Asparagine endopeptidase deletion ameliorates cognitive impairments by inhibiting proinflammatory microglial activation in MPTP mouse model of Parkinson disease

In addition to motor dysfunction, cognitive impairments have been reported to occur in patients with early-stage Parkinson's disease (PD). In this study, we examined a PD mouse model induced by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). This treatment led to the degeneration of nigrostriatal dopaminergic neurons in mice, a phenomenon that is consistent with previous studies. Besides, spatial memory and object recognition of MPTP-treated mice were impaired, as denoted by the Morris water maze (MWM) and novel object recognition (NOR) tests, respectively. Moreover, hippocampal synaptic plasticity (long-term potentiation and depotentiation) and the levels of synaptic proteins in hippocampus were decreased after MPTP treatment. We also found that MPTP resulted in the microglial activation and an inflammatory response in the striatum and hippocampus. Mammalian asparagine endopeptidase (AEP), a cysteine lysosomal protease, is involved in the cleavage and activation of Toll-like receptors (TLRs). The deletion of AEP can inhibit TLR4 in a mouse model of Alzheimer's disease, and TLR4 is upregulated in PD, inducing microglial activation and inflammation. We found that AEP deletion provided greater resistance to the toxic effects of MPTP. AEP knockout ameliorated the cognition and the synaptic plasticity defects in the hippocampus. Furthermore, AEP deletion decreased the expression of TLR4 and reduced microglial activation and the levels of several proinflammatory cytokines. Thus, we suggest that AEP plays a role in the inflammation induced by MPTP, and TLR4 might also involve in this process. AEP deletion could be a possible treatment strategy for the cognitive deficits of PD.