Claudia M. Krispel

RGS expression rate-limits recovery of rod photoresponses.

Signaling through G protein-coupled receptors (GPCRs) underlies many cellular processes, yet it is not known which molecules determine the duration of signaling in intact cells. Two candidates are G protein-coupled receptor kinases (GRKs) and Regulators of G protein signaling (RGSs), deactivation enzymes for GPCRs and G proteins, respectively. Here we investigate whether GRK or RGS governs the overall rate of recovery of the light response in mammalian rod photoreceptors, a model system for studying GPCR signaling. We show that overexpression of rhodopsin kinase (GRK1) increases phosphorylation of the GPCR rhodopsin but has no effect on photoresponse recovery. In contrast, overexpression of the photoreceptor RGS complex (RGS9-1.Gbeta5L.R9AP) dramatically accelerates response recovery. Our results show that G protein deactivation is normally at least 2.5 times slower than rhodopsin deactivation, resolving a long-standing controversy concerning the mechanism underlying the recovery of rod visual transduction.

Novel form of adaptation in mouse retinal rods speeds recovery of phototransduction.

Photoreceptors of the retina adapt to ambient light in a manner that allows them to detect changes in illumination over an enormous range of intensities. We have discovered a novel form of adaptation in mouse rods that persists long after the light has been extinguished and the rods circulating dark current has returned. Electrophysiological recordings from individual rods showed that the time that a bright flash response remained in saturation was significantly shorter if the rod had been previously exposed to bright light. This persistent adaptation did not decrease the rate of rise of the response and therefore cannot be attributed to a decrease in the gain of transduction. Instead, this adaptation was accompanied by a marked speeding of the recovery of the response, suggesting that the step that rate-limits recovery had been accelerated. Experiments on knockout rods in which the identity of the rate-limiting step is known suggest that this adaptive acceleration results from a speeding of G protein/effector deactivation.

Phosducin Regulates the Expression of Transducin βγ Subunits in Rod Photoreceptors and Does Not Contribute to Phototransduction Adaptation

For over a decade, phosducins interaction with the βγ subunits of the G protein, transducin, has been thought to contribute to light adaptation by dynamically controlling the amount of transducin heterotrimer available for activation by photoexcited rhodopsin. In this study we directly tested this hypothesis by characterizing the dark- and light-adapted response properties of phosducin knockout (Pd−/−) rods. Pd−/− rods were notably less sensitive to light than wild-type (WT) rods. The gain of transduction, as measured by the amplification constant using the Lamb-Pugh model of activation, was 32% lower in Pd−/− rods than in WT rods. This reduced amplification correlated with a 36% reduction in the level of transducin βγ-subunit expression, and thus available heterotrimer in Pd−/− rods. However, commonly studied forms of light adaptation were normal in the absence of phosducin. Thus, phosducin does not appear to contribute to adaptation mechanisms of the outer segment by dynamically controlling heterotrimer availability, but rather is necessary for maintaining normal transducin expression and therefore normal flash sensitivity in rods.

Functional comparison of RGS9 splice isoforms in a living cell

Two isoforms of the GTPase-activating protein, regulator of G protein signaling 9 (RGS9), control such fundamental functions as vision and behavior. RGS9–1 regulates phototransduction in rods and cones, and RGS9–2 regulates dopamine and opioid signaling in the basal ganglia. To determine their functional differences in the same intact cell, we replaced RGS9–1 with RGS9–2 in mouse rods. Surprisingly, RGS9–2 not only supported normal photoresponse recovery under moderate light conditions but also outperformed RGS9–1 in bright light. This versatility of RGS9–2 results from its ability to inactivate the G protein, transducin, regardless of its effector interactions, whereas RGS9–1 prefers the G protein-effector complex. Such versatility makes RGS9–2 an isoform advantageous for timely signal inactivation across a wide range of stimulus strengths and may explain its predominant representation throughout the nervous system.

Undiagnosed papilledema in a morbidly obese patient population: A prospective study

Background Idiopathic intracranial hypertension (IIH) is a rare condition that can lead to significant morbidity from visual loss. The cause of IIH is unknown, but IIH is known to be associated with obesity. Obese patients may be at particularly high risk for suffering vision loss from IIH. The purpose of the present study is to determine the prevalence of undiagnosed or asymptomatic papilledema in a population of morbidly obese individuals and to determine if these patients should undergo routine screening for papilledema. Methods Patients presenting to the UC Davis Bariatric Surgery Clinic between February 2008 and January 2011 who met the National Institutes of Health criteria for bariatric surgery were invited to participate in the study. Those patients who met the inclusion criteria and consented to the study were included. Participants were screened for IIH by nonmydriatic fundus photographs and by concerning symptoms prompting direct referral for neuro-ophthalmologic evaluation. Images were reviewed by a neuro-ophthalmologist, and patients with suspicious optic discs underwent neuro-ophthalmologic evaluation. Patients with findings consistent with IIH were sent for neurological evaluation. Results A total of 606 patients with an average body mass index of 47 kg/m2 were included in the study. Seventeen of these patients had photographic optic disc findings or symptoms suspicious for IIH. Seven of these patients did not have disc edema on clinical examination. Six patients were not evaluated in the clinic. Four of the 17 patients had subtle optic disc edema confirmed by clinical evaluation and were referred for full neurological workup. These 4 patients had normal neuroimaging, 3 of whom underwent lumbar punctures with borderline high opening pressures. All 4 patients had unremarkable visual field examinations. Fundus abnormalities other than optic disc edema were discovered in 33 patients. Conclusion Our study suggests that in a morbidly obese patient population, papilledema with significant visual loss is rare. Routine screening with fundus photography of morbidly obese patients likely is not warranted.