Apolipoprotein E4 Potentiates Amyloid β Peptide-induced Lysosomal Leakage and Apoptosis in Neuronal Cells

Abstract

We assessed the isoform-specific effects of apolipoprotein (apo) E on the response of Neuro-2a cells to the amyloid ␤ peptide (A␤1-42). As determined by the intracellular staining pattern and the release of ␤-hexosaminidase into the cytosol, apoE4-transfected cells treated with aggregated A␤1-42 showed a greater tendency toward lysosomal leakage than neo-or apoE3-transfected cells. A␤1-42 caused significantly greater cell death and more than 2-fold greater DNA fragmentation in apoE4-secreting than in apoE3-secreting or control cells. H 2 O 2 or staurosporine enhanced cell death and apoptosis in apoE4-transfected cells but not in apoE3-transfected cells. A caspase-9 inhibitor abolished the potentiation of A␤1-42-induced apoptosis by apoE4. Similar results were obtained with conditioned medium from cells secreting apoE3 or apoE4. Cells preincubated for 4 h with a source of apoE3 or apoE4, followed by removal of apoE from the medium and from the cell surface, still exhibited the isoform-specific response to A␤1-42, indicating that the potentiation of apoptosis required intracellular apoE, presumably in the endosomes or lysosomes. Studies of phospholipid (dimyristoylphosphatidylcholine) bilayer vesicles encapsulating 5-(and-6)-carboxyfluorescein dye showed that apoE4 remodeled and disrupted the phospholipid vesicles to a greater extent than apoE3 or apoE2. In response to A␤1-42, vesicles containing apoE4 were disrupted to a greater extent than those containing apoE3. These findings are consistent with apoE4 forming a reactive molecular intermediate that avidly binds phospholipid and may insert into the lysosomal membrane, destabilizing it and causing lysosomal leakage and apoptosis in response to A␤1-42.Human apolipoprotein (apo) 1 E exists in three major isoforms, apoE2, apoE3, and apoE4, which are encoded by three apoE alleles (⑀2, ⑀3, and ⑀4) (1). The ⑀4 allele is a susceptibility gene for Alzheimer's disease (2-5) and other neurodegenerative disorders (6 -9). The apoE isoforms have differential effects on neurite outgrowth in cultured neurons and on the stability of the intracellular cytoskeleton and microtubular system (10 -14). Recently, we demonstrated that a bioactive form of apoE (E4 to a greater extent than E3) can escape an intracellular membrane compartment, enter the cytosol, and interact with cytoskeletal components to form neurofibrillary tangle-like neuronal inclusions (15). Our studies in transgenic mice have indicated a role for apoE4 in neuronal degeneration and behavioral activity (16,17). In addition, apoE is critical in the deposition of amyloid ␤ peptide (A␤) in transgenic mice overproducing the amyloid precursor protein (18 -20).The neurotoxicity of A␤1-42 appears to be an important factor in the pathogenesis of Alzheimer's disease (for review, see Refs. 21 and 22). A␤1-42 is a proteolytic product of the amyloid precursor protein and is a normal component of plasma and cerebrospinal fluid (23,24). A␤ deposition or amyloid plaque formation is found in Alzheimer's disease cases (25,26) and in t...

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).…”

An apolipoprotein E variant may protect against age‐related macular degeneration through cytokine regulation

“…Current knowledge of the differential functioning among the ApoE isoforms, either in lipid metabolism or neuronal protection, does not readily provide a rationale for the observed protective role of E4 against AMD development [Nathan et al, 1994;Ji et al, 2002;Laws et al, 2003]. ApoE is expressed in photoreceptor outer-segments, the retinal ganglion layer, and both layers of Bruch's membrane.…”

“…Incubation with wild-type ApoE E3 or variant ApoE E4-enriched 3-VLDL results in differential cellular accumulation [Ji et al, 1998;Laws et al, 2003]. A two-to threefold increase in the accumulation of ApoE E3 over ApoE E4 has been observed in various cell types [Ji et al, 2002]. If mechanisms for proper clearance in the retina and Bruch's membrane are not in place, this accumulation may serve as a pathogenic basis for disease development [Ambati et al, 2003;Tuo et al, 2004b].…”

Increased tau Phosphorylation in Apolipoprotein E4 Transgenic Mice Is Associated with Activation of Extracellular Signal-regulated Kinase

“…They include modulation of the deposition and clearance of A␤ peptides and the formation of plaques (15)(16)(17)(18)(19)(20)(21), impairment of the antioxidative defense system (22), dysregulation of neuronal signaling pathways (23), altered phosphorylation of tau and NFT formation (24 -29), depletion of cytosolic androgen receptor levels in the brain (30,31), potentiation of A␤-induced lysosomal leakage and apoptosis in neuronal cells (32), and promotion of endosomal abnormalities linked to A␤ overproduction (33)(34)(35). However, the mechanisms of these apoE4-mediated detrimental effects are largely unknown, and it is not known which are the primary effects and which are subsequent or downstream effects.…”