The function of APP and its intracellular domain AICD

Despite the importance of APP in Alzheimer’s disease the physiological role of APP still remains largely unclear. This is partly due to previous research primarily focusing on the Aβ peptide itself. APP has been implicated in several physiological processes such as cell adhesion, proliferation and differentiation, as well as neurite outgrowth, synaptogenesis and synaptic plasticity (reviewed in (Zheng and Koo, 2011). This section will briefly outline the role played by APP in these processes.

1.4.2.1 APP and neurite outgrowth, synaptic plasticity and synaptogenesis

Neurite outgrowth, synaptogenesis and synaptic plasticity are some of the physiological process APP is thought to be involved in. APP is known to promote neurite outgrowth in cell culture through its role in cell adhesion. APP binds to laminin, collagen type 1 and heparin sulphate, which all influence neurite outgrowth (Kibbey et al., 1993, Beher et al., 1996). APP is also thought to be involved in cell-cell adhesion. It is known to form homo- or hetero-dimers, with trans-dimerisation able to promote cell adhesion, as well as the binding of heparin sulphate to the E1 or E2 domains of APP (Soba et al., 2005, Dahms et al., 2010). APP is also thought to influence the activity of other cell adhesion molecules such as β1-integrin, and plays an important role in regulating adult hippocampal neurogenesis (Young-Pearse et al., 2008, Wang et al., 2014a).

APP is expressed at both pre and post synaptic sites during development, as is highly expressed during periods of synaptogenesis. Therefore it is thought to be involved in regulating synaptogenesis (Dawkins and Small, 2014). APP knock out mice show neurological defects like impaired locomotor activity, which may be explained by an effect on synaptogenesis (Zheng et al., 1995). APP knock out mice also show altered

synaptic function. Mice with double knock out of APP and APLP2 have impaired neuromuscular junction formation, observed by a reduction in synaptic vesicles and impaired synaptic transmission, which may be responsible for the lethality of the APP/APLP2 double knock out (Wang et al., 2005). APP is also thought to be involved in synaptic plasticity mainly by affecting synaptic calcium homeostasis. It is thought to alter the expression of the GluR2 subunit of the AMPA receptor, a regulator of synaptic calcium permeability (Lee et al., 2010). Aged knock out mouse models of APP show defects in long term potentiation (LTP), a persistent strengthening of synapses (Ring et al., 2007). Knock in (KI) mice of the soluble ectodomain of APP (APPsα) in an APLP2 knock out background (APPsα-KI/APLP2-KO) survive to adulthood, unlike APP/APLP2 KO mice. A reduction in LTP is also observed in APPsα-KI/APLP2-KO mice (Weyer et al., 2011). The data from these mouse models reveal that APP and APLP2 are important in the function and plasticity of central synapses (Korte et al., 2012).

1.4.2.2 APP and cell signalling

The structure of APP has led to the theory that APP acts as a receptor, and therefore has a potential role in cell signalling. Due to the similarity of the proteolytic processing of APP with that of the Notch receptor, APP has been proposed to function in cell signalling in a manner similar to Notch signalling (Nakayama et al., 2011). In Notch signalling the intracellular domain of Notch is cleaved by γ-secretases and is translocated to the nucleus where it is known to activate gene transcription. The intracellular domain of APP (AICD) is also translocated to the nucleus, where it is known to control gene expression (Cupers et al., 2001). This supports the theory that APP may function in a similar way to Notch. APP may also play a role in cell signalling via its binding to G-proteins, mediated by AICD. It is thought that binding of a ligand to the ectodomain of APP may result in signal transduction via activation of the GTP- binding protein Gαo (Okamoto et al., 1995). Recently, the interaction of the insect APP homologue APPL with Gαo has been shown to be involved in the control of neuronal

migration (Ramaker et al., 2013). In support of APP’s role in cell signalling its homologue APPL in Drosophila was shown to be involved in wnt/planar cell polarity signalling (Soldano et al., 2013).

The intracellular domain of APP is known to be key to the function of the protein, and in its role as a cell signalling molecule. APP cleavage by secretases releases AICD which is known to translocate to the nucleus, where it has a role in gene expression. AICD is able to bind to Fe65 via the YENPTY motif at the C-terminus of AICD (Fiore et al., 1995). Fe65 contains two phosphotyrosine binding (PTB) domains, and binds to AICD via PTB2, the binding of which is thought to stabilise AICD (Beckett et al., 2012). Fe65 interacts with the histone acetyltransferase tat-interactive protein (Tip60) via PTB1, and together with AICD are delivered to the nucleus by dynamin mediated retrograde trafficking, where they affect gene expression (Beckett et al., 2012). AICD is reported to affect the expression of several different genes including APP itself, BACE-1, Aβ- degrading enzyme neprilysin (NEP) and epidermal growth factor receptor genes (von Rotz et al., 2004, Pardossi-Piquard et al., 2005, Belyaev et al., 2009, Zhang et al., 2007a). AICD, via its YENPTY motif is also known to bind to mDAB-1 which, when phosphorylated, is able to recruit kinases such as Src therefore providing a link between APP and phosphorylation dependent signalling pathways (De Strooper and Annaert, 2000).

APP has been linked to a wide variety of physiological functions and is probably one of the most well studied proteins. Despite this there is still currently no molecular model available for exactly how APP is involved in neurite outgrowth, synaptic plasticity, and synaptogenesis, and how it functions as a cell signalling molecule. This is primarily due to the lack of understanding of how APP can regulate such a wide variety of processes.