The success of Highly Active Antiretroviral Treatment (HAART) in combating HIV infection has resulted in a dramatic increase in the expected lifespan of HIV infected patients. Neurological deficits associated with HIV infection (collectively termed HIV associated dementia; HAD) also declined with the advent of HAART therapy, although in some cohorts, it is the incidence rather than the prevalence (which continues to increase) that has declined. This suggests that neurological dysfunction is increasing as HIV infected individuals' age. Neurons in the """"""""aged"""""""" brain are more vulnerable to toxic insult. Age-related increases in the lipid ceramide and cholesterol are thought to play key roles in increasing neuronal sensitivity to insult by priming apoptotic pathways. Ceramide signaling directly inhibits the anti-apoptotic protein Bcl-2 and decreases mitochondrial function by disrupting electron transport and promotes the generation of free radical species. Increases in cholesterol disrupt cellular signaling by decreasing the fluidity of the plasma membrane and can increase ceramide levels by slowing the conversion of ceramide to sphingomyelin. In our preliminary studies we found dramatic increases in ceramide and cholesterol in brains of HIV-1 infected patients with dementia compared to HIV-1 seropositive or seronegative controls. Because some HIV-1 proteins are thought to play key roles in neuronal dysfunction and death, we exposed human neuroglial cultures gp120 and Tat and found that these proteins increased ceramide levels. When we exposed cultures to several different insults, including Tat, cell death was dramatically reduced by inhibition of de novo ceramide synthesis, suggesting a central role for ceramide in some forms of neuronal cell death. Our findings suggest that HIV-1 infection could compound increases in ceramide and cholesterol that occur with age resulting in ceramide overload and neuronal cell death. Drugs that lower cholesterol levels (and indirectly ceramide) have therapeutic benefit in some neurodegenerative diseases such as Alzheimer's and ischemic stroke. We propose to determine the mechanisms of interaction of age, ceramides, cholesterols, gp120 and Tat that contribute to neuronal dysfunction and death in HAD. In vivo, we will test the protective effects of cholesterol reduction, inhibition of ceramide synthesis and reduction of oxidative stress in models of HIV protein-induced toxicity.
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