Involvement of inositol 1,4,5-trisphosphate-regulated stores of intracellular calcium in calcium dysregulation and neuron cell death caused by HIV-1 protein Tat

HIV-1 infection commonly leads to neuronal cell death and a debilitating syndrome known as AIDS-related dementia complex. The HIV-1 protein Tat is neurotoxic, and because cell survival as affected by the intracellular calcium concentration ([Ca²⁺](i)), we determined mechanisms by which Tat increased [Ca²⁺](i) and the involvement of these mechanisms in Tat-induced neurotoxicity. Tat increased [Ca²⁺](i) dose-dependently in cultured human fetal neurons and astrocytes. In neurons, but not astrocytes, we observed biphasic increases of [Ca²⁺](i). Initial transient increases were larger in astrocytes than in neurons and in both cell types were significantly attenuated by antagonists of inositol 1,4,5-trisphosphate (IP₃)-mediated intracellular calcium release [8-(diethylamino)octyl-3,4,5-trimethoxybenzoate HCl (TMB-8) and xestospongin], an inhibitor of receptor-G₁ protein coupling (pertussis toxin), and a phospholipase C inhibitor (neomycin). Tat significantly increased levels of IP₃ threefold. Secondary increases of neuronal [Ca²⁺](i) in neurons were delayed and progressive as a result of excessive calcium influx and were inhibited by the glutamate receptor antagonists ketamine, MK-801, (±)-2-amino-5-phosphonopentanoic acid, and 6,7-dinitroquinoxaline-2,3-dione. Secondary increases of [Ca²⁺](i) did not occur when initial increases of [Ca²⁺](i) were prevented with TMB-8, xestospongin, pertussis toxin, or neomycin, and these inhibitors as well as thapsigargin inhibited Tat-induced neurotoxicity. These results suggest that Tat, via pertussis toxin-sensitive phospholipase C activity, reduces calcium release from IP₃-sensitive intracellular stores, which leads to glutamate receptor-mediated calcium influx, dysregulation of [Ca²⁺](i), and Tat- induced neurotoxicity.