Mechanisms of HIV-tat induced neuronal apoptosis

Abstract

HIV infection of the CNS can result in neurologic dysfunction, including motor impairment and cognitive deficits, in a significant number of individuals with AIDS. NeuroAIDS is characterized by neuronal injury and loss, yet there is no evidence of HIV positive neurons. Neuronal dropout must therefore be due to the indirect effects of HIV infection of other CNS cells, through elaboration of inflammatory factors and neurotoxic viral proteins, including the viral transactivating protein tat. Tat induces apoptosis in human neurons that is dependent on the lipoprotein receptor-related protein (LRP) and the NMDA receptor (NMDAR). We show that NMDAR positive neurons initiate tat induced apoptosis through the rapid formation of a cell surface complex that includes tat, LRP, PSD-95, and NMDAR channels. Neuronal nitric oxide synthase (nNOS) is subsequently recruited to this complex and activated. Blocking LRP mediated uptake of tat, NMDAR activation or nNOS activity with specific inhibitors results in a significant reduction in tat-induced neurotoxicity, suggesting that this new complex plays an initial role in generating and amplifying tat toxicity. These data demonstrate the early mechanisms by which tat triggers apoptosis using the synaptic machinery expressed by NMDAR positive neurons. In addition, we demonstrated that tat treatment of neurons results in the association of tyrosine kinases with the NMDAR and phosphorylation of the NMDAR on multiple subunits. The NMDAR subunit 2A is rapidly phosphorylated after tat treatment in a Src kinase dependent manner, and using an in vitro kinase assay, we identified three potential Src targets within the amino acid sequence of NR2A, tyrosines 1184, 1325, and 1423. Subsequent studies using antisera and a monoclonal antibody we generated indicate that Y1325 is phosphorylated in our cultured neurons in a tat sensitive, and src dependent, manner. These data suggest that NMDAR phosphorylation is an important mechanism by which NMDAR activity may be upregulated during tat induced apoptosis. These findings may provide specific tools for understanding the neuropathogenesis of NeuroAIDS and potential therapeutic intervention.