Citrullination

Citrullination or deimination is the conversion of the amino acid arginine in a protein into the amino acid citrulline. Enzymes called peptidylarginine deiminases (PADs) replace the primary ketimine group (=NH) by a ketone group (=O).

Citrullination controls the expression of genes, particularly in the developing embryo. The immune system often attacks citrullinated proteins, leading to autoimmune diseases such as rheumatoid arthritis and multiple sclerosis.

Citrulline is not one of the 20 standard amino acids encoded by DNA in the genetic code. Instead, it is the result of a post-translational modification.

Citrullination 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.

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. Therefore, citrullination can change the structure and function of proteins.

Proteins that normally contain citrulline residues include myelin basic protein (MBP), filaggrin, and several histone proteins, while other proteins, like fibrin and vimentin, can get citrullinated during cell-death and tissue inflammation.^[1]

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.^[2]

Citrullinated vimentin may be an autoantigen in RA and other autoimmune diseases, and is used to study RA. Moreover, antibodies against mutated citrullinated vimentin (MCV) may be useful for monitoring effects of RA therapy.^[3] An ELISA system utilises genetically modified citrullinated vimentin (MCV), a naturally occurring isoform of vimentin to improve the performance of the test.^[4]

In the reaction from arginine to citrulline, one of the terminal nitrogen atoms of the arginine side chain 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.

In the nervous system

PADs are found in mammals but not in lower animals. Five PADs – PAD1, PAD2, PAD3, PAD4 and PAD6 – have been found.^[5]

PAD2 is expressed at a high level in the central nervous system (CNS), including the eye and brain. PAD transcripts have been found in the C57BL6/J mouse eyes as early as embryonic day 14.5.^[6]

The PAD isotypes differ in terms of their tissue and cellular distributions. 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 myelin basic protein. 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.^[7]

Deimination regulates gene expression through histone modifications. DNA is wrapped around histones, and the histone proteins can control DNA expression when chemical groups are added and removed. This process is known as post-translational processing or post-translational modification, because it takes place on the protein after the DNA is translated. The role of post-translational processing in gene regulation is the subject of the growing field of study, epigenetics.

One mechanism is methylation. A methyl group (CH₃) binds to an arginine on the histone protein, which can induce transcription of the DNA. When PAD converts arginine to citrulline, it no longer induces transcription.^[8] The main isotype for this is PAD4, which deiminates arginines and/or monomethylated arginines on histones 3 and 4, turning off the effects of arginine methylation.^[9]

Myelin basic protein, the main protein in myelin sheath stability, is modified this way during normal embryonic central nervous system formation, and also during myelin degeneration in demyelinating diseases such as multiple sclerosis.

Methylation and phosphorylation of MBP are active during the process of myelinogenesis. In the embryo, in early CNS development, MBP deimination plays a major role in myelin assembly. In adults, MPB deamination is found in demyelination diseases. MPB may affect different cell types in each case.^[10]

Autoimmune diseases

In rheumatoid arthritis and other autoimmune diseases, such as psoriatic arthritis, systemic lupus erythematosus and Sjögren's syndrome, autoantibodies often attack citrullinated proteins. The presence of anti-citrullinated protein antibody is a standard test for rheumatoid arthritis, and it is associated with more severe disease. Citrullinated proteins are also found in the cellular debris accompanying the destruction of cells in alzheimer disease, and after smoking cigarettes. So citrullination seems to be part of the mechanism that stimulates the immune system in autoimmune disease.^{[11][12][13][14][15]}

The first comprehensive textbook on deimination was published in 2014.^[16]

References