GABAB Receptor Subunit GB1 at the Cell Surface Independently Activates ERK1/2 through IGF-1R Transactivation

BACKGROUND: Functional GABA(B) receptor is believed to require hetero-dimerization between GABA(B1) (GB1) and GABA(B2) (GB2) subunits. The GB1 extracellular domain is required for ligand binding, and the GB2 trans-membrane domain is responsible for coupling to G proteins. Atypical GABA(B) receptor responses observed in GB2-deficient mice suggested that GB1 may have activity in the absence of GB2. However the underlying mechanisms remain poorly characterized. METHODOLOGY/PRINCIPAL FINDINGS: Here, by using cells overexpressing a GB1 mutant (GB1asa) with the ability to translocate to the cell surface in the absence of GB2, we show that GABA(B) receptor agonists, such as GABA and Baclofen, can induce ERK1/2 phosphorylation in the absence of GB2. Furthermore, we demonstrate that GB1asa induces ERK1/2 phosphorylation through Gi/o proteins and PLC dependent IGF-1R transactivation. CONCLUSIONS/SIGNIFICANCE: Our data suggest that GB1 may form a functional receptor at the cell surface in the absence of GB2

The intracellular loops of the GB2 subunit are crucial for G-protein coupling of the heteromeric gamma-aminobutyrate B

ABSTRACT The ␥-aminobutyrate B (GABA B ) receptor is the first discovered G-protein-coupled receptor (GPCR) that needs two subunits, GB1 and GB2, to form a functional receptor. The GB1 extracellular domain (ECD) binds GABA, and GB2 contains enough molecular determinants for G-protein activation. The precise role of the two subunits in G-protein coupling is investigated. GB1 and GB2 are structurally related to the metabotropic glutamate, Ca 2ϩ -sensing and other family 3 GPCRs in which the second (i2) as well as the third (i3) intracellular loop play important roles in G-protein coupling. Here, the role of the i2 loops of GB1 and GB2 in the GABA B receptor ability to activate G␣-proteins is investigated. To that aim, the i2 loops were swapped between GB1 and GB2 heptahelical domains (HDs), either in the wild-type subunits or in the chimeric subunits GB1/2 that contain the ECD of GB1 and the HD of GB2. The effect of an additional mutation within the i3 loop of GB2 that prevents coupling of the heteromeric receptor was also examined. Combinations of interest were found to be correctly addressed at the cell surface and to assemble into heteromers. Taken together our data revealed the following new information on the G-protein coupling of the heteromeric GABA B receptor: 1) the i2 loop of GB2 within the GB2 HD is required for the heteromeric GABA B receptor to couple to G-proteins, whereas the i2 loop of GB1 is not; 2) the presence of the i2 loop of GB2 within the GB1 HD is not sufficient to allow coupling of GB1; 3) the GB2 HD activates the Gqi9 protein whether it is associated with the GB2 or GB1 ECD; 4) in the combination with two GB2 HDs, each is able to couple to G-proteins; and finally, 5) the use of mutations in i2, i3, or both within the GB2 HD brings evidence for the absence of domain swapping enabling the exchange of region including i2 and i3 between the subunits

Gene expression profiling identifies sST2 as an effector of ErbB2-driven breast carcinoma cell motility, associated with metastasis

International audienceOverexpression of the ErbB2 receptor tyrosine kinase in breast cancer contributes to tumor development and is associated with poor prognosis. However, the mechanism by which ErbB2 might contribute to metastasis is not well defined. To identify genes that mediate ErbB2-driven cell motility, we performed differential gene expression analysis of ErbB2-expressing migrating breast cancer cells vs mutant ErbB2-expressing non-migrating cells. Among the genes that were specifically induced in migrating cells were known transcriptional targets of ErbB2, such as matrix metalloproteinases, and novel ErbB2 targets. Contribution of selected candidate genes to ErbB2-driven cell motility was tested by small interfering RNA targeting. Knockdown of the soluble form of ST2 (sST2), also called interleukin-1 receptor-like 1, one of the most robustly induced genes, decreased ErbB2-induced cell motility in two different cell lines. In response to ErbB2 activation, sST2 protein expression and secretion were increased. Moreover, recombinant sST2 associated with the plasma membrane and sST2-blocking antibodies reduced ErbB2-induced motility. Interestingly, cells from metastatic breast tumors secreted higher levels of sST2 than primary tumor cells. Finally, sST2 was found at high levels in the serum of metastatic breast cancer patients. Our data suggest that sST2 contributes to breast cancer cell motility and that sST2 secretion is associated with metastasis

C-terminal interaction is essential for surface trafficking but not for heteromeric assembly of GABA(b) receptors

Assembly of fully functional GABA(B) receptors requires heteromerization of the GABA(B(1)) and GABA(B(2)) subunits. It is thought that GABA(B(1)) and GABA(B(2)) undergo coiled-coil dimerization in their cytoplasmic C termini and that assembly is necessary to overcome GABA(B(1)) retention in the endoplasmatic reticulum (ER). We investigated the mechanism underlying GABA(B(1)) trafficking to the cell surface. We identified a signal, RSRR, proximal to the coiled-coil domain of GABA(B(1)) that when deleted or mutagenized allows for surface delivery in the absence of GABA(B(2)). A similar motif, RXR, was recently shown to function as an ER retention/retrieval (ERR/R) signal in K(ATP) channels, demonstrating that G-protein-coupled receptors (GPCRs) and ion channels use common mechanisms to control surface trafficking. A C-terminal fragment of GABA(B(2)) is able to mask the RSRR signal and to direct the GABA(B(1)) monomer to the cell surface, where it is functionally inert. This indicates that in the heteromer, GABA(B(2)) participates in coupling to the G-protein. Mutagenesis of the C-terminal coiled-coil domains in GABA(B(1)) and GABA(B(2)) supports the possibility that their interaction is involved in shielding the ERR/R signal. However, assembly of heteromeric GABA(B) receptors is possible in the absence of the C-terminal domains, indicating that coiled-coil interaction is not necessary for function. Rather than guaranteeing heterodimerization, as previously assumed, the coiled-coil structure appears to be important for export of the receptor complex from the secretory apparatus