The newly proposed role for ApoE in lipid antigen presentation reported by van den Elzen et al. casts a new and interesting light on the results published by Hirsch-Reinshagen et al., Koldamova et al., and Wahrle et al.. Van den Elzen et al. show that ApoE binds directly to lipid antigens and delivers them into CD1-bearing dendritic cells by receptor-mediated endocytosis much more efficiently than macropinocytosis does. This process eventually leads to the production of interferon-Aγ and other cytokines. The results in the paper point to the presentation of foreign lipids (such as bacterial pathogens), whose role in the pathogenesis of AD is not well established [Editor’s note: see ARF Live Discussion ]. However, the presentation of endogenous lipid antigens such as sulfatide could be potentially very important in...  Read more

The newly proposed role for ApoE in lipid antigen presentation reported by van den Elzen et al. casts a new and interesting light on the results published by Hirsch-Reinshagen et al., Koldamova et al., and Wahrle et al.. Van den Elzen et al. show that ApoE binds directly to lipid antigens and delivers them into CD1-bearing dendritic cells by receptor-mediated endocytosis much more efficiently than macropinocytosis does. This process eventually leads to the production of interferon-Aγ and other cytokines. The results in the paper point to the presentation of foreign lipids (such as bacterial pathogens), whose role in the pathogenesis of AD is not well established [Editor’s note: see ARF Live Discussion ]. However, the presentation of endogenous lipid antigens such as sulfatide could be potentially very important in activating microglia and astroglia as well, especially in the execution of their Aβ-clearing capacity. The sphingolipid sulfatide is the main constituent of mammalian brain lipids. In CNS it is transported by ApoE-containing lipoprotein particles and was found to be decreased in AD patients (3). Previous data support a role for ApoE as an immunomodulatory agent affecting both the innate and adaptive immune responses. For example, ApoE modulates the CNS inflammatory response by down-regulating glial secretion of inflammatory cytokines and neurotoxic mediators such as nitric oxide, which is important in exacerbating neurodegeneration. Another study demonstrated that ApoE deficiency results in impaired clearance of apoptotic cell remnants (1). A regulatory role of ABCA1 in the engulfment of apoptotic bodies was suggested about 10 years ago, at the time its cDNA was initially cloned, and now we know that such a role is being mediated, at least in part, by ApoE (2).

Although the genetic association of ApoE and Alzheimer disease has been known for more than 10 years now, and ApoE4 is indeed the only proven independent risk factor, these recent studies, including the papers in JBC (5,12) are helping to explain why ApoE is essential and how it works to prevent or facilitate Aβ aggregation, amyloid deposition, and clearance. More importantly, the fact that ABCA1 controls ApoE lipidation status and thus its proper function in the brain opens completely new directions for drug design and therapeutic interventions in Alzheimer disease (7). In this respect, it appears that the availability of brain cholesterol and phospholipids to different carriers is critical for their function, and maintaining the appropriate distribution of brain lipids rather than inhibition of their synthesis may have a protective or even therapeutic effect in AD.

References:
1. Grainger DJ, Reckless J, McKilligin E. Apolipoprotein E modulates clearance of apoptotic bodies in vitro and in vivo, resulting in a systemic proinflammatory state in apolipoprotein E-deficient mice. J.Immunol. 2004;173:6366-75. Abstract

2. Hamon Y, Broccardo C, Chambenoit O, Luciani MF, Toti F, Chaslin S, Freyssinet JM, Devaux PF, McNeish J, Marguet D, Chimini G. ABC1 promotes engulfment of apoptotic cells and transbilayer redistribution of phosphatidylserine. Nat Cell Biol. 2000 Jul;2(7):399-406. Abstract

3. Han X, Fagan AM, Cheng H, Morris JC, Xiong C, Holtzman DM. Cerebrospinal fluid sulfatide is decreased in subjects with incipient dementia. Ann Neurol. 2003 Jul;54(1):115-9. Erratum in: Ann Neurol. 2003 Nov;54(5):693. Abstract

4. Hirsch-Reinshagen V, Zhou S, Burgess BL, Bernier L, McIsaac SA, Chan JY, Tansley GH, Cohn JS, Hayden MR, Wellington CL. Deficiency of ABCA1 impairs apolipoprotein E metabolism in brain. J Biol Chem. 2004 Sep 24;279(39):41197-207. Epub 2004 Jul 21. Abstract

5. Hirsch-Reinshagen V, Maia LF, Burgess BL, Blain JF, Naus KE, McIsaac SA, Parkinson PF, Chan JY, Tansley GH, Hayden MR, Poirier J, Van Nostrand W, Wellington CL. The absence of ABCA1 decreases soluble apoE levels but does not diminish amyloid deposition in two murine models of Alzheimer's disease. J Biol Chem. 2005 Oct 5; [Epub ahead of print] Abstract

6. Koistinaho M, Lin S, Wu X, Esterman M, Koger D, Hanson J, Higgs R, Liu F, Malkani S, Bales KR, Paul SM. Apolipoprotein E promotes astrocyte colocalization and degradation of deposited amyloid-beta peptides. Nat Med. 2004 Jul;10(7):719-26. Epub 2004 Jun 13. Abstract

7. Koldamova RP, Lefterov IM, Staufenbiel M, Wolfe D, Huang S, Glorioso JC, Walter M, Roth MG, Lazo JS. 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. Epub 2004 Nov 22. Abstract

8. LaDu MJ, Pederson TM, Frail DE, Reardon CA, Getz GS, Falduto MT. Purification of apolipoprotein E attenuates isoform-specific binding to beta-amyloid. J Biol Chem. 1995 Apr 21;270(16):9039-42. Abstract

9. Shibata M, Yamada S, Kumar SR, Calero M, Bading J, Frangione B, Holtzman DM, Miller CA, Strickland DK, Ghiso J, Zlokovic BV. Clearance of Alzheimer's amyloid-ss(1-40) peptide from brain by LDL receptor-related protein-1 at the blood-brain barrier. J Clin Invest. 2000 Dec;106(12):1489-99. Abstract

10. van den Elzen P, Garg S, Leon L, Brigl M, Leadbetter EA, Gumperz JE, Dascher CC, Cheng TY, Sacks FM, Illarionov PA, Besra GS, Kent SC, Moody DB, Brenner MB. Apolipoprotein-mediated pathways of lipid antigen presentation. Nature. 2005 Oct 6;437(7060):906-10. Abstract

11. Wahrle SE, Jiang H, Parsadanian M, Legleiter J, Han X, Fryer JD, Kowalewski T, Holtzman DM. ABCA1 is required for normal central nervous system ApoE levels and for lipidation of astrocyte-secreted apoE. J Biol Chem. 2004 Sep 24;279(39):40987-93. Epub 2004 Jul 21. Abstract

12. Wahrle SE, Jiang H, Parsadanian M, Hartman RE, Bales KR, Paul SM, Holtzman DM. Deletion of Abca1 increases Abeta deposition in the PDAPP transgenic mouse model of Alzheimer's disease. J Biol Chem. 2005 Oct 5; [Epub ahead of print] Abstract

View all comments by Radosveta Koldamova
View all comments by Iliya Lefterov

In our study, we used APP23 transgenic mice in which human familial Swedish AD mutant is expressed only in neurons, and we demonstrate that targeted disruption of ABCA1 transporter increases amyloid deposition. The effect was manifested by an increased level of Aβ as well as thioflavin S-positive plaques in brain parenchyma. Moreover, the lack of ABCA1 considerably increased the level of cerebral amyloid angiopathy (CAA) in APP23/ABCA1-/- mice. The fact that the elevation of the fraction of insoluble Aβ in old APP23/ABCA1-/- mice was accompanied by no change in soluble Aβ in young APP23/ABCA1-/- mice, and no difference in APP processing supports a conclusion that ABCA1 has a bigger impact on amyloid deposition than on amyloid production. Our data are in agreement with studies from Holtzman’s (12) and Wellington’s (5) groups. They demonstrated that ABCA1 deficiency in transgenic mice expressing human APP, harboring different FAD mutations and under the control of different promoters, increases amyloid deposition. In PDAPP mice (12) there was a considerable increase...  Read more

In our study, we used APP23 transgenic mice in which human familial Swedish AD mutant is expressed only in neurons, and we demonstrate that targeted disruption of ABCA1 transporter increases amyloid deposition. The effect was manifested by an increased level of Aβ as well as thioflavin S-positive plaques in brain parenchyma. Moreover, the lack of ABCA1 considerably increased the level of cerebral amyloid angiopathy (CAA) in APP23/ABCA1-/- mice. The fact that the elevation of the fraction of insoluble Aβ in old APP23/ABCA1-/- mice was accompanied by no change in soluble Aβ in young APP23/ABCA1-/- mice, and no difference in APP processing supports a conclusion that ABCA1 has a bigger impact on amyloid deposition than on amyloid production. Our data are in agreement with studies from Holtzman’s (12) and Wellington’s (5) groups. They demonstrated that ABCA1 deficiency in transgenic mice expressing human APP, harboring different FAD mutations and under the control of different promoters, increases amyloid deposition. In PDAPP mice (12) there was a considerable increase in insoluble Aβ level and a trend toward an increase of Aβ and thioflavin S-positive deposits. In the TgSwDI/B transgenic AD model (5), a substantial increase in thioflavin S load in hippocampus and thalamus was found, although not paralleled by changes in Aβ levels as measured by ELISA. In the same study, the Wellington group used a second APP transgenic model, APP/PS, and found no change in amyloid deposition. All three studies demonstrate a considerable increase of CAA. It is remarkable that in the three studies the elevation in parenchymal amyloid and CAA was accompanied by a dramatic decrease in soluble ApoE levels in the brain. Some of the results reported in the three papers, however, do not overlap:

1. Whereas we and Wahrle et al. (12) found a considerable increase in insoluble Aβ peptides in APP23 and PDAPP mice, Hirsch-Reinshagen et al. (5) reported no change in insoluble Aβ fraction in APP/PS1 or in TgSwDI/B transgenic mice. This discrepancy could be explained by the expression of APP transgenes producing Aβ species with different ability to aggregate or propensity for clearance. In APP/PS1 and Tg-SwDI/B mice, the expression of PS1 or Swedish, Dutch, and Iowa triple-mutant APP increases the proportion of more hydrophobic Aβ peptides (5), which are known to aggregate faster and undergo inefficient clearance compared to Aβ40 peptide.

2. While ABCA1 deficiency in APP23 and Tg-SwDI/B (5) caused an increase in amyloid plaques and a trend towards increase in PDAPP mice (12), Hirsch-Reinshagen et al. did not find a difference in amyloid deposition in APP/PS1 mice (5), which were examined at the more advanced age in terms of AD pathology. One explanation could be a role for ABCA1 in the initial period of aggregation and accumulation of amyloid.

The main conclusion from these three studies (Hirsch-Reinshagen et al., Koldamova et al., and Wahrle et al.) is that there is a negative correlation between amyloid load and the level of soluble, properly lipidated ApoE in the brain. Two contrasting roles for ApoE on amyloid deposition have been proposed: one promoting amyloid deposition and another mediating Aβ clearance. The first is supported by numerous in vivo data demonstrating that in transgenic APP mice with genetically disrupted endogenous mouse ApoE, fibrillar thioflavin S-positive Aβ deposits in brain parenchyma and vasculature are virtually missing. The second one is supported by in vitro and in vivo data demonstrating that ApoE has an important role in Aβ clearance across blood-brain barrier (BBB) and by astrocytes, a process mediated primarily via LDL receptors LRP1 and LRP2 (6,9). The three present JBC papers help to explain these seemingly contradictory effects of ApoE on amyloid aggregation and clearance by the differential roles of its lipid-rich and lipid-poor states. First, ApoE binding to various LDL receptors depends on its lipidation status: Lipid-poor ApoE is a weak ligand for LRP and LDL receptors, and this could explain the decreased Aβ clearance in ABCA1-/- mice. Insufficient and poorly lipidated ApoE in brain decreases Aβ clearance and degradation, and its retention in CNS will consequently increase amyloid deposition. Second, it was demonstrated that lipid-poor ApoE is more effective than lipid-rich ApoE in promoting Aβ aggregation (8). Previous work by Holtzman’s and Wellington’s groups demonstrated that ApoE in CSF of ABCA1-/- mice, as well as ApoE secreted in the conditioned media of ABCA1-/- astrocytes, is in a lipid-poor state (4,11). Moreover, it is obvious that, as in the periphery of ABCA1-/- mice or Tangier patients, poorly lipidated ApoA-I and ApoE proteins in the brain are unstable and are subjected to fast catabolism, explaining their decreased level.

References:
1. Grainger DJ, Reckless J, McKilligin E. Apolipoprotein E modulates clearance of apoptotic bodies in vitro and in vivo, resulting in a systemic proinflammatory state in apolipoprotein E-deficient mice. J.Immunol. 2004;173:6366-75. Abstract

2. Hamon Y, Broccardo C, Chambenoit O, Luciani MF, Toti F, Chaslin S, Freyssinet JM, Devaux PF, McNeish J, Marguet D, Chimini G. ABC1 promotes engulfment of apoptotic cells and transbilayer redistribution of phosphatidylserine. Nat Cell Biol. 2000 Jul;2(7):399-406. Abstract

3. Han X, Fagan AM, Cheng H, Morris JC, Xiong C, Holtzman DM. Cerebrospinal fluid sulfatide is decreased in subjects with incipient dementia. Ann Neurol. 2003 Jul;54(1):115-9. Erratum in: Ann Neurol. 2003 Nov;54(5):693. Abstract

4. Hirsch-Reinshagen V, Zhou S, Burgess BL, Bernier L, McIsaac SA, Chan JY, Tansley GH, Cohn JS, Hayden MR, Wellington CL. Deficiency of ABCA1 impairs apolipoprotein E metabolism in brain. J Biol Chem. 2004 Sep 24;279(39):41197-207. Epub 2004 Jul 21. Abstract

5. Hirsch-Reinshagen V, Maia LF, Burgess BL, Blain JF, Naus KE, McIsaac SA, Parkinson PF, Chan JY, Tansley GH, Hayden MR, Poirier J, Van Nostrand W, Wellington CL. The absence of ABCA1 decreases soluble apoE levels but does not diminish amyloid deposition in two murine models of Alzheimer's disease. J Biol Chem. 2005 Oct 5; [Epub ahead of print] Abstract

6. Koistinaho M, Lin S, Wu X, Esterman M, Koger D, Hanson J, Higgs R, Liu F, Malkani S, Bales KR, Paul SM. Apolipoprotein E promotes astrocyte colocalization and degradation of deposited amyloid-beta peptides. Nat Med. 2004 Jul;10(7):719-26. Epub 2004 Jun 13. Abstract

7. Koldamova RP, Lefterov IM, Staufenbiel M, Wolfe D, Huang S, Glorioso JC, Walter M, Roth MG, Lazo JS. 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. Epub 2004 Nov 22. Abstract

8. LaDu MJ, Pederson TM, Frail DE, Reardon CA, Getz GS, Falduto MT. Purification of apolipoprotein E attenuates isoform-specific binding to beta-amyloid. J Biol Chem. 1995 Apr 21;270(16):9039-42. Abstract

9. Shibata M, Yamada S, Kumar SR, Calero M, Bading J, Frangione B, Holtzman DM, Miller CA, Strickland DK, Ghiso J, Zlokovic BV. Clearance of Alzheimer's amyloid-ss(1-40) peptide from brain by LDL receptor-related protein-1 at the blood-brain barrier. J Clin Invest. 2000 Dec;106(12):1489-99. Abstract

10. van den Elzen P, Garg S, Leon L, Brigl M, Leadbetter EA, Gumperz JE, Dascher CC, Cheng TY, Sacks FM, Illarionov PA, Besra GS, Kent SC, Moody DB, Brenner MB. Apolipoprotein-mediated pathways of lipid antigen presentation. Nature. 2005 Oct 6;437(7060):906-10. Abstract

11. Wahrle SE, Jiang H, Parsadanian M, Legleiter J, Han X, Fryer JD, Kowalewski T, Holtzman DM. ABCA1 is required for normal central nervous system ApoE levels and for lipidation of astrocyte-secreted apoE. J Biol Chem. 2004 Sep 24;279(39):40987-93. Epub 2004 Jul 21. Abstract

12. Wahrle SE, Jiang H, Parsadanian M, Hartman RE, Bales KR, Paul SM, Holtzman DM. Deletion of Abca1 increases Abeta deposition in the PDAPP transgenic mouse model of Alzheimer's disease. J Biol Chem. 2005 Oct 5; [Epub ahead of print] Abstract

View all comments by Radosveta Koldamova
View all comments by Iliya Lefterov

Comment on the Wahrle et al., Koldamova et al., and Hirsh-Reinshagen et al. papers
Our laboratory and the laboratories of Iliya Lefterov and Cheryl Wellington reported on the effects of ABCA1 deletion on deposition of Aβ in four different mouse models of Alzheimer disease (AD). As shown in previous work from our lab and that of Wellington’s, deletion of ABCA1 leads to poor lipidation of ApoE and large reductions in ApoE levels in the plasma, cerebrospinal fluid, and brain parenchyma. Since mouse models of AD that have reduced or no expression of mouse ApoE develop significantly less Aβ deposition and also greatly reduced deposition of thioflavin S-positive Aβ, we expected that the decreased levels of ApoE present in ABCA1 knockout mice would lead to less Aβ-related pathology in ABCA1-/- mice bred to mouse models of AD. Contrary to this hypothesis, all three laboratories found that deletion of ABCA1 either has no effect or even increases Aβ-related pathology in four different mouse models of AD. These results indicate that the poorly lipidated...  Read more

Comment on the Wahrle et al., Koldamova et al., and Hirsh-Reinshagen et al. papers
Our laboratory and the laboratories of Iliya Lefterov and Cheryl Wellington reported on the effects of ABCA1 deletion on deposition of Aβ in four different mouse models of Alzheimer disease (AD). As shown in previous work from our lab and that of Wellington’s, deletion of ABCA1 leads to poor lipidation of ApoE and large reductions in ApoE levels in the plasma, cerebrospinal fluid, and brain parenchyma. Since mouse models of AD that have reduced or no expression of mouse ApoE develop significantly less Aβ deposition and also greatly reduced deposition of thioflavin S-positive Aβ, we expected that the decreased levels of ApoE present in ABCA1 knockout mice would lead to less Aβ-related pathology in ABCA1-/- mice bred to mouse models of AD. Contrary to this hypothesis, all three laboratories found that deletion of ABCA1 either has no effect or even increases Aβ-related pathology in four different mouse models of AD. These results indicate that the poorly lipidated ApoE produced by ABCA1-/- mice may increase Aβ fibrillogenesis The papers come to similar conclusions with different mouse models and different methods, which strengthens and supports the finding of all three papers.

The Holtzman laboratory found that PDAPP (APPV717F) mice crossed onto an ABCA1-/- background have significantly more Aβ deposited in their brains and have a higher prevalence of cerebral amyloid angiopathy (CAA). There were no differences in APP processing in young PDAPP, ABCA1-/- mice that would account for the higher level of Aβ deposition. Additionally, the PDAPP, ABCA1-/- mice accumulated insoluble ApoE in plaques at a higher rate than PDAPP, ABCA1+/+ mice, suggesting that lipid-poor ApoE is not only more amyloidogenic but also that it binds to fibrillar Aβ. Using the APP23 (APPK670N, M671L) mouse model, the Lefterov group also showed that deletion of ABCA1 resulted in increased deposition of total and fibrillar (thioflavine S-positive) Aβ, which was not a result of altered APP processing. The Lefterov laboratory found significantly higher amounts of CAA and associated microhemorrhage in APP23, ABCA1-/- mice than APP23, ABCA1+/+ mice. The Wellington laboratory bred ABCA1-/- mice to two other mouse models: Tg-SwDI/B (APPK670N, M671L, E693Q, D694N) and APP/PS1 (APPK670N, M671L, PS1 DeltaE9). They did not see significant changes in soluble or insoluble Aβ in brain. However, this lack of an effect on Aβ deposition is still interesting given that the large decreases in ApoE levels in ABCA1-/- mice were expected to lead to large decreases in Aβ deposition. The Wellington group also noted increased insoluble ApoE in the ABCA1-/-, TgSwDI/B and ABCA1-/-, APP/PS1 mice compared to their respective ABCA1+/+ controls once plaques developed, again suggesting that the lipid-poor state of ApoE in ABCA1-/- mice may increase the fibrillogenesis of Aβ. Overall, the findings these papers suggest that modifying the lipidation state of ApoE in the brain may influence AD pathogenesis and be a potential treatment target.

View all comments by David Holtzman
View all comments by Suzanne Wahrle

Three papers by Hirsch-Reinshagen et al., Koldamova et al., and Wahrle et al. (1-3) have now investigated the role of ABCA1 in Alzheimer disease neuropathology in vivo. Two very important findings were common to all three groups, demonstrating that these effects are robust and hold true across specific strains and particular animal models. Firstly, all groups corroborated prior findings of significantly reduced ApoE levels in the brains of ABCA1-deficient mice. Secondly, and contrary to all expectations, the ABCA1-mediated reduction of ApoE levels did not decrease amyloid formation, as would have been expected from previous studies showing that ApoE levels determine the extent of amyloid deposition in vivo.

All three groups reported that ABCA1 deficiency led to an 80 percent reduction in soluble ApoE levels, independent of mouse strain or AD model. Impaired ApoE secretion from both primary astrocytes and microglia has been shown to occur in ABCA1-deficient cells (4) and might partially explain this phenomenon. Additionally, increased catabolism of the poorly lipidated ApoE...  Read more

Three papers by Hirsch-Reinshagen et al., Koldamova et al., and Wahrle et al. (1-3) have now investigated the role of ABCA1 in Alzheimer disease neuropathology in vivo. Two very important findings were common to all three groups, demonstrating that these effects are robust and hold true across specific strains and particular animal models. Firstly, all groups corroborated prior findings of significantly reduced ApoE levels in the brains of ABCA1-deficient mice. Secondly, and contrary to all expectations, the ABCA1-mediated reduction of ApoE levels did not decrease amyloid formation, as would have been expected from previous studies showing that ApoE levels determine the extent of amyloid deposition in vivo.

All three groups reported that ABCA1 deficiency led to an 80 percent reduction in soluble ApoE levels, independent of mouse strain or AD model. Impaired ApoE secretion from both primary astrocytes and microglia has been shown to occur in ABCA1-deficient cells (4) and might partially explain this phenomenon. Additionally, increased catabolism of the poorly lipidated ApoE particles present in ABCA1-deficient brains is likely to occur, as has been demonstrated for peripheral ApoA1 in Tangier disease (5). Given that ABCA1 is required to maintain normal brain ApoE levels, and because ApoE plays a key role in AD pathogenesis, further studies elucidating the exact mechanisms by which ABCA1 participates in brain ApoE metabolism are warranted.

The other common result to all papers is that despite a large decrease in soluble ApoE levels, no reduction in amyloid burden was observed in any of the four AD models tested. This suggests that ApoE lipidation status, which is reduced in ABCA1-deficient animals, is a crucial regulator of amyloid formation. In addition, different Aβ species and their specific deposition pattern did not influence the amyloidogenic effect of lipid-poor ApoE. ABCA1-deficient mice on either Tg-SwD/I, PDAPP, or APP23 background had increased amyloid burdens compared to their wild-type controls, suggesting that neither their specific Aβ species nor its deposition pattern (predominantly vascular in the Tg-SwD/I and parenchymal in the PDAPP and APP23 models) modulates amyloid formation in the presence of poorly lipidated ApoE. Together, all three papers demonstrate that low levels of poorly lipidated ApoE support at least as much amyloid deposition as wild-type levels of normally lipidated ApoE. How lipidation of ApoE contributes to the process of Aβ fibrillization, deposition, and clearance remains to be fully elucidated.

Although the most important findings were indeed corroborated by all groups, some differences were present and may be primarily related to methodological variables and time of analysis. Firstly, Koldamova et al. report an increase in amyloid and Aβ burden in APP23 mice that lacked ABCA1. Wahrle et al. reported a significant increase in guanidine-extractable Aβ load, but did not detect a significant change in amyloid burden. Hirsch-Reinshagen et al. report an increase in amyloid load in the Tg-SwD/I model but no detectable change in amyloid load in APP/PS1 mice that lacked ABCA1, and no change in guanidine-extractable Aβ levels in either model. Even though all groups saw no reduction in amyloid deposition despite a large decrease in ApoE levels, the differences in amyloid and Aβ load between wild-type and ABCA1-deficient mice may have been dependent on the stage of progression of AD. For example, in the APP/PS1 model, where severe pathology was present at the moment of analysis, no differences were observed in amyloid burden or guanidine-extractable Aβ levels between wild-type and ABCA1-deficient mice. In all other three models, analyzed at relatively earlier stages of disease progression, ABCA1-deficient animals did show an increase in amyloid burden when compared to wild-type controls. Further studies might clarify whether the role of ApoE in amyloid formation is most important during the initial stages of Aβ fibrillization, or whether other differences among the models tested may account for the differences in Aβ compared to amyloid burden.

A second important difference among the three studies relates to the report of a shift in ApoE distribution from a soluble to an insoluble pool. Wahrle et al. and Hirsch-Reisnhagen et al. showed an increase in guanidine-extractable pool of ApoE in ABCA1-deficient brains compared to controls. Koldamova et al., using formic acid extractions, did not observe such a phenomenon. Again, it is unclear whether this discrepancy is only of methodological nature or if singularities of the mouse model used by Koldamova et al. are responsible for this. The mechanisms underlying the shift of ApoE into an insoluble form are probably closely related to the increased amyloidogenicity of lipid-poor ApoE and are therefore an interesting subject for further studies.

References:
1. Hirsch-Reinshagen V, Maia LF, Burgess BL, Blain JF, Naus KE, McIsaac SA, Parkinson PF, Chan JY, Tansley GH, Hayden MR, Poirier J, Van Nostrand W, Wellington CL. The absence of ABCA1 decreases soluble apoE levels but does not diminish amyloid deposition in two murine models of Alzheimer's disease. J Biol Chem. 2005 Oct 5; [Epub ahead of print] Abstract

2. Koldamova R, Staufenbiel M, Lefterov I. Lack of ABCA1 considerably decreases brain Apoe level and increases amyloid deposition in APP23 mice. J Biol Chem. 2005 Oct 5; [Epub ahead of print] Abstract

3. Wahrle SE, Jiang H, Parsadanian M, Hartman RE, Bales KR, Paul SM, Holtzman DM. Deletion of Abca1 increases Aβ deposition in the PDAPP transgenic mouse model of Alzheimer's disease. J Biol Chem. 2005 Oct 5; [Epub ahead of print] Abstract

4. Hirsch-Reinshagen V, Zhou S, Burgess BL, Bernier L, McIsaac SA, Chan JY, Tansley GH, Cohn JS, Hayden MR, Wellington CL. Deficiency of ABCA1 impairs apolipoprotein E metabolism in brain. J Biol Chem. 2004 Sep 24;279(39):41197-207. Epub 2004 Jul 21. Abstract

5. Oram JF. Tangier disease and ABCA1. Biochim Biophys Acta. 2000 Dec 15;1529(1-3):321-30. Review. Abstract

View all comments by Veronica Hirsch-Reinshagen
View all comments by Cheryl Wellington

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...  Read more

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.

View all comments by Yadong Huang

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...  Read more

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:
1. Koldamova RP, Lefterov IM, Staufenbiel M, Wolfe D, Huang S, Glorioso JC, Walter M, Roth MG, Lazo JS. 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;280:4079-88. Abstract

2. Lefterov I, Bookout A, Wang Z, Staufenbiel M, Mangelsdorf D, Koldamova R. Expression profiling in APP23 mouse brain: inhibition of Abeta amyloidosis and inflammation in response to LXR agonist treatment. Mol Neurodegener. 2007;2:20. Abstract

3. Mauch DH, Nägler K, Schumacher S, Göritz C, Müller EC, Otto A, Pfrieger FW. CNS synaptogenesis promoted by glia-derived cholesterol. Science 2001;294:1354-7. Abstract

4. Riddell DR, Zhou H, Comery TA, Kouranova E, Lo CF, Warwick HK, Ring RH, Kirksey Y, Aschmies S, Xu J, Kubek K, Hirst WD, Gonzales C, Chen Y, Murphy E, Leonard S, Vasylyev D, Oganesian A, Martone RL, Pangalos MN, Reinhart PH, Jacobsen JS. The LXR agonist TO901317 selectively lowers hippocampal Abeta42 and improves memory in the Tg2576 mouse model of Alzheimer's disease. Mol Cell Neurosci. 2007;34:621-8. Abstract

5. Zelcer N, Khanlou N, Clare R, Jiang Q, Reed-Geaghan EG, Landreth GE, Vinters HV, Tontonoz P. Attenuation of neuroinflammation and Alzheimer's disease pathology by liver x receptors. Proc Natl Acad Sci U S A. 2007;104:10601-6. Abstract

View all comments by Radosveta Koldamova

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...  Read more

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:
1. Zerbinatti CV, Wahrle SE, Kim H, Cam JA, Bales K, Paul SM, Holtzman DM, Bu G. 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. Abstract

2. Koenigsknecht J, Landreth G. Microglial phagocytosis of fibrillar beta-amyloid through a beta1 integrin-dependent mechanism. J Neurosci. 2004 Nov 3;24(44):9838-46. Abstract

3. Bamberger ME, Harris ME, McDonald DR, Husemann J, Landreth GE. A cell surface receptor complex for fibrillar beta-amyloid mediates microglial activation. J Neurosci. 2003 Apr 1;23(7):2665-74. Abstract

4. 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

5. 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

6. 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

View all comments by Mary Jo LaDu

Great findings.

View all comments by Kumar Sambamurti