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

Abstract

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 (≥ 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‐κB, TPCK and SN50, which suggests an involvement of NF‐κB in the effect of the viral protein. The relatively high dose of Tat needed to observe an effect (≥ 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‐κB. 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. GLIA 33:181–190, 2001. © 2001 Wiley‐Liss, Inc.

Physiology of Microglia

“…Treatment of cultured microglia with HIV-1 regulatory protein Tat (100 ng/ml to 1 g/ml, 24 h) induced three-to sixfold increase in K v 1.3 current density (from ϳ5 to ϳ15 pA/pF at 100 mg/ml and 30 pA/pF at 1 g/ml Tat, respectively). This increase was mediated through NF-B signaling pathway and most probably required Tat entry into the cell (968). The inhibition of K v 1.3 channels with agitoxin-2 reduced the respiratory burst in cultured microglial cells treated with phorbol 12-myristate 13-acetate (446).…”

Viral dependence on cellular ion channels – an emerging anti-viral target?

“…These effects of RABV on Na + , K + and Ca 2+ channel activity may somehow act to promote virus pathogenesis, perhaps through altering the communication between neurons (a critical function of neuronal ion channels), leading to lethal consequences to the host [31]. In neuronal tissue, the K v 1.3 outward-rectifier K + channel was shown to be modulated by the HIV-encoded Tat protein [34]. HIV-Tat was found to increase the surface expression of K v 1.3, which accompanied microglial activation and Tat-induced neurotoxicity [35].…”

Regulation of ion channels and transporters by AMP-activated kinase (AMPK)

“…151,152 As NF-κB stimulates the expression of Orai1, 153,154 the inhibitory effect of AMPK on Orai1 may be partially due to downregulation of NF-κB. Channels upregulated by NF-κB further include the voltagegated K + channel Kv1.3 155 and the epithelial Cl -channel CFTR. 156 CFTR in turn downregulates the expression of the transcription factor.…”