Introduction: The liver plays a major role in regulating ammonia levels in the blood. During liver disease, loss of hepatic function leads to hyperammonemia and consequently neurotoxicity and hepatic encephalopathy. Ammonia (NH₄⁺ and NH₃) easily crosses cell membranes, including the blood brain barrier (BBB); the interface between the blood and the brain. Glutamine synthetase (GS), an enzyme which amidates glutamate to glutamine, removing ammonia, plays an important compensatory role during liver disease. GS is expressed in muscle and the brain (primarily in astrocytes) but ammonia metabolism in endothelial cells (EC) has never been thoroughly explored.
Methods: Using primary rat brain microvascular endothelial cells, GS detection was performed by rtPCR, western blot, immunohistochemistry and activity assay . In addition we evaluated GS expression in cerebral microvessels (CMV) from frontal cortex of naive rats by immunohistochemistry and western blot.
Results: ECs expressed GS mRNA, protein and activity, although less intense compared to muscle and liver (controls). In vivo results showed GS in vessels of brain slices (co-localized with caveolin ) and in CMV-enriched lysate.
Discussion: We demonstrate the presence of GS in endothelial cells both in vivo and in vitro. We speculate GS in ECs could act as a first barrier against ammonia neurotoxicity and play a compensatory role in liver disease. However, the latter has yet to be investigated. We anticipate GS in EC’s has the potential to become a new therapeutic target for treating hyperammonemia in liver disease and other hyperammonemic syndromes.
Project financed by CIHR
MO financed by MITACS, Université de Montréal