Assessment of the Effects of Altered Amyloid-Beta Clearance on Behavior Following Repeat Closed-Head Brain Injury in Amyloid-Beta Precursor Protein Humanized Mice

Traumatic brain injury (TBI) increases the risk for dementias including Alzheimer's disease (AD) and Chronic Traumatic Encephalopathy. Furthermore, both human and animal model data indicate that amyloid-beta (Aβ) peptide accumulation and its production machinery are upregulated by TBI. Considering the clear link between chronic Aβ elevation and AD as well as tau pathology, the role(s) of Aβ in TBI is of high importance. Endopeptidases including the neprilysin (NEP)-like enzymes are key mediators of Aβ clearance and may affect susceptibility to pathology post TBI. Here we use a "humanized" mouse model of Aβ production which expresses normal human APP under its natural transcriptional regulation and exposed them to a more clinically relevant repeated closed-head TBI paradigm. These transgenic mice were also crossed with mice deficient for the Aβ degrading enzymes NEP or NEP2 to assess models of reduced cerebral Aβ clearance in our TBI model. Our results show that the presence of the human form of Aβ did not exacerbate motor (Rotarod) and spatial learning/memory deficits (Morris Water Maze) post injuries while potentially reduced anxiety (Open Field) was observed. NEP and NEP2 deficiency also did not exacerbate these deficits post injuries and was associated with protection from motor (NEP and NEP2) and spatial learning/memory deficits (NEP only). These data suggest that normally regulated expression of wild-type human APP/Aβ does not contribute to deficits acutely after TBI and may be protective at this stage of injury.