Downregulation of the 18-kDa translocator protein: Effects on the ammonia-induced mitochondrial permeability transition and cell swelling in cultured astrocytes

Hepatic encephalopathy (HE) is a major neurological complication in patients with severe liver disease. While the pathogenesis of HE is unclear, elevated blood and brain ammonia levels are believed to be major etiological factors, and astrocytes appear to be the primary target of its toxicity. A notable feature of ammonia neurotoxicity is an upregulation of the 18-kDa translocator protein (TSPO) (formerly referred to as the peripheral benzodiazepine receptor or PBR), which is found on the outer mitochondrial membrane. However, the precise significance of this upregulation is unclear. To examine its potential role in ammonia-induced astrocyte dysfunction, we downregulated the TSPO using antisense oligonucleotides, and examined whether such downregulation could alter two prominent features of ammonia gliotoxicity, namely, the mitochondrial permeability transition (MPT) and astrocyte swelling. Nontransfected cultures treated with NH₄Cl (5 mM; 48 h) showed a significant increase in astrocyte cell volume (37.5%). In cultured astrocytes transfected with TSPO antisense oligonucleotides, such cell swelling was reduced to 17%, but this change was not significantly different from control cell volume. Similarly, nontransfected cultures treated with NH₄Cl (5 mM; 24 h) exhibited a 40% decline in the cyclosporin A-sensitive mitochondrial inner membrane potential (ΔΨ_m) (P < 0.01) (a measure of the MPT). By contrast, cells transfected with TSPO antisense oligonucleotides did not display a significant loss of the ΔΨ_m following ammonia exposure. Our findings highlight the important role of the TSPO in the mechanism of ammonia neurotoxicity.