The human immunodeficiency virus type-1 transcription factor Tat produces elevations in intracellular Ca²⁺ that require function of an N-methyl-D-aspartate receptor polyamine-sensitive site

Human immunodeficiency virus type-1 (HIV-1) infection is commonly associated with neuronal loss, as well as, cognitive and motor deficits collectively termed HIV-1-associated dementia (HAD). Function of the HIV-1 transcription factor Tat, activation of N-methyl-D-aspartate (NMDA)-type glutamate receptors, and subsequent rapid rises in free intracellular Ca ²⁺ have been implicated in the development of this neurological disorder. However, the role of specific NMDA receptor modulatory sites in mediating effects of Tat has not been examined. The present studies examined the ability of two variants of Tat protein (1-100 nM), Tat 1-72 and Tat 1-86, to produce rapid rises in intracellular Ca²⁺ in organotypic slice cultures of rat hippocampus. Further, these studies evaluated the role of an NMDA receptor polyamine-sensitive site in mediating Tat-induced elevations in intracellular Ca²⁺. Brief exposure (10 min) to each variant of Tat protein (>1 nM) markedly increased levels of intracellular Ca²⁺ in each region of the hippocampus to as much as 145% of controls. In contrast, exposure of cultures to a deletion mutant of Tat protein devoid of amino acids 31-61 (Tat Δ31-61) did not produce changes in intracellular Ca ²⁺ levels. Most significantly, exposure to the NMDA receptor antagonist dizocilpine (MK801 20 μM) and the polyamine site antagonist arcaine (10 μM) significantly attenuated increases in intracellular Ca ²⁺ levels when co-administered with either the Tat 1-72 or Tat 1-86 amino acid variant of Tat. Thus, exposure of the hippocampus to Tat produces increases in intracellular Ca²⁺ levels that require function of an NMDA receptor polyamine-sensitive site and this may well contribute to the neurotoxic effects of HIV-1 infection. Polyamine-sensitive portions of this receptor may then represent novel therapeutic targets in the pharmacologic treatment of HAD-related neurotoxicity.