Avian thrombocyte thrombospondin

A high molecular weight glycoprotein (450,000) was obtained from thrombin-treated duck thrombocytes by barium citrate adsorption technique followed by heparin-agarose affinity chromatography. Amino acid composition (high number of acidic amino acids and cystine) as well as carbohydrate contents (1.3 per cent hexosamine, 0.9 per cent sialic acid and 1.5 per cent hexose) showed similarity to mammalian platelet thrombospondin.     

α1β1-Integrin is an essential signal for neurite outgrowth induced by thrombospondin type 1 repeats of SCO-spondin

In the central and peripheral nervous systems a heterogeneous group of proteins constituting the thrombospondin superfamily provides a cue for axonal pathfinding. They either contain or are devoid of the tripeptide RGD, and the sequence(s) and mechanism(s) which trigger in vitro their neurite-promoting activity have remained unclear. In this study, we reconsider the problem of whether sequences present in the thrombospondin type 1 repeats (TSRs), and independent of the well-known RGD-binding site, may activate integrins and account for their neurite-promoting activity. SCO-spondin is a newly identified member of the thrombospondin superfamily, which shows a multidomain organization with a great number of TSR motifs but no RGD sequence. Previous research has implicated oligopeptides derived from SCO-spondin TSRs in in-vitro development of various neuronal cell types. In this study, we investigate whether function-blocking antibodies directed against integrin subunits can block these effects in cell line B104, cloned from a neuroblastoma of the rat central nervous system. By two different approaches: flow cytometry revealing short-term effects and cell cultures revealing long-term effects, we show that: (a) activation of cell metabolism, (b) changes in cell size and structure, and (c) neurite-promoting activity induced by TSR oligopeptides are inhibited by function-blocking antibodies to 1-subunit. Using a panel of function-blocking antibodies directed against various integrin -subunits we show that the 1-subunit might be the partner of the 1-subunit in B104 cells. Thus, we demonstrate that an original sequence within a TSR motif from SCO-spondin promotes neurite outgrowth through an intracellular signal driven by integrins, independently of an RGD-binding site.     

Structural characterization of the second TSP1-module of human thrombospondin

The TSP1-module has been first identified as the type 1 repeat of thrombospondin-1. Members of this extracellular module-family have since been shown to be present in several hundred metazoan proteins as well as in proteins of some protists. Despite the widespread occurrence and biological importance of this module-type, relatively little is known about their three-dimensional structure. To define the structural features of this important module-family, we have expressed the second TSP1-domain of human thrombospondin 1 in Escherichia coli. Amino acid sequencing of proteolytic fragments of the recombinant protein have shown that its disulfide bonds connect the six conserved cysteines in a 1-5, 2-6, 3-4 pattern. Circular dichroism studies on the recombinant protein indicate that the disulfide-bonded TSP1-module consists primarily of distorted beta-strands.     

The evolution of the thrombospondin gene family

Summary Thrombospondin-1 is an adhesive glycoprotein that is involved in cellular attachment, spreading, migration, and proliferation. To date, four genes have been identified that encode for the members of the thrombospondin gene family. These four genes are homologous to each other in the EGF-like (type 2) repeats, the calcium-binding (type 3) motifs, and the COOH-terminal. The latter has been reported to be a cell-binding domain in thrombospondin-1. Phylogenetic trees have been constructed from the multisequence alignment of thrombospondin sequences from human, mouse, chicken, and frog. Two different algorithms generate comparable results in terms of the topology and the branch lengths. The analysis indicates that an early form of the thrombospondin gene duplicated about 925 million years ago. The gene duplication that produced the thrombospondin-1 and -2 branches of the family is predicted to have occurred 583 million years ago, whereas the gene duplication that produced the thrombospondin-3 and -4 branches of the family is predicted to have occurred 644 million years ago. These results indicate that the members of the thrombospondin gene family have existed throughout the evolution of the animal kingdom and thus probably participate in functions that are common to most of its members.     

Non-peptidic Thrombospondin-1 Mimics as Fibroblast Growth Factor-2 Inhibitors: AN INTEGRATED STRATEGY FOR THE DEVELOPMENT OF NEW ANTIANGIOGENIC COMPOUNDS*

Endogenous inhibitors of angiogenesis, such as thrombospondin-1 (TSP-1), are promising sources of therapeutic agents to treat angiogenesis-driven diseases, including cancer. TSP-1 regulates angiogenesis through different mechanisms, including binding and sequestration of the angiogenic factor fibroblast growth factor-2 (FGF-2), through a site located in the calcium binding type III repeats. We hypothesized that the FGF-2 binding sequence of TSP-1 might serve as a template for the development of inhibitors of angiogenesis. Using a peptide array approach followed by binding assays with synthetic peptides and recombinant proteins, we identified a FGF-2 binding sequence of TSP-1 in the 15-mer sequence DDDDDNDKIPDDRDN. Molecular dynamics simulations, taking the full flexibility of the ligand and receptor into account, and nuclear magnetic resonance identified the relevant residues and conformational determinants for the peptide-FGF interaction. This information was translated into a pharmacophore model used to screen the NCI2003 small molecule databases, leading to the identification of three small molecules that bound FGF-2 with affinity in the submicromolar range. The lead compounds inhibited FGF-2-induced endothelial cell proliferation in vitro and affected angiogenesis induced by FGF-2 in the chicken chorioallantoic membrane assay. These small molecules, therefore, represent promising leads for the development of antiangiogenic agents. Altogether, this study demonstrates that new biological insights obtained by integrated multidisciplinary approaches can be used to develop small molecule mimics of endogenous proteins as therapeutic agents.     

Heparan sulfate modification of the transmembrane receptor CD47 is necessary for inhibition of T cell receptor signaling by thrombospondin-1

Cell surface proteoglycans on T cells contribute to retroviral infection, binding of chemokines and other proteins, and are necessary for some T cell responses to the matricellular glycoprotein thrombospondin-1. The major cell surface proteoglycans expressed by primary T cells and Jurkat T cells have an apparent M(r) > 200,000 and are modified with chondroitin sulfate and heparan sulfate chains. Thrombospondin-1 bound in a heparin-inhibitable manner to this proteoglycan and to a soluble form released into the medium. Based on mass spectrometry, knockdown, and immunochemical analyses, the proteoglycan contains two major core proteins as follows: amyloid precursor-like protein-2 (APLP2, apparent M(r) 230,000) and CD47 (apparent M(r) > 250,000). CD47 is a known thrombospondin-1 receptor but was not previously reported to be a proteoglycan. This proteoglycan isoform of CD47 is widely expressed on vascular cells. Mutagenesis identified glycosaminoglycan modification of CD47 at Ser(64) and Ser(79). Inhibition of T cell receptor signaling by thrombospondin-1 was lost in CD47-deficient T cells that express the proteoglycan isoform of APLP2, indicating that binding to APLP2 is not sufficient. Inhibition of CD69 induction was restored in CD47-deficient cells by re-expressing CD47 or an S79A mutant but not by the S64A mutant. Therefore, inhibition of T cell receptor signaling by thrombospondin-1 is mediated by CD47 and requires its modification at Ser(64).     

Monoclonal antibodies to murine thrombospondin-1 and thrombospondin-2 reveal differential expression patterns in cancer and low antigen expression in normal tissues

There is a considerable interest for the discovery and characterization of tumor-associated antigens, which may facilitate antibody-based pharmacodelivery strategies. Thrombospondin-1 and thrombospondin-2 are homologous secreted proteins, which have previously been reported to be overexpressed during remodeling typical for wound healing and tumor progression and to possibly play a functional role in cell proliferation, migration and apoptosis. To our knowledge, a complete immunohistochemical characterization of thrombospondins levels in normal rodent tissues has not been reported so far. Using antibody phage technology, we have generated and characterized monoclonal antibodies specific to murine thrombospondin-1 and thrombospondin-2, two antigens which share 62% aminoacid identity. An immunofluorescence analysis revealed that both antigens are virtually undetectable in normal mouse tissues, except for a weak staining of heart tissue by antibodies specific to thrombospondin-1. The analysis also showed that thrombospondin-1 was strongly expressed in 5/7 human tumors xenografted in nude mice, while it was only barely detectable in 3/8 murine tumors grafted in immunocompetent mice. By contrast, a high-affinity antibody to thrombospondin-2 revealed a much lower level of expression of this antigen in cancer specimens. Our analysis resolves ambiguities related to conflicting reports on thrombosponding expression in health and disease. Based on our findings, thrombospondin-1 (and not thrombospondin-2) may be considered as a target for antibody-based pharmacodelivery strategies, in consideration of its low expression in normal tissues and its upregulation in cancer.     

Who climbs the tryptophan ladder? On the structure and function of the WSXWS motif in cytokine receptors and thrombospondin repeats

For decades, a spectacular structural motif has been the focus of research in two families of animal membrane proteins: the hematopoietic cytokine type I receptors (HCR) and the thrombospondin repeat type 1 (TSR-1) domain containing proteins. Although these families include some of the best-studied and pharmaceutically most interesting human proteins, the function of the motif remains elusive. Here we show that the molecular details of the motifs are the same; that it has arisen through convergent evolution, and we argue that the same ligand binding function is maintained and suggest that the ligand can be found in the extracellular matrix (ECM). We term the motif the tryptophan ladder and suggest a function based on a comparative analysis.