The evolution of the thrombospondin gene family

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.

Determination of vWF, ADAMTS-13 and Thrombospondin-1 in Venous Thromboembolism and Relating Them to the Presence of Factor V Leiden Mutation

Thrombophilia in venous thromboembolism (VTE) is multifactorial. Von Willebrand factor (vWF) plays a major role in primary hemostasis. While elevated vWF levels are well documented in VTE, findings related to its cleaving protease (ADAMTS-13) are contradicting. The aim of this study was to determine vWF, ADAMTS-13, and the multifactorial Thrombospondin-1 (TSP-1) protein levels in patients after 3-6 months following an unprovoked VTE episode. We also explored a possible association with factor V Leiden (FVL) mutation. vWF, ADAMTS-13 and TSP-1 were analyzed using ELISA kits in 60 VTE patients and 60 controls. Patients had higher levels of vWF antigen (P = .021), vWF collagen-binding activity (P = .008), and TSP-1 protein (P < .001) compared to controls. ADAMTS-13 antigen was lower in patients (P = .046) compared to controls but ADAMTS-13 activity was comparable between the two groups (P = .172). TSP-1 showed positive correlation with vWF antigen (rho = 0.303, P = .021) and negative correlation with ADAMTS-13 activity (rho = -0.244, P = .033) and ADAMTS-13 activity/vWF antigen ratio (rho = -0.348, P = .007). A significant association was found between the presence of FVL mutation and VTE (odds ratio (OR): 9.672 (95% confidence interval (CI) 2.074-45.091- P = .004), but no association was found between the mutation and the studied proteins (P > .05). There appears to be an imbalance between vWF and ADAMTS-13 in VTE patients even after 3-6 months following the onset of VTE. We report that the odds of developing VTE in carriers of FVL mutation are 9.672 times those without the mutation, but the presence of this mutation is not associated with the studied proteins.

Stimulation with THBS4 activates pathways that regulate proliferation, migration and inflammation in primary human keratinocytes

As in other mammalian tissues, the extracellular matrix (ECM) of skin functions as mechanical support and regulative environment that guides the behavior of the cells. ECM is a gel-like structure that is primarily composed of structural and nonstructural proteins. While the content of structural proteins is stable, the level of nonstructural ECM proteins, such as thrombospondin-4 (THBS4), is dynamically regulated. In a previous work we demonstrated that THBS4 stimulated cutaneous wound healing. In this work we discovered that in addition to proliferation, THBS4 stimulated the migration of primary keratinocytes in 3D. By using a proteotransciptomic approach we found that stimulation of keratinocytes with THBS4 regulated the activity of signaling pathways linked to proliferation, migration, inflammation and differentiation. Interestingly, some of the regulated genes (eg IL37, TSLP) have been associated with the pathogenesis of atopic dermatitis (AD). We concluded that THBS4 is a promising candidate for novel wound healing therapies and suggest that there is a potential convergence of pathways that stimulate cutaneous wound healing with those active in the pathogenesis of inflammatory skin diseases.

Matricellular proteins in cutaneous wound healing

Cutaneous wound healing is a complex process that encompasses alterations in all aspects of the skin including the extracellular matrix (ECM). ECM consist of large structural proteins such as collagens and elastin as well as smaller proteins with mainly regulative properties called matricellular proteins. Matricellular proteins bind to structural proteins and their functions include but are not limited to interaction with cell surface receptors, cytokines, or protease and evoking a cellular response. The signaling initiated by matricellular proteins modulates differentiation and proliferation of cells having an impact on the tissue regeneration. In this review we give an overview of the matricellular proteins that have been found to be involved in cutaneous wound healing and summarize the information known to date about their functions in this process.

Thrombospondin-1 is a multifaceted player in tumor progression

Thrombospondins are a family of extracellular matrix (ECM) proteins. Thrombospondin-1 (TSP1) was the first member to be identified and is a main player in tumor microenvironment. The diverse functions of TSP1 depend on the interactions between its structural domains and multiple cell surface molecules. TSP1 acts as an angiogenesis inhibitor by stimulating endothelial cell apoptosis, inhibiting endothelial cell migration and proliferation, and regulating vascular endothelial growth factor bioavailability and activity. In addition to angiogenesis modulation, TSP1 also affects tumor cell adhesion, invasion, migration, proliferation, apoptosis and tumor immunity. This review discusses the multifaceted and sometimes opposite effects of TSP1 on tumor progression depending on the molecular and cellular composition of the microenvironment. Clinical implications of TSP1-related compounds are also discussed.

Serotype 3 pneumococci sequester platelet-derived human thrombospondin-1 via the adhesin and immune evasion protein Hic

Streptococcus pneumoniae serotype 3 strains emerge frequently within clinical isolates of invasive diseases. Bacterial invasion into deeper tissues is associated with colonization and immune evasion mechanisms. Thus, pneumococci express a versatile repertoire of surface proteins sequestering and interacting specifically with components of the human extracellular matrix and serum. Hic, a PspC-like pneumococcal surface protein, possesses vitronectin and factor H binding activity. Here, we show that heterologously expressed Hic domains interact, similar to the classical PspC molecule, with human matricellular thrombospondin-1 (hTSP-1). Binding studies with isolated human thrombospondin-1 and various Hic domains suggest that the interaction between hTSP-1 and Hic differs from binding to vitronectin and factor H. Binding of Hic to hTSP-1 is inhibited by heparin and chondroitin sulfate A, indicating binding to the N-terminal globular domain or type I repeats of hTSP-1. Competitive inhibition experiments with other pneumococcal hTSP-1 adhesins demonstrated that PspC and PspC-like Hic recognize similar domains, whereas PavB and Hic can bind simultaneously to hTSP-1. In conclusion, Hic binds specifically hTSP-1; however, truncation in the N-terminal part of Hic decreases the binding activity, suggesting that the full length of the α-helical regions of Hic is required for an optimal interaction.

Members of the thrombospondin gene family bind stromal interaction molecule 1 and regulate calcium channel activity

The thrombospondins (TSPs) are a family of matricellular proteins that regulate cellular phenotype through interactions with a myriad of other proteins and proteoglycans. We have identified a novel interaction of the members of the TSP gene family with stromal interaction molecule 1 (STIM1). This association is robust since it is preserved in Triton X-100, can be detected with multiple anti-TSP-1 and anti-STIM1 antibodies, and is detected in a wide range of cell types. We have also found that STIM1 co-immunoprecipitates with TSP-4 and cartilage oligomeric matrix protein (COMP), and that a recombinant version of the N-terminal domain of STIM1 binds to the signature domain of TSP-1 and COMP. The association of the TSPs with STIM1 is observed in both the presence and absence of calcium indicating that the calcium-dependent conformation of the signature domain of TSPs is not required for binding. Thus, this interaction could occur in the ER under conditions of normal or low calcium concentration. Furthermore, we observed that the expression of COMP in HEK 293 cells decreases STIM1-mediated calcium release activated calcium (CRAC) channel currents and increases arachidonic acid calcium (ARC) channel currents. These data indicate that the TSPs regulate STIM1 function and participate in the reciprocal regulation of two channels that mediate calcium entry into the cell.

Immunocamouflage of latex surfaces by grafted methoxypoly(ethylene glycol) (mPEG): proteomic analysis of plasma protein adsorption

Grafting of methoxypoly(ethylene glycol) (mPEG) to cells and biomaterials is a promising non-pharmacological immunomodulation technology. However, due to the labile nature of cells, surface-plasma interactions are poorly understood; hence, a latex bead model was studied. PEGylation of beads resulted in a density and molecular weight dependent decrease in total adsorbed protein with a net reduction from (159.9±6.4) ng cm(-2) on bare latex to (18.4±0.8) and (52.3±5.3) ng cm(-2) on PEGylated beads (1 mmol L(-1) of 2 or 20 kD SCmPEG, respectively). SDS-PAGE and iTRAQ-MS analysis revealed differential compositions of the adsorbed protein layer on the PEGylated latex with a significant reduction in the compositional abundance of proteins involved in immune system activation. Thus, the biological efficacy of immunocamouflaged cells and materials is mediated by both biophysical obfuscation of antigens and reduced surface-macromolecule interactions.

Extracellular matrix proteins and tumor angiogenesis

Tumor development is a complex process that relies on interaction and communication between a number of cellular compartments. Much of the mass of a solid tumor is comprised of the stroma which is richly invested with extracellular matrix. Within this matrix are a host of matricellular proteins that regulate the expression and function of a myriad of proteins that regulate tumorigenic processes. One of the processes that is vital to tumor growth and progression is angiogenesis, or the formation of new blood vessels from preexisting vasculature. Within the extracellular matrix are structural proteins, a host of proteases, and resident pro- and antiangiogenic factors that control tumor angiogenesis in a tightly regulated fashion. This paper discusses the role that the extracellular matrix and ECM proteins play in the regulation of tumor angiogenesis.

Thrombospondins and novel TSR-containing proteins, R-spondins, regulate bone formation and remodeling

Thrombospondins (TSPs) are a family of five secreted multimeric matricellular proteins that share homology in the type II and III repeats and carboxy-terminal region. Type I repeats, also known as properdin or thrombospondin repeats (TSRs), are found in TSP1/2, but not TSP3-5. A variety of other secreted proteins contain TSRs, including the novel extracellular molecules, R-spondins. TSP family and many TSR-containing proteins, including R-spondins, are highly expressed in skeletal tissues during development and postnatal. TSP2 regulates the osteoblast lineage, influencing bone mass and geometry, as well as response to fracture healing, ovariectomy, and mechanical loading. Compound knockout mice of TSPs have revealed important mechanistic insights. TSP1/2 knockout mice have craniofacial dysmorphism, and TSP1/3/5 compound knockout mice display growth plate abnormalities. R-spondins promote osteoblast differentiation and R-spondin-2 deficiency results in skeletal developmental defects. Overall, TSP and other TSR molecules influence multiple aspects of bone development and remodeling.

Thrombospondin-1-induced vascular smooth muscle cell migration is dependent on the hyaluronic acid receptor CD44

BACKGROUND

Thrombospondin-1 (TSP-1) induces vascular smooth muscle cell (VSMC) migration after arterial injury. TSP-1 up-regulates hyaluronic acid (HyA)-inducing genes in VSMCs. HyA also induces VSMC migration. Our hypothesis was that TSP-1-induced VSMC migration is dependent on the CD44 receptor, and that HyA and TSP-1 share migratory signaling pathways.

METHODS

VSMC migration was assessed using TSP-1, HyA, or serum-free medium as chemoattractants. VSMCs were treated with inhibitors to CD44, Ras, phosphatidylinositol-3 kinase, Raf-1 kinase, or c-SRC. TSP-1- and HyA-induced epidermal growth factor receptor (EGFR) activity was determined by enzyme-linked immunosorbent assay. Comparisons were made by the Student t test and a P value less than .05 was considered significant.

RESULTS

Inhibiting CD44 reduced TSP-1- and HyA-induced migration. Phosphatidylinositol-3 kinase and c-SRC inhibitors prevented TSP-1- and HyA-induced migration, whereas Ras and Raf-1 kinase inhibitors only affected TSP-1. TSP-1 and HyA activate the EGFR.

CONCLUSIONS

TSP-1- and HYA-induced migration share some of the same signaling pathways and the EGFR/CD44 receptors may be a common link.

Thrombospondin 1 and its mimetic peptide ABT-510 decrease angiogenesis and inflammation in a murine model of inflammatory bowel disease

OBJECTIVE

Vascular abnormalities and expression of proangiogenic factors have been repeatedly reported in inflammatory bowel disease (IBD). Thrombospondin 1 (TSP-1) is a protein well known for its antiangiogenic and anti-inflammatory properties. Using the dextran sulfate sodium (DSS) model, the role of TSP-1 in IBD has been investigated in vivo.

METHODS

TSP-1-deficient mice (TSP-1-/-) and WT mice were treated with DSS for 7 days. Disease activity indices, myeloperoxidase activity (MPO) and histology were analyzed. Microvascular density (MVD) was quantified using immunohistochemistry (IMH) with CD31 antibody. TGF-beta(1), basic FGF, VEGF, TNF-alpha and MMPs protein levels were evaluated by IMH and enzyme-linked immunoabsorbent assay (ELISA). Mice were treated with ABT-510 (Abbott Laboratories), an antiangiogenic TSP peptide, using miniosmotic pumps for 7 days.

RESULTS

TSP-1(-/-) mice had a worse clinical outcome and exhibited severe signs of rectal bleeding compared to the WT controls. The TSP-1-/- mice showed a higher level of crypt damage and deeper lesions. The grade of inflammation and the levels of MPO activity were also significantly higher in colons of TSP-1-/- mice. TSP-1-/- mice displayed higher MVD in focal areas of the colon after only 3 days of DSS treatment. Furthermore, clinical severity of the colitis and angiogenesis was significantly diminished when mice was treated with ABT-510.

CONCLUSIONS

These findings directly link TSP-1 as a protective factor in IBD and suggest antiangiogenesis treatment, including compounds such as ABT-510 as an adjuvant therapy for IBD.

Lack of thrombospondin-1 increases angiogenesis in a model of chronic inflammatory bowel disease

BACKGROUND AND AIMS

Vascular abnormalities and expression of pro-angiogenic factors are observed in inflammatory bowel diseases (IBD). In this study, the role of thrombospondin-1 (TSP-1), an antiangiogenic protein, was analyzed using the dextran sulfate sodium (DSS) model for IBD.

MATERIALS AND METHODS

Wild-type (WT) and thrombospondin-1-deficient (TSP-1(-/-)) mice were subjected to four cycles (7 days) of DSS. Basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF), transforming growth factor beta 1 (TGFbeta-1), and pro-apoptotic proteins such as Fas and its ligand (FasL) were determined by enzyme-linked immunosorbent assay. Double immunohistochemistry for cluster of differential 31 (CD31) and panendothelial cell antigen-32 antibodies was performed for detecting blood vessels. The terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling assay was also performed for identifying apoptotic cells. Inflammation, dysplasia, microvascular density (MVD), apoptotic indices (AI), protein 53 (p53), and beta-catenin expression were determined.

RESULTS

VEGF and bFGF protein levels and MVD were higher in the TSP-1(-/-) mice (p = 0.0312, p = 0.0246, and p = 0.0085, respectively). AI in the endothelial cells (EC) and FasL levels were significantly lower in TSP-1(-/-) compared to WT mice (p = 0.0042 and p = 0.0362, respectively). Dysplasia was detected in 66% of TSP-1(-/-) mice compared to 14% in WT mice. Hscores of ss-catenin and areas overexpressing p53 were higher in TSP-1(-/-) mice (p = 0.0002 and p = 0.0339, respectively).

CONCLUSION

TSP-1 may decrease angiogenesis by reducing the levels of pro-angiogenic factors and inducing apoptosis in EC through the Fas or FasL pathway. These findings, along with the increased overexpression of p53 and beta-catenin in TSP-1(-/-) mice, underline its role in carcinogenesis.

Bioinformatic analysis of adhesion proteins

Proteins that mediate cell-cell and cell-extracellular matrix (ECM) adhesion have been fundamental in the evolution of multicellular animals. Fibrillar collagens, proteoglycans, integrins, and cadherins are present in all animals from sponges to mammals, and many other adhesion proteins have arisen during animal evolution. In general, adhesion proteins are large multidomain molecules and are encoded in larger gene families in vertebrates than in invertebrates. With the increasing availability of completely sequenced genomes representing different points on the animal tree of life, bioinformatics is proving to be a very valuable approach for the analysis of the domain organization and relationships of adhesion proteins, which can direct or enhance experimental tests. Here we describe, with examples from the literature, the major methods for identifying sequence homologies; analyzing domain organization and potential for oligomerization; analyzing sequence relationships by multiple sequence alignments and phylogenetic trees, and assessing adhesion proteins as components of functional pathways and tissue systems through comparative genomics.

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.

Thrombospondin-1 controls vascular platelet recruitment and thrombus adherence in mice by protecting (sub)endothelial VWF from cleavage by ADAMTS13

The function of thrombospondin-1 (TSP-1) in hemostasis was investigated in wild-type (WT) and Tsp1-/- mice, via dynamic platelet interaction studies with A23187-stimulated mesenteric endothelium and with photochemically injured cecum subendothelium. Injected calcein-labeled WT platelets tethered or firmly adhered to almost all A23187-stimulated blood vessels of WT mice, but Tsp1-/- platelets tethered to 45% and adhered to 25.8% of stimulated Tsp1-/- vessels only. Stimulation generated temporary endothelium-associated ultralarge von Willebrand factor (VWF) multimers, triggering platelet string formation in 48% of WT versus 20% of Tsp1-/- vessels. Injection of human TSP-1 or thrombotic thrombocytopenic purpura (TTP) patient-derived neutralizing anti-ADAMTS13 antibodies corrected the defective platelet recruitment in Tsp1-/- mice, while having a moderate effect in WT mice. Photochemical injury of intestinal blood vessels induced thrombotic occlusions with longer occlusion times in Tsp1-/- venules (1027 +/- 377 seconds) and arterioles (858 +/- 289 seconds) than in WT vessels (559 +/- 241 seconds, P < .001; 443 +/- 413 seconds, P < .003) due to defective thrombus adherence, resulting in embolization of complete thrombi, a defect restored by both human TSP-1 and anti-ADAMTS13 antibodies. We conclude that in a shear field, soluble or local platelet-released TSP-1 can protect unfolded endothelium-bound and subendothelial VWF from degradation by plasma ADAMTS13, thus securing platelet tethering and thrombus adherence to inflamed and injured endothelium, respectively.

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.

The role of thrombospondins 1 and 2 in the regulation of cell-matrix interactions, collagen fibril formation, and the response to injury

Thrombospondins (TSPs) 1 and 2 are extracellular modular glycoproteins that are best known for their anti-angiogenic properties and their ability to modulate cell-matrix interactions. However, these proteins, and in particular TSP2, are pleiotropic in function and affect processes as disparate as bone growth and hemostasis. In recognition of their ability to influence a wide variety of cell functions, and in the absence of convincing evidence for their participation as integral components of extracellular structures, the term 'matricellular' has been applied to these and a small group of functionally related proteins. In this review, we focus on the role of TSP1 and 2 in two forms of injury in mice, excisional skin wounds and subcutaneously implanted biomaterials, and take advantage of mice with targeted disruptions of one or both genes to identify likely biochemical mechanisms that could account for the characteristics of the injury response in these knockout mice. In work that stems largely from our own laboratory, we show that pericellular levels of the matrix metalloproteinase, MMP2, are controlled to a large extent by TSP2 (and potentially also by TSP1), and that elevated levels of MMP2 are likely to account in part for defects as diverse as reduced cellular adhesion, abnormal collagen fibril structure, and increased endothelial cell and vascular proliferation.

Increased erythrocyte adhesion in mice and humans with hereditary spherocytosis and hereditary elliptocytosis

Mice with disruptions of the red blood cell (RBC) cytoskeleton provide severe hemolytic anemia models in which to study multiorgan thrombosis and infarction. The incidence of cerebral infarction ranges from 70% to 100% in mice with alpha-spectrin deficiency. To determine whether mutant RBCs abnormally bind adhesive vascular components, we measured adhesion of mouse and human RBCs to immobilized human thrombospondin (TSP) and laminin (LM) under controlled flow conditions. Mutant RBCs had at least 10-fold higher adhesion to TSP compared with normal RBCs (P <.006). Mutant relative to unaffected RBC adhesion to LM was significantly (P <.01) increased as well. Treatment of RBCs with the anionic polysaccharide dextran sulfate inhibited mutant RBC adhesion to TSP (P <.001). Treatment of RBCs with antibodies to CD47 or the CD47-binding TSP peptide 4N1K did not inhibit TSP adhesion of RBCs. Previously, we have shown that infarcts in alpha-spectrin-deficient sph/sph mice become histologically evident beginning at 6 weeks of age. TSP adhesion of RBCs from 3- to 4- and 6- to 8-week-old sph/sph mice was significantly higher than RBCs from adult mice (> 12 weeks old; P <.005). While the mechanism of infarction in these mice is unknown, we speculate that changes in RBC adhesive characteristics contribute to this pathology.

Characterisation of Drosophila thrombospondin defines an early origin of pentameric thrombospondins

Thrombospondins (TSPs) are multidomain oligomers that have complex roles in cell interactions and tissue organisation. The five vertebrate TSPs comprise two subgroups, A and B, that are assembled as trimers or pentamers, respectively. An invertebrate TSP was recently discovered in Drosophila melanogaster, but there is no knowledge of the oligomerisation status or properties of this molecule. We developed by bioinformatics a new dataset containing the single TSP of Drosophila melanogaster and four other newly identified invertebrate TSPs to examine the phylogenetic relationships of TSPs. These analyses clearly indicate pentamerisation as an early attribute of TSPs. We demonstrate experimentally that D.melanogaster TSP is assembled as a pentamer, has heparin-binding activity and is a component of extracellular matrix (ECM). During embryogenesis, the TSP transcript is concentrated at muscle attachment sites and is expressed by a subset of myoblasts and in imaginal discs. These novel results establish TSPs as highly conserved ECM components in both invertebrates and vertebrates and open fresh perspectives on the conservation of structure and biological function within this family.

Thrombospondin 2, a matricellular protein with diverse functions

Thrombospondin (TSP) 2 is a close relative of TSP1 but differs in its temporal and spatial distribution in the mouse. This difference in expression undoubtedly reflects the marked disparity in the DNA sequences of the promoters in the genes encoding the two proteins. The synthesis of TSP2 occurs primarily in connective tissues of the developing and growing mouse. In the adult animal the protein is again produced in response to tissue injury and in association with the growth of tumors. Despite the abnormalities in collagen fibrillogenesis, fragility of skin, and laxity of tendons and ligaments observed in the TSP2-null mouse, TSP2 does not appear to contribute directly to the structural integrity of connective tissue elements. Instead, emerging evidence supports a mode of action of TSP2 'at a distance', i.e. by modulating the activity and bioavailability of proteases and growth factors in the pericellular environment and, very likely, by interaction with cell-surface receptors. Thus, TSP2 qualifies as a matricellular protein, as defined in the introduction to this minireview series. The phenotype of TSP2-null mice has been very helpful in providing clues to the functions of TSP2. In addition to histological and functional abnormalities in connective tissues, these mice display an increased vascularity of the dermis and subdermal tissues, increased endosteal bone growth, a bleeding defect, and a marked adhesive defect of dermal fibroblasts. Our laboratory has established that TSP2 binds matrix metalloproteinase 2 (MMP2) and that the adhesive defect in TSP2-null fibroblasts results from increased MMP2 activity. The investigation of the basis for the other defects in the TSP2-null mouse is likely to yield equally interesting results.

Thrombospondin type 1 repeats interact with matrix metalloproteinase 2. Regulation of metalloproteinase activity

Thrombospondins are thought to function as inhibitors of angiogenesis. However, the mechanism(s) of this activity is not well understood. In this study, we have used the yeast two-hybrid system to identify proteins that interact with the thrombospondins 1 (TSP1) and 2 (TSP2) properdin-like type 1 repeats (TSR). One of the proteins identified that interacted with both TSR was matrix metalloproteinase 2 (MMP2). The isolated MMP2 cDNA clone encoded amino acid residues 237-633, which include the fibronectin-like gelatin binding region flanking the catalytic center and the carboxyl hemopexin-like region. Further testing of this clone demonstrated that the TSR interacted with the NH(2)-terminal region of the MMP2 that contains the catalytic domain. The protein interaction observed in yeast was further demonstrated by immunoprecipitation and Western blotting using purified intact TSP1, TSP2, MMP2, and MMP9. Although MMP2 interacted with TSP1 and TSP2 via its gelatin-binding domain or a closely mapping site, neither TSP1 nor TSP2 was degraded by MMP2 in vitro. Tissue culture and in vitro assays demonstrated that the presence of purified TSR and intact TSP1 resulted in inhibition of MMP activity. The ability of TSP1 to inhibit MMP3-dependent activation of pro-MMP9 and thrombin-induced activation of pro-MMP2 suggests that the TSPs may inhibit MMP activity by preventing activation of the MMP2 and MMP9 zymogens.

Bovine thrombospondin-2: complete complementary deoxyribonucleic acid sequence and immunolocalization in the external zones of the adrenal cortex

Given the variety of biological functions in the adrenal cortex that are controlled by ACTH, we hypothesized that some extracellular proteins act as biological relays for this systemic hormone. One candidate protein [corticotropin-induced secreted protein (CISP)] was purified from the conditioned medium of bovine adrenocortical cells on the basis of a 5- to 14-fold increase in its synthesis after the addition of ACTH. We report here the cloning of overlapping complementary DNAs that span the sequence encoding the full-length protein (1170 amino acids). The deduced CISP protein sequence is 89% identical to that of human thrombospondin-2 (TSP2), but only 61% identical to that of bovine TSP1, confirming that CISP is the bovine ortholog of TSP2. The bovine TSP2 sequence aligned perfectly with human, mouse, and chicken TSP2 sequences, except for a gap of 2 amino acids located in a linker region. All 58 cysteine residues that are conserved in other species were present in the bovine sequence as well as most of the functional domains. Most endocrine tissues (adrenal cortex, testis, ovary, and placenta) appeared to express TSP2, as determined by Western blot analysis. The highest levels of TSP2 protein were found in the adrenal cortex, followed by the heart, spleen, brain, and kidney. A differential extent of N-glycosylation or tissular proteolytic maturation may be responsible for the mol wt differences observed between bovine TSP2 detected in the medium from primary cultures and that in fresh tissue extracts. The immunohistochemical analysis of the distribution of TSP2 in the bovine adrenal gland revealed that the protein is much more abundant in the external zones (zona glomerulosa and zona fasciculata) than in the internal reticularis zone, a pattern similar to that reported for ACTH receptors. This distribution clearly suggests that TSP2 is a candidate relay protein for a subset of ACTH actions in the adrenal cortex.

Thrombospondin-1 is a mediator of the neurotypic differentiation induced by EGF in thymic epithelial cells

Thymic epithelial cell component originates from cranial neural crest as well as from endoderm and ectoderm of the third pharyngeal pouch and branchial cleft. Epidermal growth factor (EGF) has been previously shown to play a crucial role in directing thymic epithelial cells toward a neural-oriented cell fate. To identify genes that are involved in the EGF-induced neurotypic differentiation of the thymic stroma-derived TC-1S cell line, we studied EGF-treated and untreated cells by RNA fingerprinting PCR-based differential screening. We obtained 23 distinct sequences including 18 known genes and 5 sequences previously unreported, which are currently under characterization. Here, we describe the involvement of one of the isolated genes, the thrombospondin-1, as a mediator of the neurotypic differentiation induced by EGF in TC-1S cells. We show that thrombospondin-1 mRNA and protein levels are increased by EGF. Moreover, exogenous thrombospondin-1 is able to enhance the outgrowth of neurite-like processes as well as the expression of neurofilaments and neural cell adhesion molecule in TC-1S cells. These observations suggest that the up-regulation of thrombospondin-1 synthesis induced by EGF contributes to the differentiation choice of thymic epithelial cells toward a neural fate, reminiscent of their neural crest origin.

Molecular evolution of the opioid/orphanin gene family

Gene duplication is a recurring theme in the evolution of vertebrate polypeptide hormones and neuropeptides. These duplication events can lead to the formation of gene families in which divergence of function is the usual outcome. In the case of the opioid/orphanin family of genes, duplication events have proceeded along two paths: (a) an apparent duplication of function as seen in the analgesic activity of Proenkephalin and Prodynorphin end-products; and (b) divergence of function as seen in the nociceptic activity of Proorphanin end-products or the melanocortin (color change and chronic stress regulation) activity of Proopiomelanocortin end-products. Although genes coding for Proopiomelanocortin, Proenkephalin, Prodynorphin, and Proorphanin have been extensively studied in mammals, the distribution and radiation of these genes in nonmammalian vertebrates is less well understood. This review will present the hypothesis that the radiation of the opioid/orphanin gene family is the result of the duplication and divergence of the Proenkephalin gene during the radiation of the chordates. To evaluate the Proenkephalin gene duplication hypothesis, a 3'RACE procedure was used to screen for the presence of Prodynorphin-related, Proenkephalin-related, and Proorphanin-related cDNAs expressed in the brains of nonmammalian vertebrates.

Hypoxia increases thrombospondin-1 transcript and protein in cultured endothelial cells

The exposure of endothelial cells to hypoxic environments regulates the expression of a number of genes with products that are vasoactive or mitogenic for vascular tissue, including platelet-derived growth factor, endothelin-1, and endothelial nitric oxide synthase. Hypoxia is also known to alter the adhesive properties of endothelium toward a variety of blood cell types. Thrombospondin-1 (TSP-1) is a glycoprotein with major roles in cellular adhesion and vascular smooth muscle proliferation and migration. We report here that hypoxia induces TSP-1 gene and protein expression. Oxygen tensions of < or =30 torr resulted in TSP-1 transcript induction initially apparent at 1 to 6 hours, with maximal induction (6.5-fold+/-1.2-fold) within 24 to 48 hours in both human and bovine endothelial cells. TSP-1 protein levels remain elevated after 72 hours of continuous hypoxic exposure. The induction of TSP-1 steady-state transcript levels is caused in large part, if not entirely, by post-transcriptional stabilization of the TSP-1 mRNA. The TSP-1 induction by hypoxia is a graded and reversible physiologic response and can be mimicked by the use of cobalt chloride or the inhibition of nitric oxide production, suggesting both the involvement of a heme-containing oxygen sensor and a role for the endogenous production of nitric oxide in TSP-1 regulation. The effects of hypoxia both on the stabilization of the TSP-1 transcript and the stimulation of TSP-1 protein production are completely inhibited by arginine butyrate.

Thrombospondins in early Xenopus embryos: dynamic patterns of expression suggest diverse roles in nervous system, notochord, and muscle development

The thrombospondins (TSPs) are a family of extracellular matrix (ECM) glycoproteins that modulate many cell behaviors including adhesion, migration, and proliferation. Here we report the molecular cloning of the Xenopus homologs of TSP-1 and TSP-3, and the developmental patterns of expression of Xenopus TSP-1, TSP-3, and TSP-4 mRNAs. Xenopus TSP-1 and TSP-3 protein sequences each share approximately 80% amino acid identity with their mammalian counterparts. TSP-1 mRNAs are detectable at low levels in fertilized eggs indicating that this TSP is a maternally deposited transcript. Zygotic expression of TSP-1, TSP-3, and TSP-4 begins at the end of gastrulation and transcripts encoding each protein accumulate through the tadpole stages of development. Whole mount in situ hybridizations reveal that each TSP mRNA is localized in the embryo with distinct, developmentally regulated patterns of expression. TSP-1 mRNAs are detected in a wide range of tissues including the floor plate of the neural tube, epidermis, somites, notochord and, most notably, alternating rhombomeres. Transcripts encoding TSP-3 are expressed in the notochord, floor plate, sensorial layer of the epidermis and sensory epithelia. TSP-4 mRNAs are restricted to somitic mesoderm and skeletal muscle. These data suggest that the TSPs represent a functionally diverse family of ECM proteins with tissue-specific functions during embryogenesis.

Merging extracellular domains: fold prediction for laminin G-like and amino-terminal thrombospondin-like modules based on homology to pentraxins

Using a new method for construction and database searches of sequence consensus strings, we have identified a new superfamily of protein modules comprising laminin G, thrombospondin N and the pentraxin families. The conserved patterns correspond mainly to hydrophobic core residues located in central beta strands of the known three-dimensional structures of two pentraxins, the human C-reactive protein and the serum amyloid P-component. Thus, we predict a similar jellyroll fold for all members of this superfamily. In addition, the conservation of two exposed aspartate residues in the majority of superfamily members suggests hitherto unrecognised functional sites.

"Peptabody": a new type of high avidity binding protein

A new type of high avidity binding molecule, termed "peptabody" was created by harnessing the effect of multivalent interaction. A short peptide ligand was fused via a semi-rigid hinge region with the coiled-coil assembly domain of the cartilage oligomeric matrix protein, resulting in a pentameric multivalent binding molecule. In the first peptabody (Pab-S) described here, a peptide (S) specific for the mouse B-cell lymphoma BCL1 surface Ig idiotype, was selected from a phage display library. A fusion gene was constructed encoding peptide S, followed by the 24 aa hinge region from camel IgG and a modified 55 aa cartilage oligomeric matrix protein pentamerization domain. The Pab-S fusion protein was expressed in Escherichia coli in a soluble form at high levels and purified in a single step by metal-affinity chromatography. Pab-S specifically bound the BCL1 surface idiotype with an avidity of about 1 nM, which corresponds to a 2 x 10(5)-fold increase compared with the affinity of the synthetic peptide S itself. Biochemical characterization showed that Pab-S is a stable homopentamer of about 85 kDa, with interchain disulfide bonds. Pab-S can be dissociated under denaturing and reducing conditions and reassociated as a pentamer with full-binding activity. This intrinsic feature provides an easy way to combine Pab molecules with two different peptide specificities, thus producing heteropentamers with bispecific and/or chelating properties.

Domain organizations of modular extracellular matrix proteins and their evolution

Multidomain proteins which are composed of modular units are a rather recent invention of evolution. Domains are defined as autonomously folding regions of a protein, and many of them are similar in sequence and structure, indicating common ancestry. Their modular nature is emphasized by frequent repetitions in identical or in different proteins and by a large number of different combinations with other domains. The extracellular matrix is perhaps the largest biological system composed of modular mosaic proteins, and its astonishing complexity and diversity are based on them. A cluster of minireviews on modular proteins is being published in Matrix Biology. These deal with the evolution of modular proteins, the three-dimensional structure of domains and the ways in which these interact in a multidomain protein. They discuss structure-function relationships in calcium binding domains, collagen helices, alpha-helical coiled-coil domains and C-lectins. The present minireview is focused on some general aspects and serves as an introduction to the cluster.

Inhibition of breast cancer progression by an antibody to a thrombospondin-1 receptor

BACKGROUND

Thrombospondin-1 (TSP-1) is a matrix-bound adhesive glycoprotein. Breast carcinoma cells exhibit increased expression of a novel TSP-1 receptor. We evaluated the role of this receptor in breast cancer adhesion and progression.

METHODS

Adhesion assays were performed to evaluate MDA-MB-231 breast cancer cell adhesion to TSP-1 in vitro in the presence of either nonimmune immunoglobulin G(IgG) or anti-TSP-1 receptor IgG. Receptor-mediated tumor cell progression was evaluated in athymic nude mice. Mice were inoculated with MDA-MB-231 breast cancer cells and randomized to treatment with intraperitoneal injections of saline solution, nonspecific IgG antibody, or an anti-TSP-1 receptor antibody every other day for 20 days. Mice were killed at 21 days. The peritoneal cavity was examined grossly for primary tumor implantation. The liver and lungs were examined histologically for micrometastases.

RESULTS

MDA-MB-231 breast cancer cells adhered to TSP-1 in vitro. This adhesion was inhibited to 10% of control by anti-TSP-1 receptor antibody (p < 0.005). Anti-TSP-1 receptor antibody inhibited in vivo breast cancer progression. Mice treated with control IgG antibody or saline solution alone exhibited extensive intraperitoneal seeding. Only one mouse treated with the anti-TSP-1 receptor antibody exhibited any intraperitoneal tumor seeding (p < 0.01).

CONCLUSIONS

These data suggest that TSP-1 and its receptor play an important role in breast cancer progression.

Opposite regulation of thrombospondin-1 and corticotropin-induced secreted protein/thrombospondin-2 expression by adrenocorticotropic hormone in adrenocortical cells

Corticotropin-induced secreted protein (CISP) is a trimeric glycoprotein secreted by primary cultures of bovine adrenortical cells in response to adrenocorticotropic hormone (ACTH). This protein was recently purified in our laboratory, and its N-terminal amino-acid sequence revealed a significant similarity with thrombospondin-2 (TSP2). We report here the nucleotide sequence of a 386 bp RT-PCR fragment specific for CISP. The deduced protein sequence shares 84% identity with the N-terminal portion of mature human TSP2, suggesting that CISP is its bovine counterpart. Northern analysis of adrenocortical cell RNA using the above cDNA fragment as a probe revealed a 6.0 kb CISP/TSP2 mRNA whose abundance was increased nearly fivefold following a 24 h cell treatment with 10(-7) M ACTH. Under the same conditions, the expression of TSP1 mRNA was reduced by tenfold. The protein levels of TSP1 and CISP/TSP2 varied accordingly with their respective mRNA levels, as shown by immunoprecipitation and immunofluorescence experiments. Taken together, these data show that ACTH induces a dramatic shift in the pattern of adrenocortical cell thrombospondin expression from TSP1 to CISP/TSP2. This observation suggests that these two members of the thrombospondin family exert distinct biological functions in the adrenal cortex. This hypothesis is further supported by the observation that anti-CISP antibodies inhibit the maintenance of the morphological changes of bovine adrenocortical cells induced by ACTH, whereas anti-TSP1 antibodies do not.

Modeling of a five-stranded coiled coil structure for the assembly domain of the cartilage oligomeric matrix protein

The three-dimensional structure of the assembly domain of the cartilage oligomeric matrix protein (COMP) has been modeled. The model demonstrates a parallel five-stranded coiled coil and fits well with a large amount of experimental data that describe the oligomerization state, the alpha-helical conformation, the helix directionality and the properties of the (abcdefg)n repeat sequence containing apolar residues at (a) and (d) positions. Comparison of the pentamer model with the known dimer, trimer, and tetramer coiled coils revealed interactions that could mediate the switch to the formation of the pentamer coiled coil. The most distinctive feature of the pentamer model involves ion pair interactions. Charged side chains of the pentamer can form (f-g), (f-b'), and (e-c') interhelical ion pairs, which are neither experimentally observed nor modeled in the di-, tri-, and tetramers. A polar glutamine residue could be adopted at an interior (d) position of the modeled structure due to the formation of a symmetrical network of buried hydrogen bonds between five such glutamines. The pentamer model contains an axial cavity that can accept water molecules. Conformational analysis was carried out in an attempt to determine the three-dimensional structure of the disulfide bonded C-terminal region of the pentamer. Recent data on crystallization of the COMP assembly domain (Efimov et al., Proteins 24:259-262, 1996) indicate that the prediction can be tested experimentally in the near future.

Thrombospondin-4, an extracellular matrix protein expressed in the developing and adult nervous system promotes neurite outgrowth

Extracellular matrix (ECM) molecules are involved in multiple aspects of cell-to-cell signaling during development and in the adult. In nervous system development, specific recognition processes, e.g., during axonal pathfinding and synaptogenesis involve modulation and signaling by ECM components. Much less is known about their presence and possible roles in the adult nervous system. We now report that thrombospondin-4 (TSP-4), a recently discovered member of the TSP gene family is expressed by neurons, promotes neurite outgrowth, and accumulates at the neuromuscular junction and at certain synapse-rich structures in the adult. To search for muscle genes that may be involved in neuromuscular signaling, we isolated cDNAs induced in adult skeletal muscle by denervation. One of these cDNAs coded for the rat homologue of TSP-4. In skeletal muscle, it was expressed by muscle interstitial cells. The transcript was further detected in heart and in the developing and adult nervous system, where it was expressed by a wide range of neurons. An antiserum to the unique carboxyl-terminal end of the protein allowed to specifically detect TSP-4 in transfected cells in vitro and on cryostat sections in situ. TSP-4 associated with ECM structures in vitro and in vivo. In the adult, it accumulated at the neuromuscular junction and at synapse-rich structures in the cerebellum and retina. To analyze possible activities of TSP-4 towards neurons, we carried out coculture experiments with stably transfected COS cells and motor, sensory, or retina neurons. These experiments revealed that TSP-4 was a preferred substrate for these neurons, and promoted neurite outgrowth. The results establish TSP-4 as a neuronal ECM protein associated with certain synapse-rich structures in the adult. Its activity towards embryonic neurons in vitro and its distribution in vivo suggest that it may be involved in local signaling in the developing and adult nervous system.

The effect of thrombospondin on oral squamous carcinoma cell invasion of collagen

BACKGROUND

Thrombospondin (TSP), a cell matrix protein, and transforming growth factor beta (TGF-beta), a growth regulatory protein, play roles in tumor progression. The purpose of this study was to investigate the effects of TSP and TGF-beta on tumor cell invasion.

MATERIALS AND METHODS

Tumor cell invasion assays were performed using a modified Boyden chamber apparatus with collagen-coated membranes. The KB oral carcinoma cell line was studied in serum-free media. Invasion was measured as the summation of the number of cells in five representative low-power fields (x 100) traversing the collagen barrier after a 3-hour incubation period. The effects of antibodies against TSP, TGF-beta and the cysteine-serine-valine-threonine-cysteine-glycine (CSVTCG)-specific TSP receptor were also evaluated.

RESULTS

TSP caused a dose-dependent stimulation of tumor cell invasion. Antibodies against TSP, its CSVTCG-specific receptor, and TGF-beta inhibited TSP-promoted invasion by 50% to 71%.

CONCLUSIONS

TSP and its CSVTCG-specific receptor promote KB cell invasion of collagen through the production and/or activation of TGF-beta.

Thrombospondin-4 is expressed by early osteogenic tissues in the chick embryo

The thrombospondins are a family of related glycoproteins found in the embryonic extracellular matrix. To date, five members of this family have been identified. Thrombospondin-1 and thrombospondin-2 have similar primary structure, but are expressed in different tissues at different times during development. Thrombospondins-3, -4, and cartilage oligomeric protein belong to a second thrombospondin subgroup in which the carboxyl-half of each molecule is most similar to thrombospondin-1 and -2. Here we report the cloning and sequencing of a novel probe to avian thrombospondin-4. We have used this probe to determine the origins of thrombospondin-4 in the chick embryo by in situ hybridization. Thrombospondin -4 transcripts first appear in the mesenchyme surrounding bone anlage coinciding with the initial stages of osteogenesis. The expression in osteogenic tissues is transient: thrombospondin-4 mRNAs are not seen in the osteoblasts of bone collars in developing long bones. This pattern is distinct from avian thrombospondin-2 which is expressed in perichondrium and embryonic fibrous connective tissues. Our observations indicate that connective tissues are the principal site of thrombospondin-4 expression in the chick. The diverse origins of different thrombospondin gene family members imply distinctive roles for these proteins related to the growth and differentiation of cartilage, tendons, and bone.

Tendon extracellular matrix contains pentameric thrombospondin-4 (TSP-4)

In preparations of cartilage oligomeric matrix protein (COMP) from bovine tendon two contaminating polypeptides of 120 and 135 kDa were detected. N-terminal protein sequencing of these polypeptides showed homology to the N-terminus and to an internal sequence in TSP-4, respectively. TSP-4 was further enriched by heparin affinity chromatography. Electron microscopy of this sample shows primarily five armed particles with globular domains at the periphery connected to a central assembly domain in which smaller N-terminal globular domains can be resolved tightly packed at the center of the particle. We can thereby confirm the pentameric model for TSP-4 proposed by Lawler et al. [(1995) J. Biol. Chem. 270, 2809-2814], on the basis of recombinantly expressed protein. We further show that TSP-4 is abundant in tendon.

Characterization of human thrombospondin-4

The thrombospondins are a family of extracellular calcium binding proteins that are involved in cell proliferation, adhesion, and migration. We have sequenced full-length human thrombospondin-4 and characterized the recombinant protein. In contrast to Xenopus laevis thrombospondin-4, the human protein contains an RGD cell binding sequence in the third type 3 repeat. Transfection of mouse NIH3T3 fibroblasts or C2C12 myoblasts with a full-length human thrombospondin-4 cDNA results in the expression of a polypeptide with a reduced molecular weight of 140,000. In the absence of reducing agent, the expressed protein has an apparent molecular weight of 550,000. Recombinant thrombospondin-4 has been purified from the culture supernatant by heparin-Sepharose and anti-thrombospondin-4 antibody-Affi-gel affinity chromatography. Electron microscopy indicates that thrombospondin-4 is composed of five subunits with globular domains at each end. The observation of a calcium-dependent change in the electron microscopic appearance of thrombospondin-4 is consistent with limited tryptic digestion data that indicate that thrombospondin-4 is resistant to digestion in the presence of calcium. These data indicate that thrombospondin-4 is a pentameric protein that binds to heparin and calcium.

Calcium ion binding to thrombospondin 1

We have quantified the binding of Ca2+ to platelet thrombospondin 1 (TSP1) using equilibrium dialysis with 45CaCl2. Ca2+ binding to TSP1 was found to be cooperative with 10% occupancy at 15-20 microM CaCl2, 90% occupancy at 100 microM CaCl2, and a Hill coefficient of 2.4 +/- 0.2 The average apparent Kd was 52 +/- 5 microM. Maximum binding, assuming Mr = 450,000 and epsilon = 0.918 (A280/mg/ml), was 35 +/- 3 Ca2+/TSP1. This value is close to the 33 sites (11 per subunit) predicted based on homology of the epidermal growth factor (1 site) and aspartate-rich (10 sites) regions to known Ca2+ binding sequences. Ca2+ protected the aspartate-rich region from trypsin proteolysis, but not until nearly all of the Ca2+ binding sites were filled. At lower occupancy of Ca2+ binding sites, several limited tryptic digest products were obtained. This finding and the previous demonstration of extensive thiol-disulfide isomerization within the aspartate-rich regions suggest that subregions of the aspartate-rich region are stabilized in different conformers. Zn2+, Cu2+, Mn2+, Mg2+, Co2+, Cd2+, and Ba2+ were tested for their ability to modulate Ca2+ binding and protease sensitivity of TSP1. Zn2+ inhibited 40% of the Ca2+ binding but neither protected TSP1 from trypsin proteolysis, nor labilized TSP1 toward trypsin proteolysis. These results provide direct evidence for high capacity, cooperative and specific binding of Ca2+ to conformationally labile aspartate-rich repeats of TSP1.

Distinct effects of thrombospondin-1 and CISP/thrombospondin-2 on adrenocortical cell spreading

Corticotropin-induced secreted protein (CISP) is a trimeric protein secreted by bovine adrenocortical cells in response to ACTH, that is likely to represent the bovine form of thrombospondin-2 (TSP2). This study was aimed at delineating the respective effects of CISP/TSP2 and TSP1 (thrombospondin-1) on adrenocortical cell attachment and spreading. TSP1 and CISP/TSP2 were found to slightly reduce the attachment of adrenocortical cells to plastic in the presence of serum but exhibited a pronounced differential effect on cell spreading. CISP/TSP2 inhibited adrenocortical cell spreading in a dose-dependent manner (maximal effect with 40 micrograms/ml) whereas TSP1 (up to 100 micrograms/ml) did not influence this process. The inhibition of spreading was observed whether plates were coated with CISP/TSP2 alone or with a mixture of CISP/TSP2 and fibronectin. We suggest that the inhibition of in vitro adrenocortical cell spreading by CISP/TSP2 is indicative of an implication of this protein in the migration of adrenocortical cells in vivo.

Structure of homeobox-leucine zipper genes suggests a model for the evolution of gene families

Homeobox genes are present in both plants and animals. Homeobox-leucine zipper genes, however, have been identified thus far only in the small mustard plant Arabidopsis thaliana. This observation suggests that homeobox-leucine zipper genes evolved after the divergence of plants and animals, perhaps to mediate specific regulatory events. To better understand this gene family, we isolated several sequences containing the homeobox-leucine zipper motif and carried out a comparative analysis of nine homeobox-leucine zipper genes (HAT1, HAT2, HAT3, HAT4, HAT5, HAT7, HAT9, HAT14, and HAT22). Gene structures, sequence comparisons, and chromosomal locations suggest a simple model for the evolution of these genes. The model postulates that a primordial homeobox gene acquired a leucine zipper by exon capture. The nascent homeobox-leucine zipper gene then appears to have undergone a series of gene duplication and chromosomal translocation events, leading to the formation of the HAT gene family. This work has general implications for the evolution of regulatory genes.

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

Thrombospondin-1 (TSP-1) is an extracellular matrix glycoprotein that may play important roles in the morphogenesis and repair of skeletal muscle. To begin to explore the role of thrombospondin-1 in this tissue, we have examined the interactions of three rodent skeletal muscle cell lines, C2C12, G8, and H9c2, with platelet TSP-1. The cells secrete thrombospondin and incorporate it into the cell layer in a distribution distinct from that of fibronectin. Myoblasts attach and spread on fibronectin- or thrombospondin-coated substrates with similar time and concentration dependencies. Whereas cells adherent on fibronectin organize actin stress fibers, cells adherent on TSP-1 display prominent membrane ruffles and lamellae that contain radial actin microspikes. Attachment to thrombospondin-1 or the 140-kDa tryptic fragment is mediated by interactions with the type 1 repeats and the carboxy-terminal globular domain. Attachment is not inhibited by heparin, GRGDSP peptide, or VTCG peptide but is inhibited by chondroitin sulphate A. Integrins of the beta 1 or alpha V subgroups do not appear to be involved in myoblast attachment to TSP-1; instead, this process depends in part on cell surface chondroitin sulphate proteoglycans. Whereas the central 70-kDa chymotryptic fragment of TSP-1 does not support myoblast attachment, the carboxy-terminal domain of TSP-1 expressed as a fusion protein in the bacterial expression vector, pGEX, supported myoblast attachment to 30% the level of intact TSP-1. Thrombospondin-4 (TSP-4) is also present in skeletal muscle and a fusion protein containing the carboxy-terminal domain of TSP-4 also supported myoblast adhesion, although this protein was less active on a molar basis than the TSP-1 fusion protein. Thus, the carboxyterminal domain of TSP-1 appears to contain a primary attachment site for myoblasts, and this activity is present in a second member of the thrombospondin family.

The thrombospondin-like chains of cartilage oligomeric matrix protein are assembled by a five-stranded alpha-helical bundle between residues 20 and 83

The N-terminal fragment of rat cartilage oligomeric matrix protein (COMP), comprising residues 20-83, was over-expressed in E. coli and purified under non-denaturing conditions. The fragment forms pentamers similar to the assembly domain of the native protein. Its five chains can be covalently linked in vitro by oxidation of cysteines 68 and 71. The fragment adopts a predominantly alpha-helical structure as judged by circular dichroism spectroscopy. On the basis of these findings we propose the model of a five-stranded alpha-helical bundle for the assembly domain of COMP. The studied sequence is conserved in thrombospondins 3 and 4 thus raising the possibility that these proteins are also pentamers.

Evolution of genetic redundancy for advanced players

An ever expanding database on the sequence organization and repetition of genic and non-genic components of nuclear and organelle genomes reveals that the vast majority of sequences are subject to one or other mechanism of DNA turnover (gene conversion, unequal crossing over, slippage, retrotransposition, transposition and others). Detailed studies, using novel methods of experimental detection and analytical procedures, show that such mechanisms can operate one on top of another and that wide variations in their unit lengths, biases, polarities and rates create bizarre and complex patterns of genetic redundancy. The ability of these mechanisms to operate both within and between chromosomes implies that realistic models of the evolutionary dynamics of redundancy, and of the potential interaction with natural selection in a sexual species, need to consider the diffusion of variant repeats across multiple chromosome lineages, in a population context. Recently, important advances in both experimental and analytical approaches have been made along these lines. There is increasing awareness that genetic redundancy and turnover induces a molecular co-evolution between functionally interacting genetic systems in order to maintain essential functions.