Citrullination

Citrullination or deimination is the term used for the post-translational modification of the amino acid arginine in a protein into the amino acid citrulline. This reaction, shown below, is performed by enzymes called peptidylarginine deiminases (PADs). Note that citrullination of proteins is distinct from the formation of the free amino acid citrulline as part of the urea cycle or as a byproduct of enzymes of the nitric oxide synthase family.

The conversion of arginine into citrulline can have important consequences for the structure and function of proteins, since arginine is positively charged at a neutral pH, whereas citrulline is uncharged. This increases the hydrophobicity of the protein, leading to changes in protein folding.

Proteins that normally contain citrulline residues include MBP, filaggrin, and several histone proteins, while other proteins, like fibrin and vimentin, can get citrullinated during cell-death and tissue inflammation. Fibrin and fibrinogen may be favored sites for arginine deimination within rheumatoid joints. Test for presence of anti-citrullinated protein (ACP) antibodies are highly specific (88-96%) for rheumatoid arthritis (RA), about as sensitive as rheumatoid factor (70-78%) for diagnosis of RA, and are detectable from even before the onset of clinical disease.

Citrullinated vimentin is a very promising autoantigen in RA, and, more important, a very suitable tool for studying this systemic autoimmune disease. Moreover, anti-MCV antibodies may be useful for monitoring effects of RA therapy. A newly developed ELISA system utilises genetically modified citrullinated vimentin (MCV), a naturally occurring isoform of vimentin to improve the performance of the test.

In the reaction from arginine to citrulline, one of the terminal nitrogen atoms of the arginine sidechain is replaced by an oxygen. The reaction uses one water molecule and yields ammonia as a side-product:

The chemical conversion of arginine to citrulline, known as citrullination or deimination.

Citrullination in the developing nervous system

Five protein deiminases, PAD1-4 and 6 have been known to exist in mammals. PAD2 is the only one among PAD isotypes to be expressed at a high level in the central nervous system (CNS), including the eye and brain. Although the function of deimination in developing neuronal tissue is just emerging, PAD transcripts have been found in the C57BL6/J mouse eyes as early as embryonic day 14.5. The fruitfly Drosophila genome has been sequenced and like other invertebrates it lacks a PAD transcript. In higher mammals such as mouse and in humans there are 13 and 19 transcripts respectively encompassing gene and protein products for PAD1-4 and PAD6. The PAD isotypes differ in terms of their tissue and cellular distributions. Among them, the PAD2 isotype has the broadest tissue distribution and is found in myelinating cells of the CNS and in myelin, where one of its target substrates is MBP. In the normal retina, deimination is found in nearly all the retinal layers, including the photoreceptors. Deimination has been also reported in neuronal cells, such as astrocytes, microglia and oligodendrocytes, Schwann cells and neurons

The role of deimination in development has yet to be completely unraveled but so far its presence has been implicated in the regulation of gene expression through histone modifications, where it has been shown to antagonize the transcriptional induction mediated by histone bound arginine methylation. The main isotype involved in epigenetics is PAD4, which deiminates arginines and/or monomethylated arginines specifically on histones 3 and 4 antagonizing arginine methylation. There is a vast amount of work on the effect of this posttranslational modification on MBP, the main protein involved in maintaining myelin sheath stability, where its role is very dynamic, both during normal central nervous system formation, where it plays an important part in the development of myelin and also during myelin degeneration in demyelinating diseases such as Multiple Sclerosis. Whereas other posttranslational modifications such as methylation and phosphorylation of MBP are temporally correlated with the process of myelinogenesis, deimination of this protein is generally linked to the pathology of demyelination in adults. However, the degree of MBP deimination has also been found to be high in early CNS development where it plays a major physiological role in myelin assembly. This dual role for deimination in the nervous system is being investigated and although it still cannot be reconciled, perhaps it could be explained on the basis of cell type specificity of deiminases in both development and disease.