Downregulation of the 18-kDa Translocator Protein: Effects on the Ammonia-Induced Mitochondrial Permeability Transition andCell Swelling in Cultured Astrocytes

Authors: Panickar, Kiran; JAYAKUMAR, A. - UNIV MIAMI SCH MED, FL; RAMA RAO, K. - UNIV MIAMI SCH MED, FL; NORENBERG, M. - UNIV MIAMI SCH MED, FL

Submitted to: Glia
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/21/2007
Publication Date: 9/24/2007
Citation: Panickar, K.S., Polansky, M.M., Anderson, R.A. 2007. Downregulation of the 18-kDa Translocator Protein: Effects on the Ammonia-Induced Mitochondrial Permeability Transition andCell Swelling in Cultured Astrocytes. Glia.

Interpretive Summary: There are numerous conditions where diet can affect liver function that is compromised because of disease. Liver dysfunction can result in ammonia build-up in the blood and brain that causes brain swelling; and brain swelling is one of the major causes of death in acute liver failure. We developed a system to evaluate brain cells (astrocytes) in culture treated with physiological concentrations of ammonia. The cells show changes in a surface receptor protein called peripheral benzodiazepine receptor or PBR that reflect ammonia toxicity in these cultured brain cells. The mechanism by which ammonia causes swelling of these cells in culture appears mediated by PBR since blocking its activity prevents cell swelling. It is possible that this mechanism explains swelling in the whole brain during liver failure. This provides a target to reverse the deleterious effects of excess circulating blood levels of ammonia and reduce the expression of brain abnormalities. The information is important to researchers interested in preventive approaches to insure adequate brain function during critical periods of stress or disease. ARS-533s are no longer required; however, manuscripts should be reviewed by technical editor. 12/13/07

Technical Abstract: Hepatic encephalopathy (HE) is a major neurological complication in patients with severe liver disease. While the pathogenesis of HE is unclear, an elevated CNS ammonia level is believed to be a major etiological factor, and astrocytes appear to be the primary target of its toxicity. A notable feature of ammonia neurotoxicity is an upregulation of the mitochondrial peripheral benzodiazepine receptor (PBR) in cultured astrocytes as well as in vivo. The precise significance of this upregulation is unknown. To examine its role in ammonia neurotoxicity, we downregulated the PBR using antisense oliognucleotides. Since the PBR is a component of the mitochondrial permeability transition (MPT) pore, and induction of the MPT has been shown after ammonia treatment, we assessed its downregulation on ammonia-induced dissipitation of the mitochondrial inner membrane potential. A 40% decline in the membrane potential was observed in non-transfected cultures treated with NH4Cl (5 mM; 24 hr) when compared to controls (p<0.01). Treatment with antisense oligonucleotides completely reversed this decline in potential, while scrambled control antisense did not. A major consequence of ammonia toxicity is astrocyte swelling and ligands of the PBR have been shown to influence ammonia-induced astrocyte swelling. We, therefore, tested whether downregulation of the PBR would attenuate such cell swelling. Non-transfected cultures treated with NH4Cl (5 mM; 48 hr) showed a significant increase in astrocyte cell volume (39%), and this increment was significantly attenuated (by 50%) in antisense-treated cultures. Our findings indicate that ammonia-induced dissipation of the membrane potential and cell swelling are mediated, at least in part, by the PBR suggesting that the PBR plays an important role in some of the deleterious effects of ammonia on astrocytes.