Implications of neuronal Ab-uptake in Ab-induced toxicity

LUCILA SAAVEDRA ; VICTORIA MA; ELENA POSSE DE CHAVES

Madrid -Espania

Congreso; ICAD; 2006

The binding of Ab to the plasma membrane appears to be a critical step in the events leading to the development of AD. Due of its structure, Ab is able to interact with a variety of biomolecules including lipids, proteins and proteoglycans. Lipid rafts are membrane microdomains enriched in cholesterol, sphingomyelin and glycosphingolipids such as GM1. Considerable evidence suggests that rafts might represent sites of initial Ab deposition. Moreover, Ab interaction with lipid bilayers promotes Ab fibrilization in vitro. Although the molecular mechanisms of Aâ interaction with membranes have been extensively studied, little information is available regarding the requirement of Aâ uptake in Aâ-induced toxicity. Using a three-compartmented culture system we recently demonstrated that exposure of neurons to oligomeric Aâ exclusively in distal axons leads to axonal degeneration and a more pronounced nuclear apoptosis than neurons exposed in the cell bodies (Song et al, 2005). However in those studies we did not assess the need for Ab uptake for Ab-induced toxicity. Here we examined the mechanism of oligomeric Ab1-42 internalization. We treated neurons with a fluorescein-conjugated Ab1-42 peptide exclusively in the distal axon- or the cell body-containing compartments. We found that Ab1-42 is internalized mainly through the distal axons and retrogradely transported to cell bodies. Conversely, neurons treated with fluorescein- Ab1-42 exclusively in the cell bodies showed very little Ab1-42 internalization and anterograde transport was negligible. This result suggests that Ab1-42 enters the cell by a regulated mechanism. Two main internalization pathways exist: namely clathrin- and caveolae-mediated endocytosis. Both mechanisms require dynamin, but only the former is mediated by clathrin. Therefore, we expressed HA-tagged wild-type dynamin-1, HA-tagged dynK44A mutant, or T7-tagged clathrin hub in neurons and analyzed Ab1-42 uptake. Our data suggest the oligomeric Aâ1-42 internalization is independent of clathrin.