Pathophysiological roles of thrombospondin-4 in disease development

Thrombospondin-4 (TSP-4) belongs to the extracellular matrix glycoprotein family of thrombospondins (TSPs). The multidomain, pentameric structure of TSP-4 allows its interactions with numerous extracellular matrix components, proteins and signaling molecules that enable its modulation to various physiological and pathological processes. Characterization of TSP-4 expression under development and pathogenesis of disorders has yielded important insights into mechanisms underlying the unique role of TSP-4 in mediating various processes including cell-cell, cell-extracellular matrix interactions, cell migration, proliferation, tissue remodeling, angiogenesis, and synaptogenesis. Maladaptation of these processes in response to pathological insults and stress can accelerate the development of disorders including skeletal dysplasia, osteoporosis, degenerative joint disease, cardiovascular diseases, tumor progression/metastasis and neurological disorders. Overall, the diverse functions of TSP-4 suggest that it may be a potential marker or therapeutic target for prognosis, diagnosis, and treatment of various pathological conditions upon further investigations. This review article highlights recent findings on the role of TSP-4 in both physiological and pathological conditions with a focus on what sets it apart from other TSPs.

Molecular evolution of the Thrombospondin superfamily

Thrombospondins (TSPs) are multidomain, calcium-binding glycoproteins that have wide-ranging roles in vertebrates in cell interactions, extracellular matrix (ECM) organisation, angiogenesis, tissue remodelling, synaptogenesis, and also in musculoskeletal and cardiovascular functions. Land animals encode five TSPs, which assembly co-translationally either as trimers (subgroup A) or pentamers (subgroup B). The vast majority of research has focused on this canonical TSP family, which evolved through the whole-genome duplications that took place early in the vertebrate lineage. With benefit of the growth in genome- and transcriptome-predicted proteomes of a much wider range of animal species, examination of TSPs throughout metazoan phyla has revealed extensive conservation of subgroup B-type TSPs in invertebrates. In addition, these searches established that canonical TSPs are, in fact, one branch within a TSP superfamily that includes other clades designated mega-TSPs, sushi-TSPs and poriferan-TSPs. Despite the apparent simplicity of poriferans and cnidarians as organisms, these phyla encode a greater diversity of TSP superfamily members than vertebrates. We discuss here the molecular characteristics of the TSP superfamily members, current knowledge of their expression profiles and functions in invertebrates, and models for the evolution of this complex ECM superfamily.

Molecular characterization and functional analysis of Japanese flounder (Paralichthys olivaceus) thbs2 in response to lymphocystis disease virus

In mammals, a matricellular protein, thrombospondin 2 (Thbs2) has been reported to play important roles in modulating cell-matrix interactions, vascular integrity and thrombosis formation. However, the role of gene, thbs2 has not yet been studied in teleost. In the present study, this novel fish gene from Japanese flounder was cloned and its function in resistant to lymphocystis disease virus was elucidated. The Japanese flounder thbs2 encoded a 1176-amino acid protein with 91% identity to medaka. Amino acid sequence indicated that Japanese flounder Thbs2 contained 10 typical conserved domains. The thbs2 was expressed in all stages of embryo development, and in hatched larva stage, its expression was significantly higher than that in other stages (P < 0.05). The relative expression level of thbs2 was significantly higher in the head kidney, liver, blood, gill, and heart of the lymphocystis disease virus resistant fish than in sensitive fish (P < 0.05); and in muscle, this difference was at highly significant (P < 0.01). Additionally, the distribution of Thbs2 in tissue was evaluated by immunohistochemical staining. Subcellular localization analysis showed that Thbs2 was distributed throughout the cytoplasm of the cells. Taken together, our results provide new basic data for thbs2 function, especially its role in anti-lymphocystis disease virus immune response.

Sequence analysis of the thrombospondin-related adhesive protein gene and heat shock protein 70 gene of Babesia gibsoni isolated from dogs in Nanjing, China

In this study, the thrombospondin-related adhesive protein (TRAP) gene and the heat shock protein 70 (Hsp70) gene of Babesia gibsoni isolated from the naturally infected dog in the Nanjing area were cloned and sequenced. Twenty blood samples were collected from the suspected cases of babesiosis at the animal hospital of Nanjing Agriculture University. Genomic DNA was extracted from the blood samples, and the 18S rDNAs were amplified by PCR to confirm Babesia infection. As a result, 10 cases of Babesia 18S rDNA gene amplification were positive in the 20 blood samples, confirming that the 10 cases were infected with canine Babesia. The TRAP and the Hsp70 gene fragments were amplified from all 10 positive cases. The four isolates, named NJN1, NJN2, NJN3 and NJN4, were sequenced and compared with other isolates in Asian. The similarity of TRAP and Hsp70 gene sequences among four isolates in Nanjing were above 99%. The Nanjing isolates were closely related to isolates from Taiwan and Japan. Indian isolates were different form Chinese and Japanese isolates, despite the very high similarity of the 18s rRNA genes sequence. These results suggest that the TRAP and Hsp70 genes have a reference value for the genetic diversity analysis of Babesia gibsoni.

Structure of thrombospondin type 3 repeats in bacterial outer membrane protein A reveals its intra-repeat disulfide bond-dependent calcium-binding capability

Eukaryotic thrombospondin type 3 repeat (TT3R) is an efficient calcium ion (Ca²⁺) binding motif only found in mammalian thrombospondin family. TT3R has also been found in prokaryotic cellulase Cel5G, which was thought to forfeit the Ca²⁺-binding capability due to the formation of intra-repeat disulfide bonds, instead of the inter-repeat ones possessed by eukaryotic TT3Rs. In this study, we have identified an enormous number of prokaryotic TT3R-containing proteins belonging to several different protein families, including outer membrane protein A (OmpA), an important structural protein connecting the outer membrane and the periplasmic peptidoglycan layer in gram-negative bacteria. Here, we report the crystal structure of the periplasmic region of OmpA from Capnocytophaga gingivalis, which contains a linker region comprising five consecutive TT3Rs. The structure of OmpA-TT3R exhibits a well-ordered architecture organized around two tightly-coordinated Ca²⁺ and confirms the presence of abnormal intra-repeat disulfide bonds. Further mutagenesis studies showed that the Ca²⁺-binding capability of OmpA-TT3R is indeed dependent on the proper formation of intra-repeat disulfide bonds, which help to fix a conserved glycine residue at its proper position for Ca²⁺ coordination. Additionally, despite lacking inter-repeat disulfide bonds, the interfaces between adjacent OmpA-TT3Rs are enhanced by both hydrophobic and conserved aromatic-proline interactions.

N-Glycosylation of Human R-Spondin 1 Is Required for Efficient Secretion and Stability but Not for Its Heparin Binding Ability

R-spondin 1 (Rspo1) plays an essential role in stem cell biology by potentiating Wnt signaling activity. Despite the fact that Rspo1 holds therapeutic potential for a number of diseases, its biogenesis is not fully elucidated. All Rspo proteins feature two amino-terminal furin-like repeats, which are responsible for Wnt signal potentiation, and a thrombospondin type 1 (TSR1) domain that can provide affinity towards heparan sulfate proteoglycans. Using chemical inhibitors, deglycosylase and site-directed mutagenesis, we found that human Rspo1 and Rspo3 are both N-glycosylated at N137, a site near the C-terminus of the furin repeat 2 domain, and Rspo2 is N-glycosylated at N160, a position near the N-terminus of TSR1 domain. Elimination of N-glycosylation at these sites affects their accumulation in media but have no effect on the ability towards heparin. Introduction of the N-glycosylation site to Rspo2 mutant at the position homologous to N137 in Rspo1 restored full glycosylation and rescued the accumulation defect of nonglycosylated Rspo2 mutant in media. Similar effect can be observed in the N137 Rspo1 or Rspo3 mutant engineered with Rspo2 N-glycosylation site. The results highlight the importance of N-glycosylation at these two positions in efficient folding and secretion of Rspo family. Finally, we further showed that human Rspo1 is subjected to endoplasmic reticulum (ER) quality control in N-glycan-dependent manner. While N-glycan of Rspo1 plays a role in its intracellular stability, it had little effect on secreted Rspo1. Our findings provide evidence for the critical role of N-glycosylation in the biogenesis of Rspo1.

Crystal structure of R-spondin 2 in complex with the ectodomains of its receptors LGR5 and ZNRF3

The four secreted R-spondin (Rspo1-4) proteins of vertebrates function as stem cell growth factors and potentiate canonical Wnt signalling. Rspo proteins act by cross-linking members of two cell surface receptor families, complexing the stem cell markers LGR4-6 with the Frizzled-specific E3 ubiquitin ligases ZNRF3/RNF43. The consequent internalisation of the ternary LGR-Rspo-E3 complex removes the E3 ligase activity, which otherwise targets the Wnt receptor Frizzled for degradation, and thus enhances Wnt signalling. Multiple combinations of LGR4-6, Rspo1-4 and ZNRF3/RNF43 are possible, implying the existence of generic interaction determinants, but also of specific differences in complex architecture and activity. We present here a high resolution crystal structure of an ectodomain variant of human LGR5 (hLGR5ecto) complexed with a signalling competent fragment of mouse Rspo2 (mRspo2Fu1-Fu2). The structure shows that the particularly potent Rspo2 ligand engages LGR5 in a fashion almost identical to that reported for hRSPO1. Comparison of our hLGR5ecto structure with previously published structures highlights a surprising plasticity of the LGR ectodomains, characterised by a nearly 9° or larger rotation of the N-terminal half of the horseshoe-like fold relative to the C-terminal half. We also report a low resolution hLGR5-mRspo2Fu1-Fu2-mZNRF3ecto ternary complex structure. This crystal structure confirms our previously suggested hypothesis, showing that Rspo proteins cross-link LGRs and ZNRF3 into a 2:2:2 complex, whereas a 1:1:1 complex is formed with RNF43.

Differences in the resting platelet proteome and platelet releasate between healthy children and adults

Major age-related diseases such as cardiovascular disease and cancer are the primary causes of morbidity and mortality in Australia and worldwide. In our recent study characterising differences in the plasma proteome between healthy children and adults, a large number of proteins differentially expressed with age were found to be of platelet origin. This study aimed to characterise differences in the resting platelet proteome and the platelet releasate of healthy children compared to healthy adults. This study represents the setup of a procedure for the proteomic analysis of platelets from children. Differentially expressed platelet proteins were identified using Two-dimensional Differential In-Gel Electrophoresis and mass spectrometry. Significant differences in the expression of nine proteins (1.1%) in the resting platelet proteome were observed in children compared to adults. Serotransferrin, fibrinogen alpha chain, glyceraldehyde-3 phosphate dehydrogenase, serum albumin, transgelin-2, calponin-2/LIM and SH3 domain protein 1 and human chorionic gonadotropin 2039797 were up-regulated, whereas thrombospondin-1 was down-regulated in children. Eleven proteins (1.5%) were differentially expressed in the platelet releasate of children compared to adults, where transferrin was also upregulated and TSP-1 was down regulated. Identified proteins are involved in processes including tissue and organ development, cell proliferation regulation and angiogenesis. Our results provide novel insights into platelet physiology as well as growth, development and ageing in healthy individuals.

BIOLOGICAL SIGNIFICANCE

The incidence of major diseases such as cardiovascular disease (CVD) and cancer increase with increasing age and are the major causes of morbidity and mortality both in Australia and worldwide. As the aged population continues to increase dramatically, so too will the financial strains associated with the long term care of the elderly population. Compared to adults, children have a significantly lower incidence of major diseases such as thromboembolic disease. This suggests that children have a protective mechanism against the development of disease. Therefore, studies focussing on the molecular changes of proteins, the machinery of the cell, between children and adults are the key to determining the underlying mechanisms responsible for the onset of major diseases. A well-defined example of how protein expression can change with age is that of the plasma proteome. Significant differences in the expression of numerous plasma proteins between healthy children and adults have been recently demonstrated. Interestingly, a large number of differentially expressed proteins were found to be of platelet origin. This finding forms the basis for the current study, presenting as strong evidence for the age-specific differences of the platelet proteome.

Study of the sites of plasminogen molecule which are responsible for inhibitory effect of Lys-plasminogen on platelet aggregation

Plasminogen/plasmin system is involved in such important processes as thrombosis, inflammation and cancer. Plasmin and plasminogen mediate their action through plasminogen-binding proteins on the cell surface. Lys-plasminogen, but not Glu-plasminogen, shows inhibitory effect on platelet aggregation induced by ADP, collagen and thrombin in preparations of both: platelet-rich plasma and washed platelets. We have shown that the kringle domains of Lys-plasminogen mediate interaction of this proenzyme with platelet- surface proteins. The aim of the work is to study the role of certain kringle domains in the inhibitory effect of Lys-plasminogen and to determine possible plasminogen-binding proteins on the platelet surface. All studied plasminogen fragments (K1-3, K4 and K5) abolished the inhibitory effect of Lys-plasminogen on platelet aggregation. We observed that K5 was more effective than K1-3 and K4. Biotin-labeled Lys-plasminogen, Glu-plasminogen and plasminogen fragment K1-3 possessed the highest affinity for actin, whereas the binding of biotin-labeled mini-plasminogen and K4 to actin was negligible. We have suggested that inhibitory effect of Lys-plasminogen is due to the interaction of kringle domains of this proenzyme with membrane-bound proteins which are exposed on the platelet surface during activation and are involved in thrombus formation.

Computational biophysical, biochemical, and evolutionary signature of human R-spondin family proteins, the member of canonical Wnt/β-catenin signaling pathway

In human, Wnt/β-catenin signaling pathway plays a significant role in cell growth, cell development, and disease pathogenesis. Four human (Rspo)s are known to activate canonical Wnt/β-catenin signaling pathway. Presently, (Rspo)s serve as therapeutic target for several human diseases. Henceforth, basic understanding about the molecular properties of (Rspo)s is essential. We approached this issue by interpreting the biochemical and biophysical properties along with molecular evolution of (Rspo)s thorough computational algorithm methods. Our analysis shows that signal peptide length is roughly similar in (Rspo)s family along with similarity in aa distribution pattern. In Rspo3, four N-glycosylation sites were noted. All members are hydrophilic in nature and showed alike GRAVY values, approximately. Conversely, Rspo3 contains the maximum positively charged residues while Rspo4 includes the lowest. Four highly aligned blocks were recorded through Gblocks. Phylogenetic analysis shows Rspo4 is being rooted with Rspo2 and similarly Rspo3 and Rspo1 have the common point of origin. Through phylogenomics study, we developed a phylogenetic tree of sixty proteins (n = 60) with the orthologs and paralogs seed sequences. Protein-protein network was also illustrated. Results demonstrated in our study may help the future researchers to unfold significant physiological and therapeutic properties of (Rspo)s in various disease models.

Two molecular weight species of thrombospondin-2 are present in bone and differentially modulated in fractured and nonfractured tibiae in a murine model of bone healing

We report two immuoreactive species of thrombospondin-2 (TSP2), sized approximately 200 and 125 kDa, in the long bones of growing, but not skeletally mature, mice. In vitro osteoblasts secrete a 200-kDa species into the culture medium as early as day 3, and it appears in the cell-matrix layer by day 7. A 125-kDa species appears in the cell-matrix layer in parallel with mineralization; it is not detected in cell-conditioned medium. Unilateral tibial fracture induced a time-dependent upregulation of the 200-kDa species at the site of trauma. By contrast, relative levels of the 125-kDa species at the fracture site were lower than in bones from naive control animals. In the contralateral untouched control tibia, the 200-kDa species was rapidly and substantially reduced compared to bone harvested from naive control mice. Levels of the 125-kDa species in the untouched tibia declined gradually with time postfracture. TSP2 gene expression in uninjured control bone decreased modestly by 21 days postfracture. On the day of fracture, the osteoblast differentiation potential of MSCs harvested from uninjured bones decreased compared to those harvested from naive control animals. The presence of two isoforms suggests that TSP2 may undergo posttranscriptional or posttranslational processing in skeletal tissue. Our data also suggest that, in the context of trauma, the two TSP2 isforms are differentially modulated at injured and noninjured skeletal sites in an animal undergoing fracture healing.

Cellular signaling and biological functions of R-spondins

R-spondins (RSPOs) are a family of cysteine-rich secreted proteins containing a single thrombospondin type I repeat (TSR) domain. A vast amount of information regarding cellular signaling and biological functions of RSPOs has emerged over the last several years, especially with respect to their roles in the activation of the WNT signaling pathway. The identification of several classes of RSPO receptors may indicate that this family of proteins can affect several signaling cascades. Herein, we summarize the current understanding of RSPO signaling and its biological functions, and discuss its potential therapeutic implications to human diseases.

Adhesion-modulating/matricellular ECM protein families: a structural, functional and evolutionary appraisal

The thrombospondins are a family of secreted, oligomeric glycoproteins that interact with cell surfaces, multiple components of the extracellular matrix, growth factors and proteases. These interactions underlie complex roles in cell interactions and tissue homeostasis in animals. Thrombospondins have been grouped functionally with SPARCs, tenascins and CCN proteins as adhesion-modulating or matricellular components of the extracellular milieu. Although all these multi-domain proteins share various commonalities of domains, the grouping is not based on structural homologies. Instead, the terms emphasise the general observations that these proteins do not form large-scale ECM structures, yet act at cell surfaces and function in coordination with the structural ECM and associated extracellular proteins. The designation of adhesion-modulation thus depends on observed tissue and cell culture ECM distributions and on experimentally identified functional properties. To date, the evolutionary relationships of these proteins have not been critically compared: yet, knowledge of their evolutionary histories is clearly relevant to any consideration of functional similarities. In this article, we survey briefly the structural and functional knowledge of these protein families, consider the evolution of each family, and outline a perspective on their functional roles.

Soluble THSD7A is an N-glycoprotein that promotes endothelial cell migration and tube formation in angiogenesis

Background

Thrombospondin type I domain containing 7A (THSD7A) is a novel neural protein that is known to affect endothelial migration and vascular patterning during development. To further understand the role of THSD7A in angiogenesis, we investigated the post-translational modification scheme of THS7DA and to reveal the underlying mechanisms by which this protein regulates blood vessel growth.

Methodology/Principal Findings

Full-length THSD7A was overexpressed in human embryonic kidney 293T (HEK293T) cells and was found to be membrane associated and N-glycosylated. The soluble form of THSD7A, which is released into the cultured medium, was harvested for further angiogenic assays. We found that soluble THSD7A promotes human umbilical vein endothelial cell (HUVEC) migration and tube formation. HUVEC sprouts and zebrafish subintestinal vessel (SIV) angiogenic assays further revealed that soluble THSD7A increases the number of branching points of new vessels. Interestingly, we found that soluble THSD7A increased the formation of filopodia in HUVEC. The distribution patterns of vinculin and phosphorylated focal adhesion kinase (FAK) were also affected, which implies a role for THSD7A in focal adhesion assembly. Moreover, soluble THSD7A increased FAK phosphorylation in HUVEC, suggesting that THSD7A is involved in regulating cytoskeleton reorganization.

Conclusions/Significance

Taken together, our results indicate that THSD7A is a membrane-associated N-glycoprotein with a soluble form. Soluble THSD7A promotes endothelial cell migration during angiogenesis via a FAK-dependent mechanism and thus may be a novel neuroangiogenic factor.

Genetic and functional linkage between ADAMTS superfamily proteins and fibrillin-1: a novel mechanism influencing microfibril assembly and function

Tissue microfibrils contain fibrillin-1 as a major constituent. Microfibrils regulate bioavailability of TGFβ superfamily growth factors and are structurally crucial in the ocular zonule. FBN1 mutations typically cause the Marfan syndrome, an autosomal dominant disorder manifesting with skeletal overgrowth, aortic aneurysm, and lens dislocation (ectopia lentis). Infrequently, FBN1 mutations cause dominantly inherited Weill-Marchesani syndrome (WMS), isolated ectopia lentis (IEL), or the fibrotic condition, geleophysic dysplasia (GD). Intriguingly, mutations in ADAMTS [a disintegrin-like and metalloprotease (reprolysin-type) with thrombospondin type 1 motif] family members phenocopy these disorders, leading to recessive WMS (ADAMTS10), WMS-like syndrome (ADAMTS17), IEL (ADAMTSL4 and ADAMTS17) and GD (ADAMTSL2). An ADAMTSL2 founder mutation causes Musladin-Lueke syndrome, a fibrotic disorder in beagle dogs. The overlapping disease spectra resulting from fibrillin-1 and ADAMTS mutations, interaction of ADAMTS10 and ADAMTSL2 with fibrillin-1, and evidence that these ADAMTS proteins accelerate microfibril biogenesis, constitutes a consilience suggesting that some ADAMTS proteins evolved to provide a novel mechanism regulating microfibril formation and consequently cell behavior.

O-fucosylation of thrombospondin type 1 repeats

Thrombospondin type 1 repeats (TSRs) are small cysteine-rich motifs with three conserved disulfide bonds originally described as modules in the thrombospondins. Since then, TSRs have been found as tandem repeats in a wide variety of secreted and cell-surface proteins of diverse function. TSRs in many contexts are known to bind a variety of receptors and have antiangiogenic capabilities. They can be modified with O-linked fucose on serine/threonine found in the consensus, CX(2-3)(S/T)CX(2)G. Here we review what is known about O-fucosylation of TSRs and describe in detail mass spectral methods to map sites of O-fucosylation on proteins containing TSRs. These methods include techniques to identify glycosylated peptides and the relative amounts of elongated products by electrospray ionization mass spectrometry of glycopeptides.

Biological activities and molecular interactions of the C-terminal residue of thrombospondin-4, an epitome of acidic amphipathic peptides

C21, the C-terminal residue of thrombospondin-4 (TSP-4), was identified as a peptide growth factor during an investigation concerning erythropoietin-dependent, erythroid stimulating factors of endothelial origin. It is active in cultures of several human hematopoietic stem cells, skin fibroblasts and kidney epithelial cells and stimulates red cell formation in anemic mice. A method of affinity chromatography in the presence of high concentrations of Triton X-100, previously developed for identifying proteins associated with the TSP-1 receptor CD47, was utilized for the detection of C21 binding molecules and their detergent-resistant, associated partners. These experiments helped to delineate two different mechanisms of C21 action, which are compatible with its cell proliferating activity. As a cell matrix peptide, C21 binds to the osteopontin receptor CD44 and could act as an osteopontin antagonist, preventing the inhibition of primitive hematopoietic stem cell proliferation. TSP-1, another matrix protein, binds to C21 and could indirectly act as an antagonist, by shunting C21-CD44 interactions. The second mechanism is a direct effect of C21 on cell proliferation. The extremely rapid internalization and nuclear localization of the peptide could be explained by CD44-mediated internalization, followed by a microtubule-mediated transport towards the nucleus, or, eventually, direct membrane insertion. These alternative hypotheses are supported by previously observed membrane insertion of similar synthetic and viral acidic amphipathic peptides, the presence of microtubule-associated protein 1B (MAP1B) and dynactin in the triton-soluble complexes associated with C21 and the presence in such complexes of dual compartment proteins for nuclei and plasma membranes, such as MAP1B, AHNAK and CD44.

Influences of the N700S thrombospondin-1 polymorphism on protein structure and stability

Thrombospondins (THBSs) are multimodular, secreted proteins characterized by a signature domain comprising a unique set of 13 calcium-binding repeats flanked by epidermal growth factor (EGF)-like and lectin-like modules. A polymorphism that changes a conserved Asn to Ser at residue 700 in the most N-terminal calcium-binding repeat of THBS-1 (repeat 1C) is found in 8-10% of European populations and has been linked to increased risk of premature coronary artery disease. The Ser substitution leads to altered stability in the EGF-like and wire modules of the THBS-1 signature domain as assessed by differential scanning calorimetry carried out in 2 mm or 200 mum calcium. Studies of the melting profiles of the THBS-2 signature domain proteins with Asn or Ser at position 702 (homologous to 700 in THBS-1) revealed that the impact of the Ser allele is similar in both THBS-1 and THBS-2. Structure determination of the Ser(702) THBS-2 variant in 2 mm calcium showed that repeat 1C contains two bound calcium ions as in the crystal of the Asn(702) protein, including the ion that is coordinated by Asn(702), and is associated with changes in conformation of repeat 1C and the adjacent EGF-like modules. The Ser substitution leads to the decreased ability of soluble THBS-2 signature domain protein to bind 4B6.13, a conformation-sensitive monoclonal antibody that recognizes an epitope in repeat 1C. These results indicate that although THBS harboring the Ser allele binds a full complement of calcium ions, repeat 1C is altered, leading to destabilization of surrounding structures.

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).

RSPO4 is the major gene in autosomal-recessive anonychia and mutations cluster in the furin-like cysteine-rich domains of the Wnt signaling ligand R-spondin 4

Congenital anonychia is a rare autosomal-recessive disorder characterized by the absence of finger- and toenails. Recently, we and others identified the secreted Wnt signaling ligand R-spondin 4 (RSPO4) as the first gene known to be responsible for inherited anonychia. R-spondins are secreted proteins that activate the Wnt/beta-catenin signaling pathway. This puts anonychia on the growing list of congenital malformation syndromes caused by Wnt signaling pathway defects. Here, we expand the RSPO4 mutational spectrum by identification of the previously unknown mutations c.190C>T (p.Arg64Cys) in exon 2 and c.301C>T (p.Gln101X) in exon 3, thereby corroborating R-spondin 4 as the major protein in autosomal-recessive anonychia. Almost all RSPO4 mutations detected so far affect the highly conserved exons 2 and 3. Thus, we postulate that RSPO4 mutations preferentially cluster in the furin-like cysteine-rich domains of R-spondin 4, which is in line with experimental data proposing that for beta-catenin stabilization, a shortened protein comprising just these two regions is sufficient.

The thrombospondin-related protein CpMIC1 (CpTSP8) belongs to the repertoire of micronemal proteins of Cryptosporidium parvum

Bioinformatic data show that, in addition to TRAP-C1, Cryptosporidium parvum encodes 11 thrombospondin-related proteins (CpTSP2 through CpTSP12), none of which has been characterized yet. We describe herein the cloning of a 2048 bp-long sporozoite cDNA encoding CpTSP8, a type I integral membrane protein of 614 amino acids, possessing three thrombospondin type I (TSP1) repeats and one epidermal growth factor (EGF)-like domain. Transcriptionally, CpTSP8 is represented by a fully spliced and two immature mRNA forms, in which the intron is either totally or partially retained. Immunofluorescence analysis detected CpTSP8 in the apical complex of both sporozoites and type I merozoites, and showed that, upon sporozoite exposure to host cells in vitro, the protein is translocated onto the parasite surface as typical of micronemal proteins (MICs). Accordingly, double immunofluorescence localized CpTSP8 to C. parvum micronemes, prompting us to rename it CpMIC1 in agreement with the current MICs nomenclature.

Peptides derived from type I thrombospondin repeat-containing proteins of the CCN family inhibit proliferation and migration of endothelial cells

Angiogenesis, or neovascularization, is tightly orchestrated by endogenous regulators that promote or inhibit the process. The fine-tuning of these pro- and anti-angiogenic elements (the angiogenic balance) helps establish the homeostasis in tissues, and any aberration leads to pathologic conditions. The type I thrombospondin repeats are a family of protein structural elements involved in the control of angiogenesis, and some proteins containing these repeats have been identified as negative regulators of angiogenesis. Here we identify a set of 11 novel, anti-angiogenic 18-20-amino acid peptides that are derived from proteins that belong to the CCN protein family and contain type I thrombospondin motifs. We have named these peptides spondinstatin-1, cyrostatin, connectostatin, nephroblastostatin, wispostatin-2, wispostatin-3, netrinstatin-5C, netrinstatin-5D, adamtsostatin-like-4, fibulostatin-6.1, and complestatin-C6 to reflect their origin. We have shown that these peptides inhibit proliferation and migration of human umbilical vein endothelial cells in vitro. By conducting a clustering analysis of the amino acid sequences using sequence similarity criteria and of the experimental results using a hierarchical clustering algorithm, we have demonstrated that there is an underlying correlation between the sequence and activity of the identified peptides. This combination of experimental and computational approaches introduces a novel systematic framework for studying peptide activity, identifying novel peptides with anti-angiogenic activity, and designing mimetic peptides with tailored properties.

Immunochemical analysis of the structure of the signature domains of thrombospondin-1 and thrombospondin-2 in low calcium concentrations

Thrombospondins (TSPs) undergo conformational changes upon removal of calcium. The eight C-type and five N-type calcium-binding repeats of TSP-2 form a circuitous wire that, in 2 mm calcium, interacts at its ends with more N-terminal epidermal growth factor (EGF)-like modules, EGF2 and EGF3, and the C-terminal lectin-like module. These components, along with the other EGF-like module(s), form the signature domain of TSPs. Characterization of conformation-sensitive epitopes of monoclonal antibodies to human TSP-2 and its TSP-1 homolog have given insights into the structure of the signature domain in the absence of calcium. The epitope for 4B6.13 anti-TSP-2 was localized to His-722 and Leu-703 in repeat 1C of the wire; recognition only occurred in constructs that included EGF3, the rest of the wire, and the lectin-like module and in the presence of calcium. The epitope for C6.7 anti-TSP-1 was localized to Glu-609 in the EGF2 module. The C6.7 epitope was preferentially recognized when EGF2 was expressed in the context of EGF1, EGF3, the wire, and the lectin-like module. Preferential recognition of the C6.7 epitope did not require calcium. Rotary shadowing electron microscopy of TSP-1 has shown elongation of the stalk and diminution of the C-terminal globule. We propose a model whereby at low calcium concentrations the lectin-like module drops away from EGF3 concomitant with changes in conformation of the wire and loss of the 4B6.13 epitope. A critical feature of the model is interaction of repeat 12N of the wire with EGF2 in both the presence and absence of calcium.

C-Mannosylated peptides derived from the thrombospondin type 1 repeat enhance lipopolysaccharide-induced signaling in macrophage-like RAW264.7 cells

C-Mannosylation is a unique type of glycosylation occurring at the first Trp (W) in the WXXW motif, which is found in the thrombospondin type 1 repeat (TSR) of proteins. However, the biological function of C-mannosylation is not fully understood. In this study, we investigated the effect of C-mannosylated TSR-derived peptides on lipopolysaccharide (LPS)-induced signaling in macrophage-like RAW264.7 cells. The cells were stimulated with LPS in the presence or absence of chemically synthesized peptides with or without C-mannose (e.g., (C-Man)-Trp-Ser-Pro-Trp [C-Man-WSPW], C-Man-W, WSPW, etc.), then the effects of the peptides on cellular viability and signaling were examined. We found a cytotoxic effect in the cells treated with LPS and C-Man-WSPW, but not in the cells solely treated with LPS or C-Man-WSPW. We also found that production of tumor necrosis factor-alpha (TNF-alpha) was upregulated more in response to LPS plus C-Man-WSPW, than in response to LPS plus WSPW or LPS alone. Among the LPS-induced signaling pathways that induce production of TNF-alpha, the activation of c-Jun N-terminal kinase (JNK) was greatly enhanced by LPS and C-Man-WSPW, and the production of TNF-alpha was suppressed by an inhibitor for JNK. Together, these results demonstrate a novel function of the C-mannosylated TSR-derived peptide motif, to promote LPS-induced JNK signaling, and this leads to an enhancement of cytotoxicity via the upregulation of TNF-alpha production.

Anti-angiogenic peptides identified in thrombospondin type I domains

Thrombospondin 1, the prototypical protein of the thrombospondin protein family, is a potent endogenous inhibitor of angiogenesis. Although the effects of the thrombospondin 1 on neovascularization have been well studied, little is known about the anti-angiogenic potency of other proteins or peptide fragments derived from the proteins in this family. Here we identify a set of 18 novel, anti-angiogenic 17- to 20-amino acid peptides that are derived from proteins containing type I thrombospondin motifs. We have named these peptides adamtsostatin-4, adamtsostatin-16, adamtsostatin-18, cartilostatin-1, cartilostatin-2, fibulostatin-6.2, fibulostatin-6.3, papilostatin-1, papilostatin-2, properdistatin, scospondistatin, semastatin-5A.1, semastatin-5A.2, semastatin-5B, thrombostatin containing-1, thrombostatin contaning-3, thrombostatin contaning-6, and wispostatin-1 to reflect their origin. We further demonstrate that these peptides inhibit the proliferation and migration of human umbilical vein endothelial cells in vitro. The anti-proliferative and anti-migratory properties of the identified peptides may be important in maintaining angiogenic homeostasis in vivo and make these peptides suitable candidates for use as anti-angiogenic pharmaceutical agents in numerous therapeutic applications.

ABT-510, a modified type 1 repeat peptide of thrombospondin, inhibits malignant glioma growth in vivo by inhibiting angiogenesis

Anti-angiogenic therapies would be particularly beneficial in the treatment of malignant gliomas. Peptides derived from the second type 1 repeat (TSR) of thrombospondin-1 (TSP-1) have been shown to inhibit angiogenesis in non-glioma tumor models and a modified TSR peptide, ABT-510, has now entered into Phase II clinical trials of its efficacy in non-glioma tumors. As microvascular endothelial cells (MvEC) exhibit heterogeneity, we evaluated the ability of the modified TSR peptide (NAcSarGlyValDallolleThrNvalleArgProNHE, ABT-510) to inhibit malignant glioma growth in vivo and to induce apoptosis of brain microvessel endothelial cells (MvEC) propagated in vitro. We found that daily administration of ABT-510 until euthanasia (days 7 to 19), significantly inhibited the growth of human malignant astrocytoma tumors established in the brain of athymic nude mice. The microvessel density was significantly lower and the number of apoptotic MvEC was significantly higher (3-fold) in the tumors of the ABT-510-treated animals. Similar results were found using a model in which the established tumor is an intracerebral malignant glioma propagated in a syngeneic mouse model. ABT-510 treatment of primary human brain MvEC propagated as a monolayer resulted in induction of apoptosis in a dose- and time-dependent manner through a caspase-8-dependent mechanism. It also inhibited tubular morphogenesis of MvEC propagated in collagen gels in a dose- and caspase-8 dependent manner through a mechanism that requires the TSP-1 receptor (CD36) on the MvEC. These findings indicate that ABT-510 should be evaluated as a therapeutic option for patients with malignant glioma.

Depletion of the Plasmodium berghei thrombospondin-related sporozoite protein reveals a role in host cell entry by sporozoites

The malaria parasite sporozoite stage develops in the mosquito vector and is transmitted to the mammalian host by bite. Sporozoites engage in multiple interactions with vector and host tissue on the journey from their oocyst origin to their final destination inside hepatocytes. Several malaria proteins have been identified that mediate sporozoite interactions with target tissues such as secreted and surface-associated ligands CSP and TRAP, which contain a thrombospondin type 1 repeat (TSR). Recently, we identified thrombospondin-related sporozoite protein (TRSP) in Plasmodium sporozoites, which exhibits a single TSR in its putative extracellular N-terminal region and is highly conserved among Plasmodium species. Here, we show using targeted gene disruption in the rodent malaria model Plasmodium berghei, that lack of TRSP has no effect on the asexual blood stage cycle, parasite transmission to the mosquito, sporozoite development and infection of mosquito salivary glands. However, analysis of TRSP knockout sporozoites in vitro and in vivo indicates that this protein has a significant role in hepatocyte entry and therefore liver infection. Thus, TRSP is an additional TSR-containing malaria parasite protein that is mainly involved in initial infection of the mammalian host.

Inhibition of the mitogenic, angiogenic and tumorigenic activities of pleiotrophin by a synthetic peptide corresponding to its C-thrombospondin repeat-I domain

Pleiotrophin (PTN), is a heparin-dependent growth factor involved in angiogenesis and tumor growth. PTN contains a thrombospondin repeat-I (TSR-I) motif in its two beta-sheet domains that are involved in its binding to heparin and its neurite outgrowth activity. Based on the importance of the binding of PTN to heparin in its dimerization and biological activities, we have designed two synthetic peptides, P(13-39) and P(65-97) corresponding to a part of the N-terminal and C-terminal TSR-I motif of PTN, respectively. P(65-97) inhibited the mitogenic, tumorigenic and angiogenic activities of PTN, as well as the mitogenic and an angiogenic activity of fibroblast growth factor-2 (FGF-2). However, P(65-97) had no effect on the mitogenic activity of epidermal growth factor, which does not bind heparin. P(65-97) but not P(13-39) inhibited the binding of PTN and to a lesser extent of FGF-2 to heparin using an immunoassay and an optical biosensor assay and bound directly to heparin with a K(d) of 120 nM. These findings suggest that P(65-97), containing amino acids 65-97 of the TSR-I motif of the C-terminal domain of PTN, inhibits the activities of PTN and FGF-2 by virtue of its ability to bind heparin very effectively and so compete with the growth factors for their polysaccharide co-receptor.

Receptor communication within the lymphocyte plasma membrane: a role for the thrombospondin family of matricellular proteins

Lymphocytes, the principal cells of the immune system, carry out immune surveillance throughout the body by their unique capacity to constantly reposition themselves between a free-floating vascular state and a tissue state characterized by migration and frequent adhesive interactions with endothelial cells and components of the extracellular matrix. Therefore, mechanisms co-ordinating adhesion and migration with signals delivered through antigen recognition probably play a pivotal role for the regulation of lymphocyte behaviour and function. Endogenous thrombospondin-1 (TSP-1) seems to be the hub in such a mechanism for autocrine regulation of T cell adhesion and migration. TSP-1 functions as a mediator of cis interaction of vital receptors within the T lymphocyte plasma membrane, including integrins, low density lipoprotein receptor-related protein, calreticulin and integrin-associated protein.

Identification and characterization of abeta1,3-glucosyltransferase that synthesizes the Glc-beta1,3-Fuc disaccharide on thrombospondin type 1 repeats

Thrombospondin type 1 repeats (TSRs) are biologically important domains of extracellular proteins. They are modified with a unique Glcbeta1,3Fucalpha1-O-linked disaccharide on either serine or threonine residues. Here we identify the putative glycosyltransferase, B3GTL, as the beta1,3-glucosyltransferase involved in the biosynthesis of this disaccharide. This enzyme is conserved from Caenorhabditis elegans to man and shares 28% sequence identity with Fringe, the beta1,3-N-acetylglucosaminyltransferase that modifies O-linked fucosyl residues in proteins containing epidermal growth factor-like domains, such as Notch. beta1,3-Glucosyltransferase glucosylates properly folded TSR-fucose but not fucosylated epidermal growth factor-like domain or the non-fucosylated modules. Specifically, the glucose is added in a beta1,3-linkage to the fucose in TSR. The activity profiles of beta1,3-glucosyltransferase and protein O-fucosyltransferase 2, the enzyme that carries out the first step in TSR O-fucosylation, superimpose in endoplasmic reticulum subfractions obtained by density gradient centrifugation. Both enzymes are soluble proteins that efficiently modify properly folded TSR modules. The identification of the beta1,3-glucosyltransferase gene allows us to manipulate the formation of the rare Glcbeta1,3Fucalpha1 structure to investigate its biological function.

Inhibitory autoantibodies against ADAMTS-13 in patients with thrombotic thrombocytopenic purpura bind ADAMTS-13 protease and may accelerate its clearance in vivo

BACKGROUND

Many patients with acquired thrombotic thrombocytopenic purpura (TTP) harbor autoantibodies that may bind and/or inhibit ADAMTS-13 proteolytic activity and accelerate its clearance in vivo.

METHODS

To test this hypothesis, we determined ADAMTS-13 activity and antigen levels in parallel plasma samples from patients clinically diagnosed with TTP. Collagen binding, GST-VWF73 and FRETS-VWF73 assays were used to determine ADAMTS-13 activity and to detect inhibitory autoantibodies. Enzyme-linked immunosorbent assay (ELISA) and immunoprecipitation plus Western blotting (IP/WB) were used to detect total anti-ADAMTS-13 IgG (inhibitory and non-inhibitory).

RESULTS

Among 40 patients with TTP (21 idiopathic and 19 non-idiopathic), inhibitory autoantibodies were detected (by FRETS-VWF73) in 52% of idiopathic and 0% of non-idiopathic TTP patients. In contrast, non-inhibitory IgG autoantibodies were detected in 29% of idiopathic and 50% of non-idiopathic TTP patients. The concentration of inhibitory IgG autoantibody in idiopathic TTP patients was significantly higher than that of non-inhibitory IgG in either idiopathic or non-idiopathic TTP patients. Idiopathic TTP patients demonstrated significantly reduced ADAMTS-13 activity compared with non-idiopathic patients, but only slightly lower ADAMTS-13 antigen levels. Interestingly, patients with inhibitory autoantibodies exhibited significantly lower ADAMTS-13 antigen levels than those with only non-inhibitory IgG autoantibodies or no autoantibody. Serial plasma exchanges increased levels of ADAMTS-13 activity and antigen concurrently in patients with inhibitory autoantibodies.

CONCLUSION

The identification of severe ADAMTS-13 deficiency and autoantibodies or inhibitors appears to be assay-dependent; the inhibitory IgG autoantibodies, in addition to binding and inhibiting ADAMTS-13 proteolytic activity, may accelerate ADAMTS-13 clearance in vivo.

Bacterial killing by heparin-binding peptides from PRELP and thrombospondin

Antimicrobial peptides (AMP) are effector molecules of the innate immune system. A cross-functionality exists between AMPs and heparin-binding peptides. Here, we show that the peptides QPTRRPRPGTGPGRRPRPRPRP (QPT22), derived from proline arginine-rich end leucine-rich repeat protein (PRELP) and KRFKQDGGWSHWSPWSS (KRF17) from thrombospondin exert antimicrobial activities against the Gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa, as well as against the Gram-positive Bacillus subtilis. Analysis by fluorescence microscopy demonstrated that QPT22 and KRF17 bind to bacterial membranes. Particularly QPT22 was found to induce membrane leakage of liposomes. The peptides displayed no hemolytic activities, nor did they exert membrane permeabilising effects on human epithelial cells. Additional peptides derived from heparin-binding regions of laminin, vitronectin, and fibronectin exerted similar antibacterial effects. Several peptides also showed activity against Staphylococcus aureus. Thus, the data disclose a novel antimicrobial activity of heparin-binding regions of matrix glycoproteins. The findings can be utilized in the development of novel AMPs for therapeutic use.

Genetic marking of hematopoietic stem and endothelial cells: identification of the Tmtsp gene encoding a novel cell surface protein with the thrombospondin-1 domain

Using an in silico database search, we identified a novel gene encoding a cell surface molecule with a thrombospondin domain, and designated the gene as transmembrane molecule with thrombospondin module (Tmtsp). Expression profiling of Tmtsp using specific monoclonal antibodies and Venus, a variant of yellow fluorescent protein knock-in mice in the Tmtsp locus, demonstrated its specific expression in hematopoietic and endothelial cells. In lymphohematopoietic cells, Tmtsp was predominantly expressed in hematopoietic stem and progenitor cells, and the level of expression gradually declined as the cells differentiated. Venus expression faithfully traced the expression of Tmtsp, and the level of Venus expression correlated well to the in vitro hematopoietic activity as well as the in vivo bone marrow repopulating capacity. Notably, Venus expression marked the development of definitive hematopoiesis in both the extraembryonic yolk sac and the intraembryonic aorta-gonad-mesonephros (AGM) region and, in combination with CD41, strikingly promoted the enrichment of developing progenitors in the CD41(+)Venus(high) fraction at embryonic day 10.5 (E10.5). In this context, Tmtsp is a novel marker gene for primitive hematopoietic cells and endothelial cells, and Tmtsp(Venus/)(+) mice would serve as a valuable mouse model for the analysis of both embryonic and adult hematopoiesis, as well as for vascular biology.

Phylogenomic analysis of vertebrate thrombospondins reveals fish-specific paralogues, ancestral gene relationships and a tetrapod innovation

Background

Thrombospondins (TSPs) are evolutionarily-conserved, extracellular, calcium-binding glycoproteins with important roles in cell-extracellular matrix interactions, angiogenesis, synaptogenesis and connective tissue organisation. Five TSPs, designated TSP-1 through TSP-5, are encoded in the human genome. All but one have known roles in acquired or inherited human diseases. To further understand the roles of TSPs in human physiology and pathology, it would be advantageous to extend the repertoire of relevant vertebrate models. In general the zebrafish is proving an excellent model organism for vertebrate biology, therefore we set out to evaluate the status of TSPs in zebrafish and two species of pufferfish.

Results

We identified by bioinformatics that three fish species encode larger numbers of TSPs than vertebrates, yet all these sequences group as homologues of TSP-1 to -4. By phylogenomic analysis of neighboring genes, we uncovered that, in fish, a TSP-4-like sequence is encoded from the gene corresponding to the tetrapod TSP-5 gene. Thus, all TSP genes show conservation of synteny between fish and tetrapods. In the human genome, the TSP-1, TSP-3, TSP-4 and TSP-5 genes lie within paralogous regions that provide insight into the ancestral genomic context of vertebrate TSPs.

Conclusion

A new model for TSP evolution in vertebrates is presented. The TSP-5 protein sequence has evolved rapidly from a TSP-4-like sequence as an innovation in the tetrapod lineage. TSP biology in fish is complicated by the presence of additional lineage- and species-specific TSP paralogues. These novel results give deeper insight into the evolution of TSPs in vertebrates and open new directions for understanding the physiological and pathological roles of TSP-4 and TSP-5 in humans.

Intrinsic biological activity of the thrombospondin structural homology repeat in connective tissue growth factor

Connective tissue growth factor (CCN2) is a 349-residue mosaic protein that contains four structural modules (modules 1-4), which are presumptive domains for interactions with regulatory binding proteins and receptors. Module 3, corresponding to residues 199-243, is a thrombospondin structural homology repeat (TSR) and is flanked by regions that are highly susceptible to proteolytic cleavage. To test whether CCN2 module 3 (CCN2(3)) has intrinsic biological properties, it was produced recombinantly in Escherichia coli (E. coli) and examined for its effects on the function of hepatic stellate cells (HSC), the principal fibrogenic cell type in the liver. CCN2(3) stimulated dose-dependent HSC adhesion and activity of p42/p44 mitogen activated protein kinase, the latter of which was antagonized by blocking the activity of focal adhesion kinase. HSC adhesion to immobilized CCN2(3) was attributed to binding interactions with cell surface integrin alpha6beta1. As assessed by RT-PCR or Western blotting, CCN2(3) stimulated production of fibronectin and pro-collagen type IV(alpha5), both of which are downstream components of HSC-mediated fibrogenesis and which are constituents of high density matrix in fibrotic lesions. These data show that while the full length CCN2 protein is strongly associated with fibrosis and stellate cell function, key integrinbinding properties, signaling, and fibrogenic pathways are exhibited by module 3 alone. These data indicate that module 3 of CCN2 is intrinsically active and suggest that liberation of module 3 following CCN2 proteolysis may contribute to HSC-mediated fibrogenesis, as well as other CCN2-dependent processes.

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.

R-Spondin proteins: a novel link to beta-catenin activation

The R-spondin (Rspo) protein family is a recently described group of four distinct human secreted proteins. Reported activities for Rspo proteins include essential roles in vertebrate development and their ligand-type activities overlap substantially with those of the canonical Wnt ligands in that both Rspo and canonical Wnt signaling result in the activation of beta-catenin. In a general functional screen for human secreted proteins using transgenic mouse models, we identified human R-spondin1 (hRspo1) protein as a potent and specific mitogen for the gastrointestinal epithelium and demonstrated a potential therapeutic application for the protein in mouse models of cancer therapy-induced mucositis. In contrast to previous studies, our data indicated only partial overlap between Wnt and Rspo ligand activities, suggesting that there may be independent receptor/signaling pathways for Rspo proteins that intersect those of Wnt at the level of beta-catenin. Here we summarize the current reported data on the Rspo family and discuss these results in terms of alternate mechanisms of action. We have extended our observations on the potential therapeutic application of Rspo proteins by showing that all four human Rspo family members are capable of inducing epithelial proliferation and report the first non-vertebrate Rspo family member.

A conserved molecular motor drives cell invasion and gliding motility across malaria life cycle stages and other apicomplexan parasites

Apicomplexan parasites constitute one of the most significant groups of pathogens infecting humans and animals. The liver stage sporozoites of Plasmodium spp. and tachyzoites of Toxoplasma gondii, the causative agents of malaria and toxoplasmosis, respectively, use a unique mode of locomotion termed gliding motility to invade host cells and cross cell substrates. This amoeboid-like movement uses a parasite adhesin from the thrombospondin-related anonymous protein (TRAP) family and a set of proteins linking the extracellular adhesin, via an actin-myosin motor, to the inner membrane complex. The Plasmodium blood stage merozoite, however, does not exhibit gliding motility. Here we show that homologues of the key proteins that make up the motor complex, including the recently identified glideosome-associated proteins 45 and 50 (GAP40 and GAP50), are present in P. falciparum merozoites and appear to function in erythrocyte invasion. Furthermore, we identify a merozoite TRAP homologue, termed MTRAP, a micronemal protein that shares key features with TRAP, including a thrombospondin repeat domain, a putative rhomboid-protease cleavage site, and a cytoplasmic tail that, in vitro, binds the actin-binding protein aldolase. Analysis of other parasite genomes shows that the components of this motor complex are conserved across diverse Apicomplexan genera. Conservation of the motor complex suggests that a common molecular mechanism underlies all Apicomplexan motility, which, given its unique properties, highlights a number of novel targets for drug intervention to treat major diseases of humans and livestock.

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.

Identification, cloning, expression, and characterization of the gene for Plasmodium knowlesi surface protein containing an altered thrombospondin repeat domain

Proteins present on the surface of malaria parasites that participate in the process of invasion and adhesion to host cells are considered attractive vaccine targets. Aided by the availability of the partially completed genome sequence of the simian malaria parasite Plasmodium knowlesi, we have identified a 786-bp DNA sequence that encodes a 262-amino-acid-long protein, containing an altered version of the thrombospondin type I repeat domain (SPATR). Thrombospondin type 1 repeat domains participate in biologically diverse functions, such as cell attachment, mobility, proliferation, and extracellular protease activities. The SPATR from P. knowlesi (PkSPATR) shares 61% and 58% sequence identity with its Plasmodium falciparum and Plasmodium yoelii orthologs, respectively. By immunofluorescence analysis, we determined that PkSPATR is a multistage antigen that is expressed on the surface of P. knowlesi sporozoite and erythrocytic stage parasites. Recombinant PkSPATR produced in Escherichia coli binds to a human hepatoma cell line, HepG2, suggesting that PkSPATR is a parasite ligand that could be involved in sporozoite invasion of liver cells. Furthermore, recombinant PkSPATR reacted with pooled sera from P. knowlesi-infected rhesus monkeys, indicating that native PkSPATR is immunogenic during infection. Further efficacy evaluation studies in the P. knowlesi-rhesus monkey sporozoite challenge model will help to decide whether the SPATR molecule should be developed as a vaccine against human malarias.

Structure of the calcium-rich signature domain of human thrombospondin-2

Thrombospondins (THBSs) are secreted glycoproteins that have key roles in interactions between cells and the extracellular matrix. Here, we describe the 2.6-A-resolution crystal structure of the glycosylated signature domain of human THBS2, which includes three epidermal growth factor-like modules, 13 aspartate-rich repeats and a lectin-like module. These elements interact extensively to form three structural regions termed the stalk, wire and globe. The THBS2 signature domain is stabilized by these interactions and by a network of 30 bound Ca(2+) ions and 18 disulfide bonds. The structure suggests how genetic alterations of THBSs result in disease.

The two thrombospondin type I repeat domains of the heparin-binding growth-associated molecule bind to heparin/heparan sulfate and regulate neurite extension and plasticity in hippocampal neurons

HB-GAM (heparin-binding growth-associated molecule, also designated as pleiotrophin) and midkine form a two-member family of extracellular matrix proteins that bind tightly to sulfated carbohydrate structures such as heparan sulfate. These proteins are used by developing neurons as extracellular cues in axonal growth and guidance. HB-GAM was recently reported to enhance differentiation of neural stem cells. Based on the solution structure of HB-GAM, we have recently shown that HB-GAM consists of two beta-sheet domains flanked by flexible lysine-rich N- and C-terminal tails with no apparent structure. These domains are homologous to thrombospondin type I repeats present in numerous extracellular proteins that interact with the cell surface. Our findings showed that the two beta-sheet domains fold independently. We showed that the domains (but not the lysine-rich tails) in HB-GAM are required and sufficient for interaction with hippocampal neurons. The individual domains bind heparan sulfate weakly and fail to produce significant biological effects in neurite outgrowth and long term potentiation assays. The amino acids in the linker region joining the two domains may be replaced with glycines with no effect on protein function. These results suggest a co-operative action of the two beta-sheet domains in the biologically relevant interaction with neuron surface heparan sulfate.

Evidence for proteolytic cleavage of brevican by the ADAMTSs in the dentate gyrus after excitotoxic lesion of the mouse entorhinal cortex

BACKGROUND

Brevican is a member of the lectican family of aggregating extracellular matrix (ECM) proteoglycans that bear chondroitin sulfate (CS) chains. It is highly expressed in the central nervous system (CNS) and is thought to stabilize synapses and inhibit neural plasticity and as such, neuritic or synaptic remodeling would be less likely to occur in regions with intact and abundant, lectican-containing, ECM complexes. Neural plasticity may occur more readily when these ECM complexes are broken down by endogenous proteases, the ADAMTSs (adisintegrin and metalloproteinase with thrombospondin motifs), that selectively cleave the lecticans. The purpose of these experiments was to determine whether the production of brevican or the ADAMTS-cleaved fragments of brevican were altered after deafferentation and reinnervation of the dentate gyrus via entorhinal cortex lesion (ECL).

RESULTS

In the C57Bl6J mouse, synaptic density in the molecular layer of the dentate gyrus, as measured by synaptophysin levels in ELISA, was significantly attenuated 2 days (nearly 50% of contralateral) and 7 days after lesion and returned to levels not different from the contralateral region at 30 days. Immunoreactive brevican in immunoblot was elevated 2 days after lesion, whereas there was a significant increase in the proteolytic product at 7, but not 30 days post-lesion. ADAMTS activity, estimated using the ratio of the specific ADAMTS-derived brevican fragment and intact brevican levels was increased at 7 days, but was not different from the contralateral side at 2 or 30 days after deafferentation.

CONCLUSION

These findings indicate that ADAMTS activity in the dentate outer molecular layer (OML) is elevated during the initial synaptic reinnervation period (7 days after lesion). Therefore, proteolytic processing of brevican appears to be a significant extracellular event in the remodeling of the dentate after EC lesion, and may modulate the process of sprouting and/or synaptogenesis.

Molecular cloning and characterization of Fc-TSP from the Chinese shrimp Fennerpenaeus chinensis

Thrombospondins (TSPs) are extracellular, multidomain, calcium-binding glycoproteins that modulate cell behavior in homeostasis and during development, wound-healing, immune response and tumor growth of adult tissues in vertebrates. In invertebrates these proteins are a major component of cortical rods in mature oocytes. A fragment of a thrombospondin-like gene was generated by screening a subtractive cDNA library constructed from the hemocytes of Chinese shrimp, Fennerpenaeus chinensis. The full length F. chinensis cDNA of thrombospondin was cloned by 3'- and 5'-rapid amplification of cDNA ends (3'- and 5'-RACE). The complete cDNA sequence, named Fc-TSP, is 2886 bp and the open reading frame of the cDNA encodes a 938-residue protein that contains three ChtBD2 domains, an EGF domain, a TSP-3 domain and a common TSP-C (CTD) domain. The protein shares a high sequence identity with the mj-TSPa (46.3%), mj-TSPb (46.9%) and mj-TSPc (51.9%) of Marsupenaeus japonicus. The expression and distribution of Fc-TSP in both challenged and unchallenged shrimps were studied by Northern blot, RT-PCR and in situ hybridization. Northern blot analysis showed that the Fc-TSP transcripts were detected in the hemocytes, heart, intestine, stomach and ovary of both challenged and unchallenged shrimps, but the signal was much stronger in the challenged tissues. A strong hybridization signal was detected only in challenged hepatopancreas, with no signal in the unchallenged tissue. The RT-PCR showed that the Fc-TSP was detected in both challenged and unchallenged tissues including the hemocytes, heart, hepatopancreas, stomach, gills, intestine, spermary and ovary. Except for the ovary and spermary, the signal of challenged tissues was relatively stronger than that of unchallenged ones, especially in hepatopancreas. These results suggest that the thrombospondin was upregulated in the hemocytes, heart, intestine and stomach of challenged shrimp, and induced in the hepatopancreas of challenged shrimps. Therefore, Fc-TSP may be involved in the defense responses of the shrimp.

Endogenous inhibitors of angiogenesis in malignant gliomas: nature's antiangiogenic therapy

Angiogenesis is necessary for tumor growth beyond a volume of approximately 2 mm(3). This observation, along with the accessibility of tumor vessels to therapeutic targeting, has resulted in a research focus on inhibitors of angiogenesis. A number of endogenous inhibitors of angiogenesis are found in the body. Some of these are synthesized by specific cells in different organs, and others are created by extracellular proteolytic cleavage of plasma-derived or extracellular matrix-localized proteins. In this review, we focus on angiostatin, endostatin, PEX, pigment epithelial-derived factor, and thrombospondin (TSP)-1 and -2, either because these molecules are expressed in malignant glioma biopsies or because animal studies in malignant glioma models have suggested that their therapeutic administration could be efficacious. We review the known mechanisms of action, potential receptors, expression in glioma biopsy samples, and studies testing their potential therapeutic efficacy in animal models of malignant glioma. Two conclusions can be made regarding the mechanisms of action of these inhibitors: (1) Several of these inhibitors appear to mediate their antiangiogenic effect through multiple protein-protein interactions that inhibit the function of proangiogenic molecules rather than through a specific receptor-mediated signaling event, and (2) TSP-1 and TSP-2 appear to mediate their antiangiogenic effect, at least in part, through a specific receptor, CD36, which initiates the antiangiogenic signal. Although not proven in gliomas, evidence suggests that expression of specific endogenous inhibitors of angiogenesis in certain organs may be part of a host antitumor response. The studies reviewed here suggest that new antiangiogenic therapies for malignant gliomas offer exciting promise as nontoxic, growth-inhibitory agents.

Binding of ADAMTS13 to von Willebrand factor

ADAMTS13, a metalloprotease, cleaves von Willebrand factor (VWF) in plasma to generate smaller, less thrombogenic fragments. The interaction of von Willebrand factor with specific ADAMTS13 domains was characterized with a binding assay employing von Willebrand factor immobilized on a plastic surface. ADAMTS13 binding was saturable and reversible. Equilibrium binding occurred within 2 h and the half-time for dissociation was approximately 4 h. Binding to von Willebrand factor was similar with either recombinant ADAMTS13 or normal plasma ADAMTS13; plasma from a patient who lacked ADAMTS13 activity showed no binding. The stoichiometry of binding was one ADAMTS13 per two von Willebrand factor monomers, and the K(d) was 14 nm. The ADAMTS13 metalloprotease and disintegrin domains did not bind VWF detectably. ADAMTS13 truncated after the first thrombospondin type 1 repeat bound VWF with a K(d) of 206 nm, whereas ADAMTS13 truncated after the spacer domain had a K(d) of 23 nm, which is comparable with that of full-length ADAMTS13. Truncation after the eighth thrombospondin type 1 repeat reduced the binding affinity by approximately 3-fold and truncation after the seventh thrombospondin type 1 repeat in addition to the CUB domains increased the affinity for von Willebrand factor by approximately 2-fold. Therefore, the spacer domain is required for ADAMTS13 binding to von Willebrand factor. The first thrombospondin repeat also affects binding, and the C-terminal thrombospondin type 1 and CUB domains of ADAMTS13 may modulate this interaction.

Novel integrin antagonists derived from thrombospondins

Specific antagonists have been successfully developed for several different integrins. Clinical trials have been initiated to study therapeutic uses of these inhibitors in cancer, thrombosis, and inflammatory diseases. Most efforts to date have focused on the platelet integrin alphaIIbbeta3, endothelial alphavbeta3, and the leukocyte integrin alpha4beta1. However, the integrin family contains additional members with interesting tissue specificities and functional properties that could also be useful molecular targets for disease intervention. In many cases, specific recognition motifs for these integrins have not been identified, which has precluded development of specific antagonists. Our recent studies of thrombospondin-1 and thrombospondin-2 recognition by integrins have revealed novel motifs for alpha3beta1 and alpha6beta1 integrins as well as new motifs recognized by the well studied alpha4beta1 integrin. These three integrins play distinct roles in angiogenesis and its modulation by thrombospondins. This review will discuss recent insights into the specificities of alpha3beta1 and alpha6beta1 integrins, their functions in angiogenesis, and potential applications for antagonists of these integrins and of alpha4beta1 to control pathological angiogenesis and other diseases.

The antiangiogenic effect of thrombospondin-2 is mediated by CD36 and modulated by histidine-rich glycoprotein

Thrombospondins-1 and -2 (TSP-1, TSP-2) are matricellular glycoproteins with potent antiangiogenic activity. We have previously shown that the antiangiogenic activity of TSP-1 is mediated by the interaction of the type I repeats (TSR) with the receptor CD36, although other domains of TSP-1 have also been implicated. We now show that the antiangiogenic activity of TSP-2, which contains three TSRs but, unlike TSP-1, lacks the capacity to activate TGF-beta, is similarly dependent on CD36. Using the corneal pocket assay we found that TSP-2 did not inhibit bFGF-induced angiogenesis in CD36 null mice. We then demonstrated that (125)[I]-TSP-2 bound to murine macrophages and that binding was diminished by 70% by anti-CD36 antibody or by using cells from CD36 null animals. Solid-phase binding studies revealed that (125)[I]-TSP-2 bound to CD36/glutathione-S-transferase (GST) fusion proteins encoding the region spanning amino acids 93-120, but not amino acids 298-439. This 93-120 amino acid region, previously identified as the TSP-1 binding site, is homologous to domains on other TSP binding proteins, such as LIMP-2 and histidine-rich glycoprotein (HRGP). Finally, we showed with an immunoabsorbent binding assay that TSP-2 bound HRGP with high affinity and that HRGP blocked the antiangiogenic activity of TSP-2, acting like a "decoy" receptor. These data suggest that modulation of the TSR/CD36 system may play an important role in the regulation of the angiogenic "switch," and may provide a target for therapeutic interventions.

The C-terminal peptide of thrombospondin-4 stimulates erythroid cell proliferation

Erythropoietin (EPO) stimulates the production of small erythroid cell stimulating factors (molecular weight <5 kDa) in cultures of bone marrow endothelial cells. We identified a fragment of thrombospondin-4 (TSP-4) as an EPO-stimulated protein in endothelial cell lysates. Pre-incubation of the low molecular weight fractions from supernatants of EPO-treated umbilical cord endothelial cells (HUVEC) with antibodies against the C-terminal residues of TSP-1,2 and TSP-4 decreased the erythroid cell stimulating activity. The C-terminal TSP-1 section corresponding to a molecular weight lower than 6 kDa has the integrin-associated protein binding motif VVM. The corresponding TSP-4 fragment, lacking the three residue sequence VVM, has a distinctive acidic peptide comprising the last 21 amino acids (C21) with the characteristics of an amphipathic helix. C21 stimulated thymidine incorporation into bovine erythroid cells, increased cell numbers in cultures of cord blood CD36+ erythroid precursors and skin fibroblasts, and decreased HUVEC proliferation. SC21, a homologous peptide of identical amino acid composition but with interchanged residues, was non-amphipathic and had no erythroid cell stimulating activity.