Altered outward-rectifying K(+) current reveals microglial activation induced by HIV-1 Tat protein

Microglial cells are believed to be one of the key elements in the development of the HIV-related neuropathology. Not only can microglial cells be productively infected by the virus, but they are also sensitive to viral proteins. Among them, the HIV-1 regulatory protein Tat, which was shown to have neurotoxic activity, is able to promote some proinflammatory functions of microglia. Considering that microglial activation goes along with a change of ion channel profile, we aimed to study whether Tat could influence microglial electrophysiology. When microglial cultures obtained from neonatal rats were treated with Tat (> or = 100 ng/ml), whole-cell recording showed the appearance of a large outwardly rectifying current (OR) virtually absent in untreated control cells. According to voltage dependence of the kinetic variables, K(+) permeability, and pharmacological sensitivity, the Tat-induced current was due to the presence of functional Kv1.3 channels. The effect of Tat was abolished by specific anti-Tat polyclonal antibody and by heat denaturation of Tat protein, confirming that the OR enhancement was due to the viral protein. Interestingly, the OR current induced by Tat was largely prevented by two inhibitors of the transcription factor NF-kappaB, TPCK and SN50, which suggests an involvement of NF-kappaB in the effect of the viral protein. The relatively high dose of Tat needed to observe an effect (> or = 100 ng/ml) might indicate that the action of Tat required entrance of the protein into the cell, rather than being mediated by a membrane receptor. In conclusion, the HIV-1 protein Tat is able to enhance OR K(+) current in rat microglia through a mechanism involving the activation of NF-kappaB. We propose that such effect of Tat could be part of the process of microglial activation known to take place in the brain of persons with neuro-AIDS