Glutamine as a mediator of ammonia neurotoxicity: A critical appraisal

Abstract

Ammonia is a major neurotoxin implicated in hepatic encephalopathy (HE). Here we discuss evidence that many aspects of ammonia toxicity in HE-affected brain are mediated by glutamine (Gln), synthesized in excess from ammonia and glutamate by glutamine synthetase (GS), an astrocytic enzyme. The degree to which Gln is increased in brains of patients with HE was found to positively correlate with the grade of HE. In animals with HE, a GS inhibitor, methionine sulfoximine (MSO), reversed a spectrum of manifestations of ammonia toxicity, including brain edema and increased intracranial pressure, even though MSO itself increased brain ammonia levels. MSO inhibited, while incubation with Gln reproduced the oxidative stress and cell swelling observed in ammonia-exposed cultured astrocytes. Recent studies have shown that astrocytes swell subsequent to Gln transport into mitochondria and its degradation back to ammonia, which then generates reactive oxygen species and the mitochondrial permeability transition. This sequence of events led to the formulation of the "Trojan Horse" hypothesis. Further verification of the role of Gln in the pathogenesis of HE will have to account for: (1) modification of the effects of Gln by interaction of astrocytes with other CNS cells; and (2) direct effects of Gln on these cells. Recent studies have demonstrated a "Trojan Horse"-like effect of Gln in microglia, as well as an interference by Gln with the activation of the NMDA/NO/cGMP pathway by ammonia as measured in whole brain, a process that likely also involves neurons.

Fig. 1

Major steps of Gln metabolism and transport in the CNS which may contribute to, or modulate, Gln-mediated ammonia toxicity. Ammonia (NH₄⁺) is incorporated (“detoxified”) into glutamine (Gln) in astrocytes (and perhaps microglia) by the amidation of glutamate (Glu) to glutamine (Gln), a process catalyzed by glutamine synthase (GS). Gln accumulated in astrocytes i) crosses the mitochondrial membrane and is hydrolyzed by phosphate-activated glutaminase (PAG) to glutamate (Glu) and NH4⁺, and ii) exits astrocytes preferably by the system N (the SNAT3 transporter) which is controlled by extracellular Glu, and partly by system L in exchange for large neutral amino acids (NAA). The Gln that leaves astrocytes enters neurons mainly via system A to be converted to the amino acid neurotransmitters Glu and GABA, and possibly by system y⁺L (the y⁺LAT2 transporter) in exchange of Arg, which serves the purpose of modulating the availability of Arg for NO synthesis and thus may influence the rate of the NO/cGMP pathway. A portion of Gln derived from astrocytes leaves the CNS via the L and N systems that are localized in capillary endothelial cells that forms the blood-brain barrier, some of the Gln may enter the microglial cells and modulate their function.