E2F1 Mediates Death of B-amyloid-treated Cortical Neurons in a Manner Independent of p53 and Dependent on Bax and Caspase 3

Abstract

Although B-amyloid (AB) is suggested to play an important role in Alzheimer's disease, the mechanisms that control AB-evoked toxicity are unclear. We demonstrated previously that the cell cycle-related cyclin-dependent kinase 4/6/retinoblastoma protein pathway is required for AB-mediated death. However, the downstream target(s) of this pathway are unknown. We show here that neurons lacking E2F1, a transcription factor regulated by the retinoblastoma protein, are significantly protected from death evoked by AB. Moreover, p53 deficiency does not protect neurons from death, indicating that E2F1-mediated death occurs independently of p53. Neurons protected by E2F1 deficiency have reduced Bax-dependent caspase 3-like activity. However, protection afforded by E2F1, Bax, or caspase 3 deficiency is transient. In the case of E2F1, but not with Bax or caspase 3 deficiency, delayed death is accompanied by DEVD-AFC cleavage activity. Taken together, these results demonstrate the required role of E2F1, Bax, and caspase 3 in AB evoked death, but also suggest the participation of elements independent of these apoptosis regulators.Alzheimer's disease (AD) 1 is a neurodegenerative disorder characterized by memory loss and cognitive impairment (1). Although the etiology of AD is not fully understood, an increasing body of evidence suggests the importance of B-amyloid in the initiation/progression of the disease. B-amyloid (AB), a 39 -43-amino acid peptide, assembles into insoluble aggregates forming plaques characteristic of AD (2, 3). AB is formed by alternative proteolysis from an integral membrane-localized B-amyloid precursor protein (4), and its importance in AD is underscored by the identification of mutations in B-amyloid precursor protein in some familial cases of AD (5, 6). In addition, studies with in vitro cultured neurons treated with toxic forms of aggregated AB protein (7-9) as well as in vivo studies utilizing transgenic mice expressing AB (10) demonstrated neuronal loss by an apoptotic pathway. However, the mechanism by which AB causes neuronal apoptosis is not well understood.Recent evidence has suggested that cell cycle molecules that normally control cell proliferation play an important required role in some forms of neuronal death. Of particular relevance to AD, numerous reports demonstrate abnormal up-regulation of a variety of cell cycle proteins/activity in brains from AD patients. These include cyclin B, D, and E, cdc2, and Cdk4 (11-13). In addition, phosphoepitopes, including phosphorylated tau, common to both mitotic cells and degenerating AD neurons, have been described (12). In support of the potential importance of cell cycle in AD, we have shown that flavopiridol, a pharmacological inhibitor of Cdks, as well as expression of dominant negative Cdk4/6 but not 2 or 3, protects cultured cortical neurons from death evoked by AB (14). This evidence supports the idea that Cdk4/6 activation plays an obligatory role in death of neurons evoked by AB.These previous observations raise the question of the relev...

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