Identification of Small Peptides in Human Cerebrospinal Fluid upon Amyloid-ß Degradation.

BACKGROUND: Amyloid-ß (Aß) degradation in brains of Alzheimer disease patients is a crucial focus for the clarification of disease pathogenesis. Nevertheless, the mechanisms underlying Aß degradation in the human brain remain unclear. OBJECTIVE: This study aimed to quantify the levels of small C-terminal Aß fragments generated upon Aß degradation in human cerebrospinal fluid (CSF). METHODS: A fraction containing small peptides was isolated and purified from human CSF by high-pressure liquid chromatography. Degradation products of Aß C termini were identified and measured by liquid chromatography-tandem mass spectrometry. The C-terminal fragments of Aß in the conditioned medium of cultured cells transfected with the Swedish variant of ßAPP (sw ßAPP) were analyzed. These fragments in brains of PS1 I213T knock-in transgenic mice, overexpressing sw ßAPP, were also analyzed. RESULTS: The peptide fragments GGVV and GVV, produced by the cleavage of Aß40, were identified in human CSF as well as in the brains of the transgenic mice and in the conditioned medium of the cultured cells. Relative to Aß40 levels, GGVV and GVV levels were 7.6 ± 0.81 and 1.5 ± 0.18%, respectively, in human CSF. Levels of the GGVV fragment did not increase by the introduction of genes encoding neprilysin and insulin-degrading enzyme to the cultured cells. CONCLUSION: Our results indicate that a substantial amount of Aß40 in human brains is degraded via a neprilysin- or insulin-degrading enzyme-independent pathway.

Semagacestat Is a Pseudo-Inhibitor of γ-Secretase.

γ-secretase inhibitors (GSI) are drugs developed to decrease amyloid-ß peptide (Aß) production by inhibiting intramembranous cleavage of ß-amyloid protein precursor (ßAPP). However, a large phase 3 trial of semagacestat, a potential non-transition state analog (non-TSA) GSI, in patients with Alzheimer's disease (AD) was terminated due to unexpected aggravation of cognitive deficits and side effects. Here, we show that some semagacestat effects are clearly different from a phenotype caused by a loss of function of presenilins, core proteins in the γ-secretase complex. Semagacestat increases intracellular byproduct peptides, produced along with Aß through serial γ-cleavage of ßAPP, as well as intracellular long Aß species, in cell-based and in vivo studies of AD model mice. Other potential non-TSA GSIs, but not L685,458, a TSA GSI, have similar effects. Furthermore, semagacestat inhibits release of de novo intramembranous γ-byproducts to the soluble space. Thus, semagacestat is a pseudo-GSI, and therefore, the semagacestat clinical trial did not truly test the Aß hypothesis.