Protein citrullination is a type of post-translational modification (PTM) on proteins. PTM is catalyzed by enzymes after proteins are synthesized and achieved by the addition or removal of moieties on one or multiple amino acids in the protein. For example, phosphorylation is catalyzed by a class of enzymes called kinases and adds a phosphate group to certain amino acids. Citrullination is catalyzed by peptidylarginine deiminases (PADs), which remove the ketimine group from the amino acid arginine and replace it with a ketone group. These modifications change charges at the modified amino acid positions. For example, phosphorylation adds negative charges; citrullination removes positive charges. The charge changes can alter interactions between amino acids within the protein and interaction with other proteins, resulting in modulation of protein function. In physiological conditions, cells conduct PTM to regulate protein functions to meet their needs in response to various functional requirements. In pathological conditions, cellular functions often go haywire, and PTMs become dysregulated. The dysregulation leads to a disconnection between cellular functional needs and PTM, causing some PTMs to become detrimental to the health and functioning of the cell.

In neurodegenerative diseases such as Alzheimer’s disease (AD), Parkinson’s disease (PD), frontotemporal dementia (FTD), and ALS, PTMs are altered. Some of these PTMs have been intensely studied. For example, tau is a protein that forms toxic protein aggregates in AD. Tau is heavily phosphorylated to an abnormal degree both in the amount and amino acid positions. Phosphorylation at some amino acids has been shown to increase protein aggregation and its toxicity to neurons.

These research results have been used to design therapeutics targeting phosphorylated tau. In addition, phosphorylated tau has been used as a disease biomarker for diagnosis of AD and evaluation of AD progression in clinical trials.

Protein citrullination has also been shown to be increased in AD, PD, and other neurodegenerative conditions. However, whether similar changes occur in ALS is not known. Furthermore, how changes in protein citrullination affect the neurodegenerative process is also not understood. To answer these questions, our lab and Dr. Paul Thompson's lab at the Department of Biochemistry and Molecular Biotechnology, University of Massachusetts Chan Medical School, have joined forces and began to investigate protein citrullination in ALS. We initiated our research in two mouse models for ALS. Our results show that protein citrullination is increased dramatically in these mouse models. The increase is correlated with the disease progression and is colocalized with the areas of neurodegeneration in the central nervous system (CNS). Additionally, at the cellular level, there is a divergence between the neurons and glial cells. Whereas citrullination is dramatically increased in astroglia, it is decreased in neurons. Most strikingly, citrullinated proteins are almost exclusively accumulated in the protein aggregates, suggesting that citrullination may drive protein aggregation, thereby promoting the disease progression.

Currently, we are conducting studies on human ALS samples to verify the findings from the mouse models. Our preliminary results suggest that similar changes in protein citrullination also occur in human ALS. Our future studies will determine what the roles of protein citrullination are in ALS and whether protein citrullination changes can be used as biomarkers for ALS diagnosis and clinical trials.

Go to Dr. Xu's lab website.