Alzforum has been pleased to partner with the journal Science Translational Medicine in hosting a Webinar discussion of a new Alzheimer’s disease research study published 29 June 2011. The journal generously granted free access to Castellano et al. for Alzforum readers. For a brief summary of the paper, see below.
Paper Under Discussion Castellano JM, Kim J, Stewart FR, Jiang H, DeMattos RB, Patterson BW, Fagan AM, Morris JC, Mawuenyega KG, Cruchaga C, Goate AM, Bales KR, Paul SM, Bateman RJ, Holtzman DM. Human ApoE Isoforms Differentially Regulate Brain Amyloid-beta Peptide Clearance. Science Translational Medicine. 2011 June 29. Abstract
ApoE reigns supreme as the number one genetic risk factor for late-onset Alzheimer’s disease (see AlzGene Top Results), yet 18 years of research since its discovery have so far failed to clarify exactly how the apolipoprotein, when inherited in its E4 isoform, hastens disease. In today’s Science Translational Medicine, researchers led by David Holtzman at Washington University in St. Louis, Missouri, and collaborators there and elsewhere, report a series of in-vivo experiments showing directly that ApoE influences the clearance of Aβ peptide from the brain. The ApoE4 allele is the slowest at ridding the brain of soluble Aβ, ApoE3 falls in the middle, and the rare, protective ApoE2 gets rid of brain Aβ the fastest.
The scientists first report that among cognitively normal people in their fifties and sixties, those with the E4/E4 genotype were the most likely to have biomarkers of brain amyloidosis, i.e., low CSF Aβ42 and a positive PIB-PET scan, followed in descending order by people with E3/E4, E3/E3, and E3/E2 genotypes. Moving to mouse studies to study mechanism, the authors first recapitulated this finding in PDAPP/targeted replacement mice that express human ApoE2, 3, or 4. Old PDAPP/E4 mice not only had the most hippocampal and cortical Aβ deposition, followed by the E3 and then the E2 mice, but, as measured by microdialysis, they also had the highest steady-state concentration of soluble Aβ in their brains’ interstitial fluid. When the scientists halted Aβ production temporarily to measure how quickly the brain eliminated the peptide, they found that the half-life of Aβ in the brain’s interstitial fluid was longest in old PDAPP/E4 mice, followed by that in PDAPP/E3 and then PDAPP/E2 mice. The same pattern of ApoE isoform influencing brain Aβ in an E4/E3/E2 sequential effect also held true in young mice, long before they start laying down Aβ plaques. It was also true in a different strain, the PS1/APP/TRE mice. In contrast, APP processing and rates of Aβ synthesis as measured by stable isotope labeling kinetics did not vary by human ApoE isoform.
These results make a strong case, in the authors' view, that the ApoE genotype a person inherits determines when Aβ begins to accumulate in the brain by way of influencing its rate of clearance. Not the entire genetic ApoE effect is necessarily due to clearance, the authors note. Packaging of the apolipoprotein with lipids, as well as the protein’s concentration in brain, might play a role, too. But for now, they argue, this new set of data, in which mouse data closely parallel human data, provides strong in-vivo evidence that ApoE exerts its role in AD primarily via Aβ clearance. In this way, the results reinforce the rationale for therapeutic efforts aimed at removing Aβ from the brain, ideally during the preclinical phase of AD.
In this interesting and important paper, Castellano and colleagues provide clear, quantitative evidence for the effect of the different human ApoE isoforms on Aβ clearance in the brains of humanized ApoE knock-in mice carrying also the PDAPP transgene. Using their signature microdialysis assay, the authors show convincingly that the half-life of Aβ in the interstitial fluid of ApoE4 mice is significantly greater than in the E3 or E2 knock-ins. Half-life of Aβ in the different strains also correlates well with Aβ deposition in aged mice. By contrast, Aβ production was identical and independent of ApoE isoforms, demonstrating that ApoE isoforms primarily affect turnover and deposition and not Aβ generation, at least not in the PDAPP mouse model in which APP is unphysiologically overexpressed.
It remains possible that this overexpression occludes a component of physiological Aβ production that might be regulated by synaptic activity (Kamenetz et al., 2003; Bero et al., 2011) and ApoE isoforms (Chen et al., 2010), and thus could be relevant for human disease. Nevertheless, the quantitative data on Aβ turnover this study provides represents a milestone in our understanding of the molecular basis by which ApoE4 so powerfully affects the age of onset of Alzheimer’s disease.
Bero AW, Yan P, Roh JH, Cirrito JR, Stewart FR, Raichle ME, Lee JM, Holtzman DM. Neuronal activity regulates the regional vulnerability to amyloid-β deposition. Nat Neurosci. 2011 Jun;14(6):750-6. Abstract
Chen Y, Durakoglugil MS, Xian X, Herz J. ApoE4 reduces glutamate receptor function and synaptic plasticity by selectively impairing ApoE receptor recycling. Proc Natl Acad Sci U S A. 2010 Jun 29;107(26):12011-6. Abstract
Kamenetz F, Tomita T, Hsieh H, Seabrook G, Borchelt D, Iwatsubo T, Sisodia S, Malinow R. APP processing and synaptic function. Neuron. 2003 Mar 27;37(6):925-37. Abstract
This study further supports our finding, published last year, showing ApoE genotype and plasma protein concentrations are associated with brain amyloid burden. Previously using PIB-PET, we have demonstrated that ApoE4 carriers show widespread increase in Aβ deposition in cognitively normal individuals. Furthermore, we were able to show a significant relationship between plasma ApoE protein concentration and Aβ deposition in the brain. This indicates that there may be a peripheral signature.
Since then, Vuletic et al. (2008) showed a strong association between ApoE protein concentration in CSF and measures of both APP and tau metabolism in cognitively normal individuals. Together, these studies suggest intrinsic roles for ApoE gene and plasma ApoE protein in the brain amyloid cascade in older individuals at risk for AD.
Thambisetty et al. Proteome-based plasma markers of brain amyloid-β deposition in non-demented older individuals. J Alzheimers Dis. 2010;22(4):1099-109. Abstract
Vuletic et al. Apolipoprotein E highly correlates with AbetaPP- and tau-related markers in human cerebrospinal fluid. J Alzheimers Dis. 2008 Nov;15(3):409-17. Abstract
This paper provides really compelling data that one role of ApoE in AD pathogenesis is by promoting Aβ clearance. One strength of the work is that the authors use in-vivo systems to assess what is happening to the Aβ produced in the brain. The conclusion that ApoE4 mice are less competent at clearing Aβ from the CSF provides a clear model for how ApoE genotype affects risk of AD.
Like all good studies, these data also raise some interesting questions. Is the clearance of Aβ from the CSF to the blood, or into cells? Are there specific receptors that mediate this process? What other molecules besides Aβ are impaired in clearance from ApoE4 brains?
This study greatly helps our understanding of the ApoE genetic risk, and will help define the direction of future studies on other risk factors for AD.
The recent publication by Castellano et al. provides direct in-vivo evidence for the long-held hypothesis that ApoE mediates Aβ clearance in an isoform-specific manner.
Interestingly, the authors demonstrate that ApoE has no effect on Aβ production in PDAPP/TRE mice. However, Aβ transgenic mouse models overproduce Aβ, which could mask any ApoE-specific differences in Aβ production. Future studies could address whether ApoE isoforms affect Aβ production in non-demented and in late-onset AD patients, without FAD mutations.
A further interesting observation in the study is that the Aβ clearance rates are remarkably similar for young and old animals. These data are especially evident in PDAPP/ApoE4 mice, despite extensive Aβ deposition in older mice. This raises the questions: Would it be expected for Aβ clearance rates to change with age, as Aβ accumulates, and is ApoE-mediated Aβ clearance a major pathway for AD development?
There are some technical considerations that could be further clarified. Aβ clearance rates are calculated after treatment with a γ-secretase inhibitor. However, the pharmacokinetics (PKs) of the compound will greatly impact on the calculated Aβ- t1/2, and it would be optimal to know if there are any ApoE-isoform differences in plasma, ISF, or CSF drug PKs.
This important paper by the Holtzman group establishes the relationship between ApoE4 genotype and decreased clearance of the Alzheimer’s disease (AD) toxin amyloid-β peptide (Aβ) from brain, both in humans and animal models. The idea that ApoE4 diminishes elimination of Aβ from the brain is not necessarily novel, per se, and has been previously suggested by some experimental studies from the Holtzman and other groups. However, it has never been proven so convincingly and in a such complete way in humans and animal models of AD as Castellano et al. have done in their present study. The authors should be congratulated for their approach.
The translational impact of this paper to the field carries, in my opinion, true, well-needed quality to help us reveal the mystery of how ApoE4 accelerates the development of AD. Findings showing that individuals with ApoE4/ApoE4 genotype have substantially reduced levels of Aβ in the CSF compared to other ApoE genotypes are important and fascinating. Genetic modeling in mice expressing different human ApoE genotypes and human Aβ precursor protein (APP) revealed substantially elevated levels of human Aβ in the mouse brain interstitial fluid only in the presence of ApoE4 compared to ApoE2 and ApoE3, convincingly demonstrating how the ApoE4 allele accelerates the development of Aβ/amyloid pathology by diminishing Aβ clearance but not affecting its production in brain.
However, the question still persists in the field as to whether ApoE4 affects, in any other major way, the nervous system and cerebrovascular function independently and/or in parallel with the demonstrated Aβ/amyloid accumulations. More research is needed to address this question.
The important early observation that PDAPP mice hemizygous for murine ApoE (PDAPP x mouse ApoE+/-) had less Aβ and amyloid burden than those with two copies (PDAPP x mouse ApoE+/+) (Bales et al., 1997; Bales et al., 1999) is unfortunately seldom mentioned in more recent publications. In our own studies, we also noticed that PDAPP x ApoE+/- mice tended to be intermediate to PDAPP x ApoE+/+ and PDAPP x ApoE-/- in terms of Aβ and amyloid deposition (Nilsson et al., 2004). Moreover, lower Aβ deposition was seen in an independent transgenic model (Tg2576; APP-Swe) when hemizygous for murine ApoE (Holtzman et al., 2000).
If ApoE mainly serves to clear Aβ, as suggested by Castellano and coauthors, wouldn't one then expect aged PDAPP x mouse ApoE+/- to have more extensive Aβ deposition? After all, the concentration of ApoE, which is supposed to help clear Aβ, was 50 percent lower in the brains of PDAPP x mouse ApoE+/- than those of PDAPP x mouse ApoE+/+. But the experimental findings were actually the opposite.
Here, in an impressive set of experiments, Castellano et al. convincingly demonstrate that human ApoE affects steady-state levels and half-life of extracellular Aβ in an isoform-dependent manner (ApoE4 > ApoE3 > ApoE2) in young PDAPP mice.
How can the seemingly inconsistent observations with human ApoE and murine ApoE be understood? Does human ApoE only clear Aβ perhaps, while mouse ApoE only facilitates Aβ fibril formation? I doubt it.
First, it is worth reflecting upon the fact that human ApoE, which interacts with multiple receptors, is being transferred into the mouse genome environment in the experiments. The human and murine lipoprotein receptors (and also ApoE itself) do not have identical amino acid sequences and structures. Thus, human ApoE will likely not communicate perfectly in the murine genome environment, and this could trigger complex feedback mechanisms, leading to altered cholesterol metabolism and lipoprotein particle composition in human ApoE knock-in mouse brain (as compared to non-transgenic mouse brain expressing mouse ApoE). Indeed, in-vitro data (on astrocyte-based models) suggest that this warrants concern (Fagan et al., 1999).
One way to explain the inconsistent observations with human ApoE and murine ApoE would be that ApoE exerts two separate, but opposing, mechanisms, simultaneously facilitating Aβ clearance and Aβ fibril formation, but that the net lifetime effect of having ApoE4 versus ApoE3 will be increased Aβ accumulation and deposition. Two separate and opposing mechanisms would, for example, explain how human ApoE4 could initially decrease Aβ deposition as compared to ApoE-knockout in young mice (Holtzman et al., 1999), but then increase Aβ deposition in aged mice (Fagan et al., 2002). In young mice largely devoid of Aβ aggregates, the ApoE effect on extracellular Aβ clearance would have greater impact. In contrast, once Aβ aggregates are present in the extracellular space of older APP mice, the ability of ApoE to catalyze Aβ fibril formation by a direct molecular interaction would vastly overshadow its effect on Aβ clearance (Potter et al., 2001).
Multiple disparate effects of ApoE on Aβ metabolism unfortunately mean that there is no simple take-home message, such as “all presenilin and APP mutations increase Aβ42 synthesis.” Many excellent studies, such as the one by Castellano et al., in which robust effects on Aβ phenotypes are consistently seen when the ApoE gene is manipulated in transgenic models, are somewhat disregarded. Strangely, to this day, many papers have an introductory statement saying that “the mechanism of ApoE in Alzheimer’s disease is unknown” (which is hardly ever seen in papers on APP or presenilin).
Bales KR, Verina T, Dodel RC, Du Y, Altstiel L, Bender M, Hyslop P, Johnstone EM, Little SP, Cummins DJ, Piccardo P, Ghetti B, Paul SM. Lack of apolipoprotein E dramatically reduces amyloid beta-peptide deposition. Nat Genet. 1997 Nov;17(3):263-4. Abstract
Bales KR, Verina T, Cummins DJ, Du Y, Dodel RC, Saura J, Fishman CE, DeLong CA, Piccardo P, Petegnief V, Ghetti B, Paul SM. Apolipoprotein E is essential for amyloid deposition in the APP(V717F) transgenic mouse model of Alzheimer's disease. Proc Natl Acad Sci U S A. 1999 Dec 21;96(26):15233-8. Abstract
Nilsson LN, Arendash GW, Leighty RE, Costa DA, Low MA, Garcia MF, Cracciolo JR, Rojiani A, Wu X, Bales KR, Paul SM, Potter H. Cognitive impairment in PDAPP mice depends on ApoE and ACT-catalyzed amyloid formation. Neurobiol Aging. 2004 Oct;25(9):1153-67. Abstract
Holtzman DM, Fagan AM, Mackey B, Tenkova T, Sartorius L, Paul SM, Bales K, Ashe KH, Irizarry MC, Hyman BT. Apolipoprotein E facilitates neuritic and cerebrovascular plaque formation in an Alzheimer's disease model. Ann Neurol. 2000 Jun;47(6):739-47. Abstract
Fagan AM, Holtzman DM, Munson G, Mathur T, Schneider D, Chang LK, Getz GS, Reardon CA, Lukens J, Shah JA, LaDu MJ. Unique lipoproteins secreted by primary astrocytes from wild type, apoE (-/-), and human apoE transgenic mice. J Biol Chem. 1999 Oct 15;274(42):30001-7. Abstract
Holtzman DM, Bales KR, Wu S, Bhat P, Parsadanian M, Fagan AM, Chang LK, Sun Y, Paul SM. Expression of human apolipoprotein E reduces amyloid-beta deposition in a mouse model of Alzheimer's disease. J Clin Invest. 1999 Mar;103(6):R15-R21. Abstract
Fagan AM, Watson M, Parsadanian M, Bales KR, Paul SM, Holtzman DM. Human and murine ApoE markedly alters A beta metabolism before and after plaque formation in a mouse model of Alzheimer's disease. Neurobiol Dis. 2002 Apr;9(3):305-18. Abstract
Potter H, Wefes IM, Nilsson LN. The inflammation-induced pathological chaperones ACT and apo-E are necessary catalysts of Alzheimer amyloid formation. Neurobiol Aging. 2001 Nov-Dec;22(6):923-30. Abstract
I want to thank Lars Nilsson for his comments on our recent paper. I think it is important to point out that in the paper by Castellano et al., we demonstrate that human ApoE isoforms differentially influence endogenous Aβ clearance. However, we do not show whether human ApoE isoforms increase or decrease Aβ clearance relative to the absence of ApoE. I believe it is possible that human ApoE isoforms decrease Aβ clearance (E4>E3>E2) relative to no ApoE. If true, one would predict that two copies of human ApoE isoforms would result in greater amyloid deposition and fibril formation than one copy. This experiment has not yet been tested adequately in animal models. As Lars points out, two copies of mouse ApoE results is significantly greater amyloid deposition than one copy of mouse ApoE or than no copies. It will be important to see the effects of two versus one versus zero copies of human ApoE isoforms on amyloid deposition in mouse models.
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