A RhoA Signaling Pathway Regulates Dendritic Golgi Outpost Formation.

The neuronal Golgi apparatus (GA) localizes to the perinuclear region and dendrites as tubulo-vesicular structures designated Golgi outposts (GOPs). Current evidence suggests that GOPs shape dendrite morphology and serve as platforms for the local delivery of synaptic receptors. However, the mechanisms underlying GOP formation remain a mystery. Using live-cell imaging and confocal microscopy in cultured hippocampal neurons, we now show that GOPs destined to major "apical" dendrites are generated from the somatic GA by a sequence of events involving: (1) generation of a GA-derived tubule; (2) tubule elongation and deployment into the dendrite; (3) tubule fission; and (4) transport and condensation of the fissioned tubule. A RhoA-Rock signaling pathway involving LIMK1, PKD1, slingshot, cofilin, and dynamin regulates polarized GOP formation by controlling the tubule fission. Our observations identify a mechanism underlying polarized GOP biogenesis and provide new insights regarding involvement of RhoA in dendritic development and polarization.

Transport along the dendritic endoplasmic reticulum mediates the trafficking of GABAB receptors.

In neurons, secretory organelles within the cell body are complemented by the dendritic endoplasmic reticulum (ER) and Golgi outposts (GOPs), whose role in neurotransmitter receptor trafficking is poorly understood. γ-aminobutyric acid (GABA) type B metabotropic receptors (GABABRs) regulate the efficacy of synaptic transmission throughout the brain. Their plasma membrane availability is controlled by mechanisms involving an ER retention motif and assembly-dependent ER export. Thus, they constitute an ideal molecular model to study ER trafficking, but the extent to which the dendritic ER participates in GABABR biosynthesis has not been thoroughly explored. Here, we show that GABAB1 localizes preferentially to the ER in dendrites and moves long distances within this compartment. Not only diffusion but also microtubule and dynein-dependent mechanisms control dendritic ER transport. GABABRs insert throughout the somatodendritic plasma membrane but dendritic post-ER carriers containing GABABRs do not fuse selectively with GOPs. This study furthers our understanding of the spatial selectivity of neurotransmitter receptors for dendritic organelles.

Endoplasmic reticulum sorting and kinesin-1 command the targeting of axonal GABAB receptors.

In neuronal cells the intracellular trafficking machinery controls the availability of neurotransmitter receptors at the plasma membrane, which is a critical determinant of synaptic strength. Metabotropic γ amino-butyric acid (GABA) type B receptors (GABA(B)Rs) are neurotransmitter receptors that modulate synaptic transmission by mediating the slow and prolonged responses to GABA. GABA(B)Rs are obligatory heteromers constituted by two subunits, GABA(B)R1 and GABA(B)R2. GABA(B)R1a and GABA(B)R1b are the most abundant subunit variants. GABA(B)R1b is located in the somatodendritic domain whereas GABA(B)R1a is additionally targeted to the axon. Sushi domains located at the N-terminus of GABA(B)R1a constitute the only difference between both variants and are necessary and sufficient for axonal targeting. The precise targeting machinery and the organelles involved in sorting and transport have not been described. Here we demonstrate that GABA(B)Rs require the Golgi apparatus for plasma membrane delivery but that axonal sorting and targeting of GABA(B)R1a operate in a pre-Golgi compartment. In the axon GABA(B)R1a subunits are enriched in the endoplasmic reticulum (ER), and their dynamic behavior and colocalization with other secretory organelles like the ER-to-Golgi intermediate compartment (ERGIC) suggest that they employ a local secretory route. The transport of axonal GABA(B)R1a is microtubule-dependent and kinesin-1, a molecular motor of the kinesin family, determines axonal localization. Considering that progression of GABA(B)Rs through the secretory pathway is regulated by an ER retention motif our data contribute to understand the role of the axonal ER in non-canonical sorting and targeting of neurotransmitter receptors.