Regulation of neurite morphogenesis by interaction between R7 regulator of G protein signaling complexes and G protein subunit Gα13

The R7 regulator of G protein signaling family (R7-RGS) critically regulates nervous system development and function. Mice lacking all R7-RGS subtypes exhibit diverse neurological phenotypes, and humans bearing mutations in the retinal R7-RGS isoform RGS9-1 have vision deficits. Although each R7-RGS subtype forms heterotrimeric complexes with G beta(5) and R7-RGS-binding protein (R7BP) that regulate G protein-coupled receptor signaling by accelerating deactivation of G(i/o) alpha-subunits, several neurological phenotypes of R7-RGS knock-out mice are not readily explained by dysregulated G(i/o) signaling. Accordingly, we used tandem affinity purification and LC-MS/MS to search for novel proteins that interact with R7-RGS heterotrimers in the mouse brain. Among several proteins detected, we focused on G alpha(13) because it had not been linked to R7-RGS complexes before. Split-luciferase complementation assays indicated that G alpha(13) in its active or inactive state interacts with R7-RGS heterotrimers containing any R7-RGS isoform. LARG (leukemia-associated Rho guanine nucleotide exchange factor (GEF)), PDZ-RhoGEF, and p115RhoGEF augmented interaction between activated G alpha(13) and R7-RGS heterotrimers, indicating that these effector RhoGEFs can engage G alpha(13).R7-RGS complexes. Because G alpha(13)/R7-RGS interaction required R7BP, we analyzed phenotypes of neuronal cell lines expressing RGS7 and G beta(5) with or without R7BP. We found that neurite retraction evoked by G alpha(12/13)-dependent lysophosphatidic acid receptors was augmented in R7BP-expressing cells. R7BP expression blunted neurite formation evoked by serum starvation by signaling mechanisms involving G alpha(12/13) but not G alpha(i/o). These findings provide the first evidence that R7-RGS heterotrimers interact with G alpha(13) to augment signaling pathways that regulate neurite morphogenesis. This mechanism expands the diversity of functions whereby R7-RGS complexes regulate critical aspects of nervous system development and function.