Low-density lipoprotein receptor overexpression enhances the rate of brain-to-blood Aß clearance in a mouse model of ß-amyloidosis.

The apolipoprotein E (APOE)-ε4 allele is the strongest genetic risk factor for late-onset, sporadic Alzheimer's disease, likely increasing risk by altering amyloid-ß (Aß) accumulation. We recently demonstrated that the low-density lipoprotein receptor (LDLR) is a major apoE receptor in the brain that strongly regulates amyloid plaque deposition. In the current study, we sought to understand the mechanism by which LDLR regulates Aß accumulation by altering Aß clearance from brain interstitial fluid. We hypothesized that increasing LDLR levels enhances blood-brain barrier-mediated Aß clearance, thus leading to reduced Aß accumulation. Using the brain Aß efflux index method, we found that blood-brain barrier-mediated clearance of exogenously administered Aß is enhanced with LDLR overexpression. We next developed a method to directly assess the elimination of centrally derived, endogenous Aß into the plasma of mice using an anti-Aß antibody that prevents degradation of plasma Aß, allowing its rate of appearance from the brain to be measured. Using this plasma Aß accumulation technique, we found that LDLR overexpression enhances brain-to-blood Aß transport. Together, our results suggest a unique mechanism by which LDLR regulates brain-to-blood Aß clearance, which may serve as a useful therapeutic avenue in targeting Aß clearance from the brain.

Amyloid-ß (Aß) immunotherapy induced microhemorrhages are associated with activated perivascular macrophages and peripheral monocyte recruitment in Alzheimer's disease mice.

BACKGROUND: Amyloid-related imaging abnormalities (ARIA) have been identified as the most common and serious adverse events resulting from pathological changes in the cerebral vasculature during several recent anti-amyloid-ß (Aß) immunotherapy trials. However, the precise cellular and molecular mechanisms underlying how amyloid immunotherapy enhances cerebral amyloid angiopathy (CAA)-mediated alterations in vascular permeability and microhemorrhages are not currently understood. Interestingly, brain perivascular macrophages have been implicated in regulating CAA deposition and cerebrovascular function however, further investigations are required to understand how perivascular macrophages play a role in enhancing CAA-related vascular permeability and microhemorrhages associated with amyloid immunotherapy. METHODS: In this study, we examined immune responses induced by amyloid-targeting antibodies and CAA-induced microhemorrhages using histology and gene expression analyses in Alzheimer's disease (AD) mouse models and primary culture systems. RESULTS: In the present study, we demonstrate that anti-Aß (3D6) immunotherapy leads to the formation of an antibody immune complex with vascular amyloid deposits and induces the activation of CD169+ perivascular macrophages. We show that macrophages activated by antibody mediated Fc receptor signaling have increased expression of inflammatory signaling and extracellular matrix remodeling genes such as Timp1 and MMP9 in vitro and confirm these key findings in vivo. Finally, we demonstrate enhanced vascular permeability of plasma proteins and recruitment of inflammatory monocytes around vascular amyloid deposits, which are associated with hemosiderin deposits from cerebral microhemorrhages, suggesting the multidimensional roles of activated perivascular macrophages in response to Aß immunotherapy. CONCLUSIONS: In summary, our study establishes a connection between Aß antibodies engaged at CAA deposits, the activation of perivascular macrophages, and the upregulation of genes involved in vascular permeability. However, the implications of this phenomenon on the susceptibility to microhemorrhages remain to be fully elucidated. Further investigations are warranted to determine the precise role of CD169 + perivascular macrophages in enhancing CAA-mediated vascular permeability, extravasation of plasma proteins, and infiltration of immune cells associated with microhemorrhages.

Safety and biomarker effects of solanezumab in patients with Alzheimer's disease.

OBJECTIVES: To assess the safety, tolerability, pharmacokinetics, and pharmacodynamics of 12 weekly infusions of solanezumab, an anti-ß-amyloid (Aß) antibody, in patients with mild-to-moderate Alzheimer's disease. Cognitive measures were also obtained. METHODS: In this phase 2, randomized, double-blind, placebo-controlled clinical trial, 52 patients with Alzheimer's disease received placebo or antibody (100 mg every 4 weeks, 100 mg weekly, 400 mg every 4 weeks, or 400 mg weekly) for 12 weeks. Safety and biomarker evaluations continued until 1 year after randomization. Both magnetic resonance imaging and cerebrospinal fluid (CSF) examinations were conducted at baseline and after the active treatment period. The Aß concentrations were measured in plasma and CSF, and the Alzheimer's Disease Assessment Scale-cognitive portion was administered. RESULTS: Clinical laboratory values, CSF cell counts, and magnetic resonance imaging scans were unchanged by treatment, and no adverse events could be clearly related to antibody administration. Total (bound to antibody and unbound) Aß(1-40) and Aß(1-42) in plasma increased in a dose-dependent manner. Antibody treatment similarly increased total Aß(1-40) and Aß(1-42) in CSF. For patients taking 400 mg weekly, antibody treatment decreased unbound Aß(1-40) in CSF (P < .01), but increased unbound Aß(1-42) in CSF in a dose-dependent manner. The Alzheimer's Disease Assessment Scale-cognitive portion was unchanged after the 12-week antibody administration. CONCLUSIONS: Antibody administration was well tolerated with doses up to 400 mg weekly. The dose-dependent increase in unbound CSF Aß(1-42) suggests that this antibody may shift Aß equilibria sufficiently to mobilize Aß(1-42) from amyloid plaques.

Melanoma-Secreted Amyloid Beta Suppresses Neuroinflammation and Promotes Brain Metastasis.

Brain metastasis is a significant cause of morbidity and mortality in multiple cancer types and represents an unmet clinical need. The mechanisms that mediate metastatic cancer growth in the brain parenchyma are largely unknown. Melanoma, which has the highest rate of brain metastasis among common cancer types, is an ideal model to study how cancer cells adapt to the brain parenchyma. Our unbiased proteomics analysis of melanoma short-term cultures revealed that proteins implicated in neurodegenerative pathologies are differentially expressed in melanoma cells explanted from brain metastases compared with those derived from extracranial metastases. We showed that melanoma cells require amyloid beta (Aß) for growth and survival in the brain parenchyma. Melanoma-secreted Aß activates surrounding astrocytes to a prometastatic, anti-inflammatory phenotype and prevents phagocytosis of melanoma by microglia. Finally, we demonstrate that pharmacologic inhibition of Aß decreases brain metastatic burden. SIGNIFICANCE: Our results reveal a novel mechanistic connection between brain metastasis and Alzheimer's disease, two previously unrelated pathologies; establish Aß as a promising therapeutic target for brain metastasis; and demonstrate suppression of neuroinflammation as a critical feature of metastatic adaptation to the brain parenchyma. This article is highlighted in the In This Issue feature, p. 1171.

Animal models for neural diseases.

"Animal Models of Neural Disease" was the focus of General Session 5 at a 2010 scientific symposium that was sponsored jointly by the Society of Toxicologic Pathology (STP) and the International Federation of Societies of Toxicologic Pathologists (IFSTP). The objective was to consider issues that dictate the choice of animal models for neuropathology-based studies used to investigate neurological diseases and novel therapeutic agents to treat them. In some cases, no animal model exists that recapitulates the attributes of the human disease (e.g., fibromyalgia syndrome). Alternatively, numerous animal models are available for other conditions, so an essential consideration is selecting the most appropriate experimental system (e.g., Alzheimer's disease). New technologies (e.g., genetically engineered rodent models) promise the opportunity to generate suitable animal models for syndromes that currently lack any in vivo animal model, while in vitro models offer the opportunity to evaluate xenobiotic effects in specific neural cell populations. The complex nature of neurological disease requires regular reassessment of available and potential options to ensure that animal-derived data sets support translational medicine efforts to improve public health.

A gamma-secretase inhibitor decreases amyloid-beta production in the central nervous system.

OBJECTIVE: Accumulation of amyloid-beta (Abeta) by overproduction or underclearance in the central nervous system (CNS) is hypothesized to be a necessary event in the pathogenesis of Alzheimer's disease. However, previously, there has not been a method to determine drug effects on Abeta production or clearance in the human CNS. The objective of this study was to determine the effects of a gamma-secretase inhibitor on the production of Abeta in the human CNS. METHODS: We utilized a recently developed method of stable-isotope labeling combined with cerebrospinal fluid sampling to directly measure Abeta production during treatment of a gamma-secretase inhibitor, LY450139. We assessed whether this drug could decrease CNS Abeta production in healthy men (age range, 21-50 years) at single oral doses of 100, 140, or 280mg (n = 5 per group). RESULTS: LY450139 significantly decreased the production of CNS Abeta in a dose-dependent fashion, with inhibition of Abeta generation of 47, 52, and 84% over a 12-hour period with doses of 100, 140, and 280mg, respectively. There was no difference in Abeta clearance. INTERPRETATION: Stable isotope labeling of CNS proteins can be utilized to assess the effects of drugs on the production and clearance rates of proteins targeted as potential disease-modifying treatments for Alzheimer's disease and other CNS disorders. Results from this approach can assist in making decisions about drug dosing and frequency in the design of larger and longer clinical trials for diseases such as Alzheimer's disease, and may accelerate effective drug validation. Ann Neurol 2009.

Advancing combination therapy for Alzheimer's disease.

The study of Alzheimer's disease (AD) has led to an increased understanding of the multiple pathologies and pathways of the disease. As such, it has been proposed that AD and its various stages might be most effectively treated with a combination approach rather than a single therapy; however, combination approaches present many challenges that include limitations of non-clinical models, complexity of clinical trial design, and unclear regulatory requirements. The Alzheimer's Association Research Roundtable meeting on May 7-8, 2018, discussed the approaches and challenges of combination therapy for AD. Experts in the field (academia, industry, and government) provided perspectives that may help establish a path forward for the development of new combination therapies.

ApoE and clusterin cooperatively suppress Abeta levels and deposition: evidence that ApoE regulates extracellular Abeta 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.

Immunization reverses memory deficits without reducing brain Abeta burden in Alzheimer's disease model.

We have previously shown that chronic treatment with the monoclonal antibody m266, which is specific for amyloid beta-peptide (Abeta), increases plasma concentrations of Abeta and reduces Abeta burden in the PDAPP transgenic mouse model of Alzheimer's disease (AD). We now report that administration of m266 to PDAPP mice can rapidly reverse memory deficits in both an object recognition task and a holeboard learning and memory task, but without altering brain Abeta burden. We also found that an Abeta/antibody complex was present in both the plasma and the cerebrospinal fluid of m266-treated mice. Our data indicate that passive immunization with this anti-Abeta monoclonal antibody can very rapidly reverse memory impairment in certain learning and memory tasks in the PDAPP mouse model of AD, owing perhaps to enhanced peripheral clearance and (or) sequestration of a soluble brain Abeta species.

Central pharmacodynamic activity of solanezumab in mild Alzheimer's disease dementia.

INTRODUCTION: Solanezumab treatment was previously shown to significantly increase total (bound + unbound) cerebrospinal fluid (CSF) levels of amyloid ß (Aß)1-40 and Aß1-42 in patients with mild to moderate Alzheimer's disease dementia yet did not produce meaningful cognitive effects. This analysis assessed solanezumab's central nervous system target engagement by evaluating changes in CSF total and free Aß isoforms and their relationship with solanezumab exposure. METHODS: CSF Aß isoform concentrations were measured in patients with mild Alzheimer's disease dementia from a pooled EXPEDITION + EXPEDITION2 population and from EXPEDITION3. CSF solanezumab concentrations were determined from EXPEDITION3. RESULTS: Solanezumab produced statistically significant increases in CSF total Aß isoforms versus placebo, which correlated with CSF solanezumab concentration. Inconsistent effects on free Aß isoforms were observed. Solanezumab penetration into the central nervous system was low. DISCUSSION: Solanezumab administration engaged the central molecular target, and molar ratio analyses demonstrated that higher exposures may further increase CSF total Aß concentrations.

Genome-wide RNAseq study of the molecular mechanisms underlying microglia activation in response to pathological tau perturbation in the rTg4510 tau transgenic animal model.

BACKGROUND: Activation of microglia, the resident immune cells of the central nervous system, is a prominent pathological hallmark of Alzheimer's disease (AD). However, the gene expression changes underlying microglia activation in response to tau pathology remain elusive. Furthermore, it is not clear how murine gene expression changes relate to human gene expression networks. METHODS: Microglia cells were isolated from rTg4510 tau transgenic mice and gene expression was profiled using RNA sequencing. Four age groups of mice (2-, 4-, 6-, and 8-months) were analyzed to capture longitudinal gene expression changes that correspond to varying levels of pathology, from minimal tau accumulation to massive neuronal loss. Statistical and system biology approaches were used to analyze the genes and pathways that underlie microglia activation. Differentially expressed genes were compared to human brain co-expression networks. RESULTS: Statistical analysis of RNAseq data indicated that more than 4000 genes were differentially expressed in rTg4510 microglia compared to wild type microglia, with the majority of gene expression changes occurring between 2- and 4-months of age. These genes belong to four major clusters based on their temporal expression pattern. Genes involved in innate immunity were continuously up-regulated, whereas genes involved in the glutamatergic synapse were down-regulated. Up-regulated innate inflammatory pathways included NF-κB signaling, cytokine-cytokine receptor interaction, lysosome, oxidative phosphorylation, and phagosome. NF-κB and cytokine signaling were among the earliest pathways activated, likely driven by the RELA, STAT1 and STAT6 transcription factors. The expression of many AD associated genes such as APOE and TREM2 was also altered in rTg4510 microglia cells. Differentially expressed genes in rTg4510 microglia were enriched in human neurodegenerative disease associated pathways, including Alzheimer's, Parkinson's, and Huntington's diseases, and highly overlapped with the microglia and endothelial modules of human brain transcriptional co-expression networks. CONCLUSION: This study revealed temporal transcriptome alterations in microglia cells in response to pathological tau perturbation and provides insight into the molecular changes underlying microglia activation during tau mediated neurodegeneration.

Liver X receptor-mediated gene regulation and cholesterol homeostasis in brain: relevance to Alzheimer's disease therapeutics.

Liver X receptors (LXRalpha and LXRbeta) are oxysterol receptors that function as master transcription factors mediating cholesterol homeostasis in the periphery. LXRs regulate the levels of the ABCA1 and ABCG1 cholesterol transporters as well as apolipoproteins (apoE and apoC) in various cells thereby affecting cholesterol transport and metabolism. In the brain, LXRs regulate ABCA1 in both neurons and glia resulting in cholesterol efflux from these cells. In addition, the expression of apolipoprotein E (apoE), synthesized primarily by astrocytes and microglia, is also upregulated by LXR agonists. As both apoE and the ABCA1 transporter are intimately involved in amyloid-beta peptide (Abeta) transport and clearance, activation of these genes by LXR agonists in brain may have a significant impact on Abeta deposition and amyloid/neuritic plaque formation. Furthermore, LXR activation has been shown to have significant anti-inflammatory properties. Taken together, these findings suggest that brain-penetrable LXR agonists or modulators may be useful therapeutic agents for the treatment and (or) prevention of Alzheimer's disease.

Exacerbation of cerebral amyloid angiopathy-associated microhemorrhage in amyloid precursor protein transgenic mice by immunotherapy is dependent on antibody recognition of deposited forms of amyloid beta.

Passive immunization with an antibody directed against the N terminus of amyloid beta (Abeta) has recently been reported to exacerbate cerebral amyloid angiopathy (CAA)-related microhemorrhage in a transgenic animal model. Although the mechanism responsible for the deleterious interaction is unclear, a direct binding event may be required. We characterized the binding properties of several monoclonal anti-Abeta antibodies to deposited Abeta in brain parenchyma and CAA. Biochemical analyses demonstrated that the 3D6 and 10D5, two N-terminally directed antibodies, bound with high affinity to deposited forms of Abeta, whereas 266, a central domain antibody, lacked affinity for deposited Abeta. To determine whether 266 or 3D6 would exacerbate CAA-associated microhemorrhage, we treated aged PDAPP mice with either antibody for 6 weeks. We observed an increase in both the incidence and severity of CAA-associated microhemorrhage when PDAPP transgenic mice were treated with the N-terminally directed 3D6 antibody, whereas mice treated with 266 were unaffected. These results may have important implications for future immune-based therapeutic strategies for Alzheimer's disease.

TREM2-mediated early microglial response limits diffusion and toxicity of amyloid plaques.

Triggering receptor expressed on myeloid cells 2 (TREM2) is a microglial receptor that recognizes changes in the lipid microenvironment, which may occur during amyloid ß (Aß) accumulation and neuronal degeneration in Alzheimer's disease (AD). Rare TREM2 variants that affect TREM2 function lead to an increased risk of developing AD. In murine models of AD, TREM2 deficiency prevents microglial clustering around Aß deposits. However, the origin of myeloid cells surrounding amyloid and the impact of TREM2 on Aß accumulation are a matter of debate. Using parabiosis, we found that amyloid-associated myeloid cells derive from brain-resident microglia rather than from recruitment of peripheral blood monocytes. To determine the impact of TREM2 deficiency on Aß accumulation, we examined Aß plaques in the 5XFAD model of AD at the onset of Aß-related pathology. At this early time point, Aß accumulation was similar in TREM2-deficient and -sufficient 5XFAD mice. However, in the absence of TREM2, Aß plaques were not fully enclosed by microglia; they were more diffuse, less dense, and were associated with significantly greater neuritic damage. Thus, TREM2 protects from AD by enabling microglia to surround and alter Aß plaque structure, thereby limiting neuritic damage.

Apolipoprotein E markedly facilitates age-dependent cerebral amyloid angiopathy and spontaneous hemorrhage in amyloid precursor protein transgenic mice.

Cerebral amyloid angiopathy (CAA) is a common cause of brain hemorrhage in the elderly. It is found in the majority of patients with Alzheimer's disease (AD). The most common form of CAA is characterized by the deposition of the amyloid-beta (Abeta) peptide in the walls of cerebral vessels, and this deposition can lead to hemorrhage and infarction. As in AD, the epsilon4 allele of apolipoprotein E (APOE) is a risk factor for CAA. To determine the effect of apoE on CAA and associated hemorrhage in vivo, we used two amyloid precursor protein (APP) transgenic mouse models that develop age-dependent Abeta deposition: PDAPP and APPsw mice. We found that both models developed an age-dependent increase in CAA and associated microhemorrhage, with the APPsw model having an earlier and more severe phenotype; however, when APPsw and PDAPP mice were bred onto an Apoe-/- background, no CAA was detected through 24 months of age, and there was little to no evidence of microhemorrhage. Biochemical analysis of isolated cerebral vessels from both PDAPP and APPsw mice with CAA revealed that, as in human CAA, the ratio of Abeta 40:42 was elevated relative to brain parenchyma. In contrast, the ratio of Abeta 40:42 from cerebral vessels isolated from old PDAPP, Apoe-/- mice was extremely low. These findings demonstrate that murine apoE markedly promotes the formation of CAA and associated vessel damage and that the effect of apoE combined with the level of Abeta40 or the ratio of Abeta 40:42 facilitates this process.

In vivo assessment of brain interstitial fluid with microdialysis reveals plaque-associated changes in amyloid-beta metabolism and half-life.

Soluble amyloid-beta (Abeta) peptide converts to structures with high beta-sheet content in Alzheimer's disease (AD). Soluble Abeta is released by neurons into the brain interstitial fluid (ISF), in which it can convert into toxic aggregates. Because assessment of ISF Abeta levels may provide unique insights into Abeta metabolism and AD, an in vivo microdialysis technique was developed to measure it. Our Abeta microdialysis technique was validated ex vivo with human CSF and then in vivo in awake, freely moving mice. Using human amyloid precursor protein (APP) transgenic mice, we found that, before the onset of AD-like pathology, ISF Abeta in hippocampus and cortex correlated with levels of APP in those tissues. After the onset of Abeta deposition, significant changes in the ISF Abeta40/Abeta42 ratio developed without changes in Abeta1-x. These changes differed from changes seen in tissue lysates from the same animals. By rapidly inhibiting Abeta production, we found that ISF Abeta half-life was short ( approximately 2 hr) in young mice but was twofold longer in mice with Abeta deposits. This increase in half-life, without an increase in steady-state levels, suggests that inhibition of Abeta synthesis reveals a portion of the insoluble Abeta pool that is in dynamic equilibrium with ISF Abeta. This now measurable in vivo pool is a likely target for new diagnostic and therapeutic strategies.

Effect of different anti-Abeta antibodies on Abeta fibrillogenesis as assessed by atomic force microscopy.

Extensive data suggest that the conversion of the amyloid-beta (Abeta) peptide from soluble to insoluble forms is a key factor in the pathogenesis of Alzheimer's disease (AD). In recent years, atomic force microscopy (AFM) has provided useful insights into the physicochemical processes involving Abeta morphology, and it can now be used to explore factors that either inhibit or promote fibrillogenesis. We used ex situ AFM to explore the impact of anti-Abeta antibodies directed against different domains of Abeta on fibril formation. For the AFM studies, two monoclonal antibodies (m3D6 and m266.2) were incubated in solution with Abeta(1-42) with a molar ratio of 1:10 (antibody to Abeta) over several days. Fibril formation was analyzed quantitatively by determining the number of fibrils per microm(2) and by aggregate size analysis. m3D6, which is directed against an N-terminal domain of Abeta (amino acid residues 1-5) slowed down fibril formation. However, m266.2, which is directed against the central domain of Abeta (amino acid residues 13-28) appeared to completely prevent the formation of fibrils over the course of the experiment. Inhibition of fibril formation by both antibodies was also confirmed by thioflavin-T (ThT) fluorescence experiments carried out with Abeta(1-40) incubated for five days. However, unlike AFM results, ThT did not differentiate between the samples incubated with m3D6 versus m266.2. These results indicate that AFM can be not only reliably used to study the effect of different molecules on Abeta aggregation, but that it can provide additional information such as the role of epitope specificity of antibodies as potential inhibitors of fibril formation.

Abeta immunization and anti-Abeta antibodies: potential therapies for the prevention and treatment of Alzheimer's disease.

Amyloid-beta (Abeta) is a normally soluble 39-43 amino peptide. Genetic and biochemical data strongly suggest that the conversion of Abeta from soluble to insoluble forms with high beta-sheet content and its buildup in the brain is a key step in the pathogenesis of Alzheimer's disease (AD) and cerebral amyloid angiopathy (CAA). Prevention and/or reversal of this process may serve as a treatment. Methods to prevent or reverse Abeta deposition and its toxic effects would include decreasing its production, preventing its conversion to insoluble forms (e.g. inhibit beta-sheet formation) or in changing the dynamics of extracellular brain Abeta, either locally within the brain or by altering net flux of Abeta between the central nervous system (CNS) and plasma compartment. Transgenic mouse models of AD that develop age-dependent Abeta deposition, damage to the neuropil, and behavioral deficits have enabled researchers to test whether different manipulations can influence these AD-like changes. Recently, active immunization with different forms of the Abeta peptide has been shown to decrease brain Abeta deposition and improve cognitive performance in mouse models of AD. Certain peripherally administered anti-Abeta antibodies have similar effects. The mechanism(s) by which anti-Abeta antibodies result in these effects is just beginning to be elucidated. Abeta-related immune therapies in humans are an exciting new area of AD research. Understanding their detailed mechanism(s) of action and their potential usefulness awaits the results of future animal and human studies.

Apolipoprotein E dose-dependent modulation of beta-amyloid deposition in a transgenic mouse model of Alzheimer's disease.

Susceptibility to the development of Alzheimer's disease (AD) is increased for individuals harboring one or more apolipoprotein E4 (apoE4) alleles. Even though several isoform-specific effects of apoE have been identified, the relationship between biochemical function and risk factor assessment remains unknown. Our previous studies have demonstrated that there is an equilibrium between cerebral spinal fluid (CSF) and plasma beta-amyloid (Abeta) and that amyloid plaques can modify this equilibrium. Trafficking of soluble central nervous system (CNS) Abeta is a very dynamic system that almost certainly is modulated by Abeta-binding proteins. Altered trafficking of the Abeta peptide might have a dramatic consequence as to whether the peptide is metabolized or begins to deposit within the brain. To gain a better understanding of the molecular mechanisms by which apoE influences AD pathogenesis and/or Abeta trafficking, we developed PDAPP transgenic mice that express different levels of human apoE3. Analysis of the soluble CNS pools of Abeta in young mice showed an apoE3 dose-dependent decrease in Abeta levels (E3-/- > E3-/- > E3+/+). In addition to the dose-dependent effects on soluble Abeta, by 15 mo of age there were highly significant differences in the amount of deposited Abeta between the genotypes (E3-/- > E3-/- > E3+/+). These data indicate that apoE3 provides a dose-dependent protective effect against Abeta deposition. This study suggests that increasing human apoE levels in brain might be a possible therapeutic target for preventing AD.