Ramirez O, Vidal R, Tello J, Vargas K, Kindler S, Härtel S & A Couve.
J Biol Chem. May 8;284(19):13077-85. [Epub 2009 Mar 10]
Understanding the mechanisms that control synaptic efficacy through the availability of neurotransmitter receptors depends on uncovering their specific intracellular trafficking routes. Gamma-Aminobutyric acid type B (GABAB) receptors (GABABRs) are obligatory heteromers present at dendritic excitatory and inhibitory postsynaptic sites. It is unknown whether synthesis and assembly of GABABRs occur in the somatic endoplasmic reticulum (ER) followed by vesicular transport to dendrites or whether somatic synthesis is followed by independent transport of the subunits for assembly and ER export throughout the somatodendritic compartment. To discriminate between these possibilities we studied the association of GABABR subunits in dendrites of hippocampal neurons combining live fluorescence microscopy, biochemistry, quantitative colocalization, and bimolecular fluorescent complementation. We demonstrate that GABABR subunits are segregated and differentially mobile in dendritic intracellular compartments and that a high proportion of non-associated intracellular subunits exist in the brain. Assembled heteromers are preferentially located at the plasma membrane, but blockade of ER exit results in their intracellular accumulation in the cell body and dendrites. We propose that GABABR subunits assemble in the ER and are exported from the ER throughout the neuron prior to insertion at the plasmamembrane. Our results are consistent with a bulk flow of segregated subunits through the ER and rule out a post-Golgi vesicular transport of preassembled GABABRs.
Acknowledgments We thank C. Morales, M. Calegaro, L. Sandoval for technical assistance and A. Buonanno for carefully reading the manuscript. O.A.R and R.L.V. funded by MECESUP and CONICYT respectively. S.H. funded by FONDECYT 1060890. A.C funded by FONDECYT 1071001, ICM P07-048-F and HFSPO ST00105/2005-C.