Tenascin-C contains domains that independently regulate neurite outgrowth and neurite guidance

C286, an orally available retinoic acid receptor β agonist drug, regulates multiple pathways to achieve spinal cord injury repair

Retinoic acid receptor β2 (RARβ2) is an emerging therapeutic target for spinal cord injuries (SCIs) with a unique multimodal regenerative effect. We have developed a first-in-class RARβ agonist drug, C286, that modulates neuron-glial pathways to induce functional recovery in a rodent model of sensory root avulsion. Here, using genome-wide and pathway enrichment analysis of avulsed rats' spinal cords, we show that C286 also influences the extracellular milieu (ECM). Protein expression studies showed that C286 upregulates tenascin-C, integrin-α9, and osteopontin in the injured cord. Similarly, C286 remodulates these ECM molecules, hampers inflammation and prevents tissue loss in a rodent model of spinal cord contusion C286. We further demonstrate C286's efficacy in human iPSC-derived neurons, with treatment resulting in a significant increase in neurite outgrowth. Additionally, we identify a putative efficacy biomarker, S100B, which plasma levels correlated with axonal regeneration in nerve-injured rats. We also found that other clinically available retinoids, that are not RARβ specific agonists, did not lead to functional recovery in avulsed rats, demonstrating the requirement for RARβ specific pathways in regeneration. In a Phase 1 trial, the single ascending dose (SAD) cohorts showed increases in expression of RARβ2 in white blood cells correlative to increased doses and at the highest dose administered, the pharmacokinetics were similar to the rat proof of concept (POC) studies. Collectively, our data suggests that C286 signalling in neurite/axonal outgrowth is conserved between species and across nerve injuries. This warrants further clinical testing of C286 to ascertain POC in a broad spectrum of neurodegenerative conditions.

A novel splice-altering TNC variant (c.5247A > T, p.Gly1749Gly) in an Chinese family with autosomal dominant non-syndromic hearing loss

BACKGROUND

This study aims to analyze the pathogenic gene in a Chinese family with non-syndromic hearing loss and identify a novel mutation site in the TNC gene.

METHODS

A five-generation Chinese family from Anhui Province, presenting with autosomal dominant non-syndromic hearing loss, was recruited for this study. By analyzing the family history, conducting clinical examinations, and performing genetic analysis, we have thoroughly investigated potential pathogenic factors in this family. The peripheral blood samples were obtained from 20 family members, and the pathogenic genes were identified through whole exome sequencing. Subsequently, the mutation of gene locus was confirmed using Sanger sequencing. The conservation of TNC mutation sites was assessed using Clustal Omega software. We utilized functional prediction software including dbscSNV_AdaBoost, dbscSNV_RandomForest, NNSplice, NetGene2, and Mutation Taster to accurately predict the pathogenicity of these mutations. Furthermore, exon deletions were validated through RT-PCR analysis.

RESULTS

The family exhibited autosomal dominant, progressive, post-lingual, non-syndromic hearing loss. A novel synonymous variant (c.5247A > T, p.Gly1749Gly) in TNC was identified in affected members. This variant is situated at the exon-intron junction boundary towards the end of exon 18. Notably, glycine residue at position 1749 is highly conserved across various species. Bioinformatics analysis indicates that this synonymous mutation leads to the disruption of the 5' end donor splicing site in the 18th intron of the TNC gene. Meanwhile, verification experiments have demonstrated that this synonymous mutation disrupts the splicing process of exon 18, leading to complete exon 18 skipping and direct splicing between exons 17 and 19.

CONCLUSION

This novel splice-altering variant (c.5247A > T, p.Gly1749Gly) in exon 18 of the TNC gene disrupts normal gene splicing and causes hearing loss among HBD families.

What do we know about astrocytes and the antidepressant effects of DBS?

Treatment-resistant depression (TRD) is a debilitating condition that affects millions of individuals worldwide. Deep brain stimulation (DBS) has been widely used with excellent outcomes in neurological disorders such as Parkinson's disease, tremor, and dystonia. More recently, DBS has been proposed as an adjuvant therapy for TRD. To date, the antidepressant efficacy of DBS is still controversial, and its mechanisms of action remain poorly understood. Astrocytes are the most abundant cells in the nervous system. Once believed to be a "supporting" element for neuronal function, astrocytes are now recognized to play a major role in brain homeostasis, neuroinflammation and neuroplasticity. Because of its many roles in complex multi-factorial disorders, including TRD, understanding the effect of DBS on astrocytes is pivotal to improve our knowledge about the antidepressant effects of this therapy. In depression, the number of astrocytes and the expression of astrocytic markers are decreased. One of the potential consequences of this reduced astrocytic function is the development of aberrant glutamatergic neurotransmission, which has been documented in several models of depression-like behavior. Evidence from preclinical work suggests that DBS may directly influence astrocytic activity, modulating the release of gliotransmitters, reducing neuroinflammation, and altering structural tissue organization. Compelling evidence for an involvement of astrocytes in potential mechanisms of DBS derive from studies suggesting that pharmacological lesions or the inhibition of these cells abolishes the antidepressant-like effect of DBS. In this review, we summarize preclinical data suggesting that the modulation of astrocytes may be an important mechanism for the antidepressant-like effects of DBS.

Tgfbr2 in Dental Pulp Cells Guides Neurite Outgrowth in Developing Teeth

Transforming growth factor β (TGFβ) plays an important role in tooth morphogenesis and mineralization. During postnatal development, the dental pulp (DP) mesenchyme secretes neurotrophic factors that guide trigeminal nerve fibers into and throughout the DP. This process is tightly linked with dentin formation and mineralization. Our laboratory established a mouse model in which Tgfbr2 was conditionally deleted in DP mesenchyme using an Osterix promoter-driven Cre recombinase (Tgfbr2 ^cko ). These mice survived postnatally with significant defects in bones and teeth, including reduced mineralization and short roots. Hematoxylin and eosin staining revealed reduced axon-like structures in the mutant mice. Reporter imaging demonstrated that Osterix-Cre activity within the tooth was active in the DP and derivatives, but not in neuronal afferents. Immunofluorescence staining for β3 tubulin (neuronal marker) was performed on serial cryosections from control and mutant molars on postnatal days 7 and 24 (P7, P24). Confocal imaging and pixel quantification demonstrated reduced innervation in Tgfbr2 ^cko first molars at both stages compared to controls, indicating that signals necessary to promote neurite outgrowth were disrupted by Tgfbr2 deletion. We performed mRNA-Sequence (RNA-Seq) and gene onotology analyses using RNA from the DP of P7 control and mutant mice to investigate the pathways involved in Tgfbr2-mediated tooth development. These analyses identified downregulation of several mineralization-related and neuronal genes in the Tgfbr2 ^cko DP compared to controls. Select gene expression patterns were confirmed by quantitative real-time PCR and immunofluorescence imaging. Lastly, trigeminal neurons were co-cultured atop Transwell filters overlying primary Tgfbr2 ^f/f DP cells. Tgfbr2 in the DP was deleted via Adenovirus-expressed Cre recombinase. Confocal imaging of axons through the filter pores showed increased axonal sprouting from neurons cultured with Tgfbr2-positive DP cells compared to neurons cultured alone. Axon sprouting was reduced when Tgfbr2 was knocked down in the DP cells. Immunofluorescence of dentin sialophosphoprotein in co-cultured DP cells confirmed reduced mineralization potential in cells with Tgfbr2 deletion. Both our proteomics and RNA-Seq analyses indicate that axonal guidance cues, particularly semaphorin signaling, were disrupted by Tgfbr2 deletion. Thus, Tgfbr2 in the DP mesenchyme appears to regulate differentiation and the cells' ability to guide neurite outgrowth during tooth mineralization and innervation.

Interaction between Schwann cells and other cells during repair of peripheral nerve injury

Peripheral nerve injury (PNI) is common and, unlike damage to the central nervous system injured nerves can effectively regenerate depending on the location and severity of injury. Peripheral myelinating glia, Schwann cells (SCs), interact with various cells in and around the injury site and are important for debris elimination, repair, and nerve regeneration. Following PNI, Wallerian degeneration of the distal stump is rapidly initiated by degeneration of damaged axons followed by morphologic changes in SCs and the recruitment of circulating macrophages. Interaction with fibroblasts from the injured nerve microenvironment also plays a role in nerve repair. The replication and migration of injury-induced dedifferentiated SCs are also important in repairing the nerve. In particular, SC migration stimulates axonal regeneration and subsequent myelination of regenerated nerve fibers. This mobility increases SC interactions with other cells in the nerve and the exogenous environment, which influence SC behavior post-injury. Following PNI, SCs directly and indirectly interact with other SCs, fibroblasts, and macrophages. In addition, the inter- and intracellular mechanisms that underlie morphological and functional changes in SCs following PNI still require further research to explain known phenomena and less understood cell-specific roles in the repair of the injured peripheral nerve. This review provides a basic assessment of SC function post-PNI, as well as a more comprehensive evaluation of the literature concerning the SC interactions with macrophages and fibroblasts that can influence SC behavior and, ultimately, repair of the injured nerve.

Site-specific HNK-1 epitope on alternatively spliced fibronectin type-III repeats in tenascin-C promotes neurite outgrowth of hippocampal neurons through contactin-1

The human natural killer-1 (HNK-1) carbohydrate epitope, composed of a unique sulfated trisaccharide (HSO₃–3GlcAβ1–3Galβ1–4GlcNAc-R), is highly expressed during brain development and regulates higher brain function. However, it remains unclear which glycoprotein carries the HNK-1 epitope in the embryonic brain and the functional role it plays. Here, we showed that one of the major HNK-1 carrier proteins in the embryonic brain is tenascin-C (TNC), an extracellular matrix protein that regulates neurite outgrowth by interacting with the GPI-anchored protein contactin-1 (CNTN). Because the alternatively spliced fibronectin type-III (FNIII) repeats in TNC give rise to many isoforms and affect neuronal function, we evaluated neurite outgrowth of primary hippocampal neurons on purified recombinant FNIII repeats with or without the HNK-1 epitope as a substrate. We found that the presence of the HNK-1 epitope on the C domain of TNC promoted neurite outgrowth, and that this signal was mediated by CNTN, which is an HNK-1-expressing neuronal receptor. The neurite-promoting activity of the HNK-1 epitope on TNC required neuronal HNK-1 expression, which was defective in neurons lacking the glucuronyltransferases GlcAT-P and GlcAT-S. These results suggest that the HNK-1 epitope is a key modifier of TNC and CNTN in the regulation of embryonic brain development.

Tenascin-C derived signaling induces neuronal differentiation in a three-dimensional peptide nanofiber gel

The development of new biomaterials mimicking the neuronal extracellular matrix (ECM) requires signals for the induction of neuronal differentiation and regeneration. In addition to the biological and chemical cues, the physical properties of the ECM should also be considered while designing regenerative materials for nervous tissue. In this study, we investigated the influence of the microenvironment on tenascin-C signaling using 2D surfaces and 3D scaffolds generated by a peptide amphiphile nanofiber gel with a tenascin-C derived peptide epitope (VFDNFVLK). While tenascin-C mimetic PA nanofibers significantly increased the length and number of neurites produced by PC12 cells on 2D cell culture, more extensive neurite outgrowth was observed in the 3D gel environment. PC12 cells encapsulated within the 3D tenascin-C mimetic peptide nanofiber gel also exhibited significantly increased expression of neural markers compared to the cells on 2D surfaces. Our results emphasize the synergistic effects of the 3D conformation of peptide nanofibers along with the tenascin-C signaling and growth factors on the neuronal differentiation of PC12 cells, which may further provide more tissue-like morphology for therapeutic applications.

Tenascin-C: Form versus function

Tenascin-C is a large, multimodular, extracellular matrix glycoprotein that exhibits a very restricted pattern of expression but an enormously diverse range of functions. Here, we discuss the importance of deciphering the expression pattern of, and effects mediated by, different forms of this molecule in order to fully understand tenascin-C biology. We focus on both post transcriptional and post translational events such as splicing, glycosylation, assembly into a 3D matrix and proteolytic cleavage, highlighting how these modifications are key to defining tenascin-C function.

Improvement in the neural stem cell proliferation in rats treated with modified "Shengyu" decoction may contribute to the neurorestoration

ETHNOPHARMACOLOGICAL RELEVANCE

"Shengyu" decoction, a traditional Chinese medicine, has been used to treat diseases with deficit in "qi" and "blood". The modified "Shengyu" decoction (MSD) used in the present study was designed to treat traumatic brain injury (TBI) on the basis of the "Shengyu" decoction, in which additional four herbs were added. Many ingredients in these herbs have been demonstrated to be effective for the treatment of brain injury. The present study was performed to evaluate the neurorestorative effect and the underlying mechanisms of MSD on the rat brain after a TBI.

MATERIALS AND METHODS

TBI was induced in the right cerebral cortex of adult rats using Feeney's weight-drop method. Intragastrical administration of MSD (1.0 ml/200 g) was begun 6h after TBI. The neurological functions and neuronal loss in the cortex and hippocampus were determined. The levels of nerve growth-related factors GDNF, NGF, NCAM, TN-C, and Nogo-A and the number of GFAP(+)/GDNF(+), BrdU(+)/nestin(+), BrdU(+)/NeuN(+) immunoreactive cells in the brain ipsilateral to TBI were also measured. Moreover, the influences of MSD on these variables were observed at the same time.

RESULTS

We found that treatment with MSD in TBI rats ameliorated the neurological functions and alleviated neuronal loss. MSD treatment elevated the expression of GDNF, NGF, NCAM, and TN-C, and inhibited the expression of Nogo-A. Moreover, MSD treatment increased the number of GFAP(+)/GDNF(+), BrdU(+)/nestin(+), and BrdU(+)/NeuN(+) immunoreactive cells in the cortex and hippocampus.

CONCLUSION

The present results suggest that MSD treatment in TBI rats could improve the proliferation of neural stem/progenitor cells and differentiation into neurons, which may facilitate neural regeneration and tissue repair and thus contribute to the recovery of neurological functions. These effects of modified "Shengyu" decoction may provide a foundation for the use of MSD as a prescription of medicinal herbs in the traditional medicine to treat brain injuries in order to improve the neurorestoration.

Different forms of tenascin-C with tenascin-R regulate neural differentiation in bone marrow-derived human mesenchymal stem cells

Mesenchymal stem cells (MSCs) are currently thought to transdifferentiate into neural lineages under specific microenvironments. Studies have reported that the tenascin family members, tenascin-C (TnC) and tenascin-R (TnR), regulate differentiation and migration, in addition to neurite outgrowth and survival in numerous types of neurons and mesenchymal progenitor cells. However, the mechanisms by which TnC and TnR affect neuronal differentiation are not well understood. In this study, we hypothesized that different forms of tenascin might regulate the neural transdifferentiation of human bone marrow-derived mesenchymal stem cells. Human MSCs were cultured in media incorporated with soluble tenascins, or on precoated tenascins. In a qualitative polymerase chain reaction analysis, adding a soluble TnC and TnR mixture to the medium significantly enhanced the expression of neuronal and glial markers, whereas no synaptic markers were expressed. Conversely, in groups of cells treated with coated TnC, hMSCs showed neurite outgrowth and synaptic marker expression. After being treated with coated TnR, hMSCs exhibited neuronal differentiation; however, it inhibited neurite outgrowth and synaptic marker expression. A combination of TnC and TnR significantly promoted hMSC differentiation in neurons or oligodendrocytes, induced neurite formation, and inhibited differentiation into astrocytes. Furthermore, the effect of the tenascin mixture showed dose-dependent effects, and a mixture ratio of 1:1 to 1:2 (TnC:TnR) provided the most obvious differentiation of neurons and oligodendrocytes. In a functional blocking study, integrin α7 and α9β1-blocking antibodies inhibited, respectively, 80% and 20% of mRNA expression by hMSCs in the coated tenascin mixture. In summary, the coated combination of TnC and TnR appeared to regulate neural differentiation signaling through integrin α7 and α9β1 in bone marrow-derived hMSCs. Our findings demonstrate novel mechanisms by which tenascin regulates neural differentiation, and enable the use of cell therapy to treat neurodegenerative diseases.

L-serine treatment may improve neurorestoration of rats after permanent focal cerebral ischemia potentially through improvement of neurorepair

The present study was conducted to clarify whether treatment with L-serine can improve the brain repair and neurorestoration of rats after permanent middle cerebral artery occlusion (pMCAO). After pMCAO, the neurological functions, brain lesion volume, and cortical injury were determined. GDNF, NGF, NCAM L1, tenascin-C, and Nogo-A levels were measured. Proliferation and differentiation of the neural stem cells (NSCs) and proliferation of the microvessels in the ischemic boundary zone of the cortex were evaluated. Treatment with L-serine (168 mg/kg body weight, i.p.) began 3 h after pMCAO and was repeated every 12 h for 7 days or until the end of the experiment. L-Serine treatment: 1) reduced the lesion volume and neuronal loss; 2) improved the recovery of neurological functions; 3) elevated the expression of nerve growth-related factors; and 4) facilitated the proliferation of endogenous NSCs and microvessels activated after pMCAO and increased the number of new-born neurons. 5) D-cycloserine, an inhibitor of serine hydroxymethyltransferase, blunted the effects of L-serine on NSC proliferation, differentiation, microvascular proliferation. In conclusions, L-serine treatment in pMCAO rats can reduce brain injury and facilitate neurorestoration which is partly associated with the improvement of proliferation of NSCs and microvessels, reconstruction of neurovascular units and resultant neurorepair. The effects of L-serine on endogenous NSC proliferation and microvascular proliferation are partly mediated by the action of L-serine as a substrate for the production of one-carbon groups used for purine and pyrimidine synthesis and modulation of the expression of some nerve growth-related factors.

Glomerular cell cross-talk influences composition and assembly of extracellular matrix

The glomerular basement membrane (GBM) is a specialized extracellular matrix (ECM) compartment within the glomerulus that contains tissue-restricted isoforms of collagen IV and laminin. It is integral to the capillary wall and therefore, functionally linked to glomerular filtration. Although the composition of the GBM has been investigated with global and candidate-based approaches, the relative contributions of glomerular cell types to the production of ECM are not well understood. To characterize specific cellular contributions to the GBM, we used mass spectrometry-based proteomics to analyze ECM isolated from podocytes and glomerular endothelial cells in vitro. These analyses identified cell type-specific differences in ECM composition, indicating distinct contributions to glomerular ECM assembly. Coculture of podocytes and endothelial cells resulted in an altered composition and organization of ECM compared with monoculture ECMs, and electron microscopy revealed basement membrane-like ECM deposition between cocultured cells, suggesting the involvement of cell-cell cross-talk in the production of glomerular ECM. Notably, compared with monoculture ECM proteomes, the coculture ECM proteome better resembled a tissue-derived glomerular ECM dataset, indicating its relevance to GBM in vivo. Protein network analyses revealed a common core of 35 highly connected structural ECM proteins that may be important for glomerular ECM assembly. Overall, these findings show the complexity of the glomerular ECM and suggest that both ECM composition and organization are context-dependent.

Extracellular matrix alterations, accelerated leukocyte infiltration and enhanced axonal sprouting after spinal cord hemisection in tenascin-C-deficient mice

The extracellular matrix glycoprotein tenascin-C has been implicated in wound repair and axonal growth. Its role in mammalian spinal cord injury is largely unknown. In vitro it can be both neurite-outgrowth promoting and repellent. To assess its effects on glial reactions, extracellular matrix formation, and axonal regrowth/sprouting in vivo, 20 tenascin-C-deficient and 20 wild type control mice underwent lumbar spinal cord hemisection. One, three, seven and fourteen days post-surgery, cryostat sections of the spinal cord were examined by conventional histology and by immunohistochemistry using antibodies against F4/80 (microglia/macrophage), GFAP (astroglia), neurofilament, fibronectin, laminin and collagen type IV. Fibronectin immunoreactivity was significantly down-regulated in tenascin-C-deficient mice. Moreover, fourteen days after injury, immunodensity of neurofilament-positive fibers was two orders of magnitude higher along the incision edges of tenascin-C-deficient mice as compared to control mice. In addition, lymphocyte infiltration was seen two days earlier in tenascin-C-deficient mice than in control mice and neutrophil infiltration was increased seven days after injury. The increase in thin neurofilament positive fibers in tenascin-C-deficient mice indicates that lack of tenascin-C alters the inflammatory reaction and extracellular matrix composition in a way that penetration of axonal fibers into spinal cord scar tissue may be facilitated.

Regulation of axonal outgrowth and pathfinding by integrin-ECM interactions

Developing neurons use a combination of guidance cues to assemble a functional neural network. A variety of proteins immobilized within the extracellular matrix (ECM) provide specific binding sites for integrin receptors on neurons. Integrin receptors on growth cones associate with a number of cytosolic adaptor and signaling proteins that regulate cytoskeletal dynamics and cell adhesion. Recent evidence suggests that soluble growth factors and classic axon guidance cues may direct axon pathfinding by controlling integrin-based adhesion. Moreover, because classic axon guidance cues themselves are immobilized within the ECM and integrins modulate cellular responses to many axon guidance cues, interactions between activated receptors modulate cell signals and adhesion. Ultimately, growth cones control axon outgrowth and pathfinding behaviors by integrating distinct biochemical signals to promote the proper assembly of the nervous system. In this review, we discuss our current understanding how ECM proteins and their associated integrin receptors control neural network formation.

The extracellular matrix glycoprotein tenascin-C promotes locomotor recovery after spinal cord injury in adult zebrafish

Adult zebrafish, by virtue of exhibiting spontaneous recovery after spinal lesion, have evolved into a paradigmatic vertebrate model system to identify novel genes vital for successful regeneration after spinal cord injury. Due to a remarkable level of conservation between zebrafish and human genomes, such genes, once identified, could point to possibilities for addressing the multiple issues on how to deal with functional recovery after spinal cord injury in humans. In the current study, the extracellular matrix glycoprotein tenascin-C was studied in the zebrafish spinal cord injury model to assess the often disparate functions of this multidomain molecule under in vivo conditions. This in vivo study was deemed necessary since in vitro studies had shown discrepant functional effects on neurite outgrowth: tenascin-C inhibits neurite outgrowth when presented as a molecular barrier adjacent to a conducive substrate, but enhances neurite outgrowth when presented as a uniform substrate. Thus, our current study addresses the question as to which of these features prevails in vivo: whether tenascin-C reduces or enhances axonal regrowth after injury in a well accepted vertebrate model of spinal cord injury. We show upregulation of tenascin-C expression in regenerating neurons of the nucleus of median longitudinal fascicle (NMLF) in the brainstem and spinal motoneurons. Inhibition of tenascin-C expression by antisense oligonucleotide (morpholino) resulted in impaired locomotor recovery, reduced regrowth of axons from brainstem neurons and reduced synapse formation by the regrowing brainstem axons on spinal motoneurons, all vital indicators of regeneration. Our results thus point to an advantageous role of tenascin-C in promoting spinal cord regeneration, by promoting axonal regrowth and synapse formation in the spinal cord caudal to the lesion site after injury.

The extracellular matrix glycoprotein tenascin-C is beneficial for spinal cord regeneration

Tenascin-C (TNC), a major component of the extracellular matrix, is strongly upregulated after injuries of the central nervous system (CNS) but its role in tissue repair is not understood. Both regeneration promoting and inhibiting roles of TNC have been proposed considering its abilities to both support and restrict neurite outgrowth in vitro. Here, we show that spontaneous recovery of locomotor functions after spinal cord injury is impaired in adult TNC-deficient (TNC(-/-)) mice in comparison to wild-type (TNC(+/+)) mice. The impaired recovery was associated with attenuated excitability of the plantar Hoffmann reflex (H-reflex), reduced glutamatergic input, reduced sprouting of monaminergic axons in the lumbar spinal cord and enhanced post-traumatic degeneration of corticospinal axons. The degeneration of corticospinal axons in TNC(-/-) mice was normalized to TNC(+/+) levels by application of the alternatively spliced TNC fibronectin type III homologous domain D (fnD). Finally, overexpression of TNC-fnD via adeno-associated virus in wild-type mice improved locomotor recovery, increased monaminergic axons sprouting, and reduced lesion scar volume after spinal cord injury. The functional efficacy of the viral-mediated TNC indicates a potentially useful approach for treatment of spinal cord injury.

Analysis of combinatorial variability reveals selective accumulation of the fibronectin type III domains B and D of tenascin-C in injured brain

Tenascin-C (Tnc) is a multimodular extracellular matrix glycoprotein that is markedly upregulated in CNS injuries where it is primarily secreted by reactive astrocytes. Different Tnc isoforms can be generated by the insertion of variable combinations of one to seven (in rats) alternatively spliced distinct fibronectin type III (FnIII) domains to the smallest variant. Each spliced FnIII repeat mediates specific actions on neurite outgrowth, neuron migration or adhesion. Hence, different Tnc isoforms might differentially influence CNS repair. We explored the expression pattern of Tnc variants after cortical lesions and after treatment of astrocytes with various cytokines. Using RT-PCR, we observed a strong upregulation of Tnc transcripts containing the spliced FnIII domains B or D in injured tissue at 2-4 days post-lesion (dpl). Looking at specific combinations, we showed a dramatic increase of Tnc isoforms harboring the neurite outgrowth-promoting BD repeat with both the B and D domains being adjacent to each other. Isoforms containing only the axon growth-stimulating spliced domain D were also dramatically enhanced after injury. Injury-induced increase of Tnc proteins comprising the domain D was confirmed by Western Blotting and immunostaining of cortical lesions. In contrast, the FnIII modules C and AD1 were weakly modulated after injury. The growth cone repulsive A1A2A4 domains were poorly expressed in normal and injured tissue but the smallest isoform, which is also repellant, was highly expressed after injury. Expression of the shortest Tnc isoform and of variants containing B, D or BD, was strongly upregulated in cultured astrocytes after TGFbeta1 treatment, suggesting that TGFbeta1 could mediate, at least in part, the injury-induced upregulation of these isoforms. We identified complex injury-induced differential regulations of Tnc isoforms that may well influence axonal regeneration and repair processes in the damaged CNS.

Chondroitin-4-sulfation negatively regulates axonal guidance and growth

Glycosaminoglycan (GAG) side chains endow extracellular matrix proteoglycans with diversity and complexity based upon the length, composition and charge distribution of the polysaccharide chain. Using cultured primary neurons, we show that specific sulfation in the GAG chains of chondroitin sulfate mediates neuronal guidance cues and axonal growth inhibition. Chondroitin-4-sulfate (CS-A), but not chondroitin-6-sulfate (CS-C), exhibits a strong negative guidance cue to mouse cerebellar granule neurons. Enzymatic and gene-based manipulations of 4-sulfation in the GAG side chains alter their ability to direct growing axons. Furthermore, 4-sulfated chondroitin sulfate GAG chains are rapidly and significantly increased in regions that do not support axonal regeneration proximal to spinal cord lesions in mice. Thus, our findings show that specific sulfation along the carbohydrate backbone carries instructions to regulate neuronal function.

Covalently attached FGF-2 to three-dimensional polyamide nanofibrillar surfaces demonstrates enhanced biological stability and activity

Activation of fibroblast growth factor receptors (FGFRs) requires the formation of a ternary complex between fibroblast growth factors (FGFs), FGFRs, and heparan sulfate proteoglycans, which are all located on the cell surface and the basement membrane (BM)/extracellular matrix (ECM). Heparan sulfate proteoglycans appear to stabilize FGFs by inhibiting the rapid degradation of FGFs normally observed in solution. Because of the pivotal role of FGFs in proliferative and developmental pathways, a number of recent studies have attempted to engineer microenvironments to stabilize growth factors for use in applications in tissue culture and regenerative medicine. In this communication, we demonstrate that covalent linkage of FGF-2 to nanofibrillar surfaces (defined as covalently bound FGF-2) composed of a network of polyamide nanofibers resulted in the maintenance of the biological efficacy of FGF-2 when stored dry for at least 6 months at 25 degrees C or 4 degrees C. Moreover, covalently bound FGF-2 was more potent than FGF-2 in solution when measured in cellular assays of proliferation and viability using a variety of cell types. Covalently bound FGF-2 also strongly activated FGFR, extracellular signal-regulated kinase (ERK1/2), and c-fos. Hence cell-signaling molecules can be incorporated into a synthetic nanofibrillar surface, providing a novel means to enhance their stability and biological activity.

Role of DNA topoisomerase IIβ in neurite outgrowth

Failure to establish neuromuscular junctions is a major phenotype of top2beta knockout mice. However, the precise mechanism for this defect is not known. In the current study, we have investigated the role of TopIIbeta in cultured neurons. We showed that the TopII inhibitor ICRF-193 significantly blocked neurite outgrowth and growth cone formation in cultured cerebellar granule neurons (CGNs), dorsal root ganglions (DRGs) and cortical neurons (CNs). In addition, ICRF-193 also blocked neurite outgrowth and growth cone formation of PC12 cells undergoing NGF-induced differentiation. Isolated cortical neurons from top2beta knockout embryos elaborated shorter neurites than did those from their wild type counterparts, confirming the role of TopIIbeta in neurite outgrowth. Together, these results demonstrate a critical role of TopIIbeta in neurite outgrowth in cultured neurons. Furthermore, we demonstrated that neurons derived from top2beta knockout mice failed to form contacts with muscle cells in co-cultures. These results suggest that the defect in establishing neuromuscular junctions in top2beta knockout mice could be due to the lack of TopIIbeta-mediated neurite outgrowth.

Matrix metalloproteinase-19 is highly expressed in active multiple sclerosis lesions

Matrix metalloproteinases (MMPs) are proteases known for their capacity to degrade extracellular matrix (ECM) components. MMPs have been implicated in several central nervous system (CNS) diseases, including multiple sclerosis (MS). Microarray analysis has demonstrated significant increased mRNA levels of MMP-19 in chronic MS lesions, suggesting a role of MMP-19 in MS pathogenesis. Therefore, in this study, we investigated the expression pattern and cellular localization of MMP-19 protein in various well-characterized MS lesion stages. In normal control patient white matter, MMP-19 was constitutively expressed by microglia throughout the brain parenchyma, suggesting a physiological role for this MMP family member. Likewise, MMP-19 was expressed by microglia in (p)reactive MS lesions, albeit more intense. In highly active demyelinating MS lesions, parenchymal and perivascular myelin-laden macrophages were strongly immunoreactive for MMP-19, whereas reactive astrocytes were occasionally immunopositive. Astrocytes in chronic inactive lesions were weakly stained for MMP-19. In vitro, MMP-19 was expressed in cultures of primary human microglia, not in astrocyte cultures. As MMP-19 is able to degrade basement membrane constituents and other ECM proteins, it is conceivable that this relatively novel MMP family member contributes to MS pathology by remodelling the ECM of the CNS, thereby influencing leucocyte infiltration, axonal regeneration and astrogliosis.

Identification of candidate genes at the corticoseptal boundary during development

Cortical midline glia are critical to the formation of the corpus callosum during development. The glial wedge is a population of midline glia that is located at the corticoseptal boundary and expresses repulsive/growth-inhibitory molecules that guide callosal axons as they cross the midline. The glial wedge are the first cells within the cortex to express GFAP and thus may express molecules specific for glial maturation. The corticoseptal boundary is a genetically defined boundary between the cingulate cortex (dorsal telencephalon) and the septum (ventral telencephalon). The correct dorso-ventral position of this boundary is vital to the formation of both the glial wedge and the corpus callosum. Our aim was to identify genes expressed specifically within the glial wedge that might be involved in either glial differentiation, formation of the corticoseptal boundary or development of the corpus callosum. To identify such genes we have performed a differential display PCR screen comparing RNA isolated from the glial wedge with RNA isolated from control tissues such as the neocortex and septum, of embryonic day 17 mouse brains. Using 200 different combinations of primers, we identified and cloned 67 distinct gene fragments. In situ hybridization analysis confirmed the differential expression of many of the genes, and showed that clones G24F3, G39F8 and transcription factor LZIP have specific expression patterns in the telencephalon of embryonic and postnatal brains. An RNase Protection Assay (RPA) revealed that the expression of G39F8, G24F3 and LZIP increase markedly in the telencephalon at E16 and continue to be expressed until at least P0, during the period when the corpus callosum is forming.

Role of Cdc42 in neurite outgrowth of PC12 cells and cerebellar granule neurons

Inactivation of Rho GTPases inhibited the neurite outgrowth of PC12 cells. The role of Cdc42 in neurite outgrowth was then studied by selective inhibition of Cdc42 signals. Overexpression of ACK42, Cdc42 binding domain of ACK-1, inhibited NGF-induced neurite outgrowth in PC12 cells. ACK42 also inhibited the neurite outgrowth of PC12 cells induced by constitutively activated mutant of Cdc42, but not Rac. These results suggest that Cdc42 plays an important role in mediating NGF-induced neurite outgrowth of PC12 cells. Inhibition of neurite outgrowth was also demonstrated using a cell permeable chimeric protein, penetratin-ACK42. A dominant negative mutant of Rac, RacN17 inhibited Cdc42-induced neurite outgrowth of PC12 cells suggesting that Rac acts downstream of Cdc42. Further studies, using primary-cultures of rat cerebellar granule neurons, showed that Cdc42 is also involved in the neurite outgrowth of cerebellar granule neurons. Both penetratin-ACK42 and Clostridium difficile toxin B, which inactivates all members of Rho GTPases strongly inhibited the neurite outgrowth of cerebellar granule neurons. These results show that Cdc42 plays a similar and essential role in the development of neurite outgrowth of PC12 cells and cerebellar granule neurons. These results provide evidence that Cdc42 produces signals that are essential for the neurite outgrowth of PC12 cells and cerebellar granule neurons.

Neurite guidance by the FnC repeat of human tenascin-C: neurite attraction vs. neurite retention

The alternatively spliced fibronectin type-III repeat C of human tenascin-C (fnC) provides directional cues to elongating neurites in vitro. When given a choice at an interface with poly L-lysine (PLL), rat cerebellar granule neurites preferentially crossed onto fnC (defined herein as neurite attraction) whereas neurites originating on fnC preferentially remained on fnC (defined as neurite retention). Guidance motifs were further refined using synthetic peptides spanning the sequence of fnC. We found that a peptide with amino acid sequence DINPYGFTVSWMASE was sufficient to attract and retain neurites. Peptides with alterations in NPYG facilitated neurite retention but not attraction and, conversely, molecules with alterations in ASE facilitated neurite attraction but not retention. Hence neurite attraction and neurite retention mediated by fnC are separable events that can be independently regulated. This property may prove valuable for the strategic design of peptide reagents for use in strategies to facilitate directed axonal regrowth following CNS injury.

Outgrowth of neurites from NIE-115 neuroblastoma cells is prevented on repulsive substrates through the action of PAK

In the central nervous system (CNS), damaged axons are inhibited from regeneration by glial scars, where secreted chondroitin sulfate proteoglycan (CSPG) and tenascin repulse outgrowth of neurites, the forerunners of axons and dendrites. During differentiation, these molecules are thought to form boundaries for guiding neurons to their correct targets. In neuroblastoma NIE-115 cells, outgrowth of neurites on laminin could be induced by serum starvation or inhibition of RhoA by Clostridium botulinum C3 toxin. The outgrowing neurites avoided crossing onto the repulsive substrate CSPG or tenascin. This avoidance response was partially overcome on expression of membrane-targeted and kinase-inactive forms of PAK. In these cells, the endogenous PAK isoforms colocalized with actin in distinctive sites, alphaPAK in the cell center as small clusters and along the neurite shaft and betaPAK and gammaPAK in areas with membrane ruffles and filopodia, respectively. When isoform-specific N-terminal PAK sequences were introduced to interfere with PAK function, substantially more neurites crossed onto CSPG when cells contained a gammaPAK-derived peptide but not the corresponding alphaPAK- or betaPAK-derived peptide. Thus, while neurite outgrowth can be promoted by RhoA inhibition, overcoming the accompanying repulsive guidance response will require modulation of PAK activity. These results have therapeutic implications for CNS repair processes.

Characterization of Progenitor-Cell-Specific Genes Identified by Subtractive Suppression Hybridization

We have utilized subtractive suppression hybridization (SSH) to identify differentially expressed genes present in either neuroepithelial (NEP) cells or glial restricted precursor (GRP) cells. Eighteen clones enriched in GRP cells and 28 in NEP cells were identified. Five of the GRP-specific clones (tenascin C, cystatin C, GABA transporter 3, extracellular matrix molecule 2 and H2-4) were characterized further, and their glial specificity was confirmed by RT-PCR, in situ hybridization and immunocytochemistry. H2-4 (an expressed sequence tag) was shown to be part of chondroitin sulfate proteoglycan 3. Overall, our results show that SSH can be used to identify lineage- and stage-specific markers and that extracellular matrix molecules likely play important roles in the migration and differentiation of GRPs.

Lack of a genetic association between the TNXB locus and schizophrenia in a Chinese population

A recent study demonstrated that the tenascin X (TNXB) gene was associated with schizophrenia in a British population. To replicate the initial finding, we analysed two positive single nucleotide polymorphisms (SNPs), rs1009382 and rs204887 present at the TNXB locus, in a Chinese population by using PCR-based restriction fragment length polymorphism analysis. We recruited a total of 136 family trios consisting of fathers, mothers and affected offspring with schizophrenia. The transmission disequilibrium test did not show allelic association between these two SNPs and schizophrenia, and the rs1009382-rs204887 haplotypes were not associated with the illness either. The present results suggest that the TNXB locus does not appear to be associated with schizophrenia in the Chinese population. Because the TNXB gene is less than 100 kb away from the NOTCH4 locus that was also reported to be associated with schizophrenia, allelic and locus heterogeneity could be possible reasons for the failure to replicate the TNXB finding.

TNXB locus may be a candidate gene predisposing to schizophrenia

We report here on the detection of nine single nucleotide polymorphisms (SNPs) near to the NOTCH4 locus in the search for schizophrenia susceptibility genes in the class III region of the human major histocompatibility complex (MHC). We totally analyzed 122 family trios recruited in the UK. The TDT analysis demonstrated that of the nine SNPs, three were associated with schizophrenia, including rs1009382 (P = 0.00047), rs204887 (P = 0.007), and rs8283 (P = 0.015). Both rs1009382 and rs204887 are present in the TNXB locus. The rs1009382 is a non-synonymous SNP located in exon 23 of the gene and its A to G base change causes a Glu2578Gly substitution. The goodness-of-fit test showed that genotypic distribution of rs1009382 was deviated from Hardy-Weinberg equilibrium due to homozygote excess in the patient group (P = 0.01), suggesting that a double dose of a genetic risk may be involved. Possibly, rs1009382 is a candidate SNP predisposing to a schizophrenic illness. Moreover, the test for linkage disequilibrium (LD) between paired SNPs showed that the nine SNPs studied may be in the same LD block with an unexpected pattern as the strength of LD was not correlated with the distance between paired SNPs. The haplotype analysis suggested that there might be more than one disease-related allele located in the class III region of the MHC, and that these alleles possibly confer either susceptibility or resistance to schizophrenia.

Involvement of large tenascin-C splice variants in breast cancer progression

Alternative splicing of fibronectin-like type III (FNIII) repeats of tenascin-C (Tn-C) generates a number of splice variants. The distribution of large variants, typical components of provisional extracellular matrices that are up-regulated during tumor stroma remodeling, was here studied by immunoblotting and immunohistochemistry using a monoclonal antibody against the FNIII B domain (named 4C8MS) in a series of human breast cancers. Large Tn-C variants were found at only low levels in normal breast tissues, but were highly expressed at invading sites of intraductal cancers and in the stroma of invasive ductal cancers, especially at invasion fronts. There was a positive correlation between the expression of large Tn-C variants and the cell proliferation rate determined by immunolabeling of the Ki-67 antigen. Of the Tn-C recombinant fragments (all FNIII repeats or mFNIII FL, the conserved FNIII domain only, the epidermal growth factor-like domain, and the fibrinogen-like domain) which were expressed by CHO-K1 cells transfected with mouse Tn-C cDNAs, only the mFNIII FL enhanced in vitro migration and mitotic activity of mammary cancer cells derived from a Tn-C-null mouse. Addition of 4C8MS blocked the function of mFNIII FL. These findings provide strong evidence that the FNIII alternatively spliced region has important roles in tumor progression of breast cancer.

Tenascin-N: characterization of a novel member of the tenascin family that mediates neurite repulsion from hippocampal explants

Tenascin-N, a novel member of the tenascin family, was identified and shown to encode characteristic structural motifs of a cysteine-rich stretch, 3.5 epidermal growth factor-like repeats, 12 fibronectin type III homologous domains, and a fibrinogen-like domain. The third fibronectin type III homologous domain is altered by RNA splicing. Characterization of the expression of tenascin-N by in situ hybridization analysis assigned transcripts to many types of neurons in the central nervous system, to the medullary region in the kidney, and to resident macrophages of the T-cell zone in the splenic white pulp. By immunohistochemistry, tenascin-N expression is detectable in all brain regions, with a characteristic staining pattern in the hippocampus demarcating the CA3 region. Recombinantly expressed protein fragments of the alternatively spliced isoforms were presented in choice assays on patterned substrates to neurites and migrating neurons from hippocampal CA3 region explant cultures. The smaller splice variant inhibited neurite outgrowth or cell migration, whereas the longer splice form did not inhibit these functions. These observations suggest that the novel tenascin family member mediates specific repulsive properties on neurites and neurons by generating splice isoforms.

Functional peptide sequences derived from extracellular matrix glycoproteins and their receptors: strategies to improve neuronal regeneration

Peptides derived from extracellular matrix proteins have the potential to function as potent therapeutic reagents to increase neuronal regeneration following central nervous system (CNS) injury, yet their efficacy as pharmaceutical reagents is dependent upon the expression of cognate receptors in the target tissue. This type of codependency is clearly observed in successful models of axonal regeneration in the peripheral nervous system, but not in the normally nonregenerating adult CNS. Successful regeneration is most closely correlated with the induction of integrins on the surface of peripheral neurons. This suggests that in order to achieve optimal neurite regrowth in the injured adult CNS, therapeutic strategies must include approaches that increase the number of integrins and other key receptors in damaged central neurons, as well as provide the appropriate growth-promoting peptides in a "regeneration cocktail." In this review, we describe the ability of peptides derived from tenascin- C, fibronectin, and laminin-1 to influence neuronal growth. In addition, we also discuss the implications of peptide/receptor interactions for strategies to improve neuronal regeneration.

Fibronectin domains of extracellular matrix molecule tenascin-C modulate hippocampal learning and synaptic plasticity

The extracellular matrix molecule tenascin-C (TN-C) has been shown to be involved in hippocampal synaptic plasticity in vitro. Here, we describe a deficit in hippocampus-dependent contextual memory in TN-C-deficient mice using the step-down avoidance paradigm. We further show that a fragment of TN-C containing the fibronectin type-III repeats 6-8 (FN6-8), but not a fragment containing repeats 3-5, bound to pyramidal and granule cell somata in the hippocampal formation of C57BL/6J mice and repelled axons of pyramidal neurons when presented as a border in vitro. Injection of the FN6-8 fragment into the hippocampus inhibited retention of memory in the step-down paradigm and reduced levels of long-term potentiation in the CA1 region of the hippocampus. In summary, our data show that TN-C is involved in hippocampus-dependent contextual memory and synaptic plasticity and identify the FN6-8 domain as one of molecular determinants mediating these functions.

Expression of tenascin-C long isoforms is induced in the hypothalamus by FGF-1

Fibroblast growth factor (FGF)-1 modulates various brain functions, such as the hypothalamic control of feeding. In the rat, we examined the effect of intracerebroventricularly infused FGF-1 on the hypothalamic expression of tenascin-C, a selective mediator of neuron-glial interaction. In situ hybridization revealed little tenascin-C mRNA expression in control brains, but greatly increased expression in ependymal cells around the third ventricle in the FGF-1-infused rats. Reverse transcription-linked PCR analysis of hypothalamic mRNA revealed an FGF-1-induced expression not of the shortest isoform of tenascin-C, but of the long isoforms (with additional fibronectin type III-domain insertions). Quantitative analysis by real time PCR revealed that this induction was transient and dose-dependent. Specific modulation of hypothalamic neuron-glial interactions by tenascin-C may mediate FGF-1-induced feeding suppression.

Tenascin-C expression and splice variant in habu snake venom-induced glomerulonephritis

Extracellular matrix glycoprotein tenascin-C (TNC) expression is up-regulated in tissue remodeling processes such as tumorigenesis and wound healing. Mouse tenascin-C contains six alternatively spliced domains (A1, A2, A4, B, C, and D) between the fifth and the sixth type III fibronectin domains, which generate large numbers of TNC isoforms. To study TNC isoform variability of wound healing in mice, we induced glomerulonephritis by using Habu snake venom (HSV) and examined alternatively spliced regions by the reverse transcription polymerase chain reaction (RT-PCR) technique. RT-PCR products were separated into seven bands in both healthy and diseased kidneys. Among the seven bands, those containing one or five alternatively spliced domains were mainly up-regulated from 2 days to 1 week after HSV injection. Southern blotting revealed that only domain-D detected all six bands in both healthy and diseased kidneys. Furthermore, only the domain-C transcriptional level did not show an obvious change in progress following HSV injection. These results suggested that (a) the isoforms containing one or five alternatively spliced domains play important roles in the healing process of glomerulonephritis, (b) domain-D is particularly significant in kidney, and (c) domain-C may not play an important role in the healing process of HSV-induced glomerulonephritis.

Building a bridge: engineering spinal cord repair

Injuries to the spinal cord that result in disruption of axonal continuity have devastating consequences for injured patients. Current therapies that use biologically active agents to promote neuronal survival and/or growth have had modest success in allowing injured neurons to regrow through the area of the lesion. Strategies for successful regeneration will require an engineering approach. We propose the design of cell-free grafts of biocompatible materials to build a bridge across the injured area through which axons can regenerate. There are three critical regions of this bridge: the on-ramp, the surface of the bridge itself, and the off-ramp. Each of these regions has specific design requirements, which, if met, can promote regeneration of axons in the injured spinal cord. These requirements, and proposed solutions, are discussed.

Abnormal reinnervation of skeletal muscle in a tenascin-C-deficient mouse

The possible involvement of tenascin-C in the reinnervation of a skeletal muscle was investigated in the tenascin-C-deficient mouse (T-/-) produced by Saga et al. (1992; Genes Dev 6:1821-1831). The pattern of reinnervation, observed after denervation of the triangularis sterni muscle, differs in T-/- and wild-type muscles in several traits. Axonal growth and stability of terminal arbors are impaired in the T-/- muscle: Some axons in mutant muscles grow beyond their original targets and reinnervate other synaptic sites, which may become dually innervated. In contrast to wild type, polyinnervation increases with time after denervation in T-/- muscles and is still present 7 months after nerve crush. The expression of a tenascin-C mRNA product disappears between 1 and 2 months after nerve crush. Of interest is that this transcriptional regulation in T-/- muscles occurs when major alterations in the morphology of regenerating endings become obvious. These observations strongly implicate tenascin-C in the formation, maturation, and stabilization of the neuromuscular junction.

Increased expression of tenascin-C-binding epithelial integrins in human bullous keratopathy corneas

We previously found an abnormal deposition of an extracellular matrix glycoprotein, tenascin-C (TN-C), in human corneas with pseudophakic/aphakic bullous keratopathy (PBK/ABK). In this work, we studied cellular TN-C receptors in normal and PBK/ABK corneas. Cryostat sections of normal and PBK/ABK corneas were stained by immuno-fluorescence for TN-C receptors: alpha2, alpha8, alpha9, alphaVbeta3, beta1, and beta6 integrins, and annexin II. Beta6 integrin mRNA levels were assessed by semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) using beta2-microglobulin gene to normalize the samples. In PBK/ABK compared to normal corneas, relatively minor changes were observed for alpha2 and beta1 integrins, and for annexin II. Alpha8, alpha9, and beta6 subunits of TN-C receptors, alpha8beta1 alpha9beta1, and alphaVbeta6, respectively, were absent from normal central corneas but were found in the central epithelium of PBK/ABK corneas. Beta6 integrin showed the most significant accumulation. It correlated best with the expression of TN-C rather than with the expression of other alphaVbeta6 ligands, fibronectin, and vitronectin. RT-PCR analysis also showed elevated levels of beta6 mRNA in PBK/ABK compared to normal corneas. Therefore, accumulation of TN-C in PBK/ABK corneas was accompanied by an increased expression of its three binding integrins, especially alphaVbeta6 in the corneal epithelium. The interaction of tenascin-C with these integrins may contribute to the fibrotic process that occurs in PBK/ABK corneas.

Identification of a neurite outgrowth-promoting motif within the alternatively spliced region of human tenascin-C

Our work centers on understanding how the extracellular matrix molecule tenascin-C regulates neuronal growth. We have found that the region of tenascin-C containing only alternately spliced fibronectin type-III repeat D, called fnD, when used by itself, dramatically increases neurite outgrowth in culture. We used overlapping synthetic peptides to localize the neurite outgrowth-promoting site within fnD to a 15 amino acid sequence, called D5. An antibody against D5 blocked promotion of neurite outgrowth by fnD as well as tenascin-C, indicating that this peptide sequence is functional in the context of the native molecule. Further testing of shorter synthetic peptides restricted the neurite outgrowth-promoting site to eight amino acids, VFDNFVLK. Of these, "FD" and "FV" are conserved in tenascin-C sequences derived from all the species available in the GenBank. To investigate the hypothesis that FD and FV are critical for the interaction with neurons, we tested a recombinant fnD protein and synthetic peptides with alterations in FD and/or FV. These molecules did not facilitate process extension, suggesting that the conserved amino acids are required for formation of the active site in fnD. We next investigated whether VFDNFVLK could be used as a reagent to overcome the neurite outgrowth inhibitory properties of chondroitin sulfate proteoglycans, the major inhibitory molecules in the glial scar. The peptide significantly enhanced outgrowth on proteoglycans and was more effective than laminin-1, L1-Fc, or intact tenascin-C, thus demonstrating the potential applicability of tenascin-C regions as therapeutic reagents.

Protein kinase C mediates neurite guidance at an astrocyte boundary

During development, astrocytes play an active role in directing axons to their final targets. This guidance has been attributed in part to the increased expression of guidance molecules, such as tenascin-C and chondroitin sulfate proteoglycans, by boundary-forming astrocytes. We have previously used a culture model of astrocyte boundaries to demonstrate that neurites growing on permissive astrocytes alter their trajectory as they encounter less-permissive astrocytes. The present study investigated the role of the protein kinase C (PKC) family of signal transduction molecules in this form of axonal guidance. Neurons were plated onto mixed astrocyte monolayers in the presence of agents that either downregulate the phorbol ester-sensitive PKC isoforms or inhibit PKC. Both downregulation and inhibition of PKC increased the percentage of neurons that crossed onto the nonpermissive astrocytes. On astrocyte monolayers, phorbol ester modulation of PKC but not PKC inhibitors resulted in a decrease in overall neurite extension. PKC inhibitors also caused a similar alteration in the neuronal response to cell-free boundaries, at concentrations that did not inhibit neurite extension. Thus, phorbol-ester-sensitive PKC isoforms direct the guidance of neurites by astrocyte-derived matrix molecules.

The structure and function of tenascins in the nervous system

The tenascins are a family of large extracellular matrix glycoproteins that comprise five known members. Three of these, tenascin-C (TN-C) tenascin-R (TN-R) and tenascin-Y (TN-Y) are expressed in specific patterns during nervous system development and are down-regulated after maturation. The expression of TN-C, the best studied member of the family, persists in restricted areas of the nervous system that exhibit neuronal plasticity and is reexpressed after lesion. Numerous studies in vitro suggest specific roles for tenascins in the nervous system involving precursor cell migration, axon growth and guidance. TN-C has been shown to occur in a large number of isoform variants generated by combinatorial variation of alternatively spliced fibronectin type III (FNIII) repeats. This finding indicates that TN-C might specify neural microenvironments, a hypothesis supported by recent analysis of TN-C knockout animals, which has begun to reveal subtle nervous system dysfunctions.

Transplantation and gene therapy: combined approaches for repair of spinal cord injury

Motor and sensory functions are lost after spinal cord injury because neurons die or atrophy and axons fail to regenerate. Until fairly recently, it was believed that damaged neurons could not be replaced and injured axons could not regenerate, and, therefore, functions dependent on injured neurons could not be recovered. We now know that damaged neurons can be rescued by providing therapeutic factors or replaced by grafting. In addition, the adult CNS contains a population of precursor cells with a potential to generate new neural cells, whose numbers and composition can be modified by extrinsic factors. The pioneering studies of Aguayo demonstrated that CNS axons could regenerate in the right environment. Subsequent studies have revealed the identity of some of the inhibitory molecules in myelin and scar tissue, and we now have a better understanding of how the CNS environment can be modified to become more permissive to regeneration. Axons that regenerate must find an appropriate target, but it may not be essential to reestablish the precise topography for some functions to be restored. There are now new and promising strategies for delivery of therapeutic genes to protect neurons and to stimulate regeneration. The ability to engineer cells by gene therapy combines the therapeutic values of cell transplantation and gene delivery. These remarkable developments from many disciplines have generated a new level of optimism in the search for a cure for CNS injury and in particular spinal cord injury. In this review, the authors summarize recent progress in these strategies and some of the challenges that remain in elucidating the most efficacious protocols for rescuing injured neurons, encouraging regeneration of their axons, and promoting recovery of function.

The tenascin family of ECM glycoproteins: structure, function, and regulation during embryonic development and tissue remodeling

The determination of animal form depends on the coordination of events that lead to the morphological patterning of cells. This epigenetic view of development suggests that embryonic structures arise as a consequence of environmental influences acting on the properties of cells, rather than an unfolding of a completely genetically specified and preexisting invisible pattern. Specialized cells of developing multicellular organisms are surrounded by a complex extracellular matrix (ECM), comprised largely of different collagens, proteoglycans, and glycoproteins. This ECM is a substrate for tissue morphogenesis, lends support and flexibility to mature tissues, and acts as an epigenetic informational entity in the sense that it transduces and integrates intracellular signals via distinct cell surface receptors. Consequently, ECM-receptor interactions have a profound influence on major cellular programs including growth, differentiation, migration, and survival. In contrast to many other ECM proteins, the tenascin (TN) family of glycoproteins (TN-C, TN-R, TN-W, TN-X, and TN-Y) display highly restricted and dynamic patterns of expression in the embryo, particularly during neural development, skeletogenesis, and vasculogenesis. These molecules are reexpressed in the adult during normal processes such as wound healing, nerve regeneration, and tissue involution, and in pathological states including vascular disease, tumorigenesis, and metastasis. In concert with a multitude of associated ECM proteins and cell surface receptors that include members of the integrin family, TN proteins impart contrary cellular functions, depending on their mode of presentation (i.e., soluble or substrate-bound) and the cell types and differentiation states of the target tissues. Expression of tenascins is regulated by a variety of growth factors, cytokines, vasoactive peptides, ECM proteins, and biomechanical factors. The signals generated by these factors converge on particular combinations of cis-regulatory elements within the recently identified TN gene promoters via specific transcriptional activators or repressors. Additional complexity in regulating TN gene expression is achieved through alternative splicing, resulting in variants of TN polypeptides that exhibit different combinations of functional protein domains. In this review, we discuss some of the recent advances in TN biology that provide insights into the complex way in which the ECM is regulated and how it functions to regulate tissue morphogenesis and gene expression.