Photoreceptors
Heterogeneous N-terminal acylation of retinal proteins.
DeMar JC Jr, Rundle DR, Wensel TG, Anderson RE
Department of Biochemistry, Baylor College of Medicine, Houston, Texas 77030, USA.
Photoreceptor membrane proteins, phototransduction, and retinal degenerative diseases. The Friedenwald Lecture.
Molday RS
Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, Canada. molday@unixg.ubc.ca
Eye 1998;12 ( Pt 3b):526-30
Post-Golgi trafficking of rhodopsin in retinal photoreceptors.
Deretic D
Department of Ophthalmology, University of Michigan, Ann Arbor 48105, USA. dereticd@umich.edu
Rod outer segment renewal in retinal rod photoreceptors is mediated by polarised sorting of rhodopsin, and its associated proteins and lipids, on post-Golgi vesicles that bud from the trans-Golgi network and fuse with the specialised domain of the plasma membrane in the rod inner segment. This domain surrounds the cilium that connects the inner segment and the rod outer segment to which mature rhodopsin is delivered. The intracellular sorting machinery that regulates budding, targeting and fusion of rhodopsin carrier vesicles has been studied using multiple means including a newly developed cell-free assay that reconstitutes vesicle budding. These studies have revealed an essential role for small GTP-binding protein rab6, as well as the carboxyl-terminal domain of rhodopsin, in the formation of post-Golgi vesicles. In this report their role in post-Golgi trafficking of rhodopsin and the maintenance of photoreceptor cell polarity and health is discussed.
Distribution of Na+,K(+)-ATPase in photoreceptor cells of insects.
Baumann O
Institut fur Zoophysiologie und Zellbiologie, Universitat Potsdam, Germany.
Light stimulation of insect photoreceptors causes opening of cation channels and an inward current that is partially carried by Na+ ions. There is also an efflux of K+ ions upon photostimulation. Na+ and K+ gradients across the photoreceptor membrane are reestablished by the activity of the enzyme Na+,K(+)-ATPase. About two-thirds of the total amount of ATP consumed in response to a light stimulus is attributed to the activity of this ion pump, demonstrating the importance of this enzyme for photoreceptor function. Insect photoreceptor cells are polarized epithelial cells; their plasma membrane is organized into two domains having a distinct morphology, molecular composition, and function. The visual pigment rhodopsin and the molecular components of the transduction machinery are localized in the rhabdomere, an array of densely packed microvilli, whereas Na+,K(+)-ATPase resides in the nonrhabdomeric membrane. Comparative immunolocalization studies on compound eyes of diverse insect species have demonstrated subtle variations in the distribution patterns of Na+,K(+)-ATPase. These may be accounted for by differences in the mechanisms responsible for Na+,K(+)-ATPase positioning.
Nitric oxide: a review of its role in retinal function and disease.
Goldstein IM, Ostwald P, Roth S
Department of Anesthesia and Critical Care, University of Chicago, IL 60637, USA.
Nitric oxide synthase (NOS), the enzyme that catalyzes the formation of nitric oxide from L-arginine, exists in three major isoforms, neuronal, endothelial, and immunologic. Neuronal and endothelial isoforms are constitutively expressed, and require calcium for activation. Both of these isoforms can be induced (i.e., new protein synthesis occurs) under appropriate conditions. The immunologic isoform is not constitutively expressed, and requires induction usually by immunologic activation; calcium is not necessary for its activation. Neuronal and immunologic NOS have been detected in the retina. Neuronal NOS may be responsible for producing nitric oxide in photoreceptors and bipolar cells. Nitric oxide stimulates guanylate cyclase of photoreceptor rod cells and increases calcium channel currents. In the retina of cats, NOS inhibition impairs phototransduction as assessed by the electroretinogram. Inducible nitric oxide synthase, found in Muller cells and in retinal pigment epithelium, may be involved in normal phagocytosis of the retinal outer segment, in infectious and ischemic processes, and in the pathogenesis of diabetic retinopathy. Nitric oxide contributes to basal tone in the retinal circulation. To date, findings are conflicting with respect to its role in retinal autoregulation. During glucose and oxygen deprivation, nitric oxide may increase blood flow and prevent platelet aggregation, but it may also mediate the toxic effects of excitatory amino acid release. This reactive, short-lived gas is involved in diverse processes within the retina, and its significance continues to be actively studied.
Characterization and possible roles of fibroblast growth factors in retinal photoreceptor cells.
Hicks D
Centre Hospitalier Regional et Universitaire, Strasbourg, France.
There is increasing evidence that soluble polypeptide growth factors such as those belonging to the fibroblast growth factor (FGF) family play important roles in many aspects of photoreceptor cell biology, including differentiation, continued survival and pathology. At least two members of this family, acidic FGF and basic FGF, are synthesized by, bind to and have profound effects upon these highly specialized retinal first-order neurons. The present review presents an overview of the evidence accumulated to date and will try to suggest future directions for research.
Positive and negative signaling mechanisms in the regulation of photoreceptor induction in the developing Drosophila retina. Review.
Yamamoto D
Mitsubishi Kasei Institute of Life Sciences, Tokyo, Japan.
An ommatidium of a Drosophila compound eye contains eight photoreceptor cells, R1-R8. The fates of the photoreceptors are determined exclusively by inductive interactions between neuronal precursors in the cell cluster from which the ommatidium is formed. R7 induction has been extensively analysed at the molecular level. Activation of a membrane receptor tyrosine kinase (Sevenless) in the R7 precursor by a ligand (Bride of sevenless) present on the surface of R8 triggers a transduction cascade mediated by Ras, establishing the R7 fate of this cell. Other Sev-expressing cells are prevented from taking on the R7 fate by several different mechanisms. Pokkuri-mediated repression represents one such regulatory mechanism. The positive and negative signaling pathways operating in the fate determination of other photoreceptor cells are also discussed.
Na+, K(+)-ATPase isoforms in the retina.
Schneider B
Department of Pathology, University of Texas Health Science Center, San Antonio 78284.
Biophysical processes in invertebrate photoreceptors: recent progress and a critical overview based on Limulus photoreceptors.
Nagy K
Institut fur Biologie II der Rheinisch-Westfalischen Technischen Hochschule Aachen.
Limulus ventral nerve photoreceptor, a classical preparation for the study the phototransduction in invertebrate eyes, seems to have a very complex mechanism to transform light energy into a physiological signal. Although the main function of the photoreceptor is to change the membrane conductance according to the illumination, the cell has voltage-activated conductances as well. The voltage-gated conductances are matched to the light-activated ones in the sense that they make the function of the cell more efficient. The complex mechanism of phototransduction and the presence of four different voltage-gated conductance in Limulus ventral nerve photoreceptors indicate that these cells are far less differentiated than the photoreceptor cells of vertebrates. Indications accumulated in recent years support the view that the ventral photoreceptor of Limulus has different light-activated macroscopic current components, ion channels and terminal transmitters. After conclusions from macroscopic current measurements (Payne, 1986; Payne et al. 1986 a, b), direct evidence was presented by single-channel (Nagy & Stieve, 1990 a, b; Nagy, 1990 a, b) and macroscopic current measurements (Deckert et al. 1991 a, b) for three different light-activated conductances. It has been shown that two of these conductances are stimulated by two different excitation mechanisms. The two mechanisms, having different kinetics, release probably two different transmitters. One of them might be the cGMP (Johnson et al. 1986), the other one the calcium ion (Payne et al. 1986 a, b). However, the biochemical processes which link the rhodopsin molecules and the ion channels are not known. The unknown chemical details of the phototransduction result in a delay for the mathematical description of the biophysical mechanisms. More biochemical details are known about the adaptation mechanism. It was found that inositol 1,4,5-trisphosphate is a messenger for the release of calcium ions from the intracellular stores and that calcium ions are the messengers for adaptation (Payne et al. 1986 b; Payne & Fein, 1987). Concerning the mechanism of calcium release, it was revealed that a negative feedback acts on the enzyme cascade to regulate the internal calcium level and to protect the stores against complete emptying (Payne et al. 1988, 1990). Calcium ions also play an important role in the excitation mechanism. (a) In [Ca2+]i-depleted cells the light-induced current was increased after intracellular Ca2+ injection, suggesting that calcium is necessary for the transduction mechanism (Bolsover & Brown, 1985).
Molecular genetic studies of photoreceptor function using Drosophila mutants.
Pak WL
Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907.