Alzheimer's disease (AD) is an age-related neurodegenerative disease characterized by the progressive loss of synapses and neurons and by the formation of amyloid plaques and neurofibrillary tangles [1
]. Amyloid plaques are composed predominantly of the Aβ peptide, a 40 or 42 amino acid cleavage product of the amyloid precursor protein (APP). APP is a synaptic, transmembrane protein that undergoes extracellular cleavage by one of two proteases, α- or β-secretase, which results in the formation of large N-terminal extracellular fragments of secreted APP (sAPP) and smaller, membrane-bound C-terminal fragments (CTF). If the initial cleavage event occurs via β-secretase, then cleavage of the CTF by γ-secretase in the intramembrane region results in the formation of Aβ. Mutations in APP and in a component of the γ-secretase complex (presenilin) cause familial forms of AD. APP can be preferentially cleaved by α- or β-secretase depending on its localization within the cell. The majority of α-secretase activity is thought to occur on the cell surface [2
], whereas β-secretase cleavage and Aβ production is thought to occur following endocytosis of APP in the endosomal pathway [3
]. Understanding APP function, trafficking, and processing in neurons may provide valuable information in generating interventions against AD pathogenesis and its accompanying memory loss.
ApoE is the major genetic risk factor for AD (reviewed in Roses, 2006 [4
]) and is known to directly bind Aβ and co-localize with cerebral amyloid deposits in AD patients [5
]. However, the mechanism by which apoE and its receptors affect the risk for AD remains unknown. Some possible effects of apoE include altering APP processing [9
], facilitating internalization and degradation of Aβ [10
], improving clearance of Aβ into the periphery [14
], and altering neuronal toxicity of Aβ [15
]. However, some studies show detrimental effects of apoE where apoE facilitates Aβ oligomerization [16
]. Some of the contrasting evidence may be attributed to the different isoforms of apoE: human apoE2 reduces brain Aβ burden in transgenic APP mice, while human apoE4 increases brain Aβ burden [18
]. These isoform specific effects were also seen in APP transgenic mice expressing human apoE, where Aβ deposition was greatest in apoE4 APP transgenic mice compared to apoE3 and apoE2 [19
]. Recombinant human apoE at physiological levels (100 nM) has been reported to decrease Aβ production in CHO-APP751, HEK293, and primary neuron cells [21
]. However, other studies show that lipid deficient apoE4 in APP-overexpressing rat neuroblastoma B103 cells increased Aβ production compared to lipid deficient apoE3 [22
], and apoE binding to apoEr2 promoted APP endocytosis, increasing Aβ production [23
]. Thus, there is no consensus yet about how apoE affects APP processing.
We and others [24
] have used a derivative of apoE, a small peptide containing a tandem repeat of the receptor binding domain, to show the effects of apoE on neuronal signaling and APP processing. We found that apoE-derived peptide treatment increased ERK and decreased JNK activation in primary neurons [27
]. In addition, the apoE-derived peptide significantly reduced inflammation in several animal models of disease [28
], which may occur through the apoE peptide-induced decrease in c-Jun N-terminal kinase-mediated microglial activation [30
]. Furthermore, we chronically administered the apoE-derived peptide via osmotic pump and observed a consistent effect on apoE signaling, as well as on APP processing, in vivo
]. These data implicate a role for the apoE-derived peptide, regardless of isoform, in various signaling processes in neurons and glia, and these signaling processes may be related to our observed effects on altered APP processing.
In the present study, we investigated the importance of the tandem repeat in the apoE-derived peptide, and the ability of this peptide to affect APP trafficking, APP processing, and Aβ levels. We first demonstrate that a tandem repeat in the apoE-derived peptide is necessary to effect cell surface APP levels. We then show that apoE-derived peptides decrease secreted Aβ in PS1-overexpressed PS70 cells and in primary neurons. Again, the effect of the monomer was minimal in reducing secreted Aβ levels in both systems, whereas the dimer and trimer show a clear dose response. The potency of the apoE-derived peptides follows the order: trimer > dimer > monomer, which correlates with the stability of their alpha helical secondary structure as determined computationally. The effect of the dimer on APP processing was further evaluated in vivo, where a single hippocampal injection of the apoE-derived dimer peptide resulted in a significant increase in sAPPα and a significant decrease in soluble Aβ1-40 levels in wild-type mice, suggesting that the apoE dimer may promote α-secretase cleavage. Our data demonstrate a clear effect of the apoE-derived dimer peptide on APP trafficking, processing, and Aβ levels both in vitro and in vivo.