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Sadowski MJ, Pankiewicz J, Scholtzova H, Mehta PD, Prelli F, Quartermain D, Wisniewski T. Blocking the apolipoprotein E/amyloid-beta interaction as a potential therapeutic approach for Alzheimer's disease. Proc Natl Acad Sci U S A. 2006 Dec 5;103(49):18787-92. PubMed.
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Washington University
Sadowski and colleagues examine the effects of a modified Aβ peptide 12-28P that has a proline in position 18, is acetylated at the N-terminus, amidated at the C-terminus, and is made of D-amino acids. In previous studies and in this paper, it is shown that this peptide can inhibit the ApoE4-stimulated fibrillogenesis of Aβ40 as well as ApoE4 binding to Aβ40. They then administered this peptide (1mg, 3x per week) over months, systemically, to two types of APP transgenic mouse models that develop plaques. The treatment resulted in improved performance in the radial arm maze, decreased plaques, decreased Aβ levels by ELISA, and decreased CAA. The effects of the peptide in vivo appear clear, and this approach, given its systemic nature of treatment, appears quite promising. This work should stimulate greater efforts in the important area of how to utilize ApoE as a therapeutic target, given its key role in AD pathogenesis.
There are some issues regarding the mechanism of the effects that are seen that can be clarified in future studies. In regard to the effects of 12-28P, whether its actions in the brain on Aβ deposition are via ApoE could be further clarified. Is the effect of 12-28P on Aβ deposition and behavior (CNS effects) via 12-28P binding ApoE in the brain, in the blood, both, or via another effect? Does the treatment reverse existing deposits or only prevent new ones? While mouse ApoE is fibrillogenic in vivo, human ApoE isoforms, including ApoE4, delay the onset of Aβ deposition relative to the absence of ApoE. It will be useful to determine the effects of this peptide in APP/E4 mice. Finally, many treatments that can decrease Aβ in the brain affect processes which not only include ApoE/Aβ interactions but also Aβ degrading enzymes, secretases, and synaptic activity (to name but a few). It will also be important to assess 12-28P effects in regard to these types of activities.
New York University School of Medicine
We have shown that blocking the interaction of Aβ and ApoE with a synthetic peptide-Aβ12-28P that mimics the ApoE binding site on Aβ constitutes a highly promising approach for reducing Aβ accumulation in the CNS and preventing memory impairment in AD transgenic (Tg) mice (Sadowski et al., 2006). Aβ12-28P was designed to be non-toxic and non-fibrillogenic, as well as having improved BBB permeability and increased serum half-life compared to residues 12-28 of Aβ (please also see Sadowski et al., 2004). In our studies, we have demonstrated that the therapeutic effect of Aβ12-28P can be only achieved in the presence of ApoE. We have shown that Aβ12-28P does not function as a β-sheet breaker; hence, its effect on Aβ deposition cannot be mediated by direct disaggregation of Aβ deposits but by neutralizing of ApoE's effect on Aβ aggregation and its sequestration within the CNS. We have also demonstrated that Aβ12-28P does not stimulate production of anti-Aβ antibodies (since it is a very weak immunogen and is given without an adjuvant). Therefore, its therapeutic action cannot be mediated through an anti-Aβ vaccination effect.
One significant advantage of this novel approach is that blocking the ApoE/Aβ interaction not only reduces deposition of Aβ in the brain parenchyma, but also reduces the burden of cerebral amyloid angiopathy, without inducing perivascular hemorrhages. This contrasts with vaccination approaches which do not reduce cerebral amyloid angiopathy and may increase the risk of intracerebral bleeding. Observations of Pattson et al. (2006) made on brains of subjects vaccinated with AN-1792 indicated that although the burden of parenchymal deposits is effectively decreased, vascular deposits remain unchanged or may even be increased. This study also demonstrated that vaccinated subjects showed an increase in the pool of soluble Aβ. Therefore, approaches focusing on ApoE as a therapeutic target offer a unique opportunity to address both parenchymal and vascular deposition of Aβ.
Although our studies have been designed to study mainly the effect of blocking the ApoE/Aβ interaction on Aβ deposition in the CNS, there are numerous indications that this approach may offer additional therapeutic benefits. For example, several lines of evidence suggest the importance of ApoE in mediating neuronal re-uptake of Aβ via LRP receptors. This leads to neuronal Aβ accumulation and degeneration, as recently demonstrated in the elegant study by Zerbinatti et al. (2006, with Drs. Bu and Holtzman as senior authors). Therefore, blocking the ApoE/Aβ binding may potentially reduce accumulation of Aβ inside neurons and the associated toxicity. Another potential mechanism by which blocking the Aβ/ApoE interaction may have a beneficial role is in the dynamics of Aβ oligomer and Aβ fibril formation. ApoE was demonstrated to interact both with Aβ oligomers (Naslund et al., 1995) and Aβ fibrils. Future experiments have been planned to investigate how blocking ApoE/Aβ binding affects formation of oligomers and their equilibrium with fibrillar Aβ. Furthermore, studies of Bell et al. (2006, with Drs. Zlokovic and Holtzman as senior authors) indicated that binding of Aβ to ApoE reduces its efflux across the BBB. Therefore, another potential benefit of ApoE/Aβ binding antagonists would be improved clearance of Aβ across the BBB.
Unlike mouse ApoE, human ApoE exists in three different isoforms, E2, E3, and E4, which have differential effects on the risk of AD occurrence, the age of onset, and the magnitude of Aβ deposition. In vitro, all three isoforms were shown to promote Aβ fibril assembly with the ApoE4 isoform having the strongest effect. Transgenic mice studies designed to study Aβ deposition in the setting of various human ApoE have demonstrated that expression of human ApoE markedly delays Aβ deposition relative to mouse ApoE, but nevertheless, amyloid deposition occurs with an isotype gradation similar to that seen in AD patients (E4>E3>E2; Fagan et al., 2002). Therefore, although our studies have shown a benefit of blocking the ApoE/Aβ interaction, they have to be expanded to include analysis of the relative benefit in various transgenic lines expressing different human ApoE isoforms. Such studies will address the effectiveness of blocking ApoE/Aβ binding in the setting of different human ApoE isoforms, which is a prerequisite for advancing this form of treatment into clinical studies. Since it has been shown that development of fibrillar Aβ deposits is dependent on ApoE3 and E4 expression, it is likely that blocking either the Aβ/ApoE3 or E4 interaction will be beneficial. However, it is possible that the magnitude of the therapeutic response to Aβ/ApoE binding antagonists will be dependent on ApoE isotype expression.
Although ApoE has been somewhat neglected in the past decade of AD research, it constitutes a valid and attractive therapeutic target. We hope that our study will spur more extensive research toward evaluating this alternative therapeutic approach for AD, which may result in the generation and testing of novel compounds applicable to humans. The use of compounds blocking the ApoE/Aβ interaction would not preclude the additional use of other emerging treatment strategies such as secretase inhibitors or passive immunization. These Aβ targeting therapies would likely have a synergistic effect on the overall therapeutic outcome.
References:
Sadowski MJ, Pankiewicz J, Scholtzova H, Mehta PD, Prelli F, Quartermain D, Wisniewski T. Blocking the apolipoprotein E/amyloid-beta interaction as a potential therapeutic approach for Alzheimer's disease. Proc Natl Acad Sci U S A. 2006 Dec 5;103(49):18787-92. PubMed.
Sadowski M, Pankiewicz J, Scholtzova H, Ripellino JA, Li Y, Schmidt SD, Mathews PM, Fryer JD, Holtzman DM, Sigurdsson EM, Wisniewski T. A synthetic peptide blocking the apolipoprotein E/beta-amyloid binding mitigates beta-amyloid toxicity and fibril formation in vitro and reduces beta-amyloid plaques in transgenic mice. Am J Pathol. 2004 Sep;165(3):937-48. PubMed.
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. PubMed.
Fagan AM, Watson M, Parsadanian M, Bales KR, Paul SM, Holtzman DM. Human and murine ApoE markedly alters A beta metabolism before and after plaque formation in a mouse model of Alzheimer's disease. Neurobiol Dis. 2002 Apr;9(3):305-18. PubMed.
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