Immunochemical analyses of human plasma fibronectin-cytosolic transglutaminase interactions

A crucial sequence for transglutaminase type 2 extracellular trafficking in renal tubular epithelial cells lies in its N-terminal beta-sandwich domain

Transglutaminase type 2 (TG2) catalyzes the formation of an ε-(γ-glutamyl)-lysine isopeptide bond between adjacent peptides or proteins including those of the extracellular matrix (ECM). Elevated extracellular TG2 leads to accelerated ECM deposition and reduced clearance that underlie tissue scarring and fibrosis. The extracellular trafficking of TG2 is crucial to its role in ECM homeostasis; however, the mechanism by which TG2 escapes the cell is unknown as it has no signal leader peptide and therefore cannot be transported classically. Understanding TG2 transport may highlight novel mechanisms to interfere with the extracellular function of TG2 as isoform-specific TG2 inhibitors remain elusive. Mammalian expression vectors were constructed containing domain deletions of TG2. These were transfected into three kidney tubular epithelial cell lines, and TG2 export was assessed to identify critical domains. Point mutation was then used to highlight specific sequences within the domain required for TG2 export. The removal of β-sandwich domain prevented all TG2 export. Mutations of Asp(94) and Asp(97) within the N-terminal β-sandwich domain were identified as crucial for TG2 externalization. These form part of a previously identified fibronectin binding domain ((88)WTATVVDQQDCTLSLQLTT(106)). However, siRNA knockdown of fibronectin failed to affect TG2 export. The sequence (88)WTATVVDQQDCTLSLQLTT(106) within the β-sandwich domain of TG2 is critical to its export in tubular epithelial cell lines. The extracellular trafficking of TG2 is independent of fibronectin.

Mucosal tissue transglutaminase expression in celiac disease

Tissue transglutaminase (tTG) plays an important role in celiac disease pathogenesis and antibodies to tTG are a diagnostic marker of gluten-sensitive enteropathy. The aim of this study was to investigate the localization of tTG in the duodenal mucosa in control tissues and in different histological stages of celiac disease by using a commercial and a novel set of anti-tTG monoclonal antibodies, to see whether this assessment can be useful for diagnostic purpose. The distribution of tTG was firstly evaluated in 18 untreated celiac patients by using a commercial monoclonal antibody (CUB7402) against tissue transglutaminase enzyme and directed against the loop-core region of the enzyme. Thereafter, in further 30 untreated celiac patients we employed three newly characterized anti-tTG monoclonal antibodies produced against recombinant human-tTG. The epitopes recognized are located in three distinct domains of the protein corresponding to the core, C1 and C2 protein structure. Eleven age- and sex-matched patients with chronic duodenitis acted as controls. All subjects underwent upper endoscopy to obtain biopsy samples from the duodenum. Overall, we found that (i) tTG is equally expressed in CD at different stages of disease; (ii) tTG is expressed, at similar level, in CD and controls with duodenitis. Assessment of tTG level in biopsy samples by immunohistochemical methods is not useful in the clinical diagnostic work-up of CD.

External GTP-bound transglutaminase 2 is a molecular switch for chondrocyte hypertrophic differentiation and calcification

Chondrocyte maturation to hypertrophy, associated with up-regulated transglutaminase 2 (TG2) expression, mediates not only physiologic growth plate mineralization but also pathologic matrix calcification and dys-regulated matrix repair in osteoarthritic articular cartilage. TG2-/- mouse chondrocytes demonstrate markedly inhibited progression to hypertrophic differentiation in response to both retinoic acid and the chemokine CXCL1. Here, our objectives were to test if up-regulated TG2 alone is sufficient to promote chondrocyte hypertrophic differentiation and to identify TG2 molecular determinants and potential downstream signals involved. TG2 activities, regulated by nucleotides and calcium, include cross-linking of cartilage matrix proteins, binding of fibronectin, and hydrolysis of GTP and ATP. Following transfection of TG2 site-directed mutants into chondrocytic cells, we observed that wild type TG2, and TG catalytic site and fibronectin-binding mutants promoted type X collagen expression and matrix calcification consistent with chondrocyte hypertrophic differentiation. In contrast, transfected mutants of TG2 GTP binding (K173L) and externalization (Y274A) sites did not stimulate chondrocyte hypertrophy. Recombinant TG2 treatment of bovine cartilage explants demonstrated that extracellular TG2 induced hypertrophy more robustly in the GTP-bound state, confirming an essential role of TG2 GTP binding. Finally, TG2 treatment induced type X collagen in a beta1 integrin-mediated manner, associated with rapid phosphorylation of both Rac1 and p38 kinases that were inhibited by mutation of the TG2 GTP binding site. In conclusion, externalized GTP-bound TG2 serves as a molecular switch for differentiation of chondrocytes to a hypertrophic, calcifying phenotype in a manner that does not require either TG2 transamidation activity or fibronectin binding.

Transglutaminases: nature's biological glues

Transglutaminases (Tgases) are a widely distributed group of enzymes that catalyse the post-translational modification of proteins by the formation of isopeptide bonds. This occurs either through protein cross-linking via epsilon-(gamma-glutamyl)lysine bonds or through incorporation of primary amines at selected peptide-bound glutamine residues. The cross-linked products, often of high molecular mass, are highly resistant to mechanical challenge and proteolytic degradation, and their accumulation is found in a number of tissues and processes where such properties are important, including skin, hair, blood clotting and wound healing. However, deregulation of enzyme activity generally associated with major disruptions in cellular homoeostatic mechanisms has resulted in these enzymes contributing to a number of human diseases, including chronic neurodegeneration, neoplastic diseases, autoimmune diseases, diseases involving progressive tissue fibrosis and diseases related to the epidermis of the skin. In the present review we detail the structural and regulatory features important in mammalian Tgases, with particular focus on the ubiquitous type 2 tissue enzyme. Physiological roles and substrates are discussed with a view to increasing and understanding the pathogenesis of the diseases associated with transglutaminases. Moreover the ability of these enzymes to modify proteins and act as biological glues has not gone unnoticed by the commercial sector. As a consequence, we have included some of the present and future biotechnological applications of this increasingly important group of enzymes.

Transglutaminase participates in the incorporation of latent TGFbeta into the extracellular matrix of aging articular chondrocytes

TGFbeta1 is a multifunctional peptide growth factor that promotes processes associated with age-related degenerative diseases in articular cartilage. Large quantities of TGFbeta1 are stored in cartilage extracellular matrix (ECM) in a latent form (LTGFbeta1), and yet little is known about the factors that participate in the incorporation of LTGFbeta1 into the highly specialized cartilage ECM. We previously demonstrated high levels of the protein cross-linking enzyme transglutaminase (TGase) in aging articular chondrocytes and showed that this enzyme participated in LTGFbeta1 activation. This work explores the hypothesis that extracellular TGase participates in LTGFbeta1 incorporation into ECM in aging chondrocytes. We studied the effects of TGase inhibitors on TGFbeta1 levels in ECM of old and young porcine articular chondrocytes. TGase inhibitors decreased the quantity of LTGFbeta1 in the ECM in old but not in young chondrocytes to 60-70% of control values (p<.05). Fibronectin, an extracellular TGase competitive substrate, also decreased LTGFbeta1 levels in ECM (p<.01). Levels of activated TGFbeta1 also decreased in the presence of TGase inhibitors, as did levels of latent TGFbeta binding protein 1 in the cell layer. Extracellular TGase activity was present in old but not young chondrocyte cultures. These findings support a role for extracellular TGase in the incorporation of LTGFbeta1 in the ECM of aging chondrocytes.

Analysis of tissue transglutaminase function in the migration of Swiss 3T3 fibroblasts: the active-state conformation of the enzyme does not affect cell motility but is important for its secretion

Increasing evidence suggests that tissue transglutaminase (tTGase; type II) is externalized from cells, where it may play a key role in cell attachment and spreading and in the stabilization of the extracellular matrix (ECM) through protein cross-linking. However, the relationship between these different functions and the enzyme's mechanism of secretion is not fully understood. We have investigated the role of tTGase in cell migration using two stably transfected fibroblast cell lines in which expression of tTGase in its active and inactive (C277S mutant) states is inducible through the tetracycline-regulated system. Cells overexpressing both forms of tTGase showed increased cell attachment and decreased cell migration on fibronectin. Both forms of the enzyme could be detected on the cell surface, but only the clone overexpressing catalytically active tTGase deposited the enzyme into the ECM and cell growth medium. Cells overexpressing the inactive form of tTGase did not deposit the enzyme into the ECM or secrete it into the cell culture medium. Similar results were obtained when cells were transfected with tTGase mutated at Tyr(274) (Y274A), the proposed site for the cis,trans peptide bond, suggesting that tTGase activity and/or its tertiary conformation dependent on this bond may be essential for its externalization mechanism. These results indicate that tTGase regulates cell motility as a novel cell-surface adhesion protein rather than as a matrix-cross-linking enzyme. They also provide further important insights into the mechanism of externalization of the enzyme into the extracellular matrix.

Involvement of tissue transglutaminase in the stabilisation of biomaterial/tissue interfaces important in medical devices

Tissue transglutaminase (tTG) has recently been established as a novel cell surface adhesion protein that binds with high affinity to fibronectin in the pericellular matrix. In this study, we have made use of this property to enhance the biocompatibility of poly(epsilon-caprolactone) (PCL), a biomaterial currently used in bone repair. Poly(epsilon-caprolactone) discs were first coated with fibronectin and then tTG. The surface localisation of the two proteins was confirmed using ELISA and the tTG shown to be active on the surface by incorporation of biotin cadaverine into the fibronectin coating. When human osteoblasts (HOBs) were seeded onto the coated polymer surfaces in serum free medium, the surface coated with fibronectin and then tTG showed an increase in the spreading of the cells as compared to the surface coated with fibronectin alone, when analysed using environmental scanning electron microscopy. The presence of tTG had no effect on HOB cell differentiation when analysed by determining alkaline phosphatase activity. The use of tTG as a novel adhesion protein in this way may therefore have considerable potential in forming a stable tissue/biomaterial interface for application in medical devices.

Protein crosslinking, tissue transglutaminase, alternative splicing and neurodegeneration

Increasing interest and awareness of protein aggregation as being implicated in neurodegenerative processes has developed in recent years. One novel mechanism for this may be transglutaminase (TGase)-mediated protein crosslinking, that is involved in a variety of natural processes ranging from the stabilization of fibrin clots to production of the epidermal cell envelope and the fluid barrier of the skin. TGases are also implicated in both function and dysfunction of the central (CNS) and peripheral (PNS) nervous systems. The most ubiquitously expressed member of the TGase family, known as tissue TGase (tTG) or TG2, which, in addition to catalyzing the production of epsilon-lysine to gamma-glutaminyl isodipeptide bonds, serves a dual function as the G-protein Galpha(h) and is both expressed and active in PNS and CNS. It differs from other members of the TGase gene family in this regard and may implicate it in 'switches' from life or trophic signaling to those associated with apoptosis. In this regard, recent data indicate that one or more TGases are involved in neurodegenerative disorders such as the Qn/CAG repeat disorders, as well as Alzheimer's and Parkinson's diseases. As do many genes, particularly those highly expressed in the CNS, tTG undergoes alternative processing. Elevated expression and alternative splicing, resulting in a short (S) isoform of tTG with more active crosslinking activity, are associated with increased neuronal loss in affected regions in the demented brain. Our recent and novel data indicate that tTG mRNA, protein, and TGase activity are elevated in certain neurodegenerative diseases, and are accompanied by transcription of this S splice variant that results in unregulated crosslinking, unique to neurodegenerative disorders.

Loss of tissue transglutaminase as a biomarker for prostate adenocarcinoma

BACKGROUND

Additional molecular tissue biomarkers for prostate carcinoma are needed to stratify patients with clinically suspicious findings, such as an elevated prostate specific antigen (PSA) with a negative biopsy, according to risk.

METHODS

Prostate tissues from 43 cancer cases and 47 controls with no evidence of cancer were labeled for transglutaminase by immunohistochemistry. Immunoreactivity was quantified using the Autocyte Pathology Workstation. In addition, quantitative fluorescence image analysis was used to compare transglutaminase concentrations in cells obtained by fine-needle aspiration from excised prostates. Loss of gene expression was evaluated by reverse transcriptase-polymerase chain reaction and growth with 5-azacytidine.

RESULTS

Visually, benign glands from controls generally expressed tissue transglutaminase, whereas regions with adenocarcinoma generally were negative. With quantitative immunohistochemistry, 41 of 43 adenocarcinoma of the prostate (CaP) cases expressed lower mean percentage areas positive for transglutaminase than did 30 of 30 benign prostatic hyperplasia (BPH) and 17 of 17 prostatitis cases (P < 0.0001; odds ratio [OR], 1577; 95% confidence interval (CI), 74-33, 820; relative risk [RR], 25; 95% CI, 6-95). Quantitative immunofluorescence of 3277 cells collected by FNA from 19 CaP cases and 645 cells from 5 cases of BPH showed that the mean content of transglutaminase was 93 femtograms (fg) for the CaP-derived cells and 138 fg for the BPH cells (P < 0.0001). Receiver operating curve analysis of the immunohistochemistry data showed an optimized threshold produced 95% sensitivity with 100% specificity. Growth of LNCaP cells with 5-azacytidine failed to stimulate transglutaminase expression, suggesting that loss of expression was likely not attributable to promoter methylation.

CONCLUSIONS

Measurements of transglutaminase on tissue sections provides additional diagnostic information that is potentially useful for risk assessment of patients with suspicious clinical findings, such as nodules or positive PSA and negative biopsies, without overdetecting disease.

Cell surface localization of tissue transglutaminase is dependent on a fibronectin-binding site in its N-terminal beta-sandwich domain

Increasing evidence indicates that tissue transglutaminase (tTG) plays a role in the assembly and remodeling of extracellular matrices and promotes cell adhesion. Using an inducible system we have previously shown that tTG associates with the extracellular matrix deposited by stably transfected 3T3 fibroblasts overexpressing the enzyme. We now show by confocal microscopy that tTG colocalizes with pericellular fibronectin in these cells, and by immunogold electron microscopy that the two proteins are found in clusters at the cell surface. Expression vectors encoding the full-length tTG or a N-terminal truncated tTG lacking the proposed fibronectin-binding site (fused to the bacterial reporter enzyme beta-galactosidase) were generated to characterize the role of fibronectin in sequestration of tTG in the pericellular matrix. Enzyme-linked immunosorbent assay style procedures using extracts of transiently transfected COS-7 cells and immobilized fibronectin showed that the truncation abolished fibronectin binding. Similarly, the association of tTG with the pericellular matrix of cells in suspension or with the extracellular matrix deposited by cell monolayers was prevented by the truncation. These results demonstrate that tTG binds to the pericellular fibronectin coat of cells via its N-terminal beta-sandwich domain and that this interaction is crucial for cell surface association of tTG.

The requirement for basement membrane antigens in the production of human epidermal/dermal composites in vitro

The importance of a dermal element when providing permanent wound cover for skin loss has become evident as the shortcomings of pure epidermal grafts are recognized. We are developing a skin composite formed from sterilized human de-epidermized acellular dermis, keratinocytes and fibroblasts with the ultimate aim of using this composite to cover full-thickness excised burn wounds. These composites can be prepared with or without basement membrane (BM) antigens initially present on the dermis. This study investigates the importance of retaining BM antigens on the dermis to the production and appearance of these composites in vitro. Skin composites prepared from dermis with BM antigens either present or absent initially were studied throughout 3 weeks. Composites with BM antigens present initially were significantly better than those initially lacking BM antigens in: (i) the degree of epithelial cell attachment to the underlying dermis (hemidesmosomes were seen only in the former); (ii) the morphology of the epithelial layer; (iii) the consistent presence of collagen IV and laminin and the increasing expression of tenascin; and (iv) the amount of soluble collagen IV and fibronectin detected in the conditioned media. We conclude that an initial BM antigen template is vital in this skin composite model for the attachment and differentiation of the epithelial layer and for the subsequent remodelling of the BM in vitro.

Characterization of the reciprocal binding sites on human alpha-thrombin and factor XIII A-chain

Solution- and solid-phase techniques were used to probe Factor XIII A-chain-alpha-thrombin interactions. Alpha-thrombin activated Factor XIII more efficiently (Km = 0.83 +/- 0.08 x 10(-7) M; V/K = 14.90 +/- 3.20 x 10(-3) min(-1)) than beta-thrombin (Km = 6.14 +/- 1.26 x 10(-7) M; V/K = 3.30 +/- 1.00 x 10(-3) min(-1)) or gamma-thrombin (Km = 6.25 +/- 1.15 x 10(-7) M; V/K = 3.00 +/- 0.80 x 10(-3) min(-1)). Immobilized FPR-alpha-thrombin bound plasma Factor XIII (Kd = 0.17 +/- 0.04 x 10(-7) M) > Factor XIIIa (Kd = 0.69 +/- 0.18 x 10(-7) M) > liver transglutaminase (Kd = 4.73 +/- 1.01 x 10(-7) M) > Factor XIII A-chain (Kd = 49.00 +/- 9.40 x 10(-7) M). FPR-alpha-thrombin and alpha-thrombin also bound immobilized Factor XIII A-chain with affinities inversely related to protease activity: maximal binding at 1.36 x 10(-7) M and 13.6 x 10(-7) M, respectively. Plasma Factor XIII, transglutaminase, and dithiothreitol competitively inhibited Factor XIII A-chain binding to FPR-alpha-thrombin: IC50 = 1.0 x 10(-7) M, 3.0 x 10(-6) M and 1.52 x 10(-4) M, respectively. Transglutaminase also inhibited Factor XIII binding to alpha-thrombin (IC50 = 2.0 x 10(-6) M). Thrombin-binding site was localized to G38-M731 fragment of Factor XIII A-chain, probably within homologous regions (N72-A493) of transglutaminase. R320-E579 of alpha-thrombin was Factor XIII A-chain binding site. Intra-B-chain disulfides in alpha-thrombin were essential for binding but not catalytic H363 or residues R382-N394 and R443-G475. These studies propose a structural basis for Factor XIII activation, provide a regulatory mechanism for Factor XIIIa generation, and could eventually help in the development of new structure-based inhibitors of thrombin and Factor XIIIa.

Keratinocytes contract human dermal extracellular matrix and reduce soluble fibronectin production by fibroblasts in a skin composite model

Composites of human de-epidermised acellular dermis and normal adult human keratinocytes and fibroblasts were examined for the ability of cells to contract these composites. Image analysis of the outline of the composites showed that, in this model, keratinocytes alone or in the presence of fibroblasts caused highly significant contraction (of the order of 25% by day 12). There was no significant contraction of the dermis with fibroblasts alone or in the absence of cells. The presence or absence of basement membrane antigens did not influence the effect of keratinocytes on dermal contraction. Analysis of the conditioned media from these composites showed that the greatest fibronectin production was seen with fibroblasts alone in the presence of basement membrane. Keratinocytes alone produced little fibronectin irrespective of the presence of the basement membrane. If keratinocytes were present with fibroblasts, however, then fibronectin production was significantly reduced both in the presence and absence of the basement membrane, indicating that keratinocytes modify dermal fibroblast extracellular matrix production. This study shows that while keratinocytes and fibroblasts are clearly influencing each other's activity in this human skin composite model, under the circumstances we describe it is the keratinocyte and not the fibroblast which causes contraction of the human de-epidermised acellular dermis.

Hierarchies in the binding of human factor XIII, factor XIIIa, and endothelial cell transglutaminase to human plasma fibrinogen, fibrin, and fibronectin

The affinities of Factor XIII (FXIII), Factor XIIIa (FXIIIa), and cellular transglutaminase (Tg) for fibrinogen (Fgn), fibrin (Fbn), and fibronectin (Fn) were compared using a solid-phase binding assay. Initial rates of binding were as follows: FXIII bound Fbn 3-fold more than Fgn. FXIII did not bind Fn till 20 min. Increasing the ligands concentrations and binding time, resulted in weak binding of FXIII to Fn. FXIIIa bound Fbn 2-fold more than Fgn and 28-fold more than Fn. Tg bound Fn approximately 130-fold more than either Fgn or Fbn. At equilibrium, the extent of binding was determined to be as follows: FXIII bound Fbn 3-15-fold more than Fgn and 8-fold more than Fn. FXIIIa bound Fgn and Fbn equally and 12-25-fold more than Fn. FXIIIa bound Fgn or Fbn 2-fold and 25-fold greater than FXIII-Fbn and FXIII-Fgn interactions, respectively. Tg bound about equally to Fgn and Fbn and 10-20-fold less than Fn. The Kds' for FXIIIa binding to Fn, Fgn, and Fbn were 100, 23, and 19 nM, respectively. The Kd for Tg binding to Fn was 6.5 nM. The binding hierarchies are: [Tg-Fn] > [FXIIIa-Fgn] = [FXIIIa-Fbn] > [FXIII-Fbn] > [FXIII-Fgn] = [FXIIIa-Fn] > [Tg-Fbn] = [Tg-Fgn] > [FXIII-Fn]. Such hierarchies could regulate the cross-linkings by FXIIIa and Tg during hemostasis, wound healing, and cell adhesion.