Abstract
Tau accumulation and cholinergic impairment are characteristic pathologies in Alzheimer's disease (AD). However, the causal role of tau accumulation in cholinergic lesion is elusive. Here, we observed an aberrant tau accumulation in the medial septum (MS) of 3xTg and 5xFAD mice, especially in their cholinergic neurons. Overexpressing hTau in mouse MS (MShTau) for 6 months but not 3 months induced spatial memory impairment without changing object recognition and anxiety‐like behavior, indicating a specific and time‐dependent effect of MS‐hTau accumulation on spatial cognitive functions. With increasing hTau accumulation, the MShTau mice showed a time‐dependent cholinergic neuron loss with reduced cholinergic projections to the hippocampus. Intraperitoneal administration of donepezil, a cholinesterase inhibitor, for 1 month ameliorated the MS‐hTau‐induced spatial memory deficits with preservation of MS–hippocampal cholinergic pathway and removal of tau load; and the beneficial effects of donepezil was more prominent at low dose. Proteomics revealed that MS‐hTau accumulation deregulated multiple signaling pathways with numerous differentially expressed proteins (DEPs). Among them, the vacuolar protein sorting‐associated protein 37D (VP37D), an autophagy‐related protein, was significantly reduced in MShTau mice; the reduction of VP37D was restored by donepezil, and the effect was more significant at low dose than high dose. These novel evidences reveal a causal role of tau accumulation in linking MS cholinergic lesion to hippocampus‐dependent spatial cognitive damages as seen in the AD patients, and the new tau‐removal and autophagy‐promoting effects of donepezil may extend its application beyond simple symptom amelioration to potential disease modification.
Highlights
Alzheimer’s disease (AD), an irreversible and currently incurable neurodegenerative disease, is characterized neuropathologically by beta-amyloid (Aβ) plaques and neurofibrillary tangles (NFTs).[1,2,3,4] The cholinergic basal forebrain (CBF) projection neurons located within the medial septum (MS) and diagonal band Broca (DDB) are severely damaged along with the aggravation of Aβ and NFTs during AD progression.[5]
The increased phospho-tau at pT205 and pT231 was detected in both AD mouse models, and the hyperphosphorylated tau proteins were robustly accumulated in the nucleus and the cell body, respectively (Figure 1E,F)
Spatial memory loss is the early symptom of AD, and hippocampus formation takes charge of spatial cognitive functions
Summary
Proposed working model: Medial septum (MS) tau accumulation disrupts MS–hippocampus cholinergic pathway and impairs hippocampus-associated spatial memory as seen on patients with Alzheimer’s disease (AD). In addition to traditional cholinesterase inhibition, low-dose donepezil exerts novel tauremoval and autophagy-promoting effects, efficiently rescuing MS-htau-induced memory loss. Our results supplement tau toxicity and the new findings on donepezil may extend its application beyond simple symptom amelioration to potential disease modification; the latter may help physicians to optimize dosing in donepezil treatment on AD. Funding information Science and Technology Committee of China, Grant/Award Number: 2016YFC1305800; Special project of technological innovation of Hubei Province, Grant/Award Number: 2018ACA142; Natural Science Foundation of China, Grant/Award Numbers: 91949205, 31730035, 81721005; Guangdong Provincial Key S&T Program, Grant/Award Number: 018B030336001
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