Tissue regeneration and development involves highly synchronized signals both between cells and with the extracellular environment. Biomaterials can be tuned to mimic specific biological signals and control cell response(s). As a result, these materials can be used as tools to elucidate cell signaling pathways and candidate molecules involved with cellular processes. In this work, we explore enamel-forming cells, ameloblasts, which have a limited regenerative capacity. By exposing undifferentiated cells to a self-assembling matrix bearing RGDS epitopes, we elicited a regenerative signal at will that subsequently led to the identification of thrombospondin 2 (TSP2), an extracellular matrix protein that has not been previously recognized as a key player in enamel development and regeneration. Targeted disruption of the thrombospondin 2 gene (Thbs2) resulted in enamel formation with a disordered architecture that was highly susceptible to wear compared to their wild-type counterparts. To test the regenerative capacity, we injected the bioactive matrix into the enamel organ and discovered that the enamel organic epithelial cells in TSP-null mice failed to polarize on the surface of the artificial matrix, greatly reducing integrin β1 and Notch1 expression levels, which represent signaling pathways known to be associated with TSP2. These results suggest TSP2 plays an important role in regulating cell-matrix interactions during enamel formation. Exploiting the signaling pathways activated by biomaterials can provide insight into native signaling mechanisms crucial for tooth development and cell-based strategies for enamel regeneration.

BACKGROUND:Fibrosis around cardiac cell injections represents an obstacle to graft integration in cell-based cardiac repair. Thrombospondin-2 (TSP-2) is a pro-fibrotic, anti-angiogenic matricellular protein and an attractive target for therapeutic knockdown to improve cardiac graft integration and survival. METHODS:We used a TSP-2 knockout (KO) mouse in conjunction with a fetal murine cardiomyocyte grafting model to evaluate the effects of a lack of TSP-2 on fibrosis, vascular density, and graft size in the heart. RESULTS:Two weeks after grafting in the uninjured heart, fibrosis area was reduced 4.5-fold in TSP-2 KO mice, and the thickness of the peri-graft scar capsule was reduced sevenfold compared to wild-type (WT). Endothelial cell density in the peri-graft region increased 2.5-fold in the absence of TSP-2, and cardiomyocyte graft size increased by 46% in TSP-2 KO hearts. CONCLUSIONS:TSP-2 is a key regulator of fibrosis and angiogenesis following cell grafting in the heart, and its absence promotes better graft integration, vascularization, and survival. SUMMARY:Fibrosis around cardiac cell injections impairs graft integration in cell-based cardiac repair. TSP-2 is a pro-fibrotic, anti-angiogenic matricellular protein. Using a TSP-2-knockout mouse model and cardiac cell transplantation, we found significantly reduced fibrosis and increased endothelial cell density in the peri-graft region. Thus, TSP-2 is an attractive target for therapeutic knockdown to improve cardiac graft integration and survival.

Here, we define dynamic reciprocity (DR) as an ongoing, bidirectional interaction among cells and their surrounding microenvironment. In this review, we posit that DR is especially meaningful during wound healing as the DR-driven biochemical, biophysical, and cellular responses to injury play pivotal roles in regulating tissue regenerative responses. Such cell-extracellular matrix interactions not only guide and regulate cellular morphology, but also cellular differentiation, migration, proliferation, and survival during tissue development, including, e.g., embryogenesis, angiogenesis, as well as during pathologic processes including cancer, diabetes, hypertension, and chronic wound healing. Herein, we examine DR within the wound microenvironment while considering specific examples across acute and chronic wound healing. This review also considers how a number of hypotheses that attempt to explain chronic wound pathophysiology may be understood within the DR framework. The implications of applying the principles of DR to optimize wound care practice and future development of innovative wound healing therapeutics are also briefly considered.

Metaxin, a mitochondrial outer membrane protein, is critical for TNF-induced cell death in L929 cells. Its deficiency, caused by retroviral insertion-mediated mutagenesis, renders L929 cells resistance to TNF killing. In this study, we further characterized metaxin deficiency-caused TNF resistance in parallel with Bcl-XL overexpression-mediated death resistance. We did not find obvious change in mitochondria membrane potential in metaxindeficient (Metmut) and Bcl-XL-overexpressing cells, but we did find an increase in the release rate of the mitochondrial membrane potential probe rhodamine 123 (Rh123) that was preloaded into mitochondria. In addition, overexpression of a function-interfering mutant of metaxin (MetaΔTM/C) or Bcl-XL in MCF-7.3.28 cells also resulted in an acquired resistance to TNF killing and a faster rate of Rh123 release, indicating a close correlation between TNF resistance and higher rates of the dye release from the mitochondria. The release of Rh123 can be controlled by the mitochondrial membrane permeability transition (PT) pore, as targeting an inner membrane component of the PT pore by cyclosporin A (CsA) inhibited Rh123 release. However, metaxin deficiency and Bcl-XL overexpression apparently affect Rh123 release from a site(s) different from that of CsA, as CsA can overcome their effect. Though both metaxin and Bcl-XL appear to function on the outer mitochondrial membrane, they do not interact with each other. They may use different mechanisms to increase the permeability of Rh123, since previous studies have suggested that metaxin may influence certain outer membrane porins while Bcl-XL may form pores on the outer membrane. The alteration of the mitochondrial outer membrane properties by metaxin deficiency and Bcl-XL overespression, as indicated by a quicker Rh123 release, may be helpful in maintaining mitochondrial integrity.

Rationale: Thrombospondin (TSP)-4 is an extracellular protein that has been linked to several cardiovascular pathologies. However, a role for TSP-4 in vascular wall biology remains unknown. Objective: We have examined the effects of TSP-4 gene ( Thbs4 ) knockout on the development of atherosclerotic lesions in ApoE −/− mice. Methods and Results: Deficiency in TSP-4 reduced atherosclerotic lesions: at 20 weeks of age, the size of the aortic root lesions in Thbs4 −/− / ApoE −/− mice was decreased by 48% in females and by 39% in males on chow diets; in mice on Western diets, lesions in the descending aorta were reduced by 30% in females and 33% in males. In ApoE −/− mice, TSP-4 was abundant in vessel areas prone to lesion development and in the matrix of the lesions themselves. TSP-4 deficiency reduced the number of macrophages in lesions in all groups by ≥2-fold. In addition, TSP-4 deficiency reduced endothelial cell activation (expression of surface adhesion molecules) and other markers of inflammation in the vascular wall (decreased production of monocyte chemoattractant protein-1 and activation of p38). In vitro, both the adhesion and migration of wild-type macrophages increased in the presence of purified recombinant TSP-4 in a dose-dependent manner (up to 7- and 4.7-fold, respectively). These responses led to p38-MAPkinase activation and were dependent on β 2 and β 3 integrins, which recognize TSP-4 as a ligand. Conclusions: TSP-4 is abundant in atherosclerotic lesions and in areas prone to development of lesions and may influence the recruitment of macrophages by activating endothelial cells and directly interacting with macrophages to increase their adhesion and migration. Our observations suggest an important role for this matricellular protein in the local regulation of inflammation associated with atherogenesis.

The sequence encoding the N-propeptide of collagen I is characterized by significant conservation of amino acids across species; however, the function of the N-propeptide remains poorly defined. Studies in vitro have suggested that one activity of this propeptide might be to act as a feedback inhibitor of collagen I synthesis. To determine whether the N-propeptide contributed to decreased collagen content in SPARC-null mice, mice carrying a deletion of exon 2, which encodes the globular domain of the N-propeptide of collagen I, were crossed to SPARC-null animals. Mice lacking SPARC and expressing collagen I without the globular domain of the N-propeptide were viable and fertile. However, a significant number of animals developed abdominal hernias within the first 2months of life with an approximate 20% penetrance (~35% of males). The dermis of SPARC-null/exon 2-deleted mice was thinner and contained fewer large collagen fibers in comparison with wild-type or in either single transgenic animal. The average collagen fibril diameter of exon 2-deleted mice did not significantly differ from wild-type mice (WT: 87.9nm versus exon 2-deleted: 88.2nm), whereas SPARC-null/exon 2-deleted fibrils were smaller than that of SPARC-null dermis (SPARC-null: 60.2nm, SPARC-null/exon 2-deleted: 40.8nm). As measured by hydroxyproline analysis, double transgenic skin biopsies contained significantly less collagen than those of wild-type, those of exon 2-deleted, and those of SPARC-null biopsies. Acetic acid extraction of collagen from skin biopsies revealed an increase in the proportion of soluble collagen in the SPARC-null/exon 2-deleted mice. These results support a function of the N-propeptide of collagen I in facilitating incorporation and stabilization of collagen I into the insoluble ECM and argue against a primary function of the N-propeptide as a negative regulator of collagen synthesis.

Microvascular development is often perceived to result from a balance of positive and negative factors that impact signaling for proliferation and survival. The survival signaling that results from hypoxia-induced VEGF-A has been well established, but the factors that antagonize this signaling have been poorly studied. As endogenous inhibitors of angiogenesis, thrombospondins (TSPs) are likely candidates to affect survival signaling. Here we report that TSP1 antagonized microvascular survival to retinal hyperoxia, and Akt signaling in both the retina and in cultured endothelial cells. TSP1 expression is correlated with the association of the CD36 receptor with Src versus Fyn. In the presence of TSP1, CD36 is coprecipitated with Fyn as previously shown by others. However, in the absence of TSP1, there is a preferential association with Src. We now demonstrate that these Src family kinases play an important role in modulating microvascular survival in response to TSP1 by crossing tsp1(-/-) mice to the src(-/-) and fyn(-/-) mice and testing the survival of retinal blood vessels in hyperoxia. We find that tsp1(-/-), fyn(-/-), and double-mutant tsp1(-/-)/fyn(-/-) mice have a similar enhancement of capillary survival in oxygen, whereas in a tsp(-/-) background, the loss of only one allele of src restores the balance in survival and apoptosis to that of wild-type mice. Taken together, we hypothesize that TSP1 antagonizes VEGF-driven Akt survival signaling in part through the recruitment of Fyn to membrane domains containing CD36, but when TSP1 is absent, an opposing Src recruitment contributes to VEGF-driven Akt phosphorylation and capillary survival.

http://jhc.sagepub.com/content/57/4/301 The online version of this article can be found at: DOI: 10.1369/jhc.2008.952689 2009 57: 301 J Histochem Cytochem Themis R. Kyriakides Susan MacLauchlan, Eleni A. Skokos, Azin Agah, Jianmin Zeng, Weiming Tian, Jeffrey M. Davidson, Paul Bornstein and 9, and Soluble VEGF − With Increased Levels of Matrix Metalloproteinases-2 and null Wounds Is Associated − Enhanced Angiogenesis and Reduced Contraction in Thrombospondin-2

Thrombospondin-2 (TSP2) is an inhibitor of angiogenesis with pro-apoptotic and anti-proliferative effects on endothelial cells. Mice deficient in this matricellular protein display improved recovery from ischemia and accelerated wound healing associated with alterations in angiogenesis and extracellular matrix remodeling. In this study, we probed the function of TSP2 by performing a detailed analysis of dermal wounds and wound-derived fibroblasts. Specifically, we analyzed incisional wounds by tensiometry and found no differences in strength recovery between wild-type and TSP2-null mice. In addition, analysis of full-thickness excisional wounds by terminal deoxynucleotidyl transferase–mediated 2′-deoxyuridine 5′-triphosphate nick-end labeling stain and MIB-5 immunohistochemistry revealed similar numbers of apoptotic and proliferating cells, respectively. In contrast, the levels of matrix metalloproteinase (MMP)-2, MMP-9, tissue inhibitors of metalloproteinase (TIMP)-1, TIMP-2, and soluble vascular endothelial growth factor were increased in wounds of TSP2-null mice. Evaluation of the ability of TSP2-null wound fibroblasts to contract collagen gels revealed that it was compromised, even though TSP2-null wounds displayed normal myofibroblast content. Therefore, we conclude that the lack of TSP2 leads to aberrant extracellular matrix remodeling, increased neovascularization, and reduced contraction due in part to elevated levels of MMP-2 and MMP-9. These observations provide in vivo supporting evidence for a newly proposed function of TSP2 as a modulator of extracellular matrix remodeling.

Macrophages undergo fusion to form multinucleated giant cells in several pathologic conditions, including the foreign body response (FBR). We detected high levels of matrix metalloproteinase (MMP)-9 during macrophage fusion in vitro and in foreign body giant cells (FBGCs) in vivo. Wild-type (WT) bone marrow-derived macrophages were induced to fuse with IL-4 in the presence of MMP-9 function-blocking antibodies and displayed reduced fusion. A similar defect, characterized by delayed shape change and abnormal morphology, was observed in MMP-9 null macrophages. Analysis of the FBR in MMP-9 null mice was then pursued to evaluate the significance of these findings. Specifically, mixed cellulose ester disks and polyvinyl alcohol sponges were implanted s.c. in MMP-9 null and WT mice and excised 2-4 weeks later. Histochemical and immunohistochemical analyses indicated equal macrophage recruitment between MMP-9 null and WT mice, but FBGC formation was compromised in the former. In addition, MMP-9 null mice displayed abnormalities in extracellular matrix assembly and angiogenesis. Consistent with a requirement for MMP-9 in fusion, we also observed reduced MMP-9 levels in MCP-1 null macrophages, previously shown to be defective in FBGC formation. Collectively, our studies show abnormalities in MMP-9 null mice during the FBR and suggest a role for MMP-9 in macrophage fusion.

Thrombospondins (TSPs) -1 and -2 were among the first protein inhibitors of angiogenesis to be identified, a property that was subsequently attributed to the interactions of sequences in their type I repeats with endothelial cellsurface receptors. The interactions of TSPs-1 and -2 with cell-surface receptors, proteases, growth factors, and other bioactive molecules, coupled with the absence of direct structural functions that can be attributed to these matrix proteins, qualify them for inclusion in the category of 'matricellular proteins'. The phenotypes of TSP-1, TSP-2, and double TSP-1/2-null mice confirm the roles that these proteins play in the regulation of angiogenesis, and provide clues to some of the other important functions of these multi-domain proteins. One of these functions is the ability of TSP-1 to activate the latent TGF beta 1 complex, a property that is not shared by TSP-2. A major pathway by which TSP1 or TSP2 inhibits angiogenesis involves an interaction with CD 36 on endothelial cells, which leads to apoptosis of both the liganded and adjacent cells. However a homeostatic mechanism, which inhibits endothelial cell proliferation, and may be physiologically preferable under some circumstances, has also been elucidated, and involves interaction with the very low density lipoprotein receptor (VLDLR). The interaction of TSP1with its receptor, CD47, further inhibits angiogenesis by antagonizing nitric oxide signaling in endothelial and vascular smooth muscle cells. Paradoxically, there is also evidence that TSP-1 can function to promote angiogenesis. This apparent contradiction can be explained by the presence of sequences in different domains of the protein that interact with different receptors on endothelial cells. The anti-angiogenic function of TSPs has spurred interest in their use as anti-tumor agents. Currently, peptide mimetics, based on sequences in the type I repeats of TSPs that have been shown to have anti-angiogenic properties, are undergoing clinical testing.

Background— The progressive shift from a young to an aged heart is characterized by alterations in the cardiac matrix. The present study investigated whether the matricellular protein thrombospondin-2 (TSP-2) may affect cardiac dimensions and function with physiological aging of the heart. Methods and Results— TSP-2 knockout (KO) and wild-type mice were followed up to an age of 60 weeks. Survival rate, cardiac function, and morphology did not differ at a young age in TSP-2 KO compared with wild-type mice. However, >55% of the TSP-2 KO mice died between 24 and 60 weeks of age, whereas <10% of the wild-type mice died. In the absence of TSP-2, older mice displayed a severe dilated cardiomyopathy with impaired systolic function, increased cardiac dilatation, and fibrosis. Ultrastructural analysis revealed progressive myocyte stress and death, accompanied by an inflammatory response and replacement fibrosis, in aging TSP-2 KO animals, whereas capillary or coronary morphology or density was not affected. Importantly, adeno-associated virus-9 gene–mediated transfer of TSP-2 in 7-week-old TSP-2 KO mice normalized their survival and prevented dilated cardiomyopathy. In TSP-2 KO animals, age-related cardiomyopathy was accompanied by increased matrix metalloproteinase-2 and decreased tissue transglutaminase-2 activity, together with impaired collagen cross-linking. At the cardiomyocyte level, TSP-2 deficiency in vivo and its knockdown in vitro decreased the activation of the Akt survival pathway in cardiomyocytes. Conclusion— TSP-2 expression in the heart protects against age-dependent dilated cardiomyopathy.

Matricellular proteins are secreted into the extracellular environment, or matrix, but do not play a primary structural role in this location. Rather, these proteins modulate cell function by interacting with cell-surface receptors, proteases, hormones, and other bioeffector molecules, as well as with structural matrix proteins such as collagens. The term ‘matricellular’ was introduced to explain the unusual diversity of functions that were beginning to be recognized in proteins such as thrombospondin-1(TSP-1), SPARC, and tenascin-C. (Sage and Bornstein 1991; Bornstein 1995). The intent of segregating this subclass of secreted proteins was to emphasize that the extracellular environment was a major factor in regulating their synthesis, a process that had been termed ‘dynamic reciprocity’ at an earlier date (Bornstein et al. 1982). A review that summarized many of the considerations that led to the concept of matricellular proteins was published in Methods in Cell Biology (Bornstein 2002). In additional reviews (Bornstein 2001; Bornstein and Sage 2002) some of the distinguishing characteristics of matricellular proteins were summarized in greater detail. These included: 1) high levels of expression during development and in response to injury; 2) binding to many cell-surface receptors, components of the extracellular matrix, growth factors, cytokines, and proteases; 3) induction of de-adhesion or counter-adhesion in contrast to the adhesivity of most matrix proteins (Murphy-Ullrich 2001; Liu et al. 2009), and 4) a grossly normal or subtle phenotype that is observed in mice with a targeted disruption (knockout) of some matricellular protein genes. The complexity of the functions of most matricellular proteins results from the fact that these functions are, in large part, contextual, i.e. they derive from the different structural proteins, cell-surface receptors, proteases, and cytokines with which these proteins come in contact in the local environment of different tissues. The unexpected phenotypes of some matricellular protein-null mice have provided clues to the functions of matricellular proteins. Thus, for example, the abnormally shaped and sized collagen fibrils observed in TSP-2-null mice in skin and other connective tissues led to the demonstration that TSP-2 functions as a clearance factor for MMP2 in the pericellular environment (Yang et al. 2001). In turn, the elevated MMP2 levels in the pericellular environment of TSP2-null mice reduced tissue transglutaminase activity and the collagen crosslinks generated by this enzyme (Agah et al. 2005). As another example, the cataracts and increased adipogenesis encountered in SPARC-null mice led to a better understanding of extracellular matrix assembly (Brekken and Sage 2000) and SPARC signaling pathways (Nie and Sage 2009). Although a useful review of the functions of a limited number of matricellular proteins in bone biology has been published (Alford and Hankenson 2006), and Sangaletti and Colombo (2008) have surveyed the functions of matricellular proteins in inflammation and cancer, this issue of the JCCS provides a more comprehensive coverage of the field and takes advantage of recent mechanistic information that improves our understanding of how matricellular proteins function. Thus, for TSPs, Bornstein has described the mechanisms that enable TSP-1 and TSP-2 to function as both angiogenic and anti-angiogenic proteins, and the potential clinical applications of these properties; Hankenson and Delany consider the roles of TSP-2 and SPARC in bone formation; Lawler describes the effects of TSPs on the structure of the extracellular matrix; MacLauchlan and Kyriakides review the role of TSPs in wound healing, ischemia, and the foreign body response; and Schellings et al. review the roles of TSPs in cardiac remodeling. For SPARC and members of the SPARC family, Bradshaw discusses the role of SPARC in extracellular matrix assembly; Arnold and Brekken consider the role of SPARC as a regulator of tumorigenesis, and Nie and Sage summarize the evidence for SPARC as an inhibitor of adipogenesis. Some matricellar proteins are represented by single presentations. Lund et al. present the role of osteopontin in inflammatory proceses; Midwood and Orend consider the role of tenascin-C in tissue injury and tumorigenesis; Norris et al. describe the many facets of the effects of periostin on cardiac development, remodeling, and pathophysiology ; Merline et al. present evidence for the matricellular functions of small leucine-rich proteoglycans (SLRPS), and Yanagisawa et al. present data supporting the inclusion of fibulin-5 in the matricellular protein family. Finally, Eroglu summarizes the evidence for the roles of matricellular proteins in the development and function of the nervous system. Clearly, the articles mentioned above do not cover all the proteins that are now considered to be matricellular. Prominent among those not included in this review are several members of the CCN (Cyr-61, Connective tissue growth factor, and Nov family of proteins that were recently reviewed (Chen and Lau 2009; Holburn et al. 2008; and Yeger and Perbal 2007), tenascin- X (Zweers et al. 2004), the galectins (Elola et al. 2007), plasminogen activator inhibitor type 1 (PAI-1) (Maquerlot et al. 2006), and autotaxin (Dennis et al. 2008). An issue that has not been addressed in the categorization of matricellular proteins is whether to include matrix proteins, the matricellular function of which, as defined above, is revealed only when fragments of the proteins are investigated. It is now commonly recognized that new functions, not present in the intact protein, can be generated by limited physiological proteolysis (Sage 1997; Davis et al. 2000). However, it would seem preferable at this time, in the interest of brevity, to limit our discussion to matrix proteins with matricellular properties that are expressed in the intact proteins. A PubMed search of the literature, performed at the end of the 2008 calendar year, revealed that there were 348 publications that used the term ‘matricellular’ in the title and/or abstract since its first use in 1995. The increase in recent years has been almost exponential, with 65 citations listed during 2008. A number of these citations concern proteins with credentials as matricellular proteins that are incomplete or questionable, but it is reasonable to expect that the list of matricellular proteins will grow as more information is gathered about other extracellular proteins with non-structural functions.

Matrix metalloproteinase- (MMP-9) is involved in processes that occur during cutaneous wound healing such as inflammation, matrix remodeling, and epithelialization, To investigate its role in healing, full thickness skin wounds were made in the dorsal region of MMP-9-null and control mice and harvested up to 14 days post wounding. Gross examination and histological and immunohistochemical analysis indicated delayed healing in MMP-9-null mice. Specifically, MMP-9-null wounds displayed compromised reepithelialization and reduced clearance of fibrin clots. In addition, they exhibited abnormal matrix deposition, as evidenced by the irregular alignment of immature collagen fibers. Despite the presence of matrix abnormalities, MMP-9-null wounds displayed normal tensile strength. Ultrastructural analysis of wounds revealed the presence of large collagen fibrils, some with irregular shape. Keratinocyte proliferation, inflammation, and angiogenesis were found to be normal in MMP-9-null wounds. In addition, VEGF levels were similar in control and MMP-9-null wound extracts. To investigate the importance of MMP-9 in wound reepithelialization we tested human and murine keratinocytes in a wound migration assay and found that antibody-based blockade of MMP-9 function or MMP-9 deficiency retarded migration. Collectively, our observations reveal defective healing in MMP-9-null mice and suggest that MMP-9 is required for normal progression of wound closure.

The gradual disorganization of collagen fibers in the stromal connective tissue of the uterine cervix is characteristic of progressive cervical softening during pregnancy. A lack of thrombospondin (TSP) 2 has been shown to be associated with altered collagen fibril morphology of connective-tissue-rich organs such as skin and tendon. The goal of this study was to determine the role of TSP2 in cervical softening by studying a TSP2-null mouse line. Creep testing showed that, in the nonpregnant animal and on Day 10 of pregnancy, there was no difference between the cervical extensibility of the wild-type and the TSP2-deficient mice. However, by Day 14 of pregnancy, the TSP2-null mice showed 4.5-fold increase in cervical extensibility, and by Day 18, a 6.1-fold increase, when compared with wild-type mice. A further indicator of compromised cervical integrity was that, on Days 14 and 18 of pregnancy, the cervix of TSP2-null mice broke rapidly under standard loading conditions that did not break the cervix of wild-type mice. Western blotting showed that TSP2 was expressed in the cervix of mice on Days 14 and 18 of pregnancy but not on Day 10 or in the nonpregnant animal. As determined by immunohistochemistry, the amount of matrix metalloproteinase 2 (MMP2) in the cervix of TSP2-null mice increased 11-fold on Day 14 of pregnancy and 19-fold on Day 18. Thus, TSP2-null mice provide an animal model to assist in the understanding of the molecular basis of spontaneous, premature softening of the uterine cervix.

Thrombospondin-5 (TSP5) is a large extracellular matrix glycoprotein found in musculoskeletal tissues. TSP5 mutations cause two skeletal dysplasias, pseudoachondroplasia and multiple epiphyseal dysplasia; both show a characteristic growth plate phenotype with retention of TSP5, type IX collagen (Col9), and matrillin-3 in the rough endoplasmic reticulum. Whereas most studies focus on defining the disease process, few functional studies have been performed. TSP5 knockout mice have no obvious skeletal abnormalities, suggesting that TSP5 is not essential in the growth plate and/or that other TSPs may compensate. In contrast, Col9 knockout mice have diminished matrillin-3 levels in the extracellular matrix and early-onset osteoarthritis. To define the roles of TSP1, TSP3, TSP5, and Col9 in the growth plate, all knockout and combinatorial strains were analyzed using histomorphometric techniques. While significant alterations in growth plate organization were found in certain single knockout mouse strains, skeletal growth was only mildly disturbed. In contrast, dramatic changes in growth plate organization in TSP3/5/Col9 knockout mice resulted in a 20% reduction in limb length, corresponding to similar short stature in humans. These studies show that type IX collagen may regulate growth plate width; TSP3, TSP5, and Col9 appear to contribute to growth plate organization; and TSP1 may help define the timing of growth plate closure when other extracellular proteins are absent.

Thrombospondin 2 (TSP2) is a matricellular protein that inhibits angiogenesis in response to wound healing and upon implantation of a biomaterial in the body. TSP2 down-regulation by antisense cDNA has been demonstrated to increase local angiogenesis and offers the potential to increase the efficiency and life span of implanted materials and devices. In this work, siRNA (small interfering RNA)-based down-regulation of TSP2 was investigated as an alternative and possibly more potent approach for TSP2 knockdown. RNA interference (RNAi) is an effective approach to sequence-specific gene knockdown, and the delivery of siRNA into cells has been shown to significantly alter protein levels. DNA sequences encoding TSP2 siRNA were inserted into a plasmid for in vitro delivery, and TSP2 protein expression was evaluated by real-time PCR.

Thrombospondin 2 (TSP2) can inhibit angiogenesis in vitro by limiting proliferation and inducing apoptosis of endothelial cells (ECs). TSP2 can also modulate the extracellular levels of gelatinases (matrix metalloproteases, MMPs) and potentially influence the remodeling of the extracellular matrix (ECM). Here, we tested the hypothesis that by regulating MMPs, TSP2 could alter EC-ECM interactions. By using a three-dimensional angiogenesis assay, we show that TSP2, but not TSP1, limited angiogenesis by decreasing gelatinolytic activity in situ. Furthermore, TSP2-null fibroblast-derived ECM, which contains irregular collagen fibrils, was more permissive for EC migration. Investigation of the role of TSP2 in physiological angiogenesis in vivo, using excision of the left femoral artery in both TSP2-null and wild-type mice, revealed that TSP2-null mice displayed accelerated recovery of blood flow. This increase was attributable, in part, to an enhanced arterial network in TSP2-null muscles of the upper limb. Angiogenesis in the lower limb was also increased and was associated with increased MMP-9 deposition and gelatinolytic activity. The observed changes correlated with the temporal expression of TSP2 in the ischemic muscle of wild-type mice. Taken together, our observations implicate the matrix-modulating activity of TSP2 as a mechanism by which physiological angiogenesis is inhibited.