Molecular Vision 2006; 12:1699-1705 <http://www.molvis.org/molvis/v12/a194/> Received 21 August 2006 | Accepted 12 December 2006 | Published 29 December 2006

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Retinoids restore normal cyclic nucleotide sensitivity of mutant ion channels associated with cone dystrophy

Michelle L. Tetreault, Diana M. Horrigan, Jennifer A. Kim, Anita L. Zimmerman
 
 

Department of Molecular Pharmacology, Physiology & Biotechnology Brown Medical School, Providence, RI

Correspondence to: Anita L. Zimmerman, Box G-B329, Brown Medical School, Providence, RI 02912; Phone: (401) 863-2224; FAX: (401) 863-1440; email: Anita_Zimmerman@brown.edu
 
Dr. Horrigan is now at the Department of Biological Sciences, Bridgewater State College, Bridgewater, MA.

Abstract

Purpose: To determine whether inhibition of cyclic nucleotide-gated (CNG) ion channels by retinoids might be useful in treating degenerative retinal diseases in which either the CNG channels are hypersensitive to 3',5'-cyclic guanosine monophosphate (cGMP) or the photoreceptor cGMP concentration is elevated.

Methods: Patch clamp (electrophysiological) methods were used to measure activation by cGMP of wild-type human cone (hCNGA3), mutant cone (hCNGA3-N471S), and wild-type bovine rod (bCNGA1) CNG channels heterologously expressed in Xenopus oocytes. Cyclic GMP-activated currents were measured in excised, inside-out membrane patches before and after treatment with either all-trans retinal (ATR) or all-trans C22 aldehyde, which is too long to fit into the chromophore binding pocket of opsin and therefore cannot activate the visual transduction cascade.

Results: At physiological cGMP concentrations, 150 nM ATR reduced the open probability of the mutant cone CNG channel by reducing its apparent cGMP affinity to that of the normal cone channel. Furthermore, all-trans C22 aldehyde similarly inhibited the mutant cone channel as well as normal rod and cone CNG channels.

Conclusions: Our results raise the possibility that retinoids, such as all-trans C22 aldehyde, that inhibit CNG channels without affecting the transduction cascade, may be useful in treating degenerative retinal diseases in which either the cGMP concentration is elevated or the CNG channels are hypersensitive to cGMP.

Introduction

Cyclic nucleotide-gated (CNG) ion channels mediate the response to light in rod and cone photoreceptors [1-8]. These channels are opened by the direct binding of 3',5'-cyclic guanosine monophosphate (cGMP), whose free concentration is highest in the dark (about 5 μM) and decreases in the light following activation of the transduction cascade. Many retinal degenerative diseases are associated with mutations in guanylyl cyclase, phosphodiesterase, and other proteins that control the cGMP levels [9-15]. However, mutations in the rod and cone CNG channels are also associated with some forms of retinal degenerative disease [16-21]. One example is a human cone channel mutation (CNGA3-N471S) that is associated with cone dystrophy [21]. This mutant channel has been found to be hypersensitive to cGMP [22]. In addition, a number of disease-causing mutations of the human cone channel beta subunit (CNGB3) have been found to produce abnormally high sensitivity to cGMP [23].

Retinoids are key players in visual transduction [4,24,25]. The light response in rods and cones is initiated when a photon is absorbed by 11-cis retinal, which is covalently bound (via a Schiff base) to opsin. Photoisomerization leads to production of all-trans retinal (ATR), which is released from opsin and converted to all-trans retinol by retinol dehydrogenase. Interestingly, a noncovalent interaction between ATR and opsin can also stimulate transduction and is thought to participate in bleaching adaptation. Vacant opsin can also stimulate the cascade, but not as well as opsin that contains an appropriate trans retinoid [26-30].

The free concentrations of ATR and all-trans retinol in rods and cones are not known, but millimolar levels of ATR are expected to be released from the photopigment in bright light [31]. Our recent work with intact rods suggests that a large intracellular retinoid buffer protects the CNG channels from inhibition by this high level of ATR, except when the buffer is saturated with excess retinoid [32]. Furthermore, retinoid levels are altered in many retinal diseases, including Stargardt's dystrophy, some forms of retinitis pigmentosa, and macular degeneration [33,34]. In some cases, retinoid supplements have been effective at slowing the progression of disease [35-39].

In retinoid therapy, as well as in studies of retinal diseases, retinoids have been assumed to affect transduction only by regulating opsin activity. However, using excised membrane patches, we have shown that ATR, all-trans retinol, and 11-cis retinal, acting at the intracellular side of the membrane, dramatically reduce the apparent cGMP affinity of the rod CNG channel and shut it down even at saturating levels of cGMP [40-42]. At physiological cGMP concentrations (i.e., micromolar and below), ATR was found to inhibit the channel in the nanomolar range [41]. Single channel analysis showed that ATR does not change single channel conductance, but shuts channels down for long periods [41]. Inhibition was similar in homomeric (α subunit only) and heteromeric (α and β subunits) cloned channels [41], as well as native channels in rod outer segment patches [32]. Here we investigate whether application of ATR to mutant cone CNG channels that are hypersensitive to cGMP can reduce their elevated cGMP sensitivity to that of normal cone channels. We also test this channel, as well as normal rod and cone channels, for inhibition by a retinoid analog (all-trans C22 aldehyde) that does not fit into the opsin chromophore binding pocket and therefore does not activate the transduction cascade [43]. Our results raise the possibility that retinoid inhibition of CNG channels may be useful in the treatment of a variety of retinal degenerative diseases.

Methods

Heterologous expression of the channels in Xenopus oocytes was obtained by standard methods, as described by McCabe et al. [41]. CNG channel clones for the wild-type human cone α (CNGA3) and its mutant (N471S) were generously provided by Dr. Michael Varnum, Washington State University; the clone for the bovine rod α (CNGA1) was kindly provided by Dr. William N. Zagotta, University of Washington. All clones were in the pGEMHE plasmid [44]. Channel cRNA was made by in vitro transcription using Ambion's mMessage mMachine^TM kit (Austin, TX), and 20-50 ng was injected into each oocyte.

Standard patch clamp methods were used to record currents from excised, inside-out oocyte patches. Macroscopic currents were recorded at voltages ranging from -100 to +100 mV in 50 mV steps from a holding potential of 0 mV with either an Axon Instruments Axopatch 200 amplifier or a HEKA EPC 10 amplifier, using Pulse or Patchmaster software, respectively (HEKA Elektronik, Lambrecht, Germany). Data were analyzed using Igor Pro software (Wavemetrics, Inc. Lake Oswego, OR). Patch currents at each voltage were measured after weak voltage-dependent gating [45] and prior to significant ion depletion effects [46].

As previously shown, retinoid inhibition of CNG channels begins within seconds, but can take up to an hour to reach steady state [41]. A complicating factor in the retinoid experiments is that on this same time scale, the apparent cGMP affinity of excised rod channel patches can increase due to dephosphorylation by endogenous patch-associated phosphatases [47,48]. Therefore, in each experiment, the apparent cGMP affinity was monitored until it reached steady state before retinoids were applied. We found evidence for a similar, but less pronounced increase in apparent cGMP affinity in patches containing the N471S cone channels. The apparent cGMP affinity of N471S immediately after patch excision (K_1/2=7 μM) agreed with that reported by Liu and Varnum [22] but improved to 4.1 μM over tens of minutes. Consequently, ATR and C22 aldehyde were added to these patches only after allowing for this increase in apparent cGMP affinity. The wild-type cone channel did not demonstrate any significant change in apparent cGMP affinity after patch excision.

C22 aldehyde was generously provided by Dr. Rosalie Crouch, Medical University of South Carolina; synthesis of this compound was as previously described in reference [43]. ATR (Sigma, St. Louis, MO) and C22 aldehyde stocks (500 μM) were made in 100% ethanol and kept in amber glass vials covered in aluminum foil and stored at -80 or -20 °C until use. Stock purity and stability were checked by measuring the absorption peak, at 381 nm for ATR and 402 nm for C22 aldehyde, with a Beckman DU640 spectrophotometer. Patch clamp experiments were conducted in a glass petri dish. Both the bath and the pipette contained a low-divalent sodium solution consisting of: 130 mM NaCl, 500 μM EDTA, and 2 mM HEPES, at pH 7.2. Various concentrations of cGMP (Sigma) were applied to the intracellular surface using a 36-solution patch perfusion system, RSC-100 rapid solution changer (Molecular Kinetics, Pullman, WA). Currents in the absence of cGMP were subtracted as leak. Each retinoid was mixed into the bath under dim red light following guidelines published in reference [41], and currents were monitored for at least an hour until inhibition reached steady state.

Results

Some forms of cone dystrophy have been found to be associated with mutant cone CNG ion channels [19,21,49]. Liu and Varnum [22] demonstrated that the functional defect in two of these mutations (CNGA3: N471S and R563H) is a hypersensitivity to cGMP, so that the open probability of the channels is likely to be too high in the dark. This would be expected to produce abnormally high intracellular concentrations of Na⁺ and Ca²⁺, which would enter the cone through these channels and presumably contribute to cell death. For the current study, we chose to focus on N471S because it expresses well in Xenopus oocytes.

Figure 1A,B illustrate the greater cGMP sensitivity of the mutant cone channel (N471S), compared with the wild-type channel. In each case, the top family of traces was obtained in the presence of 2 mM cGMP, and the bottom family was obtained at a cGMP concentration (5 μM) expected to approximate the free concentration in rods and cones in the dark [6,50]. For both channels, 2 mM cGMP was a saturating concentration; i.e., one that gave maximal open probability. However, for the mutant channel, 5 μM cGMP gave a larger fractional current (45.6±9.4%, mean±SEM; 6 patches) than that measured for the wild-type channel (14.6±1.7%, mean±SEM; 7 patches).

Our previous work with excised membrane patches has shown that ATR dramatically inhibits the wild-type rod CNG channel, with its greatest effects at low, near physiological, cGMP concentrations [41]. If a similar inhibition occurs with the mutant cone CNG channel, it might be possible to restore normal channel activation by application of ATR or another retinoid. Figure 1C shows that application of 150 nM ATR reduced the fractional activation of the mutant channel in 5 μM cGMP to approximately that of the wild-type channel (Figure 1A). Furthermore, as shown in Figure 2, 150 nM ATR shifted the cGMP dose-response relation of the mutant channel to that of the wild-type channel within the expected physiological cGMP range (a few micromolar cGMP in the dark and less in the light). Thus, the cGMP K_1/2 changed from 4.1 μM (squares) to 12.9 μM (circles) upon addition of 150 nM ATR. This compares well with the K_1/2 of the normal channel, 12.3 μM (triangles). Although ATR also decreased the total current (by 20%) at higher levels of cGMP, this feature of ATR inhibition is not relevant for the intact cone (or rod), since these cGMP levels are much higher than those expected either for free (a few micromolar or less) or total (about 50 μM) cGMP [6,50]. Interestingly, 150 nM ATR gave about a three fold reduction in current at a cGMP concentration (5 μM) near that expected in the dark (current reduced from 45% to 14% of current obtained with maximal activation). The difference in Hill slopes of the mutant (n=1.2-1.5) and wild-type (n=1.8) channels is consistent with our observation that the apparent cGMP affinity of the mutant channel increases slightly after patch excision, whereas that of the wild type channel remains stable (see Methods). This process gives a preferential increase in current at low cGMP concentrations, with no change in the maximum cGMP-activated current, thereby making the relation more shallow. As described in the Methods, retinoids were not added to the mutant channel patches until the apparent cGMP affinity was stable.

The use of a compound like ATR to return mutant cone channel activation to normal in the physiological cGMP range has the disadvantage that it would also have effects on the transduction cascade, since it can stimulate opsin by a noncovalent interaction [26-31]. A better choice would be a retinoid that inhibits the channel but does not fit into the chromophore binding pocket of opsin. All-trans C22 aldehyde is one such compound [43]. Figure 3 shows that C22 aldehyde inhibited the N471S channel almost as effectively as did ATR. Similar results were seen in two other patches. Figure 3C presents a dose-response relation for inhibition of the N471S channel current by C22 aldehyde. The IC₅₀ for percent inhibition was 330 nM, and the Hill coefficient was 2.3. This is only slightly less effective inhibition than that measured for ATR on the wild-type human cone alpha (CNGA3) channels (IC₅₀=130 nM, n=2.3) [51]. Thus, a compound like C22 aldehyde may eventually be useful in the treatment of forms of cone dystrophy that result from hypersensitive CNG channels, such as N471S.

Retinal degenerative diseases resulting from mutant cone channels are much less common than those resulting from malfunctions in the enzyme cascade, causing abnormally high free cGMP concentrations [9-15]. These, however, also produce increases in CNG channel open probability and, therefore, increases in dark current that can lead to cell death [52]. Is it possible to treat these diseases by making the normal CNG channels less sensitive to elevated cGMP levels without altering the activity of opsin or other members of the cascade? This is a complex question that involves issues of retinoid processing, feedback control of cGMP turnover by the Ca²⁺ that enters through the CNG channels, as well as many therapeutic considerations. However, the first step in answering this question is to determine if a retinoid like C22 aldehyde is able to inhibit wild-type rod and cone channels. Figure 4 suggests that this is indeed the case. Both wild-type cone and wild-type rod CNG channels were well inhibited by the C22 aldehyde at 10 μM cGMP, which is slightly above the expected free cGMP concentration in vivo in the dark [6,50]. Inhibition of the rod channel is greater than that of the cone channel because 10 μM cGMP is a lower concentration relative to the K_1/2 for the rod channel (31 μM) compared to the cone channel (12.3 μM), and retinoids are most effective at low channel open probability and low cGMP [41].

Discussion

Our recent research has revealed a new potential role for retinoids in vision: inhibition of rod and cone CNG channels. This inhibition may be involved in the delayed dark adaptation that is seen with retinal diseases in which retinoids are elevated (e.g., Stargardt's disease and some forms of retinitis pigmentosa) [9-11,53] and may have potential as a therapeutic tool. Some forms of cone dystrophy are associated with defective cone CNG channels that are hypersensitive to cGMP [22,23]. Many other retinal diseases are associated with elevated levels of cGMP (e.g., forms of autosomal recessive retinitis pigmentosa in which there is a loss of phosphodiesterase activity, and Leber's congenital amaurosis and cone rod dystrophy, in which there are mutations in guanylyl cyclase or GCAP) [9-15]. Work by Vallazza-Deschamps and colleagues suggests that in these diseases, cell death is a direct result of too many open CNG channels, and that cell longevity can be increased by inhibiting the channels [52].

In the current study, we have shown that retinoid inhibition can recover normal cGMP activation in a hypersensitive mutant cone CNG channel associated with a form of cone dystrophy. Furthermore, this inhibition could be accomplished using submicromolar levels of all-trans C22 aldehyde, a retinoid that is too long to fit into the chromophore binding pocket of opsin. This compound also inhibited normal rod and cone CNG channels, making them less sensitive to cGMP. The ability to inhibit normal channels may be useful in treating diseases in which defects in transduction enzymes produce elevated levels of cGMP. A recent study [52] suggested that inhibitors like l-cis-diltiazem (at 10 μM) can reduce the photoreceptor apoptosis that results when CNG channel open probability is increased by high levels of cGMP. However, retinoid analogs offer an advantage over simple channel blockers like l-cis-diltiazem in that they shift the cGMP dose-response relation of the channel. Thus, retinoid analogs would tune down the open probability of the channel in the dark, while still allowing it to respond to light-induced changes in cGMP concentration.

A potential complication in the use of retinoids to treat the hypersensitive mutant cone CNG channels in cone dystrophy is that normal rod CNG channels would also be inhibited by the retinoids. Thus, rod-mediated, night vision would be sacrificed to treat cone-mediated, daylight vision. However, this sacrifice may be worth accepting, since we rely so heavily on our cone vision, and since saving the cones from degeneration may ultimately lead to prevention of total blindness. Furthermore, additional research into the mechanism of retinoid inhibition of CNG channels [42] may lead to the design of retinoid analogs that specifically target only cone or rod channels.

The retinoid concentrations for this study are stated as bath concentrations. Although the concentrations in the membrane are expected to be much higher, the bath concentration is the most relevant here for two reasons: (1) we have shown previously [41,42] that retinoids inhibit the channel at an intracellular binding site, rather than within the bilayer; and (2) bath concentrations would be most easily controlled and measured during therapeutic application of retinoids. Our recent work with intact rods has shown that bath application of retinoids can inhibit the channels, decreasing the dark current without activating the transduction cascade [32].

We suggest that CNG channel inhibition by retinoids that do not interact with opsin may be a generally useful therapeutic tool for targeting CNG channels, without affecting other players in the transduction cascade. This would allow control of the influx of harmful levels of sodium and calcium in the dark, without interfering with the photoreceptor's ability to respond to light. In addition, based on our previous work, it is likely that retinoid inhibition of CNG channels involves a specific binding site [42], so that retinoids are not expected to have generalized effects on other channels in the retina. In support of this notion, a highly homologous channel, the mouse Ether-a-go-go (EAG) channel, is not affected by ATR [54].

Several issues remain to be resolved before C22 aldehyde or similar compounds can be applied as therapeutic agents. First, although C22 does not fit into the chromophore binding pocket of opsin [43], there is currently no information on whether it interacts with other players in the transduction or visual cycle cascades, or, for that matter, with other putative retinoid binding sites on opsin. Testing for potential effects of C22 aldehyde on photoreceptor dark current would be a good first step in this regard, but biochemical studies would also be required. Second, recent work suggests that isolated rods contain a large-capacity retinoid buffer [32] that protects the CNG channels from inhibition under normal physiological conditions. Thus, endogenous retinoids produced by a bleach do not decrease the dark current of isolated rods, but bath-applied retinoids inhibit the channels and thereby reduce the dark current, presumably by overcoming the retinoid buffer capacity. It is not clear whether cones contain a similar buffer, or how close to capacity the retinoid buffers are in rods as well as cones in the intact eye, where there is continuous retinoid movement across the photoreceptor plasma membranes as part of the visual cycle.

Finally, therapeutic use of retinoids to control the activity of photoreceptor CNG channels requires selection of the best application method as well as knowledge of the metabolic processing and toxicity of the specific retinoids. Potential methods of administration of such retinoids include oral delivery, intraocular and periorbital injections [55]. Slow-release encapsulation methods produce more prolonged, uniform delivery, while reducing toxicity [56-59]. A major source of toxicity of high doses of retinoids is the conversion of retinols and retinaldehydes to retinoic acids, which regulate gene expression and can induce teratogenicity [60,61]. Retinyl esters are preferable to aldehydes and alcohols because they are more stable and less reactive, and therefore less toxic. This must be considered in designing appropriate retinoids to specifically inhibit photoreceptor CNG channels without interfering with the transduction cascade. Previous work [42] would predict that retinyl esters would be capable of inhibiting CNG channels, since both alcohol and aldehyde groups at this position permit inhibition. Thus, although further testing remains on many fronts, our results suggest that the inhibition of rod and cone CNG channels by retinoids should be kept in mind when evaluating diseases in which retinoids are elevated, and when designing therapeutic approaches for conditions in which there is an increase in cGMP concentration or in channel sensitivity to cGMP.

Acknowledgements

We thank Dr. Rosalie Crouch for providing the C22 aldehyde, and Dr. Sarah McCabe for participating in early experiments. We are also grateful to Maureen Estevez and Drs. Carter Cornwall, Clint Makino, and Rosalie Crouch for helpful discussions; and to Mandy Barragan and Jin Huang for technical assistance.

This work was supported by the National Eye Institute, NIH grant EY07774 (A.L.Z.); and NIH grant EY04939 (R.K.C.).

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