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

Cellular and Molecular Mechanisms of Pathogenesis Underlying Inherited Retinal Dystrophies

Inherited retinal dystrophies (IRDs) are congenital retinal degenerative diseases that have various inheritance patterns, including dominant, recessive, X-linked, and mitochondrial. These diseases are most often the result of defects in rod and/or cone photoreceptor and retinal pigment epithelium function, development, or both. The genes associated with these diseases, when mutated, produce altered protein products that have downstream effects in pathways critical to vision, including phototransduction, the visual cycle, photoreceptor development, cellular respiration, and retinal homeostasis. The aim of this manuscript is to provide a comprehensive review of the underlying molecular mechanisms of pathogenesis of IRDs by delving into many of the genes associated with IRD development, their protein products, and the pathways interrupted by genetic mutation.

The Transduction Cascade in Retinal ON-Bipolar Cells: Signal Processing and Disease

Our robust visual experience is based on the reliable transfer of information from our photoreceptor cells, the rods and cones, to higher brain centers. At the very first synapse of the visual system, information is split into two separate pathways, ON and OFF, which encode increments and decrements in light intensity, respectively. The importance of this segregation is borne out in the fact that receptive fields in higher visual centers maintain a separation between ON and OFF regions. In the past decade, the molecular mechanisms underlying the generation of ON signals have been identified, which are unique in their use of a G-protein signaling cascade. In this review, we consider advances in our understanding of G-protein signaling in ON-bipolar cell (BC) dendrites and how insights about signaling have emerged from visual deficits, mostly night blindness. Studies of G-protein signaling in ON-BCs reveal an intricate mechanism that permits the regulation of visual sensitivity over a wide dynamic range.

Intermolecular Interaction between Anchoring Subunits Specify Subcellular Targeting and Function of RGS Proteins in Retina ON-Bipolar Neurons

In vertebrate retina, light responses generated by the rod photoreceptors are transmitted to the second-order neurons, the ON-bipolar cells (ON-BC), and this communication is indispensible for vision in dim light. In ON-BCs, synaptic transmission is initiated by the metabotropic glutamate receptor, mGluR6, that signals via the G-protein Go to control opening of the effector ion channel, TRPM1. A key role in this process belongs to the GTPase Activating Protein (GAP) complex that catalyzes Go inactivation upon light-induced suppression of glutamate release in rod photoreceptors, thereby driving ON-BC depolarization to changes in synaptic input. The GAP complex has a striking molecular complexity. It contains two Regulator of G-protein Signaling (RGS) proteins RGS7 and RGS11 that directly act on Go and two adaptor subunits: RGS Anchor Protein (R9AP) and the orphan receptor, GPR179. Here we examined the organizational principles of the GAP complex in ON-BCs. Biochemical experiments revealed that RGS7 binds to a conserved site in GPR179 and that RGS11 in vivo forms a complex only with R9AP. R9AP and GPR179 are further integrated via direct protein-protein interactions involving their cytoplasmic domains. Elimination of GPR179 prevents postsynaptic accumulation of R9AP. Furthermore, concurrent knock-out of both R9AP and RGS7 does not reconfigure the GAP complex and completely abolishes synaptic transmission, resulting in a novel mouse model of night blindness. Based on these results, we propose a model of hierarchical assembly and function of the GAP complex that supports ON-BCs visual signaling.

LRIT3 is essential to localize TRPM1 to the dendritic tips of depolarizing bipolar cells and may play a role in cone synapse formation

Mutations in LRIT3 lead to complete congenital stationary night blindness (cCSNB). The exact role of LRIT3 in ON-bipolar cell signaling cascade remains to be elucidated. Recently, we have characterized a novel mouse model lacking Lrit3 [no b-wave 6, (Lrit3(nob6/nob6) )], which displays similar abnormalities to patients with cCSNB with LRIT3 mutations. Here we compare the localization of components of the ON-bipolar cell signaling cascade in wild-type and Lrit3(nob6/nob6) retinal sections by immunofluorescence confocal microscopy. An anti-LRIT3 antibody was generated. Immunofluorescent staining of LRIT3 in wild-type mice revealed a specific punctate labeling in the outer plexiform layer (OPL), which was absent in Lrit3(nob6/nob6) mice. LRIT3 did not co-localize with ribeye or calbindin but co-localized with mGluR6. TRPM1 staining was severely decreased at the dendritic tips of all depolarizing bipolar cells in Lrit3(nob6/nob6) mice. mGluR6, GPR179, RGS7, RGS11 and Gβ5 immunofluorescence was absent at the dendritic tips of cone ON-bipolar cells in Lrit3(nob6/nob6) mice, while it was present at the dendritic tips of rod bipolar cells. Furthermore, peanut agglutinin (PNA) labeling was severely reduced in the OPL in Lrit3(nob6/nob6) mice. This study confirmed the localization of LRIT3 at the dendritic tips of depolarizing bipolar cells in mouse retina and demonstrated the dependence of TRPM1 localization on the presence of LRIT3. As tested components of the ON-bipolar cell signaling cascade and PNA revealed disrupted localization, an additional function of LRIT3 in cone synapse formation is suggested. These results point to a possibly different regulation of the mGluR6 signaling cascade between rod and cone ON-bipolar cells.

Properties and functions of TRPM1 channels in the dendritic tips of retinal ON-bipolar cells

An increase in light intensity induces a depolarization in retinal ON-bipolar cells via a reduced glutamate release from presynaptic photoreceptor cells. The underlying transduction cascade in the dendritic tips of ON-bipolar cells involves mGluR6 glutamate receptors signaling to TRPM1 proteins that are an indispensable part of the transduction channel. Several other proteins are recognized to participate in the transduction machinery. Deficiency in many of these leads to congenital stationary night blindness, because rod bipolar cells, a subgroup of ON-bipolar cells, constitute the main route for sensory information under scotopic conditions. Here, we review the current knowledge about TRPM1 ion channels and how their activity is regulated within the postsynaptic compartment of ON-bipolar cells. The functional properties of TRPM1 channels in the dendritic compartment are not well understood as they differ substantially from those of recombinant TRPM1 channels. Critical evaluation of possible explanations of these discrepancies indicates that some key components of this transduction pathway might still not be known. The continued exploration of this pathway will yield further clinically useful insights.

Orphan Receptor GPR158 Is an Allosteric Modulator of RGS7 Catalytic Activity with an Essential Role in Dictating Its Expression and Localization in the Brain

Regulators of G protein signaling control the duration and extent of signaling via G protein-coupled receptor (GPCR) pathways by accelerating the GTP hydrolysis on G protein α subunits thereby promoting termination of GPCR signaling. A member of this family, RGS7, plays a critical role in the nervous system where it regulates multiple neurotransmitter GPCRs that mediate vision, memory, and the action of addictive drugs. Previous studies have established that in vivo RGS7 forms mutually exclusive complexes with the membrane protein RGS7-binding protein or the orphan receptor GPR158. In this study, we examine the impact of GPR158 on RGS7 in the brain. We report that knock-out of GPR158 in mice results in marked post-transcriptional destabilization of RGS7 and substantial loss of its association with membranes in several brain regions. We further identified the RGS7-binding site in the C terminus of GPR158 and found that it shares significant homology with the RGS7-binding protein. The proximal portion of the GPR158 C terminus additionally contained a conserved sequence that was capable of enhancing RGS7 GTPase-activating protein activity in solution by an allosteric mechanism acting in conjunction with the regulators of the G protein signaling-binding domain. The distal portion of the GPR158 C terminus contained several phosphodiesterase E γ-like motifs and selectively recruited G proteins in their activated state. The results of this study establish GPR158 as an essential regulator of RGS7 in the native nervous system with a critical role in controlling its expression, membrane localization, and catalytic activity.

Regulators of G-protein-signaling proteins: negative modulators of G-protein-coupled receptor signaling

Regulators of G-protein-signaling (RGS) proteins are a category of intracellular proteins that have an inhibitory effect on the intracellular signaling produced by G-protein-coupled receptors (GPCRs). RGS along with RGS-like proteins switch on through direct contact G-alpha subunits providing a variety of intracellular functions through intracellular signaling. RGS proteins have a common RGS domain that binds to G alpha. RGS proteins accelerate GTPase and thus enhance guanosine triphosphate hydrolysis through the alpha subunit of heterotrimeric G proteins. As a result, they inactivate the G protein and quickly turn off GPCR signaling thus terminating the resulting downstream signals. Activity and subcellular localization of RGS proteins can be changed through covalent molecular changes to the enzyme, differential gene splicing, and processing of the protein. Other roles of RGS proteins have shown them to not be solely committed to being inhibitors but behave more as modulators and integrators of signaling. RGS proteins modulate the duration and kinetics of slow calcium oscillations and rapid phototransduction and ion signaling events. In other cases, RGS proteins integrate G proteins with signaling pathways linked to such diverse cellular responses as cell growth and differentiation, cell motility, and intracellular trafficking. Human and animal studies have revealed that RGS proteins play a vital role in physiology and can be ideal targets for diseases such as those related to addiction where receptor signaling seems continuously switched on.

Differential function of Gγ13 in rod bipolar and ON cone bipolar cells

Heterotrimeric G-proteins (comprising Gα and Gβγ subunits) are critical for coupling of metabotropic receptors to their downstream effectors. In the retina, glutamate released from photoreceptors in the dark activates metabotropic glutamate receptor 6 (mGluR6) receptors in ON bipolar cells; this leads to activation of Go , closure of transient receptor potential melastatin 1 channels and hyperpolarization of these cells. Go comprises Gαo , Gβ3 and a Gγ. The best Gγ candidate is Gγ13, although functional data to support this are lacking. Thus, we tested Gγ13 function by generating Gng13(-/-) knockout (KO) mice, recording electroretinograms (ERG) and performing immunocytochemical staining. The amplitude of scotopic ERG b-waves in KO mice was lower than in wild-type (WT) mice. Furthermore, in both KO and WT mice, the ERG b-wave decreased with age; this decrease was much more pronounced in KO mice. By contrast, the photopic ERG b-waves in KO mice were hardly affected at any age. In KO mice retinas, immunostaining for Gβ3 and for the GTPase activating proteins RGS7, RGS11, R9AP and Gβ5 decreased significantly in rod bipolar cells but not in ON cone bipolar cells. Staining for Gαo and certain other cascade elements decreased only slightly. Analysis of our ON bipolar cDNA library showed that these cells express mRNAs for Gγ5, Gγ10 and Gγ11. Quantitative RT-PCR of retinal cDNA showed greater values for these transcripts in retinas of KO mice, although the difference was not significant. Our results suggest that Gγ13 contributes to mGluR6 signalling in rod bipolar cells more than in ON cone bipolar cells, and that this contribution includes both coupling the receptor and maintaining a stable localization of the mGluR6-related cascade elements.

R9AP targeting to rod outer segments is independent of rhodopsin and is guided by the SNARE homology domain

R9AP, the membrane anchor for transducin's GTPase-activating complex, contains targeting information within its SNARE homology domain that is both necessary and sufficient for R9AP delivery to photoreceptor outer segments. R9AP's targeting is independent of rhodopsin, the most abundant protein residing in the outer segment organelle.

In vertebrate photoreceptor cells, rapid recovery from light excitation is dependent on the RGS9⋅Gβ5 GTPase-activating complex located in the light-sensitive outer segment organelle. RGS9⋅Gβ5 is tethered to the outer segment membranes by its membrane anchor, R9AP. Recent studies indicated that RGS9⋅Gβ5 possesses targeting information that excludes it from the outer segment and that this information is overridden by association with R9AP, which allows outer segment targeting of the entire complex. It was also proposed that R9AP itself does not contain specific targeting information and instead is delivered to the outer segment in the same post-Golgi vesicles as rhodopsin, because they are the most abundant transport vesicles in photoreceptor cells. In this study, we revisited this concept by analyzing R9AP targeting in rods of wild-type and rhodopsin-knockout mice. We found that the R9AP targeting mechanism does not require the presence of rhodopsin and further demonstrated that R9AP is actively targeted in rods by its SNARE homology domain.

GPR179 is required for high sensitivity of the mGluR6 signaling cascade in depolarizing bipolar cells

Parallel visual pathways are initiated at the first retinal synapse by signaling between the rod and cone photoreceptors and two general classes of bipolar cells. For normal function, ON or depolarizing bipolar cells (DBCs) require the G-protein-coupled receptor, mGluR6, an intact G-protein-coupled cascade and the transient receptor potential melastatin 1 (TRPM1) cation channel. In addition, another seven transmembrane protein, GPR179, is required for DBC function and recruits the regulators of G-protein signaling (RGS) proteins, RGS7 and RGS11, to the dendritic tips of the DBCs. Here we use the Gpr179(nob5) mouse, which lacks GPR179 and has a no b-wave electroretinogram (ERG) phenotype, to demonstrate that despite the absence of both GPR179 and RGS7/RGS11, a small dark-adapted ERG b-wave remains and can be enhanced with long duration flashes. Consistent with the ERG, the mGluR6-mediated gating of TRPM1 can be evoked pharmacologically in Gpr179(nob5) and RGS7(-/-)/RGS11(-/-) rod BCs if strong stimulation conditions are used. In contrast, direct gating of TRPM1 by capsaicin in RGS7(-/-)/RGS11(-/-) and WT rod BCs is similar, but severely compromised in Gpr179(nob5) rod BCs. Noise and standing current analyses indicate that the remaining channels in Gpr179(nob5) and RGS7(-/-)/RGS11(-/-) rod BCs have a very low open probability. We propose that GPR179 along with RGS7 and RGS11 controls the ability of the mGluR6 cascade to gate TRPM1. In addition to its role in localizing RGS7 and RGS11 to the dendritic tips, GPR179 via a direct interaction with the TRPM1 channel alters its ability to be gated directly by capsaicin.

Localization of Cacna1s to ON bipolar dendritic tips requires mGluR6-related cascade elements

PURPOSE

L-type voltage gated calcium channels in retina localize primarily at the presynaptic active zones of photoreceptors and bipolar cells where they modulate glutamate release. However, the pore forming subunit Cacna1s of certain L-type channels is also expressed postsynaptically at the tips of ON bipolar cell dendrites where it colocalizes with mGluR6, but has an unknown function. At these dendritic tips, the components of the mGluR6 signaling cascade cluster together in a macromolecular complex, and each one's localization often depends on that of the others. Thus, we explored if Cacna1s is part of the mGluR6 complex.

METHODS

We determined Cacna1s expression by PCR using an ON bipolar library, by Western blotting, and by standard immunohistochemistry.

RESULTS

The PCR amplification confirmed expression of the transcript in ON bipolar cells, and Western blotting showed the expected bands. Immunostaining for Cacna1s was stronger in the dendritic tips of rod bipolar cells than in those of ON cone bipolar cells. This staining severely decreased in mice missing various mGluR6 cascade elements (Grm6(-/-), Gnao1(-/-), Gnb3(-/-), Gng13(-/-), and Trpm1(-/-)). During development, the ratio of the number of Cacna1s puncta to the number of presynaptic ribbons followed a sigmoidal pattern, rising rapidly from P13 to P17. The mGluR6 expression preceded that of Cacna1s and RGS11.

CONCLUSIONS

Our results show that the localization and stability of Cacna1s depend on the expression of mGluR6 and its cascade components, and they suggest that Cacna1s is part of the mGluR6 complex. We hypothesize that Cacna1s contributes to light adaptation by permeating calcium.

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.

Differential modulation of retinal ganglion cell light responses by orthosteric and allosteric metabotropic glutamate receptor 8 compounds

To investigate the role of mGluR8 in modulating the synaptic responses of retinal ganglion cells, we used a recently identified positive allosteric modulator of mGluR8, AZ12216052 (AZ) and the mGluR8-specific orthosteric agonist (S)-3,4-dicarboxyphenylglycine (DCPG). These agents were applied to whole-cell voltage-clamped ganglion cells from an isolated, superfused mouse retina preparation. DCPG reduced OFF-ganglion cell excitatory currents, whereas AZ enhanced the peak excitatory currents in ON-, OFF-, and ON-OFF-ganglion cells. The effects on ganglion cell inhibitory currents were more varied. The effects of the allosteric modulator were stronger for bright stimuli than for dim stimuli, consistent with receptor stimulation by endogenous glutamate being stronger during bright light stimulation and with mGluR8 receptors mainly being localized away from glutamate release sites, immuno-labeled with VGLUT1. The differential sensitivity of ganglion cell light responses to DCPG and AZ supports multiple sites where mGluR8 modulates the light responses of ganglion cells.

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.

"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.

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.

Gi/o signaling and the palmitoyltransferase DHHC2 regulate palmitate cycling and shuttling of RGS7 family-binding protein

R7BP (RGS7 family-binding protein) has been proposed to function in neurons as a palmitoylation-regulated protein that shuttles heterodimeric, G(i/o)α-specific GTPase-activating protein (GAP) complexes composed of Gβ5 and RGS7 (R7) isoforms between the plasma membrane and nucleus. To test this hypothesis we studied R7BP palmitoylation and localization in neuronal cells. We report that R7BP undergoes dynamic, signal-regulated palmitate turnover; the palmitoyltransferase DHHC2 mediates de novo and turnover palmitoylation of R7BP; DHHC2 silencing redistributes R7BP from the plasma membrane to the nucleus; and G(i/o) signaling inhibits R7BP depalmitoylation whereas G(i/o) inactivation induces nuclear accumulation of R7BP. In concert with previous evidence, our findings suggest that agonist-induced changes in palmitoylation state facilitate GAP action by (i) promoting Giα depalmitoylation to create optimal GAP substrates, and (ii) inhibiting R7BP depalmitoylation to stabilize membrane association of R7-Gβ5 GAP complexes. Regulated palmitate turnover may also enable R7BP-bound GAPs to shuttle between sites of low and high G(i/o) activity or the plasma membrane and nucleus, potentially providing spatio-temporal control of signaling by G(i/o)-coupled receptors.