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 ...

Alzheimer disease is associated with extracellular deposits of amyloid ␤-peptides in the brain. Amyloid ␤-peptides are generated by proteolytic processing of the ␤-amyloid precursor protein by ␤-and ␥-secretases. The cleavage by secretases occurs predominantly in postGolgi secretory and endocytic compartments and is influenced by cholesterol, indicating a role of the membrane lipid composition in proteolytic processing of the ␤-amyloid precursor protein. To analyze the role of glycosphingolipids in these processes we inhibited glycosyl ceramide synthase, which catalyzes the first step in glycosphingolipid biosynthesis. The depletion of glycosphingolipids markedly reduced the secretion of endogenous ␤-amyloid precursor protein in different cell types, including human neuroblastoma SH-SY5Y cells. Importantly, secretion of amyloid ␤-peptides was also strongly decreased by inhibition of glycosphingolipid biosynthesis. Conversely, the addition of exogenous brain gangliosides to cultured cells reversed these effects. Biochemical and cell biological experiments demonstrate that the pharmacological reduction of cellular glycosphingolipid levels inhibited maturation and cell surface transport of the ␤-amyloid precursor protein. In the glycosphingolipid-deficient cell line GM95, cellular levels and maturation of ␤-amyloid precursor protein were also significantly reduced as compared with normal B16 cells. Together, these data demonstrate that glycosphingolipids are implicated in the regulation of the subcellular transport of the ␤-amyloid precursor protein in the secretory pathway and its proteolytic processing. Thus, enzymes involved in glycosphingolipid metabolism might represent targets to inhibit the production of amyloid ␤-peptides.

The ␤-amyloid precursor protein (APP) represents a type I transmembrane glycoprotein that is ubiquitously expressed. In the brain, it is a key player in the molecular pathogenesis of Alzheimer disease. Its physiological function is however less well understood. Previous studies showed that APP is up-regulated in prostate, colon, pancreatic tumor, and oral squamous cell carcinoma. In this study, we show that APP has an essential role in growth control of pancreatic and colon cancer. Abundant APP staining was found in human pancreatic adenocarcinoma and colon cancer tissue. Interestingly, treating pancreatic and colon cancer cells with valproic acid (VPA, 2-propylpentanoic acid), a known histone deacetylase (HDAC) inhibitor, leads to up-regulation of GRP78, an endoplasmic reticulum chaperone immunoglobulin-binding protein. GRP78 is involved in APP maturation and inhibition of tumor cell growth by down-regulation of APP and secreted soluble APP␣. Trichostatin A, a pan-HDAC inhibitor, also lowered APP and increased GRP78 levels. In contrast, treating cells with valpromide, a VPA derivative lacking HDAC inhibitory properties, had no effect on APP levels. VPA did not modify the level of epidermal growth factor receptor, another type I transmembrane protein, and APLP2, a member of the APP family, demonstrating the specificity of the VPA effect on APP. Small interfering RNA-mediated knockdown of APP also resulted in significantly decreased cell growth. Based on these observations, the data suggest that APP downregulation via HDAC inhibition provides a novel mechanism for pancreatic and colon cancer therapy. ␤-Amyloid precursor protein (APP)2 is a highly conserved single transmembrane protein (type I) with a receptor-like structure and consists of a heterogeneous group of proteins migrating between 110 and 135 kDa (1, 2). The heterogeneity is due to alternative splicing, leading to eight distinct isoforms (namely APP677, APP695, APP696, APP714, APP733, APP751, APP752, and APP770), as well as by a variety of post-translational modifications, including O-and N-glycosylation, sulfation, and phosphorylation. APP isoforms exist as immature (N-glycosylated) and mature (N-and O-glycosylated, tyrosylsulfated) species. Immature APP localizes in the endoplasmic reticulum and cis-Golgi, and the mature APP form preferentially localizes in the trans-Golgi network, secretory and endocytic vesicles, and at the plasma membrane (3, 4).APP695 is the most common isoform in the central nervous system, whereas APP751 and APP770 are predominantly expressed in non-neuronal cells (5). The key event in the pathogenic cascade in Alzheimer disease is the amyloidogenic pathway characterized by subsequent cleavage of APP by the enzyme ␤-secretase and further processing by ␥-secretase, which finally leads to the generation of A␤ peptides. However, the predominant route of APP processing consists of successive cleavages by ␣-and ␥-secretases in non-neuronal cells (6, 7). The cleavage of APP at Lys 16 -Leu 17 bond by ␣-secretase within the A␤ sequence ...

Apolipoprotein E (apoE) plays a role in the pathogenesis of Alzheimer disease (AD). It is involved in the receptor-mediated cellular clearance of the amyloid beta-protein (Abeta) and in the perivascular drainage of the extracellular fluid. Microvascular changes are also associated with AD and have been discussed as a possible reason for altered perivascular drainage. To further clarify the role of apoE in the perivascular and vascular pathology in AD patients, we studied its occurrence and distribution in the perivascular space, the perivascular neuropil, and in the vessel wall of AD and control cases with and without small vessel disease (SVD). Apolipoprotein E was found in the perivascular space and in the neuropil around arteries of the basal ganglia from control and AD cases disclosing no major differences. Western blot analysis of basal ganglia tissue also revealed no significant differences pertaining to the amount of full-length and C-terminal truncated apoE in AD cases compared with controls. In contrast, Abeta occurred in apoE-positive perivascular astrocytes in AD cases but not in controls. In blood vessels, apoE and immunoglobulin G were detected within the SVD-altered vessel wall. The severity of SVD was associated with the occurrence of apoE in the vessel wall and with that of Abeta in perivascular astrocytes. These results point to an important role of apoE in the perivascular clearance of Abeta in the human brain. The occurrence of apoE and immunoglobulin G in SVD lesions and in the perivascular space suggests that the presence of SVD results in plasma-protein leakage into the brain. It is therefore tempting to speculate that apoE represents a pathogenetic link between SVD and AD.

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.