Complex formation of human thrombospondin with osteonectin

Bone Matrix Non-Collagenous Proteins in Tissue Engineering: Creating New Bone by Mimicking the Extracellular Matrix

Engineering biomaterials that mimic the extracellular matrix (ECM) of bone is of significant importance since most of the outstanding properties of the bone are due to matrix constitution. Bone ECM is composed of a mineral part comprising hydroxyapatite and of an organic part of primarily collagen with the rest consisting on non-collagenous proteins. Collagen has already been described as critical for bone tissue regeneration; however, little is known about the potential effect of non-collagenous proteins on osteogenic differentiation, even though these proteins were identified some decades ago. Aiming to engineer new bone tissue, peptide-incorporated biomimetic materials have been developed, presenting improved biomaterial performance. These promising results led to ongoing research focused on incorporating non-collagenous proteins from bone matrix to enhance the properties of the scaffolds namely in what concerns cell migration, proliferation, and differentiation, with the ultimate goal of designing novel strategies that mimic the native bone ECM for bone tissue engineering applications. Overall, this review will provide an overview of the several non-collagenous proteins present in bone ECM, their functionality and their recent applications in the bone tissue (including dental) engineering field.

Adipokines: New Potential Therapeutic Target for Obesity and Metabolic, Rheumatic, and Cardiovascular Diseases

Besides its role as an energy storage organ, adipose tissue can be viewed as a dynamic and complex endocrine organ, which produces and secretes several adipokines, including hormones, cytokines, extracellular matrix (ECM) proteins, and growth and vasoactive factors. A wide body of evidence showed that adipokines play a critical role in various biological and physiological functions, among which feeding modulation, inflammatory and immune function, glucose and lipid metabolism, and blood pressure control. The aim of this review is to summarize the effects of several adipokines, including leptin, diponectin, resistin, chemerin, lipocalin-2 (LCN2), vaspin, omentin, follistatin-like 1 (FSTL1), secreted protein acidic and rich in cysteine (SPARC), secreted frizzled-related protein 5 (SFRP5), C1q/TNF-related proteins (CTRPs), family with sequence similarity to 19 member A5 (FAM19A5), wingless-type inducible signaling pathway protein-1 (WISP1), progranulin (PGRN), nesfatin-1 (nesfatin), visfatin/PBEF/NAMPT, apelin, retinol binding protein 4 (RPB4), and plasminogen activator inhibitor-1 (PAI-1) in the regulation of insulin resistance and vascular function, as well as many aspects of inflammation and immunity and their potential role in managing obesity-associated diseases, including metabolic, osteoarticular, and cardiovascular diseases.

A holistic approach to dissecting SPARC family protein complexity reveals FSTL-1 as an inhibitor of pancreatic cancer cell growth

SPARC is a matricellular protein that is involved in both pancreatic cancer and diabetes. It belongs to a wider family of proteins that share structural and functional similarities. Relatively little is known about this extended family, but evidence of regulatory interactions suggests the importance of a holistic approach to their study. We show that Hevin, SPOCKs, and SMOCs are strongly expressed within islets, ducts, and blood vessels, suggesting important roles for these proteins in the normal pancreas, while FSTL-1 expression is localised to the stromal compartment reminiscent of SPARC. In direct contrast to SPARC, however, FSTL-1 expression is reduced in pancreatic cancer. Consistent with this, FSTL-1 inhibited pancreatic cancer cell proliferation. The complexity of SPARC family proteins is further revealed by the detection of multiple cell-type specific isoforms that arise due to a combination of post-translational modification and alternative splicing. Identification of splice variants lacking a signal peptide suggests the existence of novel intracellular isoforms. This study underlines the importance of addressing the complexity of the SPARC family and provides a new framework to explain their controversial and contradictory effects. We also demonstrate for the first time that FSTL-1 suppresses pancreatic cancer cell growth.

Current understanding of the thrombospondin-1 interactome

The multifaceted action of thrombospondin-1 (TSP-1) depends on its ability to physically interact with different ligands, including structural components of the extracellular matrix, other matricellular proteins, cell receptors, growth factors, cytokines and proteases. Through this network, TSP-1 regulates the ligand activity, availability and structure, ultimately tuning the cell response to environmental stimuli in a context-dependent manner, contributing to physiological and pathological processes. Complete mapping of the TSP-1 interactome is needed to understand its diverse functions and to lay the basis for the rational design of TSP-1-based therapeutic approaches. So far, large-scale approaches to identify TSP-1 ligands have been rarely used, but many interactions have been identified in small-scale studies in defined biological systems. This review, based on information from protein interaction databases and the literature, illustrates current knowledge of the TSP-1 interactome map.

Secreted protein acidic and rich in cysteine is a matrix scavenger chaperone

Secreted Protein Acidic and Rich in Cysteine (SPARC) is one of the major non-structural proteins of the extracellular matrix (ECM) in remodeling tissues. The functional significance of SPARC is emphasized by its origin in the first multicellular organisms and its high degree of evolutionary conservation. Although SPARC has been shown to act as a critical modulator of ECM remodeling with profound effects on tissue physiology and architecture, no plausible molecular mechanism of its action has been proposed. In the present study, we demonstrate that SPARC mediates the disassembly and degradation of ECM networks by functioning as a matricellular chaperone. While it has low affinity to its targets inside the cells where the Ca(2+) concentrations are low, high extracellular concentrations of Ca(2+) activate binding to multiple ECM proteins, including collagens. We demonstrated that in vitro, this leads to the inhibition of collagen I fibrillogenesis and disassembly of pre-formed collagen I fibrils by SPARC at high Ca(2+) concentrations. In cell culture, exogenous SPARC was internalized by the fibroblast cells in a time- and concentration-dependent manner. Pulse-chase assay further revealed that internalized SPARC is quickly released outside the cell, demonstrating that SPARC shuttles between the cell and ECM. Fluorescently labeled collagen I, fibronectin, vitronectin, and laminin were co-internalized with SPARC by fibroblasts, and semi-quantitative Western blot showed that SPARC mediates internalization of collagen I. Using a novel 3-dimensional model of fluorescent ECM networks pre-deposited by live fibroblasts, we demonstrated that degradation of ECM depends on the chaperone activity of SPARC. These results indicate that SPARC may represent a new class of scavenger chaperones, which mediate ECM degradation, remodeling and repair by disassembling ECM networks and shuttling ECM proteins into the cell. Further understanding of this mechanism may provide insight into the pathogenesis of matrix-associated disorders and lead to the novel treatment strategies.

Collagen-embedded hydroxylapatite-beta-tricalcium phosphate-silicon dioxide bone substitute granules assist rapid vascularization and promote cell growth

In the present study we assessed the biocompatibility in vitro and in vivo of a low-temperature sol-gel-manufactured SiO(2)-based bone graft substitute. Human primary osteoblasts and the osteoblastic cell line, MG63, cultured on the SiO(2) biomatrix in monoculture retained their osteoblastic morphology and cellular functionality in vitro. The effect of the biomaterial in vivo and its vascularization potential was tested subcutaneously in Wistar rats and demonstrated both rapid vascularization and good integration within the peri-implant tissue. Scaffold degradation was progressive during the first month after implantation, with tartrate-resistant acid phosphatase-positive macrophages being present and promoting scaffold degradation from an early stage. This manuscript describes successful osteoblastic growth promotion in vitro and a promising biomaterial integration and vasculogenesis in vivo for a possible therapeutic application of this biomatrix in future clinical studies.

Modulation of matrix remodeling by SPARC in neoplastic progression

SPARC is a matricellular glycoprotein that mediates interactions between cells and their microenvironment. It is produced at sites of tissue remodeling, where it regulates matrix deposition and turnover, cell adhesion, and signaling by extracellular factors, exerting profound effects on tissue architecture and cell physiology. During extensive matrix remodeling in neoplastic progression, SPARC is expressed in cancer-associated stroma and in malignant cells of some types, affecting tumor development, invasion, metastases, angiogenesis and inflammation. SPARC-induced changes in the tumor microenvironment can suppress or promote progression of different cancers depending on the tissue and cell type. Understanding the mechanism of matrix remodeling and its regulation by SPARC is essential for the development of new treatment strategies for highly aggressive cancers.

Common deleted genes in the 5q- syndrome: thrombocytopenia and reduced erythroid colony formation in SPARC null mice

The commonly deleted region (CDR) for the 5q- syndrome has been identified as a 1.5-megabase interval on human chromosome 5q32. We studied, by real-time reverse-transcription (RT)-PCR, the expression of 33 genes within the CDR that are known to be expressed in CD34+ hematopoietic stem cells. Genes in the 5q- samples that showed the most pronounced decrease in expression compared to non-5q- samples were: solute carrier family 36, member 1 (SLC36A1; 89% downregulated), Ras-GTPase-activating protein SH3 domain-binding (G3BP; 79%), antioxidant protein 1 (ATOX1; 76%), colony-stimulating factor-1 receptor precursor (CSF1R; 76%), ribosomal protein S14 (RPS14; 74%), platelet-derived growth factor receptor-beta (PDGFRB; 73%), Nef-associated factor 1 (TNIP1; 72%), secreted protein, acidic and rich in cysteine (SPARC; 71%), annexin VI (ANAX6; 69%), NSDT (66%) and TIGD (60%). We further studied the hematopoietic system in SPARC-null mice. These mice showed significantly lower platelet counts compared to wild-type animals (P=0.008). Although hemoglobin, hematocrit and mean corpuscular volume (MCV) were lower in mice lacking SPARC, differences were not statistically significant. SPARC-null mice showed a significantly impaired ability to form erythroid burst-forming units (BFU-E). However, no significant differences were found in the formation of erythroid colony-forming units (CFU-E), granulocyte/monocyte colony-forming units (CFU-GM) or megakaryocyte colony-forming units (CFU-Mk) in these animals. We conclude that many of the genes within the CDR associated with the 5q- syndrome exhibit significantly decreased expression and that SPARC, as a potential tumor suppressor gene, may play a role in the pathogenesis of this disease.

Secreted modular calcium-binding protein-1 localization during mouse embryogenesis

BM-40 is an extracellular matrix-associated protein and is characterized by an extracellular calcium-binding domain as well as a follistatin-like domain. Secreted modular calcium-binding protein-1 (SMOC-1) is a new member of the BM-40 family. It consists of two thyroglobulin-like domains, a follistatin-like domain and a new domain without known homologues and is expressed ubiquitously in many adult murine tissues. Immunofluorescence studies, as well as immunogold electron microscopy, have confirmed the localization of SMOC-1 in or around basement membranes of adult murine skin, blood vessels, brain, kidney, skeletal muscle, and the zona pellucida surrounding the oocyte. In the present work, light microscopic immunohistochemistry has revealed that SMOC-1 is localized in the early mouse embryo day 7 throughout the entire endodermal basement membrane zone of the embryo proper. SMOC-1 mRNA is synthesized, even in early stages of mouse development, by mesenchymal as well as epithelial cells deriving from all three germ layers. In embryonic stage day 12, and fetal stages day 14, 16, and 18, the protein is present in the basement membrane zones of brain, blood vessels, skin, skeletal muscle, lung, heart, liver, pancreas, intestine, and kidney. This broad and organ-specific distribution suggests multifunctional roles of SMOC-1 during mouse embryogenesis.

Thrombospondin 1, thrombospondin 2 and the eye

Thrombospondin 1 and thrombospondin 2 (TSP1 and TSP2), which comprise the subgroup A thrombospondins, are matricellular proteins. As matricellular proteins, they modulate interactions between cells and the cellular environment, regulate cell adhesion and typically are expressed during tissue formative processes. In general, TSP1 and TSP2 counter angiogenesis (including tumour angiogenesis) and play important but contrasting roles during cutaneous repair. The two proteins are involved in development, including that of the eye, although evidence suggests that they have their greatest impact during tissue production in the adult. In the normal adult eye, they tend to be found at sites of ongoing matrix synthesis or cell-matrix interactions. At these sites, the two proteins possibly influence cellular differentiation and/or basement membrane deposition. TSP1 is also present in the intraocular fluids and drainage pathway, where it may function in maintaining the anti-angiogenic environment and in intraocular pressure control, respectively. TSP1 could also be involved in ocular immune privilege. Unlike in skin wounds, where TSP1 is derived from the blood and is present only in the early phases of repair, ocular tissue damage appears to lead to protacted TSP1 synthesis by local cells. This response might help suppress angiogenesis in the transparent tissues of the eye and so lessen visual axis opacification following injury. However, TSP2, which is also produced by damaged ophthalmic tissue and may be especially important in matrix organisation, seems to augment contraction in anomalous intraocular fibrosis. Elucidating the roles of TSP1 and TSP2 in ocular physiology and pathobiology may lead to improved therapies for neovascular, neoplastic, reparative and other ophthalmic diseases.

Endogenous Osteonectin/SPARC/BM-40 Expression Inhibits MDA-MB-231 Breast Cancer Cell Metastasis

Skeletal metastases occur with high incidence in patients with breast cancer and cause long-term skeletal morbidity. Osteonectin (SPARC, BM-40) is a bone matrix factor that is an in vitro chemoattractant for breast and prostate cancer cells. Increased expression of osteonectin is found in malignant breast tumors. We infected MDA-231 breast cancer cells with an adenovirus expressing osteonectin to examine the role of osteonectin expression in breast cancer cells and its effect on metastasis, in particular to bone. Expression of osteonectin did not affect MDA-231 cell proliferation, apoptosis, migration, cell aggregation, or protease cleavage of collagen IV. However, in vitro invasion of these osteonectin-infected cells through Matrigel and colony formation on Matrigel was decreased. Interestingly, high osteonectin expression in MDA-231 cells inhibited metastasis in a dose-dependent manner to many different organs including bone. The reduction in metastasis may be due to decreased platelet-tumor cell aggregation, because exogenous osteonectin inhibited platelet aggregation in vitro and the high osteonectin expression in MDA-231 cells reduced tumor cell-induced thrombocytopenia in vivo compared with control-infected cells. These studies suggest that high endogenous expression of osteonectin in breast cancer cells may reduce metastasis via reduced invasive activity and reduced tumor cell-platelet aggregation.

Leukocyte, rather than tumor-produced SPARC, determines stroma and collagen type IV deposition in mammary carcinoma

Secreted protein, acidic and rich in cysteine (SPARC), also known as osteonectin or BM-40, is a Ca²⁺-binding matricellular glycoprotein involved in development, wound healing, and neoplasia. However, the role of SPARC in tumors is ill defined mostly because it is expressed by both tumor and stromal cells, especially inflammatory cells. We analyzed the respective roles of host- and tumor-derived SPARC in wild-type and congenic SPARC knockout (SPARC^−/−) mice on a BALB/c genetic background injected into the mammary fat pad with SPARC-producing mammary carcinoma cells derived from c-erB2 transgenic BALB/c mice. Reduced tumor growth but massive parenchyma infiltration, with large areas of necrosis and impaired vascularization were observed in SPARC^−/− mice. Immunohistochemical analysis showed a defect in collagen type IV deposition in the stroma of lobular tumors from SPARC^−/− mice. Chimeric mice expressing SPARC only in bone marrow–derived cells were able to organize peritumoral and perilobular stroma, whereas reciprocal chimeras transplanted with bone marrow from SPARC^−/− mice developed tumors with less defined lobular structures, lacking assembled collagen type IV and with a parenchyma heavily infiltrated by leukocytes. Together, the data indicate that SPARC produced by host leukocytes, rather than the tumor, determines the assembly and function of tumor-associated stroma through the organization of collagen type IV.

Spatially and temporally different expression of osteonectin and osteopontin in the infarct zone of experimentally induced myocardial infarction in rats

Osteonectin and osteopontin, two secreted matricellular proteins, have a variety of functions that are exerted through interaction with matrix components. These proteins appear in response to tissue injury. To test our hypothesis that osteopontin and osteonectin are expressed with spatially and temporally different patterns in myocardial infarct tissue, we investigated osteonectin and osteopontin expression in experimentally induced myocardial infarction in rats, in comparison with Type I collagen expression. Northern blotting demonstrated that osteonectin mRNA did not markedly increase on Day 2 after the infarction, but it increased on Days 7 and 14 by 1.7+/-0.12- and 1.8+/-0.01-fold compared to that in preligation hearts. In contrast, osteopontin mRNA was increased on Day 1 (41.9+/-11.3-fold increase) and on Day 2 (58.3+/-7.6-fold increase), and then it declined on Days 7 and 14 (24.8+/-9.0- and 13.5+/-4.7-fold increase, respectively). In situ hybridization revealed that osteonectin mRNA signals were observed in fibroblasts, myofibroblasts and macrophages around infarct necrotic tissue on Days 7 and 14. Osteopontin mRNA signals were observed in macrophages in the infarct marginal zone on Day 2. Immunopositive staining for both osteonectin and osteopontin showed the same pattern as that obtained by in situ hybridization. The time course of osteonectin mRNA was almost parallel with that of Type I collagen mRNA, while that of osteopontin was not. These results demonstrated spatially and temporally different expression patterns of osteonectin and osteopontin in myocardial infarction and suggest that osteonectin appears to be involved in the pathological course in the late phase after infarction concomitantly with Type I collagen, while osteopontin may play a role in the early phase.

Bone matrix proteins: their function, regulation, and relationship to osteoporosis

Bone is a unique tissue composed of numerous cell types entombed within a mineralized matrix each with its own unique functions. While the majority of the matrix is composed of inorganic materials, study of the organic components has yielded most of the insights into the roles and regulation of cell and tissue specific functions. The goal of this review will be to describe some of the major known organic components of the bone matrix and discuss their functions as currently perceived. The potential usefulness of bone matrix protein assays for diagnosing the status of bone diseases and our current understanding of how these proteins could be related to diseases such as osteoporosis will also be reviewed.

Effect of coronary thrombolysis on the plasma concentration of osteonectin (SPARC, BM40) in patients with acute myocardial infarction

Osteonectin is a phosphoglycoprotein exclusively located in bone and platelet alpha-granules. Human platelet-derived osteonectin is released into plasma after thrombin-induced activation. Recognizing the unique distribution of the osteonectin pool, we first sought to investigate whether osteonectin could serve as a sensitive marker of platelet activity, and identify patients with acute myocardial infarction (AMI). The second objective was to define the effects of thrombolytic therapy in these patients on the plasma concentrations of osteonectin at prespecified time points following attempted reperfusion. Osteonectin levels by ELISA were determined in AMI patients before thrombolysis and at 3, 6, 12, and 24 hours thereafter and compared with 12 healthy controls. At baseline, soluble osteonectin plasma levels were similar between controls (447. 7+/-20.6 ng/ml) and AMI patients (425.7+/-43.3 ng/mL; p=NS). A significant increase of the soluble osteonectin was observed at 3 hours after thrombolysis (519.4+/-26.9 ng/mL; p=0.03), and was followed by a decrease to baseline levels at 6 hours after attempted reperfusion. Contrary to expectations, the plasma osteonectin level in our pilot study was not a sensitive marker distinguishing patients with AMI. The early peak of soluble osteonectin at 3 hours after thrombolytic therapy is most likely not related to coronary thrombolysis per se but rather to the phasic changes of platelet activity during myocardial ischemia-reperfusion. The unquestionable platelet origin of this protein and the lack of elevated plasma levels of this alpha-granule constituent, challenge the postulate of uniform platelet activation in AMI patients.

SPARC, a matricellular glycoprotein with important biological functions

SPARC (secreted protein, acidic and rich in cysteine) is a unique matricellular glycoprotein that is expressed by many different types of cells and is associated with development, remodeling, cell turnover, and tissue repair. Its principal functions in vitro are counteradhesion and antiproliferation, which proceed via different signaling pathways. SPARC consists of three domains, each of which has independent activity and unique properties. The extracellular calcium binding module and the follistatin-like module have been recently crystallized. Specific interactions between SPARC and growth factors, extracellular matrix proteins, and cell surface proteins contribute to the diverse activities described for SPARC in vivo and in vitro. The location of SPARC in the nuclear matrix of certain proliferating cells, but only in the cytosol of postmitotic neurons, indicates potential functions of SPARC as a nuclear protein, which might be involved in the regulation of cell cycle progression and mitosis. High levels of SPARC have been found in adult eye, and SPARC-null mice exhibit cataracts at 1-2 months of age. This animal model provides an excellent opportunity to confirm and explore some of the properties of SPARC, to investigate cataractogenesis, and to study SPARC-related family proteins, e.g., SC1/hevin, a counteradhesive matricellular protein that might functionally compensate for SPARC in certain tissues.(J Histochem Cytochem 47:1495-1505, 1999)

Collagen accumulation is decreased in SPARC-null mice with bleomycin-induced pulmonary fibrosis

Secreted protein acidic and rich in cysteine (SPARC) has been shown to be coexpressed with type I collagen in tissues undergoing remodeling and wound repair. We speculated that SPARC is required for the accumulation of collagen in lung injury and that its absence would attenuate collagen accumulation. Accordingly, we have assessed levels of collagen in SPARC-null mice in an intratracheal bleomycin-injury model of pulmonary fibrosis. Eight- to ten-week-old SPARC-null and wild-type (WT) mice received bleomycin (0.0035 U/g) or saline intratracheally and were subsequently killed after 14 days. Relative levels of SPARC mRNA were increased 2.7-fold (P < 0.001) in bleomycin-treated WT lungs in comparison with saline-treated lungs. Protein from bleomycin-treated WT lung contained significantly more hydroxyproline (191.9 microg/lung) than protein from either bleomycin-treated SPARC-null lungs or saline-treated WT and SPARC-null lungs (147.4 microg/lung, 125.4 microg/lung, and 113. 0 microg/lung, respectively; P < 0.03). These results indicate that SPARC is increased in response to lung injury and that accumulation of collagen, as indicated by hydroxyproline content, is attenuated in the absence of SPARC. The properties of SPARC as a matricellular protein associated with cell proliferation and matrix turnover are consistent with its participation in the development of pulmonary fibrosis.

Regulation of human monocyte matrix metalloproteinases by SPARC

SPARC (secreted protein, acidic and rich in cysteine), also called osteonectin or BM-40, is a collagen-binding glycoprotein secreted by a variety of cells and is associated with functional responses involving tissue remodeling, cell movement and proliferation. Because SPARC and monocytes/macrophages are prevalent at sites of inflammation and remodeling in which there is connective tissue turnover, we examined the effect of SPARC on monocyte matrix metalloproteinase (MMP) production. Treatment of human peripheral blood monocytes with SPARC stimulated the production of gelatinase B (MMP-9) and interstitial collagenase (MMP-1). Experiments with synthetic peptides indicated that peptide 3.2, belonging to the alpha helical domain III of SPARC, is the major peptide mediating the MMP production by monocytes. SPARC and peptide 3.2 were also shown to induce prostaglandin synthase (PGHS)-2 as determined by Western and Northern blot analyses. The increase in PGHS-2 stimulated by SPARC or peptide 3.2 correlated with substantially elevated levels of prostaglandin E2 (PGE2) and other arachidonic acid metabolites as measured by radioimmunoassay and high performance liquid chromatography (HPLC), respectively. Moreover, the synthesis of MMP was dependent on the generation of PGE2 by PGHS-2, since indomethacin inhibited the production of these enzymes and their synthesis was restored by addition of exogenous PGE2 or dibutyryl cAMP (Bt2cAMP). These results demonstrate that SPARC might play a significant role in the modulation of connective tissue turnover due to its stimulation of PGHS-2 and the subsequent release of PGE2, a pathway that leads to the production of MMP by monocytes.

Spatiotemporal distribution of SPARC/osteonectin in developing and mature chicken retina

Expression of SPARC (Secreted Protein, Acidic, Rich in Cysteine), a counteradhesive, calcium-binding extracellular matrix (ECM) glycoprotein, is associated with several morphogenetic events during early development. In this study, changes in the spatiotemporal distribution of SPARC transcripts and the protein during chicken retinal development were documented by in situ hybridization and indirect immunofluorescence microscopy. SPARC transcripts were first detected within the proliferating neural ectoderm at embryonic day 4. 5 (E4.5), followed short thereafter (E5) by appearance of SPARC. SPARC was enriched within the inner plexiform layer (IPL) by E10 and within the outer plexiform layer (OPL) by E14, several days after these layers became morphologically distinct. Significant levels of SPARC transcripts were first observed within the ganglion cell layer (GCL) at E17 prior to accumulation of SPARC within the nerve fiber layer, seen first at E20. SPARC protein was first detected within the developing retinal pigment epithelium (RPE) at E10 and increased significantly at RPE cells ceased to proliferate and continued differentiating. Of special note was the restriction of SPARC to the basal-half of the RPE cells. SPARC transcripts were similarly distributed in the adult retina, but at lower levels than in the period just prior to hatching. In the adult retina SPARC was retained in the nerve fiber layer and present in the inner nuclear layer (INL) and outer nuclear layer (ONL), but lost from the IPL and OPL. These changes in expression pattern with time indicate that SPARC is developmentally regulated and therefore may have important function(s) in both morphological development of the retina and functioning of the mature eye.

Type I collagen-deficient Mov-13 mice do not retain SPARC in the extracellular matrix: implications for fibroblast function

The Mov-13 strain of mice was created by the insertion of the murine Moloney leukemia virus into the first intron of the alpha 1 (I) collagen gene. Consequently, Mov-13 embryos do not transcribe alpha 1 (I) collagen mRNA and lack type I collagen protein in the extracellular matrix (ECM). Homozygotes die within 12-14 days of embryonic development, in part from the rupture of large blood vessels, and also exhibit deficiencies in hematopoesis and assembly of the ECM (Lohler et al. [1984] Cell 38:597-607). Several matricellular proteins, proteoglycans, and growth factors bind to type I collagen, e.g., fibronectin, secreted protein acidic and rich in cysteine (SPARC), decorin, and transforming growth factor-beta. Here we investigate the expression and function of SPARC in the absence of type I collagen. We show that fibroblasts isolated from Mov-13 homozygous, heterozygous, and wild-type embryos transcribed and translated SPARC mRNA in vitro. However, accumulation of extracellular SPARC was severely affected in the tissues of Mov-13 homozygotes, whereas extracellular deposition of the secreted glycoproteins fibronectin and type III collagen was not altered. Since SPARC has been shown to be a regulator of cell shape, the functional consequences of the absence of extracellular SPARC were evaluated in collagen gel contraction assays. Fibroblasts isolated from homozygous Mov-13 mice did not contract native type I collagen gels as efficiently as fibroblasts from heterozygous littermates; however, addition of exogenous SPARC enhanced the contraction of collagen by homozygous Mov-13 fibroblasts. The stimulatory effect of SPARC was blocked by antibodies specific for the amino terminus of the protein. These results provide evidence that type I collagen is one of the major extracellular proteins that binds SPARC in vivo. Furthermore, the capacity of fibroblasts to contract ECM in vitro is enhanced by extracellular SPARC. We therefore propose that the remodeling of ECM by cells in vivo is regulated in part by a specific interaction between SPARC and type I collagen.

Extracellular matrix is a source of mitogenically active platelet-derived growth factor

Platelet-derived growth factor (PDGF) is a chemotactic and mitogenic agent for fibroblasts and smooth muscle cells and plays a key role in the development of atherosclerotic lesions. PDGF is produced by a number of normal and transformed cell types and occurs as homo- or heterodimers of A and B polypeptide chains. Using Chinese hamster ovary (CHO) cells transfected with various forms of PDGF, we have previously shown that PDGF A(s) (short splice version) is secreted, PDGF A(l) (long splice version) predominantly extracellular matrix-associated, and PDGF B divided between medium, cells, and matrix. In the present study we have demonstrated the mitogenic activity of matrix-localized PDGF in artificial and more physiologically relevant models by culturing Balb/c-3T3 cells (3T3), human foreskin fibroblasts (HFF), and rabbit aortic smooth muscle cells (SMC) on extracellular matrix (ECM) laid down by PDGF-expressing CHO cells and human umbilical vein endothelial cells (HUVEC). These cells responded to the local growth stimulus of PDGF-containing CHO ECM and HUVEC ECM. We showed that 3T3 cells required proteolytic activity to utilize matrix-localized PDGF, as aprotinin and epsilon-ACA inhibited growth and 3T3 cells were shown to possess plasminogen activator activity. HFF and SMC did not appear to require proteolytic activity (including metalloproteinase and serine protease activity) as a prerequisite for mitogenesis but were able to access immobilized PDGF by contact with the matrix. An understanding of the mechanisms whereby the utilization of stored PDGF is controlled in situations of excessive cellular proliferation will aid in the development of therapy for these conditions.

Association of thrombospondin-1 with osteogenic differentiation of retinal pericytes in vitro

Vascular pericytes can differentiate into osteoblast-like cells in vitro, suggesting that these cells may represent a potential source of osteoprogenitor cells in the adult. Pericyte differentiation is associated with a characteristic pattern of nodule formation and mineralisation. Nodules are formed in post-confluent cultures by the retraction of multilayered areas. Crystals of hydroxyapatite are deposited on the extracellular matrix of these nodules which then becomes mineralised. We now demonstrate that thrombospondin-1 (TSP-1) gene expression is modulated during pericyte differentiation in vitro. That is, the relative levels of TSP-1 (protein and mRNA) increased markedly during nodule formation and then decreased when mineralisation of the nodules had taken place. TSP-1 was localised throughout non-mineralised nodules but it was largely excluded from the inner mass of mineralised nodules. The production of a mineralised matrix by vascular pericytes was promoted by the presence of antibodies to TSP-1 in the culture medium and was inhibited by exogenous TSP-1. These effects did not appear to be mediated through the activation of latent TGF-beta, since neither exogenous TGF-beta nor neutralising antibodies to TGF-beta had any effect on the rate or extent of mineralisation seen in the pericyte cultures. Taken together these results suggest that high levels of TSP-1 inhibit pericyte mineralisation, supporting the view that this protein plays a role in pericyte differentiation and bone formation.

Acidic amino acid-rich sequences as binding sites of osteonectin to hydroxyapatite crystals

Osteonectin, an acidic noncollagenous protein of bone and dentin, has affinity to hydroxyapatite crystals. Binding sites to hydroxyapatite of this protein were determined by a proteolytic experiment and an in vitro binding experiment using synthetic peptide analogues. Osteonectin was adsorbed on hydroxyapatite crystals and digested with trypsin. A peptide was left adsorbed on the crystal even after the digestion. The peptide was identified as an amino terminal peptide containing glutamic acid-rich sequences, which have been assumed to be possible hydroxyapatite-binding sites. Poly glutamic acid sequences were synthesized as models of the binding sites. Glu6 peptide was bound to the hydroxyapatite with a dissociation constant of 2.4 microM. Peptides containing fewer glutamic acids had lower affinity to the crystal. Effects of these peptides on in vitro mineralization were examined by a gel system in microtiter plates. The Glu6 peptide had a positive effect on the mineralization in this system, whereas Asp6 peptide had a negative effect. These effects indicate the presence of an interaction between these peptides and mineral crystals.

Structure of a novel extracellular Ca(2+)-binding module in BM-40

The EF-hand is a highly conserved Ca(2+)-binding motif found in many cytosolic Ca(2+)-modulated proteins. Here we report the crystal structure at 2.0 A resolution of the carboxy-terminal domain of human BM-40 (SPARC, osteonectin), an extracellular matrix protein containing an EF-hand pair. The two EF-hands interact canonically but their detailed structures are unusual. In the first EF-hand a one-residue insertion is accommodated by a cis-peptide bond and by substituting a carboxylate by a peptide carbonyl as a Ca2+ ligand. The second EF-hand is stabilized by a disulphide bond. The EF-hand pair interacts tightly with an amphiphilic amino-terminal helix, reminiscent of target peptide binding by calmodulin. The present structure defines a novel protein module occurring in several other extracellular proteins.

A purine-rich sequence in the human BM-40 gene promoter region is a prerequisite for maximum transcription

BM-40 (osteonectin, SPARC [secreted protein, acidic, rich in cysteine]) is a highly conserved, matrix-associated protein that is found in basement membranes, bones and remodeling tissues throughout vertebrate evolution. We are reporting the characterization of the 5' end of the human BM-40 gene. Sequence comparison of the 5' region revealed significant homologies with the bovine and murine genes, including a purine-rich stretch composed of two boxes, GGA-box 1 and 2, separated by a pyrimidine-rich spacer element. Transfection analyses of the human BM-40 promoter provide strong evidence that this region comprises several distinct regulatory domains, to which different functions can be assigned. GGA-box 1 is thereby absolutely required and sufficient by itself for maximal BM-40 transcriptional activity, whereas the spacer element has a down-regulatory effect. Comparative transfection analyses in human cell lines, positive or negative for BM-40 transcripts, indicate that the GGA-box sequences in the human promoter, in contrast to the bovine promoter, do not significantly contribute to cell-type specific expression in human cells.

Thrombospondin 1 is expressed by proliferating mesangial cells and is up-regulated by PDGF and bFGF in vivo

Thrombospondin 1 has been shown to be linked to PDGF-mediated mesangial cell proliferation and migration in vitro, but little is known regarding its expression or regulation in glomerular disease. Experimental mesangial proliferative nephritis was induced in rats by injection of anti-Thy1 antibody. Mesangial cell proliferation was associated with de novo expression of thrombospondin 1 mRNA (detected by Northern blot and in situ hybridization) and protein (by Western blot and immunostaining). Although some thrombospondin 1 was expressed by platelets and macrophages, double labeling showed that most thrombospondin 1 mRNA and protein were expressed by proliferating alpha-actin-positive mesangial cells. Thrombospondin 1 expression in anti-Thy1 nephritis was complement-dependent and could be reduced by treatment with anti-PDGF or anti-bFGF antibodies. Thrombospondin 1 could also be induced in normal rats by infusion of PDGF and in rats which were primed with low dose anti-Thy1 antibody by infusion of PDGF of bFGF. Thus, this study demonstrates that proliferating mesangial cells express thrombospondin 1 de novo in disease and that thrombospondin 1 expression in vivo is regulated by PDGF and bFGF.

Two adjacent N-terminal glutamines of BM-40 (osteonectin, SPARC) act as amine acceptor sites in transglutaminaseC-catalyzed modification

The extracellular matrix protein BM-40 (osteonectin, SPARC) has recently been shown to be a major target for transglutaminase-catalyzed cross-linking in differentiating cartilage. In the present study we demonstrate that recombinant human BM-40 can be modified with [3H]putrescine in a 1:1 molar ratio by transglutaminaseC (tissue transglutaminase). Residues Gln3 and Gln4 were identified as major amine acceptor sites. This was confirmed with several mutant proteins, including deletions in the N-terminal domain I of BM-40, site-directed mutagenesis of the reactive glutamines, and fusion of the seven-amino acid-long N-terminal sequence (APQQEAL) to an unrelated protein. The results showed that the N-terminal target site is sufficient for modification by transglutaminase but at a low level. For high efficiency amine incorporation an intact domain I is required. The conservation of at least one of the transglutaminase target glutamines in the known vertebrate BM-40 sequences and their absence in an invertebrate homologue point to an important, but yet unknown, role of this modification in vertebrates.

Conformational changes of bovine bone osteonectin induced by interaction with calcium

To clarify calcium-induced conformational changes in bovine bone osteonectin, the protein was labeled with fluorescein isothiocyanate (FITC) in the presence and absence of calcium. By calcium titration using fluorescence spectrometry, it was demonstrated that FITC-osteonectin labeled in the presence of 2 mM CaCl2 showed a much higher affinity for calcium ions than did that labeled in the absence of calcium ions. The midpoint for completion of the increase in the intrinsic fluorescence (K0.5) of the two were 1 x 10(-7) M and 5 x 10(-7) M, respectively. By tryptic digestion and isolation of the fluorescent peptide of both FITC-osteonectins, the site of FITC-labeling was determined to be Lys174. Furthermore, it was found that the efficacy of labeling in this specific binding site was three times higher in the FITC-osteonectin labeled in the presence of 2 mM CaCl2 than in that labeled in the absence of calcium. The results indicate that in the presence of 2 mM CaCl2 the microenvironment around Lys174 of osteonectin was more open to modification than in the absence of calcium.

Transglutaminase-catalyzed matrix cross-linking in differentiating cartilage: identification of osteonectin as a major glutaminyl substrate

The expression of tissue transglutaminase in skeletal tissues is strictly regulated and correlates with chondrocyte differentiation and cartilage calcification in endochondral bone formation and in maturation of tracheal cartilage (Aeschlimann, D., A. Wetterwald, H. Fleisch, and M. Paulsson. 1993. J. Cell Biol. 120:1461-1470). We now demonstrate the transglutaminase reaction product, the gamma-glutamyl-epsilon-lysine cross-link, in the matrix of hypertrophic cartilage using a novel cross-link specific antibody. Incorporation of the synthetic transglutaminase substrate monodansylcadaverine (amine donor) in cultured tracheal explants reveals enzyme activity in the pericellular matrix of hypertrophic chondrocytes in the central, calcifying areas of the horseshoe-shaped cartilages. One predominant glutaminyl substrate (amine acceptor) in the chondrocyte matrix is osteonectin as revealed by incorporation of the dansyl label in culture. Indeed, nonreducible osteonectin-containing complexes of approximately 65, 90, and 175 kD can be extracted from mature tracheal cartilage. In vitro cross-linking of osteonectin by tissue transglutaminase gives similar products of approximately 90 and 175 kD, indicating that the complexes in cartilage represent osteonectin oligomers. The demonstration of extracellular transglutaminase activity in differentiating cartilage, i.e., cross-linking of osteonectin in situ, shows that tissue transglutaminase-catalyzed cross-linking is a physiological mechanism for cartilage matrix stabilization.

Expression of SPARC during development of the chicken chorioallantoic membrane: evidence for regulated proteolysis in vivo

SPARC is a secreted glycoprotein that has been shown to disrupt focal adhesions and to regulate the proliferation of endothelial cells in vitro. Moreover, peptides resulting from the proteolysis of SPARC exhibit angiogenic activity. Here we describe the temporal synthesis, turnover, and angiogenic potential of SPARC in the chicken chorioallantoic membrane. Confocal immunofluorescence microscopy revealed specific expression of SPARC protein in endothelial cells, and significantly higher levels of SPARC were observed in smaller newly formed blood vessels in comparison to larger, developmentally older vessels. SPARC mRNA was detected at the earliest stages of chorioallantoic membrane morphogenesis and reached maximal levels at day 13 of embryonic development. Interestingly, steady-state levels of SPARC mRNA did not correlate directly with protein accumulation; moreover, the protein appeared to undergo limited degradation during days 10-15. Incubation of [125I]-SPARC with chorioallantoic membranes of different developmental ages confirmed that extracellular proteolysis occurred during days 9-15, but not at later stages (e.g., days 17-21). Comparison of peptides produced by incubation with chorioallantoic membranes with those generated by plasmin showed an identical pattern of proteolysis. Plasmin activity was present throughout development, and in situ zymography identified sites of plasminogen activator activity that corresponded to areas exhibiting high levels of SPARC expression. Synthetic peptides from a plasmin-sensitive region of SPARC, between amino acids 113-130, stimulated angiogenesis in the chorioallantoic membrane in a dose-dependent manner; in contrast, intact SPARC was inactive in similar assays. We have shown that SPARC is expressed in endothelial cells of newly formed blood vessels in a manner that is both temporally and spatially restricted. Between days 9 and 15 of chorioallantoic membrane development, the protein undergoes proteolytic cleavage that is mediated, in part, by plasmin. SPARC peptides released specifically by plasmin induce angiogenesis in vivo. We therefore propose that SPARC acts as an intrinsic regulator of angiogenesis in vivo.

Expression of the gene encoding the extracellular matrix glycoprotein SPARC in the developing and adult mouse brain

The pattern of expression of the SPARC gene was examined during postnatal development of the mouse brain using in situ hybridization. At postnatal day 3 (P3), a strong signal representing SPARC mRNA was apparent in boundary layers such as the pia mater and the lining of the ventricles. By P12, increased levels of SPARC mRNA were noted in the cerebellum, midbrain and brain stem with a lower signal in more frontal areas, a pattern which was retained in the adult. This pronounced caudal versus frontal difference in SPARC mRNA levels was confirmed by Northern blot analysis. At P3, SPARC mRNA was detected in developing blood vessels in the cerebral cortex, suggesting a role for SPARC in angiogenesis. During development of the cerebellum, expression of SPARC mRNA became highly restricted to the Purkinje cellular layer and in the adult was localized to Bergmann glial cells rather than Purkinje neurons.

Cellular expression of bone-related proteins during in vitro osteogenesis in rat bone marrow stromal cell cultures

Rat bone marrow stromal cells comprise a heterogeneous mixture of cell lineages including osteoblastic cells. When grown in the presence of ascorbic acid, beta-glycerophosphate and 10(-8) M dexamethasone, osteoprogenitor cells within the population divide and differentiate to form bone nodules (Maniatopoulos et al., 1988, Cell Tissue Res., 254:317-330; Aubin et al., 1990, J. Bone Miner. Res., 5:S81) providing a useful model to investigate temporal and spatial changes in expression of osteoblastic markers. Immunocytochemistry was combined with Northern blotting, enzymatic assay, and radioimmunoassay to analyze the expression of bone-related proteins during the growth and differentiation sequence. By mRNA levels, protein production and/or enzymatic activity, expression of osteocalcin, bone sialoprotein, and alkaline phosphatase increased concomitantly with the development of bone nodules, while osteopontin mRNA levels decreased and those of SPARC/osteonectin did not change significantly. In older cultures with mineralizing nodules, mRNA levels for alkaline phosphatase and bone sialoprotein, but not osteocalcin, declined. Immunolabeling revealed that cells in early cultures stained poorly for SPARC/osteonectin and strongly for thrombospondin. Later, SPARC/osteonectin staining increased in most cells, while thrombospondin staining could be seen in both matrix and in cells, but with marked intercellular variability in intensity. At all time points studied, osteoblasts within bone nodules stained homogeneously for thrombospondin and alkaline phosphatase, and with marked heterogeneity of intensity amongst cells for SPARC/osteonectin and osteocalcin. Labelling with RCC455.4, a monoclonal antibody raised against rat calvaria cells which intensely labels osteoblasts and osteocytes (Turksen et al., 1992, J. Histochem. Cytochem., 40:1339-1352), co-localized with osteocalcin. Alkaline phosphatase activity and the amount of osteocalcin determined by both radioimmunoassay and immunolabelling decreased in very late cultures, a time corresponding to appearance of fully mineralized nodules. These studies indicate that the bone marrow stromal cell system is a useful model to study the temporal and spatial expression of bone-related proteins during osteogenesis and formation, mineralization, and maturation of bone nodules. Further, immunolabelling at the individual cell and single bone nodule level allowed discrimination of marked variability of expression of osteoblast markers during the differentiation sequence.

Developmental expression in the rat cerebellum of SC1, a putative brain extracellular matrix glycoprotein related to SPARC

In the nervous system, extracellular matrix (ECM) molecules have been shown to have effects on cell migration, process outgrowth and the survival of neurons. Recently we have described the molecular cloning of SC1, a putative brain extracellular matrix glycoprotein, showing partial similarity to the ECM glycoprotein SPARC/osteonectin. We have now examined the expression of SC1 during the development of the rat cerebellum at both the protein and mRNA levels. Our results indicate that SC1 is both temporally and spatially regulated during this process. Bergmann glial cells express SC1 mRNA and the resultant protein is deposited along the length of their radial fibres during the process of granule cell migration in the developing cerebellum. SC1 mRNA and protein is also found in the adult cerebellum, concentrated in the Bergmann glial cells and their radial processes, indicating that this putative ECM molecule continues to play roles in the central nervous system after migration and proliferative events have ceased.

The Caenorhabditis elegans homologue of the extracellular calcium binding protein SPARC/osteonectin affects nematode body morphology and mobility

The extracellular matrix-associated protein, SPARC (osteonectin [Secreted Protein Acidic and Rich in Cysteine]), modulates cell adhesion and induces a change in cell morphology. SPARC expression in mammals is developmentally regulated and is highest at sites of extracellular matrix assembly and remodeling such as parietal endoderm and bone. We have isolated cDNA and genomic DNA clones encoding the Caenorhabditis elegans homologue of SPARC. The gene organization is highly conserved, and the proteins encoded by mouse, human, and nematode genes are about 38% identical. SPARC consists of four domains (I-IV) based on predicted secondary structure. Using bacterial fusion proteins containing nematode domain I or the domain IV EF-hand motif, we show that, like the mammalian proteins, both domains bind calcium. In transgenic nematodes expressing a SPARC-lacZ fusion gene, beta-galactosidase staining accumulated in a striated pattern in the more heavily stained muscle cells along the body. Comparison of the pattern of transgene expression to unc-54-lacZ animals demonstrated that SPARC is expressed by body wall and sex muscle cells. Appropriate levels of SPARC are essential for normal C. elegans development and muscle function. Transgenic nematodes overexpressing the wild-type SPARC gene were abnormal. Embryos were deformed, and adult hermaphrodites had vulval protrusions and an uncoordinated (Unc) phenotype with reduced mobility and paralysis.

The functions of thrombospondin and its involvement in physiology and pathophysiology

The thrombospondin family of molecules is expressed in many different tissues. Its expression is highly regulated by different hormones and cytokines and is developmentally controlled. It can bind to many different cell types, probably via an array of receptors which are similarly regulated. The level of thrombospondins in body fluids and their distribution in tissue change in correlation with various pathological states. It is linked to the growth of primary tumors and to metastasis, to development of the atherosclerotic plaque, to malaria infection and other diseases. The role(s) of thrombospondin(s) are by and large unknown, though specific interaction seem to affect particular cell functions. The wide-spread spatial and temporal regulation, multiple interactions and correlation with major diseases imply important roles in cell function and call for concerted effort to unravel the mystery.

Thrombospondin as a mediator of cancer cell adhesion in metastasis

Thrombospondin (TSP) is a 450 kDa adhesive glycoprotein. It is present in high concentrations in the platelet alpha-granule and can readily be secreted following platelet activation where local concentrations can be increased by 3-4 orders of magnitude. TSP is also synthesized by a variety of other cells and is incorporated into their extracellular matrix. TSP is a homotrimer with a number of functional domains, at least four of which might serve as receptor recognizing regions. The amino-terminal heparin binding domain interacts with heparin, other glycosaminoglycans and glycolipids and likely recognizes specific cell surface proteoglycans. The central disulfide cross-linked region, 210 kDa non-reduced and 70 kDa reduced, contains a peptide motif CSVTCG which is apparently responsible for binding to glycoprotein IV (CD36) with high affinity. Immediately adjacent to the calcium binding region of TSP, which undergoes considerable molecular relaxation in the absence of calcium, is an RGDA sequence. TSP has been demonstrated to bind to integrins of the alpha v beta 3 and alpha IIb beta 3 class. The carboxy-terminal region of TSP also contains at least one binding epitope for a cell receptor. There are 2 well characterized genes for TSP and truncated forms of TSP have been detected which have inhibitory effects on angiogenesis. Finally, TSP can interact with fibrinogen and fibronectin, perhaps on cellular surfaces, which might serve as secondary receptor-like mechanisms for TSP binding and subsequent mediation of cell adhesion.

Localization of two binding domains for thrombospondin within fibronectin

Thrombospondin is a major glycoprotein of the platelet alpha-granule and is secreted during platelet activation. Several protease-resistant domains of thrombospondin mediate its interactions with components of the extracellular matrix including fibronectin, collagen, heparin, laminin, and fibrinogen. Thrombospondin, as well as fibronectin, is composed of several discretely located biologically active domains. We have characterized the thrombospondin binding domains of plasma fibronectin and determined the binding affinities of the purified domains; fibronectin has at least two binding sites for thrombospondin. Thrombospondin bound specifically to the 29-kDa amino-terminal heparin binding domain of fibronectin as well as to the 31-kDa non-heparin binding domain located within the larger 40-kDa carboxy-terminal fibronectin domain generated by chymotrypsin proteolysis. Platelet thrombospondin interacted with plasma fibronectin in a specific and saturable manner in blot binding as well as solid-phase binding assays. These interactions were independent of divalent cations. Thrombospondin bound to the 29-kDa fibronectin heparin binding domain with a Kd of 1.35 x 10(-9) M. The Kd for the 31-kDa domain of fibronectin was 2.28 x 10(-8) M. The 40-kDa carboxy-terminal fragment bound with a Kd of 1.65 x 10(-8) M. Heparin, which binds to both proteins, inhibited thrombospondin binding to the amino-terminal domain of fibronectin by more than 70%. The heparin effect was less pronounced with the non-heparin binding carboxy-terminal domain of fibronectin. By contrast, the binding affinity of the thrombospondin 150-kDa domain, which itself lacked heparin binding, was not affected by the presence of heparin. Based on these data, we conclude that thrombospondin binds with different affinities to two distinct domains in the fibronectin molecule.

Colocalization of thrombospondin and syndecan during murine development

Thrombospondin is an adhesive glycoprotein that is thought to play a role in tissue genesis and repair. We have used a monoclonal anti-thrombospondin antibody, designated 5G11, to localize thrombospondin in paraformaldehyde fixed, paraffin-embedded sections of developing mouse embryos. Thrombospondin expression is observed in uterine smooth muscle, endometrial glands, the decidua, and trophoblastic giant cells during the initial phase of post-implantation development in the embryo. Cardiac myocytes and neuroepithelial cells show positive staining for thrombospondin at day 8.5 of gestation, and this expression continues throughout the development of the myocardium and central nervous system. Strong staining for thrombospondin is seen in developing bone and in the liver. Thrombospondin is also observed in developing smooth muscle and skeletal muscle, as well as in a variety of epithelia, including the epidermis, small intestinal epithelium, lens epithelium, renal tubular epithelium, and the epithelium of the developing tooth. Comparison of thrombospondin staining with that of two known cell surface receptors for thrombospondin, syndecan and the vitronectin receptor, reveals remarkable colocalization of thrombospondin and syndecan in all tissues, but almost no coexpression with the vitronectin receptor. Coexpression of thrombospondin and syndecan may play an important role in cell-cell or cell-matrix interactions during development.

High-affinity and low-affinity calcium binding and stability of the multidomain extracellular 40-kDa basement membrane glycoprotein (BM-40/SPARC/osteonectin)

Reversible binding of calcium ions to a single high-affinity binding site in the 40-kDa basement membrane protein (BM-40) caused a 33% increase of alpha-helicity, an about 60% change in intrinsic fluorescence and a dramatic increase of the rate of cleavage by alpha-chymotrypsin. All these effects exhibited identical dependencies on calcium concentration from which a dissociation constant Kd = 0.6 microM was determined. Calcium release was accompanied by an increase of the frictional ratio in solution but not by denaturation which occurred at about equal guanidine.HCl concentration for both calcium-saturated and calcium-depleted protein (midpoint 1.5 M). The cleavage sites for alpha-chymotrypsin are located in or near to the EF-hand domain IV of calcium-depleted BM-40 (also known as SPARC, i.e. secreted protein acidic and rich in cysteine, and osteonectin). These and other data indicate that binding occurs in the EF-hand domain from which a large conformational change is transmitted. Low-affinity calcium-binding sites in the N-terminal glutamic-acid-rich domain I of BM-40 were identified by human leukocyte elastase which was found to cleave very specifically in the middle of this domain. From the increase of cleavage rate with increasing calcium concentration a Kd greater than or equal to 10 mM was estimated. It is suggested that variations of calcium levels in the extracellular space in this range may regulate functions of BM-40 such as collagen binding and that high-affinity binding is important for stabilization, folding and secretion during biosynthesis.

Renal tubular epithelial cells express osteonectin in vivo and in vitro

Osteonectin (SPARC, culture shock protein, BM-40) is a widely distributed glycoprotein which binds calcium and several extracellular matrix proteins, including interstitial collagens and thrombospondin, but whose physiologic role remains undefined. In the present studies, we have demonstrated that immunoreactive osteonectin is present in the distal cortical tubule and medullary tubules of murine kidney. We surveyed the renal epithelial cell lines LLC-PK1, MDCK, and OK for the expression of mRNA encoding osteonectin. We found that osteonectin mRNA is expressed by LLC-PK1 and OK cells but not by MDCK cells, as well as by adult kidney from several species. Calcitonin and vasopressin, agents which increase cAMP in these cells, were found to decrease steady-state osteonectin mRNA concentrations. We found that LLC-PK1 cells produced osteonectin protein, that the protein was localized to intracellular granules, and that the protein bound hydroxyapatite in vitro. Pulse-chase analysis revealed that osteonectin was secreted from the cell layer to the medium after a lag time of four to six hours and was secreted preferentially from the basolateral domain of the cell. The preferential secretion of the calcium-binding protein osteonectin from the renal epithelial cell is consistent with several possible functions, including a structural extracellular matrix protein, a participant in transepithelial ion transport, and an inhibitor of extracellular calcification.

Immunological screening of SPARC/Osteonectin in nonmineralized tissues

SPARC/Osteonectin is a major bone-related protein that is also present in nonmineralized tissues and in platelets. As compared to bone SPARC/Osteonectin, SPARC/Osteonectin from platelets presents a slightly lower electrophoretic mobility in SDS-PAGE and a 100-fold decreased affinity for a unique monoclonal antibody, Mab2 (Malaval et al. 1991). To check the tissular diversity of SPARC/Osteonectin, protein extracts from bovine bone, nonmineralized tissues, and platelets were screened by immunoblotting and immunoradiometric assay, with Mab2 and three other monoclonal antibodies recognizing distinct epitopes. The SPARC/Osteonectin secreted by a human osteosarcoma cell line (MG63) was also tested. In all the nonmineralized tissues tested (gut, bone marrow, tendon, mesentery, artera, lens, skin, liver, and cornea), SPARC/Osteonectin presents the same immunoreactivity and electrophoretic mobility as in bone. The heavier molecular weight and Mab2-negative form present in platelets seems to be unique to this cell type. Osteosarcoma cell extracts and conditioned media give the same results as bone extracts, indicating that the low molecular weight and Mab2-positive form of SPARC/Osteonectin present in most tissues does not result from proteolytic cleavage in the matrix, but is secreted as such. Bone and platelet SPARC/Osteonectin present different patterns of sensitivity to glycosidases, suggestive of a difference in N-glycosylation. However, these treatments do not affect the decreased affinity of Mab2 for platelet SPARC/Osteonectin, which is not likely to be related to difference in N-glycosylation.

Basement membrane proteins: structure, assembly, and cellular interactions

Basement membranes are thin layers of a specialized extracellular matrix that form the supporting structure on which epithelial and endothelial cells grow, and that surround muscle and fat cells and the Schwann cells of peripheral nerves. One common denominator is that they are always in close apposition to cells, and it has been well demonstrated that basement membranes do not only provide a mechanical support and divide tissues into compartments, but also influence cellular behavior. The major molecular constituents of basement membranes are collagen IV, laminin-entactin/nidogen complexes, and proteoglycans. Collagen IV provides a scaffold for the other structural macromolecules by forming a network via interactions between specialized N- and C-terminal domains. Laminin-entactin/nidogen complexes self-associate into less-ordered aggregates. These two molecular assemblies appear to be interconnected, presumably via binding sites on the entactin/nidogen molecule. In addition, proteoglycans are anchored into the membrane by an unknown mechanism, providing clusters of negatively charged groups. Specialization of different basement membranes is achieved through the presence of tissue-specific isoforms of laminin and collagen IV and of particular proteoglycan populations, by differences in assembly between different membranes, and by the presence of accessory proteins in some specialized basement membranes. Many cellular responses to basement membrane proteins are mediated by members of the integrin class of transmembrane receptors. On the intracellular side some of these signals are transmitted to the cytoskeleton, and result in an influence on cellular behavior with respect to adhesion, shape, migration, proliferation, and differentiation. Phosphorylation of integrins plays a role in modulating their activity, and they may therefore be a part of a more complex signaling system.

Osteonectin is an alpha-granule component involved with thrombospondin in platelet aggregation

We previously showed that thrombospondin, a major alpha-granule glycoprotein of human platelets, forms a specific complex with osteonectin, a phosphoglycoprotein originally described in bone that is also present in human platelets. The storage organelles and the function of osteonectin in platelets are still unknown. In this study, using electron microscopy in combination with immunogold staining, the major storage organelle for platelet-secreted proteins, the alpha-granules. Furthermore, osteonectin was qualitatively and quantitatively assessed by studying normal platelets and the platelets from a patient with gray platelet syndrome. Gray platelet syndrome is a rare congenital bleeding disorder characterized by a selective deficiency in morphologically recognizable platelet alpha-granules and in the alpha-granule secretory proteins. Binding of an iodinated antiosteonectin monoclonal antibody to gray platelet proteins transferred to nitrocellulose from SDS-polyacrylamide gels showed no band corresponding to osteonectin compared to control platelets. Using a polyclonal antiosteonectin antibody-based radioimmunoassay, gray platelets contained 0.2 +/- 0.03 ng osteonectin per 10(6) platelets, which is only 20% of the normal platelet content of osteonectin (0.93 +/- 0.16 ng per 10(6) platelets). Study of the localization of osteonectin to the surface of human platelets demonstrated that a radioiodinated antiosteonectin polyclonal antibody bound specifically to thrombin-stimulated platelets but not to resting platelets. Binding was concentration-dependent, saturable (1710 +/- 453 binding sites per platelet, Kd = 1 microM), and inhibited by an excess of cold antiosteonectin polyclonal antibody. No binding was observed on the surface of thrombin-stimulated gray platelets. To gain further insights into the role of osteonectin released from activated platelets, the effect of an antiosteonectin polyclonal antibody was tested on the aggregation of washed platelets. F(ab')2 fragments from the antiosteonectin polyclonal antibody inhibited in a dose-dependent manner the aggregation of collagen-stimulated, washed human platelets without affecting collagen-induced platelet serotonin release. To characterize the mechanism through which antiosteonectin F(ab')2 fragments inhibit platelet aggregation, the expression of endogenous thrombospondin (TSP) on the surface of thrombin-activated platelets was studied using 125I-labeled anti-TSP monoclonal antibody P10. The endogenous surface expression of TSP to thrombin-stimulated platelets was significantly inhibited in the presence of antiosteonectin F(ab')2 fragments (6286 +/- 2065 molecules of P10 per platelet) compared to 11,230 +/- 766 molecules of P10 per platelet in the presence of nonimmune F(ab')2 fragments.(ABSTRACT TRUNCATED AT 400 WORDS)

Recombinant expression and properties of the human calcium-binding extracellular matrix protein BM-40

A cDNA construct (approximately 1 kb) of human BM-40 in a plasmid with the cytomegalovirus promoter and enhancer was used to produce several stable clones by transfecting two human cell lines (293, HT 1080). These clones showed a high expression of exogenous 1-kb BM-40 mRNA and no or only little endogenous 2.2-kb mRNA. These clones also secreted BM-40 at high rates (5-50 micrograms ml-1 day-1) into serum-free culture medium as shown by electrophoresis, radioimmunoassay and metabolic labelling. Transfection with the plasmid and overexpression of BM-40 had no effect on cell spreading, proliferation rate and adhesion patterns to extracellular matrix substrates. Recombinant human BM-40 was purified by anion-exchange chromatography and showed the expected N-terminal sequence and amino acid composition. The protein was also identical or similar to authentic BM-40 purified from the mouse Engelbreth-Holm-Swarm tumor in hexosamine content, electrophoretic mobility, circular dichroism and binding activity for calcium and collagen IV. Reduction of both authentic and recombinant BM-40 decreased binding activity which indicates correct formation of disulfide bonds in the recombinant protein. A specific and sensitive radioimmunoassay for human BM-40 was shown to be useful for detecting small quantities of the protein in human cell culture medium and blood. No significant cross-reaction was, however, detected between human and mouse BM-40.