Glial cell interactions with tenascin-C: adhesion and repulsion to different tenascin-C domains is cell type related

Substrate-bound and soluble domains of tenascin-C regulate differentiation, proliferation and migration of neural stem and progenitor cells

Introduction

The lack of regenerative capacity of the central nervous system is one of the major challenges nowadays. The knowledge of guidance cues that trigger differentiation, proliferation, and migration of neural stem and progenitor cells is one key element in regenerative medicine. The extracellular matrix protein tenascin-C (Tnc) is a promising candidate to regulate cell fate due to its expression in the developing central nervous system and in the adult neural stem cell niches. Of special interest are the alternatively spliced fibronectin type III (FnIII) domains of Tnc whose combinatorial diversity could theoretically generate up to 64 isoforms in the mouse. A total of 27 isoforms have already been discovered in the developing brain, among others the domain combinations A1D, CD, and A124BCD.

Methods

In the present study, these domains as well as the combination of the constitutively expressed FnIII domains 7 and 8 (78) were expressed in Chinese hamster ovary cells as pseudo-antibodies fused to the Fc-fragment of a human immunoglobulin G antibody. The fusion proteins were presented to primary mouse neural stem/progenitor cells (NSPCs) grown as neurospheres, either as coated culture substrates or as soluble additives in vitro. The influence of the domains on the differentiation, proliferation and migration of NSPCs was analyzed.

Results

We observed that the domain combination A124BCD promoted the differentiation of neurons and oligodendrocytes, whereas the domain A1D supported astrocyte differentiation. The constitutively expressed domain 78 had a proliferation and migration stimulating impact. Moreover, most effects were seen only in one of the presentation modes but not in both, suggesting different effects of the Tnc domains in two- and three-dimensional cultures.

Discussion

This knowledge about the different effect of the Tnc domains might be used to create artificial three-dimensional environments for cell transplantation. Hydrogels spiked with Tnc-domains might represent a promising tool in regenerative medicine.

Tenascin-C fibronectin D domain is involved in the fine-tuning of glial response to CNS injury in vitro

Understanding processes that occur after injuries to the central nervous system is essential in order to gain insight into how the restoration of function can be improved. Extracellular glycoprotein tenascin-C (TnC) has numerous functions in wound healing process depending on the expression time, location, isoform and binding partners which makes it interesting to study in this context. We used an in vitro injury model, the mixed culture of cortical astrocytes and microglia, and observed that without TnC microglial cells tend to populate gap area in greater numbers and proliferate more, whereas astrocytes build up in the border region to promote faster gap closure. Alternatively spliced domain of TnC, fibronectin type III-like repeat D (FnD) strongly affected physiological properties and morphology of both astrocytes and microglia in this injury model. The rate of microglial proliferation in the injury region decreased significantly with the addition of FnD. Additionally, density of microglia also decreased, in part due to reduced proliferation, and possibly due to reduced migration and increased contact inhibition between enlarged FnD-treated cells. Overall morphology of FnD-treated microglia resembled the activated pro-inflammatory cells, and elevated expression of iNOS was in accordance with this phenotype. The effect of FnD on astrocytes was different, as it did not affect their proliferation, but stimulated migration of reactivated astrocytes into the scratched area 48 h after the lesion. Elevated expression and secretion of TNF-α and IL-1β upon FnD treatment indicated the onset of inflammation. Furthermore, on Western blots we observed increased intensity of precursor bands of β1 integrin and appearance of monomeric bands of P2Y12R after FnD treatment which substantiates and clarifies its role in cellular shape and motility changes. Our results show versatile functions of TnC and in particular FnD after injury, mostly contributing to ongoing inflammation in the injury region. Based on our findings, FnD might be instrumental in limiting immune cell infiltration, and promoting astrocyte migration within the injury region, thus influencing spaciotemporal organization of the wound and surrounding area.

In vitro study of novel gadolinium-loaded liposomes guided by GBI-10 aptamer for promising tumor targeting and tumor diagnosis by magnetic resonance imaging

Novel gadolinium-loaded liposomes guided by GBI-10 aptamer were developed and evaluated in vitro to enhance magnetic resonance imaging (MRI) diagnosis of tumor. Nontargeted gadolinium-loaded liposomes were achieved by incorporating amphipathic material, Gd (III) [N,N-bis-stearylamidomethyl-N'-amidomethyl] diethylenetriamine tetraacetic acid, into the liposome membrane using lipid film hydration method. GBI-10, as the targeting ligand, was then conjugated onto the liposome surface to get GBI-10-targeted gadolinium-loaded liposomes (GTLs). Both nontargeted gadolinium-loaded liposomes and GTLs displayed good dispersion stability, optimal size, and zeta potential for tumor targeting, as well as favorable imaging properties with enhanced relaxivity compared with a commercial MRI contrast agent (CA), gadopentetate dimeglumine. The use of GBI-10 aptamer in this liposomal system was intended to result in increased accumulation of gadolinium at the periphery of C6 glioma cells, where the targeting extracellular matrix protein tenascin-C is overexpressed. Increased cellular binding of GTLs to C6 cells was confirmed by confocal microscopy, flow cytometry, and MRI, demonstrating the promise of this novel delivery system as a carrier of MRI contrast agent for the diagnosis of tumor. These studies provide a new strategy furthering the development of nanomedicine for both diagnosis and therapy of tumor.

Peptidyl arginine deiminase: A novel immunohistochemical marker for liver fibrosis in patients with chronic hepatitis

Peptidylarginine deiminase (PAD) is an enzyme known to be involved in the pathogenesis of rheumatoid arthritis (RA). Since many of the molecular events present in the joints in RA also take place in the injured liver, we postulated in this study that PAD may be involved in liver fibrosis. The objectives of this study therefore were to find out if PAD could be demonstrated immunohistochemically in liver biopsies of patients with chronic hepatitis and if it is associated with METAVIR activity and fibrosis scores. Liver biopsies were obtained from 100 patients with chronic liver diseases between September 2006 and 2007. The biopsies were scored by two histopathologists according to the METAVIR activity and fibrosis scores after histological preparation. Immunohistochemistry for PAD was performed on the biopsies using a monoclonal antibody against PAD. PAD could not be demonstrated in normal liver biopsies but was found in the hepatocytes of patients with chronic hepatitis. PAD labeling could distinguish patients with no fibrosis from either F1 or F2 or F3 or F4 fibrosis. Similarly, PAD labeling could separate patients with no inflammatory activity from those with mild or moderate or severe activity. We concluded that PAD could be demonstrated immunohistochemically in liver biopsies of patients with chronic hepatitis and that its immunodetection was significantly associated with Metavir activity and fibrosis scores.

Integrin activation or alpha 9 expression allows retinal pigmented epithelial cell adhesion on Bruch's membrane in wet age-related macular degeneration

Retinal pigment epithelial cell malfunction is a causative feature of age-related macular degeneration, and transplantation of new retinal pigment epithelial cells is an attractive strategy to prevent further progression and visual loss. However, transplants have shown limited efficacy, mainly because transplanted cells fail to adhere and migrate onto pathological Bruch's membrane. Adhesion to Bruch's membrane is integrin-mediated. Ageing of Bruch's membrane leads to a decline in integrin ligands and, added to this, wet age-related macular degeneration leads to upregulation of anti-adhesive molecules such as tenascin-C. We have therefore investigated whether manipulation of integrin function in retinal pigment epithelial cells can restore their adhesion and migration on wet age-related macular degeneration-damaged Bruch's membrane. Using spontaneously immortalized human retinal pigment epithelial cells (adult retinal pigment epithelium-19), we show that adhesion and migration on the Bruch's membrane components is integrin-dependent and enhanced by integrin-activating agents manganese and TS2/16. These allowed cells to adhere and migrate on low concentrations of ligand, as would be found in aged Bruch's membrane. We next developed a method for stripping cells from Bruch's membrane so that adhesion and migration assays can be performed on its surface. Integrin activation had a moderate effect on enhancing retinal pigmented epithelial cell adhesion and migration on normal human and rat Bruch's membrane. However, on Bruch's membrane prepared from human wet age-related macular degeneration-affected eyes, adhesion was lower and integrin activation had a much greater effect. A candidate molecule for preventing retinal pigmented epithelial interaction with age-related macular degeneration-affected Bruch's membrane is tenascin-C which we confirm is present at high levels in wet age-related macular degeneration membrane. We show that tenascin-C is anti-adhesive for retinal pigmented epithelial cells, but after integrin activation, they can adhere and migrate on it using alphaVbeta3 integrin. Alternatively, we find that transduction of retinal pigmented epithelial cells with alpha9 integrin, a tenascin-C-binding integrin, led to a large increase in alpha9beta1-mediated adhesion and migration on tenascin-C. Both expression of alpha9 integrin and integrin activation greatly enhanced the ability of retinal pigment epithelial cells to adhere to tenascin-rich wet age-related macular degeneration-affected Bruch's membranes. Our results suggest that manipulation of retinal pigment epithelial cell integrins through integrin activating strategies, or expression of new integrins such as alpha9, could be effective in improving the efficacy of retinal pigment epithelial cell transplantation in wet age-related macular degeneration-affected eyes.

Subtractive hybridization identifies genes differentially expressed by olfactory ensheathing cells and neural stem cells

The in vitro differentiation of embryonic stem cells into glia has received relatively limited attention to date when compared with the interest in the generation of neurons. We are interested in a particular glial phenotype, the aldynoglia, and their differentiation from multipotential neural precursors (MNP), since this type of glia can promote neuronal regeneration. We constructed cDNA libraries from cultures of purified olfactory ensheathing cells (OEC), an aldynoglia cell type, and MNP to perform subtractive hybridization. As a result, we isolated four genes from the OEC: one tenascin C (Tn-C) isoform, Insulin-like growth factor binding protein 5 (Igfbp-5), cytochrome oxidase subunit I (COX1) and a phosphodiesterase for cyclic nucleotides (CNPase). With the exception of CNPase, these genes are expressed more strongly in the OEC than in the MNP and moreover, the expression of all four is induced when MNP were exposed to OEC conditioned media. The data suggest a role for these genes in MNP differentiation, and their products appear to represent characteristic proteins of the aldynoglia phenotype.

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.

Increased citrullination of histone H3 in multiple sclerosis brain and animal models of demyelination: a role for tumor necrosis factor-induced peptidylarginine deiminase 4 translocation

Modification of arginine residues by citrullination is catalyzed by peptidylarginine deiminases (PADs), of which five are known, generating irreversible protein structural modifications. We have shown previously that enhanced citrullination of myelin basic protein contributed to destabilization of the myelin membrane in the CNS of multiple sclerosis (MS) patients. We now report increased citrullination of nucleosomal histones by PAD4 in normal-appearing white matter (NAWM) of MS patients and in animal models of demyelination. Histone citrullination was attributable to increased levels and activity of nuclear PAD4. PAD4 translocation into the nucleus was attributable to elevated tumor necrosis factor-alpha (TNF-alpha) protein. The elevated TNF-alpha in MS NAWM was not associated with CD3+ or CD8+ lymphocytes, nor was it associated with CD68+ microglia/macrophages. GFAP, a measure of astrocytosis, was the only cytological marker that was consistently elevated in the MS NAWM, suggesting that TNF-alpha may have been derived from astrocytes. In cell cultures of mouse and human oligodendroglial cell lines, PAD4 was predominantly cytosolic but TNF-alpha treatment induced its nuclear translocation. To address the involvement of TNF-alpha in targeting PAD4 to the nucleus, we found that transgenic mice overexpressing TNF-alpha also had increased levels of citrullinated histones and elevated nuclear PAD4 before demyelination. In conclusion, high citrullination of histones consequent to PAD4 nuclear translocation is part of the process that leads to irreversible changes in oligodendrocytes and may contribute to apoptosis of oligodendrocytes in MS.

A mechanism of impaired mobility of oligodendrocyte progenitor cells by tenascin C through modification of wnt signaling

In demyelinating diseases, the mechanisms of how oligodendrocyte (OLG) progenitor cells are affected in the demyelinated area remain to be elucidated. To investigate one aspect of the mechanisms, we focused on the role of tenascin C in regulating the migratory mobility of the progenitor cells via beta-catenin. By cDNA subtraction screening, we found tenascin C expression to be increased in OLG progenitors (rat primary O2A cells). Tenascin C inhibited the migration of OLG progenitors and CG-4 cells, and beta-catenin accumulated at focal adhesions in these cells. These changes were associated with the inactivation of canonical wnt signaling. Overexpression of the wnt-signaling antagonist Dapper prevented the migration of CG-4 cells. This suggests that inactivation of the wnt signal is responsible for impaired migration of OLG caused by tenascin C. Our results suggest that tenascin C is involved in the impaired mobility of OLG progenitor cells through increased amounts of adhesion complex as well as the prevention of wnt signaling.

Dynamic neuronal-glial interactions: an overview 20 years later

After commenting on some perceived reasons why our review may have been relatively frequently cited, a brief overview is presented that first summarizes what we knew 25 years ago about the dynamic neuronal-astroglial interactions that occur in response to changes in the physiological state of the animal. The brain system in which these dynamic interactions were studied was the magnocellular hypothalamo-neurohypophysial system (mHNS) of the rat. The mHNS developed as and continues to be the model system yielding the most coherent picture of dynamic morphological changes and insights into their functional consequences. Many other brain areas, however, have more recently come under scrutiny in the search for glial-neuronal dynamisms. Outlined next are some of the questions concerning this phenomenon that led to the research efforts immediately following the initial discoveries, along with the answers, both complete and incomplete, obtained to those research questions. The basis for this first wave of follow-up research can be characterized by the phrase "what we knew we didn't know at that time." The final section is an update and brief overview of highlights of both "what we know now" and "what we now know that we don't know" about dynamic neuronal-astroglial interactions in the mHNS.

Tenascin-C expression and distribution in cultured human chondrocytes and chondrosarcoma cells

Tenascin-C (TNC) is an oligomeric glycoprotein of the extracellular matrix with several distinct isoforms variably expressed during embryogenesis, tumorogenesis, angiogenesis and wound healing. In the normal human adult, TNC is found in large concentrations in articular cartilage, suggesting tissue-specific function. The purpose of this study was to determine the specific in vitro TNC splicing patterns of articular chondrocytes and a human chondrosarcoma cell line. Cells were cultured in a three-dimensional bead system and TNC splice variant expression and distribution were examined with the use of Western blotting techniques, semi-quantitative reverse-transcription polymerase chain reaction and immunohistochemistry. At both the transcriptional and post-translational levels, the chondrocytes were found to express significantly higher levels of the smaller 220 kDa isoform (P < 0.01), which was predominantly incorporated into the matrix. The splicing pattern of the malignant cells was characterized by a higher proportion of the larger 320 kDa isoform which was extruded into the media. In vivo studies are necessary to verify the expression of the large TNC isoform in chondrosarcoma and the production and integration of the smaller isoform in normal chondroid matrix. In addition, elucidation of the biologic functions of the two major TNC isoforms may lead to the development of novel diagnostic and therapeutic approaches to chondrosarcoma.

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.

Clinical impact and functional aspects of tenascin-C expression during glioma progression

The extracellular matrix protein tenascin-C is expressed in processes like embryogenesis and wound healing and in neoplasia. Tenascin-C expression in gliomas has been described previously; however, the relation to clinical data remains inconsistent. Generally, analysis of tenascin-C function is difficult due to different alternatively spliced isoforms. Our studies focus on changes in tenascin-C expression in human gliomas, correlating these changes with tumor progression and elucidating the functional role of the glioma cell-specific tenascin-C isoform pool. Eighty-six glioma tissues of different World Health Organization (WHO) grades were analyzed immunohistochemically for tenascin-C expression. The influence of the specific tenascin-C isoforms produced by glioblastoma cells on proliferation and migration was examined in vitro using blocking antibodies recognizing all isoforms. In general, tenascin-C expression increased with tumor malignancy. Perivascular staining of tenascin-C around tumor-supplying blood vessels was observed in all glioblastoma tissues, whereas in WHO II and III gliomas, perivascular tenascin-C staining appeared less frequently. The appearance of perivascular tenascin-C correlated significantly with a shorter disease-free time. Analysis of proliferation and migration in the presence of blocking antibodies revealed an inhibition of proliferation by around 30% in all 3 glioblastoma cell cultures, as well as a decrease in migration of 30.6-46.7%. Thus we conclude that the endogenous pool of tenascin-C isoforms in gliomas supports both tumor cell proliferation and tumor cell migration. In addition, our data on the perivascular staining of tenascin-C in WHO II and III gliomas and its correlation with a shorter disease-free time suggest that tenascin-C may be a new and potent prognostic marker for an earlier tumor recurrence.

Glial tumor cell adhesion is mediated by binding of the FNIII domain of receptor protein tyrosine phosphatase beta (RPTPbeta) to tenascin C

The extracellular domain of receptor protein tyrosine phosphatase beta (RPTPbeta) is composed of several domains which mediate its interactions with distinct ligands present on the surface of either neurons or glial cells. Here, we demonstrate that the fibronectin type III domain (FNIII) of RPTPbeta binds to glial tumor-derived cell lines and primary astrocytes. We used affinity purification to isolate several proteins that specifically bind to the FNIII domain of RPTPbeta. One of these, a 240 kDa protein that was purified from U118MG glioblastoma cell, was identified as tenascin C based on the amino acid sequence of several tryptic peptides. The interaction of RPTPbeta with tenascin C was found to mediate cell adhesion. Adhesion and spreading of SF763T astrocytoma cells expressing RPTPbeta on tenascin C was specifically abolished by the addition of a soluble fragment containing the FNIII domain of the receptor. RPTPbeta-dependent cell adhesion was mediated by binding to the alternatively spliced FNIII repeats A1,2,4 (TnfnA1,2,4) of tenascin C. Furthermore, COS cells expressing RPTPbeta adhere to TnfnA1,2,4, while the parental cells did not. These results demonstrate that the FNIII domain of RPTPbeta binds to tenascin C and suggest that RPTPbeta present on glial tumor cells is a primary adhesion receptor system to the extracellular matrix.

Interactions between Candida albicans and the human extracellular matrix component tenascin-C

Tenascins are large multimeric proteins that contain repeated structural motifs that include epidermal growth factor (EGF)-like repeats, fibronectin type III repeats and a globular fibrinogen-like domain, and are involved in tissue and organ morphogenesis, as well as in adhesion and migration of cells. C. albicans germ-tubes, but not blastospores, were able to bind to soluble human tenascin-C as revealed by an indirect immunofluorescence assay. However, materials present in cell wall extracts from both morphologies attached to tenascin-C immobilized in wells of a microtiter plate. The binding specificity was demonstrated by the inhibitory effect of antibodies against C. albicans cell wall components and an anti-tenascin antibody, but not anti-laminin antibody. Fibronectin, but not fibrinogen, inhibited binding, thus indicating a role of the fibronectin type III repeats in the interaction between the fungus and tenascin-C. Binding of C. albicans cell wall materials to tenascin was RGD- and divalent cation-independent.

GDNF family members and their receptors: expression and functions in two oligodendroglial cell lines representing distinct stages of oligodendroglial development

Glial cell line-derived neurotrophic factor (GDNF), neurturin (NTN), and persephin (PSP) constitute a subfamily of transforming growth factor-betas (TGF-betas) with prominent roles in the regulation of neuron survival and differentiation. Although numerous members of the TGF-beta superfamily are important regulators of glial cell functions in health and disease, it is unknown whether any member of the GDNF subfamily may have functions in normal or pathological glial cell performances. To begin to address this issue, we have studied expression and putative functions of GDNF, NTN, PSP, and their receptors in two cell lines representing models for oligodendrocyte progenitor cells (OLI-neu) and immature oligodendrocytes (OLN-93), respectively. RT-PCR analysis revealed expression of all three growth factor mRNAs in OLI-neu and OLN-93 cells. Expression was weak in OLI-neu cells, while both NTN and PSP mRNAs were strongly expressed in OLN-93 cells. Furthermore, OLI-neu and OLN-93 cells expressed transcripts encoding the GDNF receptors Ret and GFRalpha-1. The two splice variants for GFRalpha-2 were exclusively synthesized in OLI-neu cells. Similarly, primary O-2A progenitor cells and enriched mature oligodendrocytes expressed Ret, GFRalpha-1 and GFRalpha-2 mRNAs. Both GDNF and NTN stimulated DNA synthesis monitored by BrdU incorporation of OLI-neu cells in a dose-dependent fashion. Co-administration of TGF-beta significantly reduced this effect. Similarly, PDGF co-applied with GDNF or NTN down-regulated proliferation in OLI-neu cells. In contrast, OLN-93 cells did not respond to GDNF or NTN with increased incorporation of BrdU. Expression of GDNF, NTN, and their receptors and distinct effects in two model cell lines of oligodendrocyte development suggest that functions of members of the GDNF family and their receptors may not be restricted to neurons and may be implicated in oligodendrocyte development.

Molecular tunnels and boundaries for growing axons in the anterior commissure of hamster embryos

We have analyzed the immunohistochemical expression of chondroitin sulfate proteoglycan (CSPG), fibronectin (FN), laminin (LN), tenascin (TN), and glial fibrillary acidic protein (GFAP) along the anterior commissure (AC) of hamster embryos (n=175; from embryonic day (E)12 to E16). Frozen sections were cut at different planes from embryonic brains between E12 and E16, treated for immunohistochemistry, and observed under epifluorescence microscopy. During the pre-crossing stage (E12-E13), CSPG was expressed as a sagittal stratum between the interhemispheric fissure and the prospective AC region. TN appeared rostral to the third ventricle and along the medial subventricular zone of the lateral ventricles. LN and FN both presented a faint expression, and GFAP was not detected. Although AC axons started crossing the midline region (E13.5-E14), CSPG, FN, LN, and, much less intensely, GFAP circumscribed the AC bundle, forming a tunnel through which AC fibers elongate. TN was no longer seen at the midplane but remained visible laterally. During the post-crossing stage (E14.5-E16), CSPG and TN were no longer seen at the midline, although both could be observed between the AC limbs, seeming to form boundaries for AC lateral growth. LN and FN were then absent near the AC bundle. During this late stage, GFAP expression became most intense, forming a distinct tunnel around the AC. We have shown that the expression of extracellular matrix molecules and GFAP follow a time- and space-regulated course related to AC development, plausibly representing influential factors for growth and guidance of commissural fibers.

The expression of tenascin-C along the lamprey olfactory pathway during embryonic development and following axotomy-induced replacement of the olfactory receptor neurons

Extracellular guidance molecules affect the pathway of growing axons by both attractive and repulsive interactions. Tenascin-C, a glycoprotein of the extracellular matrix, is localized along developing axonal pathways where it may function by repulsion, restricting axons within specific boundaries. The lamprey olfactory pathway offers an advantageous model for studying the role of extracellular matrix proteins in axon guidance because the entire pathway is readily seen in horizontal sections and because lesioning the olfactory nerve will induce the system into a new phase of coordinated neurogenesis and axon outgrowth. Although tenascin-C expression was absent during embryonic development, olfactory nerve fascicles contained tenascin-C-immunoreactivity (IR) during the larval stage. During retrograde degeneration, the fascicles lost tenascin-C-IR. Diffuse unfasciculated axonal processes extending from the olfactory epithelium did not express tenascin-C-IR; however, acetylated tubulin and GAP-43-IR was present, indicating axonal outgrowth. When the newly extended axons of olfactory receptor neurons converged to form fascicles, tenascin-C-IR was evident within the fascicular boundaries. The absence of tenascin-C expression when axonal process were short and diffuse, and its return when axons coalesced within fascicles, supports the view that tenascin-C functions as a boundary molecule in the olfactory pathway.

Developmental expression of the tenascin-C is altered by hypothyroidism in the rat brain

Tenascin-C is an extracellular matrix glycoprotein involved in cell adhesion and migration, and neurite outgrowth. Since these processes have been found to be under thyroid control in the developing rat brain, we have investigated the effect of congenital hypothyroidism on tenascin-C expression. At birth, in situ hybridization studies in hypothyroid rats show an abnormal up-regulation of tenascin-C in some areas (caudate-putamen, geniculate nuclei, ependymal epithelium of the lateral ventricles, hippocampus) and down-regulation in others (occipital and retrosplenial cortex, subiculum). With subsequent development, hypothyroid animals show higher tenascin-C expression also in the upper layers of the cerebral cortex and subplate, and the Bergmann glia of the cerebellum. Significantly, thyroxine treatment of hypothyroid rats led to normalization of tenascin-C levels in most areas. In agreement with the messenger RNA data, hypothyroid rats contain an uniformly higher level of immunoreactive tenascin-C protein throughout the brain, particularly in the cerebellum. Suggesting a direct cellular effect, thyroid hormone also decreases tenascin-C expression in two glial cell lines (C6, B3.1) expressing thyroid receptors. Our results show that congenital hypothyroidism causes specific alterations in the pattern of tenascin-C expression in the rat brain which may at least partially be responsible for some of the developmental disturbances observed in this syndrome.

Pervasive conformational fluctuations on microsecond time scales in a fibronectin type III domain

A novel off-resonance rotating-frame 15N NMR spin relaxation experiment is used to characterize conformational fluctuations with correlation times between 32 and 175 microseconds in the third fibronectin type III domain of human tenascin-C. Conformational fluctuations of contiguous regions of the beta-sandwich structure of the type III domain may represent collective motions, such as transient twisting or breathing of the beta-sheets. Flexibility of the loop containing the Arg-Gly-Asp (RGD) tripeptide may affect the accessibility of this motif in protein-protein interactions.

Mapping of a defined neurocan binding site to distinct domains of tenascin-C

Neurocan is a member of the aggrecan family of proteoglycans which are characterized by NH2-terminal domains binding hyaluronan, and COOH-terminal domains containing C-type lectin-like modules. To detect and enhance the affinity for complementary ligands of neurocan, the COOH-terminal neurocan domain was fused with the NH2-terminal region of tenascin-C, which contains the hexamerization domain of this extracellular matrix glycoprotein. The fusion protein was designed to contain the last downstream glycosaminoglycan attachment site and was expressed as a proteoglycan. In ligand overlay blots carried out with brain extracts, it recognized tenascin-C. The interaction was abolished by the addition of EDTA, or TNfn4,5, a bacterially expressed tenascin-C fragment comprising the fourth and fifth fibronectin type III module. The fusion protein directly reacted with this fragment in ligand blot and enzyme-linked immunosorbent assay procedures. Both tenascin-C and TNfn4,5 were retained on Sepharose 4B-linked carboxyl-terminal neurocan domains, which in BIAcore binding studies yielded a KD value of 17 nM for purified tenascin-C. We conclude that a divalent cation-dependent interaction between the COOH-terminal domain of neurocan and those fibronectin type III repeats is substantially involved in the binding of neurocan to tenascin-C.