Transglutaminases: future perspectives. Amino Acids. 2013;44:1-9. [PMID: 23283580 DOI: 10.1007/s00726-012-1431-7]

18F-labeling and initial in vivo evaluation of a Hitomi peptide for imaging tissue transglutaminase 2

INTRODUCTION

Tissue transglutaminase 2 (TG2) is a calcium-dependent enzyme which cross-links proteins. It is overexpressed in many diseases and plays a key role in tissue remodeling, including cell adhesion and migration. Overexpression of TG2 in breast cancer is a marker for patients at risk of recurrence. Non-invasive imaging of TG2 can therefore play an important role in patient management. TG2 probes labeled with the positron emitters ¹¹C and ¹⁸F have thus far not found widespread application due to purity and metabolism issues. Our approach was to radiolabel a TG2 selective, 13-mer amino acid peptide, which was modified with a 5-azidopentanoic acid group at the N-terminus via a copper free click chemistry approach.

METHODS

Radiochemistry was performed and fully automated using an iPhase FlexLab module. We produced the radiolabeling synthon [¹⁸F]FBz-DBCO from [¹⁸F]SFB and DBCO-amine. After HPLC purification, [¹⁸F]FBz-DBCO was reacted with the modified peptide and the putative radiotracer purified by HPLC. In vivo imaging using the radiolabeled amine was performed in mice bearing either TG2 expressing MDA-MB-231 or non-TG2 expressing MCF-7 xenografts as negative control. Expression of the target was confirmed using immunohistochemistry and western blot techniques.

RESULTS

We obtained 9 ± 2 GBq of the radiolabeled peptide from 55 ± 5 GBq of fluorine-18 in an overall synthesis time of 160 min from end of bombardment (EOB), including HPLC purification and reformulation. Small animal PET/MR imaging showed that visualization of MDA-MB-231 tumors using the radiolabeled peptide could only be achieved due to differences in clearance between tumor and surrounding tissue. In the MCF-7 xenograft model, radiotracer clearance from tumor and surrounding tissue occurred at a similar rate, thus making it impossible to visualize MCF-7 tumors. The presence of TG2 in MDA-MB-231 tumors and absence in MCF-7 tumors was confirmed by immunohistochemistry staining and western blot analysis.

CONCLUSION

A fully automated synthesis of a TG2 selective, 13-amino-acid peptide modified with 5-azido pentynoic acid at the N-terminal was established using [¹⁸F]FBzDBCO as a prosthetic group. Although our results show that radiolabeled peptides have potential as imaging agents for TG2, more research needs to be performed to improve radiotracer kinetics.

Zinc as a modulator of transglutaminase activity - Laboratory and pathophysiological aspects

For a whole century, citrate has been used as an in vitro anticoagulant via chelation of calcium. Later, also EDTA was introduced as an anticoagulant. An often overlooked fact is that zinc is bound to citrate and EDTA with affinities much greater than that for calcium, imposing problems in biomedical research. In vivo, proteins of the S100 family are released from leukocytes and known to bind calcium. Some of them, e.g., calprotectin, also chelate zinc. Thus, at an inflamed site, the ratio between Ca²⁺ and Zn²⁺ is changed. This mechanism is of importance for the modulation of the activation of a fascinating family of post-translationally acting calcium-dependent thiol enzymes, the transglutaminases, which are inhibited by zinc. This presentation illustrates the complexity of in vitro studies with zinc. Moreover, it exemplifies the role of Zn²⁺ in pathophysiological situations such as celiac disease and neurodegeneration.

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Citrate, EDTA and DTT bind zinc as well as calcium.

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At inflammation, calprotectin binds Zn²⁺, which leads to low concentrations of the ion.

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Zn²⁺ inhibits the activation of transglutaminases and peptidylarginine deiminases.

Polyamines in mammalian pathophysiology

Polyamines (PAs) are essential organic polycations for cell viability along the whole phylogenetic scale. In mammals, they are involved in the most important physiological processes: cell proliferation and viability, nutrition, fertility, as well as nervous and immune systems. Consequently, altered polyamine metabolism is involved in a series of pathologies. Due to their pathophysiological importance, PA metabolism has evolved to be a very robust metabolic module, interconnected with the other essential metabolic modules for gene expression and cell proliferation/differentiation. Two different PA sources exist for animals: PA coming from diet and endogenous synthesis. In the first section of this work, the molecular characteristics of PAs are presented as determinant of their roles in living organisms. In a second section, the metabolic specificities of mammalian PA metabolism are reviewed, as well as some obscure aspects on it. This second section includes information on mammalian cell/tissue-dependent PA-related gene expression and information on crosstalk with the other mammalian metabolic modules. The third section presents a synthesis of the physiological processes described as modulated by PAs in humans and/or experimental animal models, the molecular bases of these regulatory mechanisms known so far, as well as the most important gaps of information, which explain why knowledge around the specific roles of PAs in human physiology is still considered a "mysterious" subject. In spite of its robustness, PA metabolism can be altered under different exogenous and/or endogenous circumstances so leading to the loss of homeostasis and, therefore, to the promotion of a pathology. The available information will be summarized in the fourth section of this review. The different sections of this review also point out the lesser-known aspects of the topic. Finally, future prospects to advance on these still obscure gaps of knowledge on the roles on PAs on human physiopathology are discussed.

Biochemical study of sortase E2 from Streptomyces mobaraensis and determination of transglutaminase cross-linking sites

Distinct streptomycetes such as Streptomyces mobaraensis produce the protein cross-linking enzyme transglutaminase. Bioinformatic analysis predicted the occurrence of seven sortases exerting transpeptidation reactions similarly to transglutaminase. Here, we report the production and characterization of sortase E2 (Sm-SrtE2) solubilized by removal of its membrane anchor domain. Sm-SrtE2 activity was measured using pentapeptides predicted to be cell wall sorting signals of putative sortase substrate proteins. Preferred linkage to Gly₃ by Sm-SrtE2 was in the order LAETG>>LAHTG>>LAQTG~LANTG>LARTG. Chaplin 1 from S. mobaraensis was further demonstrated to be an excellent substrate of both the intrinsic Sm-SrtE2 and transglutaminase. The unexpected discovery showing Gln-62 and Gln-65 of Δ^1-50 -Sm-SrtE2 as transglutaminase cross-linking sites suggests that low enzyme stability might be due to anchor domain truncation and a disordered N terminus.

Structure of a glutamine donor mimicking inhibitory peptide shaped by the catalytic cleft of microbial transglutaminase

The protein cross-linking enzyme transglutaminase from Streptomyces mobaraensis (MTG) is frequently used to modify therapeutic proteins. In order to reveal the binding mode of glutamine donor substrates, we have now crystallized MTG covalently linked to large inhibitory peptides. A series of peptide structures were examined but DIPIGSKMTG, which was chloroacetylated at serine, was the only inhibitory molecule that resulted in an interpretable density map. We found that, besides the warhead (modified Ser6), Ile4 and Gly5 of the inhibitory peptide occupy the tight but extended hydrophobic bottom of the MTG-binding cleft. Both termini of the peptide protrude along the cleft walls almost perpendicular to the bottom of the extended cleft. This peptide model suggests a zipper-like cross-linking mechanism of self-assembled substrate proteins by MTG.

Purification and biochemical characterization of a novel transglutaminase from Mythimna separata larvae (Noctuidae, Lepidoptera)

A novel transglutaminase (MsTGase) from Mythimna separata larvae was separated and purified; its biochemical property and enzymatic catalytic activities were investigated. MsTGase was obtained chromatographically by the precipitation of Sephadex G-100 gel and DEAE-Cellulose-52 ion-exchange column with 48-fold purification and a reproducible yield of approximately 12%. Molecular weight of the MsTGase was 63.5 KDa and its N-terminal amino acid sequence was GKIEEG-LVI. Michaelis constant of the MsTGase for the substrate N-CBZ-Gln-Gly was 12.83mM with a V_max of 7.99U/mL. Optimum conditions for MsTGase activity were at 42°C and pH7.5. The enzyme didn't possess metal ion at its catalytic active site; its activity could be significantly inhibited by Mg²⁺, but activated by Ca²⁺. Chlorpyrifos and spinosad showed a strong potential to increase MsTGase activity, supporting the view that MsTGase was a novel target. Moreover, the formation of intermolecular cross-links of casein and bovine serum albumin polymerized by MsTGase in the presence of DTT was observed. These findings pave the way for future studies on the physiological role of MsTGase and the potential impact of its regulation on MsTGase-associated pest management.

Modulation of learning and memory by natural polyamines

Spermine and spermidine are natural polyamines that are produced mainly via decarboxylation of l-ornithine and the sequential transfer of aminopropyl groups from S-adenosylmethionine to putrescine by spermidine synthase and spermine synthase. Spermine and spermidine interact with intracellular and extracellular acidic residues of different nature, including nucleic acids, phospholipids, acidic proteins, carboxyl- and sulfate-containing polysaccharides. Therefore, multiple actions have been suggested for these polycations, including modulation of the activity of ionic channels, protein synthesis, protein kinases, and cell proliferation/death, within others. In this review we summarize these neurochemical/neurophysiological/morphological findings, particularly those that have been implicated in the improving and deleterious effects of spermine and spermidine on learning and memory of naïve animals in shock-motivated and nonshock-motivated tasks, from a historical perspective. The interaction with the opioid system, the facilitation and disruption of morphine-induced reward and the effect of polyamines and putative polyamine antagonists on animal models of cognitive diseases, such as Alzheimer's, Huntington, acute neuroinflammation and brain trauma are also reviewed and discussed. The increased production of polyamines in Alzheimer's disease and the biphasic nature of the effects of polyamines on memory and on the NMDA receptor are also considered. In light of the current literature on polyamines, which include the description of an inborn error of the metabolism characterized by mild-to moderate mental retardation and polyamine metabolism alterations in suicide completers, we can anticipate that polyamine targets may be important for the development of novel strategies and approaches for understanding the etiopathogenesis of important central disorders and their pharmacological treatment.

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.

A fluorescence-based array screen for transglutaminase substrates

Transglutaminases (EC 2.3.2.13) form an enzyme family that catalyzes the formation of isopeptide bonds between the γ-carboxamide group of glutamine and the ε-amine group of lysine residues of peptides and proteins. Other primary amines can be accepted in place of lysine. Because of their important physiological and pathophysiological functions, transglutaminases have been studied for 60 years. However, the substrate preferences of this enzyme class remain largely elusive. In this study, we used focused combinatorial libraries of 400 peptides to investigate the influence of the amino acids adjacent to the glutamine and lysine residues on the catalysis of isopeptide bond formation by microbial transglutaminase. Using the peptide microarray technology we found a strong positive influence of hydrophobic and basic amino acids, especially arginine, tyrosine, and leucine. Several tripeptide substrates were synthesized, and enzymatic kinetic parameters were determined both by microarray analysis and in solution.

Novel method for extracting exosomes of hepatocellular carcinoma cells

AIM: To develop a novel method for the rapid and efficient extraction of exosomes secreted by tumor cells.

METHODS: Unlike the traditional extraction method, the supernatants of cell cultures were concentrated, and the exosomes were isolated promptly and effectively using a novel nanomaterial called ExoQuick. Coomassie brilliant blue staining was used for protein quantification, and the morphology of the exosomes extracted by both methods was visualized by transmission electron microscopy. Exosome marker proteins were detected by Western blot analysis. Two potential hepatoma-associated proteins, tissue transglutaminase 2 (TGM2) and annexin A2, were analyzed.

RESULTS: The exosomes separated by the new extraction assay based on the nanomaterial were disc-shaped, intact vesicles with lipid bilayer membranes. They were approximately 30-100 nm in diameter, which is similar to the diameter of exosomes isolated by the traditional method. The protein concentration of exosomes extracted by the new method was approximately 780 μg/10⁸ cells, and therefore, it was 19 times higher than that of exosomes extracted in the traditional manner. There were differences between the total proteins of Huh-7 cells and the exosomal proteins. Typical exosome proteins, such as the transmembrane protein CD63 and heat shock protein 70, were confirmed. Two potential hepatoma-associated proteins were also identified. TGM2 was first found to exist in the exosomes of human liver cancer cells, but annexin A2 was not secreted into exosomes.

CONCLUSION: The new extraction method based on the nanomaterial is quick and efficient. The cancer-associated protein TGM2 can be secreted through an exosome-mediated non-classical secretion pathway, and it may be a valuable tumor marker.

Tissue transglutaminase expression promotes castration-resistant phenotype and transcriptional repression of androgen receptor

Many studies have supported a role for inflammation in prostate tumour growth. However, the contribution of inflammation to the development of castration-resistant prostate cancer remains largely unknown. Based on observations that aberrant expression of the proinflammatory protein tissue transglutaminase (TG2) is associated with development of drug resistance and metastatic phenotype in multiple cancer types, we determined TG2 expression in prostate cancer cells. Herein we report that human prostate cancer cell lines with low expression of androgen receptor (AR) had high basal levels of TG2 expression. Also, overexpression of TG2 negatively regulated AR mRNA and protein expression and attenuated androgen sensitivity of prostate cancer cells. TG2 expression in prostate cancer cells was associated with increased invasion and resistance to chemotherapy. Mechanistically, TG2 activated nuclear factor (NF)-κB and induced epithelial-mesenchymal transition. TG2/NF-κB-mediated decrease in AR expression resulted from transcriptional repression involving cis-interaction of NF-κB in a complex with TG2 with the 5'-untranslated region of AR. Negative regulation of AR could be partially abrogated by repression of TG2 or NF-κB (p65/RelA) by gene-specific small interfering RNA. These results suggested that a novel pathway links androgen dependence with TG2-regulated inflammatory signalling and hence may make TG2 a novel therapeutic target for the prevention and treatment of castration-resistant prostate cancer.