Passive immunization against amyloid peptide restores pattern separation deficits in early stage of amyloid pathology but not in normal aging

Aim: Aging and early Alzheimer’s disease (AD) affect pattern separation (PS) based mnemonic discrimination in humans. PS function involves the dentate gyrus (DG), a brain region producing new neurons during adulthood. Aging and AD presumably affect PS and DG function through different mechanisms, although it has never been clearly shown within the same study. Passive immunotherapy targeting β-amyloid peptides (Aβ) was used to determine the relative contribution of abnormal levels of Aβ to early PS deficits in two mouse models of aging and amyloid pathology, and potential involvement of adult neurogenesis. Methods: Female Tg2576 mice were tested in a spatial PS task from the age of three months to determine the age of onset of PS deficits. A cohort of five-month-old female Tg2576 mice and a cohort of 20-month-old male C57BL/6J mice were treated with passive immunization for four weeks, and then tested for PS performance. ELISA assays were used to quantify Aβ levels in CA3/DG regions of these mouse models. DG recruitment during PS testing was assessed with an Egr-1 ex vivo imagery. The contribution of adult-born neurons to a potential rescue of PS performances was evaluated using bromodeoxyuridine and doublecortin co-immunostainings. Results: Spatial PS deficits appeared first in four-month-old female Tg2576 mice, an early pre-plaque stage of Alzheimer pathology. Aβ immunotherapy restored PS performance in Tg2576 mice, but not in aged male C57BL/6J mice. PS impairments were associated with an overactivation of the DG in both models and a potentially abnormal level of immature adult-born neurons in Tg2576 mice. Conclusions: Alleviation of PS deficits following Aβ immunotherapy in Tg2576 mice is associated with reduced DG activation and improved adult-born neurons maturation. The absence of beneficial effects in aged mice suggests that PS deficits in aging and AD may be related to different underlying mechanisms.