Study of the intracellular domain of amyloid beta precursor protein
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Cleavage of AbetaPP by beta- and gamma-secretase yields Abeta, which is the major component of the amyloid plaques found in Alzheimer's Disease (AD), and the AbetaPP intracellular domain (AID), which may mediate AbetaPP signaling. The phosphorylation of AID at Thr668 and Tyr 682 is significantly enhanced in AD brain. These phosphorylation events may control AbetaPP function by regulating the affinity of AbetaPP for distinct binding partners.;In this study, growth factor receptor-bound protein 2 (Grb2) has been identified as an AID binding partner. Our data showed that Grb2 directly interacted with AbetaPP in a phospho-Tyr682 dependent manner. Unlike the other interactors bound to the YENPTY motif via a phosphotyrosine binding (PTB) domain, Grb2 bound to YENPTY via Src homology 2 (SH2) regions. The ratio of Abeta 42/40 was increased when Grb2 was depleted by siRNA. Given that Grb2 mediates downstream MAP kinase pathways, we evaluated the effect of MAP kinase signaling pathways on AbetaPP functions. Our study demonstrated that JNK regulates AbetaPP gene transcription, and p38 mediates the glycosylation of AbetaPP. Together with previous reports that the complex of Grb2-AbetaPP is increased in AD brains, our data suggests that Grb2 might mediate some biological and pathological functions of AbetaPP.;Neuronal protein Fe65 has the ability to bind the intracellular domain of AbetaPP via its second PTB domain. In contrast to the accepted belief, we found that interaction between Fe65 family proteins and AbetaPP family members was abolished by phosphorylation of Tyr682. Furthermore, this tyrosine was essential for AID-Fe65 nuclear signaling. Our results define a new role for Tyr682 phosphorylation in AbetaPP/Fe65 signaling pathways, and suggest that the nuclear translocation and transcriptional activity of Fe65 is regulated by both AbetaPP processing and phosphorylation. This evidence, together with the finding that other AbetaPP interactors such as ShcA, ShcC and Grb2 bind AbetaPP only if Tyr682 is phosphorylated, indicate that Tyrosine phosphorylation of AbetaPP represents a "biochemical switch" that determines the molecular composition of AbetaPP complexes. Our results demonstrated that phosphorylation of AbetaPP on Tyr682 represents a second mechanism, alternative to AbetaPP processing by secretases, that regulates AbetaPP/Fe65 downstream signaling pathways.
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