Neuronal damage and loss in patients with HIV-associated dementia (HAD) is most closely associated with macrophage infiltration and neuroinflammation of the CNS. Most of these factors stimulate changes in cell cycle regulatory machinery which determine cellular outcomes even in the absence of cell division. This has led us to propose the following overarching hypothesis: neurons in HIV-positive patients with HAD exhibit altered cell cycle protein activity and this activity determines neuronal survival in response to factors released by HIV infected macrophages. Induction of cell cycle machinery classically results in increased transcriptional activity of the E2F family of transcription factors leading to the expression of gene products necessary for DNA synthesis and progression to S-phase. Activity of the E2F family is repressed by direct interaction with the Retinoblastoma tumor suppressor protein, pRb, and its family members, an interaction that is disrupted when pRb is hyperphosphorylated. In support of our hypothesis, we have observed increased E2F1 and hyper-phosphorylated pRb in the CNS of patients with HIV encephalitis, in a simian model of disease, and in neurons in our in vitro HIV neurodegeneration model. Surprisingly, E2F1 is localized to the cytoplasm, a site inconsistent with its known transcriptional roles. Altered E2F function is further supported by altered E2F DNA binding activity in SIVE. The consistent and predominant cytoplasmic localization of E2F1 in neurons suggests a novel role for this protein. Interestingly, E2F1 has been reported to bind to a unique RNA hairpin structure and mediate stabilization of at least one mRNA species, Axin 2;however, little is known about this aspect of E2F1 function. We thus hypothesize that E2F1, acting independently of pRb, plays a novel role in modulating neurodegeneration in HIV infection via altered nucleic acid binding activity, subcellular distribution, and calpain cleavage. To test this hypothesis, we propose to: 1) determine the potential cytoplasmic role for the E2F1 nucleic acid binding domain as a mechanism modulating neurodegeneration. 2) determine the ability of calpain-cleaved E2F1 to modulate E2F1 functions such as nucleic acid binding and neuronal survival. 3) determine the role for E2F1 localization in HIV-induced neurotoxicity. Given its potentially unique function and regulation in post-mitotic neurons, we believe E2F1 may serve as a target to prevent neuronal loss in HIV associated dementia.
Patients with HIV-associated dementia exhibit increased levels of the cell cycle regulatory protein, E2F1;however, in contrast to its known roles in dividing cells, E2F1 exhibits distinct activities that are regulated by novel mechanisms in neurons. Given its potentially unique function and regulation in neurons, we believe E2F1 may serve as a target to prevent neuronal loss and subsequent cognitive decline in HIV associated dementia.
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