Apolipoprotein E4 Effects a Distinct Transcriptomic Profile and Dendritic Arbor Characteristics in Hippocampal Neurons Cultured in vitro.

The APOE gene is diversified by three alleles ε2, ε3, and ε4 encoding corresponding apolipoprotein (apo) E isoforms. Possession of the ε4 allele is signified by increased risks of age-related cognitive decline, Alzheimer's disease (AD), and the rate of AD dementia progression. ApoE is secreted by astrocytes as high-density lipoprotein-like particles and these are internalized by neurons upon binding to neuron-expressed apoE receptors. ApoE isoforms differentially engage neuronal plasticity through poorly understood mechanisms. We examined here the effects of native apoE lipoproteins produced by immortalized astrocytes homozygous for ε2, ε3, and ε4 alleles on the maturation and the transcriptomic profile of primary hippocampal neurons. Control neurons were grown in the presence of conditioned media from Apoe -/- astrocytes. ApoE2 and apoE3 significantly increase the dendritic arbor branching, the combined neurite length, and the total arbor surface of the hippocampal neurons, while apoE4 fails to produce similar effects and even significantly reduces the combined neurite length compared to the control. ApoE lipoproteins show no systemic effect on dendritic spine density, yet apoE2 and apoE3 increase the mature spines fraction, while apoE4 increases the immature spine fraction. This is associated with opposing effects of apoE2 or apoE3 and apoE4 on the expression of NR1 NMDA receptor subunit and PSD95. There are 1,062 genes differentially expressed across neurons cultured in the presence of apoE lipoproteins compared to the control. KEGG enrichment and gene ontology analyses show apoE2 and apoE3 commonly activate expression of genes involved in neurite branching, and synaptic signaling. In contrast, apoE4 cultured neurons show upregulation of genes related to the glycolipid metabolism, which are involved in dendritic spine turnover, and those which are usually silent in neurons and are related to cell cycle and DNA repair. In conclusion, our work reveals that lipoprotein particles comprised of various apoE isoforms differentially regulate various neuronal arbor characteristics through interaction with neuronal transcriptome. ApoE4 produces a functionally distinct transcriptomic profile, which is associated with attenuated neuronal development. Differential regulation of neuronal transcriptome by apoE isoforms is a newly identified biological mechanism, which has both implication in the development and aging of the CNS.

Absence of Apolipoprotein E is associated with exacerbation of prion pathology and promotes microglial neurodegenerative phenotype.

Prion diseases or prionoses are a group of rapidly progressing and invariably fatal neurodegenerative diseases. The pathogenesis of prionoses is associated with self-replication and connectomal spread of PrPSc, a disease specific conformer of the prion protein. Microglia undergo activation early in the course of prion pathogenesis and exert opposing roles in PrPSc mediated neurodegeneration. While clearance of PrPSc and apoptotic neurons have disease-limiting effect, microglia-driven neuroinflammation bears deleterious consequences to neuronal networks. Apolipoprotein (apo) E is a lipid transporting protein with pleiotropic functions, which include controlling of the phagocytic and inflammatory characteristics of activated microglia in neurodegenerative diseases. Despite the significance of microglia in prion pathogenesis, the role of apoE in prionoses has not been established. We showed here that infection of wild type mice with 22L mouse adapted scrapie strain is associated with significant increase in the total brain apoE protein and mRNA levels and also with a conspicuous cell-type shift in the apoE expression. There is reduced expression of apoE in activated astrocytes and marked upregulation of apoE expression by activated microglia. We also showed apoE ablation exaggerates PrPSc mediated neurodegeneration. Apoe-/- mice have shorter disease incubation period, increased load of spongiform lesion, pronounced neuronal loss, and exaggerated astro and microgliosis. Astrocytes of Apoe-/- mice display salient upregulation of transcriptomic markers defining A1 neurotoxic astrocytes while microglia show upregulation of transcriptomic markers characteristic for microglial neurodegenerative phenotype. There is impaired clearance of PrPSc and dying neurons by microglia in Apoe-/- mice along with increased level of proinflammatory cytokines. Our work indicates that apoE absence renders clearance of PrPSc and dying neurons by microglia inefficient, while the excess of neuronal debris promotes microglial neurodegenerative phenotype aggravating the vicious cycle of neuronal death and neuroinflammation.

Peroxiredoxin 6 mediates protective function of astrocytes in Aß proteostasis.

BACKGROUND: Disruption of ß-amyloid (Aß) homeostasis is the initial culprit in Alzheimer's disease (AD) pathogenesis. Astrocytes respond to emerging Aß plaques by altering their phenotype and function, yet molecular mechanisms governing astrocytic response and their precise role in countering Aß deposition remain ill-defined. Peroxiredoxin (PRDX) 6 is an enzymatic protein with independent glutathione peroxidase (Gpx) and phospholipase A2 (PLA2) activities involved in repair of oxidatively damaged cell membrane lipids and cellular signaling. In the CNS, PRDX6 is uniquely expressed by astrocytes and its exact function remains unexplored. METHODS: APPswe/PS1dE9 AD transgenic mice were once crossed to mice overexpressing wild-type Prdx6 allele or to Prdx6 knock out mice. Aß pathology and associated neuritic degeneration were assessed in mice aged 10 months. Laser scanning confocal microscopy was used to characterize Aß plaque morphology and activation of plaque-associated astrocytes and microglia. Effect of Prdx6 gene dose on plaque seeding was assessed in mice aged six months. RESULTS: We show that hemizygous knock in of the overexpressing Prdx6 transgene in APPswe/PS1dE9 AD transgenic mice promotes selective enticement of astrocytes to Aß plaques and penetration of plaques by astrocytic processes along with increased number and phagocytic activation of periplaque microglia. This effects suppression of nascent plaque seeding and remodeling of mature plaques consequently curtailing brain Aß load and Aß-associated neuritic degeneration. Conversely, Prdx6 haplodeficiency attenuates astro- and microglia activation around Aß plaques promoting Aß deposition and neuritic degeneration. CONCLUSIONS: We identify here PRDX6 as an important factor regulating response of astrocytes toward Aß plaques. Demonstration that phagocytic activation of periplaque microglia vary directly with astrocytic PRDX6 expression level implies previously unappreciated astrocyte-guided microglia effect in Aß proteostasis. Our showing that upregulation of PRDX6 attenuates Aß pathology may be of therapeutic relevance for AD.

Anti-prion Protein Antibody 6D11 Restores Cellular Proteostasis of Prion Protein Through Disrupting Recycling Propagation of PrPSc and Targeting PrPSc for Lysosomal Degradation.

PrPSc is an infectious and disease-specific conformer of the prion protein, which accumulation in the CNS underlies the pathology of prion diseases. PrPSc replicates by binding to the cellular conformer of the prion protein (PrPC) expressed by host cells and rendering its secondary structure a likeness of itself. PrPC is a plasma membrane anchored protein, which constitutively recirculates between the cell surface and the endocytic compartment. Since PrPSc engages PrPC along this trafficking pathway, its replication process is often referred to as "recycling propagation." Certain monoclonal antibodies (mAbs) directed against prion protein can abrogate the presence of PrPSc from prion-infected cells. However, the precise mechanism(s) underlying their therapeutic propensities remains obscure. Using N2A murine neuroblastoma cell line stably infected with 22L mouse-adapted scrapie strain (N2A/22L), we investigated here the modus operandi of the 6D11 clone, which was raised against the PrPSc conformer and has been shown to permanently clear prion-infected cells from PrPSc presence. We determined that 6D11 mAb engages and sequesters PrPC and PrPSc at the cell surface. PrPC/6D11 and PrPSc/6D11 complexes are then endocytosed from the plasma membrane and are directed to lysosomes, therefore precluding recirculation of nascent PrPSc back to the cell surface. Targeting PrPSc by 6D11 mAb to the lysosomal compartment facilitates its proteolysis and eventually shifts the balance between PrPSc formation and degradation. Ongoing translation of PrPC allows maintaining the steady-state level of prion protein within the cells, which was not depleted under 6D11 mAb treatment. Our findings demonstrate that through disrupting recycling propagation of PrPSc and promoting its degradation, 6D11 mAb restores cellular proteostasis of prion protein.

APOE Genotype Differentially Modulates Effects of Anti-Aß, Passive Immunization in APP Transgenic Mice.

BACKGROUND: APOE genotype is the foremost genetic factor modulating ß-amyloid (Aß) deposition and risk of sporadic Alzheimer's disease (AD). Here we investigated how APOE genotype influences response to anti-Aß immunotherapy. METHODS: APPSW/PS1dE9 (APP) transgenic mice with targeted replacement of the murine Apoe gene for human APOE alleles received 10D5 anti-Aß or TY11-15 isotype control antibodies between the ages of 12 and 15 months. RESULTS: Anti-Aß immunization decreased both the load of fibrillar plaques and the load of Aß immunopositive plaques in mice of all APOE backgrounds. Although the relative reduction in parenchymal Aß plaque load was comparable across all APOE genotypes, APP/ε4 mice showed the greatest reduction in the absolute Aß plaque load values, given their highest baseline. The immunization stimulated phagocytic activation of microglia, which magnitude adjusted for the post-treatment plaque load was the greatest in APP/ε4 mice implying association between the ε4 allele and impaired Aß phagocytosis. Perivascular hemosiderin deposits reflecting ensued microhemorrhages were associated with vascular Aß (VAß) and ubiquitously present in control mice of all APOE genotypes, although in APP/ε3 mice their incidence was the lowest. Anti-Aß immunization significantly reduced VAß burden but increased the number of hemosiderin deposits across all APOE genotypes with the strongest and the weakest effect in APP/ε2 and APP/ε3 mice, respectively. CONCLUSIONS: Our studies indicate that APOE genotype differentially modulates microglia activation and Aß plaque load reduction during anti-Aß immunotherapy. The APOE ε3 allele shows strong protective effect against immunotherapy associated microhemorrhages; while, conversely, the APOE ε2 allele increases risk thereof.

Blocking the apoE/Aß interaction ameliorates Aß-related pathology in APOE ε2 and ε4 targeted replacement Alzheimer model mice.

Accumulation of ß-amyloid (Aß) in the brain is essential to Alzheimer's disease (AD) pathogenesis. Carriers of the apolipoprotein E (APOE) ε4 allele demonstrate greatly increased AD risk and enhanced brain Aß deposition. In contrast, APOE ε2 allele carries show reduced AD risk, later age of disease onset, and lesser Aß accumulation. However, it remains elusive whether the apoE2 isoform exerts truly protective effect against Aß pathology or apoE2 plays deleterious role albeit less pronounced than the apoE4 isoform. Here, we characterized APPSW/PS1dE9/APOE ε2-TR (APP/E2) and APPSW/PS1dE9/APOE ε4-TR (APP/E4) mice, with targeted replacement (TR) of the murine Apoe for human ε2 or ε4 alleles, and used these models to investigate effects of pharmacological inhibition of the apoE/Aß interaction on Aß deposition and neuritic degeneration. APP/E2 and APP/E4 mice replicate differential effect of human apoE isoforms on Aß pathology with APP/E4 mice showing a several-fold greater load of Aß plaques, insoluble brain Aß levels, Aß oligomers, and density of neuritic plaques than APP/E2 mice. Furthermore, APP/E4 mice, but not APP/E2 mice, exhibit memory impairment on object recognition and radial arm maze tests. Between the age of 6 and 10 months APP/E2 and APP/E4 mice received treatment with Aß12-28P, a non-toxic, synthetic peptide homologous to the apoE binding motif within the Aß sequence, which competitively blocks the apoE/Aß interaction. In both lines, the treatment significantly reduced brain Aß accumulation, co-accumulation of apoE within Aß plaques, and neuritic degeneration, and prevented memory deficit in APP/E4 mice. These results indicate that both apoE2 and apoE4 isoforms contribute to Aß deposition and future therapies targeting the apoE/Aß interaction could produce favorable outcome in APOE ε2 and ε4 allele carriers.

Modulation of amyloid precursor protein expression reduces ß-amyloid deposition in a mouse model.

OBJECTIVE: Proteolytic cleavage of the amyloid precursor protein (APP) generates ß-amyloid (Aß) peptides. Prolonged accumulation of Aß in the brain underlies the pathogenesis of Alzheimer disease (AD) and is regarded as a principal target for development of disease-modifying therapeutics. METHODS: Using Chinese hamster ovary (CHO) APP751SW cells, we identified and characterized effects of 2-([pyridine-2-ylmethyl]-amino)-phenol (2-PMAP) on APP steady-state level and Aß production. Outcomes of 2-PMAP treatment on Aß accumulation and associated memory deficit were studied in APPSW /PS1dE9 AD transgenic model mice. RESULTS: In CHO APP751SW cells, 2-PMAP lowered the steady-state APP level and inhibited Aßx-40 and Aßx-42 production in a dose-response manner with a minimum effective concentration ≤ 0.5µM. The inhibitory effect of 2-PMAP on translational efficiency of APP mRNA into protein was directly confirmed using a 35S-methionine/cysteine metabolic labeling technique, whereas APP mRNA level remained unaltered. Administration of 2-PMAP to APPSW /PS1dE9 mice reduced brain levels of full-length APP and its C-terminal fragments and lowered levels of soluble Aßx-40 and Aßx-42 . Four-month chronic treatment of APPSW /PS1dE9 mice revealed no observable toxicity and improved animals' memory performance. 2-PMAP treatment also caused significant reduction in brain Aß deposition determined by both unbiased quantification of Aß plaque load and biochemical analysis of formic acid-extracted Aßx-40 and Aßx-42 levels and the level of oligomeric Aß. INTERPRETATION: We demonstrate the potential of modulating APP steady-state expression level as a safe and effective approach for reducing Aß deposition in AD transgenic model mice.

Differential molecular chaperone response associated with various mouse adapted scrapie strains.

Prionoses are a group of neurodegenerative diseases characterized by misfolding of cellular prion protein (PrP(C)) and accumulation of its diseases specific conformer PrP(Sc) in the brain and neuropathologically, they can be associated with presence or absence of PrP amyloid deposits. Functional molecular chaperones (MCs) that constitute the unfolded protein response include heat shock proteins and glucose-regulated protein families. They protect intracellular milieu against various stress conditions including accumulation of misfolded proteins and oxidative stress, typical of neurodegenerative diseases. Little is known about the role of MCs in pathogenesis of prionoses in mammalian prion model systems. In this study we characterized MCs response pattern in mice infected with various mouse adapted scrapie strains. Rather than uniform upregulation of MCs, we encountered two distinctly different patterns of MCs response distinguishing ME7 and 87V strains from 22L and 139A strains. ME7 and 87V strains are known for the induction of amyloid deposition in infected animals, while in mice infected with 22L and 139A strains amyloid deposits are absent. MCs response pattern similar to that associated with amyloidogenic ME7 and 87V strains was also observed in APPPS1-21 Alzheimer's transgenic mice, which represent an aggressive model of cerebral amyloidosis caused by ß-amyloid deposition. Our results highlight the probability that different mechanisms of MCs regulation exist driven by amyloidogenic and non-amyloidogenic nature of prion strains.