Holo-APP and G-protein-mediated signaling are required for sAPPα-induced activation of the Akt survival pathway

Abstract

Accumulating evidence indicates that loss of physiologic amyloid precursor protein (APP) function leads to reduced neuronal plasticity, diminished synaptic signaling and enhanced susceptibility of neurons to cellular stress during brain aging. Here we investigated the neuroprotective function of the soluble APP ectodomain sAPPα (soluble APPα), which is generated by cleavage of APP by α-secretase along the non-amyloidogenic pathway. Recombinant sAPPα protected primary hippocampal neurons and SH-SY5Y neuroblastoma cells from cell death induced by trophic factor deprivation. We show that this protective effect is abrogated in neurons from APP-knockout animals and APP-depleted SH-SY5Y cells, but not in APP-like protein 1- and 2- (APLP1 and APLP2) depleted cells, indicating that expression of membrane-bound holo-APP is required for sAPPα-dependent neuroprotection. Trophic factor deprivation diminished the activity of the Akt survival pathway. Strikingly, both recombinant sAPPα and the APP-E1 domain were able to stimulate Akt activity in wild-type (wt) fibroblasts, SH-SY5Y cells and neurons, but failed to rescue in APP-deficient neurons or fibroblasts. The ADAM10 (a disintegrin and metalloproteinase domain-containing protein 10) inhibitor GI254023X exacerbated neuron death in organotypic (hippocampal) slice cultures of wt mice subjected to trophic factor and glucose deprivation. This cell death-enhancing effect of GI254023X could be completely rescued by applying exogenous sAPPα. Interestingly, sAPPα-dependent Akt induction was unaffected in neurons of APP-ΔCT15 mice that lack the C-terminal YENPTY motif of the APP intracellular region. In contrast, sAPPα-dependent rescue of Akt activation was completely abolished in APP mutant cells lacking the G-protein interaction motif located in the APP C-terminus and by blocking G-protein-dependent signaling with pertussis toxin. Collectively, our data provide new mechanistic insights into the physiologic role of APP in antagonizing neurotoxic stress: they suggest that cell surface APP mediates sAPPα-induced neuroprotection via G-protein-coupled activation of the Akt pathway.

Dysregulated ADAM10-Mediated Processing of APP during a Critical Time Window Leads to Synaptic Deficits in Fragile X Syndrome

“…Soluble APPa is upregulated during synaptogenesis and increases synaptic density, cortical synaptogenesis, and memory retention in vivo (Bell et al, 2008;Hick et al, 2015;Roch et al, 1994;Tyan et al, 2012), and it has been proposed to have both a neurotrophic and neuroprotective effect (Milosch et al, 2014;Mü ller and Zheng, 2012). The disintegrin metalloprotease ADAM10 is the constitutive a-secretase required for sAPPa generation (Kuhn et al, 2010).…”

Amyloid Precursor Proteins Are Dynamically Trafficked and Processed during Neuronal Development

“…Most neurons within the developing Manduca CNS also contained these amphisome-like vesicles enriched in APPL (Figure 6), as noted in previous studies on Drosophila (Torroja et al, 1996). These observations suggest that the transient sequestration of APP family proteins into this compartment might provide a novel developmental mechanism for regulating interactions between the holoproteins (or their cleaved ectodomains) with other membrane-associated signaling complexes involved in neuronal growth, synaptic plasticity, and injury responses (Kogel et al, 2012; Wentzell et al, 2012; Milosch et al, 2014). …”

“…In particular, members of the APP family can function as unconventional G protein-coupled receptors (Nishimoto et al, 1993; Giambarella et al, 1997; Swanson et al, 2005), transducing responses to local cues via the heterotrimeric G protein Goα to regulate neuronal guidance (Brouillet et al, 1999; Ramaker et al, 2013, 2016). Conversely, APP-Goα interactions have been found to decrease in patients suffering from AD, suggesting that the dysregulation of normal APP-Goα signaling might provoke neuropathological responses (Shaked et al, 2009; Sola Vigo et al, 2009; Milosch et al, 2014). …”

APP Receptor? To Be or Not To Be

“…This concept has emerged from several studies, which demonstrated that accumulation or overexpression of APP-NTF (and in some instances Aβ itself [17] ) induced or exacerbated signaling associated with several classes of receptors as described above (see also Table 1). Recently, it has been shown that soluble APP and the APP-E1 domain could activate the Akt survival pathway following trophic factor depletion and that this activation involves an interaction of APP or APP-E1 domain with APP-FL [152] . In a similar fashion, Aβ interaction with APP induces G-protein-dependent enhancement of synaptic activity that involves the APP-E1 domain [153] .…”

“…As previously discussed in section 1, APP-FL possesses the structural ability to bind ligand to its own E1 domain. This association could initiate subsequent G-protein coupled signaling events by engaging motifs within the cytosolic C-terminal tail of APP [152, 153] . This scenario is reminiscent of a complex formation between a ligand and an autoreceptor.…”