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Biomedical subjects

R Ranganathan

Publications and source records attributed to R Ranganathan.

At least 37 records · Page 2Linked to original sources
PubMed1995

Arrestin binding determines the rate of inactivation of the G protein-coupled receptor rhodopsin in vivo.

G protein-coupled receptor inactivation is a crucial feature of cellular signaling systems; this process determines the catalytic lifetime of the activated receptor and is necessary for response termination. Although previous work has indicated a class of models in which several sequential steps are required for receptor inactivation, the rate-limiting event is still unclear. In this paper, we develop a theory that describes the kinetics of inactivation of the G protein-coupled receptor rhodopsin based on the rate of arrestin binding and test the theory using a combination of genetic and electrophysiological techniques in Drosophila photoreceptors. The theory quantitatively describes the inactivation kinetics of activated rhodopsin in vivo and can be independently tested with molecular and spectroscopic data. The results demonstrate that the rate of arrestin binding determines the kinetics of receptor inactivation in vivo and thus is the event that controls signal amplification at the first step of this G protein-coupled transduction cascade.

Animals↗
PubMed1994

Cytosolic calcium transients: spatial localization and role in Drosophila photoreceptor cell function.

Drosophila phototransduction is a phosphoinositide-mediated and Ca(2+)-regulated signaling cascade ideal for the dissection of feedback regulatory mechanisms. To study the roles of intracellular Ca2+ ([Ca2+]i) in this process, we developed novel techniques for the measurement of [Ca2+]i in intact photoreceptors. We genetically engineered flies that express a UV-specific rhodopsin in place of the normal rhodopsin, so that long wavelength light can be used to image [Ca2+]i changes while minimally exciting the photoreceptor cells. We show that activation with UV generates [Ca2+]i increases that are spatially localized to the rhabdomeres and that are entirely dependent on the influx of extracellular Ca2+. Application of intracellular Ca2+ chelators of varying affinities demonstrates that the Ca2+ influx initially generates a large-amplitude transient that is crucial for negative regulation. Internal Ca2+ stores were revealed by discharging them with thapsigargin. But, in contrast to proposals that IP3-sensitive stores mediate phototransduction, thapsigargin does not mimic or acutely interfere with photoexcitation. Finally, we identify a photoreceptor-specific PKC as essential for normal kinetics of [Ca2+]i recovery.

Animals↗
PubMed1993

Arrestin function in inactivation of G protein-coupled receptor rhodopsin in vivo.

Arrestins have been implicated in the regulation of many G protein-coupled receptor signaling cascades. Mutations in two Drosophila photoreceptor-specific arrestin genes, arrestin 1 and arrestin 2, were generated. Analysis of the light response in these mutants shows that the Arr1 and Arr2 proteins are mediators of rhodopsin inactivation and are essential for the termination of the phototransduction cascade in vivo. The saturation of arrestin function by an excess of activated rhodopsin is responsible for a continuously activated state of the photoreceptors known as the prolonged depolarized afterpotential. In the absence of arrestins, photoreceptors undergo light-dependent retinal degeneration as a result of the continued activity of the phototransduction cascade. These results demonstrate the fundamental requirement for members of the arrestin protein family in the regulation of G protein-coupled receptors and signaling cascades in vivo.

Amino Acid Sequence↗
PubMed1991

Photoreceptor deactivation and retinal degeneration mediated by a photoreceptor-specific protein kinase C.

The protein kinase C (PKC) family of serine-threonine kinases has been implicated in the regulation of a variety of signaling cascades. One member of this family, eye-PKC, is expressed exclusively in the Drosophila visual system. The inaC (inactivation-no-afterpotential C) locus was shown to be the structural gene for eye-PKC. Analysis of the light response from inaC mutants showed that this kinase is required for the deactivation and rapid desensitization of the visual cascade. Light adaptation was also defective in inaC mutant flies. In flies carrying the retinal degeneration mutation rdgB, absence of eye-PKC suppressed photoreceptor cell degeneration. These results indicate that eye-PKC functions in the light-dependent regulation of the phototransduction cascade in Drosophila.

Adaptation, Physiological↗
PubMed1991

A Drosophila mutant defective in extracellular calcium-dependent photoreceptor deactivation and rapid desensitization.

CALCIUM is involved in the adaptation of vertebrate photoreceptors to light and may have a similar role in invertebrate phototransduction. But the molecular mechanisms mediating this stimulus-dependent regulation are not well understood in any G protein-coupled transduction system. We have developed a preparation of isolated Drosophila photoreceptors that has allowed us to carry out an electrophysiological characterization of the light-activated response in these sensory neurons using patch-clamp techniques. We report here that extracellular calcium entering through the light-activated conductance is a key regulator of both the activation and deactivation phases of the phototransduction cascade, and that inaC mutant photoreceptors are specifically defective in the calcium-dependent deactivation mechanism. These data suggest that the light-dependent calcium influx inactivates this cascade through a biochemical pathway that requires the inaC gene product, and that this mechanism represents a molecular basis for stimulus-dependent regulation of visual transduction in Drosophila photoreceptors.

Animals↗
PubMed1991

The molecular genetics of invertebrate phototransduction.

Phototransduction, the primary event in the processing of visual stimuli, is the conversion of light energy into a change in the ionic permeabilities of the photoreceptor cell membrane. In both vertebrates and invertebrates, this process is carried out through a specialized form of a G-protein-coupled receptor cascade. The mechanisms that mediate visual excitation in the vertebrate photoreceptor have been physiologically and biochemically well characterized, and many aspects of this system have served as prototypes for other transduction cascades. However, there are still many unresolved issues in vertebrate phototransduction. The study of phototransduction in Drosophila offers a unique opportunity to make use of powerful molecular genetic techniques to identify novel transduction molecules, and then to examine the function of these molecules in vivo, in their normal cellular environment. The results of a combination of molecular, genetic, physiological and biochemical studies are beginning to produce a clearer model for the complex mechanisms involved in invertebrate visual transduction.

Animals↗
PubMed1991

An electrophoretic method to evaluate DNA polymerase activity.

We have devised an easy non-radioactive assay to evaluate the activity of six different DNA polymerases: the Klenow fragment of E.coli DNA Polymerase I, Taq DNA Polymerase, Sequenase, Moloney Murine Reverse Transcriptase, T7 DNA Polymerase and E.coli DNA Polymerase I. The method is based on the differential mobility of single stranded DNA compared to that of double-stranded DNA in agarose gel electrophoresis.

DNA↗
PubMed1989

Detection of posttransplant minimal disease chronic myelogenous leukemia by bcr rearrangement analysis.

Until recently, cytogenetic detection of the Philadelphia chromosome (Ph) was the only reliable test to diagnose chronic myelogenous leukemia (CML) and detect minimal disease or early relapse following treatment. However, the recently developed ability to detect the Ph chromosome as a rearrangement in the bcr gene of chromosome 22 permits identification of the leukemic clone comprising as little as 5% of the cell population. We present results of simultaneous cytogenetic and DNA rearrangement studies in 28 CML patients considered for or treated with bone marrow transplantation. Our results show that the molecular method is significantly more sensitive than the cytogenetic method in the detection of minimal disease Ph-positive clones.

Bone Marrow Transplantation↗