Statins Promote the Degradation of Extracellular Amyloid β-Peptide by Microglia via Stimulation of Exosome-associated Insulin-degrading Enzyme (IDE) Secretion

Abstract

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 ...

Microglia convert aggregated amyloid-β into neurotoxic forms through the shedding of microvesicles

“…Indeed, previous evidence showed that neuron-derived EMVs contain some components of the gsecretase complex, 57 whereas the insulin-degrading enzyme, which proteolyzes Ab 1-42 and Ab 1-40, has been detected among cargo proteins of microglial EMVs. 58 The significant decrease in neurotoxicity observed upon pretreatment with anti-Ab antibodies strongly supports the theory that neurotoxic Ab forms are in fact localized to the outer lipid bilayer of MVs. However, further studies are required to unequivocally define the topology of Ab species and to clarify whether Ab forms are actually associated to the extracellular membrane of shed MVs.…”

Insulin-degrading enzyme secretion from astrocytes is mediated by an autophagy-based unconventional secretory pathway in Alzheimer disease

“…Although it is indirect evidence, previous studies have shown that statin treatment increases IDE secretion, and inhibits protein prenylation. 35,41 Whether IDE secretion by statin treatment occurs through same pathway as Ab treatment should be investigated. Further, how IDE inside the lysosome is transported to the extracellular medium needs to be determined.…”

“…Prior to our work, it was known that, as IDE has no signal sequences, it is secreted from the microglia via an exosome-associated unconventional secretory pathway under statin treatment. 35 To determine whether Ab-induced IDE secretion is associated with exosomes in astrocytes, both exosomes and nonexosome fractions were isolated from vehicle-or Ab-treated ACM. Ab increased the IDE secretion both in the exosomes and nonexosome fractions ( Fig.…”

“…35 Briefly, primary astrocytes or U373MG cells grown in a 6-well plate to 70% confluency were incubated with Ab 42 or DMSO (vehicle) for 24 h. After PBS (Sigma-Aldrich, P4417) washing, cells were incubated for an additional 6 h in serum-free OPTI-MEM (GIBCO, 31985-070). The supernatant fraction (vehicle-or Ab-ACM) was then collected after centrifugation and incubated with 1 mM Ab for 12 h at 37 C in the absence or presence of 10 mg/mL bacitracin A, 0.3 ng/mL recombinant IDE (rIDE) (R&D Systems, 31985-070), or 0.5 mM insulin.…”

Phosphorylation of Amyloid-β Peptide at Serine 8 Attenuates Its Clearance via Insulin-degrading and Angiotensin-converting Enzymes

“…The half-life time of A␤ is ϳ8 -12 h in human cerebrospinal fluid and 3-4 h in the interstitial fluid of mouse brain, indicating efficient clearance mechanisms that counteract the production of A␤ (9, 10, 38). The clearance of A␤ from the brain involves internalization via pinocytosis or receptor-mediated endocytosis/phagocytosis (13-15, 39) and subsequent degradation in the endosomal/lysosomal compartments (40), transcytosis and drainage via the blood-brain barrier to the vasculature (12, 41), and proteolytic degradation of extracellular A␤ by cell surface-localized and secreted proteases (21,28,36).Microglial cells could contribute to the clearance of A␤, as they are the main phagocytes in the brain and also express several proteases at the cell surface that could degrade extracellular and internalized A␤ (27). Recent evidence demonstrated that microglia secrete substantial amounts of IDE, thereby allowing efficient degradation of monomeric A␤ variants also at some distance from the microglial cell (19,42).…”