3-Methylcrotonyl-CoA Carboxylase Expression Among Astrocytes and Neurons in the Human Brain and the Effect of Hyperglycemia on the Catabolic Flux of 13C6, 15N-Leucine in Cultured Astrocytes.

Leucine is an essential amino acid which is imported into the brain parenchyma with high capacity. Animal studies have demonstrated that leucine plays a significant role in several cellular and physiological processes in brain parenchyma. In addition to its role in protein synthesis, leucine possesses signaling and regulatory functions. Furthermore, leucine catabolism may provide brain cells with amino nitrogen for the synthesis of glutamate and glutamine with an impact on sustaining glutamatergic and GABA-ergic neurotransmission. The entry of leucine's carbon skeleton into the intermediary metabolism of astrocytes yields the production of ketone bodies and acetyl-CoA. In order to investigate the metabolic capabilities of human astrocytes regarding leucine, we enriched their culture media with 13C₆,15N-leucine and conducted a metabolomic study using liquid chromatography-mass spectrometry (LC-MS) to identify and quantify isotopically labelled metabolites. Furthermore, we employed an antiserum against 3-methylcrotonyl-CoA carboxylase (MCC), the unique enzyme in the irreversible phase of leucine catabolism, to identify MCC-expressing cells both in culture and in situ. Our results indicate that cultured human astrocytes efficiently removed leucine from the medium, which was then enriched with several compounds containing nitrogen and/or carbon atoms derived from leucine. Among the released metabolites, glutamine and citrate were the most abundant. Leucine uptake was independent of glucose concentration; however, hyperglycemic conditions stimulated the capacity for the irreversible catabolism of the leucine-derived carbon skeleton. Immunoprobing with the MCC antiserum confirmed the mitochondrial expression of MCC in astrocytes in culture as well as in situ. In addition to astrocytes, immunofluorescent double-labelling revealed the colocalization of MCC with a neuronal marker in human brain sections. This study confirms that human astrocytes are capable of catabolizing leucine and incorporating leucine-derived atoms into the intermediary metabolism. The presence of MCC in cultured astrocytes underscores their ability to convert leucine into acetyl-CoA and ketone bodies. Additionally, MCC expression in astrocytes and neurons present in brain parenchyma suggests that these cells are enzymatically equipped to catabolize leucine into compounds entering their cellular metabolism.