Zhang X, Zhang CM, Prokopenko D, Liang Y, Zhen SY, Weigle IQ, Han W, Aryal M, Tanzi RE, Sisodia SS. An APP ectodomain mutation outside of the Aβ domain promotes Aβ production in vitro and deposition in vivo. J Exp Med. 2021 Jun 7;218(6) PubMed.
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Icahn School of Medicine at Mount Sinai
Icahn School of Medicine at Mount Sinai
Rare pathogenic APP mutation causes ectodomain kinking, leading to accelerating intracellular transport, and enhancing Aβ accumulation
In this paper, Sangram Sisodia, University of Chicago, Rudolph Tanzi, Massachusetts General Hospital, Charlestown, Massachusetts, and their colleagues describe a rare new Alzheimer’s mutation, APP S198P, that resides not within or flanking the Aβ domain but instead in the Aβ precursor protein (APP) ectodomain, where it acts as a helix-breaker. Pathogenicity was validated in experiments showing enhanced accumulation of Aβ amyloid in APP S198P transgenic mouse brain. In cultured cells, APP S198P underwent accelerated ER folding and rapid delivery to endosomal-lysosomal compartments, leading to enhanced Aβ accumulation.
Precisely how is the APP S198P mutation exerting its effects on APP metabolism? Serine 198 is located in a highly flexible and extended acidic-rich domain (Ala 191-Val 290) that lies between two distinct structural domains of ~160 amino acids and ~190 amino acids in the APP 695 ectodomain, termed the E1 and E2, respectively (Coburger et al., 2013). First author Xulun Zhang and colleagues demonstrated that cellular APP C-terminal fragments (CTFs) and extracellular, soluble APP derived from the full-length APP S198P precursor containing the Swedish mutation (sAPPSwe-S198P) appeared at the earliest time points and accumulated to higher levels in pulse-chase experiments compared with the rate of production and accumulation of these metabolites derived from full-length APPSwe that harbored the wild-type serine at position 198.
Moreover, Zhang and colleagues report pulse-chase/immunoprecipitation studies suggesting that transient folding intermediates are made faster in cells expressing APPSwe S198P than in cells expressing APPSwe, a key finding that provides compelling support that the S198P variant undergoes accelerated folding. In support of this idea, pulse-chase studies using the P2-1 antibody, which is specific for a sulfhydryl-dependent structural epitope in the N-terminal E1 domain of APP, revealed that, indeed, folding of this domain occurred faster in cells expressing the APPSweS198P variant than in cells expressing APPSwe. The authors propose that the proline at position 198 enhances the rate of folding and the exit of newly synthesized APPSweS198P from the endoplasmic reticulum to the Golgi and to late compartments where BACE1 and γ-secretase are active.
How can the rate of folding of the APPSwe S198P variant be accelerated relative to APPSwe? The determining factor is the introduction of proline, an amino acid that either breaks or kinks a helix, both because it cannot donate an amide hydrogen bond (having no amide hydrogen), and because its bulky pyrrolidine ring restricts the available conformational space. To any structural biologist playing the word-association game Password, “proline” would immediately evoke the response “helix breaker.” Interestingly, an analysis of all proline residues and their local conformations extracted from the Brookhaven Protein Data bank (MacArthur and Thornton, 1991) revealed that the residue preceding proline plays an important role in determining conformation of the protein. When proline follows an aspartate residue, there is a very high probability of the α conformation being adopted (α:β=9:1). In the case of APP, the residue immediately preceding residue 198 is aspartate.
Zhang and colleagues offer a highly speculative model that the Asp-Pro pair promotes the generation of a local α-helix that enhances the rate of folding of the surrounding E1 and E2 domains and the adjacent flexible domain. Their pulse-chase studies using the sulfhydryl-dependent, structural-epitope-specific antibody, mAbP2-1 support this assertion by showing preferential binding to APPSwe S198P over APPSwe.
References:
Coburger I, Dahms SO, Roeser D, Gührs KH, Hortschansky P, Than ME. Analysis of the overall structure of the multi-domain amyloid precursor protein (APP). PLoS One. 2013;8(12):e81926. Epub 2013 Dec 4 PubMed.
MacArthur MW, Thornton JM. Influence of proline residues on protein conformation. J Mol Biol. 1991 Mar 20;218(2):397-412. PubMed.
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