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...  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 Aβ generation....  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 pathology. However, it has been previously...  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

The role of diet, and particularly dietary cholesterol, on AD risk and pathogenesis is of substantial interest and importance. The recent paper from the Lefterov and Koldamova lab that appears in the current issue of the Journal of Neuroscience sheds considerable new light on this topic, at least in mice. The authors treated APP23 mice for four months with a high-fat diet and found a remarkable fourfold increase in compact plaques in the hippocampus and cortex. There was a parallel increase in Aβ peptide levels. This is a striking demonstration of the effect of diet on amyloid deposition and clearance. Behavioral analyses revealed a diet-related impairment in memory and learning. A curious feature of the study was that there were no genotype-related differences in behavior in mice on the normal diets, a finding that conflicts with other reports. Overall, these findings verify and extend our previous understanding of the effects of high-fat intake in animal models of AD.

One of the major findings of the study is that the simultaneous treatment of the mice on the high-fat diets...  Read more

The role of diet, and particularly dietary cholesterol, on AD risk and pathogenesis is of substantial interest and importance. The recent paper from the Lefterov and Koldamova lab that appears in the current issue of the Journal of Neuroscience sheds considerable new light on this topic, at least in mice. The authors treated APP23 mice for four months with a high-fat diet and found a remarkable fourfold increase in compact plaques in the hippocampus and cortex. There was a parallel increase in Aβ peptide levels. This is a striking demonstration of the effect of diet on amyloid deposition and clearance. Behavioral analyses revealed a diet-related impairment in memory and learning. A curious feature of the study was that there were no genotype-related differences in behavior in mice on the normal diets, a finding that conflicts with other reports. Overall, these findings verify and extend our previous understanding of the effects of high-fat intake in animal models of AD.

One of the major findings of the study is that the simultaneous treatment of the mice on the high-fat diets with an agonist of Liver X Receptors (LXRs) led to a reversal of the memory deficits and a dramatic reduction in both plaque burden and Aβ peptide levels to levels observed in mice on a normal diet. They go on to show that LXRs act to stimulate ApoE levels and lipidation status, consistent with the established actions of these receptors. They argue that the net effect of LXR treatment is to stimulate Aβ clearance, and support this view with a very nice microdialysis study showing LXR-mediated reduction in Aβ levels in the living mouse.

The value of this study is that it nicely weaves together a number of experimental threads into a compelling story. Importantly, the authors have provided further validation of LXRs as a therapeutic target in Alzheimer disease.

View all comments by Gary Landreth

A high-fat diet alters cellular metabolic equilibrium and influences the risk of developing several metabolic diseases. The effect of a high-fat diet on the peripheral system is well studied, but to a much less extent in the CNS. However, in the last decade, several studies attempted to look at the effect of a high-fat diet on the brain, especially in the context of AD. These studies are important in understanding the role of a high-fat diet in the potential contribution to normal brain function and to neurodegeneration. Epidemiological and clinical data suggest that a correlation exists between lifestyle, including diet, and the development of AD (1-2). Further, experiments on animal models suggest that diet may have a direct effect on the pathology of the disease (3-5). A high-fat diet significantly aggravated Aβ and tau pathologies, decreased cognitive function, and increased dyslipidemia in transgenic APP mouse models (Tg2576, APPK670N, M671L/PS1M146V, and 3xTg-AD) (6-8). Dyslipidemia is one of the major contributing factors of all high-fat induced disease processes, and...  Read more

A high-fat diet alters cellular metabolic equilibrium and influences the risk of developing several metabolic diseases. The effect of a high-fat diet on the peripheral system is well studied, but to a much less extent in the CNS. However, in the last decade, several studies attempted to look at the effect of a high-fat diet on the brain, especially in the context of AD. These studies are important in understanding the role of a high-fat diet in the potential contribution to normal brain function and to neurodegeneration. Epidemiological and clinical data suggest that a correlation exists between lifestyle, including diet, and the development of AD (1-2). Further, experiments on animal models suggest that diet may have a direct effect on the pathology of the disease (3-5). A high-fat diet significantly aggravated Aβ and tau pathologies, decreased cognitive function, and increased dyslipidemia in transgenic APP mouse models (Tg2576, APPK670N, M671L/PS1M146V, and 3xTg-AD) (6-8). Dyslipidemia is one of the major contributing factors of all high-fat induced disease processes, and hence, the roles of liver X receptors (LXRs) in these processes are central. LXR activation with synthetic agonists significantly improves cognitive functions and Aβ-related pathology in APP Tg mouse models (9-10). Though there is evidence suggesting the role of a high-fat diet in the exacerbation, and LXRs in attenuation, of Aβ-related pathology in APP Tg models, a comprehensive study on the effect of the activation of LXR in the setting of high-fat diet-induced exacerbation of Aβ pathology and cognition is missing. Also, the field lacks a complete understanding of the mechanism of the effects of LXR agonists and how they modulate Aβ metabolism, CNS lipid metabolism, and cognitive function.

Fitz et al., in the current paper, address part of the above missing link and demonstrate that APP transgenic mice (APP23) fed for four months on a high-fat diet had significantly increased Aβ plaque load and decline in learning and memory abilities. Chronic treatment with an LXR agonist, T0, significantly decreased amyloid load, soluble and insoluble Aβ, and increased cognitive abilities caused by a high-fat diet. Further, the authors suggest that the observed amelioration of Aβ pathology is through ABCA1/ApoE clearance of Aβ, as suggested previously. This is the first work to show that high-fat diet-induced AD phenotypes can be attenuated by the activation of LXR pathways. The results of this paper also point out that activation of the LXR pathway could be a possible therapeutic target for AD and related diseases. But given the complexity of the pathways regulated by this nuclear receptor (lipid metabolism, inflammation, and innate immunity), it may be challenging to pin down all the details. Nevertheless, the work clearly shows that decrease of soluble Aβ is one of the effects of activation of the LXR pathway that is likely relevant to decreasing amyloid load. Future studies to work out all the detailed effects of LXR activation in the brain will be important to fully understand the neurobiology of this interesting pathway and how to therapeutically harness it in diseases like AD.

References:
1. Parrott, M.D., Greenwood, C.E., 2007. Dietary influences on cognitive function with aging: from high-fat diets to healthful eating. Ann. N. Y. Acad Sci. 1114, 389–397. Abstract

2. Luchsinger, J.A., Tang, M., Shea, S., Mayeux, R., 2002. Caloric intake and the risk of Alzheimer disease. Arch. Neurol. 59, 1258–1263. Abstract

3. Qin W., Chachich M., Lane M., Roth G., Bryant M. R., Ottinger M. A.,Mattison J., Ingram D., Gandy S. and Pasinetti G. M. (2006) Calorie restriction attenuates Alzheimer’s disease type brain amyloidosis in Squirrel monkeys (Saimiri sciureus). J. Alzheimers Dis.10, 417–422. Abstract

4. Qin W., Yang T., Ho L. et al. (2006b) Neuronal SIRT1 activation as a novel mechanism underlying the preservation of Alzheimer’s disease amyloid neuropathology by calorie restriction. J. Biol. Chem. 281, 21745–21754. Abstract

5. Wang J., Ho L., Qin W. et al. (2005) Caloric restriction attenuates beta-amyloid neuropathology in a mouse model of Alzheimer’s disease. FASEB J. 19, 659–661. Abstract

6. Refolo, L.M., Malester, B., LaFrancois, J., Bryant-Thomas, T., Wang, R., Tint, G.S., Sambamurti, K., Duff, K., Pappolla, M.A., 2000. Hypercholesterolemia accelerates the Alzheimer’s amyloid pathology in a transgenic mouse model. Neurobiol. Dis. 7, 321–331. Abstract

7. Ho, L., Qin,W., Pompl, P.N., Xiang, Z.,Wang, J., Zhao, Z., Peng, Y., Cambareri, G., Rocher, A., Mobbs, C.V., Hof, P.R., Pasinetti, G.M., 2004.Diet-induced insulin resistance promotes amyloidosis in a transgenic mouse model of Alzheimer’s disease. FASEB J. 18, 902–904. Abstract

8. Li, L., Cao, D., Garber, D.W., Kim, H., Fukuchi, K., 2003. Association of aortic atherosclerosis with cerebral beta-amyloidosis and learningdeficits in a mouse model of Alzheimer’s disease. Am. J. Pathol. 163, 2155–2164. Abstract

9. Koldamova RP, Lefterov IM, Staufenbiel M, Wolfe D, Huang S, Glorioso JC, Walter M, Roth MG, Lazo JS. 2003. The liver X receptor ligand T0901317 decreases amyloid beta production in vitro and in a mouse model of Alzheimer's disease. J Biol Chem. 11;280(6):4079-88. Abstract

10. Vanmierlo T, Rutten K, Dederen J, Bloks VW, van Vark-van der Zee LC, Kuipers F, Kiliaan A, Blokland A, Sijbrands EJ, Steinbusch H, Prickaerts J, Lütjohann D, Mulder M.2009. Liver X receptor activation restores memory in aged AD mice without reducing amyloid. Neurobiol Aging. 7, 321–331.

View all comments by David Holtzman
View all comments by Philip Verghese