Dementia and other milder forms of cognitive diseases of the CNS affect HIV-1 infected patients. While HAART has diminished the incidence of HIV-1-associated dementia (HAD), milder forms of neurological disease persist. Also, increased survival rates resulting from HAART have led to an increase in the prevalence of neurological disorders. HIV-induced neurodegeneration may result at least in part from changes in cellular gene expression induced by HIV-1 Tat in a variety of cell types including astrocytes and neurons. Tat is an essential viral transactivator that requires association with a cellular kinase composed of a regulatory subunit designated cyclin T1 and a catalytic subunit named CDK9. This host complex is recruited by Tat to TAR, an RNA element in the nascent HIV-1 transcript, where CDK9 is positioned to phosphorylate RNA polymerase II (RNAPII) and negative elongation factors, strongly activating HIV-1 transcription. CDK9 in association with T-type cyclins forms Positive Transcription Elongation Factors (P-TEFb) thought to be required for the expression of cellular genes. We have investigated the effect of limiting CDK9 activity in a human astrocytoma cell line. Our gene profiling analysis shows that CDK9 activity is not homogeneously required for the expression of cellular genes, but rather that inhibition of CDK9 leads to selective changes in gene expression. Among the genes downregulated we have identified various transcription factors. Our data also show that inhibition of CDK9 results in the upregulation of a subset of genes including stress and stimuli response genes (Gadd45A, Gadd45B, IL-8 and IL-12), as well as deregulation of a number of genes that have been associated with dementia (GLS, SOD2, FGF2 and CHMP2B). Cellular CDK9 is sequestered in catalytically inactive and active complexes in an equilibrium that is in place to restrict gene expression. Importantly, Tat inhibits cyclin T1/CDK9-dependent gene expression of certain genes in macrophages, dendritic cells and B cells via a competition mechanism with specific transcriptional activators. Since Tat reconfigures pools of CDK9 complexes in HIV-1 infected HeLa cells and Tat expressing primary human astrocytes and astrocytoma cells, it is likely that similar competition mechanisms operate in astrocytes and neurons with other transcription factors. Thus, the goal of this exploratory R21 proposal is to test the hypothesis that the effects of Tat in gene expression that may contribute to HAD are at least in part mediated through interference with the host CDK9 function and that restoration of normal CDK9 function should ameliorate the detrimental effects of Tat expression and HIV-1 infection in the CNS. To test this hypothesis we propose:
(Aim 1) to determine the role of cellular CDK9 in the alterations on gene expression triggered by HIV-1 Tat in astrocytes and neurons;
and (Aim 2) to determine whether the effects of Tat in gene expression can be reverted by restoring normal cellular CDK9 function in Tat expressing astrocytes and neurons. The short-term benefit of these studies is gaining insight into the molecular mechanisms by which Tat remodels cellular gene expression in astrocytes and neurons contributing to neurodegeneration. The results of these studies could set the path to subsequent structural studies to search for small compounds with the ability to bind cyclin T1 blocking Tat without altering CDK9 dependent cellular gene expression. in astrocytes and neurons
Astrocytes are the most abundant cell type in the brain and are critical for neuronal function, survival and neurogenesis. Astrocytes are infected by HIV-1, and viral proteins including Tat are made, but the virus does not productively replicate. Given their abundance and importance, astrocytes represent a very significant reservoir of infected cells that express HIV-1 proteins with neurotoxic properties. These proteins may affect astrocyte homeostasis, which in turn will influence neuronal function or might be secreted and taken up by neurons to directly affect them contributing to HAD. HIV-1 Tat is an essential viral transactivator that hijacks the cellular cyclin T1/CDK9 transcription elongation factor to promote HIV-1 replication. Tat also alters cellular gene expression, which contributes to HIV-1-associated pathogenesis by altering immune cell function. As Tat expression rearranges cellular P-TEFb complexes in human astrocytes, and these complexes are thought to play rate-limiting roles in transcription, it is hypothesized that P-TEFb deregulation by Tat induces selective changes in gene expression in astrocytes and neurons that contribute to HIV-1-associated dementia. We propose to test the hypothesis that the effects of Tat in gene expression are at least in part mediated through interference with the host CDK9 function and that restoration of normal CDK9 function should ameliorate the detrimental effects of Tat expression and HIV-1 infection in the CNS.
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