Alzheimer's disease β-amyloid peptides are released in association with exosomes

Abstract

Although the exact etiology of Alzheimer's disease (AD) is a topic of debate, the consensus is that the accumulation of ␤-amyloid (A␤) peptides in the senile plaques is one of the hallmarks of the progression of the disease. The A␤ peptide is formed by the amyloidogenic cleavage of the amyloid precursor protein (APP) by ␤-and ␥-secretases. The endocytic system has been implicated in the cleavages leading to the formation of A␤. However, the identity of the intracellular compartment where the amyloidogenic secretases cleave and the mechanism by which the intracellularly generated A␤ is released into the extracellular milieu are not clear. Here, we show that ␤-cleavage occurs in early endosomes followed by routing of A␤ to multivesicular bodies (MVBs) in HeLa and N2a cells. Subsequently, a minute fraction of A␤ peptides can be secreted from the cells in association with exosomes, intraluminal vesicles of MVBs that are released into the extracellular space as a result of fusion of MVBs with the plasma membrane. Exosomal proteins were found to accumulate in the plaques of AD patient brains, suggesting a role in the pathogenesis of AD. multivesicular bodies ͉ rafts ͉ amyloid precursor protein ͉ ␤-secretase ͉ endocytosis A lzheimer's disease (AD) is a late-onset neurological disorder with progressive loss of memory and cognitive abilities as a result of excessive neurodegeneration (1). AD is characterized by extracellular aggregates of ␤-amyloid (A␤) peptides known as amyloid plaques (2). The A␤ peptide is derived from the sequential processing of the amyloid precursor protein (APP) by ␤-and ␥-secretases. ␤-secretase [(␤-APP cleaving enzyme (BACE)] is a type-1 transmembrane aspartyl protease and is mainly localized to endosomes, lysosomes and the transGolgi network (3). ␥-Secretase is a multicomponent complex that is composed of presenilin-1͞presenilin-2, nicastrin, Aph-1, and PEN-2 (4) and is localized to the early secretory (5, 6) and the endocytic compartments (7,8). Nonamyloidogenic processing of APP involves ␣-secretase that cleaves APP inside the A␤ region, giving rise to the ␣-cleaved ectodomain, thus precluding the formation of A␤ (9). Hence, the availability of APP to either ␣-or ␤-secretase determines whether A␤ peptide will be generated. Lateral organization of membranes (10) and subcellular localization (11, 12) of the substrate and the secretases have been documented to regulate A␤ generation. Recent work suggests that ␤-secretase associates with lipid rafts, liquid-ordered domains in the membrane (13,14), and that integrity of raft domains is required for ␤-cleavage of APP to occur (ref. 10; see, however, ref. 15). ␣-Cleavage, in contrast, occurs outside raft domains (10). The ␥-secretase complex is also raft-associated (16); hence, amyloidogenic processing of APP could occur in clustered raft domains to generate A␤ (10). Inhibition of endocytosis reduces ␤-cleavage but not ␣-cleavage, suggesting that ␤-cleavage mainly occurs in endosomes (10,11,(17)(18)(19). Accumulation of A␤ peptides in extracellular...

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

“…It has been previously shown that a fraction of intracellular Ab can be released through exosomes by neurons and oligodendrocytes. 32,[45][46][47] In addition, phagocytosed Ab has been found to be re-secreted from microglia, although through an undefined mechanism. 48 We now show that microglia release neurotoxic Ab 1-42 and Ab 1-40 species in association with MVs.…”

“…Notably, microglia-derived MVs in the cerebrospinal fluid (CSF) have been recently identified as a novel biomarker of brain inflammation in humans. 30,31 The observation that typical proteins of EMVs, like flotilin, accumulate in the plaques of AD brain, 32 together with evidence that activated microglia constantly surround amyloid deposits, 33 prompted us to investigate whether EMVs may be involved in the spatiotemporal propagation of Ab pathology through the brain. Here we show that production of MVs is extremely high in patients with AD and that microglial MVs, either shed in vitro or isolated from the CSF of AD patients, promote generation of soluble neurotoxic Ab species, thereby acting as potent drivers of neuronal damage.…”

Raft association and lipid droplet targeting of flotillins are independent of caveolin

“…Despite a striking sequence similarity between flotillin-1 and flotillin-2, specific recruitment of flotillin-1 to lipid bodies upon fatty-acid loading could also underline a specific function in lipid regulation or trafficking towards these organelles. Flotillin-1, but not flotillin-2, has been shown to be enriched in maturing phagosomes (Dermine et al, 2001) and exosomes (Rajendran et al, 2006), and only flotillin-1 is specifically upregulated at least 10-fold during adipocyte differentiation (Bickel et al, 1997), suggesting that flotillin-1 could be involved in storage pathways. Finally, another possibility is that flotillins could function as a raft organizer by means of homo/hetero-oligomerization (Neumann-Giesen et al, 2004), and thus would organize or shape scaffold domains at the level of lipid droplet membranes.…”

A deadly spread: cellular mechanisms of α-synuclein transfer

“…51 Of relevance to other neurodegenerative disorders, they have been shown to transmit pathogenic prion proteins [52][53][54] and b-amyloid (Ab) aggregates. 55 Recently, a-syn-containing exosomes secreted from neuroblastoma cells overexpressing a-syn were found to transfer a-syn to other cultured neuroblastoma cells. Lysosomal inhibition of donor cells increased both exosomemediated release and transfer of a-syn 49 ( Figure 2a).…”