Glutamate-Induced Morphological And Functional Modulation To Mitochondria In Astrocytes

Astrocytes are broadly distributed in the central nervous system,they play important roles in glucose transport and metabolism,neurotransmitter uptake and recycling,extracellular environment maintenance and capillary regulation in the brain,and even the formation and maintenance of synapses.As the chief energy factory of the cells,mitochondria undergo fusion and fission as a norm.Maintaining a proper balance between the fusion and fission is pivotal to the normal morphology,distribution and function of the mitochondria.Excessive deviations from the balance between mitochondrial fusion and fission have been linked to severe neurodegenerative diseases.The proteins responsible for mitochondrial fusion and fission events have rather obvious characteristics,and are primarily located in the inner mitochondrial membrane(IMM)and outer mitochondrial membrane(OMM).The morphology and function of mitochondria are affected by many aspects,including the redox status,calcium ions and metabolic substrates.Notably,status of metabolite supply and the efficiency of ATP synthesis can also affect the dynamic of mitochondrial fusion and fission.Objective Glutamate is the most important excitatory neurotransmitter in the central nervous system.Astrocytes regulate extracellular glutamate concentration mainly through Na⁺-dependent glutamate transporters uptake which is highly sensitive to energy supply.Transported glutamate is converted to glutamine or catabolized to?-ketoglutarate to enter the tricarboxylic acid cycle.The metabolism of glutamate and its potential regulatory function have long been overshadowed by its prominent role as a neurotransmitter.Our study found that exposure to glutamate can promote mitochondrial fusion in astrocytes,increase ATP synthesis and astrocytic resistance to oxidative stress.In this study we explored the effects and significances of these morphological and functional regulations and the underlying mechanisms.Method On cultured primary astrocytes,glutamate was applied alone or in the presence of its structural analogues,glutamate receptor antagonists,glutamate transporter inhibitor and metabolic enzyme inhibitors,to study their effects on the mitochondrial morphology.Mitochondrial were labeled by immunofluorescence or fluorescent Mito-tracker,with the length of mitochondria semi-quantitatively analyzed by Image J software.ATP contents were measured as a key indicator of mitochondrial function to better explore the links between glutamate regulation of mitochondria and the roles of glutamate receptor activation,glutamate transport and various metabolic processes in astrocytes.Results(1)Exogenous glutamate stimulation significantly increased the degree of mitochondrial fusion and ATP synthesis in astrocytes,in a time-dependent manner;(2)Hydrogen peroxide(H₂O₂)stimulation led to fragmentation of mitochondria,while pretreatment of glutamate significantly enhanced mitochondrial resistance to H₂O₂;(3)Glutamate receptor antagonists could not block the glutamate-induced mitochondrial fusion;(4)Inhibition of glutamate transport eliminated the glutamate-induced mitochondrial fusion;(5)Application of exogenous glutamine and inhibition of glutamine synthesis had little effect on the mitochondrial fusion in astrocytes.Conclusion Glutamate is not only the main excitatory neurotransmitter in the central nervous system,but also an important metabolic substrate capable of regulating the morphology and function of mitochondria in astrocytes.