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 is 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‐Gi 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, induces calcium release from IP3‐sensitive intracellular stores, which leads to glutamate receptor‐mediated calcium influx, dysregulation of [Ca2+]i, and Tat‐induced neurotoxicity.