. The Alzheimer's Disease γ-Secretase Generates Higher 42:40 Ratios for β-Amyloid Than for p3 Peptides. Cell Rep. 2017 Jun 6;19(10):1967-1976. PubMed.

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  1. Siegel et al. identified the difference in the γ-secretase-mediated cleavage on APP substrate with distinct N-terminal length. Complex profiles of γ-cleavage on C83, C89, and C99 suggest that further systematic analysis for the enzymatic property of γ-secretase would be needed.

    This work highlights the pathological importance of not only Aβ' starting at Glu-11, but N-terminally truncated Aβ species. Because of limitations in biochemical detection, several labs, including ours, have utilized Swedish mutant APP, which shifts β-cleavage at +1 position, in our cell biological experiments. However, development of sensitive ELISA systems and mass spectrometry has revealed that naïve cells secrete relatively high amounts of N-terminally truncated Aβ (Seubert et al., 1992; Asami-Odaka et al., 1995; Wang et al., 1996).

    These Aβ levels/ratios might be influenced by cell type and culture conditions. Importantly, N-terminally truncated Aβ species deposited in senile plaques (Naslund et al., 1994) in addition to Aβ species modified after deposition such as pyroglutamylation. Thus, we must carefully interpret the effect of secretase modifiers in cells/mice expressing APP carrying the Swedish mutation.

    References:

    . Isolation and quantification of soluble Alzheimer's beta-peptide from biological fluids. Nature. 1992 Sep 24;359(6393):325-7. PubMed.

    . Long amyloid beta-protein secreted from wild-type human neuroblastoma IMR-32 cells. Biochemistry. 1995 Aug 15;34(32):10272-8. PubMed.

    . The profile of soluble amyloid beta protein in cultured cell media. Detection and quantification of amyloid beta protein and variants by immunoprecipitation-mass spectrometry. J Biol Chem. 1996 Dec 13;271(50):31894-902. PubMed.

    . Relative abundance of Alzheimer A beta amyloid peptide variants in Alzheimer disease and normal aging. Proc Natl Acad Sci U S A. 1994 Aug 30;91(18):8378-82. PubMed.

    View all comments by Taisuke Tomita
  2. The paper is interesting. It shows that γ-secretase processes the different APP CTF substrates to different 42/40 peptide ratios, with those produced from C89 being the highest. This unexpected observation might suggest that C89 is presented to the γ-secretase active site differently than C99 or C83.

    In determining the interaction sites of C99 by photoaffinity mapping, we have previously shown that FAD mutations in presenilin-1 can cause altered substrate positioning of C99 (Fukumori and Steiner, 2016). Similar mispositioning may particularly occur for C89, giving rise to substantial alterations in the cleavage precision/processivity compared to C99 and C83. 

    The new data indicate that interactions of γ-secretase exosites with the extracellular APP domain (also see Fukumori and Steiner, 2016) are critical for substrate cleavage, a hypothesis that Akio Fukumori and I are currently exploring further.

    View all comments by Harald Steiner
  3. The most prominent feature of this paper is that C89 is a favored APP substrate cleaved at epsilon 48 by γ-secretase to produce AICD49-99, leading to Aβ42 product line (Takami et al., 2009). It suggests that ectodomain length of substrate influences not only cleavage efficiency but also substrate cleavage sites by this enzyme.

    We previously reported cleavage efficiency and cleavage sites on APP, APLP2, and Notch substrates containing different ectodomain length (namely C99 and C83 on APP; β-cleaved APLP2 and α-cleaved APLP2 on APLP2; ∆E Notch and S2 cleaved Notch on Notch1) (Funamoto et al., 2013). In either case, γ-secretase preferentially cleaved substrates containing short ectodomain (C83, α-cleaved APLP2, and S2 cleaved Notch).

    However, we found no difference in cleavage sites close to the membrane-cytoplasmic boundary (ε, ε-like, and S3 sites) between short and long ectodomain substrates on APP, APLP2, and Notch (Funamoto et al., 2013, see supplementary Fig. S3). Probably, C89-like substrates of APLP2 and Notch may exhibit modulation of the cleavage sites as shown in C89 of APP.

    The finding by Siegel and colleagues is quite interesting. However, it is reported that the Aβ42/Aβ40 ratio is modulated by concentrations of substrate and remnant detergent (Fraering et al., 2004Yin et al., 2007). We should be careful to assess the modulation of Aβ42/Aβ40 in vitro.

    References:

    . gamma-Secretase: successive tripeptide and tetrapeptide release from the transmembrane domain of beta-carboxyl terminal fragment. J Neurosci. 2009 Oct 14;29(41):13042-52. PubMed.

    . Substrate ectodomain is critical for substrate preference and inhibition of γ-secretase. Nat Commun. 2013;4:2529. PubMed.

    . Purification and characterization of the human gamma-secretase complex. Biochemistry. 2004 Aug 3;43(30):9774-89. PubMed.

    . {gamma}-Secretase Substrate Concentration Modulates the Abeta42/Abeta40 Ratio: IMPLICATIONS FOR ALZHEIMER DISEASE. J Biol Chem. 2007 Aug 10;282(32):23639-44. PubMed.

    View all comments by Satoru Funamoto

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