Background: TREM2 is a microglial receptor, recently identified as a genetic risk factor for late onset AD. Results: Sequential proteolytic processing of TREM2 involves ectodomain shedding and intramembranous cleavage by ␥-secretase and affects signaling via its adaptor protein DAP12. Conclusion: ␥-Secretase-mediated intramembranous proteolysis modulates TREM2 signaling. Significance: Inhibition of ␥-secretase could impair TREM2 function in neuroinflammation.

Epidemiological studies indicate that intake of statins decrease the risk of developing Alzheimer disease. Cellular and in vivo studies suggested that statins might decrease the generation of the amyloid ␤-peptide (A␤) from the ␤-amyloid precursor protein. Here, we show that statins potently stimulate the degradation of extracellular A␤ by microglia. The statin-dependent clearance of extracellular A␤ is mainly exerted by insulindegrading enzyme (IDE) that is secreted in a nonconventional pathway in association with exosomes. Stimulated IDE secretion and A␤ degradation were also observed in blood of mice upon peripheral treatment with lovastatin. Importantly, increased IDE secretion upon lovastatin treatment was dependent on protein isoprenylation and up-regulation of exosome secretion by fusion of multivesicular bodies with the plasma membrane. These data demonstrate a novel pathway for the nonconventional secretion of IDE via exosomes. The modulation of this pathway could provide a new strategy to enhance the extracellular clearance of A␤. Alzheimer disease (AD)3 is associated with extracellular deposits of the amyloid ␤-peptide (A␤) and intraneuronal aggregates of hyperphosphorylated Tau protein in the brain (1). Evidence suggests that the pathogenesis of AD involves deleterious neurotoxic effects of aggregated A␤ peptides (2), which are derived by sequential proteolytic processing of the ␤-amyloid precursor protein (APP) by ␤-and ␥-secretases (3). APP can also be cleaved in a nonamyloidogenic pathway that involves initial cleavage by ␣-secretase within the A␤ domain that precludes the later generation of A␤ peptides (4). Brain A␤ levels are not only determined by the rate of production but also by different clearance mechanisms, including receptor-mediated endocytosis/phagocytosis and subsequent degradation in the endosomal/lysosomal compartment, transcytosis via the blood-brain barrier, as well as proteolytic degradation of extracellular A␤ by cell surface-localized and secreted proteases (5-10).Several studies indicated a dysregulation of lipid metabolism as an important aspect of AD-associated neurodegeneration. In particular, increased cholesterol levels seem to correlate with increased AD risk. Retrospective studies revealed the beneficial effects of statins (11). However, molecular mechanisms by which statins could offer protection against AD remain to be characterized in detail (12, 13). Most studies with cultured cells and animal models indicate that statin-mediated inhibition of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (HMGCR) could decrease the generation of A␤ by promoting nonamyloidogenic processing of APP. Other studies also showed that extraction of cholesterol from cellular membranes by cyclodextrins differentially affects A␤ generation. Although strong reduction of cholesterol decreased A␤ generation, a moderate extraction rather promoted the secretion of A␤. These effects were attributed to alterations in the distribution of APP and secretases within membrane microdomains (14 -18). In ...

Aggregation and toxicity of the amyloid β-peptide (Aβ) are considered as critical events in the initiation and progression of Alzheimer’s disease (AD). Recent evidence indicated that soluble oligomeric Aβ assemblies exert pronounced toxicity, rather than larger fibrillar aggregates that deposit in the forms of extracellular plaques. While some rare mutations in the Aβ sequence that cause early-onset AD promote the oligomerization, molecular mechanisms that induce the formation or stabilization of oligomers of the wild-type Aβ remain unclear. Here, we identified an Aβ variant phosphorylated at Ser26 residue (pSer26Aβ) in transgenic mouse models of AD and in human brain that shows contrasting spatio-temporal distribution as compared to non-phosphorylated Aβ (npAβ) or other modified Aβ species. pSer26Aβ is particularly abundant in intraneuronal deposits at very early stages of AD, but much less in extracellular plaques. pSer26Aβ assembles into a specific oligomeric form that does not proceed further into larger fibrillar aggregates, and accumulates in characteristic intracellular compartments of granulovacuolar degeneration together with TDP-43 and phosphorylated tau. Importantly, pSer26Aβ oligomers exert increased toxicity in human neurons as compared to other known Aβ species. Thus, pSer26Aβ could represent a critical species in the neurodegeneration during AD pathogenesis.Electronic supplementary materialThe online version of this article (doi:10.1007/s00401-016-1546-0) contains supplementary material, which is available to authorized users.

The molecular mechanisms of beta-amyloidogenesis in sporadic Alzheimer's disease are still poorly understood. To reveal whether aging-associated increases in brain oxidative stress and inflammation may trigger onset or progression of beta-amyloid deposition, a transgenic mouse (Tg2576) that express the Swedish double mutation of human amyloid precursor protein (APP) was used as animal model to study the developmental pattern of markers of oxidative stress and APP processing. In Tg2576 mouse brain, cortical levels of soluble beta-amyloid (1-40) and (1-42) steadily increased with age, but significant deposition of fibrillary beta-amyloid in cortical areas did not occur before postnatal age of 10 months. The slope of increase in cerebral cortical beta-secretase (BACE1) activities in Tg2576 mice between ages of 9 and 13 months was significantly higher as compared to that of the alpha-secretase, while the expression level of BACE1 protein and mRNA did not change with age. The activities of superoxide dismutase and glutathione peroxidase in cortical tissue from Tg2576 mice steadily increased from postnatal age 9-12 months. The levels of cortical nitric oxide, and reactive nitrogen species demonstrated peak values around 9 months of age, while the level of interleukin-1beta steadily increased from postnatal month 13 onwards. The developmental temporal coincidence of increased levels of reactive nitrogen species and antioxidative enzymes with the onset of beta-amyloid plaque deposition provides further evidence that developmentally and aging-induced alterations in brain oxidative status exhibit a major factor in triggering enhanced production and deposition of beta-amyloid, and potentially predispose to Alzheimer's disease.

Alzheimer's disease (AD) is the most frequent cause of dementia. There is compelling evidence that the proteolytic processing of the amyloid precursor protein (APP) and accumulation of amyloid-β (Aβ) peptides play critical roles in AD pathogenesis. Due to limited access to human neural tissue, pathogenetic studies have, so far, mostly focused on the heterologous overexpression of mutant human APP in non-human cells. In this study, we show that key steps in proteolytic APP processing are recapitulated in neurons generated from human embryonic and induced pluripotent stem cell-derived neural stem cells (NSC). These human NSC-derived neurons express the neuron-specific APP(695) splice variant, BACE1, and all members of the γ-secretase complex. The human NSC-derived neurons also exhibit a differentiation-dependent increase in Aβ secretion and respond to the pharmacotherapeutic modulation by anti-amyloidogenic compounds, such as γ-secretase inhibitors and nonsteroidal anti-inflammatory drugs. Being highly amenable to genetic modification, human NSCs enable the study of mechanisms caused by disease-associated mutations in human neurons. Interestingly, the AD-associated PS1(L166P) variant revealed a partial loss of γ-secretase function, resulting in the decreased production of endogenous Aβ40 and an increased Aβ42/40 ratio. The PS1(L166P) mutant is also resistant to γ-secretase modulation by nonsteroidal anti-inflammatory drugs. Pluripotent stem cell-derived neurons thus provide experimental access to key steps in AD pathogenesis and can be used to screen pharmaceutical compounds directly in a human neuronal system.