Despite its spot on top of the pile of genetic risk factors for late onset Alzheimer disease, ApoE has remained an enigma on the pathophysiological level. Much study has been devoted to unraveling the interactions between ApoE and amyloid-β (Aβ), with results that were sometimes surprising, but always indicating a complex role for ApoE in Aβ homeostasis in the brain. Cutting through the complications, a report in Neuron last week brings a refreshingly simple take on the ApoE/Aβ relationship. Gary Landreth and colleagues at Case Western Reserve University in Cleveland, Ohio, present evidence that ApoE stimulates the degradation of soluble Aβ in the brain. This activity is isoform-dependent, with ApoE4 being the least effective at supporting Aβ degradation, a result that could explain why E4 is associated with up to a 25-times higher risk of AD than other alleles (see ARF related news story). In addition, the pro-proteolytic activity of ApoE depends on its lipidation status, a result that helps make sense of recent studies showing dramatically increased Aβ deposition in mice lacking ABCA1, the enzyme responsible for lipidating ApoE (see ARF related news story). The results also strengthen the idea that was floated several years ago (see ARF related news story) that liver X receptor agonists, which boost expression of both ApoE and ABCA1, have the potential to be developed as anti-amyloid therapies.

In a set of conceptually straightforward experiments, first author Qingguang Jiang and colleagues investigated the ability of brain microglia in culture to take up and degrade soluble Aβ42 peptide. After establishing that the intracellular protease neprilysin destroys Aβ42, they found that this destruction could be enhanced by adding ApoE to microglial cultures, or by inducing expression of ApoE and ABCA1 by treatment with a liver X receptor (LXR) agonist. The requirement for ApoE seems critical, since microglia from ApoE knockout mice showed a significant impairment in Aβ42 degradation, which was restored by adding exogenous ApoE to the cultures. None of the manipulations affected Aβ uptake, only degradation.

The degradation-enhancing activity of ApoE was dependent on the protein isoform, and on its lipidation status. In microglia from ApoE-/- mice, addition of any of the isoforms stimulated Aβ degradation, but ApoE2 was the most effective, while ApoE4 was the least. This tracks with the risk of AD in people carrying different ApoE alleles, where E2 confers the lowest risk, and E4 the highest. Adequate lipidation of ApoE mattered, too, as microglia from ABCA1 knockout mice displayed diminished Aβ degradation.

In addition to intracellular degradation pathways, microglia can effect degradation of extracellular Aβ by secreting insulin-degrading enzyme (IDE), an Aβ protease. However, astrocytes make and secrete the majority of ApoE in the brain, so the investigators wondered whether these cells, too, could secrete IDE. They found that in cultures of astrocytes, Aβ was indeed degraded extracellularly by IDE, and this activity depended on lipidated ApoE. Conditioned medium from ABCA1 knockout astrocytes was worse at degrading Aβ than conditioned medium from wild-type cells. In addition, lipidated ApoE stimulated Aβ breakdown by purified IDE.

All of the in vitro data point to a beneficial effect of ApoE on brain amyloid levels. To determine if this was the case in vivo, the authors treated one-year-old Tg2576 mice that express human amyloid precursor protein (APP) bearing the Swedish mutation with the LXR agonist GW3965. After four months of treatment, ApoE and ABCA1 protein levels had risen about twofold compared to untreated animals. While the treatment had no effect on APP levels or processing, the treated mice showed a 50 percent reduction in plaque number, a 67 percent reduction in plaque load, a 64 percent reduction in total Aβ and lower plaque-related inflammation in the hippocampus. The reduction in amyloid load was accompanied by improved memory performance in the contextual fear conditioning procedure.

“The present study documents a mechanism through which ApoE stimulates the degradation of soluble Aβ peptides within the brain,” the authors conclude. They favor a model where ApoE interacts with Aβ, and chaperones its proteolysis both inside microglia and in the extracellular space. They cannot, however, rule out other actions of ApoE, like changes in membrane lipid composition that might enhance intracellular proteolysis of Aβ. And how might the role of ApoE in Aβ clearance tie in with other functions of the protein that are potentially related to AD pathology, including its roles in neuronal survival, inflammation, and lipid trafficking in the brain? While these aspects of the ApoE/Aβ partnership remain to be clarified, the current data help explain recent findings that overexpressing ABCA1 in APP mice can prevent amyloid deposition (see ARF related news story) and provide additional support for the idea that boosting ApoE levels might be a good thing for the amyloid-beset brain.—Pat McCaffrey.

Reference:
Jiang Q, Lee CYD, Mandrekar S, Wilkinson B, Cramer P, Zelcer N, Mann K, Lamb B, Willson TM, Collins JL, Richardson JC, Smith JD, Comery TA, Riddell D, Holtzman DM, Tontonoz P, Landreth GE. ApoE promotes the proteolytic degradation of Abeta. Neuron. 2008 June 12;58:681-693. Abstract

Comments

Make a Comment

To make a comment you must login or register.

Comments on this content

  1. The study by Jiang et al. is very interesting and important for better understanding Aβ-dependent roles of ApoE in the pathogenesis of Alzheimer disease. The authors presented strong evidence supporting a previously unappreciated action of ApoE in stimulating the proteolytic degradation of Aβ both extracellularly and in microglia. Their study also demonstrated that the lipidation status of ApoE was crucial for its ability to stimulate Aβ degradation, which is consistent with a previous observation that the lack of ABCA1, which leads to the formation of poorly lipidated ApoE particles, increased Aβ levels and deposition in brains of human APPFAD-expressing mice. Furthermore, a therapeutically important observation in this study is that treatment of human APPFAD transgenic mice with an LXR agonist dramatically reduced brain Aβ load and rescued the contextual memory deficits, probably by enhancing ApoE expression and its lipidation and, thus, Aβ degradation.

    As for many other important studies, several questions remained unanswered in this study:

    1. How does ApoE and its lipidation status affect Aβ degradation in microglia? If ApoE and its lipidation status do not affect Aβ uptake by microglia, as demonstrated in the paper, the action of ApoE must occur within the protease degradation pathways in the cell. Does ApoE or its lipidation status directly affect neprilysin activity? Does ApoE or its lipidation status affect Aβ conformation, leading to increased susceptibility to neprilysin?

    2. If ApoE deficiency leads to decreased Aβ degradation in microglia, as demonstrated in the paper, one would predict Aβ accumulation in microglia in human APPFAD transgenic mice lacking murine ApoE. Does this actually occur in those mice?

    3. How does ApoE and its lipidation status affect Aβ degradation extracellularly by IDE? Again, is this related to any ApoE’s direct effect on IDE activity or to its potential effect on Aβ conformation?

    4. For the differential effects of ApoE isoforms on Aβ degradation in microglia, is this because of the structural differences among three isoforms or because of their differential lipidation abilities? Following the study by Jiang et al., experimentally answering all or some of these questions should shed light on better understanding the roles of ApoE in both physiological and pathophysiological pathways related to Aβ catabolism and the pathogenesis of Alzheimer disease.

  2. The inheritance of ApoE ε4 is so far the only discovered risk factor for late-onset AD, but the role of different ApoE isoforms is not clear yet. In a recent article published in Neuron, Qingguang Jiang et al. (working at Gary Landreth’s laboratory, Case Western Reserve University) report that ApoE plays a role in facilitating the proteolytic clearance of soluble Aβ from the brain. The capacity of ApoE to promote Aβ degradation is isoform specific and dependent upon its lipidation status.

    ApoE is lipidated by the ATP-binding cassette transporter ABCA1, which acts in all cell types to transfer both phospholipids and cholesterol to ApoA-I in the periphery, and both ApoA-I and ApoE in brain. In this way, the lipidated ApoE, as well as ApoA-I, transport cholesterol and other lipids from astrocytes, which are necessary to maintain the synaptic plasticity and remodeling (3), to neurons. Three independent studies have already reported that global deletion of Abca1 in APP transgenic mice resulted in increased levels of amyloid deposition without a significant effect on Aβ generation. In addition, when applied in vivo, LXR ligands decreased Aβ levels in APP expressing mice in correlation with an increased ApoA-I and ApoE levels in their brains (2). Similar treatment of a different AD model line had a pronounced effect on cognitive performance (4). Moreover, in a recent article, the Landreth and Tontonoz groups demonstrated that APP transgenic mice with global deletion of LXRα or LXRβ exhibit a significant increase in Aβ plaque pathology and neuroinflammation (5). The outcomes of these studies strongly suggested that ABCA1 and LXR regulate ApoE and ApoA-I lipidation, which in turn affects either Aβ aggregation or Aβ clearance.

    In this article, Qingguang Jiang et al. try to answer the question how ABCA1 acts to enhance the clearance of Aβ from the brain. In a series of elegant experiments, the authors demonstrate that Aβ degradation was substantially decreased in cultured Abca1-/- as well as ApoE-/- microglia. In contrast, LXR ligand treatment of microglia had an increasing effect on Aβ degradation. Moreover, they found that the extracellular degradation of Aβ which is mediated by insulin-degrading enzyme, is facilitated by ApoE-containing lipoprotein complexes produced by astrocytes. Therefore, the process of Aβ degradation in the extracellular space is also dependent on ApoE lipidation status. To prove the effect in vivo, the authors treated Tg2576 mice with the synthetic LXR ligand GW3965 and found decreased amyloid deposition, as well as improved learning and memory performance. Collectively, these in vivo studies confirm previously published results with another LXR ligand in young (1,4) and older mice (2) and provide additional indication that LXR agonists have ameliorating effect on AD phenotype.

    While the data as provided in the paper demonstrate a mechanism for ApoE facilitated clearance of Aβ from brain, substantiate the role of LXR in neurodegeneration, and suggest that LXR agonists may represent a novel therapeutic approach for AD, some questions remain to be answered:

    1. The effects of ApoE lipidation, Abca1, and LXR in vitro have been tested mostly on the clearance of soluble Aβ. It will be interesting to see if these factors have the same effect on the clearance of fibrillar Aβ.

    2. If the lack of ApoE has similar effects on Aβ in vitro as compromised ApoE lipidation (no functional Abca1 as a primary cause), why is the AD phenotype of Abca1-/- mice so different from those of ApoE-/- mice?

    3. Is it possible that lipidated and lipid-free ApoE have different effects on Aβ aggregation that, in turn, could affect Aβ clearance, thus explaining the different phenotype in ApoE-/- and Abca1-/- (APP tg) mice?

    These are difficult questions, but the answer will increase our understanding for the role of LXR target genes, specifically ABCA1 and APOE in AD.

    References:

    . The liver X receptor ligand T0901317 decreases amyloid beta production in vitro and in a mouse model of Alzheimer's disease. J Biol Chem. 2005 Feb 11;280(6):4079-88. PubMed.

    . Expression profiling in APP23 mouse brain: inhibition of Abeta amyloidosis and inflammation in response to LXR agonist treatment. Mol Neurodegener. 2007;2:20. PubMed.

    . CNS synaptogenesis promoted by glia-derived cholesterol. Science. 2001 Nov 9;294(5545):1354-7. PubMed.

    . The LXR agonist TO901317 selectively lowers hippocampal Abeta42 and improves memory in the Tg2576 mouse model of Alzheimer's disease. Mol Cell Neurosci. 2007 Apr;34(4):621-8. PubMed.

    . Attenuation of neuroinflammation and Alzheimer's disease pathology by liver x receptors. Proc Natl Acad Sci U S A. 2007 Jun 19;104(25):10601-6. PubMed.

  3. Clearance of Aβ by neurons via an LRP-mediated pathway dependent on ApoE has been demonstrated (1). Clearance of fibrillar Aβ in THP-1 monocytes and microglia via scavenger-like receptors has also been shown (2,3). Both of these processes are mediated by cell-surface receptors. In the current paper, Jiang et al. propose an Aβ clearance mechanism independent of cell-surface receptors. They demonstrate that Aβ clearance occurs via proteolytic degradation in microglia (neprilysin) and extracellularly (IDE) by an ApoE-dependent process. Furthermore, this degradation requires lipidation of ApoE, presumably by ABCA1 as the addition of LXR agonists increases Aβ degradation. They also show the expected isoform differences in the ability of human ApoE to rescue the degradation of Aβ in primary microglia (E2>E3>E4).

    However, at least one major question remains unclear. If degradation of Aβ, both in microglia and extracellularly, depends on ApoE, one would expect ApoE-knockout (ApoE-KO) mice to have increased levels of amyloid deposition and Aβ pathology. However, it has been previously shown (4) that ApoE-KO mice and mice expressing human ApoE crossed with APP transgenic mice show reduced levels of amyloid deposition (5,6). It will be important to reconcile these findings to gain a clearer picture of the role of ApoE in Aβ-clearance mechanisms.

    A final note. Fluorescently labeled “soluble Aβ42 preparations” are used in the current studies to monitor Aβ uptake and trafficking/catabolism by neural cells in vitro. While this reagent will be an essential tool to the field, a structural/functional characterization of these labeled particles is needed to confirm their usefulness as a biochemical tool.

    References:

    . Apolipoprotein E and low density lipoprotein receptor-related protein facilitate intraneuronal Abeta42 accumulation in amyloid model mice. J Biol Chem. 2006 Nov 24;281(47):36180-6. PubMed.

    . Microglial phagocytosis of fibrillar beta-amyloid through a beta1 integrin-dependent mechanism. J Neurosci. 2004 Nov 3;24(44):9838-46. PubMed.

    . A cell surface receptor complex for fibrillar beta-amyloid mediates microglial activation. J Neurosci. 2003 Apr 1;23(7):2665-74. PubMed.

    . Lack of apolipoprotein E dramatically reduces amyloid beta-peptide deposition. Nat Genet. 1997 Nov;17(3):263-4. PubMed.

    . 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. PubMed.

    . 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. PubMed.

  4. Great findings.

References

News Citations

  1. AD Genetics—Problems and Promise
  2. ABCA1 Loss Lowers ApoE, Not Amyloid; New ApoE Immunology
  3. ApoE Catalyst Conference Explores Drug Development Opportunities
  4. Paper Alert—ABCA1 Protects Against Amyloid Deposition

Paper Citations

  1. . ApoE promotes the proteolytic degradation of Abeta. Neuron. 2008 Jun 12;58(5):681-93. PubMed.

External Citations

  1. ApoE

Further Reading

Papers

  1. . ApoE promotes the proteolytic degradation of Abeta. Neuron. 2008 Jun 12;58(5):681-93. PubMed.

Primary Papers

  1. . ApoE promotes the proteolytic degradation of Abeta. Neuron. 2008 Jun 12;58(5):681-93. PubMed.