Abstract
The acquired immunodeficiency syndrome (AIDS) frequently involves the central nervous system (CNS) and manifests as dementia due to encephalitis or diffuse neurodegeneration. Human immunodeficiency virus type 1 (HIV-1) proteins, potentially transported into the CNS by mononuclear inflammatory cells, have been implicated in the etiology of this HIV-1 associated neurological dysfunction. Here we investigate the neurotoxicity of the essential HIV-1 regulator protein Tat in vivo after microinfusion into the rat brain and in vitro using PC12, NG108-15, and GT17 neuronal cell lines. Infusion of either chemically synthesized Tat (Tat86) or recombinant Tat (rTat) into the striatal gray matter in Sprague-Dawley rats resulted in postural deviation ipsilateral to the infusion, a clinical presentation in rats associated with complete striatal dysfunction. Histologic examination 3 days after infusion revealed massive necrosis in the area of the distribution of the infusion. Infusion of heat denatured rTat, peptide Tat49-58, or peptide Tat57-86 did not result in clinically or histologically detectable brain damage. After 3 days incubation in vitro, the lethal dose for half (LD50) of PC 12 cells due to rTat was 5 μg/ml. The LD50 for Tat86 under the same conditions was 10 μg/ml. Tat49-58 and Tat57-86 peptides were not toxic in vitro even at 10-fold higher doses. At 5 μg/ml, rTat was toxic to 100% of GT17 cells after 24 hr. At 5 μg/ml, Tat86 was toxic to 90% of the NG108-15 cells after 7 days of treatment. Prior experiments have shown Tat49-58 is specifically recognized by a cell surface protein that mediates Tat uptake. Here we show that Tat toxicity is inhibited by cotreatment with excess Tat49-58, suggesting Tat neurotoxicity requires binding to its surface ligand. Chloroquine, which increases nuclear accumulation of Tat, enhances Tat toxicity to PC 12 cells, suggesting that Tat internalization is a required step in the mechanism of its toxicity. © 1995 Wiley-Liss, Inc.