Tools for the detection and quantitation of protein transglutamination

Organophosphorus Pesticides Promote Protein Cross-Linking

Exposure to organophosphorus pesticides (OP) can have chronic adverse effects that are independent of inhibition of acetylcholinesterase, the classic target for acute OP toxicity. In pure proteins, the organophosphorus pesticide chlorpyrifos oxon induces a cross-link between lysine and glutamate (or aspartate) with loss of water. Tubulin is particularly sensitive to OP-induced cross-linking. Our goal was to explore OP-induced cross-linking in a complex protein sample, MAP-rich tubulin from Sus scrofa and to test 8 OP for their capacity to promote isopeptide cross-linking. We treated 100 μg of MAP-rich tubulin with 100 μM chlorpyrifos, chlorpyrifos oxon, methamidophos, paraoxon, diazinon, diazoxon, monocrotophos, or dichlorvos. Each sample was separated using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and stained with Coomassie blue. Five gel slices (at about 30, 50, 150, and 300 kDa, and the top of the separating gel) were removed from the lanes for each of the eight OP samples and from untreated control lanes. These gel slices were subjected to in-gel trypsin digestion. MSMS fragmentation spectra of the tryptic peptides were examined for isopeptide cross-links. Sixteen spectra yielded convincing evidence for isopeptide cross-linked peptides. Ten were from the chlorpyrifos oxon reaction, 1 from dichlorvos, 1 from paraoxon, 1 from diazinon, and 3 from diazoxon. It was concluded that catalysis of protein cross-linking is a general property of organophosphorus pesticides and pesticide metabolites. Data are available via ProteomeXchange with identifier PXD034529.

Chlorpyrifos oxon crosslinking of amyloid beta 42 peptides is a new route for generation of self-aggregating amyloidogenic oligomers that promote Alzheimer's disease

Epidemiological evidence suggests that people chronically exposed to organophosphorus pesticides are at increased risk of neurodegenerative disease. Covalently linked amyloid beta dimers have been isolated from the brains of Alzheimer's patients. The toxic forms of amyloid beta are amyloid dimers that spontaneously oligomerize. In the present report we treated recombinant and synthetic amyloid beta (1-42) with 1 mM chlorpyrifos oxon or 1 mM paraoxon. The trypsin-digested samples were analyzed by liquid chromatography tandem mass spectrometry on an Orbitrap Fusion Lumos mass spectrometer. Data were searched with Protein Prospector software. We found two new types of crosslinks in amyloid dimers. An isopeptide Asp-Asp link occurred between the N-terminal amine of Asp 1 in one peptide and the beta carboxyl group of Asp 1 in another peptide. An Asp-Arg link occurred between the guanidino group of Arg 5 in one peptide and the beta carboxyl group of Asp 1 in another peptide. These results show that the active metabolites of the pesticides chlorpyrifos and parathion catalyze the crosslinking of amyloid beta (1-42) into toxic dimers. It was concluded that the increased risk of neurodegenerative disease in people exposed to organophosphorus pesticides could be explained by the crosslinking activity of these chemicals. Data are available via ProteomeXchange with identifier PXD034163.

Dichlorvos-induced formation of isopeptide crosslinks between proteins in SH-SY5Y cells

Chlorpyrifos oxon catalyzes the crosslinking of proteins via an isopeptide bond between lysine and glutamic acid or aspartic acid in studies with purified proteins. Our goal was to determine the crosslinking activity of the organophosphorus pesticide, dichlorvos. We developed a protocol for examining crosslinks in a complex protein mixture consisting of human SH-SY5Y cells exposed to 10 μM dichlorvos. The steps in our protocol included immunopurification of crosslinked peptides by binding to anti-isopeptide antibody 81D1C2, stringent washing of the immobilized complex, release of bound peptides from Protein G agarose with 50% acetonitrile 1% formic acid, liquid chromatography tandem mass spectrometry on an Orbitrap Fusion Lumos mass spectrometer, Protein Prospector searches of mass spectrometry data, and manual evaluation of candidate crosslinked dipeptides. We report a low quantity of dichlorvos-induced KD and KE crosslinked proteins in human SH-SY5Y cells exposed to dichlorvos. Cells not treated with dichlorvos had no detectable KD and KE crosslinked proteins. Proteins in the crosslink were low abundance proteins. In conclusion, we provide a protocol for testing complex protein mixtures for the presence of crosslinked proteins. Our protocol could be useful for testing the association between neurodegenerative disease and exposure to organophosphorus pesticides.

Mapping of Transglutaminase-2 Sites of Human Salivary Small Basic Proline-Rich Proteins by HPLC-High-Resolution ESI-MS/MS

Because of the distinctive features of the oral cavity, the determination of the proteins involved in the formation of the "oral protein pellicle" is demanding. The present study investigated the susceptibility of several human basic proline-rich peptides, named P-H, P-D, P-F, P-J, and II-2, as substrates of transglutaminase-2. The reactivity of the P-C peptide and statherin was also investigated. Peptides purified from human whole saliva were incubated with the enzyme in the presence or in the absence of monodansyl-cadaverine. Mass spectrometry analyses of the reaction products highlighted that P-H and P-D (P₃₂ and A₃₂ variants) were active substrates, II-2 was less reactive, and P-F and P-J showed very low reactivity. P-C and statherin were highly reactive. All of the peptides formed cyclo derivatives, and only specific glutamine residues were involved in the cycle formation and reacted with monodansyl-cadaverine: Q₂₉ of P-H, Q₃₇ of P-D, Q₂₁ of II-2, Q₄₁ of P-C, and Q₃₇ of statherin were the principal reactive residues. One or two secondary glutamine residues of only P-H, P-D P₃₂, P-C, and statherin were hierarchically susceptible to the reaction with monodansyl-cadaverine. MS and MS/MS data were deposited to the ProteomeXchange Consortium ( http://www.ebi.ac.uk/pride ) via the PRIDE partner repository with the data set identifier PXD014658.

Chlorpyrifos oxon promotes tubulin aggregation via isopeptide cross-linking between diethoxyphospho-Lys and Glu or Asp: Implications for neurotoxicity

Exposure to organophosphorus toxicants (OP) can have chronic adverse effects that are not explained by inhibition of acetylcholinesterase, the cause of acute OP toxicity. We therefore hypothesized that OP-induced chronic illness is initiated by the formation of organophosphorus adducts on lysine residues in proteins, followed by protein cross-linking and aggregation. Here, Western blots revealed that exposure to the OP chlorpyrifos oxon converted porcine tubulin from its original 55-kDa mass to high-molecular-weight aggregates. Liquid chromatography–tandem MS analysis of trypsin-digested samples identified several diethoxyphospho-lysine residues in the OP-treated tubulin. Using a search approach based on the Batch Tag program, we identified cross-linked peptides and found that these chemically activated lysines reacted with acidic amino acid residues creating γ-glutamyl-ϵ-lysine or aspartyl-ϵ-lysine isopeptide bonds between β- and α-tubulin. Of note, these cross-linked tubulin molecules accounted for the high-molecular-weight aggregates. To the best of our knowledge, this is the first report indicating that chlorpyrifos oxon–exposed tubulin protein forms intermolecular cross-links with other tubulin molecules, resulting in high-molecular-weight protein aggregates. It is tempting to speculate that chronic illness from OP exposure may be explained by a mechanism that starts with OP adduct formation on protein lysines followed by protein cross-linking. We further speculate that OP-modified or cross-linked tubulin can impair axonal transport, reduce neuron connections, and result in neurotoxicity.

Transglutaminse 2 and EGGL, the protein cross-link formed by transglutaminse 2, as therapeutic targets for disabilities of old age

Aging of the extracellular matrix (ECM), the protein matrix that surrounds and penetrates the tissues and binds the body together, contributes significantly to functional aging of tissues. ECM proteins become increasingly cross-linked with age, and this cross-linking is probably important in the decline of the ECM's function. This article reviews the role of ε-(γ-glutamyl)-lysine (EGGL), a cross-link formed by transglutaminase enzymes, and particularly the widely expressed isozyme transglutaminase 2 (TG2), in the aging ECM. There is little direct data on EGGL accumulation with age, and no direct evidence of a role of EGGL in the aging of the ECM with pathology. However, several lines of circumstantial evidence suggest that EGGL accumulates with age, and its association with pathology suggests that this might reflect degradation of ECM function. TG activity increases with age in many circumstances. ECM protein turnover is such that some EGGL made by TG is likely to remain in place for years, if not decades, in healthy tissue, and both EGGL and TG levels are enhanced by age-related diseases. If further research shows EGGL does accumulate with age, removing it could be of therapeutic benefit. Also reviewed is the blockade of TG and active removal of EGGL as therapeutic strategies, with the conclusion that both have promise. EGGL removal may have benefit for acute fibrotic diseases, such as tendinopathy, and for treating generalized decline in ECM function with old age. Extracellular TG2 and EGGL are therefore therapeutic targets both for specific and more generalized diseases of aging.

Detection of transglutaminase activity using click chemistry

Transglutaminase 2 (TG2) is a Ca²⁺-dependent enzyme able to catalyze the formation of ε(γ-glutamyl)-lysine crosslinks between polypeptides, resulting in high molecular mass multimers. We have developed a bioorthogonal chemical method for the labeling of TG2 glutamine-donor proteins. As amine-donor substrates we used a set of azide- and alkyne-containing primary alkylamines that allow, after being crosslinked to glutamine-donor proteins, specific labeling of these proteins via the azide-alkyne cycloaddition. We demonstrate that these azide- and alkyne-functionalized TG2 substrates are cell permeable and suitable for specific labeling of TG2 glutamine-donor substrates in HeLa and Movas cells. Both the Cu(I)-catalyzed and strain promoted azide-alkyne cycloaddition proved applicable for subsequent derivatization of the TG2 substrate proteins with the desired probe. This new method for labeling TG2 substrate proteins introduces flexibility in the detection and/or purification of crosslinked proteins, allowing differential labeling of cellular proteins.

Fluorescent probes of tissue transglutaminase reveal its association with arterial stiffening

Tissue transglutaminase (TG2) catalyzes the crosslinking of proteins. TG2 has been implicated in fibrosis and vascular calcification, both of which lead to a common feature of aging known as arterial stiffness. In order to probe the role of TG2 in arterial rigidification, we have prepared a fluorescent irreversible inhibitor as a probe for TG2 activity (RhodB-PGG-K(Acr)-LPF-OH). This probe was synthesized on solid support, characterized kinetically (k(inact) = 0.68 min⁻¹, K(I) = 79 μM), and then used to stain the aorta from rats used as a model of isolated systolic hypertension (ISH). Interestingly, TG2 activity was thus shown to increase over 4 weeks of the hypertension model, corresponding with the previously observed increase in arterial stiffness. These results clearly suggest an association between TG2 and the phenomenon of arterial rigidification.

Transglutaminase-mediated intramolecular cross-linking of membrane-bound alpha-synuclein promotes amyloid formation in Lewy bodies

The alpha-synuclein immunopositive and chaotrope-insoluble material from human brains with Lewy body pathology was analyzed by mass spectrometry. From the proteinase K-cleavable peripheral fraction of Lewy bodies, which was densely cross-linked by gamma-glutamyl-epsilon-lysine bonds between HspB1 and ubiquitin in a pattern similar to neurofibrillary tangles (Nemes, Z., Devreese, B., Steinert, P. M., Van Beeumen, J., and Fésüs, L. (2004) FASEB J. 18, 1135-1137), 53 proteins were identified. In the core of Lewy bodies only alpha-synuclein was found, and it contained a low amount of intramolecular cross-links between Gln-99 and Lys-58. In vitro cross-linking of alpha-synuclein by transglutaminases 1-3 and 5 produced a heterogeneous population of variably cross-linked alpha-synucleins in solution, which inhibited the aggregation of the protein into amyloid. However, in the presence of phosphatidylserine-rich membranes and micromolar calcium concentrations, the cross-linking by transglutaminases 1, 2, and 5 showed specificity toward the utilization of Gln-99 and Lys-58. As shown by thioflavin T fluorescence monitoring, the formation of this cross-link accelerated the aggregation of native alpha-synuclein. Chemical cross-linking of residues 58-99 triggered amyloid formation, whereas such bonding of residues 99 to 10 was inhibitory. Our findings reveal the pivotal role of membrane attachment and transglutaminase-mediated intermolecular cross-linking for the propagative misfolding and aggregation of alpha-synuclein.

A nonradioactive dot blot assay for transglutaminase activity

Aberrant transglutaminase (TG) activity has been implicated in the pathology of numerous diseases, including Huntington's disease and Alzheimer's disease. To fully characterize the role of TGs in these disorders, it is important that simple quantifiable assays be made available. The most commonly used assay currently employed requires significant time and a radioactive substrate. The assay described here uses a biotinylated substrate in conjunction with a dot blot apparatus to eliminate the use of radioactive substrates and allows relative transglutaminase activity to be measured simultaneously with minimal sample preparation in a large number of samples containing purified enzyme, cell extracts, or tissue homogenates.

The end product of transglutaminase crosslinking: simultaneous quantitation of [Nepsilon-(gamma-glutamyl) lysine] and lysine by HPLC-MS3

Transglutaminases catalyze the formation of Nepsilon-(gamma-glutamyl) isodipeptide crosslinks between proteins. These enzymes are thought to participate in a number of diseases, including neurological disease and cancer. A method associating liquid chromatography and multiple stage mass spectrometry has been developed for the simultaneous quantitation of [Nepsilon-(gamma-glutamyl) lysine] isodipeptide and lysine on an ion trap mass spectrometer. Highly specific detection has been achieved in MS3 mode. The method includes a derivatization step consisting of butylation of carboxylic groups and acetylation of amide groups, a liquid-liquid extraction, and a 19-min separation on a 100x2.1-mm Beta-basic C18 column with an acetonitrile gradient elution. 13C6-(15)N2 isotopes of the isodipeptide and the lysine serve as internal standards. The assay was linear in the range of 50 pmol/ml to 75 nmol/ml for the isodipeptide and the range of 10 nmol/ml to 3.5 micromol/ml for the lysine, with correlation coefficients greater than 0.99 for both ions. Intra- and inter-day coefficients of variation ranged from 3.5 to 15.9%. The method was successfully applied to human biological samples known to be crosslinked by transglutaminase such as cornified envelopes of epidermis, fibrin, and normal and Huntington disease brain.

Reactivity of fish and microbial transglutaminases on glutaminyl sites of peptides derived from threadfin bream myosin

Fish liver transglutaminase (FTG), a Ca(2+)-dependent enzyme, exhibits different characteristics from the Ca(2+)-independent microbial transglutaminase (MTG), leading to potential differences in their substrate specificity and reactivity. The ability of these enzymes to catalyze isopeptide bond formation by incorporating 5-(biotinamido)pentylamine (BPNH2) into peptides derived by tryptic digestion of threadfin bream (TB)-myosin was investigated to identify reaction sites and substrate specificity using a peptidomic strategy. BPNH2 was incorporated into TB-myosin peptides to a greater extent by MTG than FTG. Peptides derived from TB-myosin heavy chain (MHC) shared highest similarity to amberjack-MHC on the basis of a Mascot database search. Amino acid sequences and modification sites of BPNH2-tagged peptides were identified by tandem mass spectrometry based on the amberjack-MHC sequence. The BPNH2 modification sites catalyzed by both TGases were at the myosin rod. Most of the BPNH2 peptides contained charged amino acids (E, R, K) at the glutaminylamide site of reactive glutamine (Q*). The alpha-acrylamide site of Q* contained E, F, or L on peptides catalyzed by both enzymes, I, Q, or A on peptides catalyzed only by FTG, and V on a peptide catalyzed only by MTG. These results demonstrate the different structural requirements for glutaminyl substrates between these two enzymes.

Amine modification of digested peptide at C-terminal end during protein digestion by protease

We recently reported that C-terminal polyamine modification occurs when proteins are digested with trypsin in the presence of polyamine [Biochem. Biophys. Res. Commun., 356, 159-162 (2007)]. In the present study, the characteristics of this C-terminal modification in the presence of protease and amine were investigated. When hemoglobin (HB) was digested with trypsin in the presence of N-(2-pyridyl)-1,4-diaminobutane (Py4), formation of the modified peptide was dependent on time and on HB or Py4 concentration. When synthetic peptide was treated with trypsin in the presence of Py4, ca. 0.1% of the peptide was modified with Py4. When HB or cytochrome C was treated with a range of serine proteases in the presence of various amines (Py4, N-(2-pyridyl)-1,3-diaminopropane, tranexamic acid, isonicotinic acid hydrazide and ampicillin), the modified peptide was detected in all cases tested, thus suggesting that amine modification widely accompanies digestion by proteases.

Oligomerization of KCC2 correlates with development of inhibitory neurotransmission

The neuron-specific K+-Cl- cotransporter KCC2 extrudes Cl- and renders GABA and glycine action hyperpolarizing. Thus, it plays a pivotal role in neuronal inhibition. Development-dependent KCC2 activation is regulated at the transcriptional level and by unknown posttranslational mechanisms. Here, we analyzed KCC2 activation at the protein level in the developing rat lateral superior olive (LSO), a prominent auditory brainstem structure. Electrophysiology demonstrated ineffective KCC2-mediated Cl- extrusion in LSO neurons at postnatal day 3 (P3). Immunohistochemical analyses by confocal and electron microscopy revealed KCC2 signals at the plasma membrane in the somata and dendrites of both immature and mature neurons. Biochemical analysis demonstrated mature glycosylation pattern of KCC2 at both stages. Immunoblot analysis of the immature brainstem demonstrated mainly monomeric KCC2. In contrast, three KCC2 oligomers with molecular masses of approximately 270, approximately 400, and approximately 500 kDa were identified in the mature brainstem. These oligomers were sensitive to sulfhydryl-reducing agents and resistant to SDS, contrary to the situation seen in the related Na+-(K+)-Cl- cotransporter. In HEK-293 cells, coexpressed hemagglutinin-tagged KCC2 assembled with histidine-tagged KCC2, demonstrating formation of homomers. Based on these findings, we conclude that the oligomers represent KCC2 dimers, trimers, and tetramers. Finally, immunoblot analysis identified a development-dependent increase in the oligomer/monomer ratio from embryonic day 18 to P30 throughout the brain that correlates with KCC2 activation. Together, our data indicate that the developmental shift from depolarization to hyperpolarization can be determined by both increased gene expression and KCC2 oligomerization.

Protein modification in aging: an update

Post-translational modifications of proteins are an important biologic tool for the production of various protein species from a single gene, which may vary in conformation, function, biologic half-life and complex formation with other proteins. The present minireview summarizes a few selected research observations important for the role of post-translational modifications in biologic aging and age-related diseases, including farnesylation, methylglyoxal-derivatization, transglutaminase pathways and the formation of 3-nitrotyrosine and 2-oxo-histidine in vivo.