The integrin alpha5beta1 regulates chondrocyte hypertrophic differentiation induced by GTP-bound transglutaminase 2

TGM2 accelerates migration and differentiation of BMSCs by activating Wnt/β-catenin signaling

BACKGROUND

Transglutaminase 2 (TGM2) is a gene previously reported to be associated with the differentiation of bone marrow mesenchymal stem cells (BMSCs). The study was developed to reveal the impact of TGM2 on the migration and differentiation of BMSCs.

METHODS

Cells were isolated from bone marrow of mice and then the surface antigens were identified by flow cytometry. Wound healing assays were conducted to assess the migratory ability of BMSCs. The mRNA levels of TGM2 and osteoblast-associated genes (ALP, OCN, and RUNX2) were subjected to RT-qPCR analysis, and protein levels of these genes as well as β-catenin were quantitated by western blotting. Alizarin red staining was conducted for detection of osteogenic ability. The activation of Wnt signaling was assessed by TOP/FOP flash assays.

RESULTS

Surface antigens were positively identified in MSCs, indicating good multidirectional differentiation ability of cells. TGM2 silencing suppressed BMSC migration while weakening mRNA and protein levels of osteoblast-associated genes. While TGM2 overexpression exerts the opposite impact on cell migration and expression levels of osteoblast-associated genes. Additionally, overexpressed TGM2 promotes the mineralization of BMSCs according to results of Alizarin red staining. Moreover, TGM2 activated the Wnt/β-catenin signaling, and DKK1 (an inhibitor of Wnt signaling) reversed the promoting influence of TGM2 on cell migration and differentiation.

CONCLUSION

TGM2 promotes the migration and differentiation of BMSCs via activation of the Wnt/β-catenin signaling.

Transglutaminase 2 inhibitors attenuate osteoarthritic degeneration of TMJ-osteoarthritis by suppressing NF-κB activation

BACKGROUND

The temporomandibular joint osteoarthritis (TMJ-OA) is characterized by progressive cartilage degradation, subchondral bone erosion, and chronic pain, leading to articular damage and chewing dysfunction. Studies have shown that interleukin-1β (IL-1β) plays a critical role in the development of TMJ-OA. Transglutaminase 2 (TG2) has been identified as a marker of chondrocyte hypertrophy and IL-1β was able to increase TG2 expression in chondrocytes. Therefore, the aim of this study was to explore the ability of TG2 inhibitors to suppress TMJ-OA progression.

METHODS

Firstly, toluidine blue staining, cell counting kit-8 assay, immunocytofluorescent staining and western blot were used to investigate the anti-inflammatory effects of TG2 inhibitors in IL-1β-stimulated murine chondrocytes and the underlying mechanisms. Afterwards, micro-CT analysis, histological staining, immunohistochemical and immunohistofluorescent staining were used to evaluate the therapeutic efficacy of TG2 inhibitors in monosodium iodoacetate (MIA)-induced TMJ-OA in rats.

RESULTS

TG2 inhibitors suppressed the IL-1β-induced upregulation of COX-2, iNOS, MMP-13, and MMP-3 and reversed the IL-1β-induced proteoglycan loss in chondrocytes through inhibiting NF-κB activation. Consistently, the MIA-induced upregulation of MMP-13 and MMP-3, and loss of structural integrity of the articular cartilage and subchondral bone were markedly reversed by TG2 inhibitors via inhibiting NF-κB activation.

CONCLUSIONS

TG2 inhibitors demonstrated a potent therapeutic efficacy on cartilage and subchondral bone structures of TMJ-OA by reducing inflammation and cartilage degradation through suppressing NF-κB activation.

Calcium relieves fluoride-induced bone damage through the PI3K/AKT pathway

Bone is the main target of fluorosis, and it has been perfectly elaborated that a moderate dosage of calcium (Ca) can alleviate bone fluorosis. However, whether Ca can alleviate fluorosis through the phosphatidylinositol 3 kinase (PI3K)/protein kinase B (AKT) signaling pathway has not yet been reported. Hence, we evaluated the histopathological structure, the imbalance of the biochemical index of bone metabolism, and the expression levels of PI3K/AKT apoptosis signaling pathway-related genes in rats treated with sodium fluoride (NaF, F) and/or calcium carbonate (CaCO3) for 120 days. Our results suggest that 100 mg L-1 NaF induced histopathological injury as alkaline phosphatase (ALP) and tartrate-resistant acid phosphatase (StrACP) activity increased, with a decrease in the serum Ca levels (p < 0.05). Moreover, the results of qRT-PCR and western blotting showed that F increased the expression levels of transglutaminase 2 (TGM2), focal adhesion kinase (FAK), PI3K, AKT, forkhead box O1 (Foxo1), Bcl-2 interacting mediator of cell death (BIM), Bcl2-associated x protein (Bax) and Caspase 3 (p < 0.05, p < 0.01). It also decreased the expression of AnnexinA5 (Anxa5), 3'-phosphoinositide-dependent kinase 1 (PDK1) and B-cell lymphoma-2 (Bcl-2) (p < 0.05, p < 0.01), which finally activated the PI3K/AKT pathway. On the other hand, CaCO3 supplementation reversed the histopathological injury along with the levels of ALP, StrACP and serum Ca, alleviating the gene expression levels of PI3K/AKT pathway-related markers. Altogether, we can conclude that CaCO3 supplementation mitigated F-induced bone damage via the PI3K/AKT signaling pathway.

Tissue transglutaminase_variant 2-transduced mesenchymal stem cells and their chondrogenic potential

As cartilage is incapable of self-healing upon severe degeneration because of the lack of blood vessels, cartilage tissue engineering is gaining importance in the treatment of cartilage defects. This study was designed to improve cartilage tissue regeneration by expressing tissue transglutaminase variant 2 (TGM2_v2) in mesenchymal stem cells (MSC) derived from bone marrow of rats. For this purpose, rat MSCs transduced with TGM2_v2 were grown and differentiated on three-dimensional polybutylene succinate (PBSu) and poly-l-lactide (PLLA) blend scaffolds. The transduced cells could not only successfully express the short form transglutaminase-2, but also deposited the protein onto the scaffolds. In addition, they could spontaneously produce cartilage-specific proteins without any chondrogenic induction, suggesting that TGM2_v2 expression provided the cells the ability of chondrogenic differentiation. PBSu:PLLA scaffolds loaded with TGM2_v2 expressing MSCs could be used in repair of articular cartilage defects.

Transglutaminase 2 in cartilage homoeostasis: novel links with inflammatory osteoarthritis

Transglutaminase 2 (TG2) is highly expressed during chondrocyte maturation and contributes to the formation of a mineralised scaffold by introducing crosslinks between extracellular matrix (ECM) proteins. In healthy cartilage, TG2 stabilises integrity of ECM and likely influences cartilage stiffness and mechanistic properties. At the same time, the abnormal accumulation of TG2 in the ECM promotes chondrocyte hypertrophy and cartilage calcification, which might be an important aspect of osteoarthritis (OA) initiation. Although excessive joint loading and injuries are one of the main causes leading to OA development, it is now being recognised that the presence of inflammatory mediators accelerates OA progression. Inflammatory signalling is known to stimulate the extracellular TG2 activity in cartilage and promote TG2-catalysed crosslinking of molecules that promote chondrocyte osteoarthritic differentiation. It is, however, unclear whether TG2 activity aims to resolve or aggravate damages within the arthritic joint. Better understanding of the complex signalling pathways linking inflammation with TG2 activities is needed to identify the role of TG2 in OA and to define possible avenues for therapeutic interventions.

Regulation of α5 and αV Integrin Expression by GDF-5 and BMP-7 in Chondrocyte Differentiation and Osteoarthritis

The Integrin β1 family is the major receptors of the Extracellular matrix (ECM), and the synthesis and degradation balance of ECM is seriously disrupted during Osteoarthritis (OA). In this scenario, integrins modify their pattern expression and regulate chondrocyte differentiation in the articular cartilage. Members of the Transforming growth factor beta (Tgf-β) Superfamily, such as Growth differentiation factor 5 (Gdf-5) and Bone morphogenetic protein 7 (Bmp-7), play a key role in joint formation and could regulate the integrin expression during chondrocyte differentiation and osteoarthritis progression in an experimental OA rat model. Decrease of α5 integrin expression in articular cartilage was related with chondrocyte dedifferentiation during OA progression, while increase of α1, α2, and α3 integrin expression was related with fibrous areas in articular cartilage during OA. Hypertrophic chondrocytes expressed αV integrin and was increased in the articular cartilage of rats with OA. Integrin expression during chondrocyte differentiation was also analyzed in a micromass culture system of mouse embryo mesenchymal cells, micromass cultures was treated with Gdf-5 or Bmp-7 for 4 and 6 days, respectively. Gdf-5 induced the expression of the α5 sub-unit, while Bmp-7 induced the expression of the αV sub-unit. This suggests a switch in signaling for prehypertrophic chondrocyte differentiation towards hypertrophy, where Gdf-5 could maintain the articular chondrocyte phenotype and Bmp-7 would induce hypertrophy. Decrease of Ihh expression during late stages of OA in rat model suggest that the ossification in OA rat knees and endochondral ossification could be activated by Bmp-7 and αV integrin in absence of Ihh. Thus, chondrocyte phenotype in articular cartilage is similar to prehypetrophic chondrocyte in growth plate, and is preserved due to the presence of Indian hedgehog (Ihh), Gdf-5 and α5 integrin to maintain articular cartilage and prevent hypertrophy.

Strategies to minimize hypertrophy in cartilage engineering and regeneration

Due to a blood supply shortage, articular cartilage has a limited capacity for self-healing once damaged. Articular chondrocytes, cartilage progenitor cells, embryonic stem cells, and mesenchymal stem cells are candidate cells for cartilage regeneration. Significant current attention is paid to improving chondrogenic differentiation capacity; unfortunately, the potential chondrogenic hypertrophy of differentiated cells is largely overlooked. Consequently, the engineered tissue is actually a transient cartilage rather than a permanent one. The development of hypertrophic cartilage ends with the onset of endochondral bone formation which has inferior mechanical properties. In this review, current strategies for inhibition of chondrogenic hypertrophy are comprehensively summarized; the impact of cell source options is discussed; and potential mechanisms underlying these strategies are also categorized. This paper aims to provide guidelines for the prevention of hypertrophy in the regeneration of cartilage tissue. This knowledge may also facilitate the retardation of osteophytes in the treatment of osteoarthritis.

Transglutaminase regulation of cell function

Transglutaminases (TGs) are multifunctional proteins having enzymatic and scaffolding functions that participate in regulation of cell fate in a wide range of cellular systems and are implicated to have roles in development of disease. This review highlights the mechanism of action of these proteins with respect to their structure, impact on cell differentiation and survival, role in cancer development and progression, and function in signal transduction. We also discuss the mechanisms whereby TG level is controlled and how TGs control downstream targets. The studies described herein begin to clarify the physiological roles of TGs in both normal biology and disease states.

Endostatin inhibits Callus remodeling during fracture healing in mice

Information on the impact of endogenous anti-angiogenic factors on bone repair is limited. The hypothesis of the present study was endostatin, an endogenous inhibitor of angiogenesis, disturbs fracture healing. We evaluated this hypothesis in a closed femoral fracture model studying two groups of mice, one that was treated by a daily injection of 10 µg recombinant endostatin subcutaneously (n = 38) and a second one that received the vehicle for control (n = 37). Histomorphometric analysis showed a significantly increased callus formation in endostatin-treated animals at 2 and 5 weeks post-fracture. This was associated with a significantly higher callus tissue fraction of cartilage and fibrous tissue at 2 weeks and a significantly higher fraction of bone at 5 weeks post-fracture. Biomechanical testing revealed a significantly higher torsional stiffness in the endostatin group at 2 weeks. For both groups, we could demonstrate the expression of the endostatin receptor unit integrin alpha5 in endothelial cells, osteoblasts, osteoclasts, and chondrocytes at 2 weeks. Immunohistochemical fluorescence staining of CD31 showed a lower number of blood vessels in endostatin-treated animals compared to controls. The results of the present study indicate endostatin promotes soft callus formation but inhibits callus remodeling during fracture healing most probably by an inhibition of angiogenesis.

rFN/Cad-11-modified collagen type II biomimetic interface promotes the adhesion and chondrogenic differentiation of mesenchymal stem cells

Properties of the cell-material interface are determining factors in the successful function of cells for cartilage tissue engineering. Currently, cell adhesion is commonly promoted through the use of polypeptides; however, due to their lack of complementary or modulatory domains, polypeptides must be modified to improve their ability to promote adhesion. In this study, we utilized the principle of matrix-based biomimetic modification and a recombinant protein, which spans fragments 7-10 of fibronectin module III (heterophilic motif) and extracellular domains 1-2 of cadherin-11 (rFN/Cad-11) (homophilic motif), to modify the interface of collagen type II (Col II) sponges. We showed that the designed material was able to stimulate cell proliferation and promote better chondrogenic differentiation of rabbit mesenchymal stem cells (MSCs) in vitro than both the FN modified surfaces and the negative control. Further, the Col II/rFN/Cad-11-MSCs composite stimulated cartilage formation in vivo; the chondrogenic effect of Col II alone was much less significant. These results suggested that the rFN/Cad-11-modified collagen type II biomimetic interface has dual biological functions of promoting adhesion and stimulating chondrogenic differentiation. This substance, thus, may serve as an ideal scaffold material for cartilage tissue engineering, enhancing repair of injured cartilage in vivo.

Transglutaminases: key regulators of cancer metastasis

The ability to metastasize represents the most important characteristic of malignant tumors. The biological details of the metastatic process remain somewhat unknown, due to difficulties in studying tumor cell behaviour with high spatial and temporal resolution in vivo. Several lines of evidence involve transglutaminases (TGs) in the key stages of tumor progression cascade, even though the molecular mechanisms remain controversial. TG expression and activity display a different role in the primary tumor or in metastatic cells. In fact, TG expression is low in the primary tumor mass, but augmented when cells acquire the metastatic phenotype. Nevertheless, in other cases, the use of inducers of TG transamidating activity seems to contrast tumor cell plasticity, migration and invasion. In the following review, the function of TGs in cancer cell migration into the extracellular matrix, adhesion to the capillary endothelium and its basement membrane, invasion and angiogenesis is discussed.

WISP3-IGF1 interaction regulates chondrocyte hypertrophy

WISP3 (Wnt induced secreted protein 3) is a multi-domain protein of mesenchymal origin. Mutations in several domains of WISP3 cause PPRD (progressive pseudo rheumatoid dysplasia), which is associated with cartilage loss and restricted skeletal development. Despite several studies focusing on the functional characterization of WISP3, the molecular details underlying the course of PPRD remain unresolved. We are interested in analyzing the function of WISP3 in the context of cartilage integrity. The current study demonstrates that WISP3 binds to insulin-like growth factor 1 (IGF1) and inhibits IGF1 secretion. Additionally, WISP3 curbs IGF1-mediated collagen X expression, accumulation of reactive oxygen species (ROS) and alkaline phosphatase activity, all of which are associated with the induction of chondrocyte hypertrophy. Interestingly, both IGF1 and ROS in turn trigger an increase in WISP3 expression. Together, our results are indicative of an operational WISP3-IGF1 regulatory loop whereby WISP3 preserves cartilage integrity by restricting IGF1-mediated hypertrophic changes in chondrocytes, at least partly, upon interaction with IGF1.

Cellular functions of tissue transglutaminase

Transglutaminase 2 (TG2 or tissue transglutaminase) is a highly complex multifunctional protein that acts as transglutaminase, GTPase/ATPase, protein disulfide isomerase, and protein kinase. Moreover, TG2 has many well-documented nonenzymatic functions that are based on its noncovalent interactions with multiple cellular proteins. A vast array of biochemical activities of TG2 accounts for its involvement in a variety of cellular processes, including adhesion, migration, growth, survival, apoptosis, differentiation, and extracellular matrix organization. In turn, the impact of TG2 on these processes implicates this protein in various physiological responses and pathological states, contributing to wound healing, inflammation, autoimmunity, neurodegeneration, vascular remodeling, tumor growth and metastasis, and tissue fibrosis. TG2 is ubiquitously expressed and is particularly abundant in endothelial cells, fibroblasts, osteoblasts, monocytes/macrophages, and smooth muscle cells. The protein is localized in multiple cellular compartments, including the nucleus, cytosol, mitochondria, endolysosomes, plasma membrane, and cell surface and extracellular matrix, where Ca(2+), nucleotides, nitric oxide, reactive oxygen species, membrane lipids, and distinct protein-protein interactions in the local microenvironment jointly regulate its activities. In this review, we discuss the complex biochemical activities and molecular interactions of TG2 in the context of diverse subcellular compartments and evaluate its wide ranging and cell type-specific biological functions and their regulation.

Tissue-specific responses to loss of transglutaminase 2

Of the eight catalytic transglutaminases (TGs), transglutaminase 2 (TG2) has been the most comprehensively studied due to its ubiquitous expression in multiple cell types. Despite the observed critical role for this enzyme in multiple biological processes in vitro, TG2 knockout mouse models have shown no severe developmental phenotypes, suggesting compensation by other TGs. To begin characterization of the compensating mechanisms, we analyzed total transamidating activity and expression patterns of all catalytically active TGs in seven different tissues/organs from wild-type and TG2 knockout mice. Inhibitory analysis with TG2-specific inhibitor KCC-009 suggests that relative contribution of TG2 in total transamidating activity differs in various tissues. Accordingly, our data indicate tissue-specific mechanisms of compensation for the loss of TG2, including transcriptional compensation in heart and liver versus functional compensation in aorta, kidney and skeletal/cartiagenous tissues. On the contrary, no compensation has been detected in skeletal muscle, suggesting a limited role for the TG2-mediated transamidation in normal development of this tissue.

Transglutaminase 2 as a biomarker of osteoarthritis: an update

Osteoarthritis is a progressive joint disease characterized by cartilage degradation and bone remodelling. Under physiologic conditions, articular cartilage displays a stable chondrocyte phenotype, whereas in osteoarthritis a chondrocyte hypertrophy develops near the sites of cartilage surface damage and associates to the pathologic expression of type X collagen. Transglutaminases (TGs) include a family of Ca(2+)-dependent enzymes that catalyze the formation of γ-glutamyl cross-links. Their substrates include a variety of intracellular and extracellular macromolecular components. TGs are ubiquitously and abundantly expressed and implicated in a variety of physiopathological processes. TGs activity is modulated by inflammatory cytokines. TG2 (also known as tissue transglutaminase) mediates the hypertrophic differentiation of joint chondrocytes and interleukin-1-induced calcification. Histomorphometrical and biomolecular investigations document increased TG2 expression in human and experimental osteoarthritis. Consequently, the level of TG2 expression may represent an adjuvant additional marker to monitor tissue remodelling occurring in osteoarthritic joint tissue. Experimental induction of osteoarthritis in TG2 knockout mice is followed from reduced cartilage destruction and increased osteophyte formation compared to wild-type mice, suggesting a different influence on joint bone and cartilage remodelling. The capacity of transamidation by TG2 to regulate activation of latent TGF-β seems to have a potential impact on the regulation of inflammatory response in osteoarthritic tissues. Additional studies are needed to define TG2-regulated pathways that are differently modulated in osteoblasts and chondrocytes during osteoarthritis.

TG2, a novel extracellular protein with multiple functions

TG2 is multifunctional enzyme which can be secreted to the cell surface by an unknown mechanism where its Ca(2+)-dependent transamidase activity is implicated in a number of events important to cell behaviour. However, this activity may only be transient due to the oxidation of the enzyme in the extracellular environment including its reaction with NO probably accounting for its many other roles, which are transamidation independent. In this review, we discuss the novel roles of TG2 at the cell surface and in the ECM acting either as a transamidating enzyme or as an extracellular scaffold protein involved in cell adhesion. Such roles include its ability to act as an FN co-receptor for β integrins or in a heterocomplex with FN interacting with the cell surface heparan sulphate proteoglycan syndecan-4 leading to activation of PKCα. These different properties of TG2 involve this protein in various physiological processes, which if not regulated appropriately can also lead to its involvement in a number of diseases. These include metastatic cancer, tissue fibrosis and coeliac disease, thus increasing its attractiveness as both a therapeutic target and diagnostic marker.

Transglutaminase 2 regulates early chondrogenesis and glycosaminoglycan synthesis

The expression pattern for tissue transglutaminase (TG2) suggests that it regulates cartilage formation. We analyzed the role of TG2 in early stages of chondrogenesis using differentiating high-density cultures of mesenchymal cells from chicken limb bud as a model. We demonstrate that TG2 promotes cell differentiation towards a pre-hypertrophic stage without inducing precocious hypertrophic maturation. This finding, combined with distinctive up-regulation of extracellular TG2 in the pre-hypertrophic cartilage of the growth plate, indicates that TG2 is an autocrine regulator of chondrocyte differentiation. We also show that TG2 regulates synthesis of the cartilaginous glycosaminoglycan (GAG)-rich extracellular matrix. Elevated levels of TG2 down-regulate xylosyltransferase activity which mediates the key steps in chondroitin sulfate synthesis. On the contrary, inhibition of endogenous transglutaminase activity in differentiating chondrogenic micromasses results in increased GAG deposition and enhancement of early chondrogenic markers. Regulation of GAG synthesis by TG2 appears independent of TGF-β activity, which is a downstream mediator of the TG2 functions in some biological systems. Instead, our data suggest a major role for cAMP/PKA signaling in transmitting TG2 signals in early chondrogenic differentiation. In summary, we demonstrate that matrix synthesis and early stages of chondrogenic differentiation are regulated through a novel mechanism involving TG2-dependent inhibition of PKA. These findings further advance understanding of cartilage formation and disease, and contribute to cartilage bioengineering.

Novel insights into the mechanism of decreased expression of type X collagen in human mesenchymal stem cells from patients with osteoarthritis cultured on nitrogen-rich plasma polymers: implication of cyclooxygenase-1

Recent evidence indicates that a major drawback of current cartilage- and intervertebral disc (IVD) tissue engineering is that human mesenchymal stem cells (MSCs) from patients with osteoarthritis rapidly express type X collagen (COL10A1), a marker of late stage chondrocyte hypertrophy associated with endochondral ossification. We recently demonstrated that COL10A1 expression was inhibited in MSCs from patients with osteoarthritis cultured on nitrogen-rich plasma polymerized (PPE:N) coatings. Here, we sought to understand the mechanisms of action of this effect by culturing MSCs on PPE:N surfaces in the presence of different inhibitors of kinases and cyclooxygenases. The effect of PPE:N surfaces on COL10A1 expression was found to be mimicked by the cyclooxygenase inhibitor NPPB, but not by daphnetin (an inhibitor of protein kinases) nor by genistein (an inhibitor of tyrosine kinases). COL10A1 expression was also suppressed by the specific cyclooxygenase-1 (COX-1: SC-560) and 5-lipoxygenase (5-LOX: MK-866) inhibitors, but not by COX-2 (COX-2 inhibitor 2) and 12-LOX (baicalein) inhibitors. Finally, the incubation of MSCs on PPE:N surfaces inhibited the expression of COX-1 while 5-LOX was not expressed in these cells. Taken together, these results indicate that PPE:N surfaces inhibit COL10A1 expression via the suppression of COX-1.

Transglutaminases and disease: lessons from genetically engineered mouse models and inherited disorders

The human transglutaminase (TG) family consists of a structural protein, protein 4.2, that lacks catalytic activity, and eight zymogens/enzymes, designated factor XIII-A (FXIII-A) and TG1-7, that catalyze three types of posttranslational modification reactions: transamidation, esterification, and hydrolysis. These reactions are essential for biological processes such as blood coagulation, skin barrier formation, and extracellular matrix assembly but can also contribute to the pathophysiology of various inflammatory, autoimmune, and degenerative conditions. Some members of the TG family, for example, TG2, can participate in biological processes through actions unrelated to transamidase catalytic activity. We present here a comprehensive review of recent insights into the physiology and pathophysiology of TG family members that have come from studies of genetically engineered mouse models and/or inherited disorders. The review focuses on FXIII-A, TG1, TG2, TG5, and protein 4.2, as mice deficient in TG3, TG4, TG6, or TG7 have not yet been reported, nor have mutations in these proteins been linked to human disease.

Beta1 integrin deficiency results in multiple abnormalities of the knee joint

The lack of beta1 integrins on chondrocytes leads to severe chondrodysplasia associated with high mortality rate around birth. To assess the impact of beta1 integrin-mediated cell-matrix interactions on the function of adult knee joints, we conditionally deleted the beta1 integrin gene in early limb mesenchyme using the Prx1-cre transgene. Mutant mice developed short limbed dwarfism and had joint defects due to beta1 integrin deficiency in articular regions. The articular cartilage (AC) was structurally disorganized, accompanied by accelerated terminal differentiation, altered shape, and disrupted actin cytoskeleton of the chondrocytes. Defects in chondrocyte proliferation, cytokinesis, and survival resulted in hypocellularity. However, no significant differences in cartilage erosion, in the expression of matrix-degrading proteases, or in the exposure of aggrecan and collagen II cleavage neoepitopes were observed between control and mutant AC. We found no evidence for disturbed activation of MAPKs (ERK1/2, p38, and JNK) in vivo. Furthermore, fibronectin fragment-stimulated ERK activation and MMP-13 expression were indistinguishable in control and mutant femoral head explants. The mutant synovium was hyperplastic and frequently underwent chondrogenic differentiation. beta1-null synoviocytes showed increased proliferation and phospho-focal adhesion kinase expression. Taken together, deletion of beta1 integrins in the limb bud results in multiple abnormalities of the knee joints; however, it does not accelerate AC destruction, perturb cartilage metabolism, or influence intracellular MAPK signaling pathways.

Transglutaminase 2 cross-linking of matrix proteins: biological significance and medical applications

This review summarises the functions of the enzyme tissue transglutaminase (TG2) in the extracellular matrix (ECM) both as a matrix stabiliser through its protein cross-linking activity and as an important cell adhesion protein involved in cell survival. The contribution of extracellular TG2 to the pathology of important diseases such as cancer and fibrosis are discussed with a view to the potential importance of TG2 as a therapeutic target. The medical applications of TG2 are further expanded by detailing the use of transglutaminase cross-linking in the development of novel biocompatible biomaterials for use in soft and hard tissue repair.

Factor XIIIA mobilizes transglutaminase 2 to induce chondrocyte hypertrophic differentiation

Two transglutaminases (TGs), factor XIIIA (FXIIIA) and TG2, undergo physiologic upregulation in growth plate hypertrophic chondrocytes, and pathological upregulation in osteoarthritic cartilage. Externalization of guanine-nucleotide-bound TG2 drives chondrocyte maturation to hypertrophy, a state linked to matrix remodeling and calcification. Here, we tested the hypothesis that FXIIIA also promotes hypertrophic differentiation. Using human articular chondrocytes, we determined that extracellular FXIIIA induced chondrocyte hypertrophy associated with rapid movement of TG2 to the cell surface. Site-directed mutagenesis revealed that FXIIIA Pro37 bordering the thrombin endoproteolytic Arg38-Gly39 site, but not intrinsic TG catalytic activity, were necessary for FXIIIA to induce chondrocyte hypertrophy. TGs have been demonstrated to interact with certain integrins and, during osteoarthritis (OA), alpha1beta1 integrin is upregulated and associated with hypertrophic chondrocytes. FXIIIA engaged alpha1beta1 integrin in chondrocytes. Antibody crosslinking of alpha1beta1 integrin mobilized TG2. Conversely, an alpha1beta1-integrin-specific blocking antibody inhibited the capacity of FXIIIA to induce TG2 mobilization to the cell surface, phosphorylation of p38 MAP kinase, and chondrocyte hypertrophy. Our results identify a unique functional network between two cartilage TG isoenzymes that accelerates chondrocyte maturation without requirement for TG-catalyzed transamidation by either TG.

Expression of integrin subunits in the herniated intervertebral disc

Integrins are a class of cell adhesion molecules that regulate interactions between cells and their extracellular matrix (ECM). Several specific integrin receptors have been identified in intervertebral discs, including the fibronectin-binding integrin receptors alpha(5) beta(1), alpha(v) beta(3) and the collagen-binding integrin receptors alpha(1) beta(1), alpha(2) beta(1), and, alpha(v) beta(1). But the integrins expressions in degenerated intervertebral discs are still unknown. In our study, the expressions of alpha(1), alpha(2), alpha(5), alpha(v), beta(1), beta(3) integrin subunits, collagens, and fibronectin in normal and herniated intervertebral discs of human were determined. Specimens of human lumbar intervertebral discs were divided into 3 groups: normal discs (n = 10), protrusion (n = 15), and extrusion (n = 15). Real-time quantitative reverse transcription polymerase chain reaction (RT-PCR) and immunoprecipitation were used to evaluate the alpha(1), alpha(2), alpha(5),alpha(v), beta(1), and beta(3) integrin subunits messenger ribonucleic acid (mRNA) and protein expressions. RT-PCR was also performed to measure the mRNA level of collagen I, collagen II, and fibronectin. The expressions of alpha(5) and beta(1) subunits were increased in herniated discs, especially in the discs of extrusion. But as to alpha(1), alpha(2), alpha(v) and beta(3), their expressions had no difference among the discs. Fibronectin, whose binding integrin receptor was alpha(5) beta(1) was also increased. And in herniated discs, the collagen I was increased, but the collagen II was decreased. The expressions of some integrin subunits were increased in herniated discs. These results may be attributed to the interaction between cells and the ECM in the process of degeneration.