Graduation Year

2011

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Medical Sciences

Major Professor

Jun Tan

Co-Major Professor

Huntington Potter

Keywords

Alzheimer's, dementia, gp120, HIV-1, microglia, Tat

Abstract

Prevalence of HIV-associated cognitive impairment is rising, the worst form of which is HIV-associated dementia (HAD). The disease is fuiled by a chronic innate type pro-inflammatory response in the brain which is highly dependent upon the activation of microglia. We first created an in vitro model of HAD composed of cultured microglial cells synergistically activated by the addition of IFN-gamma and the HIV-1 coat glycoprotein, gp120. This activation, as measured by TNF-alpha and NO release, is synergistically attenuated through the alpha7nAChR and p44/42 MAPK system by pretreatment with nicotine, and the cholinesterase inhibitor, galantamine. As these medications have been FDA approved, and over time, have shown only minor improvement in neurodegenerative disease for a limited period, we next sought to explore natural compounds that may attenuate HAD mediated inflammation and related pathology. This inflammation is a key moderator of A-#914; plaque deposition in the brain. Indeed it is likely a contributing factor as epidemiological data suggests significant numbers of HIV survivors are at elevated risk of developing Alzheimer's disease (AD). HIV-1 Tat-induced A-beta deposition, tau phosphorylation, and subsequent neuronal death could be risk factors for subsequent AD and/or HAD. Recent reports suggest green tea-derived (-)-epigallocatechin-3-gallate (EGCG) can attenuate neuronal damage mediated by conditions such as brain ischemia. In order to investigate the therapeutic potential of EGCG to mitigate the neuronal damage characteristic of HAD, IFN- gamma was evaluated for its ability to enhance well-known neurotoxic properties of HIV-1 proteins gp120 and Tat in primary neurons and mice. Indeed, IFN-gamma enhanced the neurotoxicity of gp120 and Tat via increased JAK/STAT signaling. Additionally, primary neurons pretreated with a JAK1 inhibitor, or those from STAT1-deficient mice, were largely resistant to the IFN- gamma-enhanced neurotoxicity of gp120 and Tat. Moreover, EGCG treatment of primary neurons from normal mice reduced IFN-gamma-enhanced neurotoxicity of gp120 and Tat by inhibiting JAK/STAT1 pathway activation. EGCG was also found to mitigate the neurotoxic properties of HIV-1 proteins in the presence of IFN-#947; in vivo. To explore the mechanism by which HIV may augment AD-like pathology, we found HIV-1 Tat protein inhibits microglial uptake of A-beta-1-42 peptide, a process enhanced by IFN-#947; and rescued by EGCG. To mimic the HAD clinical condition, we generated mice with HIV-1 Tat-induced AD-like pathology by cross-breeding HIV-1 Tat expressing mice (expressed under control of GFAP, Doxycline inducible promoter) with the PSAPP mouse model of AD. To simulate chronic Tat secretion over we used an optimized dose of 54 mg/kg/day on a biweekly basis over three months Tat significantly induced neuron degeneration and tau phosphorylation in Tat transgenic mice, dox dependently (P<0.001). Similar effects at the chronic 54 mg/kg/day dose were observed in PSAPP/Tat mice induced with dox. These mice also showed significantly more A-beta deposition (P < 0.05), neurodegeneration, neuronal apoptotic signaling, and phospho-tau than PSAPP mice (P < 0.05). In conclusion, HIV-1 Tat significantly promotes AD-like pathology in PSAPP/Tat mice. This model may provide a framework in which to identify new mechanisms involved in cognitive impairment in the HIV infected population, and possible treatments. Additional works will be needed to fully characterize the mechanism(s) of HIV- induced amyloid deposition, and to uncover viral mechanisms promoting AD-like pathology in general.

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