HIV-1 protein Tat induces apoptosis of hippocampal neurons by a mechanism involving caspase activation, calcium overload, and oxidative stress

Patients infected with HIV-1 often exhibit cognitive deficits that are rela ted to progressive neuronal degeneration and cell death. The protein Tat, w hich is released from HIV-1-infected cells, was recently shown to be toxic toward cultured neurons. We now report that Tat induces apoptosis in cultur ed embryonic rat hippocampal neurons. Tat induced caspase activation, and t he caspase inhibitor zVAD-fmk prevented Tat-induced neuronal death. Tat ind uced a progressive elevation of cytoplasmic-free calcium levels, which was followed by mitochondrial calcium uptake and generation of mitochondrial-re active oxygen species (ROS). The intracellular calcium chelator BAPTA-AM an d the inhibitor of mitochondrial calcium uptake ruthenium red protected neu rons against Tat-induced apoptosis. zVAD-fmk suppressed Tat-induced increas es of cytoplasmic calcium levels and mitochondrial ROS accumulation, indica ting roles for caspases in the perturbed calcium homeostasis and oxidative stress induced by Tat. An inhibitor of nitric oxide synthase, and the perox ynitrite scavenger uric acid, protected neurons against Tat-induced apoptos is, indicating requirements for nitric oxide production and peroxynitrite f ormation in the cell death process. Finally, Tat caused a delayed and progr essive mitochondrial membrane depolarization, and cyclosporin A prevented T at-induced apoptosis, suggesting an important role for mitochondrial membra ne permeability transition in Tat-induced apoptosis. Collectively, our data demonstrate that Tat can induce neuronal apoptosis by a mechanism involvin g disruption of calcium homeostasis, caspase activation, and mitochondrial calcium uptake and ROS accumulation. Agents that interupt this apoptotic ca scade may prove beneficial in preventing neuronal degeneration and associat ed dementia in AIDS patients. (C) 1998 Academic Press.