The Sushi Domains of GABA_BReceptors Function as Axonal Targeting Signals

Abstract

GABA B receptors are the G-protein-coupled receptors for GABA, the main inhibitory neurotransmitter in the brain. Two receptor subtypes, GABA B(1a,2) and GABA B(1b,2) , are formed by the assembly of GABA B1a and GABA B1b subunits with GABA B2 subunits. The GABA B1b subunit is a shorter isoform of the GABA B1a subunit lacking two N-terminal protein interaction motifs, the sushi domains. Selectively GABA B1a protein traffics into the axons of glutamatergic neurons, whereas both the GABA B1a and GABA B1b proteins traffic into the dendrites. The mechanism(s) and targeting signal(s) responsible for the selective trafficking of GABA B1a protein into axons are unknown. Here, we provide evidence that the sushi domains are axonal targeting signals that redirect GABA B1a protein from its default dendritic localization to axons. Specifically, we show that mutations in the sushi domains preventing protein interactions preclude axonal localization of GABA B1a . When fused to CD8␣, the sushi domains polarize this uniformly distributed protein to axons. Likewise, when fused to mGluR1a the sushi domains redirect this somatodendritic protein to axons, showing that the sushi domains can override dendritic targeting information in a heterologous protein. Cell surface expression of the sushi domains is not required for axonal localization of GABA B1a . Altogether, our findings are consistent with the sushi domains functioning as axonal targeting signals by interacting with axonally bound proteins along intracellular sorting pathways. Our data provide a mechanistic explanation for the selective trafficking of GABA B(1a,2) receptors into axons while at the same time identifying a well defined axonal delivery module that can be used as an experimental tool.

Biphasic Effects of Cannabinoids in Anxiety Responses: CB1 and GABAB Receptors in the Balance of GABAergic and Glutamatergic Neurotransmission

“…Additionally, GABA B receptors on glutamatergic terminals were shown to reduce glutamatergic transmission in the hippocampus (Gassmann et al, 2004). Further investigations confirmed the presynaptic localization of a considerable fraction of the GABA B receptors formed by GABA B1a and GABA B2 subunits (Biermann et al, 2010). Remarkably, genetic deletion of GABA B receptors leads to an anxiogenic phenotype.…”

Sushi domains confer distinct trafficking profiles on GABA B receptors

“…The SDs form an axonal targeting signal for the preferential transport of R1aR2 receptors, although R1aR2 and R1bR2 receptors can also be found in the cell soma and dendrites (19). The interaction partners that are important for axonal targeting have not yet been identified.…”

“…GIRK cells and hippocampal neurons (14)(15)(16)(17)(18)(19)(20)(21) were imaged and analyzed as described previously (20) using ImageJ (version 1.40g) and Origin (version 6; OriginLab).…”

“…These heteromeric subtypes exhibit different subcellular compartmentalization with the SDs acting as an axonal-targeting sequence to deliver R1aR2 more efficiently to axons compared with R1bR2 (19). This would, in part, explain the differential pre-and postsynaptic signaling properties of the two subtypes of the GABA B receptor.…”

Hippocampal GABAB(1a) Receptors Constrain Generalized Contextual Fear

“…GABA A receptors are ionotropic, fast acting, and primarily postsynaptic (Enna, 2007;Rudolph et al, 2001;Sieghart, 2006), whereas GABA B receptors are metabotropic receptors formed by heterodimers of GABA B(1) and GABA B(2) subunits (Jacobson et al, 2007b). Within the GABA B1 family, receptors can be classified as GABA B(1a) or GABA B(1b) based on the presence of two sushi domains within the N-terminus (Biermann et al, 2010;Gassmann and Bettler, 2012). GABA B(1b) is predominately postsynaptic whereas the GABA B(1a) subunit is predominately presynaptic and located on glutamatergic terminals (Jacobson et al, 2007b), contributing to presynaptic inhibition of glutamate release.…”