Faculty Bibliography

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.

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 PrP^Sc, a disease specific conformer of the prion protein. Microglia undergo activation early in the course of prion pathogenesis and exert opposing roles in PrP^Sc mediated neurodegeneration. While clearance of PrP^Sc 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 PrP^Sc 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 PrP^Sc and dying neurons by microglia in Apoe^-/- mice along with increased level of proinflammatory cytokines. Our work indicates that apoE absence renders clearance of PrP^Sc 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.

BACKGROUND:) 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:AD transgenic mice were once crossed to mice overexpressing wild-type Prdx6 allele or to Prdx6 knock out mice. Aβ pathology and associated neuritic degeneration were assessed in mice aged 10 months. Laser scanning confocal microscopy was used to characterize Aβ 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:AD transgenic mice promotes selective enticement of astrocytes to Aβ 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 Aβ load and Aβ-associated neuritic degeneration. Conversely, Prdx6 haplodeficiency attenuates astro- and microglia activation around Aβ plaques promoting Aβ deposition and neuritic degeneration. CONCLUSIONS:We identify here PRDX6 as an important factor regulating response of astrocytes toward Aβ plaques. Demonstration that phagocytic activation of periplaque microglia vary directly with astrocytic PRDX6 expression level implies previously unappreciated astrocyte-guided microglia effect in Aβ proteostasis. Our showing that upregulation of PRDX6 attenuates Aβ pathology may be of therapeutic relevance for AD.

PrP^Sc is an infectious and disease-specific conformer of the prion protein, which accumulation in the CNS underlies the pathology of prion diseases. PrP^Sc replicates by binding to the cellular conformer of the prion protein (PrP^C) expressed by host cells and rendering its secondary structure a likeness of itself. PrP^C is a plasma membrane anchored protein, which constitutively recirculates between the cell surface and the endocytic compartment. Since PrP^Sc engages PrP^C 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 PrP^Sc 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 PrP^Sc conformer and has been shown to permanently clear prion-infected cells from PrP^Sc presence. We determined that 6D11 mAb engages and sequesters PrP^C and PrP^Sc at the cell surface. PrP^C/6D11 and PrP^Sc/6D11 complexes are then endocytosed from the plasma membrane and are directed to lysosomes, therefore precluding recirculation of nascent PrP^Sc back to the cell surface. Targeting PrP^Sc by 6D11 mAb to the lysosomal compartment facilitates its proteolysis and eventually shifts the balance between PrP^Sc formation and degradation. Ongoing translation of PrP^C 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 PrP^Sc and promoting its degradation, 6D11 mAb restores cellular proteostasis of prion protein.

BACKGROUND: APOE genotype is the foremost genetic factor modulating beta-amyloid (Abeta) deposition and risk of sporadic Alzheimer's disease (AD). Here we investigated how APOE genotype influences response to anti-Abeta immunotherapy. METHODS: APPSW/PS1dE9 (APP) transgenic mice with targeted replacement of the murine Apoe gene for human APOE alleles received 10D5 anti-Abeta or TY11-15 isotype control antibodies between the ages of 12 and 15 months. RESULTS: Anti-Abeta immunization decreased both the load of fibrillar plaques and the load of Abeta immunopositive plaques in mice of all APOE backgrounds. Although the relative reduction in parenchymal Abeta plaque load was comparable across all APOE genotypes, APP/epsilon4 mice showed the greatest reduction in the absolute Abeta 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/epsilon4 mice implying association between the epsilon4 allele and impaired Abeta phagocytosis. Perivascular hemosiderin deposits reflecting ensued microhemorrhages were associated with vascular Abeta (VAbeta) and ubiquitously present in control mice of all APOE genotypes, although in APP/epsilon3 mice their incidence was the lowest. Anti-Abeta immunization significantly reduced VAbeta burden but increased the number of hemosiderin deposits across all APOE genotypes with the strongest and the weakest effect in APP/epsilon2 and APP/epsilon3 mice, respectively. CONCLUSIONS: Our studies indicate that APOE genotype differentially modulates microglia activation and Abeta plaque load reduction during anti-Abeta immunotherapy. The APOE epsilon3 allele shows strong protective effect against immunotherapy associated microhemorrhages; while, conversely, the APOE epsilon2 allele increases risk thereof.