A Distinct ER/IC γ-Secretase Competes with the Proteasome for Cleavage of APP

Abstract

The deposition of amyloid-beta peptides (Abeta) in senile plaques (SPs) is a central pathological feature of Alzheimer's disease (AD). Since SPs are composed predominantly of Abeta1-42, which is more amyloidogenic in vitro, the enzymes involved in generating Abeta1-42 may be particularly important to the pathogenesis of AD. In contrast to Abeta1-40, which is generated in the trans-Golgi network and other cytoplasmic organelles, intracellular Abeta1-42 is produced in the endoplasmic reticulum/intermediate compartment (ER/IC), where it accumulates in a stable insoluble pool. Since this pool of insoluble Abeta1-42 may play a critical role in AD amyloidogenesis, we sought to determine how the production of intracellular Abeta is regulated. Surprisingly, the production of insoluble intracellular Abeta1-42 was increased by a putative gamma-secretase inhibitor as well as by an inhibitor of the proteasome. We further demonstrate that this increased generation of Abeta1-42 in the ER/IC is due to a reduction in the turnover of Abeta-containing APP C-terminal fragments. We conclude that the proteasome is a novel site for degradation of ER/IC-generated APP fragments. Proteasome inhibitors may augment the availability of APP C-terminal fragments for gamma-secretase cleavage and thereby increase production of Abeta1-42 in the ER/IC. Based on the organelle-specific differences in the generation of Abeta by gamma-secretase, we conclude that intracellular ER/IC-generated Abeta1-42 and secreted Abeta1-40 are produced by different gamma-secretases. Further, the fact that a putative gamma-secretase inhibitor had opposite effects on the production of secreted and intracellular Abeta may have important implications for AD drug design.

The Protease Inhibitor, MG132, Blocks Maturation of the Amyloid Precursor Protein Swedish Mutant Preventing Cleavage by β-Secretase

“…In previous studies, peptide aldehydes such as MG132 were thought to inhibit A␤ and p3 peptide secretion by blocking ␥-secretase cleavage of the COOH-terminal fragment of APP (12,13,17,18). This conclusion was based on the observation that CTFs were detectable in cells incubated with peptide aldehydes.…”

“…Other investigators reported that lactacystin increased secretion of both A␤40 and A␤42 from SH-SY5Y cells (14). The proposed explanation for these increases is that inhibition of the proteasome prevents degradation of both full-length and COOH-terminal fragments of APP making more of each available for cleavage by the ␣-and ␥-secretases, respectively (18). We reasoned that, if lactacystin disrupted ER-associated degradation of APPSw in a manner similar to other transmembrane proteins (25)(26)(27)(28), then more full-length protein would also be available for cleavage by ␤-secretase.…”

“…The highly specific proteasomal inhibitor, lactacystin, in- creases APP␣ and intracellular and secreted A␤42 (18,23,24). Other investigators reported that lactacystin increased secretion of both A␤40 and A␤42 from SH-SY5Y cells (14).…”

“…MG132 was one of the most potent peptide aldehyde inhibitors of A␤ peptide secretion (15,18). We hypothesized that MG132 was inhibiting A␤ peptide secretion at high concentrations by interfering with ␤-secretase cleavage of APP.…”

“…The increase in A␤ peptide secretion at low concentrations of peptide aldehydes has been postulated to result from inhibition of degradation of the CTFs generated by BACE cleavage, making more of them available for ␥-secretase cleavage (17). The inhibition of A␤ peptide secretion with higher concentrations of peptide aldehydes is attributed to an impairment of ␥-secretase cleavage of the CTFs (12,13,17,18).…”