Spinal cord injury-activated C/EBPβ-AEP axis mediates cognitive impairment through APP C586/Tau N368 fragments spreading

Spinal cord injury (SCI) leads to mental abnormalities such as dementia and depression; however, the molecular mechanism of SCI-induced dementia remains a matter of debate. Asparagine endopeptidase (AEP) mediated dementia by enhancing amyloid plaque and Tau hyperphosphorylation, indicating that it played an important role in neurodegeneration. Here we revealed that SCI stimulated AEP activation in mice with T9 contusion injury. Activated-AEP cleaved APP and Tau, resulting in APP C586 and Tau N368 formations, and consequentially accelerated Aβ deposit and Tau hyperphosphorylation, respectively. At 9 months following injury, mice demonstrated a severe deterioration in cognitive-behavioral function, which was corroborated by the presence of accumulated AD-specific pathologies. Surprisingly, activated AEP was found in the brains of mice with spinal cord injury. In contrast, AEP knockout reduced SCI-induced neuronal death and neuroinflammation, resulting in cognitive-behavioral restoration. Interestingly, compared to the full-length proteins, truncated Tau N368 and APP C586 were easier to bind to each other. These AEP-processed fragments can not only be induced to pre-formed fibrils, but also amplified their abilities of spreading and neurotoxicity in vitro. Furthermore, as a critical transcription factor of AEP, C/EBPβ was activated in injured spinal cord. Elevated C/EBPβ level, as well as microglia population and inflammatory cytokines were also noticed in the cortex and hippocampus of SCI mice. These neuroinflammation pathologies were close related to the amount of Tau N368 and APP C586 in brain. Moreover, administration with the AEP-specific inhibitor, compound #11, was shown to decelerate Aβ accumulation, tauopathy and C/EBPβ level in both spinal cord and brain of SCI mice. Thus, this study highlights the fact that spinal cord injury is a potential risk factor for dementia, as well as the possibility that C/EBPβ-AEP axis may play a role in SCI-induced cognitive impairment.