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Identification of Isopeptides Between Human Tissue Transglutaminase and Wheat, Rye, and Barley Gluten Peptides

ABSTRACT: Celiac disease (CD) is a chronic immune-mediated enteropathy of the small intestine, which is triggered by the ingestion of storage proteins (gluten) from wheat, rye, and barley in genetically predisposed individuals. Human tissue transglutaminase (TG2) plays a central role in the pathogenesis of CD, because it is responsible for specific gluten peptide deamidation and covalent crosslinking between glutamine and lysine, resulting in the formation of Nε-(γ-glutamyl)-lysine isopeptide bonds. The resulting TG2-gluten peptide complexes are assumed to cause the secretion of anti-TG2-autoantibodies, but the underlying mechanisms are only partly known. To gain more insight into the structures of these complexes, the aim of our study was to identify TG2-gluten isopeptides. We applied a reciprocal proteomics strategy including discovery-driven as well as targeted proteomics to complex wheat, rye and barley gluten hydrolysates that had been incubated with TG2. In total, we detected 29 TG2-gluten isopeptides, involving seven selected TG2 lysine residues (K205, K265, K429, K468, K590, K600, K677). Several gluten peptides carried known B-cell epitopes and/or T-cell epitopes, either intact 9-mer core regions or partial sequences, as well as sequences bearing striking similarities to already known epitopes. These novel insights into the molecular structures of TG2-gluten peptide complexes may help clarify their physiological relevance in the initiation of CD autoimmunity and the role of anti-TG2 autoantibodies

ORGANISM(S): Homo Sapiens Triticum Aestivum Secale Cereale Hordeum Vulgare

SUBMITTER: Barbara Lexhaller

PROVIDER: PXD017693 | panorama | Thu Aug 27 00:00:00 BST 2020

REPOSITORIES: PanoramaPublic

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Identification of Isopeptides Between Human Tissue Transglutaminase and Wheat, Rye, and Barley Gluten Peptides.

Lexhaller Barbara B Ludwig Christina C Scherf Katharina Anne KA

Scientific reports 20200504 1

Celiac disease (CD) is a chronic immune-mediated enteropathy of the small intestine, which is triggered by the ingestion of storage proteins (gluten) from wheat, rye, and barley in genetically predisposed individuals. Human tissue transglutaminase (TG2) plays a central role in the pathogenesis of CD, because it is responsible for specific gluten peptide deamidation and covalent crosslinking, resulting in the formation of N<sup>ε</sup>-(γ-glutamyl)-lysine isopeptide bonds. The resulting TG2-glute ...[more]

PMID: 32367038

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Project description:Celiac disease (CD) is a chronic inflammation of the small intestine triggered by the ingestion of gluten in genetically predisposed individuals. Tissue transglutaminase (TG2) is a key factor in the pathogenesis of CD, because it catalyzes both the deamidation of specific glutamine residues and the formation of covalent Nε-(γ-glutamyl)-lysine isopeptide crosslinks resulting in TG2-gluten peptide complexes. These complexes are thought to activate B cells causing the secretion of anti-TG2 autoantibodies that serve as diagnostic markers for CD, although their pathogenic role remains unclear. To gain more insight into the molecular structures of TG2-gluten peptide complexes, we developed a reciprocal proteomic analysis strategy, which facilitates the comprehensive identification of isopeptides in a complex protein hydrolysate. The workflow consists of four steps: (1) performance of the model reaction between TG2 and gluten peptide(s) followed by tryptic hydrolysis, clean-up and untargeted nLC-MS/MS analysis (2), prediction of tryptic TG2-derived lysine peptides that serve as potential isopeptide modifications (3) the search for isopeptides by configuring the TG2-modifications in MaxQuant and search against the α-gliadin fasta file and (4) the reciprocal search for isopeptides by configuring the gluten peptide(s) as modification in MaxQuant and search against the TG2 fasta file. After verification by manual data curation and visualization, this strategy enabled the identification of 26 different isopeptides involving 19 TG2 lysine residues, only six of which were known previously. Experiments with different TG2-gluten peptide molar ratios revealed that K-425, K-590, K-600 and K-649 were the most preferred lysine residues involved in isopeptide crosslinking. Further, expanding the model system to three gluten peptides allowed the localization of the preferred glutamine crosslinking sites. The described strategy is generally applicable to any hydrolysate containing isopeptides and these new insights into the structure of TG2-gluten peptide complexes may help clarify the role of extracellular TG2 in CD autoimmunity and in other inflammatory diseases.

Project description:Within the complex wheat flour proteome, the gluten proteins have attracted most of the attention because of their importance in determining the functional properties of wheat flour doughs and their roles in human health conditions such as celiac disease and food allergies. However, certain non-gluten proteins also trigger immunological responses but may be present in flour in low amounts or obscured by the more abundant gluten proteins in two-dimensional gels of total protein preparations. Non-gluten proteins were preferentially extracted from the flour with a dilute salt solution and separated by two-dimensional gel electrophoresis. Proteins in 172 gel spots were identified by tandem mass spectrometry after cleavage with trypsin or chymotrypsin. Fifty-seven different types of non-gluten proteins were identified, including 14 types that are known or suspected immunogenic proteins. The predominant proteins in 18 gel spots were gluten proteins. Transgenic wheat lines in which specific groups of gluten proteins were suppressed by RNA interference were used to estimate the amount of carry-over of gluten proteins in the salt-soluble protein fraction. Analysis of salt-soluble proteins from a transgenic line missing omega-1,2 gliadins demonstrated that certain omega-1,2 gliadins were present in large amounts in the salt-soluble fraction and obscured relatively small amounts of beta-amylase and protein disulfide isomerase. In comparison, analysis of a transgenic line in which alpha gliadins were absent revealed that glyceraldehyde-3 phosphate dehydrogenase was a moderately abundant protein that co-migrated with several alpha gliadins. The proteomic map of the non-gluten protein fraction of wheat flour developed in this study complements a proteomic map of the total flour proteins developed previously for the same cultivar. Knowing the identities of low abundance proteins in the flour as well as proteins that are hidden by some of the major gluten proteins on two-dimensional gels is critical for studies aimed at assessing the immunogenic potential of wheat flour and determining how the growth conditions of the plants affect the levels of specific immunogenic proteins in the flour.

Project description:Background & Aims: Traditional celiac disease diagnostics based on histomorphometric evaluations are liable to misinterpretations due to the common technical flaws e.g. wrong orientation of the biopsy and subjective errors are relatively common e.g. interobserver errors. We envisioned that there is a need for molecular histomorphometric tool that obviates these error sources yielding an objective ratio instead. Gene expression determine the state of a tissue, so the changes in expression may be associated with the severity of lesions during CD and can be used to classify it. Methods: 15 CD patients, who have been at least one year on gluten-free diet, were enrolled. All participants were biopsied before and after the gluten challenge (10 weeks, 4 grams of gluten daily). 6 healthy non-CD individuals were included as controls. Biopsies were taken on PAXgene biological fixative and embedded in paraffin and Vh/Cd ratio was assessed. RNA was extracted from the same sections and subjected to genome-wide 3’RNA-sequencing. Sequencing data was used to determine differentially expressed genes and regression model, that successfully describes the mucosal damage, was created and tested in independent material. Results: 167 differentially expressed genes were identified in healthy vs. treated CD comparisons with 117 genes downregulated and 50 genes upregulated. 415 differentially expressed genes were identified in Post gluten challenge to Treated CD comparisons with 195 genes downregulated and 220 genes upregulated. 119 genes whose expression highly correlates to Vh/Cd ratio (Spearman’s rank correlation coefficient, |rho|>0.7) were identified. Gene ontology analyses show that genes involved in cellular response to cytokines, including interferons, were over-represented. Stepwise regression allowed us computationally cut down the number of genes, that describe Vh/Cd ratio changes, to 7 and IELs number to 5. Created models describe 98.9% of observed Vh/Cd and 97.5% of IELs number variabilities; there is a strong correlation between the model’s predicted and observed ratios. Conclusions: Adoption of molecular histomorphometry, with our selected set of target genes, is quantitative and reliable way of estimating gluten-induced mucosal injury and inflammation. By including this technology one can overcome the typical shortcomings common in celiac disease diagnostics based on traditional histomorphometry analyses alone. Likewise, molecular histomorphometry is a promising instrument when incorporated in clinical trials where assessing drug efficacy on mucosal health is paramount. In addition, despite deemed healthy, based on traditional histomorphometric analyses, celiac patients on gluten free diet have significantly distinctive molecular histomorphometric pattern when compared to healthy controls.

Dataset Information

Identification of Isopeptides Between Human Tissue Transglutaminase and Wheat, Rye, and Barley Gluten Peptides

Publications

Identification of Isopeptides Between Human Tissue Transglutaminase and Wheat, Rye, and Barley Gluten Peptides.

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