The Experts below are selected from a list of 210 Experts worldwide ranked by ideXlab platform

Pavel P. Philippov - One of the best experts on this subject based on the ideXlab platform.

Alapakkam P. Sampath - One of the best experts on this subject based on the ideXlab platform.

  • Behavioural and physiological limits to vision in mammals.
    Philosophical transactions of the Royal Society of London. Series B Biological sciences, 2017
    Co-Authors: Greg D. Field, Alapakkam P. Sampath
    Abstract:

    Human vision is exquisitely sensitive-a dark-adapted observer is capable of reliably detecting the absorption of a few quanta of light. Such sensitivity requires that the sensory receptors of the Retina, Rod photoreceptors, generate a reliable signal when single photons are absorbed. In addition, the Retina must be able to extract this information and relay it to higher visual centres under conditions where very few Rods signal single-photon responses while the majority generate only noise. Critical to signal transmission are mechanistic optimizations within Rods and their dedicated Retinal circuits that enhance the discriminability of single-photon responses by mitigating photoreceptor and synaptic noise. We describe behavioural experiments over the past century that have led to the appreciation of high sensitivity near absolute visual threshold. We further consider mechanisms within Rod photoreceptors and dedicated Rod circuits that act to extract single-photon responses from cellular noise. We highlight how these studies have shaped our understanding of brain function and point out several unresolved questions in the processing of light near the visual threshold.This article is part of the themed issue 'Vision in dim light'.

  • Coordinated control of sensitivity by two splice variants of Gαo in Retinal ON bipolar cells
    The Journal of general physiology, 2010
    Co-Authors: Haruhisa Okawa, Lutz Birnbaumer, Johan Pahlberg, Fred Rieke, Alapakkam P. Sampath
    Abstract:

    The high sensitivity of scotopic vision depends on the efficient Retinal processing of single photon responses generated by individual Rod photoreceptors. At the first synapse in the mammalian Retina, Rod outputs are pooled by a Rod “ON” bipolar cell, which uses a G-protein signaling cascade to enhance the fidelity of the single photon response under conditions where few Rods absorb light. Here we show in mouse Rod bipolar cells that both splice variants of the Go α subunit, Gαo1 and Gαo2, mediate light responses under the control of mGluR6 receptors, and their coordinated action is critical for maximizing sensitivity. We found that the light response of Rod bipolar cells was primarily mediated by Gαo1, but the loss of Gαo2 caused a reduction in the light sensitivity. This reduced sensitivity was not attributable to the reduction in the total number of Go α subunits, or the altered balance of expression levels between the two splice variants. These results indicate that Gαo1 and Gαo2 both mediate a depolarizing light response in Rod bipolar cells without occluding each other’s actions, suggesting they might act independently on a common effector. Thus, Gαo2 plays a role in improving the sensitivity of Rod bipolar cells through its action with Gαo1. The coordinated action of two splice variants of a single Gα may represent a novel mechanism for the fine control of G-protein activity.

Elena N. Gorodovikova - One of the best experts on this subject based on the ideXlab platform.

Anand Swaroop - One of the best experts on this subject based on the ideXlab platform.

  • Cone-Rod homeobox CRX controls presynaptic active zone formation in photoreceptors of mammalian Retina
    Human Molecular Genetics, 2018
    Co-Authors: Juthaporn Assawachananont, Soo-young Kim, Koray Kaya, Jérôme E. Roger, Robert Fariss, Anand Swaroop
    Abstract:

    In the mammalian Retina, Rod and cone photoreceptors transmit the visual information to bipolar neurons through highly specialized ribbon synapses. We have limited understanding of regulatory pathways that guide morphogenesis and organization of photoreceptor presynaptic architecture in the developing Retina. While neural Retina leucine zipper (NRL) transcription factor determines Rod cell fate and function, cone-Rod homeobox (CRX) controls the expression of both Rod- and cone-specific genes and is critical for terminal differentiation of photoreceptors. A comprehensive immunohistochemical evaluation of Crx-/- (null), CrxRip/+ and CrxRip/Rip (models of dominant congenital blindness) mouse Retinas revealed abnormal photoreceptor synapses, with atypical ribbon shape, number and length. Integrated analysis of Retinal transcriptomes of Crx-mutants with CRX- and NRL-ChIP-Seq data identified a subset of differentially expressed CRX target genes that encode presynaptic proteins associated with the cytomatrix active zone (CAZ) and synaptic vesicles. Immunohistochemistry of Crx-mutant Retina validated aberrant expression of REEP6, PSD95, MPP4, UNC119, UNC13, RGS7 and RGS11, with some reduction in Ribeye and no significant change in immunostaining of RIMS1, RIMS2, Bassoon and Pikachurin. Our studies demonstrate that CRX controls the establishment of CAZ and anchoring of ribbons, but not the formation of ribbon itself, in photoreceptor presynaptic terminals.

  • quantification of oxygen consumption in Retina ex vivo demonstrates limited reserve capacity of photoreceptor mitochondria
    Investigative Ophthalmology & Visual Science, 2015
    Co-Authors: Keshav Kooragayala, Norimoto Gotoh, Tiziana Cogliati, Jacob Nellissery, Talia R Kaden, Stephanie French, Robert S Balaban, Raul Covian, Anand Swaroop
    Abstract:

    Defects in mitochondrial respiration and oxidative phosphorylation are associated with pleiotropic phenotypes, which, at least in part, can be attributed to aberrant generation of reactive oxygen (O2) and nitrogen species.1–4 Mitochondrial dysfunction has been correlated with the accumulation of cellular damage in aging and in late-onset neuRodegenerative diseases.5–8 Tissues with a high metabolic requirement, such as the Retina, are especially vulnerable to mitochondrial damage, which is implicated as a causal factor in neuronal cell death in Retinal and macular degeneration.9,10 Within the Retina, Rod and cone photoreceptors exhibit high energy metabolism and associated O2 consumption, primarily driven by ion transport and synaptic transmission in the dark and by cGMP turnover in the light.11–14 Quantification of O2 consumption rate (OCR) is generally adopted as a readout of overall mitochondrial function.15 Measurements of respiration have traditionally been conducted with the O2-consuming Clark-type electRode on mitochondria preparations,16 which require sample volumes in the milliliter range and continuous stirring of samples in suspension, with loss of cellular/tissue context. The Clark-type electRode has also been used to measure O2 consumption in the intact rat Retina, detecting higher O2 consumption in the outer Retina compared to inner Retina, with downward or upward changes, respectively, during light adaptation.17 MicroelectRodes that are O2-sensitive and intraRetinal measurements taken at differential tissue depths18 confirmed that photoreceptors have the highest O2 consumption.18 Finally, fresh mouse Retina slices in a flow culture system have been used to determine the metabolic dependence of photoreceptors on glucose as a fuel.19 A limitation of all of the above experimental conditions is that they only permit evaluation of one Retina/sample at a time and are not readily amenable for simultaneous and high throughput evaluation of multiple experimental conditions. Microplate-based assays using an extracellular flux (XF) analyzer (Seahorse XF Analyzer; Seahorse Bioscience, Billerica, MA, USA) allow parallel comparison of different samples in small microliter volumes without the need for sample suspension, continuous stirring, or large measuring volumes in the order of ∼2 mL, as is the case with typical Clark-type electRode systems. The XF analyzer (Seahorse Bioscience) has been used to determine the effects of Ca++ and oxidative stress on mitochondrial function in the photoreceptor-like cell line 661W.20 Using the same technology, mitochondrial function has been examined and compared in acutely derived sections from several brain areas of a rat model of migraine.21 Direct comparative OCR determination has not yet been accomplished in the freshly isolated mouse Retina. Here, we intRoduce an improved protocol for microplate-based measurement of O2 consumption in acute Retina samples. Photoreceptors account for the majority of O2 consumption in the Retina,22 with the entire inner Retina consuming at most 17% of the amount used by photoreceptors.13 Thus, the measurement of O2 consumption in the intact Retina largely reflects photoreceptor respiration. Our newly optimized method directly evaluates Retinal mitochondrial function and should be valuable for assessing alterations associated with Retinal dysfunction and/or disease.

Norma M. Giusto - One of the best experts on this subject based on the ideXlab platform.

  • Effect of light on phosphatidate phosphohydrolase activity of Retina Rod outer segments: the role of transducin.
    Archives of biochemistry and biophysics, 2000
    Co-Authors: Susana J. Pasquaré, Gabriela Alejandra Salvador, Marta Elena Roque, Norma M. Giusto
    Abstract:

    Abstract The aim of the present paper is to evaluate the modulation of phosphatidate phosphohydrolase (PAPase) and diacylglyceride lipase (DGL) activities in bovine Rod outer segment (ROS) under dark and light conditions and to evaluate the role of transducin (T) in this phenomenon. In dark-adapted ROS membranes exposed to light, PAPase activity is inhibited by 20% with respect to the activity found under dark conditions. To determine whether the Retinal G protein, T, participates in the regulation of PAPase activity in these membranes, the effects of GTPγS and GDPβS on enzyme activity were examined. Under dark conditions in the presence of GTPγS, which stabilizes T in its active form (Tα + Tβγ), enzyme activity was inhibited and approached control values under light conditions. GDPβS, on the other hand, which stabilizes the inactive state of T (Tαβγ), stimulated PAPase activity by 36% with respect to control light conditions. ADP-ribosylation by cholera and pertussis toxin was also studied. In ADP-rybosilated ROS membranes with pertussis toxin under dark conditions, PAPase activity was 36% higher than the activity found under control light conditions. ADP-ribosylation by CTx, on the other hand, inhibited PAPase activity by 22%, with respect to dark control conditions, mimicking light effect. The effects of GTPγS and GDPβS and conditions of ADP-ribosylation by PTx and CTx on DGL activity were similar to those of PAPase activities. Based on NEM sensitivity we have also demonstrated that the PAPase present in ROS is the PAP 2 isoform. Our findings therefore suggest that light inhibition of PAP 2 in ROS is a transducin-mediated mechanism.

  • Acyl-CoA:lysophosphatidylcholine Acyltransferase Activity in Bovine Retina Rod Outer Segments
    Archives of biochemistry and biophysics, 1997
    Co-Authors: P.i. Castagnet, Norma M. Giusto
    Abstract:

    Abstract In the present paper the properties of acyl-CoA:lysophosphatidylcholine acyltransferase activity associated with Rod outer segments (ROS) have been studied. Under adequate experimental conditions, ROS acyl-CoA:lysophosphatidylcholine acyltransferase activity presented a maximum at pH 7.0. The enzyme was able to incorporate as much as 60% of the label offered as [1- 14 C]oleoyl-CoA into phosphatidylcholine after 5 min of incubation. The use of varying concentrations of oleoyl-CoA and 46 μ m lysophosphatidylcholine gave an apparent K m value for oleoyl-CoA of 100 μ m and a V max value of 153 nmol × h −1 × (mg protein) −1 . The use of varying concentrations of lysophosphatidylcholine and 100 μ m oleoyl-CoA gave an apparent K m value for lysophosphatidylcholine of 27 μ m and a V max value of 155 nmol × h −1 × (mg protein) −1 . The enzyme was inhibited by 25% when ROS membranes were incubated in the presence of 10 m m MgCl 2 . The acyltransferase was able to incorporate other acyl-CoAs (palmitoyl-CoA and arachidonoyl-CoA) into ROS phospholipids and to acylate other lysophospholipids but less efficiently than lysophosphatidylcholine. Lysophoshatidylcholine was preferentially acylated with arachidonic acid followed by oleic acid and, less efficiently, with palmitic acid.The high specific activity of acyl-CoA lysophosphatidylcholine acyltransferase found in purified ROS compared to the activity found in other subcellular fractions of the bovine Retina suggests that this enzymatic activity is native to the ROS.