New Type Of Protein Modification Linked To Aging

Credit: Michael Ströck

Proteins are constantly being damaged by various chemical interactions and sugars are known to drive a damaging process called glycation. It turns out a reaction called carbamylation may also contribute to the aging process

Proteins in our bodies are constantly being damaged by various uncontrolled chemical reactions. Most people will be familiar with the fact that free radicals and other reactive oxygen species can lead to oxidative damage (see our previous article here). In addition to this proteins are being attacked by sugars and other reactive carbonyl compounds and isocyanate, as well as undergoing spontaneous fragmentation, deamidation, isomerization, racemization, misfolding and aggregation. Several of these modifications have been found to increase with age and their accumulation may actually be one of the drivers of aging.  

What is carbamylation?

In a new study published in the Proceedings of the National Academy of Sciences (PNAS) researchers for the first time demonstrate that the level of proteins modified by isocyanurate increase with age in 3 animal species (mice, cattle and humans). Isocyanurate is formed in the body from the waste product urea and from environmental sources such as cigarette smoke and dietary sources. Surprisingly, the dietary sources of isocyanurate are mostly healthy foods like vegetables. Several end products can result from the protein modification by isocyanurate but collectively these are known as carbamylation-derived products. In this study the researchers focussed on the most characteristic carbamylation-derived product, homocitrulline, formed by the carbamylation of the amino acid lysine.  

Credit: Sven Bulterijs 

Why is it harmful?

The first indication that modification of proteins by isocyanurate may have negative health effects came in the 1970s from the observation that patients suffering from sickle cell anemia treated with urea or cyanate (two precursors of isocyanurate) developed cataracts. This treatment interestingly depended on the fact that the urea or cyanate induced carbamylation of hemoglobin, the protein responsible for transporting oxygen in the blood. People suffering from sickle cell anemia have a mutated form of hemoglobin that is aggregation-prone leading to defective red blood cells and a decreased capacity to transport oxygen around. Carbamylation of the hemoglobin reduces its aggregation proneness and hence improves the sickle cell anemia defect.  

In the new study the researchers measured the level of one specific carbamylation-derived product known as homocitrulline. The levels of homocitrulline increased with age in the skin of mice, cattle and humans. Two year old mice had a 29-fold increased level of homocitrulline compared to one day old mice. In humans the age-related increase of homocitrulline is less extreme (old humans had an 8-fold higher level than young humans) but the absolute levels were higher (old mice had 2.2 mmol/mol Lys versus 5.5 mmol/mol Lys in old humans). The researchers also observed an age-related increase in the level of homocitrulline in human elastin and in the bones and tail collagen of mice.

What extent is homocitrulline damage important in aging?

If the level of homocitrulline is much lower than that of other forms of protein damage then its contribution to the aging process would be minimal. To answer this question, the researchers measured the level of homocitrulline and CML in the skin samples of the three species. CML is one of the more abundant end products resulting from the Maillard reaction (read more here). It was found that in all three species the level of homocitrulline was higher than that of CML (between 2.6 and 3.2-fold higher). Next the researchers looked if the accumulation of homocitrulline correlated with life expectancy in the 3 different species. Mice have the shortest life span of the three species and indeed have the highest rate of homocitrulline accumulation. Humans on the other hand have the longest life span and also the slowest accumulation rate of homocitrulline. 

Carbamylated proteins have been tied to arthritis

Isocyanurate levels are increased in chronic and end-stage renal diseases leading to higher levels of carbamylation products in their blood. This increase in carbamylation may be one, of the undoubtedly multiple, reasons why people with end-stage kidney disease have a higher risk of heart disease and an increase in mortality. Other studies have already found that carbamylated LDL is a risk factor for heart disease. Carbamylated proteins have also been linked to rheumatoid arthritis, an inflammatory joint disease. Injecting homocitrulline containing peptides into the joints of mice leads to arthritis, and antibodies against carbamylated proteins have also been found in rheumatoid arthritis patients.    

References:

Gorisse L, Pietrement C, Vuiblet V, Schmelzer CEH, Köhler M, Duca L, Debelle L, Fornès P, Jaisson S, Gillery P (2016). Protein carbamylation is a hallmark of aging. Proc Natl Acad Sci USA 113(5): 1191-1196.  
Verbrugge FH, Tang WHW, Hazen SL (2015). Protein carbamylation and cardiovascular disease. Kidney Int 88: 474-478.