Two types of transglutaminase in the PC12 pheochromocytoma cell line. Stimulation by sodium butyrate

Transglutaminases are a class of enzymes capable of covalently cross-linking both intracellular and extracellular proteins. The activity of tissue transglutaminase is known to decrease precipitously following neoplastic transformation, and it has been hypothesized that transglutaminase may be involved in growth regulation. We have found that the differentiation promoter sodium butyrate is able to cause a marked increase in transglutaminase activity in PC12 pheochromocytoma cells in a time- and dose-dependent manner. This increased transglutaminase activity is associated with growth arrest, as well as with striking morphological changes including increased cell adhesion. The transglutaminase induced by sodium butyrate appears to be tissue transglutaminase, based on its cytosolic localization, thermal lability at basic pH, and elution profile on anion-exchange chromatography. Untreated PC12 cells contain only small amounts of transglutaminase which resembles epidermal transglutaminase, an enzyme previously described only in skin. In contrast to sodium butyrate, nerve growth factor did not stimulate tissue transglutaminase in PC12 cells, although it, too, caused growth arrest. It is hypothesized that transglutaminase may be involved in certain morphological changes accompanying cellular differentiation and neoplastic transformation, rather than in growth regulation per se.     

Ca2+: a stabilizing component of the transglutaminase activity of Galphah (transglutaminase II)

Galphah (transglutaminase type II; tissue transglutaminase) is a bifunctional enzyme with transglutaminase (TGase) and guanosine triphosphatase (GTPase) activities. The GTPase function of Galphah is involved in hormonal signaling and cell growth while the TGase function plays an important role in apoptosis and in cross-linking extracellular and intracellular proteins. To analyze the regulation of these dual enzymatic activities we examined their calcium-dependence and thermal stability in enzymes from several cardiac sources (mouse heart, and normal, ischemic and dilated cardiomyopathic human hearts). The GTP binding activity of Galphah was markedly inhibited by Ca2+ whereas the TGase activity was strongly stimulated, suggesting that Ca2+ acts as a regulator, switching Galphah from a GTPase to a TGase. The TGase function of Galphah of both mouse and human hearts was more thermostable in the presence of Ca2+.     

In vitro modification of betaine-homocysteine S-methyltransferase by tissue-type transglutaminase

Transglutaminases catalyze the cross-linking and amine incorporation of proteins, and are implicated in various biological phenomena. To elucidate the physiological roles of transglutaminase at the molecular level, we need to identify its physiological protein substrates and clarify the relationship between transglutaminase modification of protein substrates and biological responses. Here we examined whether betaine-homocysteine S-methyltransferase (BHMT: EC 2.1.1.5) can be a substrate of tissue-type transglutaminase by in vitro experiments using porcine liver BHMT and guinea pig liver transglutarninase. Guinea pig liver transglutaminase incorporated 5-(biotinamido) pentylamine and [3H] histamine into BHMT in a time-dependent manner. Putrescine and spermidine also seemed to be incorporated into BHMT by transglutaminase. In the absence of the primary amines, BHMT subunits were cross-linked intra- and intermolecularly. BHMT activity was decreased significantly through the cross-linking by transglutaminase. Histamine incorporation slightly reduced the BHMT activity. Peptide fragments of BHMT containing the glutamine residues reactive for transglutaminase reaction were isolated through biotin labelling, proteinase digestion, biotin-avidin a affinity separation, and reverse phase HPLC. The results of amino acid sequence analyses of these peptides and sequence homology alignment with other mammalian liver BHMT subunits showed that these reactive glutamine residues were located in the region near the carboxyl terminal of porcine BHMT subunit. These results suggested that the liver BHMT can be modified by tissue-type transglutaminase and its activity is regulated repressively by the modification, especially by the cross-linking. This regulatory reaction might be involved in the regulation of homocysteine metabolism in the liver.     

Characterization of transglutaminase activity in rabbit tracheal epithelial cells. Regulation by retinoids

Rabbit tracheal epithelial cells undergo terminal cell division, start to express a squamous phenotype, and form cross-linked envelopes when reaching the plateau phase of the growth curve. This terminal differentiation is accompanied by a 20-30-fold increase in the activity of the cross-linking enzyme transglutaminase. This activity is found almost solely in the particulate fraction of homogenized cells and can be solubilized by nonionic detergents. This transglutaminase crossreacts with a monoclonal antibody raised against type I transglutaminase, but does not react with an antiserum against type II transglutaminase. The tracheal transglutaminase contains a protein subunit of approximately 92 kDa. The omission of epidermal growth factor from the medium or the addition of fetal bovine serum, conditions that induce terminal cell division and expression of a squamous phenotype, enhance transglutaminase activity. High calcium concentrations only stimulate transglutaminase activity after the cells become committed to terminal cell division. Retinoids, which inhibit the expression of the squamous phenotype but not terminal cell division, inhibit the enhancement in transglutaminase activity induced by either confluency or serum, indicating that this enzyme activity is under the control of retinoids. Some retinoids are active at concentrations as low as 10(-12) M. The ability of retinoids to inhibit transglutaminase activity correlates well with their capacity to bind to the retinoic acid-binding protein. Our results show that the increase in transglutaminase activity correlates with the induction of the terminal differentiated phenotype and suggest that this enzyme can function as a marker for this program of differentiation of rabbit tracheal epithelial cells in culture. Our results identify the transglutaminase as type I transglutaminase and are in agreement with the concept that this transglutaminase is involved in the formation of cross-linked envelopes.     

Lipoprotein (a) is a substrate for factor XIIIa and tissue transglutaminase.

The mechanisms which mediate deposition of lipoprotein (a) (Lp(a)), an atherogenic lipoprotein particle, onto the vessel wall and cell surfaces are unknown. An irreversible deposition of Lp(a) may require the presence of enzymes that catalyze its binding to surface-oriented structures. Transglutaminases catalyze cross-linking of proteins as well as incorporation of primary amines into protein substrates. We studied whether tissue transglutaminase and/or activated Factor XIII (plasma derived or recombinant FXIIIa) incorporate primary amines into Lp(a). In the presence of Ca2+, Factor XIIIa and tissue transglutaminase catalyze incorporation of monodansylcadaverine or [14C]putrescine into purified Lp(a) in a specific and time-dependent manner. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis demonstrated that monodansylcadaverine became incorporated into the apo(a) portion of Lp(a). Lp(a) purified from five different donors showing different apo(a) phenotypes were substrates for tissue transglutaminases (TG). Western blot analysis confirmed that apo(a) was the major monodansylcadaverine carrying protein moiety of Lp(a). Tissue TG also extensively cross-linked the apo(a) portion of the Lp(a) particle. Characterization of the specificity of tissue TG showed that fibronectin, alpha 2-plasmin inhibitor, and apo(a) could be readily labeled with monodansylcadaverine by tissue TG, but other proteins including low density lipoprotein, IgG, alpha 1-proteinase inhibitor, and albumin showed poor or no reactivity. Direct comparison of Lp(a) with low density lipoprotein showed that apoB 100 was a poor substrate for transglutaminases. Recombinant apolipoprotein (a) proved to be an excellent substrate for TGs in that 1 mol of recombinant apolipoprotein (a) incorporated as much as 15 mol of [14C]putrescine, which corresponded to five times the amount of amine incorporated into Lp(a). The susceptibility of Lp(a) to transglutaminases suggests a mechanism whereby the interaction of Lp(a) with surface receptors and other surface oriented structures could be enzymatically altered.     

Lysosomotropic agents and inhibitors of cellular transglutaminase stimulate dome formation, a differentiated characteristic of MDCK kidney epithelial cell cultures

Dome formation is a manifestation of transepithelial fluid transport in cell culture, a differentiated characteristic of transporting epithelia. A dramatic increase in numbers of domes in confluent MDCK kidney epithelial cell cultures was noted after addition of Friend cell inducers such as hexamethylane bisacetamide (HMBA) (Lever, 1979b). In the present study, we show that primary amines such as methylamine, ethylamine, and dansyl cadaverine also stimulate dome formation. These compounds largely prevented the marked decrease in numbers of spontaneously occurring domes which occurred when cultures were switched from medium containing 10% serum to medium containing serum concentrations below 0.2%. Many of these primary amines are not only lysosomotropic agents but also potent inhibitors of transglutaminase activity when assayed in MDCK cell extracts, at concentrations correlating with those effective in stimulation of dome formation. Other lysosomotropic agents such as chloroquine and secondary and tertiary amines stimulated dome formation yet did not inhibit transglutaminase. Induction of domes by HMBA differed in several properties from that stimulated by amines and did not involve fluctuations in transglutaminase activity. These findings suggest that lysosomal functions modulate serum stimulation of dome formation in epithelial cells by a pathway distinct from that triggered by HMBA.     

"Tissue" transglutaminase in animal development

The "tissue" transglutaminase is a multifunctional enzyme that in its cross-linking configuration catalyzes Ca2+ -dependent reactions resulting in post-translational modification of proteins by establishing epsilon(gamma-glutamyl) lysine cross-links and/or covalent incorporation of biogenic amines (di- and poly-amines and histamine) into proteins. Several laboratories have shown that in Vertebrates, "tissue" transglutaminase (tTG) gene expression specifically characterizes cells undergoing apoptosis or programmed cell death (PCD). The Ca2+ -dependent activation of this enzyme leads to the formation of detergent-insoluble cross-linked protein polymers in cells undergoing PCD. This insoluble protein scaffold could stabilize the integrity of the dying cells before their clearance by phagocytosis, preventing the non-specific release of harmful intracellular components (e.g. lysosomal enzymes, nucleic acids, etc.) and consequently inflammatory responses and scar formation in bystander tissues. In this review we attempt to present an overview of the current knowledge on tTG expression and regulation in animal reproduction and development. The data available so far further strengthen the relationship existing between tTG expression and the induction of PCD.     

Expression of tissue transglutaminase in Balb-C 3T3 fibroblasts: effects on cellular morphology and adhesion

Tissue transglutaminase is a cytosolic enzyme whose primary function is to catalyze the covalent cross-linking of proteins. To investigate the functions of this enzyme in physiological systems, we have established lines of Balb-C 3T3 fibroblasts stably transfected with a constitutive tissue transglutaminase expression plasmid. Several cell lines expressing high levels of catalytically active tissue transglutaminase have been isolated and characterized. Transglutaminase-transfected cells showed morphologic features quite distinct from their nontransfected counterparts. Many of the cells showed an extended and very flattened morphology that reflected increased adhesion of the cells to the substratum. Other cells, particularly those showing the highest levels of intracellular transglutaminase expression, showed extensive membrane blebbing and cellular fragmentation. The results of these experiments suggest that the induction and activation of tissue transglutaminase may contribute both to changes in cellular morphology and adhesiveness.     

Isodityrosine cross-linking mediates insolubilization of cell walls in Chlamydomonas.

Enzymatic removal of the cell wall induces vegetative Chlamydomonas reinhardtii cells to transcribe wall genes and synthesize new hydroxyproline-rich glycoproteins (HRGPs) related to the extensins found in higher plant cell walls. A cDNA expression library made from such induced cells was screened with antibodies to an oligopeptide containing the (SP)x repetitive domains found in Chlamydomonas wall proteins. One of the selected cDNAs encodes an (SP)x-rich polypeptide that also displays a repeated YGG motif. Ascorbate, a peroxidase inhibitor, and tyrosine derivatives were shown to inhibit insolubilization of both the vegetative and zygotic cell walls of Chlamydomonas, suggesting that oxidative cross-linking of tyrosines is occurring. Moreover, insolubilization of both walls was concomitant with a burst in H2O2 production and in extracellular peroxidase activity. Finally, both isodityrosine and dityrosine were found in hydrolysates of the insolubilized vegetative wall layer. We propose that the formation of tyrosine cross-links is essential to Chlamydomonas HRGP insolubilization.     

The comparative ability of plasma and tissue transglutaminases to use collagen as a substrate

Heat denatured type I and type III calf skin collagen were found to be substrates for guinea pig liver transglutaminase (R-glutaminyl-peptide:amine gamma-glutamyl-yltransferase, EC 2.3.2.13) but not for active plasma factor XIII (factor XIIIa). Liver transglutaminase was shown to catalyse incorporation of 14C-putrescine into subunits of denatured collagen of both types, cross-linking of the latter into high molecular weight polymers and their co-cross-linking to fibrin and fibrinogen. Factor XIIIa is inactive in these respects. None of these reactions was catalysed by liver transglutaminase and plasma factor XIIIa when nondenatured collagens both soluble or in the forms of reconstituted fibrils served as substrates. Some cross-linking of cleavage products of collagen type I (obtained by treatment with collagenase from human neutrophiles) was induced by liver transglutaminase and factor XIIIa. The results indicate that although appropriate glutamine and lysine residues for a epsilon-(gamma-glutamine) lysine cross-linked formation are present in collagen, the native conformation of collagen prevents the action of liver transglutaminase and factor XIIIa.     

Effect of tissue transglutaminase on the solubility of proteins containing expanded polyglutamine repeats

The expansion of a polyglutamine (polyQ) domain in neuronal proteins is the molecular genetic cause of at least eight neurodegenerative diseases. Proteins with a polyQ domain that is greater than 40 Q (Q40) residues form insoluble intranuclear and cytoplasmic inclusions. Expanded polyQ proteins self-associate by non-covalent interactions and become insoluble. They can also be covalently cross-linked by tissue transglutaminase (TTG), a calcium-dependent enzyme present in cells throughout the nervous system. However, it remains unclear whether TTG cross-linking directly contributes to the insolubility of the expanded polyQ proteins. Using an in vitro solubility assay, we found TTG cross-linked Q62 monomers into high molecular weight soluble complexes in a calcium-dependent reaction. Inhibition of TTG cross-linking by primary amine substrates including putrescine and biotinylated pentylamine antagonized TTG's ability to form soluble complexes. In contrast, primary amines (histamine and lysine) that were less effective inhibitors of TTG cross-linking did not inhibit Q62 from becoming insoluble. In summary, TTG can increase the solubility of expanded polyQ proteins by catalyzing intermolecular cross-links. This demonstrates directly that TTG will reduce the ability of expanded polyQ proteins from becoming insoluble. Furthermore, the effectiveness of a primary amine substrate at inhibiting formation of insoluble inclusions may be related to their ability to inhibit intermolecular cross-linking by TTG.     

Studies on the mechanisms of the antibacterial action of ortho-phthalaldehyde

The reaction of ortho-phthalaldehyde (OPA) with amino acids and proteins was investigated as a possible mode of action. Bacterial pellets (obtained by centrifugation) changed colour after exposure to OPA. These colours were more intense at alkaline than acidic pH. Acidic and alkaline OPA reacted with primary amino acids to form coloured products. The reaction rate accelerated with increasing pH. OPA increased the optical density of bacterial cell suspensions (an indication of protein coagulation or microbial surface or other changes in the opacity of cell constituents). The inhibition of ethylenediaminetetraacetic acid- and sodium lauryl sulphate-induced lysis was not as great as for glutaraldehyde (GTA), possibly indicating less cross-linking of amines. Interactions with primary amino groups of the outer envelope or cell wall probably play a part in the action of OPA but the level of cross-linking associated with the outer membrane does not appear to be as extensive as that of GTA. The aromatic component might allow OPA to penetrate the outer layers of cells, thus helping to explain the very high activity of OPA against Gram-negative vegetative organisms even though the degree of cross-linking seems to be less than that seen with GTA. Thus, OPA reacts strongly with primary amines and stabilizes, to some extent, the outer membrane and cell walls of vegetative organisms and this probably accounts for part, but not necessarily all, of its lethal action.     

The use of Fluoresceincadaverine for detecting amine acceptor protein substrates accessible to active transglutaminase in living cells

The use of Fluoresceincadaverine as a primary amine donor for detecting the endogenous substrates for active transglutaminase in living cells was studied. Fluoresceincadaverine was found to be suitable for labelling cells in culture as it did not induce cytotoxicity when used at 0.5 mSmD in culture media and diffused throughout the cell. After appropriate fixation using methanol, Fluoresceincadaverine-labelled cells were observed by direct fluorescence microscopy, allowing visualization of the substrates for active transglutaminase. Simultaneous detection of transglutaminase and of Fluoresceincadaverine incorporated into proteins strongly suggested that cytosolic transglutaminase was inactive in these living cells. However, transglutaminase co- distributed with Fluoresceincadaverine-labelled structures, which resembled a lattice. Fluoresceincadaverine-labelled proteins detected by Western blotting using an anti-Fluorescein antibody showed that, in living cells, the major transglutaminase substrate migrated at an apparent molecular weight of 220 kDa, as does fibronectin. Fibronectin was found to co-distribute with Fluoresceincadaverine-labelled lattice. This confirmed that these lattice structures were extracellular and, therefore, that transglutaminase is in an active form in this compartment. This opportunity to perform morphological and biochemical analyses in the search for transglutaminase substrates in living cells should help in determining the specific function of transglutaminases in a particular cell type as well as in universal cellular events, such as apoptosis or cell growth. This revised version was published online in November 2006 with corrections to the Cover Date.     

Penetration of C8 and C9 in the C5b-9 complex across the erythrocyte membrane into the cytoplasmic space

We have developed a technique in which transglutaminase is used to measure the penetration of terminal complement proteins across the erythrocyte membrane into the cytoplasmic space. Penetration of a given terminal complement protein into the cytoplasmic space was assessed by labeling the protein of interest with radioactive iodine, forming the complement channel using the labeled protein, adding transglutaminase to only one side of the membrane, and allowing the enzyme to cross-link the susceptible proteins on that side of the membrane. Cross-linking was assessed by measuring the increase in molecular weight of the appropriate molecule on sodium dodecyl sulfate gels under reducing conditions. The results of these experiments indicate that C8 and C9 are rapidly cross-linked to high molecular weight from either the interior or the exterior of the membrane. In order to determine whether the cross-linking mediated by enzyme on the interior was occurring from within the ghosts and not via enzyme that had leaked into the extracellular medium, experiments were performed with dimethylcasein in the extracellular medium. In the presence of this protein, cross-linking of C8 and C9 from outside was negligible. Hence, if cross-linking occurs when transglutaminase is trapped inside the ghosts, it cannot be due to leakage of enzyme, but must be attributable to cross-linking from the inside. The results show that C9 definitely penetrated across the membrane into the intracellular space. With respect to C8, statistical evaluation indicates that C8 probably penetrated into the intracellular space.     

Plasma membrane transglutaminase and cornified envelope competence in cultured human keratinocytes

When confluent cultures of the transformed human keratinocyte line SV-K14 are shifted to serum-free medium the cells achieve, within 4 days, the ability to synthesize a cornified envelope after challenge with the Ca2+ ionophore A23187. During these 4 days the enzyme transglutaminase (EC 2.3.2.13), which catalyses the cross-linking of different envelope precursor proteins, is partially transferred from the cytosolic pool into the plasma membrane. The association of the enzyme with the plasma membrane proves to be an essential step in the envelope formation since a direct correlation between plasma membrane-bound transglutaminase and envelope competence is observed. Retinoids block the insertion of the enzyme and therefore prevent envelope formation.     

Expression of tissue transglutaminase in the developing chicken limb is associated both with apoptosis and endochondral ossification

The cross-linking enzyme tissue transglutaminase (tTG) participates in a variety of cellular functions. To assess its contribution to extracellular and intracellular processes during development we cloned the cDNA for chicken heart tissue transglutaminase and localized the sites of transglutaminase expression by in situ hybridization and immunohistochemistry. Compared with the chicken red blood cell transglutaminase cDNA, the heart cDNA encodes a transglutaminase with an amino-terminal truncation. The truncated enzyme retains full catalytic activity and is GTP-inhibitable. Tissue transglutaminase expression was observed in developmentally transient structures in embryonic chicken limb at day 7.5 of incubation suggesting that its expression is dynamically regulated during limb morphogenesis. The major morphogenetic events of the limb associated with transglutaminase expression were cartilage maturation during skeletal development, interdigital apoptosis, and differentiation of skeletal muscle. Maturation of the cartilage during endochondral ossification was characterized by intra- and extracellular transglutaminase accumulation in the zone of hypertrophic chondrocytes. Only intracellular enzyme could be detected in mesenchymal cells of the prospective joints, in apoptotic cells of the interdigital web, and in skeletal muscle myoblasts. An apparently constitutive expression of tissue transglutaminase was found in vascular endothelial cells corresponding to the adult expression pattern. The dynamic pattern of transglutaminase expression during morphogenesis suggests that tissue remodeling is a major trigger for transglutaminase induction.     

Iodination of Myxococcus xanthus during development

Intact cells of Myxococcus xanthus were iodinated with [125I]lactoperoxidase to permit examination of the surface components accessible to labeling during cell development. Vegetative cells, starved on a defined solid medium, aggregated, formed fruiting bodies, and produced myxospores. Cells collected at different stages were iodinated, and their proteins were analyzed by one- and two-dimensional electrophoresis and autoradiography. One-dimensional electrophoresis revealed six iodinated bands in vegetative cell extracts. During development, 10 radioactive bands were detected, 4 of which migrated to the same positions as those of vegetative cells. Only six bands were detected in purified, labeled myxospores. Of these, one band possessed mobility similar to that of labeled vegetative cell proteins, whereas the other bands possessed mobility similar to that detected in developing cells. Analysis of two-dimensional gels indicated that at least 14 proteins were iodinated in vegetative cells, one of which was intensely labeled (protein b). Another of the proteins (protein a) was labeled throughout development. During development, about 30 proteins were iodinated and the prominently labeled ones were designated c, d, e, f, and g. The latter two (proteins f and g) were not detected in purified, iodinated myxospores. The data indicated a pronounced change in surface structure during development; some of the change may be involved in cellular interaction during aggregation.     

Overexpressed transglutaminase 5 triggers cell death

Summary. Transglutaminases are a class of nine different proteins involved in many biological phenomena such as differentiation, tissue repair, endocytosis. Transglutaminase 5 was originally cloned from skin keratinocytes, and a partial biochemical characterization showed its involvment in skin differentiation. Here we demonstrate that transglutaminase 5 is able to induce cell death when intracellularly overexpressed. Transfected cells show enzymatic activity, as demonstrated by fluoresceincadaverine staining. Transfected cells died due to the formation of hypodiploid DNA content, indicating the induction of cell death under these pharmacological conditions. We also show that the primary sequence of transglutaminase 5 contains GTP binding domains which are similar to those in transglutaminase 2. This raises the possibility that transglutaminase 5 is regulated by GTP in a similar fashion to transglutaminase 2.     

Activation of transglutaminase in mu-calpain null erythrocytes

Intracellular transglutaminases (protein-glutamine: amine gamma-glutamyltransferase, EC 2.3.2.13) are calcium-dependent thiol enzymes that catalyze the covalent cross-linking of proteins, including those in the erythrocyte membrane. Several studies suggest that the activation of some transglutaminases is positively regulated by the calcium-dependent cysteine protease, mu-calpain. Using mu-calpain null (Capn1(-/-)) mouse erythrocytes, we demonstrate that the activation of soluble as well as membrane-bound forms of transglutaminase (TG2) in mouse erythrocytes was independent of mu-calpain. Also, the absence of mu-calpain or any detectable cysteine protease did not affect the transglutaminase activity in the erythrocyte lysate. Our studies also identify physiological substrates of mu-calpain in the erythrocyte membrane and show that their cleavage has no discernible effect on the transglutaminase mediated cross-linking of membrane proteins. Taken together, these data suggest the existence of a calpain-independent mechanism for the activation of transglutaminase 2 by calcium ions in the mouse erythrocytes and presumably also in non-erythroid cells.     

Detection and characterization, using fluoresceincadaverine, of amine acceptor protein substrates accessible to active transglutaminase expressed by rabbit articular chondrocytes

The purpose of this study was to investigate the implication of transglutaminases in the biology of articular chondrocytes. Transglutaminase activity measurements performed on cell lysates showed that a transglutaminase was present in chondrocytes in primary culture and that it was strongly activated by limited proteolysis. In chondrocytes dedifferentiated by subculture or retinoic acid treatment, this transglutaminase appeared to be downregulated, while type II transglutaminase expression was induced. However, protein levels, mRNA steady-state levels or transglutaminase activity in whole-cell lysates do not necessarily reflect the activity present in living cells, as it is strongly regulated. Therefore, Fluoresceincadaverine, a fluorescent polyamine, was used for detecting amine acceptor protein substrates accessible to active transglutaminase in living cells. After incubation of chondrocytes with Fluoresceincadaverine, dedifferentiated cells exhibited an extracellula r labelling, while chondrocytes in primary culture did not, unless thrombin was added to the culture medium. In contrast, Fluoresceincadaverine labelling was not detected in the cytosol, although the transglutaminases were also partly cytosolic. By confocal microscopy and Western blot analysis of labelled cells in culture, fibronectin was shown to be the main substrate for both transglutaminases. The transglutaminases present in articular chondrocytes may, therefore, contribute to the organization and the stabilization of their extracellular matrix.