Accelerated protein damage in brains of PIMT+/− mice; a possible model for the variability of cognitive decline in human aging

Abstract

Isoaspartate formation is a common type of protein damage normally kept in check by the repair enzyme protein-L-isoaspartyl methyltransferase (PIMT). Mice with a knockout of the gene (Pcmt1) for this enzyme (KO −/−) exhibit a pronounced neuropathology with fatal epileptic seizures at 30-60 days. HZ (+/−) mice have 50% of the PIMT activity found in WT (+/+) mice, but appear normal. To see if HZ mice exhibit accelerated aging at the molecular level, we compared brain extracts from HZ and WT mice at 8 months and 2 years with regard to (a) PIMT activity, (b) isoaspartate levels, and (c) activity of an endogenous PIMT substrate, creatine kinase B. PIMT activity declined modestly with age in both genotypes. Isoaspartate was significantly higher in HZ than WT mice at eight months and more so at two years, rising 5X faster in HZ males, and 3X faster in females. Creatine kinase activity decreased with age and was always lower in the HZ mice. These findings suggest the individual variation of human PIMT levels may significantly influence the course of age-related CNS dysfunction.

Polymorphic Variants of Human Protein l-Isoaspartyl Methyltransferase Affect Catalytic Activity, Aggregation, and Thermal Stability

“…Interestingly, female ϩ/Ϫ mice accumulate approximately three times more isoaspartyl damage over the same time period but show no apparent reduction in lifespan. In WT mice, PIMT activity shows a modest decrease over the lifespan (21), but in postmortem human brain samples, the highest PIMT activities are found in the eldest (23). This latter observation suggests that a higher inherited PIMT activity in humans may promote longevity.…”

“…Brain extracts of PIMT ϩ/Ϫ mice have 50 -55% of the PIMT activity found in WT (ϩ/ϩ) mice. Between 8 months and 2 years, male ϩ/Ϫ mice accumulate six times more isoaspartyl damage than ϩ/ϩ males (21) and have a median lifespan that is ϳ6 weeks shorter (22). Interestingly, female ϩ/Ϫ mice accumulate approximately three times more isoaspartyl damage over the same time period but show no apparent reduction in lifespan.…”

Insight of brain degenerative protein modifications in the pathology of neurodegeneration and dementia by proteomic profiling

“…Although N-deamidation is a spontaneous process, host cells employ a repair mechanism that prevents the excessive accumulation of deamidated residues in proteins. This repair mechanism is mediated by the enzyme L-isoaspartyl (D-aspartyl) methyltransferase (PIMT) [19], which is most highly expressed in mammalian brain tissues [182], and exhibits decreasing activity in aging mice [183]. Accordingly, PIMT-deficient mice suffer neuropathology and fatal epileptic seizures at 30-60 days after birth [184, 185].…”

Aspartate Glycosylation Triggers Isomerization to Isoaspartate

“…7,11a,12,16 Our findings show that OGT can catalyze aspartate to isoaspartate isomerization in a model substrate, and one might expect that other enzymes that glycosylate aspartate will also promote this process as long as the local sequence permits succinimide formation. While we do not yet know if OGT modifies aspartate in any cellular proteins, we note that PARPs naturally glycosylate aspartic acid residues; 8,17 this activity may, therefore, lead to succinimide intermediates in some cases.…”