Cell-type specific adhesive interactions of skeletal myoblasts with thrombospondin-1

Involvement of phosphatidylserine receptors in the skeletal muscle regeneration: therapeutic implications

Sarcopenia is a progressive loss of muscle mass and strength with a risk of adverse outcomes such as disability, poor quality of life, and death. Increasing evidence indicates that diminished ability of the muscle to activate satellite cell-dependent regeneration is one of the factors that might contribute to its development. Skeletal muscle regeneration following myogenic cell death results from the proliferation and differentiation of myogenic stem cells, called satellite cells, located beneath the basal lamina of the muscle fibres. Satellite cell differentiation is not a satellite cell-autonomous process but depends on signals provided by the surrounding cells. Infiltrating macrophages play a key role in the process partly by clearing the necrotic cell debris, partly by producing cytokines and growth factors that guide myogenesis. At the beginning of the muscle regeneration process, macrophages are pro-inflammatory, and the cytokines produced by them trigger the proliferation and differentiation of satellite cells. Following the uptake of dead cells, however, a transcriptionally regulated phenotypic change (macrophage polarization) is induced in them resulting in their transformation into healing macrophages that guide resolution of inflammation, completion of myoblast differentiation, myoblast fusion and growth, and return to homeostasis. Impaired efferocytosis results in delayed cell death clearance, delayed macrophage polarization, prolonged inflammation, and impaired muscle regeneration. Thus, proper efferocytosis by macrophages is a determining factor during muscle repair. Here we review that both efferocytosis and myogenesis are dependent on the cell surface phosphatidylserine (PS), and surprisingly, these two processes share a number of common PS receptors and signalling pathways. Based on these findings, we propose that stimulating the function of PS receptors for facilitating muscle repair following injury could be a successful approach, as it would enhance efferocytosis and myogenesis simultaneously. Because increasing evidence indicates a pathophysiological role of impaired efferocytosis in the development of chronic inflammatory conditions, as well as in impaired muscle regeneration both contributing to the development of sarcopenia, improving efferocytosis should be considered also in its management. Again applying or combining those treatments that target PS receptors would be expected to be the most effective, because they would also promote myogenesis. A potential PS receptor-triggering candidate molecule is milk fat globule-EGF-factor 8 (MFG-E8), which not only stimulates PS-dependent efferocytosis and myoblast fusion but also promotes extracellular signal-regulated kinase (ERK) and Akt activation-mediated cell proliferation and cell cycle progression in myoblasts.

Alternative splicing controls cell lineage-specific responses to endogenous innate immune triggers within the extracellular matrix

The identification of barely more than 20,000 human genes was amongst the most surprising outcomes of the human genome project. Alternative splicing provides an essential means of expanding the proteome, enabling a single gene to encode multiple, distinct isoforms by selective inclusion or exclusion of exons from mature mRNA. However, mis-regulation of this process is associated with most human diseases. Here, we examine the impact of post-transcriptional processing on extracellular matrix function, focusing on the complex alternative splicing patterns of tenascin-C, a molecule that can exist in as many as 500 different isoforms. We demonstrate that the pro-inflammatory activity of this endogenous innate immune trigger is controlled by inclusion or exclusion of a novel immunomodulatory site located within domains AD2AD1, identifying this as a mechanism that prevents unnecessary inflammation in healthy tissues but enables rapid immune cell mobilization and activation upon tissue damage, and defining how this goes awry in autoimmune disease.

Thrombospondin-4 in tissue remodeling

Thrombospondin-4 (TSP-4) belongs to the thrombospondin protein family that consists of five highly homologous members. A number of novel functions have been recently assigned to TSP-4 in cardiovascular and nervous systems, inflammation, cancer, and the motor unit, which have attracted attention to this extracellular matrix (ECM) protein. These newly discovered functions set TSP-4 apart from other thrombospondins. For example, TSP-4 promotes angiogenesis while other TSPs either prevent it or have no effect on new blood vessel growth; TSP-4 reduces fibrosis and collagen production while TSP-1 and TSP-2 promote fibrosis in several organs; unlike other TSPs, TSP-4 appears to have some structural functions in ECM. The current information about TSP-4 functions in different organs and physiological systems suggests that this evolutionary conserved protein is a major regulator of the extracellular matrix (ECM) organization and production and tissue remodeling during the embryonic development and response to injury. In this review article, we summarize the properties and functions of TSP-4 and discuss its role in tissue remodeling.

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.

Control of organization and function of muscle and tendon by thrombospondin-4

Thrombospondins (TSPs) are multifunctional proteins that are deposited in the extracellular matrix where they directly affect the function of vascular and other cell types. TSP-4, one of the 5 TSP family members, is expressed abundantly in tendon and muscle. We have examined the effect of TSP-4 deficiency on tendon collagen and skeletal muscle morphology and function. In Thbs4(-/-) mice, tendon collagen fibrils are significantly larger than in wild-type mice, and there is no compensatory over-expression of TSP-3 and TSP-5, the two TSPs most highly homologous to TSP-4, in the deficient mice. TSP-4 is expressed in skeletal muscle, and higher levels of TSP-4 protein are associated with the microvasculature of red skeletal muscle with high oxidative metabolism. Lack of TSP-4 in medial soleus, red skeletal muscle with predominant oxidative metabolism, is associated with decreased levels of several specific glycosaminoglycan modifications, decreased expression of a TGFβ receptor beta-glycan, decreased activity of lipoprotein lipase, which associates with vascular cell surfaces by binding to glycosaminoglycans, and decreased uptake of VLDL. The soleus muscle is smaller and hind- and fore-limb grip strength is reduced in Thbs4(-/-) mice compared to wild-type mice. These observations suggest that TSP-4 regulates the composition of the ECM at major sites of its deposition, tendon and muscle, and the absence of TSP-4 alters the organization, composition and physiological functions of these tissues.

THBS4, a novel stromal molecule of diffuse-type gastric adenocarcinomas, identified by transcriptome-wide expression profiling

Gastric adenocarcinomas can be divided into two major histological types, the diffuse and intestinal type (Laurén classification). Since they diverge in many clinical and molecular characteristics, it is widely accepted that they represent distinct disease entities that may benefit from different therapeutic approaches. Gene expression profiling studies have identified numerous genes that are differentially expressed between them. However, none of these studies covered the whole transcriptome and the published gene lists reveal little overlap, raising the need for further, more comprehensive analyses. Here, we present the first transcriptome-wide expression profiling study comparing the two types (diffuse n=19, intestinal n=24), which identified >1000 genes that are differentially expressed. Among them, thrombospondin 4 (THBS4) showed the strongest correlation to histological type, with vast overexpression in the diffuse type. Quantitative real-time PCR validated this strong overexpression and revealed that intestinal tumors generally lack THBS4 expression. Immunohistochemistry demonstrated THBS4 overexpression on the protein level (n=10) and localized THBS4 to the stromal aspect. Its expression was primarily observed within the extracellular matrix surrounding the tumor cells, with the highest intensities found in regions of high tumor cell density and invasion. Intestinal tumors and matched non-neoplastic gastric epithelium and stroma did not feature any relevant THBS4 expression in a preliminary selection of analyzed cases (n=5). Immunohistochemical colocalization and in vitro studies revealed that THBS4 is expressed and secreted by cancer-associated fibroblasts. Furthermore, we show that THBS4 transcription in fibroblasts is stimulated by tumor cells. This study is the first to identify THBS4 as a powerful marker for diffuse-type gastric adenocarcinomas and to provide an initial characterization of its expression in the course of this disease.

The BRAFV600E mutation: what is it really orchestrating in thyroid cancer?

BRAFV600E is a constitutively active onco-kinase and is the most common genetic alteration in papillary thyroid carcinoma (PTC), and in anaplastic thyroid carcinoma as well, albeit at a lower frequency. The BRAFV600E mutation in some studies has been significantly associated with extra-thyroidal extension, metastases, recurrence, and mortality in patients with PTC. A recent genome-wide expression profiling approach (Gene Set Enrichment Analysis (GSEA)) and in vitro and in vivo functional studies revealed that BRAFV600E affects extracellular matrix composition (i.e. increased expression of some collagens and laminins) and promotes thyroid cancer migration and invasion. BRAFV600E through the phospho-MEK1/2 and phospho-ERK1/2 pathway may control a network of genes crucial in integrating and regulating the extracellular and intracellular signaling in thyroid cancer cells, which may be fundamental to trigger an abnormal cell differentiation/totipotency and shape/polarity, and contribute to tumor aggressiveness mechanisms (i.e. cell adhesion, migration, and invasion). Increasing our knowledge of BRAFV600E-modulated ECM genes and targeting the subset of genes essential for tumor aggressiveness will help establish a novel paradigm for treatment of thyroid cancers harboring BRAFV600E. Furthermore, identifying downstream events from the BRAFV600E/ERK1/2 pathway will eventually identify novel biomarkers that can be used to correlate with disease outcome and overall survival.

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).

CD47 knockout mice exhibit improved recovery from spinal cord injury

Recent data have implicated thrombospondin-1 (TSP-1) signaling in the acute neuropathological events that occur in microvascular endothelial cells (ECs) following spinal cord injury (SCI) (Benton et al., 2008b). We hypothesized that deletion of TSP-1 or its receptor CD47 would reduce these pathological events following SCI. CD47 is expressed in a variety of tissues, including vascular ECs and neutrophils. CD47 binds to TSP-1 and inhibits angiogenesis. CD47 also binds to the signal regulatory protein (SIRP)α and facilitates neutrophil diapedesis across ECs to sites of injury. After contusive SCI, TSP-1(-/-) mice did not show functional improvement compared to wildtype (WT) mice. CD47(-/-) mice, however, exhibited functional locomotor improvements and greater white matter sparing. Whereas targeted deletion of either CD47 or TSP-1 improved acute epicenter vascularity in contused mice, only CD47 deletion reduced neutrophil diapedesis and increased microvascular perfusion. An ex vivo model of the CNS microvasculature revealed that CD47(-/-)-derived microvessels (MVs) prominently exhibit adherent WT or CD47(-/-) neutrophils on the endothelial lumen, whereas WT-derived MVs do not. This implicates a defect in diapedesis mediated by the loss of CD47 expression on ECs. In vitro transmigration assays confirmed the role of SIRPα in neutrophil diapedesis through EC monolayers. We conclude that CD47 deletion modestly, but significantly, improves functional recovery from SCI via an increase in vascular patency and a reduction of SIRPα-mediated neutrophil diapedesis, rather than the abrogation of TSP-1-mediated anti-angiogenic signaling.

The BRAFV600E mutation: what is it really orchestrating in thyroid cancer?

BRAFV600E is a constitutively active onco-kinase and is the most common genetic alteration in papillary thyroid carcinoma (PTC), and in anaplastic thyroid carcinoma as well, albeit at a lower frequency. The BRAFV600E mutation in some studies has been significantly associated with extra-thyroidal extension, metastases, recurrence, and mortality in patients with PTC. A recent genome-wide expression profiling approach (Gene Set Enrichment Analysis (GSEA)) and in vitro and in vivo functional studies revealed that BRAFV600E affects extracellular matrix composition (i.e. increased expression of some collagens and laminins) and promotes thyroid cancer migration and invasion. BRAFV600E through the phospho-MEK1/2 and phospho-ERK1/2 pathway may control a network of genes crucial in integrating and regulating the extracellular and intracellular signaling in thyroid cancer cells, which may be fundamental to trigger an abnormal cell differentiation/totipotency and shape/polarity, and contribute to tumor aggressiveness mechanisms (i.e. cell adhesion, migration, and invasion). Increasing our knowledge of BRAFV600E-modulated ECM genes and targeting the subset of genes essential for tumor aggressiveness will help establish a novel paradigm for treatment of thyroid cancers harboring BRAFV600E. Furthermore, identifying downstream events from the BRAFV600E/ERK1/2 pathway will eventually identify novel biomarkers that can be used to correlate with disease outcome and overall survival.

Role of thrombospondin 1 in macrophage inflammation in dysferlin myopathy

Muscle inflammation can be a prominent feature in several muscular dystrophies. In dysferlin myopathy, it is mainly composed of macrophages. To understand the origin of inflammation in dysferlin-deficient muscle, we analyzed soluble factors involved in monocyte chemotaxis released by myoblasts and myotubes from control and dysferlinopathy patients using a transwell system. Dysferlin-deficient myotubes released more soluble factors involved in monocyte chemotaxis compared with controls (p < 0.001). Messenger RNA microarray analysis showed a 3.2-fold increase of thrombospondin 1 (TSP-1) expression in dysferlin-deficient myotubes. Retrotranscriptasepolymerase chain reaction analysis, ELISA, and immunohistochemistry confirmed these results. Dysferlin mRNA knockdown with short-interfering RNA in normal myogenic cells resulted in TSP-1 mRNA upregulation and increased chemotaxis. Furthermore, monocyte chemotaxis was decreased when TSP-1 was blocked by specific antibodies. In muscle biopsies from dysferlinopathy patients, TSP-1 expression was increased in muscle fibers but not in biopsies of patientswith other myopathies with inflammation; TSP-1 was seen in some macrophages in all samples analyzed. Taken together, the data demonstrate that dysferlin-deficient muscle upregulates TSP-1 in vivoand in vitro and indicate that endogenous chemotactic factors arecrucial to the sustained inflammatory process observed in dysferlinopathies.

Cartilage oligomeric matrix protein promotes cell attachment via two independent mechanisms involving CD47 and alphaVbeta3 integrin

Cartilage oligomeric matrix protein (COMP) is a pentameric approximately 524 kDa multidomain extracellular matrix protein and is the fifth member of the thrombospondin family. COMP is abundantly expressed in proliferating and hypertrophic chondrocytes of the growth plate, articular cartilage, synovium, tendon, and ligament. The spatial localization of COMP highlights its importance in the phenotypes of pseudoachondroplasia (PSACH) and multiple epiphyseal dysplasia (MED), COMP disorders that are characterized by disproportionate short stature, brachydactyly, scoliosis, early-onset osteoarthritis, and joint hypermobility. In this study, the role of COMP in ligament was investigated with a series of cell attachment assays using ligament cells binding to COMP. A dose-dependent cell attachment activity was found, which was inhibited by a peptide containing the SFYVVMWK amino acid sequence derived from the globular C-terminal domain of COMP. This activity was independent of the recently described RGD-dependent attachment activity. Function-blocking antibodies to CD47 and alphaVbeta3 integrin reduced cell attachment to COMP, implicating the participation of these cell surface molecules in COMP cell binding. Immunofluorescence studies showed that cell attachment to COMP induced the formation of lamellae containing F-actin microspikes associated with fascin. We propose that COMP promotes cell attachment via two independent mechanisms involving cell surface CD47 and alphaVbeta3 integrin and that a consequence of cell attachment to COMP is the specific induction of fascin-stabilized actin microspikes.

The mouse C2C12 myoblast cell surface N-linked glycoproteome: identification, glycosite occupancy, and membrane orientation

Endogenous regeneration and repair mechanisms are responsible for replacing dead and damaged cells to maintain or enhance tissue and organ function, and one of the best examples of endogenous repair mechanisms involves skeletal muscle. Although the molecular mechanisms that regulate the differentiation of satellite cells and myoblasts toward myofibers are not fully understood, cell surface proteins that sense and respond to their environment play an important role. The cell surface capturing technology was used here to uncover the cell surface N-linked glycoprotein subproteome of myoblasts and to identify potential markers of myoblast differentiation. 128 bona fide cell surface-exposed N-linked glycoproteins, including 117 transmembrane, four glycosylphosphatidylinositol-anchored, five extracellular matrix, and two membrane-associated proteins were identified from mouse C2C12 myoblasts. The data set revealed 36 cluster of differentiation-annotated proteins and confirmed the occupancy for 235 N-linked glycosylation sites. The identification of the N-glycosylation sites on the extracellular domain of the proteins allowed for the determination of the orientation of the identified proteins within the plasma membrane. One glycoprotein transmembrane orientation was found to be inconsistent with Swiss-Prot annotations, whereas ambiguous annotations for 14 other proteins were resolved. Several of the identified N-linked glycoproteins, including aquaporin-1 and beta-sarcoglycan, were found in validation experiments to change in overall abundance as the myoblasts differentiate toward myotubes. Therefore, the strategy and data presented shed new light on the complexity of the myoblast cell surface subproteome and reveal new targets for the clinically important characterization of cell intermediates during myoblast differentiation into myotubes.

The interaction of Thrombospondins with extracellular matrix proteins

The thrombospondins (TSPs) are a family of five matricellular proteins that appear to function as adapter molecules to guide extracellular matrix synthesis and tissue remodeling in a variety of normal and disease settings. Various TSPs have been shown to bind to fibronectin, laminin, matrilins, collagens and other extracellular matrix (ECM) proteins. The importance of TSP-1 in this context is underscored by the fact that it is rapidly deposited at the sites of tissue damage by platelets. An association of TSPs with collagens has been known for over 25 years. The observation that the disruption of the TSP-2 gene in mice leads to collagen fibril abnormalities provided important in vivo evidence that these interactions are physiologically important. Recent biochemical studies have shown that TSP-5 promotes collagen fibril assembly and structural studies suggest that TSPs may interact with collagens through a highly conserved potential metal ion dependent adhesion site (MIDAS). These interactions are critical for normal tissue homeostasis, tumor progression and the etiology of skeletal dysplasias.

Chemical synthesis and biotinylation of the thrombospondin domain TSR2

The type 1 repeat domain from thrombospondin has potent antiangiogenic activity and a structurally interesting fold, making it an attractive target for protein engineering. Chemical synthesis is an attractive approach for studying protein domains because it enables the use of unnatural amino acids for site-specific labeling and detailed structure-function analysis. Here, we demonstrate the first total chemical synthesis of the thrombospondin type 1 repeat domain by native chemical ligation. In addition to the natural domain, five sites for side chain modification were evaluated and two were found to be compatible with oxidative folding. Several challenges were encountered during peptide synthesis due to the functional complexity of the domain. These challenges were overcome by the use of new solid supports, scavengers, and the testing of multiple ligation sites. We also describe an unusual sequence-specific protecting group migration observed during cleavage resulting in +90 Da and +194 Da adducts. Synthetic access to this domain enables the synthesis of a number of variants that can be used to further our understanding of the biochemical interaction network of thrombospondin and provide insight into the structure and function of this important antitumorogenic protein domain.

Phosphatidylserine-positive erythrocytes bind to immobilized and soluble thrombospondin-1 via its heparin-binding domain

Phosphatidylserine (PS)-dependent erythrocyte adhesion to endothelium and subendothelial matrix components is mediated in part via thrombospondin (TSP). Although TSP exhibits multiple cell-binding domains, the PS-binding site on TSP is unknown. Because a cell-binding domain for anionic heparin is located at the amino-terminus, we hypothesized that PS-positive red blood cells (PS(+ve)-RBCs) bind to this domain. We demonstrate that both heparin and its low-molecular-weight derivative enoxaparin (0.5-50 u/mL) inhibited PS(+ve)-RBC adhesion to immobilized TSP in a concentration-dependent manner (21% to 77% inhibition, P < 0.05). Preincubation of immobilized TSP with an antibody against the heparin-binding domain blocked PS(+ve)-RBC adhesion to TSP. Antibodies that recognize the collagen- and the carboxy-terminal CD47-binding domain on TSP had no effect on this process. Although preincubation of PS(+ve)-RBCs with TSP peptides from the heparin-binding domain that contained the specific heparin-binding motif KKTRG inhibited PS(+ve)-erythrocyte adhesion to matrix TSP (P < 0.001), these peptides in the immobilized form supported PS-mediated erythrocyte adhesion. A TSP-peptide that lacks the binding motif neither inhibited nor supported PS(+ve)-RBC adhesion. Additional experiments show that soluble TSP also interacted with PS(+ve)-RBCs via its heparin-binding domain. Our results demonstrate that PS-positive erythrocytes bind to both immobilized and soluble TSP via its heparin-binding domain and that both heparin and enoxaparin, at clinically relevant concentrations, block this interaction. Other studies have shown that heparin inhibited P-selectin- and soluble-TSP-mediated sickle erythrocyte adhesion to endothelial cells. Our results, taken together with the previously documented findings, provide a rational basis for clinical use of heparin or its low-molecular-weight derivatives as therapeutic agents in treating vaso-occlusive pain in patients with sickle cell disease.

Identification of erythroid-enriched gene expression in the mouse embryonic yolk sac using microdissected cells

Little is known about the genes that control the embryonic erythroid program. Laser capture microdissection was used to isolate primitive erythroid precursors and epithelial cells from frozen sections of the embryonic day 9.5 yolk sac. The RNA samples were amplified and labeled for hybridization to Affymetrix GeneChip Mouse Genome 430A 2.0 arrays. Ninety-one genes are expressed significantly higher in erythroid than in epithelial cells. Ingenuity pathway analysis indicates that many of these erythroid-enriched genes cluster in highly significant biological networks. One of these networks contains RBTN2/LMO2, SCL/TAL1, and EKLF/KLF1, three of the very few genes required for primitive erythropoiesis. Quantitative real-time polymerase chain reaction was used to verify that platelet factor 4, reelin, thrombospondin-1, and muscleblind-like 1 mRNA is erythroid-enriched. These genes have established roles in development or differentiation in other systems, and are, therefore, good candidates for regulating primitive erythropoiesis. These results provide a catalog of genes expressed during primitive erythropoiesis.

Structures of thrombospondins

Thrombospondins are large secreted, multimodular, calcium-binding glycoproteins that have complex roles in mediating cellular processes. Determination of high-resolution structures of thrombospondins has revealed unique and interesting protein motifs. Here, we review this progress and discuss implications for function. By combining structures of modules from thrombospondins and related extracellular proteins it is now possible to prepare an overall model of the structure of thrombospondin-1 and thrombospondin-2 and discern features of other thrombospondins. (Part of a multi-author Review).

Extracellular matrix retention of thrombospondin 1 is controlled by its conserved C-terminal region

Thrombospondins (TSPs) are an evolutionarily ancient family of extracellular calcium-binding glycoproteins. The five mammalian TSPs collectively have important roles in angiogenesis and vascular biology, synaptogenesis, wound repair and connective tissue organisation. Their complex functions relate to the multiple postsecretion fates of TSPs that can involve endocytic uptake, proteolysis or retention within the extracellular matrix (ECM). Surprisingly, the molecular and cellular mechanisms by which TSPs become retained within the ECM are poorly understood. We hypothesised that the highly conserved TSP C-terminal domain mediates ECM retention. We report that ECM incorporation as insoluble punctate deposits is an evolutionarily conserved property of TSPs. ECM retention of TSP1 is mediated by the C-terminal region in trimeric form, and not by C-terminal monomer or trimers of the N-terminal domain or type 1 repeats. Using a novel mRFP-tagged TSP1 C-terminal trimer, we demonstrate that ECM retention involves the RGD site and a novel site in the L-lectin domain with structural similarity to the ligand-binding site of cargo transport proteins. CD47 and beta1 integrins are dispensable for ECM retention, but beta1 integrins enhance activity. These novel data advance concepts of the molecular processes that lead to ECM retention of TSP1.

Thrombospondin-4 and matrix three-dimensionality in axon outgrowth and adhesion in the developing retina

Thrombospondin-4 (TSP-4), a large pentameric glycoprotein of the extracellular matrix, has been described as a neurite outgrowth-promoting molecule. However, the means by which TSP-4 promotes neurite outgrowth in the developing eye is unclear. Here we show that TSP-4 is present at the appropriate time in development and displays a localization pattern within the developing mouse retina consistent with a role in retinal ganglion cell (RGC) neurite outgrowth. Furthermore, results indicate that while TSP-4 alone does not support adhesion or neurite extension, it enhances the ability of laminins to promote adhesion and neurite outgrowth of embryonic retinal cells. The mechanism of enhancement is, in part, based on the ability of TSP-4 to enhance the three-dimensionality and/or clustering of laminins within the substrate matrix. These results support a model where TSP-4 acts as an organizer of adhesive and axon outgrowth-promoting molecules in the ECM to optimize retinal ganglion cell responses.

The thrombospondin-1 N700S polymorphism is associated with early myocardial infarction without altering von Willebrand factor multimer size

The N700S polymorphism of thrombospondin-1 (TSP-1) has been identified as a potential genetic risk factor for myocardial infarction (MI). In a large case-control study of 1425 individuals who survived a myocardial infarction prior to age 45, the N700S polymorphism was a significant risk factor for myocardial infarction in both homozygous (odds ratio [OR] 1.9, 95% confidence interval [CI] 1.1-3.3, P = .01) and heterozygous carriers of the S700 allele (OR 1.4, 95% CI 1.1-3.3, P = .01). TSP-1 has been shown to reduce von Willebrand factor (VWF) multimer size, and the domain responsible for VWF-reducing activity has been localized to the calcium-binding C-terminal sequence. As the N700S polymorphism was previously shown to alter the function of this domain, we investigated whether the altered VWF-reducing activity of TSP-1 underlies the observed prothrombotic phenotype. The TSP1 N700S polymorphism did not influence VWF multimer size in patients homozygous for either allele nor was there a significant reduction of VWF multimer size following incubation with recombinant N700S fragments or platelet-derived TSP-1.

Function-blocking antithrombospondin-1 monoclonal antibodies

BACKGROUND

Thrombospondin-1 (TSP-1) has been implicated in many different processes based in part on inhibitory activities of anti-TSP-1 monoclonal antibodies (mAbs).

OBJECTIVE

To map epitopes of 13 anti-TSP-1 mAbs to individual modules or groups of modules spanning TSP-1 and the closely related TSP-2 homolog.

RESULTS

The mapping has led to assignment or reassignment of the epitopes of four mAbs, refinement of the epitopes of six mAbs, and confirmation of the epitopes of the remaining three mAbs. ESTs10, P12, and MA-II map to the N-terminal domain; 5G11, TSP127.6, and ESTs12 to the third properdin module; C6.7, HB8432, and P10 to epidermal growth factor (EGF)-like modules 1 and/or 2; and A6.1, mAb133, MA-I, and D4.6 to the calcium-binding wire module. A6.1, which recognizes a region of the wire that is identical in mouse and human TSP-1, reacts with TSP-1 from both species, and also reacts weakly with human TSP-2. Two other mouse antihuman TSP-1 mAbs, A4.1 and D4.6, also react with mouse TSP-1.

CONCLUSIONS

Consideration of previous literature and mapping of epitopes of inhibitory mAbs suggest that biological activities are present throughout TSP-1, including the EGF-like modules that have not been implicated in the past. Because the epitopes for 10 of the antibodies likely are within 18 nm of one another in calcium-replete TSP-1, some of the inhibitory effects may result from steric hindrance. Such seems to be the case for mAb133, which binds the calcium-binding wire but is still able to interfere with the activation of latent TGF-beta by the properdin modules.

Biophysical characterization of the signature domains of thrombospondin-4 and thrombospondin-2

The signature domain of thrombospondins consists of tandem epidermal growth factor-like modules, 13 calcium-binding repeats, and a lectin-like module. Although very similar, the signature domains of thrombospondin-1 and -2 differ in several potentially important ways from the domains of thrombospondin-3, -4, and -5. We have compared matching recombinant segments representing the signature domains of thrombospondin-2 and -4. In the presence of 2 mM CaCl2, the far UV circular dichroism spectra of thrombospondin-2 and -4 constructs contain a strong negative band at 202 nm, but only the thrombospondin-2 construct has a band at 216 nm. Chelation of calcium shifted the negative bands to lower magnitudes. Titrations of the spectra demonstrated lower cooperativity and affinity for binding of calcium to thrombospondin-4 compared with thrombospondin-2. Atomic absorption spectroscopy demonstrated that the thrombospondin-4 constructs bind seven less calcium than the thrombospondin-2 construct at 0.6 mM CaCl2. In 2 mM CaCl2, the near UV circular dichroism spectra of thrombospondin-2, but not thrombospondin-4, contain a positive band at 292 nm that disappears upon calcium chelation. Intrinsic fluorescence spectra for both proteins were also sensitive to calcium, but the changes were simpler and more marked for thrombospondin-2 than for thrombospondin-4. In differential scanning calorimetry, the thrombospondin-2 construct melted in two distinct transitions at 53.5 and 81.8 degrees C, whereas the first transition for thrombospondin-4 constructs was observed at 63.5 degrees C. Thus, the studies revealed significant differences between the signature domains of thrombospondin-2 and thrombospondin-4 in calcium binding, fine structure, and inter-modular interactions.

Mechanism and effect of thrombospondin-4 polymorphisms on neutrophil function

High-throughput genomic technology identified an association between a single nucleotide polymorphism (SNP), a proline (P387) rather than the predominant alanine (A387) at position 387 in thrombospondin-4 (TSP-4) and premature myocardial infarction. The inflammatory hypothesis of atherosclerosis invokes a prominent role of leukocytes and cytokines in pathogenesis. As the expression of TSP-4 by vascular cells permits its exposure to circulating leukocytes, the interactions of human neutrophils (polymorphonuclear leukocytes [PMNs]) with both TSP-4 variants were investigated. Phorbol 12-myristate 13-acetate (PMA)-stimulated PMNs adhered and migrated well and equally on the TSP-4 variants. Integrin alpha(M)beta2 was identified as the TSP-4 receptor mediating these responses, and the 3 epidermal growth factor (EGF)-like domains of TSP-4 harboring the SNPs interacted with the alpha(M)I-domain. Despite the similarity in these responses, the P387 variant induced more robust tyrosine phosphorylation of the stress-related mitogen-activated protein kinases (MAPKs): p38MAPK and c-Jun NH2-terminal kinase (JNK), as well as signal transducer and activator of transcription-1 (STAT1) and heat shock protein 27 (HSP27) than the A387 variant. Additionally, cells adherent to P387 TSP-4 variant released 4-fold more H2O2 and secreted 2-fold more interleukin 8 (IL-8) as compared with the A387. H2O2 release and p38MAPK activation were totally inhibited by blockade of alpha(M)beta2. Thus, alpha(M)beta2 plays a central role in proinflammatory activities of TSP-4 (P387) and may contribute to the prothrombotic phenotype associated with this variant.

Transcriptional profile of a myotube starvation model of atrophy

Skeletal muscle wasting is a pervasive phenomenon that can result from a wide range of pathological conditions as well as from habitual muscular inactivity. The present work describes a cell-culture condition that induces significant atrophy in skeletal muscle C2C12 myotubes. The failure to replenish differentiation media in mature myotubes leads to rapid atrophy (53% in diameter), which is referred to here as starvation. Affymetrix microarrays were used to develop a transcriptional profile of control (fed) vs. atrophied (nonfed) myotubes. Myotube starvation was characterized by an upregulation of genes involved in translational inhibition, amino acid biosynthesis and transport, and cell cycle arrest/apoptosis, among others. Downregulated genes included several structural and regulatory elements of the extracellular matrix as well as several elements of Wnt/frizzled and TGF-β signaling pathways. Interestingly, the characteristic transcriptional upregulation of the ubiquitin-proteasome system, calpains, and cathepsins known to occur in multiple in vivo models of atrophy were not seen during myotube starvation. With the exception of the downregulation of extracellular matrix genes, serine protease inhibitor genes, and the upregulation of the translation initiation factor PHAS-I, this model of atrophy in cell culture has a transcriptional profile quite distinct from any study published to date with atrophy in whole muscle. These data show that, although the gross morphology of atrophied muscle fibers may be similar in whole muscle vs. myotube culture, the processes by which this phenotype is achieved differ markedly.

Type 3 repeat/C-terminal domain of thrombospondin-1 triggers caspase-independent cell death through CD47/alphavbeta3 in promyelocytic leukemia NB4 cells

By means of its antiangiogenic activity, thrombospondin-1 (TSP-1) exerts indirect antitumoral action on solid tumors. Here, we investigated potential antitumor action in an in vitro cell model for promyelocytic leukemia (NB4-LR1), resistant to retinoid maturation. Purified soluble TSP-1 added to cultures induced a strong dose-dependent growth inhibition and a slowly developing maturation-independent cell death. Recombinant fragments of TSP-1 allowed mapping of these activities to its type 3 repeat/C-terminal domain, features that are distinct from those of TSP-1 action on solid tumors, previously ascribed to the type 1 repeat domain. Cell death in leukemia was characterized as a caspase-independent mechanism, without DNA fragmentation, but phosphatidylserine externalization followed by membrane permeabilization. Mitochondria membrane depolarization was inherent to TSP-1 action but did not produce release of death-promoting proteins (eg, noncaspase apoptosis regulators, apoptosis-induced factor [AIF], endonuclease G, or Omi/HtrA2 or the caspase regulators, cytochrome c or second mitochondrial activator of caspase/direct inhibitor of apoptosis protein-binding protein with low isoelectric point [Smac/DIABLO]). Although detected, reactive oxygen species (ROS) production was likely not involved in the death process. Finally, receptor agonist RFYVVM and RGD peptides indicated that TSP-1 death effects are mediated by membrane receptors CD47 and alphavbeta3. These results demonstrated a new domain-specific antitumoral activity of TSP-1 on a leukemia cell line, which extends TSP-1 therapeutic potential outside the area of vascularized solid tumors.

Functions of the conserved thrombospondin carboxy-terminal cassette in cell-extracellular matrix interactions and signaling

Thrombospondins (TSPs) are extracellular, multidomain, calcium-binding glycoproteins that function at cell surfaces, in extracellular matrix (ECM) and as bridging molecules in cell-cell interactions. TSPs are multifunctional and modulate cell behavior during development, wound-healing, immune response, tumor growth and in the homeostasis of adult tissues. TSPs are assembled as oligomers that are composed of homologous polypeptides. In all the TSP polypeptides, the most highly-conserved region is the carboxyl-region, which contains a characteristic set of domains comprising EGF domains, TSP type 3 repeats and a globular carboxy-terminal domain. This large region is termed here the thrombospondin carboxy-terminal cassette (TSP-CTC). The strong conservation of the TSP-CTC suggests that it may mediate ancestral functions that are shared by all TSPs. This review summarizes the current knowledge of the TSP-CTC and areas of future interest.

The N-terminus of thrombospondin: the domain stands apart

Thrombospondin 1 (TSP1) was first recognized as a thrombin-sensitive protein associated with platelet membranes. It is secreted by numerous cell types and its expression is predominant in areas of active tissue remodeling. Thrombospondins 1 and 2 are large, trimeric, matricellular proteins, composed of multiple structural motifs which interact with a diverse array of receptors and molecules. Thrombospondin's capacity to bind multiple receptors renders it multifunctional. The functions of its isolated domains can be overlapping or contradictory. In this review, we focus on the N-terminus of the molecule, first recognized for its strong heparin binding properties and characterized by its susceptibility to proteolytic cleavage from the stalk region of thrombospondin. The N-terminus, called the heparin binding domain (HBD), interacts with a variety of macromolecules including heparan sulfate proteoglycans at the membrane and in the matrix, LDL receptor-related protein (LRP), sulfated glycolipids, calreticulin, and integrins. The HBD mediates endocytosis of thrombospondin. It functions both as a soluble and an insoluble modulator of cell adhesion and motility. In contrast to thrombospondin, the HBD has pro-angiogenic activity. We propose that the HBD of thrombospondins 1 and 2 are found primarily in the cellular microenvironment in conditions of cellular injury, stress and tissue remodeling and that the HBD conveys multiple signals involved in cellular adaptation to injury.

Coronary artery disease and the thrombospondin single nucleotide polymorphisms

GeneQuest was a high throughput, large-scale analysis of single nucleotide polymorphisms (SNPs) to identify gene associated with familial, premature coronary artery disease and myocardial infarction. The three SNPs showing the highest and most significant associations with disease were all members of the thrombospondin gene family, thrombospondin-1, thrombospondin-2 and thrombospondin-4. These unanticipated associations have kindled efforts to understand how the three SNPs influence the structures and functions of the thrombospondins. The SNP in thrombospondin-1 and thrombospondin-4 reside in their coding regions and result in single amino acid changes: in thrombospondin-1, the predominant asparagine at position 700 is changed to a serine while, in thrombospondin-4, it is a change of an alanine to a proline at position 387. The SNP in thrombospondin-2 is a base change in the 3'-untranslated region of the mRNA. At this early stage of investigation, predictive analyses suggest that the substitutions in thrombospondin-2 and thrombospondin-4 should alter structure, and there is direct evidence to indicate that the thrombospondin-1 SNP alters conformational stability. In addition, profound differences in the function of the thrombospondin-4 SNP variants have been identified with respect to their capacity to support endothelial cell adhesion and proliferation. While substantial additional information is needed to understand if and how the polymorphic forms of the thrombospondins affect coronary artery disease, the data assembled to date suggest marked effects of these SNPs on the structures and functions of the thrombospondins, which are consistent with induction of a proatherogenic and prothrombotic phenotype.

The Tryptophan-rich Motifs of the Thrombospondin Type 1 Repeats Bind VLAL Motifs in the Latent Transforming Growth Factor-β Complex

Transforming growth factor-beta (TGF-beta) is secreted as a latent complex of the latency-associated peptide (LAP) and the mature domain, which must be activated for TGF-beta to signal. We previously identified thrombospondin 1 (TSP1) as a physiologic activator of TGF-beta in vitro and in vivo. The WSXW sequences in the type 1 repeats of TSP1 interact with the mature domain of TGF-beta, and WSXW peptides inhibit TSP1-mediated activation by blocking TSP1 binding to the TGF-beta latent complex. However, the binding site for the WSXW sequence was not identified. In this report, we show that the WSXW sequences bind the (61)VLAL sequence in mature TGF-beta and also bind (77)VLAL in LAP. A glutathione S-transferase (GST) fusion protein of the second TSP1 type 1 repeat (GST-TSR2) binds immobilized VLAL peptide. VLAL peptides inhibit binding of LAP and mature TGF-beta to soluble GST-TSR2 and immobilized WSXW peptide. VLAL peptide inhibits TSP1-mediated activation of recombinant and endothelial cell-derived latent TGF-beta. Furthermore, TGF-beta or LAP deleted in the VLAL sequence fails to bind immobilized WSXW or soluble GST-TSR2, indicating that binding to both VLAL sequences is important for association of TSP1 and the latent complex. Additionally, TSP1 is unable to activate latent TGF-beta when VLAL is deleted from the mature domain. These data show that the WSXW motif binds VLAL on both LAP and mature TGF-beta, and these interactions are critical for TSP1-mediated activation of the TGF-beta latent complex.

Structure of a thrombospondin C-terminal fragment reveals a novel calcium core in the type 3 repeats

Thrombospondins (TSPs) are extracellular regulators of cell-matrix interactions and cell phenotype. The most highly conserved region of all TSPs are the calcium-binding type 3 (T3) repeats and the C-terminal globular domain (CTD). The crystal structure of a cell-binding TSP-1 fragment, spanning three T3 repeats and the CTD, reveals a compact assembly. The T3 repeats lack secondary structure and are organised around a core of calcium ions; two DxDxDGxxDxxD motifs per repeat each encapsulate two calcium ions in a novel arrangement. The CTD forms a lectin-like beta-sandwich and contains four strictly conserved calcium-binding sites. Disruption of the hairpin structure of T3 repeats 6 and 7 decreases protein secretion and stability. The availability for cell attachment of an RGD motif in T3 repeat 7 is modulated by calcium loading. The central architectural role of calcium explains how it is critical for the functions of the TSP C-terminal region. Mutations in the T3 repeats of TSP-5/COMP, which cause two human skeletal disorders, are predicted to disrupt the tertiary structure of the T3-CTD assembly.

Interaction of fascin and protein kinase Calpha: a novel intersection in cell adhesion and motility

Coordination of protrusive and contractile cell-matrix contacts is important for cell adhesion and migration, but the mechanisms involved are not well understood. We report an unexpected direct association between fascin, an actin-bundling component of filopodia, microspikes and lamellipodial ribs, and protein kinase Calpha (PKCalpha), a regulator of focal adhesions. The association is detectable by protein-protein binding in vitro, by coimmunoprecipitation from cell extracts, and in live cells as fluorescence resonance energy transfer detected by fluorescence imaging lifetime microscopy. The interaction is physiologically regulated by the extracellular matrix context of cells, depends on activation of PKCalpha and is mediated by the C1B domain of PKCalpha. Strikingly, a fascin mutant, fascin S39D, associates constitutively with PKCalpha. Through use of a newly developed set of membrane-permeable peptides that separately inhibit either fascin/PKCalpha or fascin/actin binding, we have uncovered that specific blockade of the fascin/PKCalpha interaction increases cell migration on fibronectin in conjunction with increased fascin protrusions and remodeling of focal adhesions. These results identify the fascin-PKCalpha interaction as an important novel intersection in the regulation and networking of cell-matrix contacts.

Distribution of thrombospondin-4 in the bovine eye

PURPOSE

The purpose of this study was to analyze the distribution of thrombospondin-4 (TSP-4) in the bovine eye.

METHODS

Anterior and posterior segments of the bovine eyes were sectioned and stained by the indirect immunofluorescence method with an anti-TSP-4 antibody. The tissues were analyzed by reverse-transcription-polymerase chain reaction (RT-PCR) to determine where the TSP-4 mRNA is produced.

RESULTS

Immunohistochemical staining for TSP-4 indicated the presence of TSP-4 in the cornea (epithelium, basement membrane, and keratocytes), conjunctiva (epithelium and stroma), aqueous ducts, sclera, iris (stroma), ciliary processes and muscle, trabecular meshwork, Bruch's membrane, retina, lamina cribrosa, and optic nerve, and in all blood vessel walls. TSP-4 mRNA was expressed by the cells in all structures.

CONCLUSIONS

TSP-4 is widely distributed in the bovine eye where it may play a role in the functions of basement membranes in various tissues. It is abundant in the trabecular and uveo-scleral pathways and may play a role in the regulation of aqueous outflow resistance.

Thrombospondin-1-induced vascular smooth muscle cell chemotaxis: the role of the type 3 repeat and carboxyl terminal domains

Thrombospondin-1 (TSP-1), an acute phase reactant implicated in vascular disease, is a matricellular glycoprotein with six domains that confer different functions. The authors have shown TSP-1 induces vascular smooth muscle cell (VSMC) chemotaxis via extracellular signal-regulated kinases-1 and -2 (ERK) and p38 kinase (p38) and that a fusion protein of the carboxyl terminal (COOH) and type 3 repeat (T3) domains independently induce VSMC chemotaxis. The purpose of this study was to determine whether COOH-, T3-induced VSMC chemotaxis, or both, is dependent upon ERK or p38 activation. To determine if the T3, COOH, or type 2 repeat domain (T2, control domain not associated with chemotaxis) activate ERK, p38, or both, VSMCs were exposed to each fusion protein (20 microg/ml for 15, 30, 60, or 120 min), serum-free media (SFM, negative control), or TSP-1 (20 microg/ml for 30 min, positive control). Western immunoblotting was performed for activation studies. Using a microchemotaxis chamber, VSMCs pre-incubated in SFM, DMSO (vehicle control), PD98059 (10 microM), or SB202190 (10 microM) were exposed to each domain, TSP-1, or SFM. After 4 h (37 degrees C), migrated VSMCs were recorded as cells/five fields (400 x) and analyzed by paired t-test. ERK was activated by T2, T3, and COOH. However, p38 was activated by T3 and COOH, but not T2. T3 and COOH-induced VSMC chemotaxis were inhibited by PD98059 or SB202190, but more completely by SB202190. The T2 domain had no effect on VSMC chemotaxis. These results suggest activation of the p38 pathway may be more specific than ERK for COOH- and T3-induced VSMC chemotaxis.

Further characterization of BC3H1 myogenic cells reveals lack of p53 activity and underexpression of several p53 regulated and extracellular matrix-associated gene products

To catalog factors that may contribute to the completion of myogenesis, we have been looking for molecular differences between BC3H1 and C2C12 cells. Cells of the BC3H1 tumor line, though myogenic, are nonfusing, and withdraw from the cell cycle only reversibly, whereas cells of the C2C12 line fuse, differentiate terminally, and express several muscle-specific gene products that BC3H1 cells do not. Relative to C2C12 cells, BC3H1 cells underaccumulated cyclin-dependent kinase inhibitor p21 and underaccumulated transcripts for p21, GADD45, CDO, decorin, osteopontin, H19, fibronectin, and thrombospondin-1 (tsp-1). Levels of accumulation of H19, tsp-1, and larger isoforms of fibronectin messenger ribonucleic acid (mRNA) were found to increase in response to expression of myogenic regulatory factors as shown by their accumulation in differentiated myogenically converted 10T1/2 cells but not in 10T1/2 fibroblasts. BC3H1s accumulated a temperature-insensitive, geldanamycin-sensitive, misfolded form of p53 incapable of transactivating a p53 responsive reporter, consistent with underexpression of p21, GADD45, and tsp-1. BC3H1 and C2C12 cells were similar with respect to upregulation of p27 protein, downregulation of mitogen-activated protein kinase phosphatase-1 (MKP-1) protein, upregulation of retinoblastoma (Rb) mRNA, and nuclear localization of hypophosphorylated Rb. Cells of both lines expressed the muscle-specific 1b isoform of MEF2D. Although nonfusing in the short term, after more than 18 d in differentiation medium, some cultures of BC3H1 cells formed viable multinucleated cells in which the nuclei did not reinitiate synthesis of DNA in response to serum. Our findings suggest participation of tsp-1 and specific isoforms of fibronectin in myogenesis and suggest additional avenues of research in myogenesis and oncogenesis.

Trimeric assembly of the C-terminal region of Thrombospondin-1 or Thrombospondin-2 is necessary for cell spreading and fascin spike organisation

Thrombospondin-1 (TSP-1) and the highly related protein thrombospondin-2(TSP-2) are trimeric extracellular molecules that have complex roles in wound healing, angiogenesis and matrix organisation. At the cellular level, TSP-1 supports cell adhesion and migration by the organisation of fascin spike cytoskeletal structures. To define the molecular requirements for assembly of fascin spikes by thrombospondins, we developed a panel of recombinant protein units of TSP-1 and TSP-2; these were designed according to the domain boundaries and included matched monomeric and trimeric units. These proteins were tested for their effects on cell attachment and fascin spike organisation using C2C12 skeletal myoblasts and vascular smooth muscle cells. In monomeric units, cell attachment activity was localised to the type 1 repeats or type 3 repeats/C-terminal globule, and both regions need to be present in the same molecule for maximal activity. On a molar basis, cell-attachment activities with monomeric units were low compared with intact TSP-1, and no monomeric unit induced cell spreading. Trimeric versions of the type 1 repeats were more adhesive but did not induce cell spreading. Strikingly, trimers that contained the type 3 repeats/C-terminal globule of either TSP-1 or TSP-2 supported cell spreading and fascin spike organisation, producing a similar activity to intact TSP-1. We conclude that trimeric assembly of the highly conserved TSP C-terminal region is necessary for organisation of the fascin-based cytoskeletal structures that are needed for thrombospondin-induced cell motility.

Thrombospondins: multifunctional regulators of cell interactions

Thrombospondins are secreted, multidomain macromolecules that act as regulators of cell interactions in vertebrates. Gene knockout mice constructed for two members of this family demonstrate roles in the organization and homeostasis of multiple tissues, with particularly significant activities in the regulation of angiogenesis. This review discusses the functions of thrombospondins with regard to their cellular mechanisms of action and highlights recent advances in understanding how multifactorial molecular interactions, at the cell surface and within extracellular matrix, produce cell-type-specific effects on cell behavior and the organization of matrix and tissues.

A role for syndecan-1 in coupling fascin spike formation by thrombospondin-1

An important role of cell matrix adhesion receptors is to mediate transmembrane coupling between extracellular matrix attachment, actin reorganization, and cell spreading. Thrombospondin (TSP)-1 is a modulatory component of matrix expressed during development, immune response, or wound repair. Cell adhesion to TSP-1 involves formation of biochemically distinct matrix contacts based on stable fascin spikes. The cell surface adhesion receptors required have not been identified. We report here that antibody clustering of syndecan-1 proteoglycan specifically transduces organization of cortical actin and fascin bundles in several cell types. Transfection of COS-7 cells with syndecan-1 is sufficient to stimulate cell spreading, fascin spike assembly, and extensive protrusive lateral ruffling on TSP-1 or on syndecan-1 antibody. The underlying molecular mechanism depends on glycosaminoglycan (GAG) modification of the syndecan-1 core protein at residues S45 or S47 for cell membrane spreading and on the VC2 region of the cytoplasmic domain for spreading and fascin spike formation. Expression of the VC2 deletion mutant or GAG-negative syndecan-1 showed that syndecan-1 is necessary in spreading and fascin spike formation by C2C12 cells on TSP-1. These results establish a novel role for syndecan-1 protein in coupling a physiological matrix ligand to formation of a specific matrix contact structure.

C-mannosylation and O-fucosylation of the thrombospondin type 1 module

Thrombospondin-1 (TSP-1) is a multidomain protein that has been implicated in cell adhesion, motility, and growth. Some of these functions have been localized to the three thrombospondin type 1 repeats (TSRs), modules of approximately 60 amino acids in length with conserved Cys and Trp residues. The Trp residues occur in WXXW patterns, which are the recognition motifs for protein C-mannosylation. This modification involves the attachment of an alpha-mannosyl residue to the C-2 atom of the first tryptophan. Analysis of human platelet TSP-1 revealed that Trp-368, -420, -423, and -480 are C-mannosylated. Mannosylation also occurred in recombinant, baculovirally expressed TSR modules from Sf9 and "High Five" cells, contradictory to earlier reports that such cells do not carry out this reaction. In the course of these studies it was appreciated that the TSRs in TSP-1 undergo a second form of unusual glycosylation. By using a novel mass spectrometric approach, it was found that Ser-377, Thr-432, and Thr-489 in the motif CSX(S/T)CG carry the O-linked disaccharide Glc-Fuc-O-Ser/Thr. This is the first protein in which such a disaccharide has been identified, although protein O-fucosylation is well described in epidermal growth factor-like modules. Both C- and O-glycosylations take place on residues that have been implicated in the interaction of TSP-1 with glycosaminoglycans or other cellular receptors.

The C-terminal domain of thrombospondin-1 induces vascular smooth muscle cell chemotaxis

OBJECTIVE

Thrombospondin-1 (TSP-1), an acute-phase reactant implicated in vascular disease, is a 420-kd multifunctional glycoprotein chemotactic for vascular smooth muscle cells (VSMCs). TSP-1 has six domains of repeating homologous amino acid sequences: N-terminal, procollagen homology, type 1 repeat, type 2 repeat, type 3 repeat/RGD (T3), and C-terminal (COOH). The purpose of this experiment was to determine which domains of TSP-1 induce VSMC chemotaxis.

METHODS

A modified Boyden Chamber chemotaxis assay was used to assess VSMC migration. Serum-free medium, TSP-1, or each of the fusion proteins (10 and 20 microg/mL) synthesized for the different domains were placed in the bottom wells. Quiescent bovine aortic VSMCs (50,000) were placed in the top wells. After 4 hours at 37 degrees C, migrated VSMCs were recorded as cells per five fields (400x) and analyzed with the paired t test. To verify the fusion protein data, we performed chemotaxis assays with antibodies to each of the domains (25 microg/mL) combined with TSP-1 (20 microg/mL) in the bottom wells and VSMCs in the top wells.

RESULTS

The COOH domain significantly stimulated VSMC chemotaxis (P = <.001). To a lesser extent, the N-terminal and T3 domains also induced chemotaxis (P <.05). However, only the anti-COOH antibody (C6.7) and the anti-integrin-associated protein portion of COOH antibody (D4.6) significantly inhibited TSP-1-induced VSMC chemotaxis (by 85% and 92%, respectively).

CONCLUSIONS

These results implicate the COOH domain as the portion of the TSP-1 molecule primarily responsible for VSMC chemotaxis. This experiment suggests that future strategies in the prevention of VSMC migration, an initial step in the development of vascular lesions, may involve selective inhibition of the COOH domain of TSP-1.

Direct determination of glycosylation sites in O-fucosylated glycopeptides using nano-electrospray quadrupole time-of-flight mass spectrometry

O-Fucosylation is an unusual posttranslational modification present in several proteins that play important roles in physiological processes such as coagulation, cell signaling and metastasis. Although the exact function of the modification is still unclear, the number of proteins found to be modified is increasing, and there is a need for further structural and functional analyses. Here we report on a rapid and straightforward approach in the analysis of glycosylation status and determination of glycosylation sites in O-fucosylated glycopeptides using nano-electrospray quadrupole time-of-flight (nano-ESI Q-TOF) mass spectrometry. In a single measurement of previously chemically untreated O-fucosylated peptides originating from the thrombospondin-1 repeats, we were able to determine the glycosylation status of the analyzed peptide, the glycosylation site, and the glycan structure. The abundance of glycosylated peptide fragment ions in MS(2) spectra suggests that nano-ESI Q-TOF mass spectrometry can be used as a general approach in structural studies of O-fucosylation in proteins.

Cartilage oligomeric matrix protein (thrombospondin-5) is expressed by human vascular smooth muscle cells

Cartilage oligomeric matrix protein (COMP/thrombospondin [TSP]-5) belongs to the thrombospondin gene family and is an extracellular glycoprotein found predominantly in cartilage and tendon. To date, there is limited evidence of COMP/TSP-5 expression outside of the skeletal system. The aim of the present study was to investigate the expression of COMP/TSP-5 in cultured human vascular smooth muscle cells and human arteries. COMP/TSP-5 mRNA and protein expression was detected in cultured human vascular smooth muscle cells with both Northern blotting and immunoprecipitation. Serum, as well as transforming growth factor (TGF)beta1 and TGF-beta3, stimulated COMP/TSP-5 mRNA expression. COMP/TSP-5 was detected in normal as well as atherosclerotic and restenotic human arteries with immunohistochemistry. The majority of COMP/TSP-5 was expressed in close proximity to vascular smooth muscle cells. In vitro attachment assays demonstrated strong adhesion of smooth muscle cells to COMP/TSP-5-coated surfaces, with the majority of cells spreading and forming stress fibers. In addition, COMP/TSP-5 supported the migration of smooth muscle cells in vitro. The present study shows that COMP/TSP-5 is present in human arteries and may play a role in the adhesion and migration of vascular smooth muscle cells during vasculogenesis and in vascular disease settings such as atherosclerosis.

The cell biology of thrombospondin-1

Thrombospondin-1 (TSP-1) is a matricellular protein that regulates cellular phenotype during tissue genesis and repair. It acts as a molecular facilitator by bringing together cytokines, growth factors, matrix components, membrane receptors and extracellular proteases. TSP-1 binds to a wide variety of integrin and non-integrin cell surface receptors. The binding sites for these receptors on TSP-1 are dispersed throughout the molecule, with most domains binding multiple receptors. In some cases, TSP-1 binds to multiple receptors concurrently, and recent data indicate that there is cross-talk between the receptor systems. Thus, TSP-1 may function to direct the clustering of receptors to specialized domains for adhesion and signal transduction.

Residues F16-G33 and A784-N823 within platelet thrombospondin-1 play a major role in binding human neutrophils: evaluation by two novel binding assays

The thrombospondin-1 (TSP1) structural requirements within its heparin-binding domain (HBD)(30 kd) or within the other domains of the molecule (450 kd) that interact with neutrophils (PMNs) have not been delineated. Synthetic peptides based on the HBD, a TSP1 proteolytic fragment lacking the HBD, a large C-terminal domain of TSP1 (210 kd), a TSP1 recombinant fragment (rTSP1(784-932)), and a monoclonal antibody directed against the TSP1 type 3 repeats (mAb D4.6) were utilized to map such structural requirements on TSP1. Synthetic peptides containing a heparin-binding motif and encompassing residues F16-G33 or A74-S95 of TSP1 competed quantitatively with iodine 125-labeled TSP1 for binding to heparinagarose beads. However, only F16-G33 was a competitor of TSP1 binding to PMNs, suggesting that the sequence F16-G33 within the HBD plays a role in PMN binding. The interaction site within the 450-kd fragment was further narrowed. A TSP1 -derived proteolytic fragment (210 kd), a recombinant TSP1 fragment (rTSP1(784-932)), and a type 3 repeat anti-TSP1 monoclonal antibody (mAb D4.6) competed for the binding of 125I-labeled TSP1 to PMNs. The N-terminal of rTSP1(784-932) and C-terminal sequence analysis of TSP1-210 kd delineated the structural requirements for the second binding region for PMNs-namely, residues A784-N823.

Cell-matrix adhesions differentially regulate fascin phosphorylation

Cell adhesion to individual macromolecules of the extracellular matrix has dramatic effects on the subcellular localization of the actin-bundling protein fascin and on the ability of cells to form stable fascin microspikes. The actin-binding activity of fascin is down-regulated by phosphorylation, and we used two differentiated cell types, C2C12 skeletal myoblasts and LLC-PK1 kidney epithelial cells, to examine the hypothesis that cell adhesion to the matrix components fibronectin, laminin-1, and thrombospondin-1 differentially regulates fascin phosphorylation. In both cell types, treatment with the PKC activator 12-tetradecanoyl phorbol 13-acetate (TPA) or adhesion to fibronectin led to a diffuse distribution of fascin after 1 h. C2C12 cells contain the PKC family members alpha, gamma, and lambda, and PKCalpha localization was altered upon cell adhesion to fibronectin. Two-dimensional isoelectric focusing/SDS-polyacrylamide gels were used to determine that fascin became phosphorylated in cells adherent to fibronectin and was inhibited by the PKC inhibitors calphostin C and chelerythrine chloride. Phosphorylation of fascin was not detected in cells adherent to thrombospondin-1 or to laminin-1. LLC-PK1 cells expressing green fluorescent protein (GFP)-fascin also displayed similar regulation of fascin phosphorylation. LLC-PK1 cells expressing GFP-fascin S39A, a nonphosphorylatable mutant, did not undergo spreading and focal contact organization on fibronectin, whereas cells expressing a GFP-fascin S39D mutant with constitutive negative charge spread more extensively than wild-type cells. In contrast, C2C12 cells coexpressing S39A fascin with endogenous fascin remained competent to form microspikes on thrombospondin-1, and cells that expressed fascin S39D attached to thrombospondin-1 but did not form microspikes. Blockade of PKCalpha activity by TPA-induced down-regulation led to actin association of wild-type fascin in fibronectin-adherent C2C12 and LLC-PK1 cells but did not alter the distribution of S39A or S39D fascins. The association of fascin with actin in fibronectin-adherent cells was also evident in the presence of an inhibitory antibody to integrin alpha5 subunit. These novel results establish matrix-initiated PKC-dependent regulation of fascin phosphorylation at serine 39 as a mechanism whereby matrix adhesion is coupled to the organization of cytoskeletal structure.

Inhibition of angiogenesis by thrombospondin-1 is mediated by 2 independent regions within the type 1 repeats

BACKGROUND

Suppression of tumor growth by thrombospondin-1 (TSP-1) has been associated with its ability to inhibit neovascularization. The antiangiogenic activity of TSP-1, as defined by cornea pocket assays, was previously mapped to the amino-terminal portion of the protein within the procollagen region and the type 1 repeats.

METHODS AND RESULTS

We evaluated the specificity and efficacy of different regions of TSP-1 using recombinant fragments of the protein on chorioallantoic membrane (CAM) angiogenesis and endothelial cell proliferation assays. In both assays, fragments containing the second and third type 1 repeats but not the procollagen region inhibited angiogenesis and endothelial cell proliferation. To further define the sequences responsible for the angiostatic effect of TSP-1, we used synthetic peptides. The CAM assay defined 2 sequences that independently suppressed angiogenesis. The amino-terminal end of the type 1 repeats showed higher potency for inhibiting angiogenesis driven by basic fibroblast growth factor (FGF-2), whereas the second region equally blocked angiogenesis driven by either FGF-2 or vascular endothelial growth factor (VEGF). Modifications of the active peptides revealed the specific amino acids required for the inhibitory response. One sequence included the conserved tryptophan residues in the amino-terminal end of the second and third type 1 repeats, and the other involved the amino acids that follow the CSVTCG sequence in the carboxy-terminus of these repeats. Both inhibition in the CAM assay and inhibition of breast tumor xenograft growth in nude mice were independent of the TGF-beta-activating sequence located in the second type 1 repeat.

CONCLUSIONS

These results indicate that the type 1 repeats of TSP-1 contain 2 subdomains that may independently inhibit neovascularization. They also identify 2 independent pathways by which TSP-1 can block FGF-2 and VEGF angiogenic signals on endothelial cells.

The Carboxy-Terminal Cell-Binding Domain of Thrombospondin Is Essential for Sickle Red Blood Cell Adhesion

Sickle red blood cells (SS-RBCs) have enhanced adhesion to the plasma and subendothelial matrix protein thrombospondin-1 (TSP) under conditions of flow in vitro. TSP has at least four domains that mediate cell adhesion. The goal of this study was to map the site(s) on TSP that binds SS-RBCs. Purified TSP proteolytic fragments containing either the N-terminal heparin-binding domain, or the type 1, 2, or 3 repeats, failed to sustain SS-RBC adhesion (&lt;10% adhesion). However, a 140-kD thermolysin TSP fragment, containing the carboxy-terminal cell-binding domain in addition to the type 1, 2, and 3 repeats fully supported the adhesion of SS-RBCs (126% ± 25% adhesion). Two cell-binding domain adhesive peptides, 4N1K (KRFYVVMWKK) and 7N3 (FIRVVMYEGKK), failed to either inhibit or support SS-RBC adhesion to TSP. In addition, monoclonal antibody C6.7, which blocks platelet and melanoma cell adhesion to the cell-binding domain, did not inhibit SS-RBC adhesion to TSP. These data suggest that a novel adhesive site within the cell binding domain of TSP promotes the adhesion of sickle RBCs to TSP. Furthermore, soluble TSP did not bind SS-RBCs as detected by flow cytometry, nor inhibit SS-RBC adhesion to immobilized TSP under conditions of flow, indicating that the adhesive site on TSP that recognizes SS-RBCs is exposed only after TSP binds to a matrix. We conclude that the intact carboxy-terminal cell-binding domain of TSP is essential for the adhesion of sickle RBCs under flow conditions. This study also provides evidence for a unique adhesive site within the cell-binding domain that is exposed after TSP binds to a matrix.

Thrombospondin-1 and neural crest cell migration

Using a monoclonal antibody raised against human platelet thrombospondin, we found anti-thrombospondin immunoreactivity in the extracellular matrix of avian embryos, coincident with the ventral pathways followed by trunk neural crest cells. To confirm that the antibody recognized thrombospondin-1 and to determine the tissue of origin of the thrombospondin matrix, a thrombospondin-1 cRNA probe was used for whole mount in situ hybridization. This probe revealed thrombospondin-1 mRNAs in the developing myotome before and during neural crest cell migration. The effect of thrombospondin-1 on neural crest cell migration, morphology, and adhesion was assayed in vitro. Quail trunk neural crest cells cultured on 4 microg/ml of thrombospondin-1 migrate at 1.14 +/- 0.54 microm/min, which is significantly greater than the rate of cell migration on tissue culture plastic. Using a shaker-based adhesion assay, a significantly greater number of neural crest cells remain attached to dishes coated with 4 microg/ml of thrombospondin-1 than to tissue culture plastic alone. The number of neural crest cells that remain attached to 4 microg/ml of thrombospondin-1 is similar to the number that remain attached to dishes coated with 10 microg/ml of fibronectin. These observations indicate that neural crest cells migrate through a thrombospondin-filled extracellular matrix, and that thrombospondin-1 promotes neural crest cell migration and adhesion. Thus, thrombospondin-1 is the first somite-derived extracellular matrix molecule with properties consistent with a role in the promotion of migration into the anterior somite, as opposed to the repulsion of neural crest cells from the posterior half of the somite.

Expression of thrombospondin-1 in ischemia-induced retinal neovascularization

Thrombospondin-1 is an extracellular matrix protein that inhibits endothelial cell proliferation, migration, and angiogenesis. This study was performed to investigate the role of thrombospondin-1 in ischemic retinal neovascularization. In a murine model of retinal neovascularization, thrombospondin-1 mRNA was increased from postnatal day 13 (P13), with a threefold peak response observed on P15, corresponding to the time of development of retinal neovascularization. Prominent expression of thrombospondin-1 was observed in neovascular cells, specifically, cells adjacent to the area of nonperfusion. It has been suggested that vascular endothelial growth factor (VEGF) plays a major role in ischemia-induced retinal neovascularization of this model, so we studied the effects of VEGF on thrombospondin-1 expression. In bovine retinal microcapillary endothelial cells, VEGF induced a biphasic response of thrombospondin-1 expression; VEGF decreased thrombospondin-1 mRNA 0.41-fold after 4 hours, whereas it increased, with a threefold peak response, after 24 hours. VEGF-induced endothelial cell proliferation was completely inhibited by exogenous thrombospondin-1 and increased by 37.5% with anti-thrombospondin-1 antibody. The present findings suggest that, in the ischemic retina, retinal neovascular cells increase thrombospondin-1 expression, and VEGF may stimulate endogenous thrombospondin-1 induction, which inhibits endothelial cell growth. VEGF-mediated thrombospondin-1 induction in ischemia-induced angiogenesis may be a negative feedback mechanism.