Inhibitors of the mitochondrial permeability transition reduce ammonia‐induced cell swelling in cultured astrocytes

Abstract

Ammonia is the principal neurotoxin implicated in the pathogenesis of hepatic encephalopathy, and astrocytes are the neural cells predominantly affected in this condition. Astrocyte swelling (cytotoxic edema) represents a critical component of the brain edema in acute form of hepatic encephalopathy (acute liver failure, ALF). Although mechanisms of astrocyte swelling by ammonia are not completely understood, cultured astrocytes exposed to pathophysiological levels of ammonia develop the mitochondrial permeability transition (mPT), a process that was shown to result in astrocyte swelling. Cyclosporin A (CsA), a traditional inhibitor of the mPT, was previously shown to completely block ammonia-induced astrocyte swelling in culture. However, the efficacy of CsA to protect cytotoxic brain edema in ALF is problematic because it poorly crosses the blood-brain barrier, which is relatively intact in ALF. We therefore examined the effect of agents that block the mPT but are also known to cross the blood-brain barrier, including pyruvate, magnesium, minocycline, and trifluoperazine on the ammonia-induced mPT, as well as cell swelling. Cultured astrocytes exposed to ammonia for 24 hr displayed the mPT as demonstrated by a CsA-sensitive dissipation of the mitochondrial inner membrane potential. Pyruvate, minocycline, magnesium, and trifluoperazine significantly blocked the ammonia-induced mPT. Ammonia resulted in a significant increase in cell volume, which was blocked by the above-mentioned agents to a variable degree. A regression analysis indicated a high correlation between the effectiveness of reducing the mPT and cell swelling. Our data suggest that all these agents have therapeutic potential in mitigating brain edema in ALF.

Interaction of oxidative stress, astrocyte swelling and cerebral ammonia toxicity

“…From this it was concluded that mitochondrial glutamine uptake and subsequent cleavage of glutamine by glutaminase elevates mitochondrial ammonia levels, which in turn stimulates ROS production via induction of the mitochondrial permeability transition (MPT) [22,23]. Ammonia-induced astrocyte swelling was considered as a consequence of mitochondrial glutamine uptake, glutamine cleavage, MPT induction and ROS formation [24 ]. However, this view is difficult to reconcile with the fact that ammonia in cultured astrocytes induces an almost instantaneous ROS/RNS production and astrocyte swelling [12,15], whereas MPT-induction and glutamine accumulation occur thereafter [25,26].…”

Ammonia production, excretion, toxicity, and defense in fish: a review

“…Interests in studying ammonia toxicity in mammals arise from the fact that liver failure in human leads to the development of neurological abnormalities collectively referred to as hepatic encephalopathy, and ammonia is a key neurotoxin implicated in this condition (see Häussinger and Schliess, 2008; Lemberg and Fernandez, 2009 for reviews). More recent studies in mammals have implicated oxidative stress and mitochondrial permeability transition in the mechanism of ammonia neurotoxicity (Norenberg et al, 2005; Jayakumar et al, 2006; Pichili et al, 2007; Reddy et al, 2009; Görg et al, 2010; Häussinger and Görg, 2010). The mitochondrial permeability transition involves the opening of a pore in the inner mitochondrial membrane that leads to a collapse of ionic gradients, resulting in mitochondrial dysfunction.…”

Diets in Encephalopathy

“…It is the astrocyte (the glial cells of the central nervous system) that is pivotal in providing adequate nutrition to neurons. 2 Fig. 1 shows the metabolism of ammonia and its role in HE and inflammation.…”