Research On The Mechanism Of GABA_B Receptor Coupling To G₁₃ Protein To Regulate Synaptic Function And Apoptosis

γ-aminobutyric acid（GABA）is one of the main inhibitory neurotransmitters in the central nervous system,and its metabotropic receptor GABA_B receptor belongs to class C G protein-coupled receptor（GPCR）.GABA_B receptor is abundantly expressed throughout the brain in both pre-and post-synaptic compartments where they mediate slow and prolonged inhibition of synaptic transmission to modulate various brain functions.The agonists of GABA_B receptor are in use to treat spasms,alcohol addition and cataplexy associated with narcolepsy.GABA_B receptor is considered to be an attractive drug target for neurological and psychiatric disorders.Therefore,the research on the signaling pathways involved in GABA_B receptor is particularly important.Previous studies believed that GABA_B receptor acts exclusively through Gi/o protein.Through activation of Gi/o protein,GABA_B receptor regulates mitogen-activated protein kinase（MAPK）signaling,such as extracellular signal-regulated protein kinase 1/2（ERK1/2）.However,whether GABA_B receptor triggers signaling through the activation of other MAPKs such as c-Jun N-terminal kinase（JNK）remains unknown.GABA_B receptor couples to Gi/o protein can also induce phosphorylation of various downstream signaling molecules,including type 1 insulin-like growth factor receptor（IGF-1R）,cyclic adenosine monophosphate response element binding protein,focal adhesion kinase,Src kinase,protein kinase B,protein kinase C,and calmodulin-dependent protein kinase Ⅱ,which are critical for GABA_B receptor-mediated neuroprotection,up-regulation of fragile X mental retardation protein and long-term synaptic plasticity.Thus,in addition to Gi/o protein,whether GABA_B receptor actives other types of G protein is also the focus of the present reseach.To systematically reveal the MAPK signaling network mediated by GABA_B receptor and the cell functions involved,cerebellar granule neurons were used as the cell model in this study.And the unique mechanism of which GABA_B receptor activates the MAPK pathway was discovered: The GABA_B receptor,which has been thought to couple to Gi/o protein,can also activates two small guanosine triphosphatases Rho A and Rac1 by coupling to G₁₃ protein,thereby activates JNK.Further research found that GABA_B receptor agonists activate G₁₃ with slower kinetics and lower potency than Gi/o.It is suggested that GABA_B receptor couples to two types of G proteins in different modes.In deepth,the G₁₃/JNK pathway is shown to promote phosphorylation and accumulation of the scaffold protein PSD95 in the postsynaptic density（PSD）region to affect synaptic function.This pathway is also shown to be necessary for GABA_B receptor-mediated neuroprotection in granule neurons,acting then in synergy with the Gi/o/IGF-1R pathway that previously reported to co-regulate cell apoptosis.In summary,the present reseach analyzed the regulation mechanism of GABA_B receptor on JNK MAPK pathway,confirmed the important role of G₁₃ protein in it,and revealed the key role of G₁₃/JNK pathway on specific cell functions.It complements the signaling network related to GABA_B receptor,provides new clues for a better understanding of the various functions of GABA_B receptor in the brain,and opens up new directions for related drugs development.