A Rapid Transglutaminase Assay for High-Throughput Screening Applications

A synthetic heparin mimetic that allosterically inhibits factor XIa and reduces thrombosis in vivo without enhanced risk of bleeding

BACKGROUND

Human factor XIa (FXIa) is an actively pursued target for development of safer anticoagulants. Our long-standing hypothesis has been that allosterism originating from heparin-binding site(s) on coagulation enzymes is a promising approach to yield safer agents.

OBJECTIVES

To develop a synthetic heparin mimetic as an inhibitor of FXIa so as to reduce clot formation in vivo but not carry high bleeding risk.

METHODS

We employed a gamut of methods involving synthetic chemistry, biophysical biochemistry, enzyme assays, blood and plasma coagulation assays, and in vivo thrombosis models in this work.

RESULTS

Sulfated chiro-inositol (SCI), a non-saccharide mimetic of heparin, was synthesized in three steps in overall yields of >50%. SCI inhibited FXIa with potency of 280 nmol/L and preferentially engaged FXIa's heparin-binding site to conformationally alter its active site. SCI inhibition of FXIa could be rapidly reversed by common antidotes, such as protamine. SCI preferentially prolonged plasma clotting initiated with recalcification, rather than thromboplastin, alluding to its intrinsic pathway-based mechanism. Human blood thromboelastography indicated good ex vivo anticoagulation properties of SCI. Rat tail bleeding and maximum-dose-tolerated studies indicated that no major bleeding or toxicity concerns for SCI suggesting a potentially safer anticoagulation outcome. FeCl3 -induced arterial and thromboplastin-induced venous thrombosis model studies in the rat showed reduced thrombus formation by SCI at 250 μg/animal, which matched enoxaparin at 2500 μg/animal.

CONCLUSIONS

Overall, SCI is a highly promising, allosteric inhibitor of FXIa that induces potent anticoagulation in vivo. Further studies are necessary to assess SCI in animal models mimicking human clinical indications.

The In Vitro Effects of Pentamidine Isethionate on Coagulation and Fibrinolysis

Pentamidine is bis-oxybenzamidine-based antiprotozoal drug. The parenteral use of pentamidine appears to affect the processes of blood coagulation and/or fibrinolysis resulting in rare but potentially life-threatening blood clot formation. Pentamidine was also found to cause disseminated intravascular coagulation syndrome. To investigate the potential underlying molecular mechanism(s) of pentamidine's effects on coagulation and fibrinolysis, we studied its effects on clotting times in normal and deficient human plasmas. Using normal plasma, pentamidine isethionate doubled the activated partial thromboplastin time at 27.5 µM, doubled the prothrombin time at 45.7 µM, and weakly doubled the thrombin time at 158.17 µM. Using plasmas deficient of factors VIIa, IXa, XIa, or XIIa, the concentrations to double the activated partial thromboplastin time were similar to that obtained using normal plasma. Pentamidine also inhibited plasmin-mediated clot lysis with half-maximal inhibitory concentration (IC50) value of ~3.6 μM. Chromogenic substrate hydrolysis assays indicated that pentamidine inhibits factor Xa and plasmin with IC50 values of 10.4 µM and 8.4 µM, respectively. Interestingly, it did not significantly inhibit thrombin, factor XIa, factor XIIIa, neutrophil elastase, or chymotrypsin at the highest concentrations tested. Michaelis-Menten kinetics and molecular modeling studies revealed that pentamidine inhibits factor Xa and plasmin in a competitive fashion. Overall, this study provides quantitative mechanistic insights into the in vitro effects of pentamidine isethionate on coagulation and fibrinolysis via the disruption of the proteolytic activity of factor Xa and plasmin.

Cystamine and Disulfiram Inhibit Human Transglutaminase 2 via an Oxidative Mechanism

The catalytic activity of transglutaminase 2 (TG2), a ubiquitously expressed mammalian enzyme, is regulated by multiple post-translational mechanisms. Because elevated activity of TG2 in the extracellular matrix is associated with organ-specific diseases such as celiac disease and renal fibrosis, there is growing therapeutic interest in inhibitors of this enzyme. Cystamine, a symmetric disulfide compound, is one of the earliest reported TG2 inhibitors. Despite its widespread use as a tool compound to block TG2 activity in vitro and in vivo, its mechanism of action has remained unclear. Here, we demonstrate that cystamine irreversibly inhibits human TG2 ( k_inh/ K_i = 1.2 mM^-1 min^-1) via a mechanism fundamentally distinct from those proposed previously. Through mass spectrometric disulfide mapping and site-directed mutagenesis, we show that cystamine promotes the formation of a physiologically relevant disulfide bond between Cys370 and Cys371 that allosterically abrogates the catalytic activity of human TG2. This discovery led us to evaluate clinically useful thiol → disulfide oxidants for TG2 inhibitory activity. It is demonstrated that disulfiram, a relatively safe oral thiuram disulfide, is a fairly potent TG2 inhibitor ( k_inh/ K_i = 8.3 mM^-1 min^-1) and may therefore provide a practical tool for clinically validating this emerging therapeutic target in intestinal disorders such as celiac disease.

Allosteric Inhibition of Factor XIIIa. Non-Saccharide Glycosaminoglycan Mimetics, but Not Glycosaminoglycans, Exhibit Promising Inhibition Profile

Factor XIIIa (FXIIIa) is a transglutaminase that catalyzes the last step in the coagulation process. Orthostery is the only approach that has been exploited to design FXIIIa inhibitors. Yet, allosteric inhibition of FXIIIa is a paradigm that may offer a key advantage of controlled inhibition over orthosteric inhibition. Such an approach is likely to lead to novel FXIIIa inhibitors that do not carry bleeding risks. We reasoned that targeting a collection of basic amino acid residues distant from FXIIIa’s active site by using sulfated glycosaminoglycans (GAGs) or non-saccharide GAG mimetics (NSGMs) would lead to the discovery of the first allosteric FXIIIa inhibitors. We tested a library of 22 variably sulfated GAGs and NSGMs against human FXIIIa to discover promising hits. Interestingly, although some GAGs bound to FXIIIa better than NSGMs, no GAG displayed any inhibition. An undecasulfated quercetin analog was found to inhibit FXIIIa with reasonable potency (efficacy of 98%). Michaelis-Menten kinetic studies revealed an allosteric mechanism of inhibition. Fluorescence studies confirmed close correspondence between binding affinity and inhibition potency, as expected for an allosteric process. The inhibitor was reversible and at least 9-fold- and 26-fold selective over two GAG-binding proteins factor Xa (efficacy of 71%) and thrombin, respectively, and at least 27-fold selective over a cysteine protease papain. The inhibitor also inhibited the FXIIIa-mediated polymerization of fibrin in vitro. Overall, our work presents the proof-of-principle that FXIIIa can be allosterically modulated by sulfated non-saccharide agents much smaller than GAGs, which should enable the design of selective and safe anticoagulants.

A fluorescence anisotropy-based assay for determining the activity of tissue transglutaminase

Tissue transglutaminase (TGase 2) is the most abundantly expressed enzyme of the transglutaminase family and involved in a large variety of pathological processes, such as neurodegenerative diseases, disorders related to autoimmunity and inflammation as well as tumor growth, progression and metastasis. As a result, TGase 2 represents an attractive target for drug discovery and development, which requires assays that allow for the characterization of modulating agents and are appropriate for high-throughput screening. Herein, we report a fluorescence anisotropy-based approach for the determination of TGase 2's transamidase activity, following the time-dependent increase in fluorescence anisotropy due to the enzyme-catalyzed incorporation of fluorescein- and rhodamine B-conjugated cadaverines 1-3 (acyl acceptor substrates) into N,N-dimethylated casein (acyl donor substrate). These cadaverine derivatives 1-3 were obtained by solid-phase synthesis. To allow efficient conjugation of the rhodamine B moiety, different linkers providing secondary amine functions, such as sarcosyl and isonipecotyl, were introduced between the cadaverine and xanthenyl entities in compounds 2 and 3, respectively, with acyl acceptor 3 showing the most optimal substrate properties of the compounds investigated. The assay was validated for the search of both irreversible and reversible TGase 2 inhibitors using the inactivators iodoacetamide and a recently published L-lysine-derived acrylamide and the allosteric binder GTP, respectively. In addition, the fluorescence anisotropy-based method was proven to be suitable for high-throughput screening (Z' factor of 0.86) and represents a non-radioactive and highly sensitive assay for determining the active TGase 2 concentration.

Allosteric inhibition of factor XIa. Sulfated non-saccharide glycosaminoglycan mimetics as promising anticoagulants

Recent development of sulfated non-saccharide glycosaminoglycan mimetics, especially sulfated pentagalloyl glucopyranoside (SPGG), as potent inhibitors of factor XIa (FXIa) (J. Med. Chem. 2013; 56:867-878 and J. Med. Chem. 2014; 57:4805-4818) has led to a strong possibility of developing a new line of factor XIa-based anticoagulants. In fact, SPGG represents the first synthetic, small molecule inhibitor that appears to bind in site remote from the active site. Considering that allosteric inhibition of FXIa is a new mechanism for developing a distinct line of anticoagulants, we have studied SPGG's interaction with FXIa with a goal of evaluating its pre-clinical relevance. Comparative inhibition studies with several glycosaminoglycans revealed the importance of SPGG's non-saccharide backbone. SPGG did not affect the activity of plasma kallikrein, activated protein C and factor XIIIa suggesting that SPGG-based anticoagulation is unlikely to affect other pathways connected with coagulation factors. SPGG's effect on APTT of citrated human plasma was also not dependent on antithrombin or heparin cofactor II. Interestingly, SPGG's anticoagulant potential was diminished by serum albumin as well as factor XI, while it could be reversed by protamine or polybrene, which implies possible avenues for developing antidote strategy. Studies with FXIa mutants indicated that SPGG engages Lys529, Arg530 and Arg532, but not Arg250, Lys252, Lys253 and Lys255. Finally, SPGG competes with unfractionated heparin, but not with polyphosphates and/or glycoprotein Ibα, for binding to FXIa. These studies enhance understanding on the first allosteric inhibitor of FXIa and highlight its value as a promising anticoagulant.

Tissue transglutaminase: an emerging target for therapy and imaging

Tissue transglutaminase (transglutaminase 2) is a multifunctional enzyme with many interesting properties resulting in versatile roles in both physiology and pathophysiology. Herein, the particular involvement of the enzyme in human diseases will be outlined with special emphasis on its role in cancer and in tissue interactions with biomaterials. Despite recent progress in unraveling the different cellular functions of transglutaminase 2, several questions remain. Transglutaminase 2 features in both confirmed and some still ambiguous roles within pathological conditions, raising interest in developing inhibitors and imaging probes which target this enzyme. One important prerequisite for identifying and characterizing such molecular tools are reliable assay methods to measure the enzymatic activity. This digest Letter will provide clarification about the various assay methods described to date, accompanied by a discussion of recent progress in the development of inhibitors and imaging probes targeting transglutaminase 2.

Characterization of recombinant Zea mays transglutaminase expressed in Pichia pastoris and its impact on full and non-fat yoghurts

BACKGROUND

Transglutaminases catalyze post-translational modification of proteins by ε-(γ-glutamyl) links and covalent amide bonds. Research on properties and applications of plant transglutaminases is less developed than in animals and micro-organisms. In a previous study, optimized Zea mays transglutaminase was purified from recombinant Pichia pastoris strain. The main objective of the present study was to characterize this enzyme and assess its effect on the properties of yoghurt.

RESULTS

The purified recombinant transglutaminase presented a Km of 3.98 µmol L(-1) and a Vmax of 2711 min(-1) by the fluorometric method. The enzyme was stable after incubation for 30 min below 50 °C and over a broad pH range of 5-8 at -20 °C for 12 h. The results showed that the crosslinking reaction catalyzed by this enzyme could effectively improve the properties of full and non-fat yoghurts. Also, the properties of non-fat yoghurt could be improved similar to the full-fat product by recombinant transglutaminase.

CONCLUSION

The application of recombinant transglutaminase in yoghurt indicated that this enzyme could be used as a substitute for microbial transglutaminase in the production of yoghurt, thus providing experimental evidence for the future application of plant transglutaminases in the food industry.

Nanochannel-based electrochemical assay for transglutaminase activity

A novel nanochannel-based electrochemical approach to determine transglutaminase activity by using mesoporous silica thin film-coated electrodes.

Biotechnological applications of transglutaminases

In nature, transglutaminases catalyze the formation of amide bonds between proteins to form insoluble protein aggregates. This specific function has long been exploited in the food and textile industries as a protein cross-linking agent to alter the texture of meat, wool, and leather. In recent years, biotechnological applications of transglutaminases have come to light in areas ranging from material sciences to medicine. There has also been a substantial effort to further investigate the fundamentals of transglutaminases, as many of their characteristics that remain poorly understood. Those studies also work towards the goal of developing transglutaminases as more efficient catalysts. Progress in this area includes structural information and novel chemical and biological assays. Here, we review recent achievements in this area in order to illustrate the versatility of transglutaminases.

Self-assembly of tissue transglutaminase into amyloid-like fibrils using physiological concentration of Ca2+

Tissue transglutaminase (tTG or TG2) is a member of the transglutaminase family that catalyzes calcium dependent formation of isopeptide bonds. It has been shown that the expression of TG2 is elevated in neurodegenerative diseases such as Parkinson's, Huntington's, and Alzheimer's. We have investigated the self-assembly of TG2 in vitro. First, using software, hot spots, which are prone for aggregation, were identified in domain 2 of the enzyme. Next we expressed and purified recombinant TG2 and its truncated version that contains only the catalytic domain, and examined their amyloidogenic behavior in various conditions including different temperatures and pHs, in the presence of metal ions and Guanosine triphosphate (GTP). To analyze various stages leading to TG2 fibrillation, we employed various techniques including Thioflavin T (ThT) binding assay, Congo-Red, birefringence, Circular Dichroism (CD), 8-anilino-1-naphthalene sulfonic acid (ANS) binding, Transmission Electron Microscopy (TEM) and Atomic Force Microscopy (AFM). Our results indicated that using low concentrations of Ca(2+), TG2 self-assembled into amyloid-like fibrils; this self-assembly occurred at the physiological temperature (37 °C) and at a higher temperature (57 °C). The truncated version of TG2 (domain 2) also forms amyloid-like fibrils only in the presence of Ca(2+). Because amyloid formation has occurred with domain 2 alone where no enzymatic activity was shown, self-cross-linking by the enzyme was ruled out as a mechanism of amyloid induction. The self-assembly of TG2 was not significant with magnesium and zinc ions, indicating specificity of the self-assembly for calcium ions. The calcium role in self-assembly of TG2 into amyloid may be extended to other proteins with similar biophysical properties to produce novel biomaterials.

Fluorescent substrates for polyamine catabolic enzymes and transport

The most widely used methods for measuring polyamine enzyme activities are radioisotope methods that measure the radioactivity of compounds produced from radiolabeled substrate by the enzyme reaction. Several fluorescent polyamines have been developed for the measurement of the polyamine transport system (PTS) or transglutaminase. Although fluorophores in the fluorescent polyamines may affect the affinity of the polyamine moiety to the enzyme protein, the assays that use fluorescent substrate are sensitive and simple for common laboratory usage.In this chapter, the uses of dansyl polyamines with a simple high-performance liquid chromatography system for the measurement of the PTS and polyamine catabolic enzymes including spermidine/spermine N¹-acetyltransferase and N¹-acetylpolyamine oxidase are described.

EGF potentiated oncogenesis requires a tissue transglutaminase-dependent signaling pathway leading to Src activation

EGF receptor (EGFR) signaling in human cancers elicits changes in protein-expression patterns that are crucial for potentiating tumor growth. Identifying those proteins with expression regulated by the EGFR and determining how they contribute to malignancy is fundamental for the development of more effective strategies to treat cancer. Here, we show that tissue transglutaminase (tTG) is one such protein. EGF up-regulates tTG expression in human breast-cancer cells, and knock-downs of tTG or the treatment of breast cancer cells with a tTG inhibitor blocks their EGF-stimulated anchorage-independent growth. We further show that the combined actions of Ras and Cdc42, leading to the activation of PI 3-kinase and NFkappaB, provide a mechanism by which EGF can up-regulate tTG in breast-cancer cells. Moreover, overexpression of wild-type tTG, but not its transamidation-defective counterpart, fully mimics the growth advantages afforded by EGF to these cancer cells. Surprisingly, the tTG-promoted growth of breast-cancer cells is dependent on its ability to activate the Src tyrosine kinase as an outcome of a complex formed between tTG and the breast-cancer marker and intermediate filament protein keratin-19. These findings identify tTG as a key participant in an EGFR/Src-signaling pathway in breast-cancer cells and a potential target for inhibiting EGFR-promoted tumor progression.

Quantitative and rapid analysis of transglutaminase activity using protein arrays in mammalian cells

We developed a novel on-chip activity assay using protein arrays for quantitative and rapid analysis of transglutami-nase activity in mammalian cells. Transglutaminases are a family of Ca2+-dependent enzymes involved in cell regulation as well as human diseases such as neurodegenerative disorders, inflammatory diseases and tumor progression. We fabricated the protein arrays by immobilizing N,N'-dimethylcasein (a substrate) on the amine surface of the arrays. We initiated transamidating reaction on the protein arrays and determined the transglutaminase activity by analyzing the fluorescence intensity of biotinylated casein. The on-chip transglutaminase activity assay was proved to be much more sensitive than the [3H]putrescine-incorporation assay. We successfully applied the on-chip assay to a rapid and quantitative analysis of the transgluta-minase activity in all-trans retinoic acid-treated NIH 3T3 and SH-SY5Y cells. In addition, the on-chip transglutaminase activity assay was sufficiently sensitive to determine the transglutaminase activity in eleven mammalian cell lines. Thus, this novel on-chip transglutaminase activity assay was confirmed to be a sensitive and high-throughput approach to investigating the roles of transglutaminase in cellular signaling, and, moreover, it is likely to have a strong potential for monitoring human diseases.

Development of an isoenzyme-specific colorimetric assay for tissue transglutaminase 2 cross-linking activity

Tissue transglutaminase (TGase 2) belongs to the multigene transglutaminase family of Ca2+-dependent protein cross-linking enzymes. Based on the transamidation activity of TGase 2, a novel colorimetric assay has been developed using covalently coupled spermine to carboxy-substituted polystyrene plates and biotinylated pepT26, an excellent acyl-donor substrate, highly specific for TGase 2. The assay is based on the incorporation of the gamma-carboxamide of glutamine of pepT26 into the immobilized spermine. The amount of biotinylated pepT26 bound to the plate, as measured by the activity of streptavidin-peroxidase, is directly proportional to the TGase activity. The colorimetric procedure showed a good correlation (r=0.995) with the commonly used radiometric filter paper method for TGase2, and provides linear dose-response curves over a wide range of hrTGase2 concentrations (2.5-40 microU/ml). In addition, the assay shows higher sensitivity when compared with our previous TG-colorimetric test (more than 50-fold increase) and other existing assays. PepT26 displays strong reactivity with TGase 2, and no reactivity with TGases 1, 3, and FXIII. The procedure constitutes a rapid, TG2-specific, sensitive, and nonisotopic method for the measurement of TGase 2 activity in as low as 4ng of hrTGase 2 and purified guinea pig liver transglutaminase, and 1.25mug of guinea pig liver extracts.

Screening, purification, and identification of a copper-dependent FITC-binding protein in human plasma: albumin

In this study, a protein purified by fluorescein isothiocyanate (FITC)-affinity chromatography from human plasma was identified as albumin by MALDI-TOF-MS. Albumin was found to conjugate with FITC-labeled molecules through a copper-dependent reaction. The formation of this complex was confirmed by methods including a newly developed "charcoal-based fluorescence assay" (CFA), gel-filtration, affinity chromatography, and ultrafiltration. The binding was identified as disulfide bridge formation. This is the first to demonstrate that copper induces a covalent binding of FITC-labeled molecules with albumin. In addition, the developed CFA method facilitates the screening of small fluorescent dyes binding to macromolecules.