Homophilic Interactions of the Amyloid Precursor Protein (APP) Ectodomain Are Regulated by the Loop Region and Affect -Secretase Cleavage of APP

Institut für Chemie und Biochemie, Freie Universität Berlin, 14195 Berlin, Germany.
Journal of Biological Chemistry (Impact Factor: 4.57). 04/2008; 283(11):7271-9. DOI: 10.1074/jbc.M708046200
Source: PubMed


We found previously by fluorescence resonance energy transfer experiments that amyloid precursor protein (APP) homodimerizes in living cells. APP homodimerization is likely to be mediated by two sites of the ectodomain and a third site within the transmembrane sequence of APP. We have now investigated the role of the N-terminal growth factor-like domain in APP dimerization by NMR, biochemical, and cell biological approaches. Under nonreducing conditions, the N-terminal domain of APP formed SDS-labile and SDS-stable complexes. The presence of SDS was sufficient to convert native APP dimers entirely into monomers. Addition of an excess of a synthetic peptide (APP residues 91-116) containing the disulfide bridge-stabilized loop inhibited cross-linking of pre-existing SDS-labile APP ectodomain dimers. Surface plasmon resonance analysis revealed that this peptide specifically bound to the N-terminal domain of APP and that binding was entirely dependent on the oxidation of the thiol groups. By solution-state NMR we detected small chemical shift changes indicating that the loop peptide interacted with a large protein surface rather than binding to a defined pocket. Finally, we studied the effect of the loop peptide added to the medium of living cells. Whereas the levels of alpha-secretory APP increased, soluble beta-cleaved APP levels decreased. Because Abeta40 and Abeta42 decreased to similar levels as soluble beta-cleaved APP, we conclude either that beta-secretase binding to APP was impaired or that the peptide allosterically affected APP processing. We suggest that APP acquires a loop-mediated homodimeric state that is further stabilized by interactions of hydrophobic residues of neighboring domains.

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Available from: Tobias Bethge, Nov 18, 2015
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    • "Despite advances in our understanding of amyloid-b precursor protein (APP) processing in Alzheimer disease, its normal physiological function has proven more difficult to elucidate (Turner et al., 2003; Anliker and Müller, 2006). Several membrane-anchored proteins interact with the extracellular domain of APP and modulate APP processing (Rice et al., 2013), such as APP family members themselves (Soba et al., 2005; Kaden et al., 2008), TAG-1 (Ma et al., 2008), Notch family members, the Nogo-66 receptor (Wolfe and Guénette, 2007), BRI2 (Fotinopoulou et al., 2005; Matsuda et al., 2005) and Pancortins (Rice et al., 2012). Another class of binding partners for APP comprises extracellular matrix components, such as collagen, laminin, fibulin-1 and heparan sulfates (Turner et al., 2003; Reinhard et al., 2005), and NgCAM and contactins (Osterfield et al., 2008). "
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    • "It is yet to be determined the functional effects mediated by homo-and heterodimerisation of the APP family members, although it has been hypothesized that cis-dimerisation of membrane bound APP with sAPPa may allow activation of intracellular signalling pathways (Gralle et al. 2009; Reinhard et al. 2005). Alternatively cis dimerisation may simply influence the processing of APP, possibly by changing its conformational state, thereby decreasing its amyloidogenic processing by preventing access of b-secretase to its cleavage site (Kaden et al. 2008). It is possible that sAPPa may act in a similar manner to known growth factors, whereby heparin is required to facilitate binding of APP to a co-receptor, through the APP96-110 region. "
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