APP can be phosphorylated at multiple sites in both extracellular and intracellular domains (reviewed by [
131]). Among these, the phosphorylation at the threonine residue within the VT
668PEER motif (Thr
668) in the APP intracellular domain (Figure
1) has received most of the attention. Several kinases have been implicated in this phosphorylation event, including cyclin-dependent kinase 5 (CDK5), c-Jun N-terminal kinase 1 (JNK1) and JNK3, CDK1/CDC2 kinase and GSK3β [
132‐
135]. Phosphorylation at this residue has been reported to result in several outcomes. First, it has been implicated to regulate APP localization to the growth cones and neurites [
134,
136], a finding consistent with the preferential transport of Thr
668 phosphorylated APP to nerve terminals [
137]. Second, phosphorylation at Thr
668 has been reported to contribute to Aβ generation, a finding consistent with an increase of Thr
668 phosphorylated APP fragments in brains of AD individuals [
138]. Third, Thr
668 phosphorylation leads to resistance of APP to be cleaved by caspases between Asp
664 and Ala
665 residues, an event that has been proposed to result in increased vulnerability to neuronal death (see below). Fourth, phosphorylation at Thr
668 leads to a conformational change in the APP cytoplasmic domain such that interaction with the cytoplasmic adaptor Fe65 through the distal YENPTY motif [
139] is altered, thereby affecting the proposed nuclear signaling activity of the APP-Fe65 complex [
140]. As the YENPTY motif has been shown to bind several other cytosolic adaptor proteins, it is not surprising then that Thr
668 phosphorylation has also been reported to modulate APP interaction with Mint-1/X11a [
141]. Lastly, following phosphorylation, it has been shown that the peptidyl-propyl cis/trans isomerase Pin1 catalyzes the cis to trans isomerization of the Thr
668-Pro
669 bond and this is predicted to alter APP conformation [
142], possibly related to the Fe65 or Mint-1/X11a interaction with APP. In support of this idea, it was shown that loss of Pin1 in mice resulted in accumulation of hyperphosphorylated tau and increased Aβ levels [
142,
143], two features that should accelerate AD pathology in the brain. Nevertheless, knockin mice replacing the Thr668 with a non-phosphorylatable Ala residue did not result in substantive changes in either APP localization or in the levels of Aβ in brain [
144], raising the question whether Thr
668 phosphorylation plays a significant role in regulating APP trafficking and Aβ generation
in vivo.
In addition to Thr
668 phosphorylation, the highly conserved APP intracellular domain has been shown to bind to numerous proteins (reviewed in [
145,
146]). Of particular interest and relevance to this review, the Y
682ENPTY motif is required to interact with various adaptor proteins, including Mint-1/X11a (and the family members Mint-2 and Mint-3, so named for their ability to interact with Munc18), Fe65 (as well as Fe65 like proteins Fe65L1 and Fe65L2) and c-Jun N-terminal kinase (JNK)-interacting protein (JIP), through the phosphotyrosine-binding (PTB) domain. The Y
682 has been shown to modulate APP processing in vivo [
147]. Of interest is the finding that Fe65 acts as a functional linker between APP and LRP (another type I membrane protein containing two NPXY endocytosis motifs) in modulating endocytic APP trafficking and amyloidogenic processing [
148].