RGS7 and -11 complexes accelerate the ON-bipolar cell light response

The visual ON pathway, which responds to increases in light intensity, originates at the first retinal synapse between rod and cone photoreceptors and ON-bipolar cells (ON-BPCs). Light stimulation causes photoreceptors to hyperpolarize, thereby reducing the rate of glutamate release into the synaptic cleft. ON-BPCs depolarize in response to the light-induced decrease in synaptic glutamate. This unusual sign-inverting synaptic response is mediated by a metabotropic glutamate receptor, mGluR6,1 which is negatively coupled to the activation of a TRPM1-containing cation channel via the G protein Go.2–4 Deactivation of mGluR6 leads to depolarization within 100 ms,5 allowing this signaling cascade to maintain a high degree of temporal resolution. The kinetics of most heterotrimeric G protein-based signaling cascades are limited by the rate of GTP hydrolysis by the G protein α subunit. Generally, the response time is shortened by the GTPase accelerating activity provided by regulators of G protein signaling (RGS) proteins.6 Based on time constraints, RGS activity would seem to be required for the ON-BPC light response, since the intrinsic rate of GTP hydrolysis by Gαo is slow, requiring tens of seconds.7,8 The situation is similar for phototransduction in the photoreceptor outer segments. In this case, GTP hydrolysis by transducin is accelerated by interaction with RGS9–1, a member of the R7 subfamily of RGS proteins, in conjunction with its obligate binding partner Gβ5. The RGS9–1/Gβ5 complex is tethered to the disc membrane by association with a third protein, R9AP.9 Rods from transgenic mice lacking RGS9–1,10 Gβ5,11 or R9AP12 show normal activation of the photoresponse, but a profoundly slowed time course of deactivation. RGS9 is one member of a large family of more than 30 RGS proteins.13 The defining structural element of an RGS protein is the RGS domain, a conserved ∼120 amino acid sequence, through which it interacts with Gα subunits to stimulate GTP hydrolysis.14 RGS proteins in the R7 subfamily, which includes RGS6, -7, -9, and -11,15 are characterized by a G protein γ subunit-like (GGL) domain, which facilitates interaction with a unique G protein beta subunit, Gβ5. The R7 RGS proteins also contain a disheveled/EGL-10/pleckstrin homology (DEP) domain through which they can bind to G protein-coupled receptors,16 as well as the membrane-anchoring proteins, R7BP and R9AP.9,17–20 We and others have previously identified two RGS proteins of the R7 subfamily, RGS7 and -11, that co-localize with mGluR6 in the tips of ON-BPC dendrites,21–24 where they are poised to play a role in the mGluR6-based signaling pathway. In this study, we investigated the role of RGS7 and -11 complexes in the mGluR6 signal transduction pathway in mouse ON-BPCs. We compared ON-BPC responses between wild-type (WT) mice and genetically modified mice lacking RGS11 (RGS11−/−), or with a deletion mutation in RGS7 (RGS7Δ/Δ), or both. The targeted mutation in RGS7 is a deletion of exon 11, which encodes 33 amino acids spanning an interdomain region of unknown function and the first 9 amino acids of the GGL domain.25 The functionality of this deletion mutant remains to be determined. We report that the absence of RGS11 results in a delayed onset of ON-BPC light responses and increased photopic amplitudes compared with WT mice. The RGS7 deletion alone has no measurable effect on the ON-BPC responses, but it enhances the effect of the lack of RGS11 in RGS7Δ/Δ/RGS11−/− double-mutant mice.