No abstract

The 19-transmembrane multisubunit γ-secretase complex generates the amyloid β-peptide (Aβ) of Alzheimer's disease (AD) by intramembrane proteolysis of the β-amyloid precursor protein (APP). Despite substantial advances in elucidating how this protein complex functions, the effect of the local membrane lipid microenvironment on γ-secretase cleavage of substrates is still poorly understood. Using detergent-free proteoliposomes to reconstitute purified human γ-secretase, we examined the effects of fatty acyl (FA) chain length, saturation and double-bond isomerization, and membrane lipid polar headgroups on γ-secretase function. We analyzed γ-secretase activity and processivity [i.e., sequential cleavages in the APP transmembrane domain that convert longer Aβ species (e.g., Aβ 46 ) into shorter ones (e.g., Aβ 40 )] by quantifying the APP intracellular domain (AICD) and various Aβ peptides, including via a bicine/urea gel system that detects multiple Aβ lengths. These assays revealed several trends. (1) Switching from a cis to a trans isomer of a monounsaturated FA chain in phosphatidylcholine (PC) increased γactivity, did not affect Aβ 42 :Aβ 40 ratios, but decreased the ratio of long (≥42) versus short (≤41) Aβ peptides. ( 2) Increasing the FA carbon chain length (14, 16, 18, and 20) increased γ-activity, reduced longer Aβ species, and reduced the Aβ 42 :Aβ 40 ratio. (3) Shifting the position of the double bond in 18:1(Δ9-cis) PC to the Δ6 position substantially reduced activity. (4) Gangliosides increased γ-activity but decreased processivity, thus elevating the Aβ 42 :Aβ 40 ratio. (5) Phosphatidylserine decreased γ-activity but increased processivity. ( 6) Phosphatidylinositol strongly inhibited γ-activity. Overall, our results show that subtle changes in membrane lipid composition can greatly influence γ-secretase activity and processivity, suggesting that relatively small changes in lipid membrane composition may affect the risk of AD at least as much as presenilin or APP mutations do.

No abstract

The biological underpinnings linking stress to Alzheimer's disease (AD) risk are poorly understood. We investigated how corticotrophin releasing factor (CRF), a critical stress response mediator, influences amyloid-b (Ab) production. In cells, CRF treatment increases Ab production and triggers CRF receptor 1 (CRFR1) and c-secretase internalization. Co-immunoprecipitation studies establish that c-secretase associates with CRFR1; this is mediated by b-arrestin binding motifs. Additionally, CRFR1 and c-secretase co-localize in lipid raft fractions, with increased c-secretase accumulation upon CRF treatment. CRF treatment also increases c-secretase activity in vitro, revealing a second, receptorindependent mechanism of action. CRF is the first endogenous neuropeptide that can be shown to directly modulate c-secretase activity. Unexpectedly, CRFR1 antagonists also increased Ab. These data collectively link CRF to increased Ab through c-secretase and provide mechanistic insight into how stress may increase AD risk. They also suggest that direct targeting of CRF might be necessary to effectively modulate this pathway for therapeutic benefit in AD, as CRFR1 antagonists increase Ab and in some cases preferentially increase Ab42 via complex effects on c-secretase.