Oxidative Damage to the TCA Cycle Enzyme MDH1 Dysregulates Bioenergetic Enzymatic Activity in the Aged Murine Brain

Aging
can have profound effects on the mammalian brain leading
to neurodegeneration and cognitive impairment. The brain has exceptionally
high-energy requirements and is particularly susceptible to damage
within its bioenergetic pathways. Here, we asked how the bioenergetic
proteome of the murine brain changed with age and how this might affect
brain function. Using label-free LC-MS/MS proteomics for the discovery
phase and quantitative multiple reaction monitoring LC-MRM-MS/MS for
the validation phase, we found dysregulated expression of multiple
components of the tricarboxylic acid cycle, which is key for mitochondrial
energy production, including SULA2, IDH1, IDH2, SDHB, PDHB, MDH1,
FH1, and NDUFS3, in old murine brains. We also saw that the oxidoreductases,
thioredoxin and glutaredoxin, were significantly down-regulated in
the old mouse brain and showed through MS that this correlated with
the accumulation of trioxidation in the key metabolic enzyme MDH1
at Cys137. 3D modeling of MDH1 predicted that the damaged sites were
located at the protein active zone, and enzymatic kinetic analysis
confirmed that MDH1 function was significantly reduced in the old
mouse brain. These findings identify the tricarboxylic acid cycle
as a key target of degenerative protein modifications with deleterious
effects on the aging brain’s bioenergetic function.