Glutamate Dehydrogenase Is Important for Ammonia Fixation and Amino Acid Homeostasis in Brain During Hyperammonemia.

Caroline M Voss,Lene Arildsen,Jakob D Nissen,Peter Ott,Helle S Waagepetersen,Arne Schousboe,Anne B Walls,Pierre Maechler,Hendrik Vilstrup

doi:10.3389/fnins.2021.646291

Abstract

Impaired liver function may lead to hyperammonemia and risk for hepatic encephalopathy. In brain, detoxification of ammonia is mediated mainly by glutamine synthetase (GS) in astrocytes. This requires a continuous de novo synthesis of glutamate, likely involving the action of both pyruvate carboxylase (PC) and glutamate dehydrogenase (GDH). An increased PC activity upon ammonia exposure and the importance of PC activity for glutamine synthesis has previously been demonstrated while the importance of GDH for generation of glutamate as precursor for glutamine synthesis has received little attention. We therefore investigated the functional importance of GDH for brain metabolism during hyperammonemia. To this end, brain slices were acutely isolated from transgenic CNS-specific GDH null or litter mate control mice and incubated in aCSF containing [U-13C]glucose in the absence or presence of 1 or 5 mM ammonia. In another set of experiments, brain slices were incubated in aCSF containing 1 or 5 mM 15N-labeled NH4Cl and 5 mM unlabeled glucose. Tissue extracts were analyzed for isotopic labeling in metabolites and for total amounts of amino acids. As a novel finding, we reveal a central importance of GDH function for cerebral ammonia fixation and as a prerequisite for de novo synthesis of glutamate and glutamine during hyperammonemia. Moreover, we demonstrated an important role of the concerted action of GDH and alanine aminotransferase in hyperammonemia; the products alanine and α-ketoglutarate serve as an ammonia sink and as a substrate for ammonia fixation via GDH, respectively. The role of this mechanism in human hyperammonemic states remains to be studied.

Highlights

Ammonia is a product of protein and amino acid metabolism and in the healthy liver ammonia is irreversibly removed by formation of urea which is subsequently excreted in the urine
The amount of alanine was significantly increased in hippocampal slices from CNSGlud1−/− mice while the contents of glutamate, glutamine, aspartate, and GABA were similar in the two genotypes (Table 1)
We have demonstrated a prominent role of glutamate dehydrogenase (GDH) for ammonia fixation during hyperammonemia

Summary

Introduction

Ammonia is a product of protein and amino acid metabolism and in the healthy liver ammonia is irreversibly removed by formation of urea which is subsequently excreted in the urine. Decreased liver function leads to impaired urea synthesis capacity (Vilstrup, 1984) and when porto-systemic shunts are present, ammonia-rich blood from the intestines bypasses the liver. These disturbances result in systemic and cerebral hyperammonemia, associated with a risk for hepatic encephalopathy, Brain Glutamate Dehydrogenase During Hyperammonemia which is the most deleterious complication to liver diseases. Ammonia is toxic to the brain through mechanisms involving secondarily disturbed metabolism and eventually neurotransmission, manifesting as hepatic encephalopathy This is a neuropsychiatric syndrome, which occurs as a complication to acute and chronic liver failure, with symptoms ranging from mild cognitive impairment to coma (Vilstrup et al, 2014). With very high ammonia concentrations in acute liver failure there is a high risk for cerebral herniation (Ott et al, 2005)

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