Recent advances in characterization of citrullination and its implication in human disease research: From method development to network integration

Abstract

Post‐translational modifications (PTM) of proteins increase the functional diversity of the proteome and have been implicated in the pathogenesis of numerous diseases. The most widely understood modifications include phosphorylation, methylation, acetylation, O‐linked/N‐linked glycosylation, and ubiquitination, all of which have been extensively studied and documented. Citrullination is a historically less explored, yet increasingly studied, protein PTM which has profound effects on protein conformation and protein‐protein interactions. Dysregulation of protein citrullination has been associated with disease development and progression. Identification and characterization of citrullinated proteins is highly challenging, complicated by the low cellular abundance of citrullinated proteins, making it difficult to identify and quantify the extent of citrullination in samples, coupled with challenges associated with development of mass spectrometry (MS)‐based methods, as the corresponding mass shift is relatively small, +0.984 Da, and identical to the mass shift of deamidation. The focus of this review is to discuss recent advancements of citrullination‐specific MS approaches and integration of the potential methodology for improved citrullination identification and characterization. In addition, the association of citrullination in disease networks is also highlighted.

The Role of Citrullination Modification in CD4+ T Cells in the Pathogenesis of Immune-Related Diseases

“…Although the main lesions occur in the synovium of peripheral joints, in at least 40% of rheumatoid arthritis patients, this disease may affect other organs, manifesting as specific or nonspecific extra-articular inflammatory presentations, with systemic implications [52]. Citrullination is considered a major cause of the onset of RA, and citrullinated proteins are the primary targets of antibodies in RA patients [53]. The pathogenesis of rheumatoid arthritis (RA) is associated with an increase in citrullinated protein levels mediated by PAD2 in synovial fluid [54].…”

Mass Spectrometry-Based Precise Identification of Citrullinated Histones via Limited Digestion and Biotin Derivative Tag Enrichment

“…This conversion is catalyzed by protein arginine deiminases (PADs), a group of enzymes which mainly localize in the nucleus and are known to target core histones H2A, H3, and H4, as well as linker histone H1 (Figure S1). − Modification of the positively charged arginine to an uncharged citrulline affects histone conformation and alters intramolecular and intermolecular interactions, resulting in looser chromatin structure, impacting chromatin function and gene expression. , This epigenetic mechanism has also been reported to contribute to the pathogenesis of many diseases including cancer, , rheumatoid arthritis, systemic lupus erythematosus, , Sjögren’s disease, and multiple sclerosis . For instance, citrullination of histone H3 and H4 by PAD4 is linked to cell death, and subsequent activation of inflammatory pathways can initiate nuclear fragmentation, a critical factor in carcinogenesis. ,, In addition, histone citrullination has a close relationship with the formation of neutrophil extracellular traps (NETs), which act as an important component of the immune system by trapping bacteria or pathogens. , Although histone citrullination has profound effects on various physiological and pathological processes, there are few established methods that enable large-scale identification of citrullinated proteins for the study of their cellular distribution and function, ultimately limiting our understanding of protein citrullination-associated disease mechanisms …”