Primary Research On Trafficking Pathway Of GABA_B Receptor

GABAB receptor is a major inhibitory neurotransmitter receptor in the central nervous systenm.This receptor is found in either pre- or postsynaptic elements in various types of neurons. As such, GABAB receptors play important role in brain function and its involvement in various types of epilepsy as well as in nociception and drug addiction. GABAB receptors belongs to the the classIII GPCRs,together with metabotropic glutamate(mGlu),extracellular Ca2+-sensing, and some pheromone and taste receptor. Whereas mGlu and Ca2+-sensing receptors exist as homodimers, the GABAB receptor is a heterodimer composed of two homologous subunits, GABAB1(GB1) and GABAB2(GB2). So far, only the heterodimeric form of the GABAB receptor has been shown to activate G-proteins efficiently. Although only GB1 binds GABA, GB2 masks an intracellular retention signal of GB1 such that GB1 reaches the cell surface only when associated with GB2. GB2 HD contains all the determinants required for G-protein coupling and plays a pivotal role in G-protein activation by the heterodimer.Cell surface expression of transmembrane protein is strictly regulated。The trans-Golgi network is a key station that mediates sorting of proteins into distinct transport pathway. Subsequent movement of proteins to the plasma membrane appear to exist not only small, pleiomorphic transport containers, but also relatively large tubular-saccular carriers that travel along cytoskeletal tracks. This production and movement of these membranous structures are typiclly described as constitutive. This regulation is thought to underlie inportant biological phenomena such as synaptic plasticity. Therefore, GABAB receptor provide a wonderful model to explore the molecular mechanism of cellular transport in relative to the homodimer.In this study, the mobility of GB2-GFP and GB1-GFP+GB2-WT which comes from cerebellum granule cells is compared. Evanescent-field microscopy and Gaussian-fit-based single particle tracking were used to follow the three-dimensional trajectories of single particle. By fitting appropriate equations to particular mean squared displacement of single vesicles, three different motion modes were revealed for both of them. Quantitative analysis showed that the number of GB2-GFP undergoing random diffusion and directed diffusion was much more thant GB1-GFP+GB2-WT. Furthermore, it was compared that three-dimensional diffusion coefficients for them. The median diffusion coefficient of GB2-GFP is 2.605±1.3729-3μm2/s and GB1-GFP+GB2 is 4.88±0.9375×10-3μm 2/s. Although the precise molecular mechanism of GABAB receptor remains unclear, our results reveal a significant difference between them in their mobility, suggesting there are different molecular mechanisms underlying the intracellular transport of them.