Brain Proteomics Supports the Role of Glutamate Metabolism and Suggests Other Metabolic Alterations in Protein l‑Isoaspartyl Methyltransferase (PIMT)-Knockout Mice

Protein l-isoaspartyl methyltransferase
(PIMT) repairs
the isoaspartyl residues (isoAsp) that originate from asparagine deamidation
and aspartic acid (Asp) isomerization to Asp residues. Deletion of
the gene encoding PIMT in mice (Pcmt1) leads to isoAsp
accumulation in all tissues measured, especially in the brain. These
PIMT-knockout (PIMT-KO) mice have perturbed glutamate metabolism and
die prematurely of epileptic seizures. To elucidate the role of PIMT
further, brain proteomes of PIMT-KO mice and controls were analyzed.
The isoAsp levels from two of the detected 67 isoAsp sites (residue
98 from calmodulin and 68 from glyceraldehyde-3-phosphate dehydrogenase)
were quantified and found to be significantly increased in PIMT-KO
mice (p < 0.01). Additionally, the abundance of
at least 151 out of the 1017 quantified proteins was found to be altered
in PIMT-KO mouse brains. Gene ontology analysis revealed that many
down-regulated proteins are involved in cellular amino acid biosynthesis.
For example, the serine synthesis pathway was suppressed, possibly
leading to reduced serine production in PIMT-KO mice. Additionally,
the abundances of enzymes in the glutamate–glutamine cycle
were altered toward the accumulation of glutamate. These findings
support the involvement of PIMT in glutamate metabolism and suggest
that the absence of PIMT also affects other processes involving amino
acid synthesis and metabolism.