Human immunodeficiency virus (HIV) infection of the CMS is an increasingly prevalent complication of AIDS. HIV infection in the CMS causes HIV-associated dementia (HAD) or milder forms of neurocognitive impairment in 10-20% of AIDS patients. Although highly active antiretroviral therapy (HAART) reduces HIV RNA levels in plasma and cerebrospinal fluid (CSF) and improves neurocognitive function, most antiviral drugs cannot cross the BBB. Thus, the CMS is a reservoir for long-term viral persistence. The overall goal of this proposal is to understand mechanisms that underlie HIV neurotropism. The specific aims are: 1) to identify specific amino acid variants in the gp120 C3 region that enhance membrane fusion mediated by brain-derived Envs from patients with HIV-associated dementia;2) to determine whether specific C3 variants that enhance membrane fusion activity of brain-derived Envs also increase the capacity of HIV to replicate in macrophages/microglia and induce neuronal apoptosis in primary brain cultures;and 3) to characterize mechanisms by which C3 variants that enhance HIV macrophage/microglia tropism exert their effects on membrane fusion. Eight gp120 C3 region amino acid variants associated with brain compartmentalization were identified through analysis of matched brain- and lymphoid-derived Env amino acid sequences. These C3 variants will be examined for the ability to enhance fusion in cells with reduced CD4/CCR5 expression and fusion kinetics in macrophages/microglia. The role of variants in macrophage/microglia replication and neuronal apoptosis will be studied for variants that demonstrate enhancement of fusion and/or fusion kinetics. Potential mechanisms by which C3 variants might contribute to neurotropism and/or neurovirulence will be investigated for variants selected based on results of the first two specific aims. Using parental and mutant Envs, we will investigate receptor binding site exposure and post-receptor binding conformational changes in HIV Env. These studies are expected to contribute to the understanding of the influence of specific HIV Env C3 amino acid variants on neurotropism and neurovirulence. Furthermore, these studies will provide insight into HIV Env-receptor interactions and post- receptor binding events that determine the efficiency of membrane fusion and represent targets for entry inhibitors.