Amyloid-?-dependent impairment of adult hippocampal neurogenesis in a mouse model of recurrent herpes simplex virus type-1 infection

Defects in adult hippocampal neurogenesis have been proposed to play a crucial role in cognitive dysfunction associated with Alzheimer's disease (AD). However, if amyloid-? protein (A?) affects neurogenesis is still unclear, likely due to the use of different experimental conditions (e.g. cellular/animal models, A? isoforms, concentrations, and aggregation profiles) leading to conflicting results. Here we investigated the effects of A? on adult hippocampal neurogenesis by using an experimental paradigm of Herpes Simplex Virus type-1 (HSV-1) infection of neural stem/progenitor cells (NSCs), that we previously reported to trigger Amyloid Precursor Protein (APP) cleavage andintracellular A? accumulation in neuronal cells (Piacentini et al., 2011, 2015). Cultured NSCs were highly permissive to HSV-1 and, once infected, they accumulated A? and exhibited reduced proliferation (-30% vs. mock-infected cells, p<0.05, assessed by BrdU incorporation, Ki67 immunoreactivity and Sox2 mRNA expression) and impaired neuronal differentiation (-35%; p<0.01) in favor of glial phenotype (evaluated by immunoreactivity for the neuronal marker MAP2, the glial marker GFAP and the expression of the pro-neuronal genes Mash-1 and NeuroD1). The effects of HSV-1 on cultured NSCs were due to A? accumulation. Indeed, they were reverted by the presence of either ?/?-secretase inhibitors preventing A? production or the 4G8 antibody counteracting the action of intracellular A?, in infected cells. Moreover, extracellular oligomeric A?42 (200 nM), able to enter NSCs, mimicked the effects of HSV-1. Impaired adult neurogenesis was also observed in the hippocampal dentate gyrus of an in vivo model of recurrent HSV-1 infections, that we recently set up and characterized (De Chiara et al., 2019). Specifically, immunofluorescence experiments revealed a reduction of BrdU+ NSCs in the subgranular zone (SGZ) and of cells double-labelled for BrdU and the immature neuronal marker doublecortin (DCX) in the granule cell layer of HSV-1- vs. mock-infected mice (-28% and -41%, respectively, p<0.01). Western blot experiments also revealed a reduction of NeuroD1 and DCX protein expression at hippocampal level (-40% and -47%, respectively, vs. mock; p<0.05). Impaired neurogenesis correlated with A? accumulation in nestin+ cells of the SGZ of infected mice. In fact, it was not detectable in infected APP-/- mice unable to produce A?, despite the presence of HSV-1 in their brains. Overall, these results suggest that HSV-1-induced A? accumulation impairs adult hippocampal neurogenesis, likely contributing to AD-like phenotype occurring in mice subjected to multiple HSV-1 reactivations.