Intracellular and secreted Alzheimer β-amyloid species are generated by distinct mechanisms in cultured hippocampal neurons

Cerebral plaques containing β-amyloid (βA4) represent an invariant pathological feature of Alzheimer disease (AD). βA4 is proteolytically generated from its parent molecule, amyloid precursor protein (APP). In nonneuronal cells βA4 has been shown to be secreted via a pH-sensitive and endocytosis-dependent pathway, and this process, when occurring in the brain, is considered to play an important role in AD. In neurons the mechanisms of βA4 production are not known. Here we have analyzed these mechanisms by expressing human APP and its mutant versions in hippocampal neurons using the Semliki forest virus system. We show that these cells initially generate two pools of βA4, an extracellular and an intracellular, and only the extracellular pool is produced via a pH-sensitive and endocytosis-dependent pathway. Thus, hippocampal neurons are able to utilize an alternate pathway to produce intracellular βA4. We also show that a common feature of two types of APP mutations (“Swedish” and “London”) implicated in early-onset AD is their increased production of C-terminally elongated βA4 (β(42)), both intra- and extracellularly. Since neurons are the only cells that produce substantial levels of intracellular βA4 and also the main victims in AD, these findings may provide an important link between βA4 and neurodegeneration.