. ABAD directly links Abeta to mitochondrial toxicity in Alzheimer's disease. Science. 2004 Apr 16;304(5669):448-52. PubMed.

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  1. In this article, Lustbader and colleagues investigate what happens when Aβ interacts with ABAD (Aβ-binding alcohol dehydrogenase), the only protein found to interact with Aβ in a yeast two-hybrid screen [1]. In the current work, the authors showed that ABAD and Aβ colocalized to mitochondria by electron microscopy, and could be coimmunoprecipitated from a mitochondrial preparation. Aβ caused the cofactor NAD+ to be excluded from the crystal structure of ABAD, likely explaining the previously observed [2] inhibition of enzymatic activity by Aβ. ABAD levels were increased in pathologically affected areas of AD brain. This is potentially deleterious, because the presence of ABAD exacerbated the cytotoxicity of exogenous Aβ or of expressing a doubly mutated APP, resulting in increased free radical production, cytochrome c release, and DNA fragmentation. Moreover, mice coexpressing ABAD and mutant APP exhibited learning deficits. An ABAD “decoy peptide,” elegantly designed based on the crystal structure, attenuated Aβ-induced cytotoxicity, presumably by preventing the interaction of Aβ and bona fide ABAD.

    This is a tantalizing contribution from several points of view. First, it provides a new therapeutic target. Protection from Aβ-induced toxicity was previously obtained by use of an anti-ABAD F(ab')2 [1] and by catalytically inactivating ABAD [2], confirming the results of the ABAD “decoy peptide” approach. The “decoy peptide” has the added advantage that it might prevent other pathologic interactions of Aβ, not just that with ABAD. Second, although the final pathologic hallmarks of Alzheimer’s disease, amyloid plaques, are extracellular, a growing literature suggests that the intracellular accumulation of Aβ may be pathogenetically important [3]. Lustbader and colleagues have suggested a specific molecular target and mechanism through which intracellular Aβ could be toxic. Third, this work links amyloid with the apparently unrelated world of mitochondrial dysfunction and oxidative stress. Mitochondrial dysfunction and oxidative stress are among the earliest events in Alzheimer’s disease [4, 5] and transgenic APP animal models [6], but the mechanisms relating these to Aβ physiology have been unclear.

    At the same time, a few cautions are appropriate. A large body of work localizes intracellular APP and Aβ to the secretory or endocytic pathways. Only one prior study suggested that APP could be targeted to mitochondria [7], and Aβ was not mentioned. Thus, the mitochondrial localization of Aβ needs to be confirmed. The rabbit anti-A IgG used to visualize Aβ in mitochondria is not characterized or referenced. Could it cross-react with APP (in agreement with Anandatheerthavarada et al.) or a yet unidentified mitochondrial epitope (does it stain mitochondria in an APP knockout mouse)[7]? Second, there is not yet sufficient evidence to conclude that the pathologic ABAD-Aβ interaction is the one occurring in mitochondria. The authors have previously shown that ABAD is also localized to the ER and, after exposure to Aβ, the inner surface of the plasma membrane [1,2]. The ABAD “decoy peptide” presumably disrupts the ABAD-Aβ interaction at all sites, not just in mitochondria. Thus, toxicity from one of these other sites cannot be excluded. Indeed, in cells coexpressing ABAD and APPV717F, the most intense colocalization of ABAD with hydroxynonenal and malondialydehyde (markers of lipid oxidation) was subplasmalemmal [2].

    View all comments by Michael Lin
  2. This is an interesting but still incomplete story. This work dates back several years, when the authors found an unexpected ability of the Aβ peptide to bind to the enzyme alcohol dehydrogenase (ADH). It appears that the binding is relatively specific when compared with other peptides, although I am surprised that these peptides don't bind to other proteins non-specifically. The authors had a great opportunity to study the interaction between Aβ and the so-called ABAD protein when they apparently cocrystallized the two. Unfortunately they could not see the Aβ peptide in the complex, so it is impossible to say where Aβ actually binds, or why it blocks the ability of NAD to bind to ABAD.

    The paper shows that Aβ and ABAD localize in, around, or next to mitochondria, but not that it is primarily inside the mitochondria, and the immunoelectron microscopy data do not resolve this question. It is too early to suggest that this latest observation offers therapeutic potential, but one hopes it may with further, more definitive data.

    View all comments by Vincent Marchesi
  3. It has been suggested previously that ERAB (aka ABAD) residues 99-108 contain the Aβ binding domain (Milton et al 2001) so it's nice to see that confirmed using different techniques.

    View all comments by Nathaniel Milton
  4. This is the first article I've seen that provides a considerably complete mechanism for the toxicity of β amyloid in Alzheimer's disease. It ties together evidence of Aβ binding to ABAD, mitochondrial stress, and free radical involvement long implicated in Alzheimer's disease.

    Are there any suggestions as to how the proposed mechanism may affect synaptic function prior to cell death? (Reference implicating synaptic dysfunction in Alzheimer's is included.)

    View all comments by Robyn Mansfield
  5. ABAD—The New/Old Good/Bad Guy in Alzheimer's Disease Lustbader and colleagues [1] present a complex potential mechanism for the role of amyloid β in Alzheimer's disease (AD) pathology. The authors created a crystal form of amyloid β-binding alcohol dehydrogenase (ABAD) and amyloid β that demonstrates that both molecules interact and accumulate inside mitochondria. They suggested that this interaction increases oxidative stress, mitochondrial dysfunction, and cell death occurring in AD.

    AD, one of the most devastating age-related neurodegenerative diseases, is associated with oxidative stress, altered energy metabolism, and mitochondrial impairment [1-15]. Postmortem studies revealed a decline in the activities of pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase [16], key enzymes in energy metabolism that are localized in mitochondria. Furthermore, defects on cytochrome oxidase have also been described [17]. Furthermore, it was demonstrated that Aβ peptides and/or Ca2+ induce the opening of mitochondrial permeability transition pore (a process that results in a nonselective increase in the permeability of the inner mitochondrial membrane to solutes smaller than 1.5 kDa) [18,19]. Moreover, Cardoso et al. [20] showed that amyloid β toxicity requires functional mitochondria.

    A previous study [21] implicated endoplasmic reticulum amyloid β-binding protein (ERAB) as a participant in causing neuronal dysfunction in AD. In the same, study the authors observed a strong neuronal ERAB reactivity in the brains of patients with AD, yet it is almost absent in normal brain. Moreover, ERAB is found near amyloid β plaques, and the cellular toxicity of amyloid β can be reduced by blocking ERAB and increased by its overexpression. Furthermore, Oppermann et al. [22] demonstrated that ERAB is localized in the endoplasmic reticulum and mitochondria suggesting a complex interaction with components of the programmed cell death machinery.

    ABAD is a member of the family of short chain dehydrogenase/reductase and ERAB has structural homology with short-chain alcohol dehydrogenase (hydroxysteroid dehydrogenases and acetyl-CoA reductases). Furthermore, both appear to potentiate cell stress induced by amyloid β, as evidenced by increased oxidative stress [22, 23]. So, together these findings may suggest that both ABAD and ERAB may interplay in the amyloid β-associated oxidative stress mechanisms or both may be the same entity with two different names. However, these findings suggest a possible target for the development of new therapeutic strategies envisaging the prevention or reduction of the oxidative stress cascade typical of AD pathology.

    View all comments by Maria Santos