The endosomal trafficking of apolipoprotein E3 and E4 in cultured human brain neurons and astrocytes

Cell type-specific roles of APOE4 in Alzheimer disease

The ɛ4 allele of the apolipoprotein E gene (APOE), which translates to the APOE4 isoform, is the strongest genetic risk factor for late-onset Alzheimer disease (AD). Within the CNS, APOE is produced by a variety of cell types under different conditions, posing a challenge for studying its roles in AD pathogenesis. However, through powerful advances in research tools and the use of novel cell culture and animal models, researchers have recently begun to study the roles of APOE4 in AD in a cell type-specific manner and at a deeper and more mechanistic level than ever before. In particular, cutting-edge omics studies have enabled APOE4 to be studied at the single-cell level and have allowed the identification of critical APOE4 effects in AD-vulnerable cellular subtypes. Through these studies, it has become evident that APOE4 produced in various types of CNS cell - including astrocytes, neurons, microglia, oligodendrocytes and vascular cells - has diverse roles in AD pathogenesis. Here, we review these scientific advances and propose a cell type-specific APOE4 cascade model of AD. In this model, neuronal APOE4 emerges as a crucial pathological initiator and driver of AD pathogenesis, instigating glial responses and, ultimately, neurodegeneration. In addition, we provide perspectives on future directions for APOE4 research and related therapeutic developments in the context of AD.

The Contributions of the Endolysosomal Compartment and Autophagy to APOEɛ4 Allele-Mediated Increase in Alzheimer's Disease Risk

Apolipoprotein E4 (APOE4), although yet-to-be fully understood, increases the risk and lowers the age of onset of Alzheimer's disease (AD), which is the major cause of dementia among elderly individuals. The endosome-lysosome and autophagy pathways, which are necessary for homeostasis in both neurons and glia, are dysregulated even in early AD. Nonetheless, the contributory roles of these pathways to developing AD-related pathologies in APOE4 individuals and models are unclear. Therefore, this review summarizes the dysregulations in the endosome-lysosome and autophagy pathways in APOE4 individuals and non-human models, and how these anomalies contribute to developing AD-relevant pathologies. The available literature suggests that APOE4 causes endosomal enlargement, increases endosomal acidification, impairs endosomal recycling, and downregulates exosome production. APOE4 impairs autophagy initiation and inhibits basal autophagy and autophagy flux. APOE4 promotes lysosome formation and trafficking and causes ApoE to accumulate in lysosomes. APOE4-mediated changes in the endosome, autophagosome and lysosome could promote AD-related features including Aβ accumulation, tau hyperphosphorylation, glial dysfunction, lipid dyshomeostasis, and synaptic defects. ApoE4 protein could mediate APOE4-mediated endosome-lysosome-autophagy changes. ApoE4 impairs vesicle recycling and endosome trafficking, impairs the synthesis of autophagy genes, resists being dissociated from its receptors and degradation, and forms a stable folding intermediate that could disrupt lysosome structure. Drugs such as molecular correctors that target ApoE4 molecular structure and enhance autophagy may ameliorate the endosome-lysosome-autophagy-mediated increase in AD risk in APOE4 individuals.

Apolipoprotein E intersects with amyloid-β within neurons

Apolipoprotein E4 (ApoE4) is the most important genetic risk factor for Alzheimer's disease (AD). Among the earliest changes in AD is endosomal enlargement in neurons, which was reported as enhanced in ApoE4 carriers. ApoE is thought to be internalized into endosomes of neurons, whereas β-amyloid (Aβ) accumulates within neuronal endosomes early in AD. However, it remains unknown whether ApoE and Aβ intersect intracellularly. We show that internalized astrocytic ApoE localizes mostly to lysosomes in neuroblastoma cells and astrocytes, whereas in neurons, it preferentially localizes to endosomes-autophagosomes of neurites. In AD transgenic neurons, astrocyte-derived ApoE intersects intracellularly with amyloid precursor protein/Aβ. Moreover, ApoE4 increases the levels of endogenous and internalized Aβ42 in neurons. Taken together, we demonstrate differential localization of ApoE in neurons, astrocytes, and neuron-like cells, and show that internalized ApoE intersects with amyloid precursor protein/Aβ in neurons, which may be of considerable relevance to AD.

A Review of APOE Genotype-Dependent Autophagic Flux Regulation in Alzheimer's Disease

Autophagy is a basic physiological process maintaining cell renewal, the degradation of dysfunctional organelles, and the clearance of abnormal proteins and has recently been identified as a main mechanism underlying the onset and progression of Alzheimer's disease (AD). The APOE ɛ4 genotype is the strongest genetic determinant of AD pathogenesis and initiates autophagic flux at different times. This review synthesizes the current knowledge about the potential pathogenic effects of ApoE4 on autophagy and describes its associations with the biological hallmarks of autophagy and AD from a novel perspective. Via a remarkable variety of widely accepted signaling pathway markers, such as mTOR, TFEB, SIRT1, LC3, p62, LAMP1, LAMP2, CTSD, Rabs, and V-ATPase, ApoE isoforms differentially modulate autophagy initiation; membrane expansion, recruitment, and enclosure; autophagosome and lysosome fusion; and lysosomal degradation. Although the precise pathogenic mechanism varies for different genes and proteins, the dysregulation of autophagic flux is a key mechanism on which multiple pathogenic processes converge.

APOE4 is associated with cognitive and pathological heterogeneity in patients with Alzheimer's disease: a systematic review

Possession of the ε4 allele of apolipoprotein E (APOE) is the primary genetic risk factor for the sporadic form of Alzheimer’s disease (AD). While researchers have extensively characterized the impact that APOE ε4 (APOE4) has on the susceptibility of AD, far fewer studies have investigated the phenotypic differences of patients with AD who are APOE4 carriers vs. those who are non-carriers. In order to understand these differences, we performed a qualitative systematic literature review of the reported cognitive and pathological differences between APOE4-positive (APOE4+) vs. APOE4-negative (APOE4−) AD patients. The studies performed on this topic to date suggest that APOE4 is not only an important mediator of AD susceptibility, but that it likely confers specific phenotypic heterogeneity in AD presentation, as well. Specifically, APOE4+ AD patients appear to possess more tau accumulation and brain atrophy in the medial temporal lobe, resulting in greater memory impairment, compared to APOE4− AD patients. On the other hand, APOE4− AD patients appear to possess more tau accumulation and brain atrophy in the frontal and parietal lobes, resulting in greater impairment in executive function, visuospatial abilities, and language, compared to APOE4+ AD patients. Although more work is necessary to validate and interrogate these findings, these initial observations of pathological and cognitive heterogeneity between APOE4+ vs. APOE4− AD patients suggest that there is a fundamental divergence in AD manifestation related to APOE genotype, which may have important implications in regard to the therapeutic treatment of these two patient populations.

Dysregulation of Exosome Cargo by Mutant Tau Expressed in Human-induced Pluripotent Stem Cell (iPSC) Neurons Revealed by Proteomics Analyses

Accumulation and propagation of hyperphosphorylated Tau (p-Tau) is a common neuropathological hallmark associated with neurodegeneration of Alzheimer's disease (AD), frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17), and related tauopathies. Extracellular vesicles, specifically exosomes, have recently been demonstrated to participate in mediating Tau propagation in brain. Exosomes produced by human induced pluripotent stem cell (iPSC)-derived neurons expressing mutant Tau (mTau), containing the P301L and V337M Tau mutations of FTDP-17, possess the ability to propagate p-Tau pathology after injection into mouse brain. To gain an understanding of the mTau exosome cargo involved in Tau pathogenesis, these pathogenic exosomes were analyzed by proteomics and bioinformatics. The data showed that mTau expression dysregulates the exosome proteome to result in 1) proteins uniquely present only in mTau, and not control exosomes, 2) the absence of proteins in mTau exosomes, uniquely present in control exosomes, and 3) shared proteins which were significantly upregulated or downregulated in mTau compared with control exosomes. Notably, mTau exosomes (not control exosomes) contain ANP32A (also known as I1PP2A), an endogenous inhibitor of the PP2A phosphatase which regulates the phosphorylation state of p-Tau. Several of the mTau exosome-specific proteins have been shown to participate in AD mechanisms involving lysosomes, inflammation, secretases, and related processes. Furthermore, the mTau exosomes lacked a substantial portion of proteins present in control exosomes involved in pathways of localization, vesicle transport, and protein binding functions. The shared proteins present in both mTau and control exosomes represented exosome functions of vesicle-mediated transport, exocytosis, and secretion processes. These data illustrate mTau as a dynamic regulator of the biogenesis of exosomes to result in acquisition, deletion, and up- or downregulation of protein cargo to result in pathogenic mTau exosomes capable of in vivo propagation of p-Tau neuropathology in mouse brain.

Endosomal Traffic Jams Represent a Pathogenic Hub and Therapeutic Target in Alzheimer's Disease

While clues have existed that endosomal trafficking is associated with Alzheimer's disease (AD), whether it plays a central role in the disease and if so how has remained unknown. Here we rely on recent genetic and cellular findings to construct a model proposing that traffic jams in the early endosome can act as an upstream pathogenic hub in AD. We also rely on an independent series of findings to suggest how the traffic jams can act as a unified mediator of downstream pathophysiology. The model predicts, therefore, that interventions designed to unjam the endosome carry high therapeutic promise.

Apolipoprotein E4 Elicits Lysosomal Cathepsin D Release, Decreased Thioredoxin-1 Levels, and Apoptosis

The major genetic risk factor for Alzheimer’s disease (AD), apolipoprotein E4 (ApoE4), has been suggested to have detrimental effects on neurons, including direct toxicity via apoptosis. Thioredoxin-1 (Trx1) is an endogenous antioxidant protein important for redox regulation and participates in the regulation of apoptosis through the inhibition of apoptosis signal-regulating kinase-1 (Ask-1). In this study, we have investigated the effects of ApoE on Trx1 in the brain. Our results showed that the protein levels of Trx1 were reduced in the hippocampus of ApoE4 targeted replacement (TR) mice compared to ApoE3 TR mice. The reduction was also seen in vitro after treatment of both human primary cortical neurons and neuroblastoma cells with human recombinant ApoE4 (rApoE4). Furthermore, ApoE4 caused a disruption of lysosomal integrity and a shift in the localization of Cathepsin D, an enzyme known to degrade Trx1. ApoE4 treatment induced in addition apoptosis through translocation of Death-domain associated protein-6 (Daxx) from the nucleus to the cytosol, suggesting an activation of the Ask-1 pathway. This toxicity was prevented by overexpression of Trx1 and other endogenous Ask-1 inhibitors. Our data suggests that down-regulation of Trx1 is involved in the toxicity caused by ApoE4. An activated ASK-1 pathway might indeed make cells more vulnerable to other insults such as amyloid-β, which could partially explain the mechanism behind the strongest genetic risk factor for AD.

The Endosomal-Lysosomal Pathway Is Dysregulated by APOE4 Expression in Vivo

Possession of the ε4 allele of apolipoprotein E (APOE) is the major genetic risk factor for late-onset Alzheimer's disease (AD). Although numerous hypotheses have been proposed, the precise cause of this increased AD risk is not yet known. In order to gain a more comprehensive understanding of APOE4's role in AD, we performed RNA-sequencing on an AD-vulnerable vs. an AD-resistant brain region from aged APOE targeted replacement mice. This transcriptomics analysis revealed a significant enrichment of genes involved in endosomal–lysosomal processing, suggesting an APOE4-specific endosomal–lysosomal pathway dysregulation in the brains of APOE4 mice. Further analysis revealed clear differences in the morphology of endosomal–lysosomal compartments, including an age-dependent increase in the number and size of early endosomes in APOE4 mice. These findings directly link the APOE4 genotype to endosomal–lysosomal dysregulation in an in vivo, AD pathology-free setting, which may play a causative role in the increased incidence of AD among APOE4 carriers.

Monocytic elastase-mediated apolipoprotein-E degradation: Potential involvement of microglial elastase-like proteases in apolipoprotein-E proteolysis in brains with Alzheimers disease

Impaired clearance of soluble Aβ (amyloid-β) promotes Aβ aggregation in brains with Alzheimer's disease (AD), while apolipoprotein-E (ApoE) in microglia mediates Aβ clearance. We studied the protease responsible for ApoE(4) degradation in human peripheral monocyte extracts, which are from the same lineage as microglia. We detected the hydrolytic activity for ApoE(4) in high-salt extracts with 2 M NaCl and found that the activity was inhibited by a serine protease inhibitor and an elastase-specific inhibitor, but not by other protease inhibitors. The extracts exhibited higher activity for the elastase substrate, and we followed the activity with ion-exchange and gel-filtration chromatography. Through silver staining, we partially purified a protein of 28 kDa, which was clarified as elastase by liquid chromatography-tandem mass spectrometry. These observations suggest that elastase is the key protease for ApoE(4) degradation. We also detected ApoE(4) hydrolytic activity in high-salt extracts in mouse microglial (BV-2) cell lysates, and showed that the ApoE(4) fragments by the BV-2 extracts differed from the fragments by the monocyte extracts. Though the ApoE(4) degradation by the extracts was not inhibited with elastase-specific inhibitors, it was inhibited by an elastase-specific monoclonal antibody, suggesting that elastase-like proteases in microglia differ from those of monocytes. Immunohistochemistry revealed that both elastase and ApoE were expressed in the senile plaques of brains with AD. In vitro studies also disclosed the localization of elastase in the microglial cell line, BV-2. Our results suggest that elastase-like proteases in the microglial cells surrounding Aβ plaques are responsible for ApoE degradation in the brain.

Altered Cholesterol Intracellular Trafficking and the Development of Pathological Hallmarks of Sporadic AD

Compared to the rare familial early onset Alzheimer's disease (AD) that results from gene mutations in AbPP and presenilin-1, the pathogenesis of sporadic AD is much more complex and is believed to result from complex interactions between nutritional, environmental, epigenetic and genetic factors. Among those factors, the presence APOE4 is still the single strongest genetic risk factor for sporadic AD. However, the exact underlying mechanism whereby apoE4 contributes to the pathogenesis of sporadic AD remains unclear. Here, we discuss how altered cholesterol intracellular trafficking as a result of apoE4 might contribute to the development of pathological hallmarks of AD including brain deposition of amyloid beta (Ab), neurofibrillary tangles, and synaptic dysfunction.

Role of LDL cholesterol and endolysosomes in amyloidogenesis and Alzheimer's disease

The pathogenesis of late-onset sporadic Alzheimer's disease (AD) is believed to result from complex interactions between nutritional, environmental, epigenetic and genetic factors. Among those factors, altered circulating cholesterol homeostasis, independent of the APOE genotype, continues to be implicated in brain deposition of amyloid beta protein (Aβ) and the pathogenesis of AD. It is believed that trafficking of amyloid beta precursor protein (AβPP) into endolysosomes appears to play a critical role in determining amyloidogenic processing of AβPP because this is precisely where two enzymes critically important in AβPP metabolism are located; beta amyloid converting enzyme (BACE-1) and gamma secretase enzyme. We have shown that elevated levels of LDL cholesterol promote AβPP internalization, disturb neuronal endolysosome structure and function, and increase Aβ accumulation in neuronal endolysosomes. Here, we will further discuss the linkage between elevated levels of LDL cholesterol and AD pathogenesis, and explore the underlying mechanisms whereby elevated levels of plasma LDL cholesterol promote amyloidogenesis.

Reconsider Alzheimer's disease by the 'calpain-cathepsin hypothesis'--a perspective review

Alzheimer's disease (AD) is characterized by slowly progressive neuronal death, but its molecular cascade remains elusive for over 100 years. Since accumulation of autophagic vacuoles (also called granulo-vacuolar degenerations) represents one of the pathologic hallmarks of degenerating neurons in AD, a causative connection between autophagy failure and neuronal death should be present. The aim of this perspective review is at considering such underlying mechanism of AD that age-dependent oxidative stresses may affect the autophagic-lysosomal system via carbonylation and cleavage of heat-shock protein 70.1 (Hsp70.1). AD brains exhibit gradual but continual ischemic insults that cause perturbed Ca(2+) homeostasis, calpain activation, amyloid β deposition, and oxidative stresses. Membrane lipids such as linoleic and arachidonic acids are vulnerable to the cumulative oxidative stresses, generating a toxic peroxidation product 'hydroxynonenal' that can carbonylate Hsp70.1. Recent data advocate for dual roles of Hsp70.1 as a molecular chaperone for damaged proteins and a guardian of lysosomal integrity. Accordingly, impairments of lysosomal autophagy and stabilization may be driven by the calpain-mediated cleavage of carbonylated Hsp70.1, and this causes lysosomal permeabilization and/or rupture with the resultant release of the cell degradation enzyme, cathepsins (calpain-cathepsin hypothesis). Here, the author discusses three topics; (1) how age-related decrease in lysosomal and autophagic activities has a causal connection to programmed neuronal necrosis in sporadic AD, (2) how genetic factors such as apolipoprotein E and presenilin 1 can facilitate lysosomal destabilization in the sequential molecular events, and (3) whether a single cascade can simultaneously account for implications of all players previously reported. In conclusion, Alzheimer neuronal death conceivably occurs by the similar 'calpain-hydroxynonenal-Hsp70.1-cathepsin cascade' with ischemic neuronal death. Blockade of calpain and/or extra-lysosomal cathepsins as well as scavenging of hydroxynonenal would become effective AD therapeutic approaches.

Self-association and stability of the ApoE isoforms at low pH: implications for ApoE-lipid interactions

Apolipoprotein E (apoE) isoforms are known to differentially accumulate in the lysosomes of neuronal cells, and the deleterious effects of the apoE4 isoform in Alzheimer's disease may relate to its properties at the low lysosomal pH. However, the effect of pH on the molecular properties of full-length apoE is unclear. Here we examine the pH dependence of the monomer-dimer-tetramer reaction, of lipid binding, and of the stability of the three major apoE isoforms. Using FRET measurements, we find that the association-dissociation behavior of apoE proteins changes dramatically with changes in pH. At pH 4.5, approximating the pH of the lysosome, rate constants for association and dissociation are 2-10 times faster than those at pH 7.4. Aggregation beyond the tetrameric form is also more evident at lower pH values. Stability, as measured by urea denaturation at pH 4.5, is found to be considerably greater than that at neutral pH and to be isoform dependent. Lipid binding, as measured by turbidity clearance of unilamellar vesicles of DMPC, is faster at acidic pH values and consistent with our previous hypothesis that it is only the monomeric form of apoE that binds lipid tightly. Since apoE is more stable at pH 4.5 than at neutral pH, the more rapid apoE-lipid interactions at low pH are not correlated with the stability of the apoE isoforms, but rather to the faster association-dissociation behavior. Our results indicate that pathological behavior of apoE4 may arise from altered molecular properties of this protein at the acidic pH of the lysosome.

Following activation of the amyloid cascade, apolipoprotein E4 drives the in vivo oligomerization of amyloid-β resulting in neurodegeneration

According to the amyloid hypothesis, the accumulation of oligomerized amyloid-β (Aβ) is a primary event in the pathogenesis of Alzheimer's disease (AD). The trigger of the amyloid cascade and of Aβ oligomerization in sporadic AD, the most prevalent form of the disease, remains elusive. Here, we examined the hypothesis that apolipoprotein E4 (ApoE4), the most prevalent genetic risk factor for AD, triggers the accumulation of intraneuronal oligomerized Aβ following activation of the amyloid cascade. We investigated the intracellular organelles that are targeted by these processes and govern their pathological consequences. This revealed that activation of the amyloid cascade in vivo by inhibition of the Aβ degrading enzyme neprilysin specifically results in accumulation of Aβ and oligomerized Aβ and of ApoE4 in the CA1 neurons of ApoE4 mice. This was accompanied by lysosomal and mitochondrial pathology and the co-localization of Aβ, oligomerized Aβ, and ApoE4 with enlarged lysosomes and of Aβ and oligomerized Aβ with mitochondria. The time course of the lysosomal effects paralleled that of the loss of CA1 neurons, whereas the mitochondrial effects reached an earlier plateau. These findings suggest that ApoE4 potentiates the pathological effects of Aβ and the amyloid cascade by triggering the oligomerization of Aβ, which in turn, impairs intraneuronal mitochondria and lysosomes and drives neurodegeneration.

Generation of an immortalized astrocyte cell line from H-2Kb-tsA58 mice to study the role of astrocytes in brain metastasis

Astrocytes play a critical role in maintaining cerebral homeostasis and their dysregulation is thought to contribute to the pathogenesis of several diseases, including brain cancer and metastasis. Similar to the human disease, we found that lung and melanoma metastases in the mouse brain are accompanied by a reactive gliosis. To begin to study the biology of astrocytes and examine how these cells might contribute to metastasis formation and progression in the brain, we generated a conditionally immortal astrocyte cell line from H-2Kb-tsA58 mice. Astrocytes grown in culture expressed glial fibrillary acid protein (GFAP), glutamate receptor 1, and the N-methyl-D-aspartate (NMDA) receptor. Astrocytes also expressed the glial-specific transporters excitatory amino acid transporter 1 (EAAT1) and EAAT2. Astrocytes grown under permissive conditions (33 degrees C) expressed SV40 large T antigen and had a doubling time of 36 h, whereas expression of SV40 large T antigen was negligible in astrocytes grown at 37 degrees C for 72 h, which coincided with a plateau in cell division. In a co-culture assay with human lung adenocarcinoma cells (PC14-PE6), astrocytes activated programs in the tumor cells that signal for cell division and survival. Hence, the immortalized cell line will be useful for studying the role of astrocytes in disease processes in the brain, such as metastasis.

Recycling of apolipoprotein E is not associated with cholesterol efflux in neuronal cells

After receptor-mediated endocytosis of apolipoprotein E (apoE)-containing lipoproteins in hepatocytes, the isoform apoE3 is efficiently recycled in a process which is associated with cholesterol efflux. Recycling and cholesterol efflux are greatly reduced when apoE4 is the only isoform present. ApoE is the main apolipoprotein in cerebrospinal fluid, and it plays a pivotal role in maintaining cholesterol homeostasis in the brain. The isoform apoE4 is associated with an increased risk of Alzheimer's disease and it has been postulated that high intracellular cholesterol levels promote the amyloidogenic processing of amyloid precursor protein. Therefore we investigated the cellular processing of different apoE isoforms as well as the associated cholesterol efflux in the murine neuronal cell line HT-22. Uptake of apoE3-containing lipoproteins resulted in the expected recycling while, as seen in non-neuronal cells, recycling of apoE4 was significantly reduced. However, despite these differences in apoE recycling, there was no difference in rates of cholesterol efflux. Therefore we conclude that in this neuronal cell model the reduced recycling of apoE4 does not affect cellular cholesterol metabolism.

Proteomic analysis of cathepsin B- and L-deficient mouse brain lysosomes

Cathepsins B and L are lysosomal cysteine proteases which have been implicated in a variety of pathological processes such as cancer, tumor angiogenesis, and neurodegeneration. However, only a few protein substrates have thus far been described and the mechanisms by which cathepsins B and L regulate cell proliferation, invasion, and apoptosis are poorly understood. Combined deficiency of both cathepsins results in early-onset neurodegeneration in mice reminiscent of neuronal ceroid lipofuscinoses in humans. Therefore, we intended to quantify accumulated proteins in brain lysosomes of double deficient mice. A combination of subcellular fractionation and LC-MS/MS using isobaric tagging for relative and absolute quantitation (iTRAQ) allowed us to simultaneously assess wildtype and cathepsin B(-/-)L(-/-) cerebral lysosomes. Altogether, 19 different proteins were significantly increased in cathepsin B(-/-)L(-/-) lysosomes. Most elevated proteins had previously been localized to neuronal biosynthetic, recycling/endocytic or lysosomal compartments. A more than 10-fold increase was observed for Rab14, the Delta/Notch-like epidermal growth factor-related receptor (DNER), calcyon, and carboxypeptidase E. Intriguingly, immunohistochemistry demonstrated that Rab14 and DNER specifically stain swollen axons in double deficient brains. Since dense accumulations of expanded axons are the earliest phenotypic and pathognomonic feature of cathepsin B(-/-)L(-/-) brains, our data suggest a role for cathepsins B and L in recycling processes during axon outgrowth and synapse formation in the developing postnatal central nervous system.

Cathepsin D-mediated proteolysis of apolipoprotein E: possible role in Alzheimer's disease

Proteolysis of apolipoprotein E (apoE) may be involved in the pathogenesis of Alzheimer's disease (AD). We previously identified aspartic protease(s) as possibly contributing to the proteolysis of apoE in human brain homogenates. The current study used biochemical and immunohistochemical methods to examine whether cathepsin D (catD) and cathepsin E (catE), candidate aspartic proteases, may be involved in apoE proteolysis. CatD was found to proteolyze both lipid-free recombinant full-length human apoE and lipidated human plasma full-length apoE (apoE4/dipalmitoylphosphatidylcholine-reconstituted discs). CatE was found to proteolyze lipid-free recombinant human apoE to a much greater extent than lipidated apoE. This proteolysis, as well as proteolysis of human apoE added to brain homogenates from apoE-deficient mice, was inhibited by pepstatin A (an aspartic protease inhibitor), but not by phenylmethanesulfonyl fluoride (a serine protease inhibitor). The major apoE fragment obtained with catD included the receptor-binding domain and had an apparent molecular weight similar to that found in human brain homogenates. There was little immunoreactivity for catE in AD brain tissue sections. In contrast, qualitative and quantitative analyses of immunostained sections of the frontal cortex revealed that catD and apoE are colocalized in a subset of predominantly dense-core neuritic plaques and in some neurofibrillary tangles. A positive correlation was observed between estimated duration of illness and the percentage of apoE-positive plaques that were also catD-positive. These results suggest that aspartic proteases, catD in particular, may be involved in proteolysis of apoE and perhaps contribute to the generation of apoE fragments previously implicated in AD pathology.

Correction of the neuropathogenic human apolipoprotein E4 (APOE4) gene to APOE3 in vitro using synthetic RNA/DNA oligonucleotides (chimeraplasts)

Apolipoprotein E (apoE) is a multifunctional circulating 34-kDa protein, whose gene encodes single-nucleotide polymorphisms linked to several neurodegenerative diseases. Here, we evaluate whether synthetic RNA/DNA oligonucleotides (chimeraplasts) can convert a dysfunctional gene, APOE4 (C, A and E, T, Cys112Arg), a risk factor for Alzheimer's disease and other neurological disorders, into wild-type APOE3. In preliminary experiments, we treated recombinant Chinese hamster ovary (CHO) cells stably secreting apoE4 and lymphocytes from a patient homozygous for the epsilon 4 allele with a 68-mer apoE4-to-apoE3 chimeraplast, complexed to the cationic delivery reagent, polyethyleneimine. Genotypes were analyzed after 48 h by routine polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) and by genomic sequencing. Clear conversions of APOE4 to APOE3 were detected using either technique, although high concentrations of chimeraplast were needed (> or =800 nM). Spiking experiments of PCR reactions or CHO-K1 cells with the chimeraplast confirmed that the repair was not artifactual. However, when treated recombinant CHO cells were passaged for 10 d and then subcloned, no conversion could be detected when >90 clones were analyzed by locus-specific PCR-RFLP. We conclude that the apparent efficient repair of the APOE4 gene in CHO cells or lymphocytes 48 h post-treatment is unstable, possibly because the high levels of chimeraplast and polyethyleneimine that were needed to induce nucleotide substitution are cytotoxic.

Progress toward identification of protease activity involved in proteolysis of apolipoprotein e in human brain

Apolipoprotein E (apoE) genotype is the single most important genetic risk factor for the most common (sporadic) form of Alzheimer's disease (AD). Increasing evidence supports the hypothesis that the presence of the E4 isoform of this cholesterol-binding protein contributes directly to disease risk, age of onset, and severity of the neuropathology. For example, studies in transgenic mice demonstrate that apoE is necessary for the formation of plaques with neuritic pathology. The precise mechanism by which apoE contributes to the disease remains unknown. However, several lines of investigation from a number of laboratories now point to a role for proteolytic fragments of apoE in the formation of both plaques and tangles, the two pathological hallmarks of the disease. In particular, the C-terminal portion of apoE has been implicated in binding to amyloid and is localized to plaques. The N-terminal domain, on the other hand, is neurotoxic in culture and has been localized to, and implicated in the formation of, neurofibrillary tangles. These results suggest that inhibition of apoE proteolysis is a potential therapeutic strategy for AD. Using human brain homogenates, we have determined that proteolysis of apoE is greatest at acidic pH and can be inhibited by compounds targeting aspartic proteases. The feasibility of screening candidate inhibitors is supported by both ELISA and immunoblotting methods. Future studies will use a combination of in vitro and in vivo assays to test the efficacy of the most effective compounds for their ability to inhibit apoE proteolysis in human brain and apoE transgenic mouse brain tissue.

Murine synaptosomal lipid raft protein and lipid composition are altered by expression of human apoE 3 and 4 and by increasing age

Apolipoprotein E (apoE) 4 and aging are risk factors for Alzheimer's disease (AD). Mice expressing human apoE4 and aged wild-type mice show a similarity in the transbilayer distribution of cholesterol in synaptic plasma membranes (SPMs) but differ markedly compared with apoE3 mice and young mice. The largest changes in cholesterol distribution were observed in the SPM exofacial leaflet where there was a doubling of cholesterol. Lipid rafts are thought to be associated with the exofacial leaflet, and we proposed that lipid raft protein and lipid composition would be associated with apoE genotype and age. Lipid rafts were isolated from synaptosomes of different age groups (2, 12, 24 months) of mice expressing human apoE3 and apoE4. Lipid raft markers, alkaline phosphatase (ALP), flotillin-1, cholesterol and sphingomyelin (SM) were examined. Lipid rafts of young apoE4 mice were more similar to older mice as compared with young apoE3 mice in reductions in alkaline phosphatase activity and flotillin-1 abundance. Lipid raft cholesterol and sphingomyelin levels were not significantly different between the young apoE3 and apoE4 mice but cholesterol levels of lipid rafts did increase with age in both genotypes. Results of the present study demonstrate that the two risk factors for Alzheimer's disease, apoE4 genotype and increasing age have similar effects on brain lipid raft protein markers and these findings support the notion that the transbilayer distribution of cholesterol is associated with lipid raft function.

Androgens, ApoE, and Alzheimer's Disease

Increasing evidence indicates that there are reductions in estrogen and androgen levels in aged men and women. These hormonal reductions might be risk factors for cognitive impairments and the development of Alzheimer's disease (AD). Aged people show improved cognition after treatments with sex steroids. Therefore, ongoing clinical AD trials have been designed to evaluate the potential benefits of estrogen therapy in women and testosterone therapy in men. Apolipoprotein E (apoE) plays an important role in the metabolism and redistribution of lipoproteins and cholesterol. The three major human apoE isoforms, apoE2, apoE3, and apoE4, differ in their effects on AD risk and pathology. Here I review various mechanisms proposed to mediate the differential effects of apoE isoforms on brain function and highlight the potential contribution of detrimental isoform-dependent effects of apoE on androgen- and androgen receptor (AR)-mediated pathways. I also discuss potential interactions of androgens with other AD-related factors.

Diversity in the mechanisms of neuronal cell death

Neurons may die as a normal physiological process during development or as a pathological process in diseases. The best-understood mechanism of neuronal cell death is apoptosis, which is regulated by an evolutionarily conserved cellular pathway that consists of the caspase family, the Bcl-2 family, and the adaptor protein Apaf-1. Apoptosis, however, may not be the only cellular mechanism that regulates neuronal cell death. Neuronal cell death may exhibit morphological features of autophagy or necrosis, which differ from that of the canonical apoptosis. This review evaluates the evidence supporting the existence of alternative mechanisms of neuronal cell death and proposes the possible existence of an evolutionarily conserved pathway of necrosis.

Endocytic pathway alterations in human hippocampus after global ischemia and the influence of APOE genotype

Apolipoprotein epsilon4 (apoE, protein; APOE, gene) allele is the most important genetic risk factor for development of Alzheimer's disease and is also associated with poor outcome after brain injury. Although the mechanisms underlying this susceptibility are currently unknown, recent experimental evidence suggests that APOE genotype may influence activity in the endocytic pathway of neurons. This study determined whether alterations in the endocytic pathway occurred in medial temporal lobe sections after brain injury because of cardiorespiratory arrest and whether these alterations were influenced by APOE genotype. Antibodies to two proteins involved in endocytosis, rabaptin-5 and rab4, were used as markers of endocytic pathway activity. Alterations in immunoreactivity were examined in medial temporal lobe sections in the postmortem brain of patients who experienced an episode of global ischemia and in controls. After global ischemia there was a marked increase in immunoreactivity of both endocytic markers, rabaptin-5 and rab4, in neurons, and to a lesser extent in glia compared to controls. Furthermore, possession of an APOE epsilon4 allele was associated with specific alterations in the endocytic pathway. After global ischemia, there was no influence of APOE genotype on the extent of rabaptin-5 immunoreactivity. However, there was a statistically significant influence of APOE genotype on the extent of rab4 immunoreactivity in response to global ischemia. These results indicate marked alterations in the endocytic pathway after global ischemia that are dependent on APOE genotype. This may underlie the important influence of APOE genotype on brain injury and disease.

Apolipoproteins and aging: emerging mechanisms

One of the goals of modern medicine is to foster successful aging. In order to age successfully, one must accomplish two things: first, survive; and second, survive with good health and a sharp mind. In this discussion of apolipoproteins and aging, the focus will be on apolipoprotein E (apoE), a protein with three common isoforms, which has a large impact on longevity and successful aging. One variant of apoE (E4) is associated with increased risk for heart disease, stroke and Alzheimer's disease (AD). In addition, some of the potential mechanisms for the observed effects of apoE on aging will be discussed.

Molecular biology of apolipoprotein E

Apolipoprotein E, first identified 26 years ago as a serum protein that mediates extracellular cholesterol transport, is now known to regulate multiple additional metabolic pathways. Several clinically important disorders of the vasculature and brain are differentially caused, or modified, by the three isoforms of this protein. Apolipoprotein E was previously believed to traffic exclusively through binding cell surface receptors, endocytosis, and hydrolysis. However, recent studies reveal a variety of additional physiologically important roles for apolipoprotein E that are mediated through interactions with different families of receptors, through binding other proteins, and through other intracellular trafficking pathways and second messengers. Much research is now directed toward identifying those pathways of apolipoprotein E metabolism that are differentially regulated by the various isoforms of apolipoprotein E, with the goal of identifying the particular molecular pathways that result in vascular and neurologic disorders.