Dissecting Newly Discovered Corneal Glutamine and Ammonia Metabolism Pathways

Title Dissecting Newly Discovered Corneal Glutamine and Ammonia Metabolism Pathways
Author, Co-Author Wenlin Zhang, Shimin Li, Joseph Bonanno
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Abstract We recently discovered that corneal endothelium (CE) can utilize glutamine as an alternative energy source for ATP production (Zhang, W, ARVO E-Abstract 2578, 2015). In fact, corneal endothelium ATP levels can be sustained by glutamine alone. These results provided novel insights to optimizing corneal graft storage medium and the pathogenesis of a number of Endothelial Corneal Dystrophies. In this study, we used RT-PCR to compare enzymes and transporters involved in glutamine metabolism in CE, in order to examine the possible route of glutamine consumption. In a human CE cell line, several glutamine transporters were expressed including: SLC1A5, SLC6A19, SLC7A5, SLC7A8, SLC38A1, SLC38A2, SLC38A3, SLC38A5, SLC38A7 and SLC38A8. Na+-glutamine co-transporters (SLC1A5, SLC38A1), glutamine/large neutral amino acids antiporter (SLC7A5) and Na+-glutamine/H+ antiporter had the highest expression. Human CE tissue mitochondrial glutaminase (GLS1 and GLS2) show significantly higher expression than the membrane glutaminase equivalent g-glutamyl-transpeptidase. Taken together this indicates that human CE take up glutamine directly from the aqueous humor into mitochondria for deamination and further metabolism. Mouse corneal endothelial cells show a similar expression pattern. Ammonia is the major byproduct of glutamine metabolism, so we also examined ammonia transporter/channel expression in mouse corneal epithelium, keratocytes and endothelium. Ammonia transporter SLC4A11 is expressed in all three corneal layers, whereas ammonia channel AQP8 was only expressed in keratocytes and endothelium consistent with mitochondrial abundance in these cell types. Interestingly, keratocytes also express glutamine synthase, a putative ammonia-detoxifying enzyme. This suggests a metabolic stromal-endothelium interaction so that ammonia produced in CE can be immediately detoxified by stromal keratocytes.
Affiliation of Co-Authors Indiana University, Indiana Univeristy