RGS7 and-11 Complexes Accelerate the ON-Bipolar Cell Light Response

Department of Veterinary and Comparative Anatomy, Pharmacology, and Physiology, Washington State University, Pullman, Washington, USA.
Investigative ophthalmology & visual science (Impact Factor: 3.4). 09/2009; 51(2):1121-9. DOI: 10.1167/iovs.09-4163
Source: PubMed


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.
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.
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.
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.

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Available from: Catherine W Morgans
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    • "There is another piece of puzzle required to be considered in regards to the origin of the b-wave delay: the delay is also observed in the knockout of RGS11, the established R9AP partner in ON-bipolar cells [20], [32], [33], [38], and in the mouse in which RGS7 function was altered by hypomorphic deletion [32], [33]. Though it could not be completely ruled out that small fractions of RGS11 and RGS7 reside in photoreceptors [39], immunolocalization results argue that it is not the case [19], [21], [32], [33], [38], including the study in which the specificity of immunostaining was established with the RGS11 knockout control [19]. Therefore, the b-wave delay phenotype caused by RGS11 knockout and the RGS7 mutant may be potentially caused by different mechanisms than the phenotype observed in RGS9−/− and R9AP−/− mice. "
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    ABSTRACT: 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.
    Preview · Article · Nov 2011 · PLoS ONE
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    • "Glutamate, released from vertebrate photoreceptors, activates retinal ON bipolar neurons through a sign inverting mechanism mediated by the GRM6 – G o α/Gβ 5 -nyctalopin- RGS7/11 transduction pathway (Nomura et al., 1994; Dhingra et al., 2002; Rao et al., 2007; Chen et al., 2010; Zhang et al., 2010). While all the steps in the ON bipolar transduction cascade remain to be determined, the main cation channel downstream from the heteromeric G protein has been identified as TRPM1, a member of the melastatin TRP channel family (Morgans et al., 2009; Koike et al., 2010). "
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    ABSTRACT: The light response in retinal ON bipolar cells is associated with disinhibition of current flow through cation channels recently identified as type 1 members of the melastatin transient receptor potential (TRPM) family. We determined the developmental expression of Trpm1 in the wild type C57BL/6, DBA/2J, DBA2J-Gpnmb mouse retinas and in Pde6brd1 retinas characterized by degeneration of rod photoreceptors. Trpm1 mRNA in wild type retinas was low at birth but exhibited progressive increases in abundance up to early adulthood at postnatal day 21 (P21). Retinal Trpm1 mRNA content did not decrease following loss of photoreceptors. At P21, TRPM1-immunopositive perikarya migrated into the outer nuclear layer. The TRPM1 protein was trafficked to discrete postsynaptic puncta in wild type retinas whereas in adult Pde6brd1 mouse retinas, TRPM1 translocated to bipolar perikarya and bar-like structures in the distal inner nuclear layer. These findings show that expression and localization of the TRPM1 in the mouse retina is plastic, modulated by use-dependence and availability of sustained excitatory input.
    Full-text · Article · Nov 2010 · Vision research
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    • "Light reduces glutamate stimulation of mGlu6 receptors, leading to opening of cation channels and depolarization of ONbipolar cells. The effect of light is amplified by inactivation of the mGlu6 receptor signalling mediated by the transmembrane protein R9AP, which facilitates the GTPase activity of RGS11 complexed with the Gb 5 subunit of Go (Cao et al., 2009; Masuho et al., 2010; Zhang et al., 2010). mGlu6 receptor knockout mice show a loss of ON responses but an unchanged response to light (Masu et al., 1995), and a mouse screened for the lack of the b-wave at the electroretinogram showed a splice error in the Grm6 and the lack of mGlu6 receptors in the retina (Maddox et al., 2008). "
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    ABSTRACT: 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'.
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