How changes in APP processing by secretase
How changes in APP processing by γ-secretase lead to AD is still not well understood. Deposition of amyloid beta (Aβ) peptide fragments of the amyloid precursor protein (APP) in amyloid plaques and hyperphosphorylated tau in neurofibrillary tangles are the cellular hallmarks of AD (O’Brien and Wong, 2011). The amyloid hypothesis proposes that it is the accumulation of extracellular toxic forms of Aβ as both soluble oligomers and insoluble plaques that is the primary pathologic driver of the disease (Hardy and Selkoe, 2002). However, causal links between production of potentially toxic longer forms of Aβ peptides, amyloid deposition, and disease initiation have not been definitively demonstrated and remain contentious (De Strooper and Karran, 2016).
Nascent APP proteins mature through the constitutive secretory pathway from endoplasmic reticulum and the Golgi apparatus and are transported to the plasma membrane (Thinakaran and Koo, 2008). Endosomal and lysosomal compartments are involved in APP processing and perturbation of the endolysosomal system can lead to altered APP processing, Aβ peptide production and amyloid accumulation (Nixon, 2017). Conversely, increased Cholic acid of APP or expression of mutant APP in fibroblasts leads to dysfunction of the endolysosomal system (Cataldo et al., 2008). AD mutations in PSEN1 lead to altered lysosomal function, including reduced acidification through altered PSEN1-mediated recruitment of the Vo ATPase to the lysosome, which in turns leads to defects in neuronal macroautophagy (Lee et al., 2010). Moreover, PSEN1 deficiency has been shown to alter lysosomal calcium storage and release (Coen et al., 2012, Neely Kayala et al., 2012). While the appearance of pathological changes in the endolysosomal and autophagy systems is an important early feature of AD, whether these changes are causes or consequences of primary drivers of neurodegenerative processes remain unclear (Peric and Annaert, 2015).
To address this question, we investigated how AD mutations in APP and PSEN1 affect the endolysosomal and autophagy systems in human-stem-cell-derived cortical neurons. We and others have previously shown that mutations in APP and PSEN1 lead to immediate changes in APP processing, with either increased relative or absolute amounts of longer forms of Aβ peptides (Yagi et al., 2011, Muratore et al., 2014, Moore et al., 2015). We now find that mutations in both APP and PSEN1 causal for monogenic, early onset AD, lead to major defects in lysosome function and autophagy in induced pluripotent stem cell (iPSC)-derived human neurons, with APP mutations also disrupting endosomal function. Reducing production of C-terminal APP by inhibition of BACE1 rescued these phenotypes in both APP and PSEN1 mutant neurons, and defects in the lysosomal and autophagic systems due to PSEN1 mutations are prevented by CRISPR knockout of APP. These data demonstrate that compromised proteolysis of the C-terminal of APP by γ-secretase by autosomal dominant mutations in both PSEN1 and APP causes dysfunction of the endolysosomal and autophagy systems in human neurons and is a driver of neurodegeneration in familial AD.
Discussion We report here that mutations in APP and PSEN1 that are causal for early onset AD lead to major defects in lysosome function and autophagy in human neurons. The rapid appearance of these defects indicates that they are a direct effect of those mutations, and not a secondary consequence of a primary degenerative process, such as protein aggregation. Consistent with this, we find that inhibition of γ-secretase causes lysosomal and autophagy dysfunction in healthy human neurons. APP and PSEN1 mutant neurons accumulate APP-β-CTF and reducing production of C-terminal APP by inhibition of BACE1 rescued lysosome-autophagy phenotypes in both APP and PSEN1 mutant neurons. Defects in the lysosome and autophagy systems due to PSEN1 mutations are prevented by CRISPR knockout of APP, demonstrating that lysosome and autophagy dysfunction in PSEN1 mutant neurons are dependent on the presence of APP. These data demonstrate that compromised proteolysis of the C-terminal of APP by γ-secretase by autosomal dominant mutations in PSEN1 and APP causes dysfunction of the lysosomal and autophagy systems in human neurons and represents a potential driver of neurodegeneration in monogenic AD.