Protein deimination in aging and age-related diseases with ocular manifestations

Deimination refers to the conversion of protein-bound arginines into citrulline. It has been established as a posttranslational modification due to the lack of any known tRNA carrier for citrulline, as well as the presence of deiminases that are capable of catalyzing this modification in vitro. There is no known enzyme that can revert protein-bound citrulline into arginine, rendering it a relatively long-term modification. Elevated deimination has been found in neuronal tissues in a number of neurodegenerative diseases including multiple sclerosis and glaucoma. Observations in the retina, a tissue where the retinal ganglion cell layer lacks a substantial presence of astroglial cells, demonstrated that elevated and reduced deimination occurs simultaneously in astroglial cells and neurons, respectively. Such opposite effects are expected to complicate therapeutic strategies, necessitating cell-specific delivery systems for perturbation of deiminases that catalyze deimination in neuronal tissues. In this review, we will briefly discuss the occurrence of deimination with normal aging, the importance of deimination in diseases, and the effect of deimination on mRNA transport in neuronal tissue. Elevated deimination induces proteolysis via modification of protein structures, while reduced deimination affects protein synthesis and the outgrowth of dendrites in neurons.