A large number of studies have explored the intracellular sites of Aβ production, mostly in AD models. Aβ42 and Aβ40 monomers have been previously demonstrated in ER[6–8], TGN and post-TGN secretory vesicles, mitochondria, endosomes, lysosomes, multivesicular bodies (MVB), and cytosol[12, 45–47]. However, little is known about intracellular localization of Aβ in normal conditions, when Aβ is not overproduced.
In this study, we demonstrated that in differentiated neuroblastoma cells cultured under normal in vitro conditions, only little Aβ (including Aβ42 and Aβ40) showed colocalization with organelles such as TGN, Golgi-derived vesicles, early and late endosomes, lysosomes, or exocytotic vesicles, while the greater part of Aβ was located in the cytosol or in undetermined compartments.
The absence of major Aβ immunoreactivity in these cellular compartments, in which it was found in AD, as well as in cellular and in vivo AD models, suggests that, under normal conditions, this peptide is either relocated, or degraded, or secreted extracellularly. The fact that lysosomes showed little Aβ immunoreactivity would suggest that cells are able to perform a rapid proteolytic digestion of this peptide under normal biological conditions. In support of this hypothesis, we have previously shown that inhibition of lysosomal enzymes induces Aβ accumulation within the lysosomal compartment.
In addition, we have found that inhibition of exocytosis by TeNT induced a general increase of intracellular Aβ, both intra- and extralysosomal. As we previously reported, the intralysosomal Aβ accumulation can be mediated by enhanced Aβ autophagy. It is also possible that inhibition of exocytosis results in Aβ accumulation along the secretory pathway, including ER, Golgi apparatus, transport visicles and secretory vesicles.
Although under normal conditions late endosomes and lysosomes seem to be free of Aβ, this is not the case for AD neurons, in which Aβ has been demonstrated intralysosomally[10, 27, 28]. It is not clear what causes these changes and how Aβ relocation to lysosomes contributes to the pathogenesis of AD. One possible explanation is that oxidative stress might enhance autophagy, leading to intralysosomal Aβ accumulation, consequent lysosomal membrane damage and release of lysosomal enzymes to the cytosol, culminating in apoptosis[29, 30].
In AD, Aβ has been shown to accumulate within lysosomes, apparently promoting neuronal death through lysosomal destabilization[22, 25, 49]. As we previously demonstrated, intralysosomal Aβ accumulation can be triggered by oxidative stress and consequent activation of macroautophagy[29, 30]. On the other hand, Aβ has been shown to induce oxidant-mediated autophagic cell death in cultured cells, while antioxidants can protect cells from Aβ-mediated oxidative damage.
The fact that in the majority of AD cases there is no consistent overproduction of Aβ suggests that deficits in its degradation could lie behind the pathogenesis of the disease. On the other hand, intracellular accumulation of Aβ is proposed to compromise normal neuronal function in AD. Our findings demonstrate that, under normal conditions, intracellular Aβ (including Aβ42 and Aβ40) is mainly associated with cytosolic structures and, to a large extent, is secreted from the cells. They may also suggest that deficits in secretion or lysosomal processing would result in intracellular Aβ accumulation and its translocation to the cellular organelles, as seen in AD and its models[12, 21, 52, 53]. Our finding may contribute to better understanding of AD pathogenesis, and may help develop new therapeutic strategies against AD (reviewed in).