Cyclic GMP directly regulates a cation conductance in membranes of bovine rods by a cooperative mechanism

Inherited Retinal Degeneration: PARP-Dependent Activation of Calpain Requires CNG Channel Activity

Inherited retinal degenerations (IRDs) are a group of blinding diseases, typically involving a progressive loss of photoreceptors. The IRD pathology is often based on an accumulation of cGMP in photoreceptors and associated with the excessive activation of calpain and poly (ADP-ribose) polymerase (PARP). Inhibitors of calpain or PARP have shown promise in preventing photoreceptor cell death, yet the relationship between these enzymes remains unclear. To explore this further, organotypic retinal explant cultures derived from wild-type and IRD-mutant mice were treated with inhibitors specific for calpain, PARP, and voltage-gated Ca²⁺ channels (VGCCs). The outcomes were assessed using in situ activity assays for calpain and PARP and immunostaining for activated calpain-2, poly (ADP-ribose), and cGMP, as well as the TUNEL assay for cell death detection. The IRD models included the Pde6b-mutant rd1 mouse and rd1*Cngb1^−/− double-mutant mice, which lack the beta subunit of the rod cyclic nucleotide-gated (CNG) channel and are partially protected from rd1 degeneration. We confirmed that an inhibition of either calpain or PARP reduces photoreceptor cell death in rd1 retina. However, while the activity of calpain was decreased by the inhibition of PARP, calpain inhibition did not alter the PARP activity. A combination treatment with calpain and PARP inhibitors did not synergistically reduce cell death. In the slow degeneration of rd1*Cngb1^−/− double mutant, VGCC inhibition delayed photoreceptor cell death, while PARP inhibition did not. Our results indicate that PARP acts upstream of calpain and that both are part of the same degenerative pathway in Pde6b-dependent photoreceptor degeneration. While PARP activation may be associated with CNG channel activity, calpain activation is linked to VGCC opening. Overall, our data highlights PARP as a target for therapeutic interventions in IRD-type diseases.

D-cis-Diltiazem Can Produce Oxidative Stress in Healthy Depolarized Rods In Vivo

Purpose

New perspectives are needed to understand decades of contradictory reports on the neuroprotective effects of the Cav1.2 L-type calcium channel blocker d-cis-diltiazem in retinitis pigmentosa (RP) models. Here, we address, in vivo, the following two knowledge gaps regarding d-cis-diltiazem's actions in the murine outer retina: (1) do normal mouse rods contain d-cis-diltiazem-insensitive Cav1.2 L-type calcium channels? (2) Can d-cis-diltiazem modify the normal rod redox environment?

Methods

First, transretinal Cav1.2 L-type calcium channels were noninvasively mapped with manganese-enhanced magnetic resonance imaging (MRI) following agonist Bay K 8644 in C57BL/6 (B6) and in Cav1.2 L-type calcium channel BAY K 8644-insensitive mutant B6 mice. Second, d-cis-diltiazem-treated oxidative stress-vulnerable (B6) or -resistant [129S6 (S6)] mice were examined in vivo (QUEnch-assiSTed [QUEST] MRI) and in whole retina ex vivo (lucigenin). Retinal thickness was measured using MRI.

Results

The following results were observed: (1) manganese uptake patterns in BAY K 8644-treated controls and mutant mice identified in vivo Cav1.2 L-type calcium channels in inner and outer retina; and (2) d-cis-diltiazem induced rod oxidative stress in dark-adapted B6 mice but not in light-adapted B6 mice or dark-adapted S6 mice (QUEST MRI). Oxidative stress in vivo was limited to inferior outer retina in dark-adapted B6 mice approximately 1-hour post d-cis-diltiazem. By approximately 4 hours post, only superior outer retina oxidative stress was observed and whole retinal superoxide production was supernormal. All groups had unremarkable retinal thicknesses.

Conclusions

D-cis-diltiazem's unexpectedly complex spatiotemporal outer retinal oxidative stress pattern in vivo was dependent on genetic background and rod membrane depolarization, but not apparently dependent on Cav1.2 L-type calcium channels, providing a potential rationale for contradictory results in different RP models.

Photoreceptor Guanylate Cyclase (GUCY2D) Mutations Cause Retinal Dystrophies by Severe Malfunction of Ca2+-Dependent Cyclic GMP Synthesis

Over 100 mutations in GUCY2D that encodes the photoreceptor guanylate cyclase GC-E are known to cause two major diseases: autosomal recessive Leber congenital amaurosis (arLCA) or autosomal dominant cone-rod dystrophy (adCRD) with a poorly understood mechanism at the molecular level in most cases. Only few mutations were further characterized for their enzymatic and molecular properties. GC-E activity is under control of neuronal Ca²⁺-sensor proteins, which is often a possible route to dysfunction. We investigated five recently-identified GC-E mutants that have been reported in patients suffering from arLCA (one large family) and adCRD/maculopathy (four families). Microsatellite analysis revealed that one of the mutations, c.2538G > C (p.K846N), occurred de novo. To better understand the mechanism by which mutations that are located in different GC-E domains develop different phenotypes, we investigated the molecular consequences of these mutations by expressing wildtype and mutant GC-E variants in HEK293 cells. Analyzing their general enzymatic behavior, their regulation by Ca²⁺ sensor proteins and retinal degeneration protein 3 (RD3) dimerization domain mutants (p.E841K and p.K846N) showed a shift in Ca²⁺-sensitive regulation by guanylate cyclase-activating proteins (GCAPs). Mutations in the cyclase catalytic domain led to a loss of enzyme function in the mutant p.P873R, but not in p.V902L. Instead, the p.V902L mutation increased the guanylate cyclase activity more than 20-fold showing a high GCAP independent activity and leading to a constitutively active mutant. This is the first mutation to be described affecting the GC-E catalytic core in a complete opposite way.

Speract-receptor interaction and the modulation of ion transport in Strongylocentrotus purpuratus sea urchin sperm

We are studying the regulation of ion transport in sperm physiology. Sperm ion permeability is modulated by components from the outer layer of the egg which, depending on the species, regulate sperm motility, Chemotaxis and the acrosome reaction (AR). This reaction is required for sperm to fertilise the egg in many species from sea urchins to man (Darszon et al., 1999).

Speract, a decapeptide from the external layer of Strongylocentrotus purpuratus sea urchin eggs, influences sperm respiration, motility and possibly the AR. Signal transduction starts when speract binds to a protein of 77 kDa closely coupled to sperm guanylyl cyclase (Garbers, 1989). Our recent receptor binding experiments using fluorescent-labelled speract (fluorescein and rhodamine) have allowed estimates of the association (kon 2.4 × 107 M−1s−1) and dissociation rate constants (koff 1.3 × 10−4 s−1). Furthermore, studies with fluorescent speract analogues indicate that the receptor undergoes conformational changes that depend on intracellular pH (pHi). The overall results are consistent with the possibility that speract may induce in sea urchin sperm a hyperactivated-like flagellar movement inside the jelly coat to accelerate sperm penetration through this layer.

Ion Channels in the Eye: Involvement in Ocular Pathologies

The eye is the sensory organ of vision. There, the retina transforms photons into electrical signals that are sent to higher brain areas to produce visual sensations. In the light path to the retina, different types of cells and tissues are involved in maintaining the transparency of avascular structures like the cornea or lens, while others, like the retinal pigment epithelium, have a critical role in the maintenance of photoreceptor function by regenerating the visual pigment. Here, we have reviewed the roles of different ion channels expressed in ocular tissues (cornea, conjunctiva and neurons innervating the ocular surface, lens, retina, retinal pigment epithelium, and the inflow and outflow systems of the aqueous humor) that are involved in ocular disease pathophysiologies and those whose deletion or pharmacological modulation leads to specific diseases of the eye. These include pathologies such as retinitis pigmentosa, macular degeneration, achromatopsia, glaucoma, cataracts, dry eye, or keratoconjunctivitis among others. Several disease-associated ion channels are potential targets for pharmacological intervention or other therapeutic approaches, thus highlighting the importance of these channels in ocular physiology and pathophysiology.

Diminished vision in healthy aging is associated with increased retinal L-type voltage gated calcium channel ion influx

Extensive evidence implicates an increase in hippocampal L-type voltage-gated calcium channel (L-VGCC) expression, and ion influx through these channels, in age-related cognitive declines. Here, we ask if this “calcium hypothesis" applies to the neuroretina: Is increased influx via L-VGCCs related to the well-documented but poorly-understood vision declines in healthy aging? In Long-Evans rats we find a significant age-related increase in ion flux through retinal L-VGCCs in vivo (manganese-enhanced MRI (MEMRI)) that are longitudinally linked with progressive vision declines (optokinetic tracking). Importantly, the degree of retinal Mn²⁺ uptake early in adulthood significantly predicted later visual contrast sensitivity declines. Furthermore, as in the aging hippocampus, retinal expression of a drug-insensitive L-VGCC isoform (α_1D) increased – a pattern confirmed in vivo by an age-related decline in sensitivity to L-VGCC blockade. These data highlight mechanistic similarities between retinal and hippocampal aging, and raise the possibility of new treatment targets for minimizing vision loss during healthy aging.

The styryl dye FM1-43 suppresses odorant responses in a subset of olfactory neurons by blocking cyclic nucleotide-gated (CNG) channels

Many olfactory receptor neurons use a cAMP-dependent transduction mechanism to transduce odorants into depolarizations. This signaling cascade is characterized by a sequence of two currents: a cation current through cyclic nucleotide-gated channels followed by a chloride current through calcium-activated chloride channels. To date, it is not possible to interfere with these generator channels under physiological conditions with potent and specific blockers. In this study we identified the styryl dye FM1-43 as a potent blocker of native olfactory cyclic nucleotide-gated channels. Furthermore, we characterized this substance to stain olfactory receptor neurons that are endowed with cAMP-dependent transduction. This allows optical differentiation and pharmacological interference with olfactory receptor neurons at the level of the signal transduction.

Effects of calcium ion, calpains, and calcium channel blockers on retinitis pigmentosa

Recent advances in molecular genetic studies have revealed many of the causative genes of retinitis pigmentosa (RP). These achievements have provided clues to the mechanisms of photoreceptor degeneration in RP. Apoptosis is known to be a final common pathway in RP and, therefore, a possible therapeutic target for photoreceptor rescue. However, apoptosis is not a single molecular cascade, but consists of many different reactions such as caspase-dependent and caspase-independent pathways commonly leading to DNA fractionation and cell death. The intracellular concentration of calcium ions is also known to increase in apoptosis. These findings suggest that calpains, one of the calcium-dependent proteinases, play some roles in the process of photoreceptor apoptosis and that calcium channel antagonists may potentially inhibit photoreceptor apoptosis. Herein, the effects of calpains and calcium channel antagonists on photoreceptor degeneration are reviewed.

Do calcium channel blockers rescue dying photoreceptors in the Pde6b ( rd1 ) mouse?

Retinitis pigmentosa (RP) is a genetically heterogeneous set of blinding diseases that affects more than a million people worldwide. In humans, ~5-8% of recessive and dominant RP cases are caused by nonsense mutations in the Pde6b gene coding for the ss-subunit of the rod photoreceptor cGMP phosphodiesterase 6 (PDE6-ss). The study of the disease has been greatly aided by the Pde6b ( rd1 ) (rd1) mouse model of RP carrying a null PDE6ss allele. Degenerating rd1 rods were found to experience a pathological increase in intracellular calcium concentration ('Ca overload') when they enter the apoptotic process at postnatal day 10. A 1999 study suggested that the Ca(2+) channel antagonist D-cis diltiazem delays the kinetics of rd1 rod degeneration, conferring partial rescue of scotopic vision. Subsequent reports were mixed: whereas several studies failed to replicate the original results, others appeared to confirm the neuroprotective effects of Ca(2+) channel antagonists such as diltiazem, nilvadipine and verapamil. We discuss the discrepancies between the results of different groups and suggest plausible causes for the discordant results. We also discuss potential involvement of recently identified Ca(2+)-dependent mechanisms that include protective calcium ATPase mechanisms, ryanodine and IP3 calcium stores, and store operated channels in Pde6b ( rd1 ) neurodegeneration.

Presence of cyclic nucleotide-gated channels in the rat urethra and their involvement in nerve-mediated nitrergic relaxation

We have addressed the distribution of cGMP-gated channels (CNG) in the rat urethra for the first time, as well as their putative role in mediating of the relaxation elicited by electrical field stimulation of nitrergic nerves. Functional studies have shown that specifically blocking CNG with L-cis-diltiazem leads to the rapid inhibition of urethral relaxation induced either by nitric oxide (NO) released by the nerves or by soluble guanylate cyclase activated with YC-1. By contrast, nerve-mediated noradrenergic contractions were only slowly and mildly reduced by L-cis-diltiazem. This effect was mimicked by lower concentrations of the D-diltiazem isomer, probably due to the nonspecific inhibition of voltage-dependent calcium channels. However, D-diltiazem did not affect relaxation responses. The expression of heteromeric retinal-like CNGA1 channels was demonstrated by conventional PCR on mRNA from the rat urethra. These channels were located in a subpopulation of intramuscular interstitial cells of Cajal (ICC) as well as in smooth muscle cells, although they were less abundant in the latter. CNG channels could not be visualized in any nervous structure within the urethral wall, in agreement with the emerging view that a subset of ICC serves as a target for NO. These channels could provide a suitable ionic mechanism to associate the changes in cytosolic calcium with the activation of the nitric NO-cGMP pathway and relaxation although the precise mechanisms involved remain to be elucidated.

Calcium homeostasis and cone signaling are regulated by interactions between calcium stores and plasma membrane ion channels

Calcium is a messenger ion that controls all aspects of cone photoreceptor function, including synaptic release. The dynamic range of the cone output extends beyond the activation threshold for voltage-operated calcium entry, suggesting another calcium influx mechanism operates in cones hyperpolarized by light. We have used optical imaging and whole-cell voltage clamp to measure the contribution of store-operated Ca²⁺ entry (SOCE) to Ca²⁺ homeostasis and its role in regulation of neurotransmission at cone synapses. Mn²⁺ quenching of Fura-2 revealed sustained divalent cation entry in hyperpolarized cones. Ca²⁺ influx into cone inner segments was potentiated by hyperpolarization, facilitated by depletion of intracellular Ca²⁺ stores, unaffected by pharmacological manipulation of voltage-operated or cyclic nucleotide-gated Ca²⁺ channels and suppressed by lanthanides, 2-APB, MRS 1845 and SKF 96365. However, cation influx through store-operated channels crossed the threshold for activation of voltage-operated Ca²⁺ entry in a subset of cones, indicating that the operating range of inner segment signals is set by interactions between store- and voltage-operated Ca²⁺ channels. Exposure to MRS 1845 resulted in ∼40% reduction of light-evoked postsynaptic currents in photopic horizontal cells without affecting the light responses or voltage-operated Ca²⁺ currents in simultaneously recorded cones. The spatial pattern of store-operated calcium entry in cones matched immunolocalization of the store-operated sensor STIM1. These findings show that store-operated channels regulate spatial and temporal properties of Ca²⁺ homeostasis in vertebrate cones and demonstrate their role in generation of sustained excitatory signals across the first retinal synapse.

How vision begins: an odyssey

Retinal rods and cones, which are the front-end light detectors in the eye, achieve wonders together by being able to signal single-photon absorption and yet also able to adjust their function to brightness changes spanning 10(9)-fold. How these cells detect light is now quite well understood. Not surprising for almost any biological process, the intial step of seeing reveals a rich complexity as the probing goes deeper. The odyssey continues, but the knowledge gained so far is already nothing short of remarkable in qualitative and quantitative detail. It has also indirectly opened up the mystery of odorant sensing. Basic science aside, clinical ophthalmology has benefited tremendously from this endeavor as well. This article begins by recapitulating the key developments in this understanding from the mid-1960s to the late 1980s, during which period the advances were particularly rapid and fit for an intricate detective story. It then highlights some details discovered more recently, followed by a comparison between rods and cones.

Amiloride-insensitive Na+and fluid absorption in the mammalian distal lung

The ability of the distal lung epithelia to actively transport Na+, with Cl−and water following, from the alveolar spaces inversely correlates with morbidity and mortality of infants, children, and adults with alveolar pulmonary edema. It is now recognized, in contrast to many other Na+transporting epithelia, that at least half of this active transport is not sensitive to amiloride, which inhibits the epithelial Na+channel. This paper reviews amiloride-insensitive Na+and fluid transport in the mammalian distal lung unit under basal conditions and speculates on potential explanations for this amiloride-insensitive transport. It also provides new information, using primary cultures of rat fetal distal lung epithelia and alveolar type II cells grown under submersion and air-liquid interface culture conditions, regarding putative blockers of this transport.

Detection and localization of a putative cyclic-GMP-activated channel protein in the protozoan ciliate Stentor coeruleus

Immunoblotting and immunocytochemical assays were employed to identify and localize a channel protein activated by cyclic GMP (cGMP) in the protozoan ciliate Stentor coeruleus. Analysis of whole-cell homogenate with antibodies raised against the alpha-subunit of the cGMP-activated channel protein from bovine rod outer segments and against cGMP revealed four major protein bands with molecular masses of 40 kDa, 63 kDa, and over 120 kDa, which bound cGMP. However, only a cGMP-binding protein of 63 kDa, corresponding to the alpha-subunit of the cGMP-activated ion channel protein from bovine rod outer segments, was found in the ciliate cortex fraction. The functional cGMP-activated channel protein was also shown to be present in the cortex fraction of S. coeruleus by patch-clamp measurements of artificial liposomes. Incorporation of the cortex fraction into liposomes resulted in the appearance of ion channel activity related to cGMP. The reconstituted protein channels were strongly inhibited by l-cis-diltiazem, a known potent blocker of many types of cyclic-nucleotide-activated channels. The results presented here are the first demonstration of the existence and localization of a putative cGMP-activated channel protein in the ciliate S. coeruleus. Cyclic-nucleotide-activated channel proteins are nonspecific cation channels which mediate the receptor potentials in photoreceptor cells and in cells of the olfactory epithelium. On the basis of these data, we suggest that the 63 kDa protein identified in Stentor coeruleus is also a cGMP-activated ion channel and that it may be involved as an effector in the photosensory transduction pathway leading to the motile photophobic response in this ciliate protist.

Calcium permeability and flux through osmosensory transduction channels of isolated rat supraoptic nucleus neurons

Hypertonic stimuli delivered into the supraoptic nucleus provoke neuropeptide release from the somata of magnocellular neurosecretory cells (MNCs) in the presence of tetrodotoxin, suggesting that such stimuli can increase intracellular calcium concentration ([Ca2+]i) in the absence of action potentials. We therefore examined whether the stretch-inhibited cation (SIC) channels of MNCs can mediate calcium influx. Whole-cell recordings were made in MNCs isolated from the supraoptic nuclei of adult rats. Measurements of reversal potentials in different solutions revealed that the current induced by a suction-evoked decrease in cell volume (ISIC) displays a selectivity sequence for monovalent cations of K+>Cs+>Na+>NMDG+. The permeability of SIC channels to Ca2+, relative to Na+, was approximately 5. In the presence of physiological concentrations of external Na+ and K+, the amplitude of inward ISIC was reduced dose-dependently by external Ca2+ with an IC50 of 4.9 mM. This was not due to reduced suction-evoked volume changes or to an accumulation of [Ca2+]i. Confocal imaging of cytoplasmic Calcium Green-1 fluorescence revealed that activation of ISIC significantly increases [Ca2+]i in physiological solutions. This effect is absent in Ca2+-free solution, or when Gd3+ (300 microM) is added to Ca2+-containing solution. Part of this effect is inhibited in the presence of dantrolene (10 microM) and heparin (4 mg/mL), suggesting that release from intracellular Ca2+ stores participates in suction-evoked Ca2+ signalling. These observations indicate that SIC channels are highly permeable to Ca2+, mediate significant Ca2+ entry and release of Ca2+ from internal stores under conditions when the volume of MNCs is decreased.

Excessive activation of cyclic nucleotide-gated channels contributes to neuronal degeneration of photoreceptors

In different animal models, photoreceptor degeneration was correlated to an abnormal increase in cGMP concentration. The cGMP-induced photoreceptor toxicity was demonstrated by applying the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine on retinal explants. To assess the role of cGMP-gated channels in this cGMP toxicity, the Ca(2+) channel blockers verapamil and L- and D-diltiazem, which block cGMP-gated channels with different efficacies, were applied to in vitro animal models of photoreceptor degeneration. These models included: (i) adult rat retinal explants incubated with zaprinast, a more specific inhibitor of the rod phosphodiesterase than 3-isobutyl-1-methylxanthine and (ii) rd mouse retinal explants. Photoreceptor apoptosis was assessed by terminal dUTP nick end labelling and caspase 3 activation. Effects of the blockers on the synaptic rod Ca(2+) channels were measured by patch-clamp recording. In the zaprinast-induced photoreceptor degeneration model, both diltiazem isomers rescued photoreceptors whereas verapamil had no influence. Their neuroprotective efficacy was correlated to their inhibition of cGMP-gated channels (l-diltiazem>d-diltiazem>verapamil=0). In contrast, all three Ca(2+) channel blockers suppressed rod Ca(2+) channel currents similarly. This suppression of the currents by the diltiazem isomers was very weak (16.5%) at the neuroprotective concentration (10 microm). In rd retinal explants, both diltiazem isomers also slowed down rod degeneration in contrast to verapamil. L-diltiazem exhibited this effect at concentrations ranging from 1 to 20 microm. This study further supports the photoreceptor neuroprotection by diltiazem particularly in the rd mouse retina, whereas the absence of neuroprotection by verapamil further suggests the role of cGMP-gated channel activation in the induction of photoreceptor degeneration.

Voltage-Gated Channels and Calcium Homeostasis in Mammalian Rod Photoreceptors

Recent reports on rod photoreceptor neuroprotection by Ca2+channel blockers have pointed out the need to assess the effect of these blockers on mammalian rods. However, in mammals, rod electrophysiological characterization has been hampered by the small size of these photoreceptors, which were instead extensively studied in nonmammalian vertebrates. To further characterize ionic conductances and to assess the pharmacology of Ca2+channels in mammalian rods, freshly dissociated pig rod photoreceptors were recorded with the whole cell patch-clamp technique. Rod cells expressed 1) a hyperpolarization-activated inward-rectifying conductance ( Ih) sensitive to external Cs+; 2) a sustained outward K+current ( IK) sensitive to tetraethylammonium; 3) a sustained voltage-gated Ca2+current ( ICa) sensitive to benzothiazepine (diltiazem) and phenylalkylamine (verapamil) derivatives; 4) a Ca2+-activated Cl−current ( ICl(Ca)); and 5) a plasma membrane Ca2+-ATPase. The Ca2+current showed a range of activation from positive potentials to –60 mV with a maximum between –30 and –20 mV. In contrast to other L-type Ca2+channels, rod Ca2+channels were blocked at similar and relatively high concentrations by the diltiazem isomers and verapamil. The biphasic dose-response for d-diltiazem confirmed the low sensitivity of Ca2+channels for the molecule. The ATPase, which was localized at the axon terminal, was found to contribute to Ca2+extrusion. These results suggest that the electrophysiological features of rod photoreceptors had been preserved during evolution from nonmammalian vertebrates to mammals. This work indicates further that mammalian rods express nonclassic L-type Ca2+channels, showing a low sensitivity to the diltiazem isomers used in neuroprotective studies.

Second messenger cascade of glial responses evoked by interneuron activity and by a myomodulin peptide in the leech central nervous system

The giant glial cell in the neuropil of segmental ganglia of the leech Hirudo medicinalis responds to the activity of the Leydig interneuron and to a peptide of the myomodulin family, the presumed transmitter mediating the Leydig neuron-to-giant glial cell transmission, with a membrane hyperpolarization due to an increased membrane K+ conductance [Britz et al. (2002) Glia, 38, 215-227]. We have now studied the second messenger cascade initiated by Leydig neuron stimulation and by the endogenous myomodulin (MMHir) in the voltage-clamped giant glial cell. Glial responses to both stimuli are mediated by a G-protein-coupled receptor linked to adenylyl cyclase by the following criteria: (i) injection of GDP-beta-S, but not GDP, resulted in an irreversible decrease of the glial responses to both stimuli; (ii) the responses to both stimuli were reversibly inhibited by the adenylyl cyclase inhibitor SQ22,536; and (3) bath-applied di-butyryl-cyclic AMP, but not di-butyryl-cyclic GMP, elicited an outward current, which reduced the responses elicited by neuronal stimulation or myomodulin. A cocktail of protein kinase (PK) inhibitors (H-8, KT5720), the PKA antagonist Rp-cAMPS, or presumed inhibitors of cyclic nucleotide channels, LY83583 and l-cis-diltiazem, had no effect on the glial responses. Our results suggest that Leydig neuron stimulation and MMHir activate a cAMP-mediated K+ conductance in the glial cell, which appeared neither to be due to the activation of PKA nor of known cyclic nucleotide-gated channels directly.

Tuning outer segment Ca2+ homeostasis to phototransduction in rods and cones

Cone photoreceptors respond to light with less sensitivity, faster kinetics and adapt over a much wider range of intensities than do rods. These differences can be explained, in part, by the quantitative differences in the molecular processes that regulate the cytoplasmic free Ca2+ concentration in the outer segment of both receptor types. Ca2+ concentration is regulated through the kinetic balance between the ions' influx and efflux and the action of intracellular buffers. Influx is passive and mediated by the cyclic-GMP gated ion channels. In cones, Ca2+ ions carry about 35% of the ionic current flowing through the channels in darkness. In rods, in contrast, this fraction is about 20%. We present a kinetic rate model of the ion channels that helps explain the differences in their Ca2+ fractional flux. In cones, but not in rods, the cGMP-sensitivity of the cyclic GMP-gated ion channels changes with Ca2+ at the concentrations expected in dark-adapted photoreceptors. Ca2+ efflux is active and mediated by a Na+ and K+-dependent exchanger. The rate of Ca2+ clearance mediated by the exchanger in cones, regardless of the absolute size of their outer segment is of the order of tens of milliseconds. In rod outer segments, and again independently of their size, Ca2+ clearance rate is of the order of hundreds of milliseconds to seconds. We investigate the functional consequences of these differences in Ca2+ homeostasis using computational models of the phototransduction signal in rods and cones. Consistent with experimental observation, differences in Ca2+ homeostasis can make the cone's flash response faster and less sensitive to light than that of rods. In the simulations, however, changing Ca2+ homeostasis is not sufficient to recreate authentic cone responses. Accelerating the rate of inactivation (but NOT activation) of the enzymes of the transduction cascade, in addition, to changes in Ca2+ homeostasis are needed to explain the differences between rod and cone photosignals. The large gain and precise kinetic control of the electrical photoresponse of rod and cone retinal receptors suggested a long time back that phototransduction is mediated by cytoplasmic second messengers that, in turn, control membrane ionic conductance. (1) The unquestionable identification of cyclic GMP as the phototransduction messenger, however, did not come until the mid 1980's with the discovery that the light-regulated membrane conductance in both rods and cones is gated by this nucleotide (2-4) and is, in fact, an ion channel. (7) The cyclic nucleotide gated (CNG) channels, now we know, are not just the compliant targets of light-dependent change in cytoplasmic cGMP, but actively participate in the regulation transduction through Ca2+ feedback signals. The precise magnitude and time course of the concentration changes of cGMP and Ca2+ in either rods or cones remains controversial. It is clear, however, that whereas cGMP directly controls the opening and closing of the plasma membrane channels, Ca2+ controls the light-sensitivity and kinetics of the transduction signal. (8,9) The modulatory role of Ca2+ is particularly apparent in the process of light adaptation: in light-adapted rods or cones, the transduction signal generated by a given flash is lower in sensitivity and faster in time course than in dark-adapted cells. Light adaptation is compromised if Ca2+ concentration changes are attenuated by cytopiasmic Ca2+ buffers (8,10,11) and does not occur if Ca2+ concentration changes are prevented by manipulation of the solution bathing the cells. (2,4) Several Ca2+-dependent biochemical reactions have been identified in photoreceptors, among them: 1. ATP-dependent deactivation. (15,16) 2 Phodopsin phospshorylation, through the action of recoverin (S-modulin). (17-19) 3. Catalytic activity of guanylyl cyclase, (20-22) through the action of GCAP proteins. (23,24,25) 4. cGMP-sensitivity of the CNG channels. (26-29,30) A challenge in contemporary phototransduction research is to understand the details of these reactions and their role in the control of the phototransduction signal. Transduction signals in cone photoreceptors are faster, lower in light sensitivity, and more robust in their adaptation features than those in rods (for review see refs. 31;32). A detailed molecular explanation for these differences is not at hand. However, biochemical and electrophysiological (33) studies indicate that the elements in the light-activated pathway that hydrolyzes cGMP are quantitatively similar in their function in rods and cones and unlikely to account for the functional differences. Also, within the limited exploration completed todate, the Ca2+-dependence of guanylyl cyclase (34) and visual pigment phosphorylation (19) do not differ in rods and cones. On the other hand, data accumulated over the past few years indicate that cytoplasmic Ca2+ homeostasis, while controlled through essentially identical mechanisms it is quantitatively very different in its features in the two photoreceptor types. Both Ca2+ influx through CNG channels and the rate of Ca2+ clearance from the outer segment differ between the two receptor cells. Also, the Ca2+-dependent modulation of cGMP sensitivity is larger in extent in cones than in rods. Most significantly, the concentration range of this Ca2+ dependence overlaps the physiological range of light-dependent changes in cytoplasmic Ca2+ level in cones, but not in rods. We briefly review some of the evidence that supports these assertions and we then provide a quantitative analysis of the possible significance of these known differences. We conclude that while differences in Ca2+ homeostasis contribute importantly to explaining the differences between the two receptor types, they are alone not sufficient to explain the differences in the photoreceptor's response. It is likely that Ca2+-independent inactivation of the transduction cascade enzymes is more rapid in cones than in rods.

[7-(Dialkylamino)coumarin-4-yl]methyl-Caged Compounds as Ultrafast and Effective Long-Wavelength Phototriggers of 8-Bromo-Substituted Cyclic Nucleotides

[7-(Dimethylamino)coumarin-4-yl]methyl (DMACM) and [7-(diethylamino)coumarin-4-yl]methyl (DEACM) esters of 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP) and 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP) are described as novel caged compounds for 8-bromo-substituted cyclic nucleotides. Synthesis is accomplished by treatment of the free acids of the cyclic nucleotides with the corresponding 7(dialkylamino)-substituted 4(diazomethyl)coumarins. Irradiation of the DMACM- and DEACM-caged cyclic nucleotides with UV light stimulates the release of the cyclic nucleotides within roughly a nanosecond. The new caged compounds are resistant to hydrolysis in aqueous buffers and exhibit long-wavelength absorption properties with maxima at 400 nm, high extinction coefficients, and high quantum yields (0.15-0.31). Their favorable properties render these compounds the most efficient and rapid phototriggers of 8-bromo-substituted cyclic nucleotides known. The usefulness of the compounds for physiological studies under nondamaging light conditions was examined in HEK293 cells expressing the alpha subunit of the cyclic-nucleotide-gated (CNG) channel of cone photoreceptors (CNGA3) and of olfactory neurons (CNGA2) by using confocal laser scanning microscopy and the patch clamp technique.

Pharmacological strategies to block rod photoreceptor apoptosis caused by calcium overload: a mechanistic target-site approach to neuroprotection

PURPOSE

Photoreceptor apoptosis and resultant visual deficits occur in humans and animals with inherited, and disease-, injury- and chemical-induced retinal degeneration. Our aims were three-fold: 1) to determine the kinetics of rod apoptosis and Ca2+ overload in Pde6b9rd1) mice and developmentally lead-exposed rats, 2) to establish a pathophysiologically-relevant model of Ca2+ overload/rod-selective apoptosis in isolated rat retina and 3) to examine different mechanistic based neuroprotective strategies that would abrogate or mollify rod Ca2+ overload/apoptosis.

METHODS

Retinal morphometry and elemental calcium content ([Ca]) determined the kinetics of rod apoptosis and Ca2+ overload. A multiparametric analysis of apoptosis including rod [Ca], a live/dead assay, rod oxygen consumption, cytochrome c immunoblots and caspase assays was combined with pharmacological studies of an isolated rat retinal model of rod-selective Ca2+ overload/apoptosis.

RESULTS

Ca2+ overload preceded rod apoptosis in mice and rats, although the extent and kinetics in each differed significantly. The isolated rat model of rod Ca2+ overload/apoptosis showed that blockade of Ca2+ entry through rod cGMP-activated channels with L-cis diltiazem was partially neuroprotective, whereas blockade of Ca2+ entry into rods through L-type Ca2+ channels with D-cis diltiazem or verapamil provided no protection. Inhibition of the mitochondrial Na+/Ca2+ exchanger with D-cis diltiazem provided no protection. CsA and NIM811, mitochondrial permeability transition pore (mPTP) inhibitors, blocked all Ca(2+)-induced apoptosis, whereas the caspase-3 inhibitor DEVD-fmk only blocked the downstream cytochrome c-induced apoptosis.

CONCLUSIONS

The successful pharmacological neuroprotective strategies for rod Ca2+ overload/apoptosis targeted the rod cGMP-activated channels or mPTP, but not the rod L-type Ca2+ channels.

Dequalinium: a novel, high-affinity blocker of CNGA1 channels

Cyclic nucleotide-gated (CNG) channels have been shown to be blocked by diltiazem, tetracaine, polyamines, toxins, divalent cations, and other compounds. Dequalinium is an organic divalent cation which suppresses the rat small conductance Ca(2+)-activated K(+) channel 2 (rSK2) and the activity of protein kinase C. In this study, we have tested the ability of dequalinium to block CNGA1 channels and heteromeric CNGA1+CNGB1 channels. When applied to the intracellular side of inside-out excised patches from Xenopus oocytes, dequalinium blocks CNGA1 channels with a K(1/2) approximately 190 nM and CNGA1+CNGB1 channels with a K(1/2) approximately 385 nM, at 0 mV. This block occurs in a state-independent fashion, and is voltage dependent with a zdelta approximately 1. Our data also demonstrate that dequalinium interacts with the permeant ion probably because it occupies a binding site in the ion conducting pathway. Dequalinium applied to the extracellular surface also produced block, but with a voltage dependence that suggests it crosses the membrane to block from the inside. We also show that at the single-channel level, dequalinium is a slow blocker that does not change the unitary conductance of CNGA1 channels. Thus, dequalinium should be a useful tool for studying permeation and gating properties of CNG channels.

Cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels first identified in retinal photoreceptors and olfactory sensory neurons (OSNs). They are opened by the direct binding of cyclic nucleotides, cAMP and cGMP. Although their activity shows very little voltage dependence, CNG channels belong to the superfamily of voltage-gated ion channels. Like their cousins the voltage-gated K+ channels, CNG channels form heterotetrameric complexes consisting of two or three different types of subunits. Six different genes encoding CNG channels, four A subunits (A1 to A4) and two B subunits (B1 and B3), give rise to three different channels in rod and cone photoreceptors and in OSNs. Important functional features of these channels, i.e., ligand sensitivity and selectivity, ion permeation, and gating, are determined by the subunit composition of the respective channel complex. The function of CNG channels has been firmly established in retinal photoreceptors and in OSNs. Studies on their presence in other sensory and nonsensory cells have produced mixed results, and their purported roles in neuronal pathfinding or synaptic plasticity are not as well understood as their role in sensory neurons. Similarly, the function of invertebrate homologs found in Caenorhabditis elegans, Drosophila, and Limulus is largely unknown, except for two subunits of C. elegans that play a role in chemosensation. CNG channels are nonselective cation channels that do not discriminate well between alkali ions and even pass divalent cations, in particular Ca2+. Ca2+ entry through CNG channels is important for both excitation and adaptation of sensory cells. CNG channel activity is modulated by Ca2+/calmodulin and by phosphorylation. Other factors may also be involved in channel regulation. Mutations in CNG channel genes give rise to retinal degeneration and color blindness. In particular, mutations in the A and B subunits of the CNG channel expressed in human cones cause various forms of complete and incomplete achromatopsia.

Participation of a K(+) channel modulated directly by cGMP in the speract-induced signaling cascade of strongylocentrotus purpuratus sea urchin sperm

Speract, a decapeptide from Strongylocentrotus purpuratus sea urchin eggs, transiently stimulates a membrane guanylyl cyclase and activates a K(+)-selective channel that hyperpolarizes sperm. However, previous studies of sperm and of sperm membrane vesicles reached conflicting conclusions about the mechanisms that open these channels. We find that speract hyperpolarizes and increases the cGMP content of flagellar vesicles. We confirm previous findings that intravesicular GTPgammaS and GTP enhance this hyperpolarization, but not GDPbetaS. The G protein activators AlF(-)(4) and mastoparan also are ineffective. Thus, it is unlikely that a G protein participates in the speract response. In contrast, hyperpolarization responses to speract are increased by 3-isobutyl-1-methylxanthine, which preferentially inhibits cGMP-selective phosphodiesterases of sperm, and the 8Br-cGMP derivative hyperpolarizes vesicles in the absence of speract. The responses to speract and to 8Br-cGMP have similar ionic selectivities (K(+) > Rb(+) > > Li(+) > Na(+)) and sensitivities to the channel blockers 4-aminopiridine and 3, 4-dichlorobenzamil, indicating that they likely result from opening of the same K(+) channel. Inhibitors that preferentially inhibit cAMP-selective phosphodiesterases do not alter responses to speract, and permeant cAMP analogs do not hyperpolarize vesicles. In addition, inhibitors of protein kinases and phosphatases fail to alter vesicle hyperpolarization by speract. The increase in vesicular cGMP content produced by speract therefore may directly mediate opening of the channel that hyperpolarizes sperm membrane vesicles. Similar mechanisms presumably operate in intact sperm.

Retinitis pigmentosa: rod photoreceptor rescue by a calcium-channel blocker in the rd mouse

Retinitis pigmentosa is an inherited degenerative disease of photoreceptors leading to blindness. A well-characterized model for this disease is provided by the retinal degeneration mouse, in which the gene for the rod cGMP phosphodiesterase is mutated, as in some affected human families. We report that D-cis-diltiazem, a calcium-channel blocker that also acts at light-sensitive cGMP-gated channels, rescued photoreceptors and preserved visual function in the retinal degeneration mouse. The long record of diltiazem prescription in cardiology should facilitate the design of clinical trials for some forms of retinitis pigmentosa.

Measurement of intracellular Ca2+ changes using novel caged cyclic nucleotides and confocal laser scanning microscopy

The intention of this study is to explore the applicability of confocal microscopy in conjunction with the use of caged cyclic nucleotide derivatives. The methodological potential of UV laser confocal microscopy has been assessed. It is shown that illumination of a single cell or a small area of a single cell is possible, whereby the intracelluar Ca2+ signal is measured at illuminated and non-illuminated cells. Such measurements do not have a high time resolution because of the specific system parameters. However, with an N2 pulse laser (not part of the standard microscope set-up), Ca2+ signals with a time resolution of around 100 ms have been measured. This facilitates investigation of the kinetics of Ca2+ influx. Intracellular Ca2+ measurements at HEK293 and sperm cells have been made here. For sperm cells the advantages of confocal microscopy are best evidenced in conjunction with the use of caged cyclic nucleotides; a cyclic nucleotide-gated Ca2+ influx at the tail of these cells has thereby been demonstrated for the first time.

Spanning binding sites on allosteric proteins with polymer-linked ligand dimers

One approach to drug design involves determination of the structure of binding sites on target proteins to provide templates for ligand construction. Alternatively, random combinations of chemical groups can be used to generate diverse molecules for screening in the search for effective compounds. Here we report a strategy for developing potent ligands for proteins with multiple binding sites, which combines elements of both approaches: 'polymer-linked ligand dimers', in which two ligands are joined by a polymer chain of variable length. We find that polymer-linked ligand dimers containing two cyclic GMP moieties are up to a thousand times more potent than cyclic GMP in activating cyclic-nucleotide-gated channels and cGMP-dependent protein kinase. Each target protein responds optimally to a polymer-linked ligand dimer with a different average polymer length, even though their cyclic-nucleotide-binding sites are conserved. The tuning of polymer-linked ligand dimers indicates that each protein has a unique spacing of binding sites and provides an estimate of the distance between these sites. As optimal ligands are selected empirically, the polymer-linked ligand dimer strategy enables potent and selective agents to be identified without requiring previous structural information about the target proteins.

Molecular and pharmacological analysis of cyclic nucleotide-gated channel function in the central nervous system

Most functional studies of cyclic nucleotide-gated (CNG) channels have been confined to photoreceptors and olfactory epithelium, in which CNG channels are abundant and easy to study. The widespread distribution of CNG channels in tissues throughout the body has only recently been recognized and the functions of this channel family in many of these tissues remain largely unknown. The molecular biological and pharmacological properties of the CNG channel family are summarized in order to put in context studies aimed at probing CNG channel functions in these tissues using pharmacological and genetic methods. Compounds have now been identified that are useful in distinguishing CNG channel activated pathways from cAMP/cGMP dependent-protein kinases or other pathways. The ways in which these interact with CNG channels are understood and this knowledge is leading to the identification of more potent and more specific CNG channel subtype-specific agonists or antagonists. Recent molecular and genetic analyses have identified novel roles of CNG channels in neuronal development and plasticity in both invertebrates and vertebrates. Targeting CNG channels via specific drugs and genetic manipulation (such as knockout mice) will permit better understanding of the role of CNG channels in both basic and higher orders of brain function.

An unusual cGMP pathway underlying depolarizing light response of the vertebrate parietal-eye photoreceptor

All cellular signaling pathways currently known to elevate cGMP involve the activation of a guanylyl cyclase to synthesize cGMP. Here we describe an exception to this rule. In the vertebrate parietal eye, the photoreceptors depolarize to light under dark-adapted conditions, unlike rods and cones but like most invertebrate photoreceptors. We report that the signaling pathway for this response involves a rise in intracellular cGMP resulting from an inhibition of the phosphodiesterase that hydrolyzes cGMP. Furthermore, this phosphodiesterase is driven by an active G protein in darkness. These results indicate an antagonistic control of the phosphodiesterase by two G proteins, analogous to the Gs/Gi control of adenylyl cyclase. Our findings demonstrate an unusual phototransduction mechanism and at the same time indicate that signaling involving cyclic nucleotides is more elaborate than previously known.

Substituted cGMP analogs can act as selective agonists of the rod photoreceptor cGMP-gated cation channel

Cyclic nucleotide-gated (CNG) channels are expressed in many cell types in both the nervous system and nonexcitable tissues. In order to understand the roles of cGMP-gated channels, and to distinguish actions of cGMP mediated through CNG channels from those through cGMP-dependent protein kinase (G-kinase), several new cGMP analogs were tested for potency as CNG channel agonists. Using Xenopus oocytes expressing the rat rod cGMP-gated ion channel alpha-subunit, we showed that an analog containing a pCPT group at the 8-position, 8-pCPT-cGMP, was 80 times more potent than cGMP and 14 times more potent than 8-Br-cGMP. 8-pCPT-cGMP is the most potent CNG channel agonist so far described and also has the advantages of much better membrane permeability as well as much higher resistance to PDE-hydrolysis, as compared with 8-Br-cGMP. Modification of both 8-Br-cGMP and 8-pCPT-cGMP by introduction of a sulphur atom into the cyclic phosphate group gave smaller changes in agonist efficiency. Both Sp-8-Br-cGMPS and Sp-8-pCPT-cGMPS acted as agonists of CNG channels and are also G-kinase activators. In contrast, Rp-8-Br-cGMPS was a channel agonist, with an EC50 of 173.5 microM, but a G-kinase antagonist with a Ki of 4 microM. Finally, Rp-8-pCPT-cGMPS was a channel agonist and showed additional noncompetitive antagonist activity at higher concentrations. The results suggest that 8-pCPT-cGMPS is a highly potent photoreceptor CNG channel agonist with high membrane permeability and PDE-resistance and furthermore Rp-8-Br-cGMPS can be used to test whether the actions of cGMP are selectively mediated by CNG channels.

Mechanism of tetracaine block of cyclic nucleotide-gated channels

Local anesthetics are a diverse group of ion channel blockers that can be used to probe conformational changes in the pore. We examined the effects of the local anesthetic tetracaine on rod and olfactory cyclic nucleotide-gated channels expressed from subunit 1 in Xenopus oocytes. We found that 40 microM tetracaine effectively blocked the bovine rod channel but not the rat olfactory channel at saturating concentrations of cGMP. By testing chimeric channels containing regions of sequence from both rod and olfactory channels, we found that determinants of apparent affinity for tetracaine at saturating cGMP did not map to any one region of the channel sequence. Rather, the differences in apparent affinity could be explained by differences between the chimeras in the free energy of the opening allosteric transition. If a channel construct (such as the rod channel) spent appreciable time in the closed state at saturating cGMP, then it had a high apparent affinity for tetracaine. If, on the other hand, a channel construct (such as the olfactory channel) spent little time in the closed state at saturating cGMP, then it had a low apparent affinity for tetracaine. Furthermore, tetracaine became more effective at low concentrations of cGMP and at saturating concentrations of cAMP, conditions which permit the channels to spend more time in the closed configuration. These results were well fit by a model in which tetracaine binds more tightly to the closed channel than to the open channel. Dose-response curves for tetracaine in the presence of saturating cGMP are well fit with a Michaelis-Menten binding scheme indicating that a single tetracaine molecule is sufficient to produce block. In addition, tetracaine block is voltage dependent with an effective z delta of +0.56. These data are consistent with a pore-block hypothesis. The finding that tetracaine is a state-dependent pore blocker suggests that the inner mouth of the pore of cyclic nucleotide-gated channels undergoes a conformational change during channel opening.

Molecular cloning, functional expression and chromosomal localization of a human homolog of the cyclic nucleotide-gated ion channel of retinal cone photoreceptors

We have cloned from human retina a cyclic nucleotide-gated (CNG) ion channel that is distinct from the one found in rod photoreceptors. This channel protein is highly homologous to the CNG channel that recently has been cloned from bovine testis and kidney and has been shown to be present in retinal cone photoreceptors. When expressed in human embryonic kidney cells, the protein forms functional ion channels with properties broadly similar to those described for the cloned bovine channel. The gene for this channel resides on chromosome 2.

Molecular mechanisms of cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) channels are highly specialized to carry out their unique role in cell signalling. Significant progress has been made in the last several years determining the molecular mechanisms for these specializations. The activation of the channels begins with the binding of cyclic nucleotide to a domain in the carboxyl terminal region. This binding, in turn, produces an induced fit of the protein that involves a movement of the C-helix portion of the binding domain. The induced fit of the binding domain is coupled to an allosteric conformational change that opens the channel pore. The pore is formed primarily from the sequence between the S5 and S6 segments. A single glutamic acid in the pore represents the binding site for multiple monovalent cations, the blocking site for external divalent cations, and the site for the effect of protons on permeation.

Cyclic Nucleotide-Gated Channels, Nitric Oxide, and Neural Function

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels that are activated by direct interaction of the second messengers cAMP and cGMP. Once activated, they provide for membrane depolarization and Ca2+ influx into cells. The functions of CNG channels are tightly coupled to the mechanisms by which cAMP and cGMP are regulated in neurons, namely through activation of G-protein–coupled membrane receptors and through the nitric oxide/guanylyl cyclase signaling system. These functions are best understood in sensory neurons of the vertebrate visual and olfactory system, where CNG channels are critical components of the signal transduction apparatus. The family of known CNG channels is expanding, and there is now increasing evidence that these channels are also present in many other areas of the nervous system. Based on their role in sensory neurons, a functional framework for the role of CNG channels in the CNS is provided.

Calcium homeostasis in vertebrate retinal rod outer segments

The outer segments of vertebrate retinal rod photoreceptors (ROS) exhibit dynamic Ca2+ fluxes. In darkness, Ca2+ continuously enters via the light-sensitive, cGMP-gated channels and this requires the presence of a powerful Ca2+ extrusion mechanism in the ROS plasma membrane. Our laboratory has characterized a Na/Ca+K exchanger in the ROS plasma membrane, which utilizes both inward Na+ gradient and outward K+ gradient to extrude Ca2+. Here, I review our work on the functional properties of the Na/Ca+K exchanger including the stoichiometry, ion binding sites and regulation of Ca2+ transport via Na/Ca+K exchange. Inactivation of the Ca2+ extrusion mode of the Na/Ca+K exchanger will be discussed as a mechanism to prevent lowering of cytosolic free Ca2+ to undesirably low values of < 1 nM that are expected from the coupling stoichiometry of the Na/Ca+K exchanger and that are expected to occur when Ca2+ influx via the cGMP-gated channels is interrupted during saturation of rod photoreceptors in bright light. This review also reexamines the contribution of internal Ca2+ stores (i.e. disks) to Ca2+ homeostasis in ROS.