A Dominant Role for FE65 (APBB1) in Nuclear Signaling

Abstract

FE65 has been described as an adaptor protein; its partners include the ␤-amyloid precursor protein (APP) and Tip60 (a histone acetyltransferase). Recent evidence suggests that APP may function in a nuclear signaling pathway via formation of APP-FE65-Tip60 complexes. The evidence is largely based on experiments in which APP/Tip60 is fused to the DNA binding domain of a yeast transcriptional factor Gal4 (Gal4DB) that can activate a reporter gene only when FE65 is coexpressed. One interpretation of published experiments has not yet been tested; however, there is the possibility that FE65 itself is the dominant transcriptional activator, whereas APP and Tip60 serve merely as positive/negative modulators or bridges for connecting FE65 to Gal4DB. To test this possibility, we fused Gal4DB directly to either end of FE65 and assessed their nuclear signaling in the presence or absence of exogenous APP/Tip60 or after knockdown of endogenous APP/Tip60. We found that FE65-Gal4DB by itself was able to trigger robust reporter activities. Increasing levels of APP could not further augment the reporter activity, but knocking down endogenous APP or interrupting FE65-APP binding reduced the signaling by up to 2-fold. The magnitudes of the reporter activities did not correlate with relative FE65 affinities for APP. Both overexpression and knockdown experiments showed that Tip60 plays a negative role. The results are consistent with the notion that FE65 is the key agent of Gal4DB-mediated transcriptional transactivation, whereas Tip60 is an FE65-associated repressor. Although APP may have modest stimulating effects, apparently there is no absolute requirement for that molecule for the nuclear signaling pathway.FE65 is a brain-enriched protein and has the ability to interact with several different proteins via its three protein-protein interaction domains as follows: a WW domain and two phosphotyrosine interaction domains (PID 2 domains) (1, 2). PID1 interacts with Tip60 (Tatinteractive protein, 60 kDa); PID2 is the main region bound by the APP intracellular domain (AICD). These interactions may have impacts on APP processing (1, 3, 4), membrane dynamics (including axonal projections and neuronal positioning) (5-7), learning and memory (8), and transcriptional transactivation (9).APP is the best studied protein in the field of Alzheimer research because causal relationships have been established with mutations in APP and presenilins, enzymes involved in APP processing (10 -12). Proteolytic fragments of APP are also key components of Alzheimer pathology (13). Despite extensive research efforts, however, APP functions are poorly understood. The protein undergoes two consecutive cleavages at sites near or within its transmembrane domain (12). The first cleavages (by ␣-/␤-secretases) shed APP ectodomains into extracellular environments, perhaps leading to modulations of cell proliferation and adhesion, neurite outgrowth, and synaptogenesis (6, 14,15). A subsequent cleavage (by ␥-secretases complexes) liberates P3 or the ␤-amyloid peptid...

Physiological Functions of APP Family Proteins

“…g-secretase cleavage then releases Fe65 together with AICD, thereby allowing Fe65 to enter the nucleus and to interact with Tip60 (Cao and Südhof 2004); (2) Hass and Yankner revealed that PS-dependent AICD production is not required for the APP signaling activity as it proceeds normally in PS null cells and on PS inhibitor treatment (Hass and Yankner 2005). Instead, the investigators provided an alternative pathway involving Tip60 phosphorylation; (3) a later report documented that the proposed signaling activity is, in fact, executed by Fe65 independently of APP (Yang et al 2006). Last, the link of Fe65 to chromatin remodeling instead of transcription suggests that APP may not act on specific genes, but rather modulates the overall transcriptional state of a cell (Giliberto et al 2008).…”

FE65 regulates and interacts with the Bloom syndrome protein in dynamic nuclear spheres – potential relevance to Alzheimer's disease

“…However, the identification of several putative target genes have been controversially discussed (Baek et al, 2002;Bimonte et al, 2004;Müller et al, 2007;Telese et al, 2005;von Rotz et al, 2004). Elucidation of the role of FE65 in a gene expression complex is further complicated by the observation that FE65 itself triggers robust gene expression in a reporter assay (Yang et al, 2006). Microscopy studies revealed that the above described complex consisting of AICD, FE65 and TIP60, is visible as speckles in the nucleus (Muresan and Muresan, 2004;von Rotz et al, 2004).…”

Evidence for a role of the amyloid precursor protein in thyroid carcinogenesis

“…The AICD contains a conserved YENPTY motif that enables interaction with a number of binding proteins via their phosphotyrosine-binding domain, for example with Fe65, JNK interacting protein ( JIP1) and ShC proteins (De Strooper & Annaert 2000). In particular, the APP adaptor protein Fe65 is involved in nuclear signalling of APP and in cell damage response (von Rotz et al 2004, Yang et al 2006. In addition, APP has been linked to the pro-apoptotic JNK pathway via JIP1 (Matsuda et al 2001, Taru et al 2002b.…”