Reactivity of Apolipoprotein E4 and Amyloid β Peptide

Abstract

We previously demonstrated that apolipoprotein E4 (apoE4) potentiates lysosomal leakage and apoptosis induced by amyloid ␤ (A␤) peptide in cultured Neuro-2a cells and hypothesized that the low pH of lysosomes accentuates the conversion of apoE4 to a molten globule, inducing reactive intermediates capable of destabilizing cellular membranes. Here we report that neutralizing lysosomal pH with bafilomycin or NH 4 Cl abolished the apoE4 potentiation of A␤-induced lysosomal leakage and apoptosis in Neuro-2a cells. Consistent with these results, apoE4 at acidic pH bound more avidly to phospholipid vesicles and disrupted them to a greater extent than at pH 7.4. Comparison of "Arctic" mutant A␤, which forms multimers, and GM6 mutant A␤, which remains primarily monomeric, showed that aggregation is essential for apoE4 to potentiate A␤-induced lysosomal leakage and apoptosis. Both apoE4 and A␤1-42 had to be internalized to exert these effects. Blocking the low density lipoprotein receptor-related protein with small interfering RNA abolished the enhanced effects of apoE4 and A␤ on lysosomes and apoptosis. In cultured Neuro-2a cells, A␤1-42 increased lysosome formation to a greater extent in apoE3-or apoE4-transfected cells than in Neo-transfected cells, as shown by immunostaining for lysosome-associated membrane protein 1. Similarly, in transgenic mice expressing apoE and amyloid precursor protein, hippocampal neurons displayed increased numbers of lysosomes. Thus, apoE4 and A␤1-42 may work in concert in neurons to increase lysosome formation while increasing the susceptibility of lysosomal membranes to disruption, release of lysosomal enzymes into the cytosol, and neuronal degeneration. Alzheimer disease (AD)2 is a debilitating neurodegenerative disease. With no effective treatment available, the incidence of AD is likely to increase as the population continues to age. Although in most cases the exact cause is unknown, two proteins have been implicated in its pathogenesis: apolipoprotein E4 (apoE4) and amyloid ␤ (A␤).ApoE has important functions in lipid transport in the blood and in the redistribution of lipids among cells in the brain (1). One of its three common isoforms (2, 3), apoE4, is the major genetic risk factor for AD (4 -6). Moreover, the ⑀4 allele is associated with impaired central nervous system repair after injury and in other neurodegenerative diseases (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22).The A␤ peptide, typically 39 -43 amino acids, is derived from the proteolytic cleavage of the amyloid precursor protein (APP) (23,24). Amyloid plaques, a pathological hallmark of AD, are extracellular deposits of A␤ (25,26). Several findings suggest that apoE, another constituent of plaques, may modulate plaque formation or alter the availability of A␤ for plaque formation. Lipid-free apoE4 avidly complexes with A␤ (27), possibly inducing plaques, whereas lipidated apoE3 facilitates A␤ clearance (28 -32) and may reduce plaque formation. ApoE also modulates the cleavage of APP by ␥-secretase (33).S...

Apolipoprotein E4: A causative factor and therapeutic target in neuropathology, including Alzheimer’s disease

“…In cultured Neuro-2a cells, we showed that apoE4 enhances A␤-induced lysosomal leakage and apoptotic cell death to a much greater extent (2-to 4-fold) than apoE3 (74,127).…”

Membrane Localization of β-Amyloid 1–42 in Lysosomes

“…Previous reports have suggested that the lysosomal accumulation of A␤42 may play an important role in neuronal cell death and that the loss of lysosomal membrane impermeability is involved in A␤42 pathogenesis (22,23,68,69), but what is essential for the A␤42-induced lysosomal damage remains to be shown. We confirmed the view that A␤42 applied extracellularly leads to fluorescence diffusion of endocytosed punctate Lucifer Yellow and lysosomal enzyme leakage.…”

“…Treatment with alkalizing drugs, such as chloroquine and BafA1, obviously ameliorated lysosome damage and cell death. A recent publication by Ji et al (68,69) reported that apoE4 and A␤42 might work in concert at lower pH to increase the susceptibility of lysosomes to disruption. They also pointed out that the question of whether the apoE4/A␤42 complex or the two molecules, independently, destabilize the membranes still remains to be resolved.…”

The emerging roles of protein homeostasis‐governing pathways in Alzheimer's disease

“…Possible mechanisms may be that the expression of autophagy‐related genes decreases with (Omata, Lim, Akao, & Tsuda, 2014), as well as ApoE4 induces membrane disruption of lysosomes, release of lysosomal enzymes, leading to dysfunction of autophagic machinery (Ji, Müllendorff, Cheng, Miranda, & Huang, 2006). …”