Retina-specific GTPase accelerator RGS11/G beta 5S/R9AP is a constitutive heterotrimer selectively targeted to mGluR6 in ON-bipolar neurons

Macromolecular composition dictates receptor and G protein selectivity of regulator of G protein signaling (RGS) 7 and 9-2 protein complexes in living cells

Regulator of G protein signaling (RGS) proteins play essential roles in the regulation of signaling via G protein-coupled receptors (GPCRs). With hundreds of GPCRs and dozens of G proteins, it is important to understand how RGS regulates selective GPCR-G protein signaling. In neurons of the striatum, two RGS proteins, RGS7 and RGS9-2, regulate signaling by μ-opioid receptor (MOR) and dopamine D2 receptor (D2R) and are implicated in drug addiction, movement disorders, and nociception. Both proteins form trimeric complexes with the atypical G protein β subunit Gβ5 and a membrane anchor, R7BP. In this study, we examined GTPase-accelerating protein (GAP) activity as well as Gα and GPCR selectivity of RGS7 and RGS9-2 complexes in live cells using a bioluminescence resonance energy transfer-based assay that monitors dissociation of G protein subunits. We showed that RGS9-2/Gβ5 regulated both Gi and Go with a bias toward Go, but RGS7/Gβ5 could serve as a GAP only for Go. Interestingly, R7BP enhanced GAP activity of RGS7 and RGS9-2 toward Go and Gi and enabled RGS7 to regulate Gi signaling. Neither RGS7 nor RGS9-2 had any activity toward Gz, Gs, or Gq in the absence or presence of R7BP. We also observed no effect of GPCRs (MOR and D2R) on the G protein bias of R7 RGS proteins. However, the GAP activity of RGS9-2 showed a strong receptor preference for D2R over MOR. Finally, RGS7 displayed an four times greater GAP activity relative to RGS9-2. These findings illustrate the principles involved in establishing G protein and GPCR selectivity of striatal RGS proteins.

The relationship between slow photoresponse recovery rate and temporal resolution of vision

The rate at which photoreceptors recover from excitation is thought to be critical for setting the temporal resolution of vision. Indeed, mutations in RGS9 (regulator of G-protein signaling 9) and R9AP (RGS9 anchor protein) proteins mediating rapid photoresponse recovery impair patients' ability to see moving objects. In this study, we analyzed temporal properties of retinal sensitivity and spatiotemporal aspects of visual behavior in R9AP knock-out mice. Surprisingly, we have found that this knock-out does not affect dim-light vision mediated by rods acting as single-photon counters. Under these conditions, vision was also unaffected in mice overexpressing R9AP in rods, which causes accelerated photoresponse recovery. However, in brighter light, slow photoresponse recovery in rods and cones impaired visual responses to high temporal frequency stimuli, as reported for the daylight vision of human patients. Therefore, the speed of photoresponse recovery can affect temporal resolution and motion detection when photoreceptors integrate signals from multiple photons but not when they act as single-photon counters.

GIRK channel modulation by assembly with allosterically regulated RGS proteins

G-protein-activated inward-rectifying K(+) (GIRK) channels hyperpolarize neurons to inhibit synaptic transmission throughout the nervous system. By accelerating G-protein deactivation kinetics, the regulator of G-protein signaling (RGS) protein family modulates the timing of GIRK activity. Despite many investigations, whether RGS proteins modulate GIRK activity in neurons by mechanisms involving kinetic coupling, collision coupling, or macromolecular complex formation has remained unknown. Here we show that GIRK modulation occurs by channel assembly with R7-RGS/Gβ5 complexes under allosteric control of R7 RGS-binding protein (R7BP). Elimination of R7BP occludes the Gβ5 subunit that interacts with GIRK channels. R7BP-bound R7-RGS/Gβ5 complexes and Gβγ dimers interact noncompetitively with the intracellular domain of GIRK channels to facilitate rapid activation and deactivation of GIRK currents. By disrupting this allosterically regulated assembly mechanism, R7BP ablation augments GIRK activity. This enhanced GIRK activity increases the drug effects of agonists acting at G-protein-coupled receptors that signal via GIRK channels, as indicated by greater antinociceptive effects of GABA(B) or μ-opioid receptor agonists. These findings show that GIRK current modulation in vivo requires channel assembly with allosterically regulated RGS protein complexes, which provide a target for modulating GIRK activity in neurological disorders in which these channels have crucial roles, including pain, epilepsy, Parkinson's disease and Down syndrome.

Gβ3 is required for normal light ON responses and synaptic maintenance

Heterotrimeric G-proteins, comprising Gα and Gβγ subunits, couple metabotropic receptors to various downstream effectors and contribute to assembling and trafficking receptor-based signaling complexes. A G-protein β subunit, Gβ(3), plays a critical role in several physiological processes, as a polymorphism in its gene is associated with a risk factor for several disorders. Retinal ON bipolar cells express Gβ(3), and they provide an excellent system to study its role. In the ON bipolar cells, mGluR6 inverts the photoreceptor's signal via a cascade in which glutamate released from photoreceptors closes the TRPM1 channel. This cascade is essential for vision since deficiencies in its proteins lead to complete congenital stationary night blindness. Here we report that Gβ(3) participates in the G-protein heterotrimer that couples mGluR6 to TRPM1. Gβ(3) deletion in mouse greatly reduces the light response under both scotopic and photopic conditions, but it does not eliminate it. In addition, Gβ(3) deletion causes mislocalization and downregulation of most cascade elements and modulators. Furthermore, Gβ(3) may play a role in synaptic maintenance since in its absence, the number of invaginating rod bipolar dendrites is greatly reduced, a deficit that was not observed at 3 weeks, the end of the developmental period.

GPR158/179 regulate G protein signaling by controlling localization and activity of the RGS7 complexes

Interaction of RGS proteins with orphan GPCRs promotes signaling compartmentalization and specificity.

The extent and temporal characteristics of G protein–coupled receptor (GPCR) signaling are shaped by the regulator of G protein signaling (RGS) proteins, which promote G protein deactivation. With hundreds of GPCRs and dozens of RGS proteins, compartmentalization plays a key role in establishing signaling specificity. However, the molecular details and mechanisms of this process are poorly understood. In this paper, we report that the R7 group of RGS regulators is controlled by interaction with two previously uncharacterized orphan GPCRs: GPR158 and GPR179. We show that GPR158/179 recruited RGS complexes to the plasma membrane and augmented their ability to regulate GPCR signaling. The loss of GPR179 in a mouse model of night blindness prevented targeting of RGS to the postsynaptic compartment of bipolar neurons in the retina, illuminating the role of GPR179 in night vision. We propose that the interaction of RGS proteins with orphan GPCRs promotes signaling selectivity in G protein pathways.

"mGlu Receptors in the Retina" - WIREs Membrane Transport and Signaling

Glutamate, a key neurotransmitter in the vertebrate retina, acts via ionotropic and metabotropic receptors. Retina expresses mRNA for all metabotropic glutamate receptors and proteins for all but mGluR3. Every retinal cell class expresses one or more of these receptors. In general, these receptors are present presynaptically and serve to modulate synaptic transmission. While mGluRs on the photoreceptor terminal act as autoreceptors to titer glutamate levels, those on horizontal cell processes seem to shape the light response. Similarly, autoreceptors on bipolar axon terminals modulate glutamate release and the receptors on amacrine and ganglion cells modulate feedforward signals by modulating K+ or Ca2+ current to fine tune light responses. Since most of the mGluR sub-types are present in amacrine and ganglion cells that belong to many cell types, the pathways downstream of mGluRs are highly diverse with primarily modulatory effects. An exception to most mGluRs which have modulatory function is mGluR6 because it plays a key role in the feedforward transmission from photoreceptors to ON bipolar cells and is also required for the correct localization of the synaptic proteins in the dendritic tips. In humans, mutations in the gene encoding mGluR6 cause autosomal recessive night blindness. In addition, mGluRs appear to play a trophic role in development and after retinal damage, suggesting potential future therapeutic implications.

Regulators of G protein signaling RGS7 and RGS11 determine the onset of the light response in ON bipolar neurons

The time course of signaling via heterotrimeric G proteins is controlled through their activation by G-protein coupled receptors and deactivation through the action of GTPase accelerating proteins (GAPs). Here we identify RGS7 and RGS11 as the key GAPs in the mGluR6 pathway of retinal rod ON bipolar cells that set the sensitivity and time course of light-evoked responses. We showed using electroretinography and single cell recordings that the elimination of RGS7 did not influence dark-adapted light-evoked responses, but the concurrent elimination of RGS11 severely reduced their magnitude and dramatically slowed the onset of the response. In RGS7/RGS11 double-knockout mice, light-evoked responses in rod ON bipolar cells were only observed during persistent activation of rod photoreceptors that saturate rods. These observations are consistent with persistently high G-protein activity in rod ON bipolar cell dendrites caused by the absence of the dominant GAP, biasing TRPM1 channels to the closed state.

Defective retinal depolarizing bipolar cells in regulators of G protein signaling (RGS) 7 and 11 double null mice

Two members of the R7 subfamily of regulators of G protein signaling, RGS7 and RGS11, are present at dendritic tips of retinal depolarizing bipolar cells (DBCs). Their involvement in the mGluR6/Gα(o)/TRPM1 pathway that mediates DBC light responses has been implicated. However, previous genetic studies employed an RGS7 mutant mouse that is hypomorphic, and hence the exact role of RGS7 in DBCs remains unclear. We have made a true RGS7-null mouse line with exons 6-8 deleted. The RGS7(-/-) mouse is viable and fertile but smaller in body size. Electroretinogram (ERG) b-wave implicit time in young RGS7(-/-) mice is prolonged at eye opening, but the phenotype disappears at 2 months of age. Expression levels of RGS6 and RGS11 are unchanged in RGS7(-/-) retina, but the Gβ5S level is significantly reduced. By characterizing a complete RGS7 and RGS11 double knock-out (711dKO) mouse line, we found that Gβ5S expression in the retinal outer plexiform layer is eliminated, as is the ERG b-wave. Ultrastructural defects akin to those of Gβ5(-/-) mice are evident in 711dKO mice. In retinas of mice lacking RGS6, RGS7, and RGS11, Gβ5S is undetectable, whereas levels of the photoreceptor-specific Gβ5L remain unchanged. Whereas RGS6 alone sustains a significant amount of Gβ5S expression in retina, the DBC-related defects in Gβ5(-/-) mice are caused solely by a combined loss of RGS7 and RGS11. Our data support the notion that the role of Gβ5 in the retina, and likely in the entire nervous system, is mediated exclusively by R7 RGS proteins.

mGluR6 deletion renders the TRPM1 channel in retina inactive

In darkness, glutamate released from photoreceptors activates the metabotropic glutamate receptor 6 (mGluR6) on retinal ON bipolar cells. This activates the G protein G(o), which then closes transient receptor potential melastatin 1 (TRPM1) channels, leading to cells' hyperpolarization. It has been generally assumed that deleting mGluR6 would render the cascade inactive and the ON bipolar cells constitutively depolarized. Here we show that the rod bipolar cells in mGluR6-null mice were hyperpolarized. The slope conductance of the current-voltage curves and the current noise were smaller than in wild type. Furthermore, while in wild-type rod bipolar cells, TRPM1 could be activated by local application of capsaicin; in null cells, it did not. These results suggest that the TRPM1 channel in mGluR6-null rod bipolar cells is inactive. To explore the reason for this lack of activity, we tested if mGluR6 deletion affected expression of cascade components. Immunostaining for G protein subunit candidates Gα(o), Gβ(3), and Gγ(13) showed no significant changes in their expression or distribution. Immunostaining for TRPM1 in the dendritic tips was greatly reduced, but the channel was still present in the soma and primary dendrites of mGluR6-null bipolar cells, where a certain fraction of TRPM1 appears to localize to the plasma membrane. Consequently, the lack of TRPM1 activity in the null retina is unlikely to be due to failure of the channels to localize to the plasma membrane. We speculate that, to be constitutively active, TRPM1 channels in ON bipolar cells have to be in a complex, or perhaps require an unidentified factor.

Association between regulator of G protein signaling 9-2 and body weight

Regulator of G protein signaling 9-2 (RGS9-2) is a protein that is highly enriched in the striatum, a brain region that mediates motivation, movement and reward responses. We identified a naturally occurring 5 nucleotide deletion polymorphism in the human RGS9 gene and found that the mean body mass index (BMI) of individuals with the deletion was significantly higher than those without. A splicing reporter minigene assay demonstrated that the deletion had the potential to significantly decrease the levels of correctly spliced RGS9 gene product. We measured the weights of rats after virally transduced overexpression of RGS9-2 or the structurally related RGS proteins, RGS7, or RGS11, in the nucleus accumbens (NAc) and observed a reduction in body weight after overexpression of RGS9-2 but not RGS7 or 11. Conversely, we found that the RGS9 knockout mice were heavier than their wild-type littermates and had significantly higher percentages of abdominal fat. The constituent adipocytes were found to have a mean cross-sectional area that was more than double that of corresponding cells from wild-type mice. However, food intake and locomotion were not significantly different between the two strains. These studies with humans, rats and mice implicate RGS9-2 as a factor in regulating body weight.

RGS9 knockout causes a short delay in light responses of ON-bipolar cells

RGS9 and R9AP are components of the photoreceptor-specific GTPase activating complex responsible for rapid inactivation of the G protein, transducin, in the course of photoresponse recovery from excitation. The amount of this complex in photoreceptors is strictly dependent on the expression level of R9AP; consequently, the knockouts of either RGS9 or R9AP cause comparable delays in photoresponse recovery. While RGS9 is believed to be present only in rods and cones, R9AP is also expressed in dendritic tips of ON-bipolar cells, which receive synaptic inputs from photoreceptors. Recent studies demonstrated that knockouts of R9AP and its binding partner in ON-bipolar cells, RGS11, cause a small delay in ON-bipolar cell light responses manifested as a delayed onset of electroretinography b-waves. This led the authors to suggest that R9AP and RGS11 participate in regulating the kinetics of light responses in these cells. Here we report the surprising finding that a nearly identical b-wave delay is observed in RGS9 knockout mice. Given the exclusive localization of RGS9 in photoreceptors, this result argues for a presynaptic origin of the b-wave delay in this case and perhaps in the case of the R9AP knockout as well, since R9AP is expressed in both photoreceptors and ON-bipolar cells. We also conducted a detailed analysis of the b-wave rising phase kinetics in both knockout animal types and found that, despite a delayed b-wave onset, the slope of the light response is unaffected or increased, dependent on the light stimulus intensity. This result is inconsistent with a slowdown of response propagation in ON-bipolar cells caused by the R9AP knockout, further arguing against the postsynaptic nature of the delayed b-wave phenotype in RGS9 and R9AP knockout mice.

TRPM1 forms complexes with nyctalopin in vivo and accumulates in postsynaptic compartment of ON-bipolar neurons in mGluR6-dependent manner

Synaptic transmission between light-sensory photoreceptor cells and downstream ON-bipolar neurons plays an important role for vertebrate vision. This process is mediated by the G-protein-coupled receptor pathway involving glutamate receptor mGluR6 and effector channel TRPM1. The signal transmission occurs on a rapid timescale; however, the molecular organization that ensures timely signaling in this cascade is unknown. Genetic studies in human patients and animal models reveal that ON-bipolar cell signaling depends on the synaptic protein nyctalopin. We have conducted a proteomic search for proteins associated with nyctalopin in the mouse retina and identified TRPM1 as the binding partner. We further demonstrate that nyctalopin additionally interacts with mGluR6 receptor. Disruption of mGluR6 prevented targeting of TRPM1 to the postsynaptic compartment of ON-bipolar neurons. These results reveal a unique macromolecular organization of the mGluR6 cascade, where principal signaling components are scaffolded by nyctalopin, creating an organization essential for the correct localization of the signaling ensemble and ultimately intact transmission of the signal at the first visual synapse.

A role for nyctalopin, a small leucine-rich repeat protein, in localizing the TRP melastatin 1 channel to retinal depolarizing bipolar cell dendrites

Expression of channels to specific neuronal sites can critically impact their function and regulation. Currently, the molecular mechanisms underlying this targeting and intracellular trafficking of transient receptor potential (TRP) channels remain poorly understood, and identifying proteins involved in these processes will provide insight into underlying mechanisms. Vision is dependent on the normal function of retinal depolarizing bipolar cells (DBCs), which couple a metabotropic glutamate receptor 6 to the TRP melastatin 1 (TRPM1) channel to transmit signals from photoreceptors. We report that the extracellular membrane-attached protein nyctalopin is required for the normal expression of TRPM1 on the dendrites of DBCs in mus musculus. Biochemical and genetic data indicate that nyctalopin and TRPM1 interact directly, suggesting that nyctalopin is acting as an accessory TRP channel subunit critical for proper channel localization to the synapse.

Targeted deletion of one or two copies of the G protein β subunit Gβ5 gene has distinct effects on body weight and behavior in mice

We investigated the physiological role of Gβ5, a unique G protein β subunit that dimerizes with regulators of G protein signaling (RGS) proteins of the R7 family instead of Gγ. Gβ5 is essential for stability of these complexes, so that its knockout (KO)causes degradation of the entire Gβ5-R7 family. We report that the Gβ5-KO mice remain leaner than the wild type (WT) throughout their lifetime and are resistant to a high-fat diet. They have a 5-fold increase in locomotor activity, increased thermogenesis, and lower serum insulin, all of which correlate with a higher level of secreted epinephrine. Heterozygous (HET) mice are 2-fold more active than WT mice. Surprisingly, with respect to body weight, the HET mice display a phenotype opposite to that of the KO mice: by the age of 6 mo, they are ≥ 15% heavier than the WT and have increased adiposity, insulin resistance, and liver steatosis. These changes occur in HET mice fed a normal diet and without apparent hyperphagia, mimicking basic characteristics of human metabolic syndrome. We conclude that even a partial reduction in Gβ5-R7 level can perturb normal animal metabolism and behavior. Our data on Gβ5 haploinsufficient mice may explain earlier observations of genetic linkage between R7 family mutations and obesity in humans.

Regulator of G protein signaling 9-2 (RGS9-2) mRNA is up regulated during neuronal differentiation of mouse embryonic stem cells

In this study we demonstrate up-regulation of mRNA for Regulator of G protein Signaling (RGS) 6, 7, 9 and 11, R7 family RGS binding protein (R7BP) and RGS9 anchor protein (R9AP) during neuronal differentiation of mouse embryonic stem cells (mESCs). This expression pattern was most robust for RGS9 whose transcript level was low in undifferentiated mESCs but increased over 125 fold when differentiating mESCs began to exhibit a neuronal precursor cell (NPC) phenotype. In addition, we demonstrate that RGS9 mRNA is expressed in neuronal stem cells isolated from embryonic mouse cortex. The expression of RGS9 in two distinct populations of NPCs suggests that RGS9 and its accessory proteins may play an important role in neuron development.

Membrane anchoring subunits specify selective regulation of RGS9·Gbeta5 GAP complex in photoreceptor neurons

The RGS9·Gβ5 complex is the key regulator of neuronal G-protein signaling and shows remarkable selectivity of subunit composition. In retinal photoreceptors, RGS9·Gβ5 is bound to the membrane anchor R9AP and the complex regulates visual signaling. In the basal ganglia neurons, RGS9·Gβ5 is instead associated with a homologous protein, R7BP, and regulates reward circuit. Switching this selective subunit composition of the complex in rod photoreceptors allowed us to study the molecular underpinning of signaling specificity in diverse G-protein pathways. We have found that both membrane anchoring subunits play a conserved role in regulating protein levels of RGS9·Gβ5 and enhancing the ability of RGS·Gβ5 complexes to stimulate GTPase activity of G proteins. However, notable differences exist in the subcellular targeting of alternatively configured complexes. Unlike R9AP, which relies on passive targeting mechanisms for the delivery to the outer segments of the photoreceptors, R7BP is excluded from this location and is instead specifically targeted to the plasma membrane. R7BP-containing complexes could be rerouted to the outer segments, where they are capable of regulating the phototransduction cascade by the active targeting signals derived from rhodopsin. These findings illustrate the diversity of the G-protein signaling regulation by RGS·Gβ5 complexes achieved by differential recruitment of the membrane anchors.

Metabotropic glutamate receptors: from the workbench to the bedside

Metabotropic glutamate (mGlu) receptors were discovered in the mid 1980s and originally described as glutamate receptors coupled to polyphosphoinositide hydrolysis. Almost 6500 articles have been published since then, and subtype-selective mGlu receptor ligands are now under clinical development for the treatment of a variety of disorders such as Fragile-X syndrome, schizophrenia, Parkinson's disease and L-DOPA-induced dyskinesias, generalized anxiety disorder, chronic pain, and gastroesophageal reflux disorder. Prof. Erminio Costa was linked to the early times of the mGlu receptor history, when a few research groups challenged the general belief that glutamate could only activate ionotropic receptors and all metabolic responses to glutamate were secondary to calcium entry. This review moves from those nostalgic times to the most recent advances in the physiology and pharmacology of mGlu receptors, and highlights the role of individual mGlu receptor subtypes in the pathophysiology of human disorders. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.

A conserved protein interaction interface on the type 5 G protein beta subunit controls proteolytic stability and activity of R7 family regulator of G protein signaling proteins

Regulators of G protein signaling (RGS) proteins of the R7 subfamily limit signaling by neurotransmitters in the brain and by light in the retina. They form obligate complexes with the Gβ5 protein that are subject to proteolysis to control their abundance and alter signaling. The mechanisms that regulate this proteolysis, however, remain unclear. We used genetic screens to find mutations in Gβ5 that selectively destabilize one of the R7 RGS proteins in Caenorhabditis elegans. These mutations cluster at the binding interface between Gβ5 and the N terminus of R7 RGS proteins. Equivalent mutations within mammalian Gβ5 allowed the interface to still bind the N-terminal DEP domain of R7 RGS proteins, and mutant Gβ5-R7 RGS complexes initially formed in cells but were then rapidly degraded by proteolysis. Molecular dynamics simulations suggest the mutations weaken the Gβ5-DEP interface, thus promoting dynamic opening of the complex to expose determinants of proteolysis known to exist on the DEP domain. We propose that conformational rearrangements at the Gβ5-DEP interface are key to controlling the stability of R7 RGS protein complexes.

R9AP stabilizes RGS11-G beta5 and accelerates the early light response of ON-bipolar cells

The rate-limiting step in the recovery of the photoreceptor light response is the hydrolysis of GTP by transducin, a reaction that is accelerated by the RGS9-Gbeta5 complex, and its membrane anchor, R9AP. Similar complexes, including RGS7, RGS11, and Gbeta5, are found in retinal ON-bipolar cell dendrites. Here, we present evidence that R9AP is also expressed in the dendritic tips of ON-bipolar cells. Immunofluorescent staining for R9AP revealed a punctate pattern of labeling in the outer plexiform layer, where it colocalized with mGluR6. In photoreceptors, R9AP is required for proteolytic stability of the entire regulator of G protein signaling complex, and we found that genetic deletion of R9AP also results in a marked reduction in the levels of RGS11 and Gbeta5 in the bipolar cell dendrites; the level of RGS7 was unaffected, suggesting the presence of another interaction partner to stabilize RGS7. To determine the effect of R9AP deletion on the response kinetics of ON-bipolar cells, we compared the electroretinogram (ERG) between wild-type and R9AP-deficient mice. The ERG b-wave, reflecting ON-bipolar cell activity, was delayed and larger in the R9AP-deficient mice. Our data indicate that R9AP is required for stable expression of RGS11-Gbeta5 in ON-bipolar cell dendrites. Furthermore, they suggest that the RGS11-Gbeta5-R9AP complex accelerates the initial ON-bipolar cell response to light.

Nuclear localization of the G protein beta 5/R7-regulator of G protein signaling protein complex is dependent on R7 binding protein

The neuronally expressed G beta(5) subunit is the most structurally divergent among heterotrimeric G beta isoforms and unique in its ability to heterodimerize with the R7 subfamily of regulator of G protein signaling (RGS) proteins. The complex between G beta(5) and R7-type RGS proteins targets the cell nucleus by an unknown mechanism. Although the nuclear targeting of the G beta(5)/R7-RGS complex is proposed to involve the binding of R7-binding protein (R7BP), this theory is challenged by the observations that endogenous R7BP is palmitoylated, co-localizes strongly with the plasma membrane, and has never been identified in the cytosol or nucleus of native neurons or untreated cultured cells. We show here mutant RGS7 lacking the N-terminal Disheveled, EGL-10, Pleckstrin homology domain is expressed in transfected cells but, unlike wild-type RGS7, is excluded from the cell nucleus. As the Disheveled, EGL-10, Pleckstrin homology domain is essential for R7BP binding to RGS7, we studied the subcellular localization of G beta(5) in primary neurons and brain from mice deficient in R7BP. The level of endogenous nuclear G beta(5) and RGS7 in neurons and brains from R7BP knockout mice is reduced by 50-70%. These results suggest that R7BP contributes significantly to the nuclear localization of endogenous G beta(5)/R7-RGS complex in brain.

Membrane anchor R9AP potentiates GTPase-accelerating protein activity of RGS11 x Gbeta5 complex and accelerates inactivation of the mGluR6-G(o) signaling

The R7 subfamily of RGS proteins critically regulates neuronal G protein-signaling pathways that are essential for vision, nociception, motor coordination, and reward processing. A member of the R7 RGS family, RGS11, is a GTPase-accelerating protein specifically expressed in retinal ON-bipolar cells where it forms complexes with the atypical G protein beta subunit, Gbeta(5), and transmembrane protein R9AP. Association with R9AP has been shown to be critical for the proteolytic stability of the complex in the retina. In this study we report that R9AP can in addition stimulate the GTPase-accelerating protein activity of the RGS11 x Gbeta(5) complex at Galpha(o). Single turnover GTPase assays reveal that R9AP co-localizes RGS11 x Gbeta(5) and Galpha(o) on the membrane and allosterically potentiates the GTPase-accelerating function of RGS11 x Gbeta(5). Reconstitution of mGluR6-Galpha(o) signaling in Xenopus oocytes indicates that RGS11 x Gbeta(5)-mediated GTPase acceleration in this system requires co-expression of R9AP. The results provide new insight into the regulation of mGluR6-Galpha(o) signaling by the RGS11 x Gbeta(5) x R9AP complex and establish R9AP as a general GTPase-accelerating protein activity regulator of R7 RGS complexes.

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

PURPOSE

The retinal ON-bipolar cell (ON-BPC) light response is initiated upon deactivation of the metabotropic glutamate receptor mGluR6 and the G protein Go. G protein-based signaling cascades are typically accelerated by interaction of the G protein alpha subunit with a member of the regulator of G protein signaling (RGS) protein family. The goal of this study was to determine whether RGS7 and/or -11 serve this function in retinal ON-BPCs.

METHODS

Retinas from mice lacking RGS11 (RGS11(-/-)), or with a deletion mutation in RGS7 (RGS7(Delta/Delta)), or both, were compared to wild-type (WT) by immunofluorescence confocal microscopy. The retinal light response was measured with the electroretinogram (ERG). The kinetics of simulated light responses from individual rod bipolar cells were recorded by whole-cell patch-clamp electrophysiology.

RESULTS

Levels of the R7 RGS interaction partners, Gbeta5 and R9AP, were reduced in the outer plexiform layer of the RGS11(-/-) and RGS7(Delta/Delta)/RGS11(-/-) mice. ERG recordings demonstrated a delay in the rising phase of the ERG b-wave, larger photopic b-wave amplitudes, and increased scotopic threshold response sensitivity in the RGS11(-/-) and RGS7(Delta/Delta)/RGS11(-/-) mice. The ERG measured from the RGS7(Delta/Delta) retina was normal. Patch-clamp recordings of chemically simulated light responses of rod BPCs revealed a 25-ms delay in the onset of the ON-BPC response in the RGS7(Delta/Delta)/RGS11(-/-) mouse compared with the WT.

CONCLUSIONS

RGS11 plays a role in the deactivation of Galphao, which precedes activation of the depolarizing current in ON-BPCs. RGS7 must also serve a role as changes in RGS7(Delta/Delta)/RGS11(-/-) mice were greater than those in RGS11(-/-) mice.