Cerebral Phospho-Tau Acts Synergistically with Soluble Aβ42 Leading to Mild Cognitive Impairment in AAV-AD Rats

Background Alzheimer’s disease (AD) is a continuum of events beginning with an increase in brain soluble Aβ342 followed by the appearance of hyperphosphorylated tau (P-tau, asymptomatic stage). Mild Cognitive Impairment (MCI) then appears (prodromal stage). However, the individual contribution of these two soluble proteins in the onset of the first cognitive symptoms remains unclear. Objectives We sought to understand the specific impact of p-tau on the development of MCI in the AAV-AD rat model, a model of late-onset Alzheimer’s disease (LOAD) predementia. Methods We specifically reduced the phosphorylation level of tau while leaving Aβ342 levels unchanged using a DYRK1A protein kinase inhibitor, Leucettine L41, in an adeno-associated virus-based Alzheimer’s disease (AAV-AD) rat model. Leucettine L41 was administered by intraperitoneal injection at 20 mg/kg per day in AAV-AD rats from 9 (late asymptomatic phase) to 10 (prodromal phase) months of age. Results Decreased soluble forms of P-tau induced by chronic administration of Leucettine L41 did not change soluble Aβ342 levels but prevented MCI onset in 10-month-old AAV-AD rats. Conclusions The present study argues that P-tau is required to induce the development of MCI. Consistent with our previous findings that soluble Aβ342 is also required for MCI onset, the data obtained in the AAV-AD rat model confirm that the transition from the asymptomatic to the prodromal stage may be caused by the combined presence of both soluble brain forms of Aβ342 and p-tau, suggesting that the development of MCI may be the consequence of their synergistic action.