Phosphorylation regulates APP and Fe65, two key players in Alzheimer’s disease

Abstract: Alzheimer’s disease (AD) is a slow progressive neurodegenerative disease characterized by the accumulation of toxic amyloid beta (Aβ) peptide within the brain. APP plays an important role in AD, as the Aβ is formed when APP is sequentially cleaved by β- and γ-secretase. This is known as amyloidogenic processing of APP. However, non-amyloidogenic processing, in which APP is cleaved by α-secretases in the middle of the Aβ sequence, giving rise to the neuroprotective fragment sAPPα is also possible. In addition to amyloidogenic and non-amyloidogenic processing, APP can be processed along non-canonical pathways by δ, η , capase or Meprin β, resulting in numerous fragments which may have different functional properties. In paper 1, we for the first time show that phosphorylation of APP at Ser675 alters APP processing resulting in a significant decrease in the release of total sAPP and sAPPα, without affecting the plasma membrane level of APP. We further show an increased level of a slower migrating APP-CTF, similar to the expected size of β-secretase generated C99-CTF. However, no expression of the major β-secretase BACE1 was found in the model used and in the presence of metalloprotease inhibitors, generation of the slower migrating CTF was blocked. Taken together these findings suggest that Ser675 phosphorylation might promote APP processing by the metalloprotease Meprin β, an alternative β-secretase localized at the plasma membrane. How Ser675 phosphorylation could promote Meprin β cleavage is unclear, but an altered APP protein interaction could be involved as we found that mutation of Ser675 increased APP interaction with the adaptor protein Fe65. In paper 2, we wanted to elucidate more about how Fe65 is regulated and found that phosphorylated forms of Fe65 preferentially localize to the cytoplasm. Furthermore we showed that the Fe65 PTB2 domain, rather than the WW domain, plays an important role in localizing Fe65 to the nucleus. Together our results show that phosphorylation regulates both APP and Fe65, two important proteins linked to AD.