Tenascins

Tenascins are a family of large multimeric extracellular matrix (ECM) proteins. Vertebrates express four tenascins termed tenascin-C, -R, -X and -W present in their connective tissues. Each tenascin has a specific expression pattern. To the contrary of many other ECM proteins, tenascins promote only weak cell adhesion and do not activate cell spreading. They have been classified as anti-adhesive, adhesion-modulating or even repellent ECM proteins. Tenascin-C and tenascin-R deficient mice show abnormalities in the nervous system and tenascin-C deficient mice, in addition, have defects in several regenerative processes. Mice lacking tenascin-X display hyperelastic skin much like Ehlers Danlos patients with mutations in their tenascin-X gene. Since tenascin-C is highly overexpressed in tumor stroma antibodies against tenascin-C have been used in tumor diagnosis and therapy. Since tenascins are known to influence cell shape, migration and growth they represent good candidate molecules for inclusion in artificial bioengineered tissue implants.     

The evolution of tenascins and fibronectin

Tenascins are extracellular matrix glycoproteins that act both as integrin ligands and as modifiers of fibronectin-integrin interactions to regulate cell adhesion, migration, proliferation and differentiation. In tetrapods, both tenascins and fibronectin bind to integrins via RGD and LDV-type tripeptide motifs found in exposed loops in their fibronectin-type III domains. We previously showed that tenascins appeared early in the chordate lineage and are represented by single genes in extant cephalochordates and tunicates. Here we have examined the genomes of the coelacanth Latimeria chalumnae, the elephant shark Callorhinchus milii as well as the lampreys Petromyzon marinus and Lethenteron japonicum to learn more about the evolution of the tenascin gene family as well as the timing of the appearance of fibronectin during chordate evolution. The coelacanth has 4 tenascins that are more similar to tetrapod tenascins than are tenascins from ray-finned fishes. In contrast, only 2 tenascins were identified in the elephant shark and the Japanese lamprey L. japonicum. An RGD motif exposed to integrin binding is observed in tenascins from many, but not all, classes of chordates. Tetrapods that lack this RGD motif in tenascin-C have a similar motif in the paralog tenascin-W, suggesting the potential for some overlapping function. A predicted fibronectin with the same domain organization as the fibronectin from tetrapods is found in the sea lamprey P. marinus but not in tunicates, leading us to infer that fibronectin first appeared in vertebrates. The motifs that recognize LDV-type integrin receptors are conserved in fibronectins from a broad spectrum of vertebrates, but the RGD integrin-binding motif may have evolved in gnathostomes.     

Regulation of tenascin expression in bone

Tenascins regulate cell interaction with the surrounding pericellular matrix. Within bone, tenascins C and W influence osteoblast adhesion and differentiation, although little is known about the regulation of tenascin expression. In this study we examined the effect of osteogenic differentiation, bone morphogenetic protein (BMP) and Wnt growth factors, and mechanical loading on tenascin expression in osteogenic cells. Osteogenic differentiation increased tenascin C (TnC), and decreased tenascin W (TnW), expression. Both growth factors and mechanical loading increased both TnC and TnW expression, albeit via distinct signaling mechanisms. Both BMP-2 and Wnt5a induction of tenascin expression were mediated by MAP kinases. These data establish a role for BMP, Wnts, and mechanical loading in the regulation of tenascin expression in osteoblasts.     

The complexity in regulating the expression of tenascins

The tenascins are a growing family of extracellular matrix proteins of typical multidomain structure. The prototype to be discovered was tenascin-C. It shows a highly regulated expression pattern during embryonic development and is often transiently associated with morphogenetic tissue interactions during organogenesis. In the adult organism reexpression of tenascin-C occurs in tumors and many other pathological conditions. Tenascin-C expression can be regulated by many different growth factors and hormones. Furthermore, mechanical strain exerted by fibroblasts seems to induce the expression of tenascin-C. This could represent a mechanism of translating mechanical forces into protein patterns, a step of potential relevance in the organization of embryogenesis. Tenascin-C as well as tenascin-R are believed to counteract the cell adhesion and spreading activity of fibronectin, thereby facilitating cell movement.     

Connective tissues: signalling by tenascins

Different connective tissue cells secrete different types of tenascins. These glycoproteins contribute to extracellular matrix (ECM) structure and influence the physiology of the cells in contact with the tenascin containing environment. Tenascin-C expression is regulated by mechanical stress. It shows highest expression in connective tissue surrounding tumors, in wounds and in inflamed tissues where it may regulate cell morphology, growth, and migration by activating diverse intracellular signalling pathways. Thus, integrin and syndecan signalling is influenced by tenascin-C and the levels and/or activies of several proteins involved in intracellular signalling pathways are regulated by its presence. Tenascin-X is important for the proper deposition of collagen fibers in dermis and patients with a tenascin-X deficiency suffer from Ehlers Danlos syndrome. Tenascin-R (and -C) is prominent in the nervous system and has an impact on neurite outgrowth and synaptic functions, and tenascin-W is found in the extracellular matrix of bone, muscle, and kidney. Cell facts:bone: osteoblasts produce tenascin-C, -W cartilage: perichondrial cells produce tenascin-C tendon: fibroblasts produce tenascin-C smooth muscle cells produce tenascin-W, -C skeletal muscle: endo-, peri-, and epimysial fibroblasts produce tenascin-X dermal fibroblasts produce tenascin-X tumors: stromal fibroblasts produce tenascin-C wounds: fibroblasts produce tenascin-C nervous system: glial cells produce tenascin-R, -C, -X.     

Tenascin in the developing and adult human intestine

The tenascins are a family of multifunctional extracellular matrix glycoproteins subject to complex spatial and temporal patterns of expression in the course of various organogenetic processes, namely those involving epithelial-mesenchymal interactions. In the intestine, the tenascins, in particular tenascin-C, have been found to be differentially expressed in the developing and adult small intestinal and colonic mucosa as well as in neoplasm. While tenascin-C emerges as a key player likely to be involved in intestinal mucosa development, maintenance and disease, its exact role in the regulation of fundamental intestinal cell function(s) such as proliferation, migration and tissue-specific gene expression remains however to be established.     

Tenascins and the Importance of Adhesion Modulation

Tenascins are a family of extracellular matrix proteins that evolved in early chordates. There are four family members: tenascin-X, tenascin-R, tenascin-W, and tenascin-C. Tenascin-X associates with type I collagen, and its absence can cause Ehlers-Danlos Syndrome. In contrast, tenascin-R is concentrated in perineuronal nets. The expression of tenascin-C and tenascin-W is developmentally regulated, and both are expressed during disease (e.g., both are associated with cancer stroma and tumor blood vessels). In addition, tenascin-C is highly induced by infections and inflammation. Accordingly, the tenascin-C knockout mouse has a reduced inflammatory response. All tenascins have the potential to modify cell adhesion either directly or through interaction with fibronectin, and cell-tenascin interactions typically lead to increased cell motility. In the case of tenascin-C, there is a correlation between elevated expression and increased metastasis in several types of tumors.     

Nanomechanical Properties of Tenascin-X Revealed by Single-Molecule Force Spectroscopy

Tenascin-X is an extracellular matrix protein and binds a variety of molecules in extracellular matrix and on cell membrane. Tenascin-X plays important roles in regulating the structure and mechanical properties of connective tissues. Using single-molecule atomic force microscopy, we have investigated the mechanical properties of bovine tenascin-X in detail. Our results indicated that tenascin-X is an elastic protein and the fibronectin type III (FnIII) domains can unfold under a stretching force and refold to regain their mechanical stability upon the removal of the stretching force. All the 30 FnIII domains of tenascin-X show similar mechanical stability, mechanical unfolding kinetics, and contour length increment upon domain unfolding, despite their large sequence diversity. In contrast to the homogeneity in their mechanical unfolding behaviors, FnIII domains fold at different rates. Using the 10th FnIII domain of tenascin-X (TNXfn10) as a model system, we constructed a polyprotein chimera composed of alternating TNXfn10 and GB1 domains and used atomic force microscopy to confirm that the mechanical properties of TNXfn10 are consistent with those of the FnIII domains of tenascin-X. These results lay the foundation to further study the mechanical properties of individual FnIII domains and establish the relationship between point mutations and mechanical phenotypic effect on tenascin-X. Moreover, our results provided the opportunity to compare the mechanical properties and design of different forms of tenascins. The comparison between tenascin-X and tenascin-C revealed interesting common as well as distinguishing features for mechanical unfolding and folding of tenascin-C and tenascin-X and will open up new avenues to investigate the mechanical functions and architectural design of different forms of tenascins.     

Tenascins and inflammation in disorders of the nervous system

In vitro and in vivo studies on the role of tenascins have shown that the two paradigmatic glycoproteins of the tenascin family, tenascin-C (TnC) and tenascin-R (TnR) play important roles in cell proliferation and migration, fate determination, axonal pathfinding, myelination, and synaptic plasticity. As components of the extracellular matrix, both molecules show distinct, but also overlapping dual functions in inhibiting and promoting cell interactions depending on the cell type, developmental stage and molecular microenvironment. They are expressed by neurons and glia as well as, for TnC, by cells of the immune system. The functional relationship between neural and immune cells becomes relevant in acute and chronic nervous system disorders, in particular when the blood brain and blood peripheral nerve barriers are compromised. In this review, we will describe the functional parameters of the two molecules in cell interactions during development and, in the adult, in synaptic activity and plasticity, as well as regeneration after injury, with TnC being conducive for regeneration and TnR being inhibitory for functional recovery. Although not much is known about the role of tenascins in neuroinflammation, we will describe emerging knowledge on the interplay between neural and immune cells in autoimmune diseases, such as multiple sclerosis and polyneuropathies. We will attempt to point out the directions of experimental approaches that we envisage would help gaining insights into the complex interplay of TnC and TnR with the cells that express them in pathological conditions of nervous and immune systems.     

Integrin expression during epithelial migration and restratification in the tenascin-C-deficient mouse cornea.

In the unwounded cornea, tenascin-C localizes to a short stretch of the basement membrane zone at the corneoscleral junction or limbus. To determine whether the function of the limbus is affected by the absence of tenascin-C, mice possessing a deletion of tenascin-C and strain-matched wild-type mice are used in corneal debridement wounding experiments. The expression of integrins (alpha3, alpha9, and beta4) in the tenascin-C knockout corneas is evaluated by producing polyclonal cytoplasmic domain antipeptide sera and performing immunofluorescence microscopy. In addition, we evaluate the localization of several other proteins involved in wound healing, including fibronectin, laminin beta1, nidogen/entactin, and VCAM-1, in both the tenascin knockout and wild-type mice. There are no differences in healing rate, scarring, or neovascularization after corneal debridement wounds. alpha9 integrin is expressed at the limbal border of unwounded tenascin-C knockout animals and is upregulated during migration only after the larger wounds. At 8 weeks after larger wounds, the localization of alpha9 again becomes restricted to the limbal border. Results show that tenascin-C is not required for development or maintenance of the corneal limbus or for normal re-epithelialization of corneal epithelial cells after debridement wounding.     

Role of tenascins in the ECM of gliomas

Tenascins are a family of extracellular matrix molecules that are mainly expressed in embryonic development and down-regulated in adulthood. A re-expression in the adult occurs under pathological conditions such as inflammation, regeneration or neoplasia. As the most prominent member of the tenascin family, TN-C, is highly expressed in glioma tissue and rising evidence suggests that TN-C plays a crucial role in cell migration or invasion – the most fatal characteristics of glioma – also the other members of this protein family have been investigated with regard to their impact on glioma biology. For all tenascins correlations between the expression levels of the different family members and the degree of malignancy and invasiveness of glial tumors could be detected. Overall, the former and recent results in the research on glioma and tenascins point at distinct roles of each of the molecules in glioma biology and the devastating properties of these tumors.     

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.     

Long-term potentiation in vivo increases rat hippocampal tenascin-C expression

We investigated the expression of the extracellular matrix glycoprotein tenascin-C after induction of long-term potentiation (LTP) by high-frequency tetanization (HFT) in the rat dentate gyrus in vivo. Expression of tenascin-C was evaluated at the mRNA and protein levels by in situ hybridization and immunocytochemistry, respectively. Whereas no tenascin-C mRNA was detectable in control animals, an increase in tenascin-C mRNA levels was observed in the granule cell layer of the dentate gyrus 4 h after HFT. At 24 h after HFT, tenascin-C mRNA had returned to control levels. Expression of tenascin-C protein 4 h after HFT followed that of controls in that tenascin was detectable in the strata oriens and radiatum of CA1, in the molecular layer, and within a narrow area at the inner surface of the granule cell layer in the dentate gyrus. However, 24 h after HFT, additional patches of tenascin-C immunoreactivity were observed in the molecular layer of the dentate gyrus. No increase in tenascin mRNA or protein levels was detected in control animals that received no stimulation, low-frequency stimulation, or HFT in the presence of the N-methyl-D -aspartate receptor antagonist D (−)-2-amino-5-phosphonopentanoic acid or the metabotropic glutamate receptor antagonist (R,S)-α-methyl-4-carboxyphenylglycine. These observations implicate a role for tenascin-C in N-methyl-D -aspartate and metabotropic glutamate receptor–dependent changes accompanying induction and/or maintenance of LTP. © 1998 John Wiley & Sons, Inc. J Neurobiol 37: 393–404, 1998     

Phylogenetic analysis of the tenascin gene family: evidence of origin early in the chordate lineage

Background  

Tenascins are a family of glycoproteins found primarily in the extracellular matrix of embryos where they help to regulate cell proliferation, adhesion and migration. In order to learn more about their origins and relationships to each other, as well as to clarify the nomenclature used to describe them, the tenascin genes of the urochordate Ciona intestinalis, the pufferfish Tetraodon nigroviridis and Takifugu rubripes and the frog Xenopus tropicalis were identified and their gene organization and predicted protein products compared with the previously characterized tenascins of amniotes.     

Tenascin in the human intervertebral disc: alterations with aging and disc degeneration

Our objective for this study was to determine the presence and distribution of tenascin in the human intervertebral disc. The tenascins are a family of extracellular matrix proteins with repeated structural domains homologous to epidermal growth factor, fibronectin type III and the fibrinogens. Little is known about the presence of this protein in the disc. Ten normal human discs donated from subjects newborn to 15 years old, 10 control discs from adult donors aged 24-41 years, and 11 surgical disc specimens from patients aged 26-76 years were examined for immunolocalization of tenascin. In young discs, tenascin was localized throughout the annulus; in the nucleus, localization was confined to pericellular matrix. In adult control and degenerating disc specimens, tenascin in the annulus was localized primarily in pericellular matrix regions encircling either single cells or clusters of disc cells; in rare instances localization was more diffuse in the intraterritorial matrix. In young, healthy disc, tenascin was abundant throughout the annulus. In contrast, degenerating discs in adults showed a localization restricted to the pericellular, and rarely, more restricted intraterritorial matrix. These observations indicate that changes in the amount and distribution of tenascin may have a role in disc aging and degeneration, possibly by modulating fibronectin-disc-cell interactions, and causing alterations in the shape of disc cells.     

Tenascin-C: Its functions as an integrin ligand

This review summarizes the experimental evidence of tenascin-C/integrin interactions, emphasizing the identification of integrin binding sites and the effects of specific interactions on cell behavior. At least four integrins appear to bind to the third fibronectin-type 3 domain of tenascin-C: α9β1, αVβ3, α8β1 and αVβ6. The α9β1 integrin recognizes a highly conserved IDG motif in this domain, while the others recognize an RGD motif. There is also significant evidence that the collagen receptor α2β1 can bind to tenascin-C, but the interacting site is unknown. Tenascin-C interactions with α9β1 and αVβ3 can promote cell proliferation and interactions with αVβ3 can also inhibit apoptosis. Interactions with α7β1 integrin, which may bind to the alternatively spliced domain of tenascin-C, and α9β1 integrin are able to influence the differentiation of mesenchymal stem cells into the neuronal lineage. This illustrates the potential for using our knowledge of tenascins and their integrin receptors in stem cell-based therapies.     

Stromal expression of tenascin is inversely correlated to epithelial differentiation of hormone dependent tissues

We were previously investigating the expression of the extracellular matrix glycoprotein tenascin in normal and malignant endometrial tissues of humans and rodents. These studies suggested that the expression of tenascin was induced by proliferating epithelia (normal and particularly malignant) and was downregulated with their differentiation. The aim of this study was to investigate the hormone dependency of tenascin expression in (a) the transplantable EnDA endometrial tumor model with or without estrogen deprivation (overiectomy) of the animals, (b) DMBA-induced rat mammary tumors with or without a hormonal treatment of the animals [ovariectomy, antiestrogen (tamoxifen) or antiprogestin (ZK 98299) treatment] and (c) in the rat prostate of untreated or androgen deprived animals (orchiectomy, flutamide-, casodex- or cyproterone acetate (CPA)- treatment). 1. Estrogen withdrawal by ovariectomy did not affect tenascin expression in transplantable EnDA endometrial adenocarcinoma, meaning the entire extracellular space of the stromal mesenchyme was decorated by tenascin immunoreactivity. 2. In untreated DMBA-induced rat mammary tumors almost the entire extracellular space of the stroma was stained by tenascin immunoreactivity. Ovariectomy and antiestrogen treatment did not affect tenascin expression. In contrast, antiprogestin treatment induced terminal differentiation of mammary tumor cells and in parallel downregulated tenascin expression. 3. In the normal rat prostate no tenascin was detectable by immunocytochemistry. However, following androgen deprivation we found tenascin expression in the stroma of the prostate. The most prominent expression was observable after CPA-treatment, possibly due to its progestagenic potency. In conclusion, the hormones and antihormones tested show no direct effect on the stromal expression of tenascin. However, proliferative activity and a low degree of differentiation of the epithelium induces tenascin expression, whereas epithelial differentiation apparently shuts down tenascin expression. Preliminary in vitro studies suggest that paracrine acting growth factors trigger the hormonal regulation of tenascin expression.