Aberrant synaptic transmission in the hippocampal CA3 region and cognitivedeterioration in protein-repair enzyme-deficient mice

L-aspartate is the amino-acid residue most susceptible to spontaneous isome rization. This denaturation causes an alteration in the biological activity of the protein and is regarded as an aging process of the protein. Protein L-isoaspartyl methyltransferase (PIMT) repairs this posttranslational modi fication and thus is implicated in retarding the aging process of proteins. PIMT is highly expressed in the brain, and its deficiency results in progr essive epilepsy after 4 weeks of age, with a fatal seizure in mice. Here we report the pathophysiological role of this repair system in the hippocampa l slice of PIMT-deficient mice. The hippocampal mossy fiber-CA3 synapses of PIMT-deficient mice showed hyperexcitation that was repressed by a gamma - aminobutyric acid (GABA)(A) receptor agonist muscimol. In addition, the mos sy fiber-CA3 synapses failed to show long-term potentiation or paired-pulse facilitation. No abnormality, however, was observed in Schaffer collateral -CA1 synapses or in perforant path-dentate gyrus synapses. Electron microsc opic study revealed aberrant distribution of synaptic vesicles in the mossy fiber terminals and vacuolar degeneration at the axon hillock of dentate g ranule cells in PIMT-deficient mice. Furthermore, the PIMT-deficient mice s howed impaired spatial memory in Morris water maze test and exhibited fewer anxiety-related behaviors in the elevated-plus test. These results suggest that the mossy fiber-CA3 system is vulnerable to aspartate isomerization a nd that the PIMT-mediated repair system is essential for maintenance of nor mal functions of the hippocampus. Hippocampus 2001;11:287-298. (C) 2001 Wil ey-Liss, Inc.