Cyclic nucleotide gated channels

cGMP manipulation in cardiometabolic disease: chances and challenges

PURPOSE OF REVIEW

Cardiovascular and metabolic diseases are closely linked and commonly occur in the same patients. This review focuses on the cyclic guanosine monophosphate (cGMP) system and its crosstalk between metabolism and the cardiovascular system.

RECENT FINDINGS

Recent studies suggest that cGMP, which serves as second messenger for nitric oxide and for natriuretic peptides, improves oxidative metabolism and insulin signaling. The clinical evidence is particularly strong for the natriuretic peptide branch of the cGMP system. Clinical trials suggested improvements in insulin sensitivity and reductions in the risk of progressing to type 2 diabetes mellitus. However, further studies are needed.

SUMMARY

Enhancing cGMP signaling through nonpharmacological or pharmacological means may improve glucose metabolism in addition to affecting the cardiovascular system. However, excessive cGMP production could have significant unwanted cardiovascular and metabolic effects.

Novel phosphodiesterase-5 (PDE5) inhibitors in the alleviation of erectile dysfunction due to diabetes and ageing-induced oxidative stress

BACKGROUND

Diabetes and ageing are associated with erectile dysfunction (ED). Under these conditions, increased oxidative stress produces superoxide ions, which in turn suppresses the nitric oxide-cyclic guanosine monophosphate pathway. The phophodiesterase-5 (PDE5) inhibitors still remain ineffective in 15 - 57% of impotent men.

OBJECTIVE

To identify the efficacy of conventional and novel PDE5 inhibitors on oxidative stress involved in diabetes and ageing.

METHODS

An electronic search of the literature was conducted to review published information relating diabetes and ageing to ED and to cover late-stage developments of novel PDE5 inhibitors.

RESULTS/CONCLUSION

Safe and more efficacious alternatives that can modulate PDE5 levels in ED regarding oxidative stress conditions are needed. Included in this review are updates on the new PDE5 inhibitors avanafil, udenafil, SLx-2101, and mirodenafil.

Dimerization of Nitric Oxide-sensitive Guanylyl Cyclase Requires the α1 N Terminus

The enzyme nitric oxide-sensitive guanylyl cyclase is an obligate heterodimer consisting of an alpha and a beta subunit. Whereas the C-terminal parts of the subunits have been shown to be sufficient for catalysis, regulation was assigned to the N termini. The central domains have been postulated to be responsible for the formation of alphabeta heterodimers. Here, we have analyzed dimerization by precipitation of various N- and C-terminally truncated alpha(1) mutants with beta(1) wild type or deletion mutants thereof after coexpression in the baculovirus/Sf9 system. In contrast to the current hypothesis, our analysis revealed that an N-terminal region of the alpha(1) subunit (amino acids 61-128) is mandatory for quantitative dimerization. The central domain (amino acids 367-462) contributes but is not sufficient to mediate robust alphabeta interaction. Wild type-like binding of the identified minimum dimerization region of alpha(1) (amino acids 61-462) requires the N-terminal and central region of beta(1) (amino acids 1-385). Furthermore, we observed an unequal stability of the alpha(1) and beta(1) subunit. Whereas beta(1) forms heme containing homodimers and is stable, alpha(1) appears to be prone to misfolding and degradation when heterodimerization is impaired by deletion of important sequences.

The VGL-Chanome: A Protein Superfamily Specialized for Electrical Signaling and Ionic Homeostasis

Complex multicellular organisms require rapid and accurate transmission of information among cells and tissues and tight coordination of distant functions. Electrical signals and resulting intracellular calcium transients, in vertebrates, control contraction of muscle, secretion of hormones, sensation of the environment, processing of information in the brain, and output from the brain to peripheral tissues. In nonexcitable cells, calcium transients signal many key cellular events, including secretion, gene expression, and cell division. In epithelial cells, huge ion fluxes are conducted across tissue boundaries. All of these physiological processes are mediated in part by members of the voltage-gated ion channel protein superfamily. This protein superfamily of 143 members is one of the largest groups of signal transduction proteins, ranking third after the G protein-coupled receptors and the protein kinases in number. Each member of this superfamily contains a similar pore structure, usually covalently attached to regulatory domains that respond to changes in membrane voltage, intracellular signaling molecules, or both. Eight families are included in this protein superfamily-voltage-gated sodium, calcium, and potassium channels; calcium-activated potassium channels; cyclic nucleotide-modulated ion channels; transient receptor potential (TRP) channels; inwardly rectifying potassium channels; and two-pore potassium channels. This article identifies all of the members of this protein superfamily in the human genome, reviews the molecular and evolutionary relations among these ion channels, and describes their functional roles in cell physiology.

All-trans-retinal is a closed-state inhibitor of rod cyclic nucleotide-gated ion channels

Rod vision begins when 11-cis-retinal absorbs a photon and isomerizes to all-trans-retinal (ATR) within the photopigment, rhodopsin. Photoactivated rhodopsin triggers an enzyme cascade that lowers the concentration of cGMP, thereby closing cyclic nucleotide–gated (CNG) ion channels. After isomerization, ATR dissociates from rhodopsin, and after a bright light, this release is expected to produce a large surge of ATR near the CNG channels. Using excised patches from Xenopus oocytes, we recently showed that ATR shuts down cloned rod CNG channels, and that this inhibition occurs in the nanomolar range (aqueous concentration) at near-physiological concentrations of cGMP. Here we further characterize the ATR effect and present mechanistic information. ATR was found to decrease the apparent cGMP affinity, as well as the maximum current at saturating cGMP. When ATR was applied to outside-out patches, inhibition was much slower and less effective than when it was applied to inside-out patches, suggesting that ATR requires access to the intracellular surface of the channel or membrane. The apparent ATR affinity and maximal inhibition of heteromeric (CNGA1/CNGB1) channels was similar to that of homomeric (CNGA1) channels. Single-channel and multichannel data suggest that channel inhibition by ATR is reversible. Inhibition by ATR was not voltage dependent, and the form of its dose–response relation suggested multiple ATR molecules interacting per channel. Modeling of the data obtained with cAMP and cGMP suggests that ATR acts by interfering with the allosteric opening transition of the channel and that it prefers closed, unliganded channels. It remains to be determined whether ATR acts directly on the channel protein or instead alters channel–bilayer interactions.

A method to determine the relative cAMP permeability of connexin channels

Here we present a method by which gap junction-mediated intercellular diffusion of adenosine 3',5'-cyclic monophosphate (cAMP) molecules can be monitored in "real-time" and the cAMP permeability of different gap junction channels can be compared. Intercellular cAMP diffusion was investigated throughout this study in human HeLa cells coexpressing murine connexin45 and cyclic nucleotide-gated (CNG) ion channels. The CNG channels were used as cAMP sensors, since CNG channel activation led to an increase of the cytosolic Ca2+ concentration, which was monitored by Ca2+ imaging. A cAMP gradient was generated between two contacting cells by restricting the photolysis of caged cAMP to only one cell. The intercellular diffusion of cAMP was measured by the increase in Ca2+ concentration in the neighboring cell. We developed a standardization procedure for the Ca2+ signal which allowed estimation of the amount of cAMP that diffused from cell to cell. The number of gap junction channels between each cell pair investigated was determined by double whole-cell patch-clamp measurements. On the basis of these data we calculated how many gap junction channels contributed to the diffusion of a certain amount of cAMP. The new method can be used to compare the selective permeabilities of different gap junction channels for cAMP and for cGMP which also activates the CNG channel.

Regulation of the rod photoreceptor cyclic nucleotide-gated channel

The cGMP-gated channel of rod photoreceptors plays a key role in phototransduction by controlling the flow of cations into the outer segment in response to light-induced changes in cGMP. The channel is a heterotetramer composed of alpha-subunits required for channel activity and beta-subunits that are important in modulating the activity of channel. Earlier studies have shown that exogenous calmodulin binds to the beta-subunit of the channel and modulates the sensitivity of the channel for cGMP in a calcium dependent manner. In addition unidentified Ca2+-dependent endogenous proteins have been reported to modulate the activity of the frog rod channel. In this paper, we investigated whether endogenous calmodulin and other Ca2+ binding proteins interact with and modulate the cGMP-gated channel in bovine rod outer segments. Using immunoaffinity techniques in conjunction with ion flux assays, we show that endogenous calmodulin, but not other Ca2+ dependent proteins, binds and modulates the rod cGMP-gated channel in bovine rod outer segments. We also show that the beta-subunit of the channel is phosphorylated by endogenous and exogenous casein kinase 2. This posttranslational modification, however, does not alter the sensitivity of the channel for cGMP.

Electrophysiological analysis of cloned cyclic nucleotide-gated ion channels

Electrophysiological studies were conducted on the cloned plant cyclic nucleotide-gated ion channels AtCNGC2 and AtCNGC1 from Arabidopsis, and NtCBP4 from tobacco (Nicotiana tobacum). The nucleotide coding sequences for these proteins were expressed in Xenopus laevis oocytes or HEK 293 cells. Channel characteristics were evaluated using voltage clamp analysis of currents in the presence of cAMP. AtCNGC2 was demonstrated to conduct K(+) and other monovalent cations, but exclude Na(+); this conductivity profile is unique for any ion channel not possessing the amino acid sequence found in the selectivity filter of K(+)-selective ion channels. Application of cAMP evoked currents in membrane patches of oocytes injected with AtCNGC2 cRNA. Direct activation of the channel by cyclic nucleotide, demonstrated by application of cyclic nucleotide to patches of membranes expressing such channels, is a hallmark characteristic of this ion channel family. Voltage clamp studies (two-electrode configuration) demonstrated that AtCNGC1 and NtCBP4 are also cyclic nucleotide-gated channels. Addition of a lipophilic analog of cAMP to the perfusion bath of oocytes injected with NtCBP4 and AtCNGC1 cRNAs induced inward rectified, noninactivating K(+) currents.

Zaprinast, an inhibitor of cGMP-selective phosphodiesterases, enhances the secretion of TNF-alpha and IL-1beta and the expression of iNOS and MHC class II molecules in rat microglial cells

Proinflammatory cytokines produced by activated glial cells may in turn augment the immune/inflammatory reactions of glial cells through autocrine and paracrine routes. The NO/cGMP signaling represents one of the reactions of activated glial cells. We investigated whether the production of proinflammatory cytokines by glial cells is affected by NO-dependent downstream cGMP signaling. In primary cultures of mixed astrocytes and microglial cells, zaprinast (0.1 mM), an inhibitor of cGMP-selective phosphodiesterases, enhanced the basal and LPS (1.0 microg/ml)-induced secretion of TNF-alpha and IL-1beta. Zaprinast also enhanced NO production induced by LPS or IFN-gamma (100 U/ml), and in microglial cell cultures, but not in astrocyte cultures, zaprinast enhanced the basal and the IFN-gamma-induced production of the cytokines, TNF-alpha and IL-1beta, and of NO. This upregulation by zaprinast was partially inhibited by KT5823 (1.0 microM), an inhibitor of protein kinase G. The LPS-induced production of TNF-alpha, IL-1beta, and NO was inhibited by ODQ (50 microM), an inhibitor of soluble guanylyl cyclase, and by KT5823. Immunohistochemical analysis of mixed glial cell cultures showed that LPS/IFN-gamma-induced iNOS expression and the enhanced expression of iNOS by zaprinast were restricted to microglial cells. Zaprinast enhanced the IFN-gamma (200 U/ml)-induced expression of MHC Class II molecules in astrocytes and microglial cells in mixed cultures, but did not enhance this IFN-gamma-induced expression in pure astrocytes, which lacked paracrine TNF-alpha from microglial cells. Summarizing, zaprinast, which is associated with cGMP/protein kinase G signaling, may augment central immune/inflammatory reactions, possibly via the increased production of TNF-alpha and IL-1beta by activated microglial cells.

Ion channels and their functional role in vascular endothelium

Endothelial cells (EC) form a unique signal-transducing surface in the vascular system. The abundance of ion channels in the plasma membrane of these nonexcitable cells has raised questions about their functional role. This review presents evidence for the involvement of ion channels in endothelial cell functions controlled by intracellular Ca(2+) signals, such as the production and release of many vasoactive factors, e.g., nitric oxide and PGI(2). In addition, ion channels may be involved in the regulation of the traffic of macromolecules by endocytosis, transcytosis, the biosynthetic-secretory pathway, and exocytosis, e.g., tissue factor pathway inhibitor, von Willebrand factor, and tissue plasminogen activator. Ion channels are also involved in controlling intercellular permeability, EC proliferation, and angiogenesis. These functions are supported or triggered via ion channels, which either provide Ca(2+)-entry pathways or stabilize the driving force for Ca(2+) influx through these pathways. These Ca(2+)-entry pathways comprise agonist-activated nonselective Ca(2+)-permeable cation channels, cyclic nucleotide-activated nonselective cation channels, and store-operated Ca(2+) channels or capacitative Ca(2+) entry. At least some of these channels appear to be expressed by genes of the trp family. The driving force for Ca(2+) entry is mainly controlled by large-conductance Ca(2+)-dependent BK(Ca) channels (slo), inwardly rectifying K(+) channels (Kir2.1), and at least two types of Cl( -) channels, i.e., the Ca(2+)-activated Cl(-) channel and the housekeeping, volume-regulated anion channel (VRAC). In addition to their essential function in Ca(2+) signaling, VRAC channels are multifunctional, operate as a transport pathway for amino acids and organic osmolytes, and are possibly involved in endothelial cell proliferation and angiogenesis. Finally, we have also highlighted the role of ion channels as mechanosensors in EC. Plasmalemmal ion channels may signal rapid changes in hemodynamic forces, such as shear stress and biaxial tensile stress, but also changes in cell shape and cell volume to the cytoskeleton and the intracellular machinery for metabolite traffic and gene expression.

Non-synaptic ion channels in insects--basic properties of currents and their modulation in neurons and skeletal muscles

Insects are favoured objects for studying information processing in restricted neuronal networks, e.g. motor pattern generation or sensory perception. The analysis of the underlying processes requires knowledge of the electrical properties of the cells involved. These properties are determined by the expression pattern of ionic channels and by the regulation of their function, e.g. by neuromodulators. We here review the presently available knowledge on insect non-synaptic ion channels and ionic currents in neurons and skeletal muscles. The first part of this article covers genetic and structural informations, the localization of channels, their electrophysiological and pharmacological properties, and known effects of second messengers and modulators such as neuropeptides or biogenic amines. In a second part we describe in detail modulation of ionic currents in three particularly well investigated preparations, i.e. Drosophila photoreceptor, cockroach DUM (dorsal unpaired median) neuron and locust jumping muscle. Ion channel structures are almost exclusively known for the fruitfly Drosophila, and most of the information on their function has also been obtained in this animal, mainly based on mutational analysis and investigation of heterologously expressed channels. Now the entire genome of Drosophila has been sequenced, it seems almost completely known which types of channel genes--and how many of them--exist in this animal. There is much knowledge of the various types of channels formed by 6-transmembrane--spanning segments (6TM channels) including those where four 6TM domains are joined within one large protein (e.g. classical Na+ channel). In comparison, two TM channels and 4TM (or tandem) channels so far have hardly been explored. There are, however, various well characterized ionic conductances, e.g. for Ca2+, Cl- or K+, in other insect preparations for which the channels are not yet known. In some of the larger insects, i.e. bee, cockroach, locust and moth, rather detailed information has been established on the role of ionic currents in certain physiological or behavioural contexts. On the whole, however, knowledge of non-synaptic ion channels in such insects is still fragmentary. Modulation of ion currents usually involves activation of more or less elaborate signal transduction cascades. The three detailed examples for modulation presented in the second part indicate, amongst other things, that one type of modulator usually leads to concerted changes of several ion currents and that the effects of different modulators in one type of cell may overlap. Modulators participate in the adaptive changes of the various cells responsible for different physiological or behavioural states. Further study of their effects on the single cell level should help to understand how small sets of cells cooperate in order to produce the appropriate output.

Genetic analysis of the Drosophila Gs(alpha) gene

One of the best understood signal transduction pathways activated by receptors containing seven transmembrane domains involves activation of heterotrimeric G-protein complexes containing Gs(alpha), the subsequent stimulation of adenylyl cyclase, production of cAMP, activation of protein kinase A (PKA), and the phosphorylation of substrates that control a wide variety of cellular responses. Here, we report the identification of "loss-of-function" mutations in the Drosophila Gs(alpha) gene (dgs). Seven mutants have been identified that are either complemented by transgenes representing the wild-type dgs gene or contain nucleotide sequence changes resulting in the production of altered Gs(alpha) protein. Examination of mutant alleles representing loss-of-Gs(alpha) function indicates that the phenotypes generated do not mimic those created by mutational elimination of PKA. These results are consistent with the conclusion reached in previous studies that activation of PKA, at least in these developmental contexts, does not depend on receptor-mediated increases in intracellular cAMP, in contrast to the predictions of models developed primarily on the basis of studies in cultured cells.

Cyclic nucleotide analogs as biochemical tools and prospective drugs

Cyclic AMP (cAMP) and cyclic GMP (cGMP) are key second messengers involved in a multitude of cellular events. From the wealth of synthetic analogs of cAMP and cGMP, only a few have been explored with regard to their therapeutic potential. Some of the first-generation cyclic nucleotide analogs were promising enough to be tested as drugs, for instance N(6),O(2)'-dibutyryl-cAMP and 8-chloro-cAMP (currently in clinical Phase II trials as an anticancer agent). Moreover, 8-bromo and dibutyryl analogs of cAMP and cGMP have become standard tools for investigations of biochemical and physiological signal transduction pathways. The discovery of the Rp-diastereomers of adenosine 3',5'-cyclic monophosphorothioate and guanosine 3',5'-cyclic monophosphorothioate as competitive inhibitors of cAMP- and cGMP-dependent protein kinases, as well as subsequent development of related analogs, has proven very useful for studying the molecular basis of signal transduction. These analogs exhibit a higher membrane permeability, increased resistance against degradation, and improved target specificity. Furthermore, better understanding of signaling pathways and ligand/protein interactions has led to new therapeutic strategies. For instance, Rp-8-bromo-adenosine 3',5'-cyclic monophosphorothioate is employed against diseases of the immune system. This review will focus mainly on recent developments in cyclic nucleotide-related biochemical and pharmacological research, but also highlights some historical findings in the field.

Role of cAMP in the short-term modulation of a neuromuscular system in aplysia

Neuromuscular synapses in buccal muscle I3a of Aplysia are modulated by the small cardioactive peptide (SCP), a peptide cotransmitter that is intrinsic to the motor neurons, and by serotonin (5-HT) released from modulatory neurons that are extrinsic to the motor circuit. Although the modulation of excitatory junction potentials (EJPs) and contractions by 5-HT and SCP has been studied extensively in this muscle, little is known about the mechanisms that underlie the modulation. 5-HT and SCP, at 1 microM, were found to potently increase the level of cAMP in I3a. Therefore we investigated whether the activation of the cAMP pathway was sufficient to modulate EJPs and contractions. The direct activation of adenylyl cyclase with forskolin increased the level of cAMP, facilitated EJPs, and potentiated contractions. Indeed, the short-term effects of forskolin were very similar to all aspects of the short-term effects of 5-HT and SCP. Membrane-permeable cAMP analogues also mimicked the effects of 5-HT and SCP on EJPs and contractions. However, it seems likely that some effects of 5-HT are also mediated through other second-messenger pathways because low concentrations of 5-HT modulate EJPs and contractions but do not significantly increase cAMP levels in I3a. It is possible that lower concentrations of 5-HT function through receptors linked to protein kinase C (PKC) because phorbol, an activator of PKC, modulated EJPs and contractions without increasing the levels of cAMP. In conclusion, we provide evidence that pharmacological agents that activate the cAMP pathway mimicked most of the effects of 5-HT or SCP and that more than one second-messenger system appears to be involved in the modulation of the I3a neuromuscular system.