Maia Parsadanian

P-glycoprotein deficiency at the blood-brain barrier increases amyloid-β deposition in an Alzheimer disease mouse model

Accumulation of amyloid-beta (Abeta) within extracellular spaces of the brain is a hallmark of Alzheimer disease (AD). In sporadic, late-onset AD, there is little evidence for increased Abeta production, suggesting that decreased elimination from the brain may contribute to elevated levels of Abeta and plaque formation. Efflux transport of Abeta across the blood-brain barrier (BBB) contributes to Abeta removal from the brain. P-glycoprotein (Pgp) is highly expressed on the luminal surface of brain capillary endothelial cells and contributes to the BBB. In Pgp-null mice, we show that [I]Abeta40 and [I]Abeta42 microinjected into the CNS clear at half the rate that they do in WT mice. When amyloid precursor protein-transgenic (APP-transgenic) mice were administered a Pgp inhibitor, Abeta levels within the brain interstitial fluid significantly increased within hours of treatment. Furthermore, APP-transgenic, Pgp-null mice had increased levels of brain Abeta and enhanced Abeta deposition compared with APP-transgenic, Pgp WT mice. These data establish a direct link between Pgp and Abeta metabolism in vivo and suggest that Pgp activity at the BBB could affect risk for developing AD as well as provide a novel diagnostic and therapeutic target.

ApoE and Clusterin Cooperatively Suppress Aβ Levels and Deposition: Evidence that ApoE Regulates Extracellular Aβ Metabolism In Vivo

Apolipoprotein E (apoE) and clusterin can influence structure, toxicity, and accumulation of the amyloid-beta (Abeta) peptide in brain. Both molecules may also be involved in Abeta metabolism prior to its deposition. To assess this possibility, we compared PDAPP transgenic mice that develop age-dependent Abeta accumulation in the absence of apoE or clusterin as well as in the absence of both proteins. apoE(-/-) and clusterin(-/-) mice accumulated similar Abeta levels but much less fibrillar Abeta. In contrast, apoE(-/-)/clusterin(-/-) mice had both earlier onset and markedly increased Abeta and amyloid deposition. Both apoE(-/-) and apoE(-/-)/clusterin(-/-) mice had elevated CSF and brain interstitial fluid Abeta, as well as significant differences in the elimination half-life of interstitial fluid Abeta measured by in vivo microdialysis. These findings demonstrate additive effects of apoE and clusterin on influencing Abeta deposition and that apoE plays an important role in regulating extracellular CNS Abeta metabolism independent of Abeta synthesis.

Expression of human apolipoprotein E reduces amyloid-β deposition in a mouse model of Alzheimer's disease

The epsilon4 allele of apolipoprotein E (apo E) is associated with an increased risk for developing Alzheimers disease (AD). This may be due to interactions between apo E and the amyloid-beta protein (Abeta). To assess the effects of human apo E isoforms on Abeta deposition in vivo, we bred apo E3 and apo E4 hemizygous (+/-) transgenic mice expressing apo E by astrocytes to mice homozygous (+/+) for a mutant amyloid precursor protein (APPV717F) transgene that develop age-dependent AD neuropathology. All mice were on a mouse apo E null (-/-) background. By nine months of age, APPV717F+/-, apo E-/- mice had developed Abeta deposition, and, as reported previously, the quantity of Abeta deposits was significantly less than that seen in APPV717F+/- mice expressing mouse apo E. In contrast to effects of mouse apo E, similar levels of human apo E3 and apo E4 markedly suppressed early Abeta deposition at nine months of age in APPV717F+/- transgenic mice, even when compared with mice lacking apo E. These findings suggest that human apo E isoforms decrease Abeta aggregation or increase Abeta clearance relative to an environment in which mouse apo E or no apo E is present. The results may have important implications for understanding mechanisms underlying the link between apo E and AD.

Clusterin promotes amyloid plaque formation and is critical for neuritic toxicity in a mouse model of Alzheimer's disease

Studies have shown that clusterin (also called apolipoprotein J) can influence the structure and toxicity of amyloid-β (Aβ) in vitro. To determine whether endogenous clusterin plays a role in influencing Aβ deposition, structure, and toxicity in vivo, we bred PDAPP mice, a transgenic mouse model of Alzheimers disease, to clusterin−/− mice. By 12 months of age, PDAPP, clusterin−/− mice had similar levels of brain Aβ deposition as did PDAPP, clusterin+/+ mice. Although Aβ deposition was similar, PDAPP, clusterin−/− mice had significantly fewer fibrillar Aβ (amyloid) deposits than PDAPP mice expressing clusterin. In the absence of clusterin, neuritic dystrophy associated with the deposited amyloid was markedly reduced, resulting in a dissociation between fibrillar amyloid formation and neuritic dystrophy. These findings demonstrate that clusterin markedly influences Aβ structure and neuritic toxicity in vivo and is likely to play an important role in Alzheimers disease pathogenesis.

Overexpression of ABCA1 reduces amyloid deposition in the PDAPP mouse model of Alzheimer disease

APOE genotype is a major genetic risk factor for late-onset Alzheimer disease (AD). ABCA1, a member of the ATP-binding cassette family of active transporters, lipidates apoE in the CNS. Abca1(-/-) mice have decreased lipid associated with apoE and increased amyloid deposition in several AD mouse models. We hypothesized that mice overexpressing ABCA1 in the brain would have increased lipidation of apoE-containing lipoproteins and decreased amyloid deposition. To address these hypotheses, we created PrP-mAbca1 Tg mice that overexpress mouse Abca1 throughout the brain under the control of the mouse prion promoter. We bred the PrP-mAbca1 mice to the PDAPP AD mouse model, a transgenic line overexpressing a mutant human amyloid precursor protein. PDAPP/Abca1 Tg mice developed a phenotype remarkably similar to that seen in PDAPP/Apoe(-/-) mice: there was significantly less amyloid beta-peptide (Abeta) deposition, a redistribution of Abeta to the hilus of the dentate gyrus in the hippocampus, and an almost complete absence of thioflavine S-positive amyloid plaques. Analyses of CSF from PrP-mAbca1 Tg mice and media conditioned by PrP-mAbca1 Tg primary astrocytes demonstrated increased lipidation of apoE-containing particles. These data support the conclusions that increased ABCA1-mediated lipidation of apoE in the CNS can reduce amyloid burden and that increasing ABCA1 function may have a therapeutic effect on AD.

Deletion of Abca1 Increases Aβ Deposition in the PDAPP Transgenic Mouse Model of Alzheimer Disease

Apolipoprotein E (apoE) genotype has a major influence on the risk for Alzheimer disease (AD). Different apoE isoforms may alter AD pathogenesis via their interactions with the amyloid β-peptide (Aβ). Mice lacking the lipid transporter ABCA1 were found to have markedly decreased levels and lipidation of apoE in the central nervous system. We hypothesized that if Abca1-/- mice were bred to the PDAPP mouse model of AD, PDAPP Abca1-/ mice would have a phenotype similar to that of PDAPP Apoe+/- and PDAPP Apoe-/- mice, which develop less amyloid deposition than PDAPP Apoe+/+ mice. In contrast to this prediction, 12-month-old PDAPP Abca -/- mice had significantly higher levels of hippocampal Aβ, and cerebral amyloid angiopathy was significantly more common compared with PDAPP Abca1+/+ mice. Amyloid precursor protein (APP) C-terminal fragments were not different between Abca1 genotypes prior to plaque deposition in 3-month-old PDAPP mice, suggesting that deletion of Abca1 did not affect APP processing or Aβ production. As expected, 3-month-old PDAPP Abca1-/- mice had decreased apoE levels, but they also had a higher percentage of carbonate-insoluble apoE, suggesting that poorly lipidated apoE is less soluble in vivo. We also found that 12-month-old PDAPP Abca1-/- mice had a higher percentage of carbonate-insoluble apoE and that apoE deposits co-localize with amyloid plaques, demonstrating that poorly lipidated apoE co-deposits with insoluble Aβ. Together, these data suggest that despite substantially lower apoE levels, poorly lipidated apoE produced in the absence of ABCA1 is strongly amyloidogenic in vivo.

Human Apolipoprotein E4 Alters the Amyloid-β 40:42 Ratio and Promotes the Formation of Cerebral Amyloid Angiopathy in an Amyloid Precursor Protein Transgenic Model

Alzheimers disease (AD) is characterized by the aggregation and deposition of the normally soluble amyloid-β (Aβ) peptide in the extracellular spaces of the brain as parenchymal plaques and in the walls of cerebral vessels as cerebral amyloid angiopathy (CAA). CAA is a common cause of brain hemorrhage and is found in most patients with AD. As in AD, the ϵ4 allele of the apolipoprotein E (apoE) gene (APOE) is a risk factor for CAA. To determine the effect of human apoE on CAA in vivo, we bred human APOE3 and APOE4 “knock-in” mice to a transgenic mouse model (Tg2576) that develops amyloid plaques as well as CAA. The expression of both human apoE isoforms resulted in a delay in Aβ deposition of several months relative to murine apoE. Tg2576 mice expressing the more fibrillogenic murine apoE develop parenchymal amyloid plaques and CAA by 9 months of age. At 15 months of age, the expression of human apoE4 led to substantial CAA with very few parenchymal plaques, whereas the expression of human apoE3 resulted in almost no CAA or parenchymal plaques. Additionally, young apoE4-expressing mice had an elevated ratio of Aβ 40:42 in brain extracellular pools and a lower 40:42 ratio in CSF, suggesting that apoE4 results in altered clearance and transport of Aβ species within different brain compartments. These findings demonstrate that, once Aβ fibrillogenesis occurs, apoE4 favors the formation of CAA over parenchymal plaques and suggest that molecules or treatments that increase the ratio of Aβ 40:42 may favor the formation of CAA versus parenchymal plaques.

Pomegranate juice decreases amyloid load and improves behavior in a mouse model of Alzheimer's disease

Although there are no proven ways to delay onset or slow progression of Alzheimers disease (AD), studies suggest that diet can affect risk. Pomegranates contain very high levels of antioxidant polyphenolic substances as compared to other fruits and vegetables. Polyphenols have been shown to be neuroprotective in different model systems. We asked whether dietary supplementation with pomegranate juice (PJ) would influence behavior and AD-like pathology in a transgenic mouse model. Transgenic mice (APP(sw)/Tg2576) received either PJ or sugar water control from 6 to 12.5 months of age. PJ-treated mice learned water maze tasks more quickly and swam faster than controls. Mice treated with PJ had significantly less (approximately 50%) accumulation of soluble Abeta42 and amyloid deposition in the hippocampus as compared to control mice. These results suggest that further studies to validate and determine the mechanism of these effects, as well as whether substances in PJ may be useful in AD, should be considered.

Overexpression of Low-Density Lipoprotein Receptor in the Brain Markedly Inhibits Amyloid Deposition and Increases Extracellular Aβ Clearance

Apolipoprotein E (APOE) is the strongest genetic risk factor for Alzheimers disease (AD). Previous studies suggest that the effect of apoE on amyloid-beta (A beta) accumulation plays a major role in AD pathogenesis. Therefore, understanding proteins that control apoE metabolism may provide new targets for regulating A beta levels. LDLR, a member of the LDL receptor family, binds to apoE, yet its potential role in AD pathogenesis remains unclear. We hypothesized that LDLR overexpression in the brain would decrease apoE levels, enhance A beta clearance, and decrease A beta deposition. To test our hypothesis, we created several transgenic mice that overexpress LDLR in the brain and found that apoE levels in these mice decreased by 50%-90%. Furthermore, LDLR overexpression dramatically reduced A beta aggregation and enhanced A beta clearance from the brain extracellular space. Plaque-associated neuroinflammatory responses were attenuated in LDLR transgenic mice. These findings suggest that increasing LDLR levels may represent a novel AD treatment strategy.

Rapid Microglial Response Around Amyloid Pathology after Systemic Anti-Aβ Antibody Administration in PDAPP Mice

Aggregation of amyloid-β (Aβ) peptide in the brain in the form of neuritic plaques and cerebral amyloid angiopathy (CAA) is a key feature of Alzheimers disease (AD). Microglial cells surround aggregated Aβ and are believed to play a role in AD pathogenesis. A therapy for AD that has entered clinical trials is the administration of anti-Aβ antibodies. One mechanism by which certain anti-Aβ antibodies have been proposed to exert their effects is via antibody-mediated microglial activation. Whether, when, or to what extent microglial activation occurs after systemic administration of anti-Aβ antibodies has not been fully assessed. We administered an anti-Aβ antibody (m3D6) that binds aggregated Aβ to PDAPP mice, an AD mouse model that was bred to contain fluorescent microglia. Three days after systemic administration of m3D6, there was a marked increase in both the number of microglial cells and processes per cell visualized in vivo by multiphoton microscopy. These changes required the Fc domain of m3D6 and were not observed with an antibody specific to soluble Aβ. These findings demonstrate that some effects of antibodies that recognize aggregated Aβ are rapid, involve microglia, and provide insight into the mechanism of action of a specific passive immunotherapy for AD.