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Apolipoprotein E4 (ApoE4), the Goliath of late-onset Alzheimer's disease genes (see AlzGene top results), is an enigma. Its protein clearly plays a role in AD and could be a therapeutic target, but scientists are not sure whether they should raise or lower it in the brain. While some studies suggest that more ApoE, regardless of isoform, would be beneficial for plaque clearance, a new paper by David Holtzman and colleagues at the Washington University School of Medicine in St. Louis, Missouri, provides strong evidence that less ApoE is better. They report that, in mice, halving the genetic dose of either human ApoE3 or ApoE4 results in fewer amyloid plaques and less microglial activation than have transgenic mice expressing two copies of the genes. The results, published December 7 in the Journal of Neuroscience, imply that ApoE-lowering treatments have a place among proposed AD therapies.

"Decreasing ApoE resulted in a massive decrease in amyloid-β pathology and inflammation," said Holtzman. "It's the first study to show how the dosage of human ApoE affects amyloid-related pathology," he added. Previous studies found a similar dose-dependent relationship in mice expressing their own ApoE gene (see Bales et al., 1997 and Holtzman et al., 2000).

First author Jungsu Kim and colleagues crossed APPswe/PS1(L166P) mice with knock-in mouse models that replace the endogenous ApoE gene with human sequences for ApoE3 or ApoE4. The researchers designed the crosses to get APP transgenic mice with either one or two copies of the human genes. Compared to mice with two copies, haploinsufficient mice, with only one human ApoE gene, had about half the amount of ApoE mRNA in the cortex. They also had less than half the insoluble Aβ40 and Aβ42, a dramatic reduction in plaques and fibrillar Aβ, and 95 percent less microglial activation. This dose-dependent effect was true for both ApoE4, the strongest risk allele for AD, and ApoE3, considered the allele with normal risk. Holtzman said they found a similar result for the ApoE2 allele, which is thought to be protective against Alzheimer's, but these data are not published yet. While mice that carried the ApoE4 allele had far greater Aβ accumulation than did those carrying the ApoE3 allele, decreasing the dose of either allele cut down on pathology.

The results of this study go against the theory that raising ApoE levels would aid in plaque clearance. Researchers previously observed that transgenic mice carrying the human ApoE4 allele have less ApoE and more Aβ deposition than mice expressing ApoE3 (see Bales et al., 2009). Humans homozygous for the ApoE4 also have less plasma ApoE than those with other alleles (see Gupta et al., 2011). Those observations led to a widespread hypothesis that dropping ApoE levels, of any allele, leads to more pathology and that boosting ApoE, regardless of isoform, might be a reasonable therapeutic strategy.

The current paper directly tests that hypothesis and finds that the opposite is true, said Yadong Huang, University of California, San Francisco. "The therapeutic implication is quite significant—it changes the direction of the ApoE as a target," he told ARF. That was Holtzman’s contention as well. "If you want to develop a treatment to affect ApoE and decrease pathology, this study suggests you'd want to decrease ApoE levels, assuming it didn't cause some other problem in the brain," he told Alzforum. His group did not notice anything unusual in the mice with just one copy, he said, but that would need further exploration. Huang noted that the lowered protein level in these mice is a lifelong change. Further tests are necessary to verify that an ApoE-dampening treatment administered in adulthood would have the same effect, he added.

Holtzman said that the current results fit with the possibility that ApoE impairs the clearance of Aβ (see ARF Webinar and ARF related news story on Castellano et al., 2011). "ApoE isoforms appear to regulate the clearance of Aβ from the brain, and they do it differentially," Holtzman told ARF. While all isoforms seem to slow clearance, he said, ApoE4 seems to be the slowest, which may be why it confers the most genetic risk. "ApoE is probably also directly affecting Aβ aggregation itself," he added, though it is not yet clear how.

For some scientists, the results were unexpected. "It's a bit surprising that decreasing ApoE provided this much of a dramatic effect," said Steven Paul, Weill Cornell Medical College, New York City. Paul was not involved in the study. "There's a lot of evidence that increasing lipidated ApoE in models is beneficial," he told Alzforum. Holtzman agrees. He said raising the amount of lipidated ApoEs is another story entirely. Compounds that hike the amount of cholesterol and phospholipids present in each ApoE-containing lipoprotein particle (raising ApoE's "lipidation state") reportedly lessen plaque load in transgenic mice (see ARF related news story on Jiang et al., 2008). These compounds both increase expression of ApoE, and simultaneously stimulate production of molecules such as ABCA1 (ATP-binding cassette A1, which loads ApoE with lipids) that alter the protein's lipidation state. "When ApoE is more lipidated in the brain, it appears to either increase Aβ clearance or decrease its aggregation," Holtzman told Alzforum. "If one increases ApoE levels but also increases its lipidation state, that is very different from just increasing ApoE."

For therapeutic purposes, altering the lipidation state of ApoE with drugs, such as nuclear liver X receptor agonists, could be just as important, and a potentially more viable therapeutic strategy than altering the level of the protein alone, said Gary Landreth, Case Western Reserve University in Cleveland, Ohio. "The biologically relevant species are the ApoE-based high-density lipoproteins (HDLs)," he said. Simultaneously increasing both ApoE expression and its lipidation could clear plaques just as well as lowering ApoE, he contended.

Holtzman's next steps will be to raise the level of ApoE in the brain (without altering the lipidation state) to see if, as the current experiment predicts, Aβ levels go up. He will also look to repeat his results in other animal models and work to figure out the mechanisms behind how ApoE lowering in turn alters Aβ deposition. Given the genetic importance of ApoE in AD, "This puzzle is unequivocally the most important riddle to solve in the field of Alzheimer's disease," said Paul.

The ApoE puzzle has implications for other disorders as well. Its E2 allele, while protective against AD, was reported to heighten the risk of sporadic Parkinson's disease, frontotemporal dementia, and to cause earlier onset of Huntington's disease. In addition, a recent paper by Conceicao Bettencourt, University of the Azores in Ponta Delgada, Portugal, and colleagues, published in the December 12 Archives of Neurology, reports that the ApoE2 allele lowers the onset age for Machado-Joseph Disease (MJD), the most common form of spinocerebellar ataxia. People inheriting a nucleotide expansion in ataxin 3, which causes the disease, and who had an ApoE2 allele got the disease, on average, five years earlier than those carrying ApoE3 or ApoE4 alleles, and had a five times higher risk of developing the disease before the age of 39. The authors pointed out that ApoE2 has a 50- to 100-fold weaker affinity for low-density lipoprotein receptors in astrocytes than ApoE3 and ApoE4. This "could contribute to the altered homeostasis of cholesterol in the brain, which may ultimately be associated with the earlier manifestation of MJD in ε2 allele carriers," they wrote.—Gwyneth Dickey Zakaib.

References:
Kim J, Jiang H, Park S, Eltorai AEM, Stewart FR, Yoon H, Basak JM, Finn MB, Holtzman D. Haploinsufficiency of Human APOE Reduces Amyloid Deposition in a Mouse Model of Amyloid-β Amyloidosis. The Journal of Neuroscience, 2011 December 10; 31(49):18007–12. Abstract

Bettencourt C, Raposo M, Kazachkova N, Cymbron T, Santos C, Kay T, Vasconcelos J, Maciel P, Donis KC, Saraiva-Pereira ML, Jardim LB, Sequeiros J, Lima M. The APOE 2 Allele Increases the Risk of Earlier Age at Onset in Machado-Joseph Disease. Arch Neurol. 2011 December;68(12):1580-3. Abstract

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  1. Kim et al. have given us an important insight into the relationship between plasma ApoE levels and Alzheimer’s disease pathogenesis that, as the authors point out, will drive therapeutic strategies in a 180-degree different direction than currently envisaged. Rather than raising the level of a presumptively beneficial ApoE isoform, any ApoE, whether E3 or E4, should be decreased. Missing still from this discussion is the E2 isoform, as well as behavioral studies of the mice. Nonetheless, the dramatic effects on amyloid seem to raise some exciting therapeutic possibilities.

    Therapeutic strategies that have focused on disrupting the amyloid cascade in symptomatic patients to date have had less than promising outcomes. Regarding neuropathological stages, one could argue that the “amyloid stage” has nearly peaked by the time a patient presents with MCI, so that, as Bray Hyman has pointed out, intervention following this stage is attacking a target that may have been relevant for disease initiation but may no longer be as relevant for disease progression (see Hyman, 2011). Targeting patients early in the amyloid cascade process implies identifying patients earlier than the expression of MCI, i.e., preclinically. During this clinically silent period, there is neuropathological, neuroimaging, and even neuropsychological evidence of disease progression. Therefore, in genetically susceptible individuals with biomarker evidence of preclinical (amyloid stage) AD, interventions to lower plasma ApoE levels might provide the right treatment, for the right target, at the right time.

    View all comments by Richard Caselli
  2. The new reports by Kim et al. are very intriguing, as they provide evidence that contradicts over a decade of research indicating that ApoE is required for Aβ deposition. Given the novelty of these results, a primary concern is that the authors establish their controversial findings using a transgenic mouse model (APPSwe/PS1-L166P) with relatively little provenance in the field (it was created in 2006), without first determining or discussing the effects of this PS1 mutation on endogenous murine ApoE or the effects of the complete loss of ApoE. Regardless, the authors show diminished plaque deposition as ApoE levels decrease. The authors interpret this data as suggesting that lowering the levels of ApoE may attenuate AD progression via decreasing the level of plaques. This interpretation is interesting, given that total plaque load has recently been shown to correlate poorly with AD-associated cognitive and behavioral deficits. In addition, recent work from our lab indicates that ApoE2 may also increase plaque deposition, consistent with clinical findings by Kawas' group that ApoE2 increases diffuse plaques in the oldest old. Collectively, our work suggests a role for plaque morphology rather than overt deposition. Therefore, without direct comparisons to ApoE2, it may be difficult for the authors of this study to speculate fully about the true effects of ApoE modulation in AD progression.

    Rather, soluble dimers and/or oligomers are the more neurotoxic Aβ species, and ApoE may have distinct functional interactions with each of these pools. Therefore, it would have been interesting if the authors had addressed not only the levels, but the solubility of ApoE and Aβ. The selective lower levels of ApoE4 may be explained by an increase in GuHCl-soluble ApoE4 due to its association with amyloid deposits, which may have led to confounding interpretations in this study.

    Finally, the authors cite this data as supporting evidence that removal of ApoE protein may be a viable therapeutic approach for AD. However, mRNA levels are not different between ApoE3 and ApoE4, consistent with significant post-translational modifications that may increase or decrease ApoE stability in vivo. Modification of ApoE at the gene level is one approach, but should be undertaken with extreme caution. It would likely result in ubiquitous, undesirable, unexpected effects, due to the innumerable roles ApoE plays in different biological systems. Thus, while interesting, these data require a large amount of follow-up research, and we should interpret these findings with discretion.

    View all comments by Katherine Youmans
  3. This is an interesting paper, showing that the concentration of human ApoE, irrespective of the isoforms (ApoE3 or ApoE4), plays a role in determining the extent of brain Aβ accumulation in a mouse model of AD. The authors have tested the effect of human ApoE gene dose on amyloid pathology in APPPS1-21 mice expressing two copies of ApoE (ApoE3/3 or ApoE4/4) or one copy of ApoE (ApoE3/- or ApoE4/-). Compared to their respective homozygous mice, the ApoE3/- and ApoE4/- mice had decreased Aβ pathology and less microgliosis. These interesting data not only reinforce the ApoE isoform-specific effect on Aβ pathology (E4>E3>E2) and on the clearance of Aβ from brain (E2>E3>E4) as reported by this and other groups, but also extend our knowledge on the role of ApoE gene dose in AD. While in ApoE3/3 mice, brain ApoE levels were about twofold greater than in ApoE4/4 mice, brain Aβ levels (soluble and insoluble) were about twofold greater in the ApoE4/4 mice, confirming the role of ApoE4 in promoting accumulation of brain Aβ. In contrast, in the mice expressing one copy of these ApoE isoforms (ApoE3/- or ApoE4/-), brain Aβ levels were similar in both groups of mice, while Aβ plaque load and fibrillar plaque load were greater in ApoE4/- mice compared to ApoE3/- mice. There was a clear ApoE dose-dependent effect on CD45 load. These data suggest that reducing human brain ApoE levels will reduce Aβ pathology and neuroinflammation in adult brains. However, further work is needed to confirm whether altering brain ApoE levels in adult mice and in humans affects Aβ levels and AD pathology.

    View all comments by Rashid Deane
  4. Usually, I’m very skeptical and critical about studies in transgenic mice, which many researchers call AD models. This study by Dr. Holtzman and fellow researchers is, however, an exception. Not because the conclusions of this study essentially confirm observations in my own recent studies in patients with sporadic AD, but because this study is one of a handful that use transgenic animals appropriately. Here, they use mice to reinforce or reject observations made in humans rather than use human studies to test observations made of animals, as is most often the case.

    In general, there are two main views on the role of apolipoprotein E (ApoE). In simple terms, the first argues that expression of ApoE is reduced in the brain of ApoE4 carriers, and this is the reason this allele is an AD risk factor. This is based on ApoE protein measurement in postmortem brains and several CSF and plasma studies—one reported very recently—and subsequent in-vitro and transgenic studies (Gupta et al., 2011). Frankly, the numbers of publications that seem to support this view are strikingly in the majority. Some researchers profess using “ApoE3 mimicking peptide” as a treatment strategy to compensate for low ApoE protein in the brain of AD.

    The second view suggests that high expression/accumulation of ApoE protein, particularly in ApoE4 AD carriers, confers the risk of AD associated with this allele. To my limited knowledge, a promoter study (Law et al., 2002) and two main papers (and several posters/oral presentations at different scientific meetings) by our group sum up the overall support for this second view. I would like to mention here that at least two additional studies supporting the second view are underway, both of which are based on human subjects.

    By confirming the main findings of our studies (Lane et al., 2008; Darreh-Shori et al., 2009; Darreh-Shori et al., 2009; Darreh-Shori et al., 2010; Darreh-Shori et al., 2011; Darreh-Shori et al., 2011) and the promoter study by Law, Holtzman, and colleagues address, in my opinion, the most important issue in AD, and by so doing, contribute substantially to reversing the mainstream view about ApoE4’s roles in AD. This study shows that a high level of ApoE protein is the major driving force for Aβ deposition in the AD brain.

    Given the impact which the two completely opposite views on ApoE might have on choosing new preventive and or therapeutic intervention strategies, it is imperative to know which is accurate. We cannot afford anymore disappointing negative (and occasionally harmful) clinical trials in AD.

    ApoE4 was recognized quite long ago as the major genetic risk factor for sporadic AD, which comprises over 90 percent of all AD cases. Later, the majority (if not all) genetic studies in AD, regardless of their population size, repeatedly churned out ApoE as the main genetic player in the etiology of AD. Still, judging from the number of publications and candidate drug trials, the strategy of reducing Aβ production dominates the field, although there is really no good evidence for increased Aβ production in the brains of sporadic AD patients. This is the main reason for my negative view about transgenic animal “models of AD.” They are, as Dr. Holtzman and colleagues very appropriately point out, simply a model of Aβ amyloidosis (at best they are a model of a specific familiar form of AD, e.g., APPswe for the Swedish genetic form of AD).

    The next most important research objective, in my opinion, is to deduce the native biological activity/role of Aβ peptides in the human brain. Truly, it does not make sense that nature would spend millions of years of evolution to bring about the universal expression of Aβ precursor protein (APP) in combination with such sophisticated cleavage machinery just to prevent Homo sapiens from reaching advanced age with intact cognitive ability.

    In this context, studies by the Holtzman group (Cirrito et al., 2005) and ours (Darreh-Shori et al., 2009; Darreh-Shori et al., 2009; Darreh-Shori et al., 2010; Darreh-Shori et al., 2011; Darreh-Shori et al., 2011) portray a complex interplay between cholinesterases and Aβ peptides in which high ApoE protein level seems to have a major pathological influence. Our findings suggest the presence of a molecular complex, which we have termed BAβACs (Butyrylcholineesterase/Acetylcholineesterase-Aβ-ApoE Complexes).

    Returning to the paper of Dr. Holtzman and colleagues, there is, however, a peculiar observation that does not make sense, and, as a matter of fact, directly counter-argues the main conclusion of the study. The ApoE3/3 mice show twofold higher ApoE protein levels in the brain than ApoE4/4 mice. In contrast, they show about two- to threefold lower insoluble Aβ40 and 42 in the brain than ApoE4 mice. Thus, within this same study, this contradictory observation puts the main finding in question. The impact of this contradictory observation is far too crucial to let it go, despite the explanatory remarks by the authors.

    Since ApoE4 rather than ApoE3 is the AD risk factor, this observation may reflect poorer response of mouse ApoE receptors to human ApoE4 than ApoE3 (mouse ApoE is structurally like human ApoE3), or it may indicate a flaw in the ApoE protein analyses.

    A clue actually is present in Fig.1A, which shows that both mice have essentially identical ApoE3 and 4 mRNA expressions. With the reservation that mRNA level and protein expression do not always go hand in hand, it hints at a plausible confounding source in the ApoE protein analysis.

    For instance, measuring “levels of PBS soluble ApoE” (see the legend of Fig. 1) by no means will reflect the amounts of ApoE protein in the brain, since these are by definition very lipophilic proteins. Measurement of ApoE protein in plasma may, in this case, have been a better measure of the relative differences among the genotype groups, particularly since ApoE4, due to its closed-arm tertiary conformation, may be more lipophilic than ApoE3 isoprotein. This may additionally lead to a different extraction efficiency of ApoE3 compared to ApoE4 from the brain tissue homogenates, which consist mainly of fat/lipids, as well as differential loss to the surface of tubes and pipettes, etc., during homogenization, which can be very substantial (Hesse et al., 2000).

    Another unnecessary caveat in this regard is the use of iso-specific ApoE ELISA (mentioned in Fig.1). This will provide no important information, since the genotype of the animals is already known. It may, however, have potentially introduced systemic errors in comparative ApoE level analyses. Distinct antibody pairs may produce dissimilar data due to their relative affinity to the standards (which could be peptide, recombinant, or purified ApoE protein?) versus ApoE in the samples. These are important because ApoE3 is more likely to produce dimers than ApoE4, which means that one capturing antibody may retain, simultaneously, two binding epitopes for the detecting of antibody in ApoE3 ELISA, thereby giving a higher signal per available binding sites. All of these issues may be a confounding source for the striking discrepancy between the clear impact of ApoE hemizygosity on Aβ deposition, and the contradictory result of twofold higher ApoE protein but still having about two- to threefold fewer insoluble Aβ peptides in ApoE3 than ApoE4 mice. Hopefully, the authors will clarify these confounding issues in further work.

    View all comments by Taher Darreh-Shori

References

Webinar Citations

  1. Slow Aβ Clearance Is ApoE4’s Modus Operandi in Late-Onset AD

News Citations

  1. Paper Alert: ApoE Affects Alzheimer's Risk via Aβ Clearance
  2. ApoE’s Secret Revealed? Protein Promotes Aβ Degradation

Paper Citations

  1. . Lack of apolipoprotein E dramatically reduces amyloid beta-peptide deposition. Nat Genet. 1997 Nov;17(3):263-4. PubMed.
  2. . Apolipoprotein E facilitates neuritic and cerebrovascular plaque formation in an Alzheimer's disease model. Ann Neurol. 2000 Jun;47(6):739-47. PubMed.
  3. . Human APOE isoform-dependent effects on brain beta-amyloid levels in PDAPP transgenic mice. J Neurosci. 2009 May 27;29(21):6771-9. PubMed.
  4. . Plasma apolipoprotein E and Alzheimer disease risk: the AIBL study of aging. Neurology. 2011 Mar 22;76(12):1091-8. PubMed.
  5. . Human apoE isoforms differentially regulate brain amyloid-β peptide clearance. Sci Transl Med. 2011 Jun 29;3(89):89ra57. PubMed.
  6. . ApoE promotes the proteolytic degradation of Abeta. Neuron. 2008 Jun 12;58(5):681-93. PubMed.
  7. . Haploinsufficiency of human APOE reduces amyloid deposition in a mouse model of amyloid-β amyloidosis. J Neurosci. 2011 Dec 7;31(49):18007-12. PubMed.
  8. . The APOE ε2 allele increases the risk of earlier age at onset in Machado-Joseph disease. Arch Neurol. 2011 Dec;68(12):1580-3. PubMed.

Other Citations

  1. APPswe/PS1(L166P) mice

External Citations

  1. AlzGene top results

Further Reading

Papers

  1. . Haploinsufficiency of human APOE reduces amyloid deposition in a mouse model of amyloid-β amyloidosis. J Neurosci. 2011 Dec 7;31(49):18007-12. PubMed.
  2. . The APOE ε2 allele increases the risk of earlier age at onset in Machado-Joseph disease. Arch Neurol. 2011 Dec;68(12):1580-3. PubMed.

News

  1. ApoE Disrupts Brain Networks, Helps Microglia Clear Aβ
  2. St. Louis: ApoE—A Clearer View of its Role In AD?
  3. ApoE’s Secret Revealed? Protein Promotes Aβ Degradation
  4. Australia Report: Aβ Toxicity, ApoE
  5. San Diego: The Curious Case of ApoE, Cholesterol, and Cognition
  6. Paper Alert: ApoE Affects Alzheimer's Risk via Aβ Clearance

Webinars

  1. Slow Aβ Clearance Is ApoE4’s Modus Operandi in Late-Onset AD

Primary Papers

  1. . Haploinsufficiency of human APOE reduces amyloid deposition in a mouse model of amyloid-β amyloidosis. J Neurosci. 2011 Dec 7;31(49):18007-12. PubMed.
  2. . The APOE ε2 allele increases the risk of earlier age at onset in Machado-Joseph disease. Arch Neurol. 2011 Dec;68(12):1580-3. PubMed.