developmental stages [190]. Glypican mRNA levels were specifically identified in cerebellar granule cells, large motor neurons in the brainstem, and in CA3 pyramidal cells of the hippocampus [187].

There is increasing evidence that Gpc’s act as regulators of cell migration, axonal pathfinding, synaptogenesis, and structural plasticity. In brain, neuronal Gpc are regulated by N-methyl-D-aspartate and metabotropic receptor activation [191]. Gpc’s serve as cell-surface receptors for neurocan, which is involved in the promotion of neurite outgrowth [192]; axon guidance, and visual system function [193]; brain patterning and neurogenesis [194]; cell cycle progression [195], as well as in the regulation of brain size [196].

Alzheimer’s Disease

A role for Gpc’s in AD is well established. Glypican-1 (Gpc-1), was shown to be abundantly expressed in cerebral amyloid angiopathy [197–199]. Gpc-1 was also expressed in neuritic plaques and neurofibrillary tangles in AD [200]. Further, Gpc-1 binds to APP and such binding inhibits APP-induced neurite outgrowth in chick sympathetic neurons [201].

Cultured cortical neurons from APP KO mice showed an increased nitric oxide-mediated degradation of Gpc-1, as compared to brain tissue and neurons from wild-type mice, suggesting a nitroxyl anion-catalyzed degradation of Gpc-1 in AD [202]. Increased levels of Gpc-1 were also identified in astrocyte cultures following exposure to Aβ [203]. Further, when human brain pericytes were cultured in the presence of Aβ, Gpc-1 protein and mRNA expression were increased, suggesting that Aβ modulates the cellular expression of Gpc-1 [204]. Such modulation may prevent Aβ-mediated cellular toxicity in AD, possibly by enhancing heparan sulfate accumulation [204], suggesting that enhanced levels of Gpc’s may be neuroprotective in AD.

Neoplasms

Gpc’s have also been implicated in tumor progression. Gpc-1 is highly expressed in gliomas and such expression was shown to induce DNA replication, suggesting a mutagenic activity for Gpc-1. Gpc-1 expression also led to a significant downregulation of the tumor suppressors pRb, Cip/Kip cyclin-dependent kinase inhibitors (CKIs), and CDH1, as well as to an upregulation of the pro-oncogenic proteins cyclin E, cyclin-dependent kinase 2, Skp2, and Cdt1. These Gpc-1-induced changes were accompanied by a significant reduction in all types of D cyclins [205, 206]. These findings suggest that Gpc-1 plays an important role in glioma [205, 206].

Parkinson’s Disease and Ischemia Conditions

The direct infusion of glypican into an infarct cavity for 7 days, beginning 7 days after middle cerebral artery occlusion in rats, increased levels of the microtubule-associated protein-2 immunoreactivity in the peri-infarct region and ameliorated the middle cerebral artery occlusion associated behavioral abnormalities [207]. Intrastriatal transplantation of Gpc4 into a 6-OHDA rat model of Parkinson’s disease (PD) (which exhibit a reduction in Gpc4 level), was shown to improve motor behavior [208].

Hepatic Encephalopathy

In recent studies, we observed a decrease in Gpc-4 and 6 in ammonia-treated cultured astrocytes (unpublished observations), and in brains of rats treated with thioacetamide (a model of heaptic encephalopathy) by immunofluorescence as described previously by us [16] (Figs. 2, 3). Further, enhancing Gpc-4 and 6 levels by blocking oxidative stress and NF-κB activation in ammonia-treated astrocytes, and adding the CM from such procedure to neurons, diminished the loss of synaptophysin induced by CM from ammonia-treated astrocytes (unpublished observation). Astrocytic Gpc-4 and -6 were also shown to promote the formation of excitatory synapses through the activation of GluA1 AMPA receptors subunit [209–211]. These findings collectively suggest a potential role of Gpc’s in the pathogenesis of various neurological disorders.

Fig. 2
figure 2

Glypican-4 protein levels in astrocytes from cerebral cortex of normal and TAA-treated adult rats. Glial fibrillary acidic protein (GFAP, green, astrocytes), glypican-4 (red), and DAPI (blue, nuclei). The co-localization of glypican-4 and GFAP illustrates the presence of glypican-4 in adult rat brain astrocytes (a–d). Additionally, treatment of rats with the liver toxin TAA, reduced the glypican-4 levels in astrocytes (e–h). I, quantification of glypican-4 levels after TAA. Scale bar 10 µm

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Fig. 3
figure 3

Glypican-6 protein levels in astrocytes from cerebral cortex of normal and TAA-treated adult rats. Glial fibrillary acidic protein (GFAP, red, astrocytes), glypican-6 (green), and DAPI (blue, nuclei). The co-localization of glypican-6 and GFAP illustrates the presence of glypican-6 in adult rat brain astrocytes (a–d). Additionally, treatment of rats with the liver toxin TAA, reduced the glypican-6 levels in astrocytes (e–h). I, quantification of glypican-6 levels after TAA. Scale bar 15 µm

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CCN Family of Proteins

This family of proteins consists of a complex of six proteins (CCN1-6) encoded from the CCN (CYR61/CTGF/NOV) family of genes. These proteins interact with cell surface receptors and receptor ligands to induce intracellular signaling. They regulate cell adhesion, proliferation, migration, chemotaxis, cell survival, differentiation, angiogenesis, chondrogenesis, tumorigenesis and wound healing [212, 213]. Except CCN6, most of these proteins are present in adult brain. CCN1 is present in the frontal, temporal and occipital cortices, hippocampus and caudate, and is strongly expressed in the spinal cord [214]. CCN2 expression is similar to CCN1, but is lower in the hippocampus, caudate and corpus callosum [215]. It is also present in cortical glial cells [216]. CCN3 is present in cortex, hippocampus, amygdala and spinal cord [214, 217]. CCN4 is found in cerebral cortex, cerebellum, cortical endothelial cells and Purkinje neurons. CCN5 is present in the adult cortex, cerebellum and spinal cord neurons [215 and references therein].

While the presence of CCN family of proteins has been associated with various neurological disorders, including neoplasms, ischemia, amyotrophic lateral sclerosis, trauma and alzheimer’s disease [215], their precise involvement in these conditions is not clear. In addition to the CCN family of proteins, changes in osteopontin, small leucine-rich proteoglycans (SLRPs) in rat anterior pituitary, Fibulin-5 and Galectins 1 and 3 have also been found in various neurological conditions [218–228]. Their role in the pathogenesis in these conditions, however, is not yet well established.

Summary/Perspectives

MCPs play a major role in the pathogenesis of various neurological disorders, including Alzheimer’s disease, amyotrophic lateral sclerosis, ischemia, traumatic brain inquiry, hepatic encephalopathy, Down’s syndrome, autism, multiple sclerosis, neoplasms, Parkinson disease and epilepsy. Increased MCPs levels have also been identified in neoplasms, ischemia and Alzheimer’s disease, while decreased levels were identified in hepatic encephalopathy and traumatic brain inquiry (Table 1). The reversal of MCP levels by genetic manipulation, or by the inhibition of upstream signaling, was shown to reduce disease progression in these conditions.

Table 1 Matricellular proteins (MCPs) in various neurological conditions
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While the reason for alterations in MCPs expression and their involvement in various neurological disorders remain unclear, it possible that several factors, including the duration and severity of the insult; the concentration of agents used in these investigations to induce a disease process (e.g., Aβ peptides); or the animal model used, may have varied in these studies, possibly accounting for the differences in MCPs protein expression. Regardless, a better understanding of the role of MCPs in these neurological conditions should aid in the identification of novel therapeutic agents for these disorders.

Relative to treatment strategies aimed at diminishing/preventing neurological disorders associated with decreased levels of MCP, agents that enhance such levels have been shown to reduce the severity of these conditions, including, hepatic encephalopathy, traumatic brain injury and cerebral ischemia. Additionally, small-molecules based on a CD36-binding peptide sequence derived from TSP-1 are currently being tested for their efficacy in the treatment of some neurological disorders (e.g., neoplasms, ischemia). One analog of TSP-1, ABT-510, which exhibited potent pro-apoptotic activity in glioma cell lines, was subsequently shown to exhibit therapeutic benefit against several malignancies. ABT-510 is currently in phase II clinical trials for the treatment of various CNS neoplasms (e.g., astrocytomas).

Alterations in MCPs expression appear to be critically involved in the pathogenesis of many neurological conditions, including Alzheimer’s disease, amyotrophic lateral sclerosis, ischemia, trauma, hepatic encephalopathy, Down’s syndrome, autism, multiple sclerosis, neoplasms, Parkinson disease and epilepsy. We anticipate that a better understanding of the role of MCPs in these disorders will provide valuable insights into disease pathogenesis that may ultimately lead to improved therapeutic outcomes for patients with these conditions.