Calcium-dependent regulation of cyclic GMP phosphodiesterase by a protein from frog retinal rods.

In vertebrate photoreceptors, light reduces cyclic GMP concentration and closes cGMP-activated channels to induce a hyperpolarizing response. As Ca2+ can permeate the channels and the Na(+)-Ca2+ exchanger continuously extrudes Ca2+, closure of the channel results in a reduction of the inter-rod Ca2+ concentration. This is believed to be one of the mechanisms of light-adaptation produced by activation of guanylate cyclase. Effects of Ca2+ on the cGMP phosphodiesterase (PDE) have been reported, but their physiological significance has remained unclear. We have perfused the inside-out preparation of a frog rod outer segment (I/O ROS, originally termed truncated ROS, and find that Ca2+ in a physiological range regulates the light-activation of PDE. Therefore, PDE regulation by Ca2+ must be involved in light-adaptation in rods. The effect is mediated by a newly found protein which binds to disk membranes at high Ca2+ concentrations and prolongs PDE activation.     

ATP mediates rapid reversal of cyclic GMP phosphodiesterase activation in visual receptor membranes

Weak or strong lights will activate visual receptor rod disk membrane (RDM) cyclic GMP phosphodiesterase (PDE) in the presence of GTP cofactor. A similarly activated GTPase can exhaust small amounts of initially present GTP to deactivate the PDE. However, further additions of GTP reactivate PDE without more light, and deactivation by simple GTP depletion takes minutes or more, even at GTP concentrations 100 to 1,000 times lower than physiological levels. A more rapid deactivation mechanism must exist if modulation of cytoplasmic cyclic GMP by light is to play a role on the time scale (seconds) of events in vision. We report here that ATP is essential to such rapid control and that its presence permits multiple cycles of activation-deactivation. The complete control mechanism seems to involve gamma phosphate transfer from both ATP and GTP.     

Induction by cyclic GMP of cationic conductance in plasma membrane of retinal rod outer segment

Vertebrate rod photoreceptors hyperpolarize when illuminated, due to the closing of cation-selective channels in the plasma membrane. The mechanism controlling the opening and closing of these channels is still unclear, however. Both 3',5'-cyclic GMP and Ca2+ ions have been proposed as intracellular messengers for coupling the light activation of the photopigment rhodopsin to channel activity and thus modulating light-sensitive conductance. We have now studied the effects of possible conductance modulators on excised 'inside-out' patches from the plasma membrane of the rod outer segment (ROS), and have found that cyclic GMP acting from the inner side of the membrane markedly increases the cationic conductance of such patches (EC50 30 microM cyclic GMP) in a reversible manner, while Ca2+ is ineffective. The cyclic GMP-induced conductance increase occurs in the absence of nucleoside triphosphates and, hence, is not mediated by protein phosphorylation, but seems rather to result from a direct action of cyclic GMP on the membrane. The effect of cyclic GMP is highly specific; cyclic AMP and 2',3'-cyclic GMP are completely ineffective when applied in millimolar concentrations. We were unable to recognize discrete current steps that might represent single-channel openings and closings modulated by cyclic GMP. Analysis of membrane current noise shows the elementary event to be 3 fA with 110 mM Na+ on both sides of the membrane at a membrane potential of -30 mV. If the initial event is assumed to be the closure of a single cyclic GMP-sensitive channel, this value corresponds to a single-channel conductance of 100 fS. It seems probable that the cyclic GMP-sensitive conductance is responsible for the generation of the rod photoresponse in vivo.     

Effects on the photoresponse of calcium buffers and cyclic GMP incorporated into the cytoplasm of retinal rods

It is generally accepted that the light response in retinal rods involves a reduction of ionic permeability (predominantly to Na+) in the plasma membrane of the outer segment and that this is mediated by an internal messenger which diffuses between the disk and plasma membranes. There is controversy, however, over the identity of the diffusible substance; two alternative schemes have received widespread support (for review see refs 1,2). According to the 'calcium hypothesis', light stimulates the release into the cytoplasm of calcium, leading to the blockage of channels which are normally open in darkness, whereas based on the 'cyclic nucleotide hypothesis', cyclic GMP causes the opening of channels in the dark, but is hydrolysed by a light-activated phosphodiesterase. We report here effects of introducing calcium buffers and cyclic GMP into the rod cytoplasm by means of a patch pipette, which seem to be inconsistent with the calcium hypothesis.     

Cyclic GMP increases photocurrent and light sensitivity of retinal cones

Like retinal rods, cone photoreceptors contain cyclic GMP and light-activated phosphodiesterase. The cGMP phosphodiesterase cascade is thought to mediate phototransduction in rods. Biochemical assays of nucleotide content in cone-dominant retinas, however, have failed to demonstrate light-induced changes in cGMP. Changes in cyclic AMP following light exposure have been reported, leading to the suggestion that in cone phototransduction cAMP assumes a role analogous to that played by cGMP in rods. Cyclic GMP introduced from tight-seal pipettes into isolated cones of the larval tiger salamander, Ambystoma tigrinum, rapidly increases light-modulated membrane current more than 10-fold. In cones, as in rods, cGMP also causes an approximately 10-fold increase in photocurrent duration and a 5- to 10-fold increase in light-sensitivity. Cyclic AMP has no effect on cone photocurrents under the same conditions. Because cGMP has similar effects on photocurrent magnitude and kinetics in both rods and cones, we conclude that cGMP plays corresponding roles in transduction in both vertebrate photoreceptor classes.     

Prevention of natural motoneurone cell death by dibutyryl cyclic GMP

Natural neuronal cell death is a well-described developmental phenomenon common to many nerve centres in a variety of animal species. Neuronal survival has been shown to depend on the presence and size of the available target tissue and it has been suggested that neuronal survival is dependent on successful competition for either a limited number of synaptic sites or a limited amount of trophic factor(s). In the lateral motor column of the lumbar spinal cord in the chick embryo, the period of axon elongation and innervation of the periphery has been shown to precede that of natural motoneurone cell death. While muscle contractile activity appears to regulate the extent of motoneurone death, to date the intracellular molecular events that initiate and regulate the developmental process of natural neuronal cell death or, more importantly, neuronal survival are unknown. Our earlier studies suggested that either contact or association between spinal cord processes and muscle cells during neuromuscular junction formation in vivo leads to an increase in cyclic GMP in whole spinal cord. We now show that treatment of chick embryos with the membrane-permeable cyclic GMP analogue, dibutyryl cyclic GMP during the period of natural motoneurone cell death prevents greater than 58% of natural motoneurone cell death in the lumbar lateral motor column.     

Opposite effects of cyclic GMP and cyclic AMP on Ca2+ current in single heart cells

The slow inward Ca2+ current, ICa, is fundamental in the initiation of cardiac contraction and neurohormonal regulation of cardiac function. It is increased by beta-adrenergic agonists, which stimulate synthesis of cyclic AMP (cAMP) and cAMP-dependent phosphorylation. The neurotransmitter acetylcholine reduces ICa by an unknown mechanism. There is strong evidence that acetylcholine reduces ICa by decreasing adenylate cyclase activity, but cGMP has also been implicated as ACh stimulates cGMP accumulation and activates cGMP-dependent protein kinase. Application of cGMP decreases contractile force, decreases Ca flux, shortens the duration of action potentials and inhibits Ca-dependent action potentials. Other studies, however, have concluded that cGMP levels do not correlate with contractile force and that cGMP has no effect on ICa. We have therefore examined the effects of intracellular perfusion of cGMP on ICa using isolated, voltage-clamped cells from frog ventricle. We find that cGMP has negligible effects on basal ICa, but greatly decreases the ICa that had been elevated by beta-adrenergic agonists or by intracellular perfusion with cAMP. The decrease of ICa is mediated by cAMP hydrolysis via a cGMP-stimulated cyclic nucleotide phosphodiesterase.     

Insulin trigger, cyclic AMP-dependent activation and phosphorylation of a plasma membrane cyclic AMP phosphodiesterase

Regulation of blood glucose levels by the liver is primarily achieved by the action of two peptide hormones, insulin and glucagon, which bind to specific receptors associated with the hepatocyte plasma membrane. Whilst the molecular action of glucagon at the level of the cell plasma membrane in activating adenylate cyclase is relatively well understood, we know little, if anything, of the molecular consequences of insulin occupying its receptor. We demonstrate here that insulin, at physiologically relevant concentrations, can trigger the cyclic AMP-dependent activation and phosphorylation of a low Km cyclic AMP phosphodiesterase attached to the liver plasma membrane. Such an effect may in part explain the ability of insulin to inhibit the increase in cellular cyclic AMP content that glucagon alone produces by activation of adenylate cyclase. Our observation that basal, intracellular cyclic AMP levels are insufficient to allow insulin to activate the cyclic AMP phosphodiesterase, yet those cyclic AMP levels achieved after exposure of the cells to glucagon are sufficient, gives a molecular rationale to Butcher and Sutherland's proposal that it is necessary to first elevate cellular cyclic AMP levels before they can be depressed by insulin.     

Control of Ca2+ in rod outer segment disks by light and cyclic GMP

Photons absorbed in vertebrate rods and cones probably cause electrochemical changes at the photoreceptor plasma membrane by changing the cytoplasmic concentration of a diffusible transmitter substance, reducing the Na+ current flowing into the outer segment of the cell in the dark, to produce the observed membrane hyperpolarization that is the initial excitatory response. Cyclic GMP has been proposed as the transmitter because a light-activated cyclic GMP phosphodiesterase (PDE) has been found in rod disk membranes and because intracellularly injected cyclic GMP reduces rod membrane potentials. Free Ca2+ has also been proposed because increasing external [Ca2+] quickly and reversibly reduces the dark current and divalent cationophores increase the Ca2+ sensitivity. Ca2+ efflux from rod outer segments (ROS) of intact retinas occurs simultaneously with light responses. Vesicles prepared from ROS disk membranes become more permeable on illumination, releasing trapped ions or molecules, but intact outer segment disks have not previously been found to store sufficient Ca2+ in darkness and to release enough in light to meet the theoretical requirements for control of the dark current by varying cytoplasmic Ca2+ (refs 14-18). We now report experiments that show the required Ca2+ storage and release from rod disk membranes suspended in media containing high-energy phosphate esters and electrolytes approximating the cytoplasmic composition of live rod cells. Cyclic GMP stimulates Ca2+ uptake by ROS disks in such media.     

Diurnal cycles in serotonin acetyltransferase activity and cyclic GMP content of cultured chick pineal glands

Levels of serotonin N-acetyltransferase (NAT: acetul CoA:arylamine N-acetyltransferase; EC 2.1.1.5.) activity in the chick pineal gland exhibit a marked diurnal variation in birds kept under a diurnal cycle of ilumination. Activity begins to rise rapidly at the start of the dark phase of the cycle and reaches maximum levels at mid-dark phase about 25-fold greater than the minimum basal level at mid-light phase. Thereafter, the level of activity declines to the basal level about the start of the light phase. This diurnal cycle in chick pineal NAT activity found in vivo has recently been reproduced in vitro with intact glands incubated in organ culture. The mechanism of the 'biological clock' which regulates these variations in level of chick pineal NAT activity is unknown. However, I now report that chick pineal glands cultured under a diurnal cycle of illumination exhibit a diurnal cycle in content of cyclic GMP which roughly parallels the cycles in NAT activity. In contrast, there was no correlation between variations in pineal content of cyclic AMP and in level of NAT activity.     

Defect in cyclic AMP phosphodiesterase due to the dunce mutation of learning in Drosophila melanogaster

Cyclic AMP is an intracellular mediator ('second messenger') in the nervous and endocrine control of cellular function, regulating different processes in different cell types. Although evidence is incomplete, it seems that cyclic AMP enhances the calcium-mediated release of neurotransmitter in some neurones. A simple form of memory in the mollusc Aplysia is probably encoded as a cyclic AMP-induced enhancement of neurotransmission at certain synapses of the central nervous system. The possibility that cyclic AMP participates in learning mechanisms may be explored using genetic mutants. For this purpose the fruitfly Drosophila is suitable as it is genetically well characterized and can learn through olfaction, vision or taste. We show here that independent searches for mutations of olfactory learning and of cyclic AMP metabolism, and for mutations causing female infertility have each led to the same gene--the dunce gene. Our evidence indicates that the normal dunce gene may specify a cyclic AMP phosphodiesterase.     

Photoreceptor degeneration in inherited retinal dystrophy delayed by basic fibroblast growth factor

Numerous inherited retinal degenerations exist in animals and humans, in which photoreceptors inexplicably degenerate and disappear. In RCS rats with inherited retinal dystrophy, the mutant gene is expressed in the retinal pigment epithelial (RPE) cell, and leads to the loss of photoreceptor cells. Photoreceptors can be rescued from degeneration if they are juxtaposed to wild-type RPE cells in experimental chimaeras or by the transplantation of RPE cells from normal rats. In both cases, the rescue effect extends beyond the immediate boundaries of the normal RPE cells, suggesting trophic action of a diffusible factor(s) from the normal RPE cells. We considered that the fibroblast growth factors, aFGF and bFGF, might have such a trophic role as they are found in the retina and RPE cells; bFGF acts as a neurotrophic agent after axonal injury in several regions of the central nervous system, and bFGF induces retinal regeneration from developing RPE cells. Here we report that subretinal injection of bFGF results in extensive rescue of photoreceptors in RCS rats for at least two months after the injection, and that intravitreal injection of bFGF results in even more widespread rescue, across almost the entire retina. The findings demonstrate for the first time that bFGF can act as a survival-promoting neurotrophic factor in a hereditary neuronal degeneration of the central nervous system.     

Mutations in the human retinal degeneration slow gene in autosomal dominant retinitis pigmentosa.

The murine retinal degeneration slow (rds) gene is a semidominant mutation with a phenotype having rod and cone photoreceptors that develop abnormally and then slowly degenerate. The phenotype is a possible model for retinitis pigmentosa, one of the scores of hereditary human retinal degenerations, which is also characterized by photoreceptor degeneration. We report here three mutations of the human homologue of the rds gene (RDS) that cosegregate with autosomal dominant retinitis pigmentosa in separate families. Our results indicate that some cases of autosomal dominant retinitis pigmentosa are due to mutations at the RDS locus.     

Identification of a photoreceptor-specific mRNA encoded by the gene responsible for retinal degeneration slow (rds)

Mutant mice homozygous for 'retinal degeneration slow' (rds/rds) are characterized phenotypically by abnormal development of photoreceptor outer segments in the retina, followed by gradual degeneration of photoreceptors. This process of degeneration is complete by one year, with preservation of all other retinal cells. The biochemical defect that leads to the mutant phenotype is not known. Our strategy for cloning the rds gene was based upon three previously reported observations. First, the rds locus maps to chromosome 17. Second, experimental rds/rds----+/+ and rds/+----+/+ tetra-parental mice manifest patchy photoreceptor changes in the retina, suggesting that the wild-type rds locus is expressed within cells of the photoreceptor lineage. Finally, the process of degeneration is specific to photoreceptors. On the basis of these observations, we predicted that the rds mRNA is encoded by a gene on chromosome 17 and is normally expressed exclusively within photoreceptors in the retina. We here present evidence that this is the case.     

Cyclic GMP is involved in the excitation of invertebrate photoreceptors

The hyperpolarizing receptor potential in vertebrate rod photoreceptors appears to be mediated by the second messenger, cyclic GMP. Injection of cGMP into rods or application of cGMP to inside-out membrane patches activates a conductance resembling that produced by light. Light produces a rapid reduction of cGMP in living rods, leading to closure of sodium channels and membrane hyperpolarization. In most invertebrate photoreceptors the response to light is depolarizing. We have investigated whether cGMP is involved in controlling the increase in sodium conductance that underlies this depolarization. We show here that injection of cGMP into Limulus photoreceptors produces a depolarization that mimics the receptor potential. We also show that the cGMP concentration of the squid retina increases rapidly during exposure to light. These results support the hypothesis that cGMP mediates the light-induced depolarization in invertebrate photoreceptors and suggests that vertebrate and invertebrate phototransduction may be more similar than previously thought.