Relaciones en la Comunidad Educativa Claretiana

Activation of specific cell signalling transduction pathways by APP is presumably important for APP to exert physiological effects, such as neurite outgrowth (Allinquant et al 1995), neural stem cell proliferation and differentiation (Clarris et al 1995, Hu et al 2013, Masliah et al 1992) and cell viability (Murayama et al 1996). However, the mechanisms underlying these effects have not been well clarified. In this section, the role of APP as a putative cell-surface receptor is discussed, as is role of the intracellular domain of APP (AICD).

1.3.4.4.1 APP may act as a cell -surface receptor

APP has been proposed to be a cell-surface receptor, because it shares in structural, post-translational modification and proteolytic processing similarities to Notch, a cell surface receptor involved in cell growth (De Strooper et al 1999, Selkoe & Kopan

2003). APP is also proposed to be a G-protein coupled receptor in a ligand-dependent and ligand-specific signalling (Okamoto et al 1995). Furthermore, APP has been reported to activate serine/threonine kinases and to stimulate the mitogen activated protein kinase (MAPK) pathway that transfers signals from the cell surface to the nucleus (Murayama et al 1996). In addition, the extracellular matrix protein, reelin binds the E1 domain of APP, causing a reduction in Aβ production and promoting neurite outgrowth (Hoe et al 2009b, Hoe et al 2006). Moreover, the diffusible molecule netrin-1 reportedly can interact with APP and participate in the regulation of Aβ production (Lourenco et al 2009). More recently, APP was shown to act as a co- receptor in netrin-1 mediated neural navigation and commissural axon outgrowth (Rama et al 2012). Although APP interacts with extracellular matrix proteins, the idea that APP may act as a cell-surface receptor is supported by that F-spondin’s ability to bind to the E2 domain of APP, APLP1 and APLP2 (Ho & Sudhof 2004). As a signalling glycoprotein secreted by neurons, F-spondin may exert effects on neuronal development and repair (Peterziel et al 2011). However, binding of F-spondin to APP

can lead to blockage of β-secretase cleavage. Therefore, F-spondin is proposed to

regulate APP processing (Ho & Sudhof 2004). Although several physiological ligands can interact with APP, APP induced intracellular signalling transduction is the strongest evidence for the idea that APP is a cell-surface receptor (Dawkins & Small 2014).

1.3.4.4.2 Role of APP in intracellular signalling

APP is a substrate for regulated intramembrane proteolysis. This is a mechanism that regulates membrane protein activity, and has been implicated in a wide range of biological processes (Brown et al 2000, Lichtenthaler & Steiner 2007). The C-

terminal region of proteins produced by the γ-secretase translocate to the nucleus and activate gene transcription (Ebinu & Yankner 2002). The intracellular domain of APP (AICD) is suggested to translocate to the nucleus, where it regulates transcriptional activation (Cupers et al 2001, Hebert et al 2006, Lichtenthaler et al 2011), although AICD easily undergoes degradation under normal conditions (Kimberly et al 2001). However, AICD may be stabilized by interaction with adaptor proteins (Kimberly et al 2001, Small et al 2005). AICD can be produced in different forms because the ε - cleavage by γ -secretase generates AICD fragments that start further towards the C- terminus (Sastre et al 2001). Caspase-dependent AICD cleavage starts at a C-terminal position upstream of the ε- cleavage site (Lu et al 2000).

There are three sequence motifs in AICD that have been suggested to have functional

significance. The first one is the 653YTSI sequence, that is proposed to contribute to

basolateral sorting of APP (Lai et al 1998), and participates in tyrosine-mediated and clathrin-based endocytic sorting (Bonifacino & Traub 2003). The second region, the

667_{VTPEER sequence motif of AICD, has been implicated in certain}

pathophysiological processes. For example, Thr668 in the sequence was found to undergo increased phosphorylation in AD patients (Lee et al 2003). Peptidyl-prolyl cis-trans isomerase NIMA-interacting 1 (Pin1), a prolyl isomerase, may affect the turnover of APP by halting GSK3β-induced phosphorylation at Thr668, as Pin1 overexpression reduces Aβ whereas a knockout of Pin1 results in an enhanced Aβ yield (Ma et al 2012a, Pastorino et al 2006). The third functional sequence motif, YENPTY, has drawn the most attention and been studied intensively. The YENPTY region has been shown to interact with many adaptor proteins that have phosphotyrosine binding (PTB) or phosphototyrosine interacting domains (PID)

(Borg et al 1996). APP homologues all contain the YENPTY motif, which means that all APP family members may interact with similar adaptor proteins and may function similarly as transcriptional regulators (Bressler et al 1996, Zheng & Koo 2011). This is consistent with the evidence for functional redundancy among APP homologoues from APP knockout mice studies.

The YENPTY motif has been shown to be bind to a variety of adaptor proteins: X11 family proteins (Borg et al 1996), Fe65 (Fiore et al 1995), Disable-1 protein (Homayouni et al 1999), JNK interacting protein 1 (Scheinfeld et al 2002b), ShcA/C (Tarr et al 2002) and growth factor receptor-bound protein 2 (Zhou et al 2004). Fe65 was the first identified APP binding partner (Fiore et al 1995) and has been investigated intensively. Fe65 binds to the YENPTY motif in a manner that is independent of tyrosine phosphorylation (Scheinfeld et al 2002a). Fe65 overexpression results in enhanced plasma membrane translocation of APP, and increased production of sAPPα and Aβ (Sabo et al 1999). This demonstrates that the interaction of APP with FE65 probably affects APP processing and trafficking. However, FE65 expression may lead to inhibition of APP maturation as well as a reduction in Aβ production when thr668 has been phosphorylated (Ando et al 2001).

Therefore, FE65 may bind to both 667VTPEER and YENPTY to mediate FE65-

dependent gene transactivation (Cao & Sudhof 2001, Sumioka et al 2005).

The interaction of FE65 with AICD can recruit another protein, TIP60, and this AICD/FE65/TIP60 complex is suggested to be involved in transcriptional activation of several target genes (Kim et al 2003, Pardossi-Piquard et al 2005, Zhang et al 2007b). Studies of AICD’s role in cell proliferation indicate that the

AICD/FE65/TIP60 complex may work as a negative regulator of neural stem/progenitor cell (NSPC) proliferation via down regulation of the epidermal growth factor receptor (EGFR) mediated gene transcription (Ayuso-Sacido et al 2010, Zhang et al 2007b). In addition, genes that participate in cell death and apoptosis, such as P53, are suggested to be a target of AICD-dependent activation (Alves da Costa et al 2006, Checler et al 2007, Ozaki et al 2006). Cell death and apoptosis occur in transfected cells expressing AICD (Konietzko 2012, Lu et al 2000), while those expressing mutant versions that contain no FE65 binding domain show little effect on cell survival. These findings demonstrate that the interaction of AICD with FE65 may be essential for mediating P53 - induced cell death (Konietzko 2012).

Apart from targets on EGFR or P53, AICD has also been reported to form multiple protein complexes affecting APP gene transcription (von Rotz et al 2004). In addition, the intracellular domains of APP and APLP are implicated in the regulation of neprilysin, which is an Aβ-degrading enzyme (Pardossi-Piquard et al 2005). Furthermore, APP and APLP2 have been shown to regulate cholesterol metabolism via AICD nuclear signaling that may involve modulation of the lipoprotein protein receptor 1 (LRP1) promoter (Liu et al 2007).