Molecular interactions of the type 1 human immunodeficiency virus transregulatory protein Tat with N-methyl- d-aspartate receptor subunits

Abstract

We investigated the effect of type 1 human immunodeficiency virus (HIV-1) regulatory protein Tat on N-methyl- d-aspartate (NMDA) receptors expressed in Xenopus oocytes by voltage-clamp recording and its role in NMDA-mediated neurotoxicity using cultured rat hippocampal neurons. Tat (0.01–1μM) potentiated NMDA-induced currents of recombinant NMDA receptors. However, in the presence of Zn 2+, the potentiating effect of Tat was much more pronounced, indicating an additional Zn 2+-related effect on NMDA receptors. Consistently, Tat potentiated currents of the particularly Zn 2+-sensitive NR1/NR2A NMDA receptor with a higher efficacy, whereas currents from a Zn 2+-insensitive mutant were only marginally augmented. In addition, chemical-modified Tat, deficient for metal binding, did not reverse Zn 2+-mediated inhibition of NMDA responses, demonstrating that Tat disinhibits NMDA receptors from Zn 2+-mediated antagonism by complexing the cation. We therefore investigated the interplay of Tat and Zn 2+ in NMDA-mediated neurotoxicity using cultures of rat hippocampal neurons. Zn 2+ exhibited a prominent rescuing effect when added together with the excitotoxicant NMDA, which could be reverted by the Zn 2+-chelator tricine. Similar to tricine, Tat enhanced NMDA-mediated neurotoxicity in the presence of neuroprotective Zn 2+ concentrations. Double-staining with antibodies against Tat and the NR1 subunit of the NMDA receptor revealed partial colocalization of the immunoreactivities in membrane patches of hippocampal neurons, supporting the idea of a direct interplay between Tat and glutamatergic transmission. We therefore propose that release of Zn 2+-mediated inhibition of NMDA receptors by HIV-1 Tat contributes to the neurotoxic effect of glutamate and may participate in the pathogenesis of AIDS-associated dementia.