Trafficking of Cell Surface β-Amyloid Precursor Protein: Retrograde and Transcytotic Transport in Cultured Neurons

Amyloid β-protein (Aβ), the principal constituent of senile plaques seen in Alzheimer's disease (AD), is derived by proteolysis from the β-amyloid precursor protein (βPP). The mechanism of Aβ production in neurons, which are hypothesized to be a rich source of Aβ in brain, remains to be defined. In this study, we describe a detailed localization of cell surface βPP and its subsequent trafficking in primary cultured neurons. Full-length cell surface βPP was present primarily on perikarya and axons, the latter with a characteristic discontinuous pattern. At growth cones, cell surface βPP was inconsistently detected. By visualizing the distribution of βPP monoclonal antibodies added to intact cultures, βPP was shown to be internalized from distal axons or terminals and retrogradely transported back to perikarya in organelles which colocalized with fluid-phase endocytic markers. Retrograde transport of βPP was shown in both hippocampal and peripheral sympathetic neurons, the latter using a compartment culture system that isolated cell bodies from distal axons and terminals. In addition, we demonstrated that βPP from distal axons was transcytotically transported to the surface of perikarya from distal axons in sympathetic neurons. Indirect evidence of this transcytotic pathway was obtained in hippocampal neurons using antisense oligonucleotide to the kinesin heavy chain to inhibit anterograde βPP transport. Taken together, these results demonstrate novel aspects of βPP trafficking in neurons, including retrograde axonal transport and transcytosis. Moreover, the axonal predominance of cell surface βPP is unexpected in view of the recent report of polarized sorting of βPP to the basolateral domain of MDCK cells.