Diminished callus size and cartilage synthesis in alpha 1 beta 1 integrin-deficient mice during bone fracture healing

Thrombospondin-2 influences the proportion of cartilage and bone during fracture healing

Thrombospondin-2 (TSP2) is a matricellular protein with increased expression during growth and regeneration. TSP2-null mice show accelerated dermal wound healing and enhanced bone formation. We hypothesized that bone regeneration would be enhanced in the absence of TSP2. Closed, semistabilized transverse fractures were created in the tibias of wildtype (WT) and TSP2-null mice. The fractures were examined 5, 10, and 20 days after fracture using microCT, histology, immunohistochemistry, quantitative RT-PCR, and torsional mechanical testing. Ten days after fracture, TSP2-null mice showed 30% more bone by microCT and 40% less cartilage by histology. Twenty days after fracture, TSP2-null mice showed reduced bone volume fraction and BMD. Mice were examined 5 days after fracture during the stage of neovascularization and mesenchymal cell influx to determine a cellular explanation for the phenotype. TSP2-null mice showed increased cell proliferation with no difference in apoptosis in the highly cellular fracture callus. Although mature bone and cartilage is minimal 5 days after fracture, TSP2-null mice had reduced expression of collagen IIa and Sox9 (chondrocyte differentiation markers) but increased expression of osteocalcin and osterix (osteoblast differentiation markers). Importantly, TSP2-null mice had a 2-fold increase in vessel density that corresponded with a reduction in vascular endothelial growth factor (VEGF) and Glut-1 (markers of hypoxia inducible factor [HIF]-regulated transcription). Finally, by expressing TSP2 using adenovirus starting 3 days after fracture, chondrogenesis was restored in TSP2-null mice. We hypothesize that TSP2 expressed by cells in the fracture mesenchyme regulates callus vascularization. The increase in vascularity increases tissue oxemia and decreases HIF; thus, undifferentiated cells in the callus develop into osteoblasts rather than chondrocytes. This leads to an alternative strategy for achieving fracture healing with reduced endochondral ossification and enhanced appositional bone formation. Controlling the ratio of cartilage to bone during fracture healing has important implications for expediting healing or promoting regeneration in nonunions.

Comparison of the healing of open tibial fractures covered with either muscle or fasciocutaneous tissue in a murine model

The objective of this study was to compare the effects of soft tissue coverage by either muscle or fasciocutaneous tissue on the healing of open tibial fractures in a murine model. An open tibial fracture, stripped of periosteum with intramedullary fixation, was created in mice. Experimental groups were devised to allow exclusive comparison of either muscle alone or skin plus fascia in direct contact with healing bone. To exclusively assess the relative efficacy of muscle and fasciocutaneous tissue to promote healing of a fracture stripped of periosteum, a piece of sterile inert material (polytetrafluoroethylene) was positioned anteriorly, excluding skin and fascia (muscle group) or posteriorly, excluding muscle (fasciocutaneous group). Skeletal repair was assessed histologically and quantified by histomorphometry; quantitative peripheral computed tomography (pQCT) and mechanical testing using a four-point bending technique. This standardized, reproducible model allowed characterization of the morphology of open fracture healing. At 28 days postfracture, there was faster healing in the experimental muscle coverage group compared to skin and fascia alone. Furthermore, there was almost 50% more cortical bone content and a threefold stronger union beneath muscle compared to fasciocutaneous tissue (p < 0.05 by one-way ANOVA). Exclusive comparison of muscle and fasciocutaneous tissue in our novel murine model demonstrates that muscle is superior for the coverage of open tibial fractures for both the rate and quality of fracture healing.

Evolution of collagen-based adhesion systems

Collagens are large, triple-helical proteins that form fibrils and network-like structures in the extracellular matrix. The collagens may have participated in the evolution of the metazoans from their very earliest origins. Cell adhesion receptors, such as the integrins, are at least as old as the collagens. Still, the early metazoan cells might not have been able to anchor directly to collagen fibrils, since the integrin-type collagen receptors have only been identified in vertebrates. Instead, the early metazoans may have used integrin-type receptors in the recognition of collagen-binding glycoproteins. It is possible that specialized, high-avidity collagen-receptor integrins have become instrumental for the evolution of bone, cartilage, circulatory and immune systems in the chordates. In vertebrates, specific collagen-binding receptor tyrosine kinases send signals into cells after adhesion to collagen. These receptors are members of the discoidin domain receptor (DDR) group. The evolutionary history of DDRs is poorly known at this time. DDR orthologs have been found in many invertebrates, but their ability to function as collagen receptors has not yet been tested. The two main categories of collagens, fibrillar and non-fibrillar, already exist in the most primitive metazoans, such as the sponges. Interestingly, both integrin and DDR families seem to have members that favor either one or the other of these two groups of collagens.

Polymorphisms and haplotypes of integrinalpha1 (ITGA1) are associated with bone mineral density and fracture risk in postmenopausal Koreans

INTRODUCTION

ITGA1 is involved in the early remodeling of osteoarthritic cartilage and plays an essential role in the regulation of mesenchymal stem cell proliferation and cartilage production. We investigated the association between bone parameters and ITGA1 polymorphisms and their haplotype linkage disequilibrium (LD) blocks (BL_hts). Genetic susceptibility to osteoporosis was studied in 946 postmenopausal Korean women.

METHODS

We identified 67 genetic polymorphisms in ITGA1 region by direct sequencing (n = 114). Eight SNPs were genotyped to further investigate their potential involvement in osteoporosis in postmenopausal women (n = 946). Areal BMD of the lumbar spine and proximal femur was measured using dual-energy X-ray absorptiometry.

RESULTS

The SNPs, +73187C>T (exon 3) and +76969T>G (intron 5), and their BL_hts were associated with bone mineral density (BMD) at various femur sites (p = 0.009-0.05). Moreover, +159174A>C (intron 28) and its haplotype BL3_ht1 showed a highly significant association with risk of non-vertebral fracture (p = 0.002-0.005) and the minor allele of +159174A>C showed a protective effect.

CONCLUSIONS

These results are suggestive of the association of ITGA1 with osteoporosis and related risk in postmenopausal women.

The collagen family members as cell adhesion proteins

The collagen family of extracellular matrix proteins has played a fundamental role in the evolution of multicellular animals. At the present, 28 triple helical proteins have been named as collagens and they can be divided into several subgroups based on their structural and functional properties. In tissues, the cells are anchored to collagenous structures. Often the interaction is indirect and mediated by matrix glycoproteins, but cells also express receptors, which have the ability to directly bind to the triple helical domains in collagens. Some receptors bind to sites that are abundant in all collagens. However, increasing evidence indicates that the coevolution of collagens and cell adhesion mechanisms has given rise to receptors that bind to specific motifs in collagens. These receptors may also recognize the different members of the large collagen family in a selective manner. This review summarizes the present knowledge about the properties of collagen subtypes as cell adhesion proteins.

CCN2 (Connective Tissue Growth Factor) is essential for extracellular matrix production and integrin signaling in chondrocytes

The matricellular protein CCN2 (Connective Tissue Growth Factor; CTGF) is an essential mediator of ECM composition, as revealed through analysis of Ccn2 deficient mice. These die at birth due to complications arising from impaired endochondral ossification. However, the mechanism(s) by which CCN2 mediates its effects in cartilage are unclear. We investigated these mechanisms using Ccn2 ( -/- ) chondrocytes. Expression of type II collagen and aggrecan were decreased in Ccn2 (-/-) chondrocytes, confirming a defect in ECM production. Ccn2 ( -/- ) chondrocytes also exhibited impaired DNA synthesis and reduced adhesion to fibronectin. This latter defect is associated with decreased expression of alpha5 integrin. Moreover, CCN2 can bind to integrin alpha5beta1 in chondrocytes and can stimulate increased expression of integrin alpha5. Consistent with an essential role for CCN2 as a ligand for integrins, immunofluorescence and Western blot analysis revealed that levels of focal adhesion kinase (FAK) and extracellular signal-regulated kinase (ERK)1/2 phosphorylation were reduced in Ccn2 ( -/- ) chondrocytes. These findings argue that CCN2 exerts major effects in chondrocytes through its ability to (1) regulate ECM production and integrin alpha5 expression, (2) engage integrins and (3) activate integrin-mediated signaling pathways.

Comparison of cellular functionality of human mesenchymal stromal cells and PBMC

BACKGROUND

Human mesenchymal stromal cells (MSC) and PBMC play significant roles in repair processes following inflammation. Mechanisms of recruitment are still under investigation.

METHODS AND RESULTS

MIP-1alpha induced the chemotactic migration of MSC but not of PBMC. Correlating with this, 7.7% of MSC expressed the chemokine receptor CCR-1, as shown by FACS analysis. In contrast, PBMC did not express CCR-1 or CCR-2 but did express CXCR-4 (81.9%) and CCR-7 (42.2%). Setum induced the chemotaxis of both cell types, and zymosan activation increased the migration of PBMC but not of MSC. Corresponding with this, C5a induced the migration of PBMC but not of MSC. Dose-dependent and -specific adhesion to fibronectin, fibrinogen, collagen type I and collagen type II could be demonstrated for MSC; in contrast, PBMC did not adhere to any of the investigated proteins. Real-time PCR of receptor expression revealed a 12.2-fold higher expression of alphav in MSC compared with PBMC. Incubation of MSC with tumor necrosis factor-alpha (TNFalpha) induced NFkappaB activation and increased the chemotactic response to serum and adhesion to fibronedtin.

DISCUSSION

Chemotaxis and adhesion are crucial and differing cell fundtons of MSC and PBMC.

Impaired bone fracture healing in matrix metalloproteinase-13 deficient mice

Vascular and cellular invasion into the cartilage is a critical step in the fracture healing. Matrix metalloproteinase-13 (MMP-13) is a member of the zinc-dependent endopeptidase family and plays an important role in remodeling of extracellular matrix. Therefore we investigated the possible involvement of MMP-13 in a murine model of stabilized bone fracture healing. Repair of the fracture in MMP-13 deficient (MMP-13(-/-)) mice was significantly delayed and characterized by a retarded cartilage resorption in the fracture callus. Immunohistochemistry indicated severe defects in vascular penetration and chondroclast recruitment to the fracture callus in MMP-13(-/-) mice. Consistent with the observations, the chondrocyte pellets cultured from the MMP13(-/-) mice exhibited diminished angiogenic activities when the pellets were co-cultured with endothelial cells. These results suggest that MMP-13 is crucial to the process of angiogenesis during healing of fracture, especially in the cartilage resorption process.

Potential of dental mesenchymal cells in developing teeth

The tooth, composed of dentin and enamel, develops through epithelium-mesenchyme interactions. Neural crest (NC) cells contribute to the dental mesenchyme in the developing tooth and differentiate into dentin-secreting odontoblasts. NC cells are known to differentiate into chondrocytes and osteoblasts in the craniofacial region. However, it is not clear whether the dental mesenchymal cells in the developing tooth possess the potential to differentiate into a lineage(s) other than the odontoblast lineage. In this study, we prepared mesenchymal cells from E13.5 tooth germ cells and assessed their potential for differentiation in culture. They differentiated into odontoblasts, chondrocyte-like cells, and osteoblast-like cells. Their derivation was confirmed by tracing NC-derived cells as LacZ(+) cells using P0-Cre/Rosa26R mice. Using the flow cytometry-fluorescent di-beta-D-galactosidase system, which makes it possible to detect LacZ(+) cells as living cells, cell surface molecules of dental mesenchymal cells were characterized. Large number of LacZ(+) NC-derived cells expressed platelet-derived growth factor receptor alpha and integrins. Taken together, these results suggest that NC-derived cells with the potential to differentiate into chondrocyte-like and osteoblast-like cells are present in the developing tooth, and these cells may contribute to tooth organogenesis.

Comparison of standard surface chemistries for culturing mesenchymal stem cells prior to neural differentiation

A critical element of any stem cell differentiation protocol is the ability to compare its effects relative to an undifferentiated population of the same cells. In an attempt to standardize pre-differentiation conditions of adult derived mesenchymal stem cells prior to neural induction experiments, we asked what is the simplest chemical surface that supports the growth and maintenance of these cells in a pre-differentiation state. Adult bone marrow-derived rat mesenchymal stem cells (BMSCs) were expanded in vitro on Permanox Lab-Tek tissue culture treated plastic (TCP), poly-D-lysine (PDL) coated glass, PDL-laminin-1 coated glass, and untreated glass. TCP provided the best surface for maintaining morphologies generally considered to be undifferentiated, while PDL coated glass and uncoated glass provided the least suitable surfaces. Expansion of BMSCs on PDL-laminin-1 coated glass resulted in expression of nestin, a marker associated with neuronal and other progenitor cells, and therefore may confound experimental results if used as a pre-differentiation surface.

Regulation of prostate cell collagen receptors by malignant transformation

Cell adhesion receptors, including the integrin-type collagen receptors (alpha1beta1, alpha2beta1, alpha10beta1 and alpha11beta1) participate in cancer progression and invasion. Quantitative RT-PCR indicated that all 4 receptors are abundantly expressed in sarcoma-derived cell lines, whereas most carcinoma-derived cells express alpha1beta1 and alpha2beta1 only. This was surprising because alpha11beta1 has been connected previously to the progression of lung adenocarcinomas. To test the hypothesis that alpha11 expression may not persist in cultured cancer cells we analyzed fresh tissue samples of 104 total prostatectomies, keeping in mind that prostate cancer cell lines showed negligible alpha11 mRNA levels. In prostate alpha2 expression was significantly lower in poorly differentiated carcinomas when compared to benign lesions (p = 0.0331). In immunohistochemistry the protein levels of alpha2 integrin decreased significantly (p = 0.0001) and the protein levels of alpha11 subunit increased significantly (p = 0.029) with the increasing grade of carcinoma. Thus alpha11beta1 may replace alpha2beta1 during tumor progression. Our observations support the idea that alpha11beta1 may be expressed in tumors but the corresponding cell lines may lose the expression of this integrin. Previous studies have shown that in cell culture androgen receptor (AR) controls alpha2beta1 expression. We measured AR mRNA levels and the number of AR positive nuclei in the prostate samples and the results showed a significant correlation between alpha2beta1 and AR. Androgen receptors may control the mechanisms regulating integrin expression in prostate.

Regulation of Fibrosis by the Immune System

Inflammation and fibrosis are two inter-related conditions with many overlapping mechanisms. Three specific cell types, macrophages, T helper cells, and myofibroblasts, each play important roles in regulating both processes. Following tissue injury, an inflammatory stimulus is often necessary to initiate tissue repair, where cytokines released from resident and infiltrating leukocytes stimulate proliferation and activation of myofibroblasts. However, in many cases this drive stimulates an inappropriate pro-fibrotic response. In addition, activated myofibroblasts can take on the role of traditional APCs, secrete pro-inflammatory cytokines, and recruit inflammatory cells to fibrotic foci, amplifying the fibrotic response in a vicious cycle. Moreover, inflammatory cells have been shown to play contradictory roles in initiation, amplification, and resolution of fibrotic disease processes. The central role of the macrophage in contributing to the fibrotic response and fibrotic resolution is only beginning to be fully appreciated. In the following review, we discuss the fibrotic disease process from the context of the immune response to injury. We review the major cellular and soluble factors controlling these responses and suggest ways in which more specific and, hopefully, more effective therapies may be derived.

NF1 gene expression in mouse fracture healing and in experimental rat pseudarthrosis

Neurofibromatosis type 1 (NF1) is an inherited disease with an incidence of about 1:3000 worldwide. Approximately half of all patients with NF1 present osseous manifestations, which can vary from mild to severely debilitating changes such as congenital pseudarthrosis. In the present study, fracture healing of mouse tibia was followed and specimens were collected 5, 9, 14, and 22 days postoperatively. Experimental pseudarthrosis of rat was followed up to 15 weeks postoperatively. In situ hybridization and immunohistochemistry were used to demonstrate expression of NF1 tumor suppressor and phosphorylated p44/42 mitogen-activated protein kinase (MAPK), an indicator of the Ras-MAPK pathway. The results showed that ossified callus was formed in mouse fracture 22 days after the operation. The final outcome of rat pseudarthrosis was detected 9 weeks after the operation, presenting abundant cartilaginous callus at the pseudarthrosis. NF1 gene expression was noted in the maturing and in the hypertrophic cartilages during normal mouse fracture healing, and in rat pseudarthrosis. Phosphorylated p44/42 MAPK was detected in a subpopulation of the hypertrophic chondrocytes in both models. Furthermore, positive labeling for NF1 mRNA and protein was detected in endothelium in both the pseudarthrosis and in the fracture. In conclusion, NF1 gene expression and function are needed for normal fracture healing, possibly restraining excessive Ras-MAPK pathway activation.

Fibroblast Growth Factor (FGF) 18 Signals through FGF Receptor 3 to Promote Chondrogenesis

Signaling by fibroblast growth factor (FGF) 18 and FGF receptor 3 (FGFR3) have been shown to regulate proliferation, differentiation, and matrix production of articular and growth plate chondrocytes in vivo and in vitro. Notably, the congenital absence of either FGF18 or FGFR3 resulted in similar expansion of the growth plates of fetal mice and the addition of FGF18 to human articular chondrocytes in culture enhanced proliferation and matrix production. Based on these and other experiments it has been proposed that FGF18 signals through FGFR3 to promote cartilage production by chondrocytes. Its role in chondrogenesis remains to be defined. In the current work we used the limb buds of FGFR3(+/+) and FGFR3(-/-) embryonic mice as a source of mesenchymal cells to determine how FGF18 signaling affects chondrogenesis. Confocal laser-scanning microscopy demonstrated impaired cartilage nodule formation in the FGFR3(-/-) cultures. Potential contributing factors to the phenotype were identified as impaired mitogenic response to FGF18, decreased production of type II collagen and proteoglycan in response to FGF18 stimulation, impaired interactions with the extracellular matrix resulting from altered integrin receptor expression, and altered expression of FGFR1 and FGFR2. The data identified FGF18 as a selective ligand for FGFR3 in limb bud mesenchymal cells, which suppressed proliferation and promoted their differentiation and production of cartilage matrix. This work, thus, identifies FGF18 and FGFR3 as potential molecular targets for intervention in tissue engineering aimed at cartilage repair and regeneration of damaged cartilage.

The Fibril-associated Collagen IX Provides a Novel Mechanism for Cell Adhesion to Cartilaginous Matrix

Collagen IX is the prototype fibril-associated collagen with interruptions in triple helix. In human cartilage it covers collagen fibrils, but its putative cellular receptors have been unknown. The reverse transcription-PCR analysis of human fetal tissues suggested that based on their distribution all four collagen receptor integrins, namely alpha1beta1, alpha2beta1, alpha10beta1, and alpha11beta1, are possible receptors for collagen IX. Furthermore primary chondrocytes and chondrosarcoma cells express the four integrins simultaneously. Chondrosarcoma cells, as well as Chinese hamster ovary cells transfected to express alpha1beta1, alpha2beta1, or alpha10beta1 integrin as their only collagen receptor, showed fast attachment and spreading on human recombinant collagen IX indicating that it is an effective cell adhesion protein. To further study the recognition of collagen IX we produced recombinant alphaI domains in Escherichia coli. For each of the four alphaI domains, collagen IX was among the best collagenous ligands, making collagen IX exceptional compared with all other collagen subtypes tested so far. Rotary shadowing electron microscopy images of both alpha1I- and alpha2I-collagen IX complexes unveiled only one binding site located in the COL3 domain close to the kink between it and the COL2 domain. The recognition of collagen IX by alpha2I was considered to represent a novel mechanism for two reasons. First, collagen IX has no GFOGER motif, and the identified binding region lacks any similar sequences. Second, the alpha2I domain mutations D219R and H258V, which both decreased binding to collagen I and GFOGER, had very different effects on its binding to collagen IX. D219R had no effect, and H258V prevented type IX binding. Thus, our results indicate that collagen IX has unique cell adhesion properties when compared with other collagens, and it provides a novel mechanism for cell adhesion to cartilaginous matrix.

The role of very late antigen-1 in immune-mediated inflammation

The alpha1beta1 integrin, also known as "very late antigen" (VLA)-1, is normally expressed on mesenchymal cells, some epithelial cells, activated T cells, and macrophages, and interacts, via the I-domain of the extracellular domain of the alpha1 subunit, with collagen molecules in the extracellular matrix (ECM). By "outside-in" transmembranal signaling to the interior of the cell, it mediates adhesion, migration, proliferation, remodeling of the ECM, and cytokine secretion by endothelial cells, mesangial cells, fibroblasts, and immunocytes. Importantly, its expressions and functions are enhanced by inflammatory cytokines including interferon (IFN)gamma and tumor necrosis factor (TNF)alpha, thus augmenting angiogenesis and fibrosis linked, in particular, to inflammation. Moreover, within the immune system, VLA-1 marks effector memory CD4+ and CD8+ T cells that are retained in extralymphatic tissues by interactions of the integrin with collagen and produce high levels of IFNgamma. Thus, immune-mediated inflammation in vivo is inhibited by blockade of the VLA-1-collagen interaction in experimental animal models of arthritis, colitis, nephritis, and graft versus host disease (GVHD), suggesting that inhibiting the interaction of the alpha1 I-domain with its ligands or modulating "outside-in" signaling by VLA-1 would be a useful approach in the human diseases simulated by these experimental models.

Lack of integrin alpha1beta1 leads to severe glomerulosclerosis after glomerular injury

Severity of fibrosis after injury is determined by the nature of the injury and host genetic susceptibility. Metabolism of collagen, the major component of fibrotic lesions, is, in part, regulated by integrins. Using a model of glomerular injury by adriamycin, which induces reactive oxygen species (ROS) production, we demonstrated that integrin alpha1-null mice develop more severe glomerulosclerosis than wild-type mice. Moreover, primary alpha1-null mesangial cells produce more ROS both at baseline and after adriamycin treatment. Increased ROS synthesis leads to decreased cell proliferation and increased glomerular collagen IV accumulation that is reversed by antioxidants both in vivo and in vitro. Thus, we have identified integrin alpha1beta1 as a modulator of glomerulosclerosis. In addition, we showed a novel pathway where integrin alpha1beta1 modulates ROS production, which in turn controls collagen turnover and ultimately fibrosis. Because integrin alpha1beta1 is expressed in many cell types this may represent a generalized mechanism of controlling matrix accumulation, which has implications for numerous diseases characterized by fibrosis.

The collagen receptor subfamily of the integrins

The four collagen receptor integrins, alpha1beta1, alpha2beta1, alpha10beta1 and alpha11beta1, form a structurally and functionally distinct subgroup when compared to other members of the integrin family. In this review, we discuss the structures of these receptors and their differences in collagen binding and signalling function.

Integrin-mediated cell adhesion to type I collagen fibrils

In the integrin family, the collagen receptors form a structurally and functionally distinct subgroup. Two members of this subgroup, alpha(1)beta(1) and alpha(2)beta(1) integrins, are known to bind to monomeric form of type I collagen. However, in tissues type I collagen monomers are organized into large fibrils immediately after they are released from cells. Here, we studied collagen fibril recognition by integrins. By an immunoelectron microscopy method we showed that integrin alpha(2)I domain is able to bind to classical D-banded type I collagen fibrils. However, according to the solid phase binding assay, the collagen fibril formation appeared to reduce integrin alpha(1)I and alpha(2)I domain avidity to collagen and to lower the number of putative alphaI domain binding sites on it. Respectively, cellular alpha(1)beta(1) integrin was able to mediate cell spreading significantly better on monomeric than on fibrillar type I collagen matrix, whereas alpha(2)beta(1) integrin appeared still to facilitate both cell spreading on fibrillar type I collagen matrix and also the contraction of fibrillar type I collagen gel. Additionally, alpha(2)beta(1) integrin promoted the integrin-mediated formation of long cellular projections typically induced by fibrillar collagen. Thus, these findings suggest that alpha(2)beta(1) integrin is a functional cellular receptor for type I collagen fibrils, whereas alpha(1)beta(1) integrin may only effectively bind type I collagen monomers. Furthermore, when the effect of soluble alphaI domains on type I collagen fibril formation was tested in vitro, the observations suggest that integrin type collagen receptors might guide or even promote pericellular collagen fibrillogenesis.

Tomorrow's skeleton staff: mesenchymal stem cells and the repair of bone and cartilage

Stem cells are regenerating medicine. Advances in stem cell biology, and bone marrow-derived mesenchymal stem cells in particular, are demonstrating that many clinical options once thought to be science fiction may be attainable as fact. The extra- and intra-cellular signalling used by stem cells as they differentiate into lineages appropriate to their destination are becoming understood. Thus, the growth stimuli afforded by LIF, FGF-2 and HGF, as well as the complementary roles of Wnt and Dickkopf-1 in stem cell proliferation are evident. The ability to direct multi-lineage mesenchymal stem sell (MSC) potential towards an osteogenic phenotype by stimulation with Menin and Shh are important, as are the modulatory roles of Notch-1 and PPARgamma. Control of chondrocytic differentiation is effected by interplay of Brachyury, BMP-4 and TGFbeta3. Smads 1, 4 and 5 also play a role in these phenotypic expressions. The ability to culture MSC has led to their use in tissue repair, both as precursor and differentiated cell substitutes, and with successful animal models of bone and cartilage repair using MSC, their clinical use is accelerating. However, MSC also suppress some T-cell functions in transplanted hosts, and could facilitate tumour growth, so a cautious approach is needed.

Accelerated, aging-dependent development of osteoarthritis in alpha1 integrin-deficient mice

OBJECTIVE

Cell-matrix interactions regulate chondrocyte differentiation and survival. The alpha1beta1 integrin is a major collagen receptor that is expressed on chondrocytes. Mice with targeted inactivation of the integrin alpha1 gene (alpha1-KO mice) provide a model that can be used to address the role of cell-matrix interactions in cartilage homeostasis and osteoarthritis (OA) pathogenesis.

METHODS

Knee joints from alpha1-KO and wild-type (WT) BALB/c mice were harvested at ages 4-15 months. Knee joint sections were examined for inflammation, cartilage degradation, and loss of glycosaminoglycans (by Safranin O staining). Immunohistochemistry was performed to detect the distribution of alpha1 integrin, matrix metalloproteinases (MMPs), and chondrocyte apoptosis.

RESULTS

In WT mice, the alpha1 integrin subunit was detected in hypertrophic chondrocytes in the growth plate and in a subpopulation of cells in the deep zone of articular cartilage. There was a marked increase in alpha1-positive chondrocytes in the superficial and upper mid-zones in OA-affected areas in joints from old WT mice. The alpha1-KO mice showed more severe cartilage degradation, glycosaminoglycan depletion, and synovial hyperplasia as compared with the WT mice. MMP-2 and MMP-3 expression was increased in the OA-affected areas. In cartilage from alpha1-KO mice, the cellularity was reduced and the frequency of apoptotic cells was increased. These results suggest that the alpha1 integrin subunit is involved in the early remodeling process in OA cartilage.

CONCLUSION

Deficiency in the alpha1 integrin subunit is associated with an earlier deregulation of cartilage homeostasis and an accelerated, aging-dependent development of OA.

Altered fracture repair in the absence of MMP9

The regeneration of adult skeletal tissues requires the timely recruitment of skeletal progenitor cells to an injury site, the differentiation of these cells into bone or cartilage, and the re-establishment of a vascular network to maintain cell viability. Disturbances in any of these cellular events can have a detrimental effect on the process of skeletal repair. Although fracture repair has been compared with fetal skeletal development, the extent to which the reparative process actually recapitulates the fetal program remains uncertain. Here, we provide the first genetic evidence that matrix metalloproteinase 9 (MMP9) regulates crucial events during adult fracture repair. We demonstrate that MMP9 mediates vascular invasion of the hypertrophic cartilage callus, and that Mmp9(-/-) mice have non-unions and delayed unions of their fractures caused by persistent cartilage at the injury site. This MMP9- dependent delay in skeletal healing is not due to a lack of vascular endothelial growth factor (VEGF) or VEGF receptor expression, but may instead be due to the lack of VEGF bioavailability in the mutant because recombinant VEGF can rescue Mmp9(-/-) non-unions. We also found that Mmp9(-/-) mice generate a large cartilage callus even when fractured bones are stabilized, which implicates MMP9 in the regulation of chondrogenic and osteogenic cell differentiation during early stages of repair. In conclusion, the resemblance between Mmp9(-/-) fetal skeletal defects and those that emerge during Mmp9(-/-) adult repair offer the strongest evidence to date that similar mechanisms are employed to achieve bone formation, regardless of age.

Differential gene expression in the periprosthetic membrane: lubricin as a new possible pathogenetic factor in prosthesis loosening

About 10% of hip endoprostheses will loosen after 10 years. Prosthesis loosening is caused by two different pathomechanisms: aseptic loosening (AL) and septic loosening (SL). This study evaluated differences in gene expression in AL and SL. Eight hybridizations were performed on PIQOR cDNA arrays. Objects of the study were periprosthetic interface tissue samples from two patients with SL and three patients with AL. Tissue parts directly adjacent to the site of RNA isolation were analyzed immuno/histopathologically in order to overcome the problem of tissue heterogeneity. Thirty-three genes were found constantly differentially expressed, among which were cd11b, cd18, cd68, osteopontin and ferritin heavy-chain upregulated in AL and collagen types 1alpha-1, 3alpha-1, integrin alpha-1, thrombospondin2 and nidogen upregulated in SL. The most striking finding was the strong upregulation (from 20-fold to 323-fold) of megakaryocyte stimulating factor (msf) in all aseptic cases and one of the two septic cases, which was confirmed by real-time reverse transcription-polymerase chain reaction. In this study, msf is linked to prosthesis loosening for the first time. The upregulation in AL suggests an important pathogenetic role: the msf splice product lubricin is responsible for the lubrication of healthy joints, but its excellent lubrication ability may disturb the tight interaction between bone and prosthesis and thereby contribute to prosthesis loosening.

Increased bone formation in mice lacking plasminogen activators

Plasminogen activators tPA and uPA are involved in tissue remodeling, but their role in bone growth is undefined. Mice lacking tPA and uPA show increased bone formation and bone mass. The noncollagenous components of bone matrix are also increased, probably from defective degradation. This study underlines the importance of controlled bone matrix remodeling for normal endochondral ossification.

INTRODUCTION

Proteolytic pathways are suggested to play a role in endochondral ossification. To elucidate the involvement of the plasminogen activators tPA and uPA in this process, we characterized the long bone phenotype in mice deficient in both tPA and uPA (tPA-/-:uPA-/-).

MATERIALS AND METHODS

Bones of 2- to 7-day-old tPA-/-:uPA-/- and wild-type (WT) mice were studied using bone histomorphometry, electron microscopy analysis, and biochemical assessment of bone matrix components. Cell-mediated degradation of metabolically labeled bone matrix, osteoblast proliferation, and osteoblast differentiation, both at the gene and protein level, were studied in vitro using cells derived from both genotypes.

RESULTS

Deficiency of the plasminogen activators led to elongation of the bones and to increased bone mass (25% more trabecular bone in the proximal tibial metaphysis), without altering the morphology of the growth plate. In addition, the composition of bone matrix was modified in plasminogen activator deficient mice, because an increased amount of proteoglycans (2x), osteocalcin (+45%), and fibronectin (+36%) was detected. Matrix degradation assays showed that plasminogen activators, by generating plasmin, participate in osteoblast-mediated degradation of the noncollagenous components of bone matrix. In addition, proliferation of primary osteoblasts derived from plasminogen activator-deficient mice was increased by 35%. Finally, osteoblast differentiation and formation of a mineralized bone matrix were enhanced in osteoblast cultures derived from tPA-/-:uPA-/- mice.

CONCLUSIONS

The data presented indicate the importance of the plasminogen system in degradation of the noncollagenous components of bone matrix and suggest that the accumulation of these proteins in bone matrix--as occurs during plasminogen activator deficiency--may in turn stimulate osteoblast function, resulting in increased bone formation.