Fibrillar β-amyloid 1-42 alters cytokine secretion, cholinergic signalling and neuronal differentiation

Abstract

Adult neurogenesis is impaired by inflammatory processes, which are linked to altered cholinergic signalling and cognitive decline in Alzheimer's disease. In this study, we investigated how amyloid beta (Aβ)-evoked inflammatory responses affect the generation of new neurons from human embryonic stem (hES) cells and the role of cholinergic signalling in regulating this process. The hES were cultured as neurospheres and exposed to fibrillar and oligomeric Aβ(1-42) (Aβf, AβO) or to conditioned medium from human primary microglia activated with either Aβ(1-42) or lipopolysaccharide. The neurospheres were differentiated for 29 days in vitro and the resulting neuronal or glial phenotypes were thereafter assessed. Secretion of cytokines and the enzymes acetylcholinesterase (AChE), butyrylcholinesterase (BuChE) and choline acetyltransferase (ChAT) involved in cholinergic signalling was measured in medium throughout the differentiation. We report that differentiating neurospheres released various cytokines, and exposure to Aβf, but not AβO, increased the secretion of IL-6, IL-1β and IL-2. Aβf also influenced the levels of AChE, BuChE and ChAT in favour of a low level of acetylcholine. These changes were linked to an altered secretion pattern of cytokines. A different pattern was observed in microglia activated by Aβf, demonstrating decreased secretion of TNF-α, IL-1β and IL-2 relative to untreated cells. Subsequent exposure of differentiating neurospheres to Aβf or to microglia-conditioned medium decreased neuronal differentiation and increased glial differentiation. We suggest that a basal physiological secretion of cytokines is involved in shaping the differentiation of neurospheres and that Aβf decreases neurogenesis by promoting a microenvironment favouring hypo-cholinergic signalling and gliogenesis.

Keywords: Alzheimer's disease; gliogenesis; human embryonic stem cells; human microglia; inflammation; neurogenesis; neurospheres.