Cyclic nucleotide-regulated cation channels

Improved spatial direct method with gradient-based diffusion to retain full diffusive fluctuations

The spatial direct method with gradient-based diffusion is an accelerated stochastic reaction-diffusion simulation algorithm that treats diffusive transfers between neighboring subvolumes based on concentration gradients. This recent method achieved a marked improvement in simulation speed and reduction in the number of time-steps required to complete a simulation run, compared with the exact algorithm, by sampling only the net diffusion events, instead of sampling all diffusion events. Although the spatial direct method with gradient-based diffusion gives accurate means of simulation ensembles, its gradient-based diffusion strategy results in reduced fluctuations in populations of diffusive species. In this paper, we present a new improved algorithm that is able to anticipate all possible microscopic fluctuations due to diffusive transfers in the system and incorporate this information to retain the same degree of fluctuations in populations of diffusing species as the exact algorithm. The new algorithm also provides a capability to set the desired level of fluctuation per diffusing species, which facilitates adjusting the balance between the degree of exactness in simulation results and the simulation speed. We present numerical results that illustrate the recovery of fluctuations together with the accuracy and efficiency of the new algorithm.

Bacillus anthracis edema factor substrate specificity: evidence for new modes of action

Since the isolation of Bacillus anthracis exotoxins in the 1960s, the detrimental activity of edema factor (EF) was considered as adenylyl cyclase activity only. Yet the catalytic site of EF was recently shown to accomplish cyclization of cytidine 5'-triphosphate, uridine 5'-triphosphate and inosine 5'-triphosphate, in addition to adenosine 5'-triphosphate. This review discusses the broad EF substrate specificity and possible implications of intracellular accumulation of cyclic cytidine 3':5'-monophosphate, cyclic uridine 3':5'-monophosphate and cyclic inosine 3':5'-monophosphate on cellular functions vital for host defense. In particular, cAMP-independent mechanisms of action of EF on host cell signaling via protein kinase A, protein kinase G, phosphodiesterases and CNG channels are discussed.

Dual contradictory roles of cAMP signaling pathways in hydroxyl radical production in the rat striatum

Studies have suggested that cAMP signaling pathways may be associated with the production of reactive oxygen species. In this study, we examined how modifications in cAMP signaling affected the production of hydroxyl radicals in rat striatum using microdialysis to measure extracellular 2,3-dihydroxybenzoic acid (2,3-DHBA), which is a hydroxyl radical adduct of salicylate. Up to 50 nmol of the cell-permeative cAMP mimetic 8-bromo-cAMP (8-Br-cAMP) increased 2,3-DHBA in a dose-dependent manner (there was no additional increase in 2,3-DHBA at 100 nmol). Another cAMP mimetic, dibutyryl cAMP (db-cAMP), caused a nonsignificant increase in 2,3-DHBA at 50 nmol and a significant decrease at 100 nmol. Up to 20 nmol of forskolin, which is a direct activator of adenylyl cyclase, increased 2,3-DHBA, similar to the effect of 8-Br-cAMP; however, forskolin resulted in a much greater increase in 2,3-DHBA. A potent inhibitor of protein kinase A (PKA), H89 (500 μM), potentiated the 8-Br-cAMP- and forskolin-induced increases in 2,3-DHBA and antagonized the inhibitory effect of 100 nmol of db-cAMP. Interestingly, the administration of 100 nmol of 8-bromo-cGMP alone or in combination with H89 had no significant effect on 2,3-DHBA levels. Doses of 100 nmol of a preferential PKA activator (6-phenyl-cAMP) or a preferential PKA inhibitor (8-bromoadenosine-3',5'-cyclic monophosphorothionate, Rp-isomer; Rp-8-Br-cAMPS), which also inhibits the cAMP-mediated activation of Epac (the exchange protein directly activated by cAMP), suppressed or enhanced, respectively, the formation of 2,3-DHBA. Up to 100 nmol of 8-(4-chlorophenylthio)-2'-O-methyladenosine-cAMP, which is a selective activator of Epac, dose-dependently stimulated the formation of 2,3-DHBA. These findings suggest that cAMP signaling plays contradictory roles (stimulation and inhibition) in the production of hydroxyl radicals in rat striatum by differential actions of Epac and PKA. These roles might contribute to the production of hydroxyl radicals concomitant with cAMP in carbon monoxide poisoning, because the formation of 2,3-DHBA was potentiated by the PKA inhibitor H89 and suppressed by Rp-8-Br-cAMPS, which inhibits PKA and Epac.

Evolutionary and structural perspectives of plant cyclic nucleotide-gated cation channels

Ligand-gated cation channels are a frequent component of signaling cascades in eukaryotes. Eukaryotes contain numerous diverse gene families encoding ion channels, some of which are shared and some of which are unique to particular kingdoms. Among the many different types are cyclic nucleotide-gated channels (CNGCs). CNGCs are cation channels with varying degrees of ion conduction selectivity. They are implicated in numerous signaling pathways and permit diffusion of divalent and monovalent cations, including Ca(2+) and K(+). CNGCs are present in both plant and animal cells, typically in the plasma membrane; recent studies have also documented their presence in prokaryotes. All eukaryote CNGC polypeptides have a cyclic nucleotide-binding domain and a calmodulin binding domain as well as a six transmembrane/one pore tertiary structure. This review summarizes existing knowledge about the functional domains present in these cation-conducting channels, and considers the evidence indicating that plant and animal CNGCs evolved separately. Additionally, an amino acid motif that is only found in the phosphate binding cassette and hinge regions of plant CNGCs, and is present in all experimentally confirmed CNGCs but no other channels was identified. This CNGC-specific amino acid motif provides an additional diagnostic tool to identify plant CNGCs, and can increase confidence in the annotation of open reading frames in newly sequenced genomes as putative CNGCs. Conversely, the absence of the motif in some plant sequences currently identified as probable CNGCs may suggest that they are misannotated or protein fragments.

Central and Peripheral GABA(A) Receptor Regulation of the Heart Rate Depends on the Conscious State of the Animal

Intuitively one might expect that activation of GABAergic inhibitory neurons results in bradycardia. In conscious animals the opposite effect is however observed. GABAergic neurons in nucleus ambiguus hold the ability to control the activity of the parasympathetic vagus nerve that innervates the heart. Upon GABA activation the vagus nerve will be inhibited leaving less parasympathetic impact on the heart. The picture is however blurred in the presence of anaesthesia where both the concentration and type of anaesthetics can result in different effects on the cardiovascular system. This paper reviews cardiovascular outcomes of GABA activation and includes own experiments on anaesthetized animals and isolated hearts. In conclusion, the impact of changes in GABAergic input is very difficult to predict in these settings, emphasizing the need for experiments performed in conscious animals when aiming at determining the cardiovascular effects of compounds acting on GABAergic neurons.

CNGA3: a target of spinal nitric oxide/cGMP signaling and modulator of inflammatory pain hypersensitivity

A large body of evidence indicates that nitric oxide (NO) and cGMP contribute to central sensitization of pain pathways during inflammatory pain. Here, we investigated the distribution of cyclic nucleotide-gated (CNG) channels in the spinal cord, and identified the CNG channel subunit CNGA3 as a putative cGMP target in nociceptive processing. In situ hybridization revealed that CNGA3 is localized to inhibitory neurons of the dorsal horn of the spinal cord, whereas its distribution in dorsal root ganglia is restricted to non-neuronal cells. CNGA3 expression is upregulated in the superficial dorsal horn of the mouse spinal cord and in dorsal root ganglia following hindpaw inflammation evoked by zymosan. Mice lacking CNGA3 (CNGA3(-/-) mice) exhibited an increased nociceptive behavior in models of inflammatory pain, whereas their behavior in models of acute or neuropathic pain was normal. Moreover, CNGA3(-/-) mice developed an exaggerated pain hypersensitivity induced by intrathecal administration of cGMP analogs or NO donors. Our results provide evidence that CNGA3 contributes in an inhibitory manner to the central sensitization of pain pathways during inflammatory pain as a target of NO/cGMP signaling.

cAMP activates hyperpolarization-activated Ca2+ channels in the pollen of Pyrus pyrifolia

Many signal-transduction processes in plant cells have been suggested to be triggered by signal-induced opening of calcium ion (Ca(2+)) channels in the plasma membrane. Cyclic nucleotides have been proposed to lead to an increase in cytosolic free Ca(2+) in pollen. However, direct recordings of cyclic-nucleotide-induced Ca(2+) currents in pollen have not yet been obtained. Here, we report that cyclic AMP (cAMP) activated a hyperpolarization-activated Ca(2+) channel in the Pyrus pyrifolia pollen tube using the patch-clamp technique, which resulted in a significant increase in pollen tube protoplast cytosolic-Ca(2+) concentration. Outside-out single channel configuration identified that cAMP directly increased the Ca(2+) channel open-probability without affecting channel conductance. cAMP-induced currents were composed of both Ca(2+) and K(+). However, cGMP failed to mimic the cAMP effect. Higher cytosolic free-Ca(2+) concentration significantly decreased the cAMP-induced currents. These results provide direct evidence for cAMP activation of hyperpolarization-activated Ca(2+) channels in the plasma membrane of pollen tubes, which, in turn, modulate cellular responses in regulation of pollen tube growth.

1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one induces cell cycle arrest and apoptosis in HeLa cells by preventing microtubule polymerization

1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) is known as a specific inhibitor of soluble guanylyl cyclase (sGC). Previously, however, ODQ was reported to induce cell death via sGC-dependent and sGC-independent means in a variety of cell types. The aim of this study was to investigate the mechanism by which ODQ induces cell death in HeLa cells. Treatment of HeLa cells with ODQ induced a concentration-dependent decrease in cell viability over the range from 10 to 100 μM. DNA fragmentation and fluorescence-activated cell sorting analysis using annexin V and propidium iodide staining revealed that ODQ triggered apoptosis at concentrations of 50 and 100 μM within 24 to 48 h. The addition of 8-Br-cGMP in the presence of ODQ failed to rescue HeLa cells from death, suggesting that the inhibition of sGC was not responsible for the pro-apoptotic action of ODQ. ODQ arrested the cell cycle at the G2/M phase and caused disassembly of the microtubule network. This process was reversed by dithiothreitol. In addition, ODQ was shown to inhibit the polymerization of purified tubulin, and this was also prevented by dithiothreitol. These results indicate that ODQ inhibits microtubule assembly by direct oxidation of tubulin, induces cell cycle arrest at the G2/M phase, and triggers apoptosis in HeLa cells.

Protein kinase a in cancer

In the past, many chromosomal and genetic alterations have been examined as possible causes of cancer. However, some tumors do not display a clear molecular and/or genetic signature. Therefore, other cellular processes may be involved in carcinogenesis. Genetic alterations of proteins involved in signal transduction have been extensively studied, for example oncogenes, while modifications in intracellular compartmentalization of these molecules, or changes in the expression of unmodified genes have received less attention. Yet, epigenetic modulation of second messenger systems can deeply modify cellular functioning and in the end may cause instability of many processes, including cell mitosis. It is important to understand the functional meaning of modifications in second messenger intracellular pathways and unravel the role of downstream proteins in the initiation and growth of tumors. Within this framework, the cAMP system has been examined. cAMP is a second messenger involved in regulation of a variety of cellular functions. It acts mainly through its binding to cAMP-activated protein kinases (PKA), that were suggested to participate in the onset and progression of various tumors. PKA may represent a biomarker for tumor detection, identification and staging, and may be a potential target for pharmacological treatment of tumors.

Cyclic AMP-mediated immune regulation--overview of mechanisms of action in T cells

The canonical second messenger cAMP is well established as a potent negative regulator of T cell immune function. Through protein kinase A (PKA) it regulates T cell function at the level of transcription factors, members of the mitogen-activated protein kinase pathway, phospholipases (PLs), Ras homolog (Rho)A and proteins involved in the control of cell cycle progression. Type I PKA is the predominant PKA isoform in T cells. Furthermore, whereas type II PKA is located at the centrosome, type I PKA is anchored close to the T cell receptor (TCR) in lipid rafts by the Ezrin-ERM-binding phosphoprotein of 50 kDa (EBP50)-phosphoprotein associated with glycosphingolipid-enriched microdomains (PAG) scaffold complex. The most TCR-proximal target for type I PKA is C-terminal Src kinase (Csk), which upon activation by raft recruitment and phosphorylation inhibits the Src family tyrosine kinases Lck and Fyn and thus functions to maintain T cell homeostasis. Recently, induction of cAMP levels in responder T cells has emerged as one of the mechanisms by which regulatory T (T(R)) cells execute their suppressive action. Thus, the cAMP-type I PKA-Csk pathway emerges as a putative target for therapeutic intervention in autoimmune disorders as well as in cancer, where T(R) cell-mediated suppression contributes to suboptimal local immune responses.

Excitatory action of vasopressin in the brain of the rat: role of cAMP signaling

Brain vasopressin plays a role in behavioral and cognitive functions and in pathological conditions. Relevant examples are pair bonding, social recognition, fear responses, stress disorders, anxiety and depression. At the neuronal level, vasopressin exerts its effects by binding to V1a receptors. In the brainstem, vasopressin can excite facial motoneurons by generating a sustained inward current which is sodium-dependent, tetrodotoxin-insensitive and voltage-gated. This effect is independent of intracellular calcium mobilization and is unaffected by phospholipase Cβ (PLCβ) or protein kinase C (PKC) inhibitors. There are two major unsolved problems. (i) What is the intracellular signaling pathway activated by vasopressin? (ii) What is the exact nature of the vasopressin-sensitive cation channels? We performed recordings in brainstem slices. Facial motoneurons were voltage-clamped in the whole-cell configuration. We show that a major fraction, if not the totality, of the peptide effect was mediated by cAMP signaling and that the vasopressin-sensitive cation channels were directly gated by cAMP. These channels appear to exclude lithium, are suppressed by 2-aminoethoxydiphenylborane (2-APB) and flufenamic acid (FFA) but not by ruthenium red or amiloride. They are distinct from transient receptor channels and from cyclic nucleotide-regulated channels involved in visual and olfactory transduction. They present striking similarities with cation channels present in a variety of molluscan neurons. To our knowledge, the presence in mammalian neurons of channels having these properties has not been previously reported. Our data should contribute to a better knowledge of the neural mechanism of the central actions of vasopressin, and may be potentially significant in view of clinical applications.

Phosphorylation and modulation of hyperpolarization-activated HCN4 channels by protein kinase A in the mouse sinoatrial node

The sympathetic nervous system increases heart rate by activating β adrenergic receptors and increasing cAMP levels in myocytes in the sinoatrial node. The molecular basis for this response is not well understood; however, the cardiac funny current (I_f) is thought to be among the end effectors for cAMP signaling in sinoatrial myocytes. I_f is produced by hyperpolarization-activated cyclic nucleotide–sensitive (HCN4) channels, which can be potentiated by direct binding of cAMP to a conserved cyclic nucleotide binding domain in the C terminus of the channels. β adrenergic regulation of I_f in the sinoatrial node is thought to occur via this direct binding mechanism, independent of phosphorylation. Here, we have investigated whether the cAMP-activated protein kinase (PKA) can also regulate sinoatrial HCN4 channels. We found that inhibition of PKA significantly reduced the ability of β adrenergic agonists to shift the voltage dependence of I_f in isolated sinoatrial myocytes from mice. PKA also shifted the voltage dependence of activation to more positive potentials for heterologously expressed HCN4 channels. In vitro phosphorylation assays and mass spectrometry revealed that PKA can directly phosphorylate at least 13 sites on HCN4, including at least three residues in the N terminus and at least 10 in the C terminus. Functional analysis of truncated and alanine-substituted HCN4 channels identified a PKA regulatory site in the distal C terminus of HCN4, which is required for PKA modulation of I_f. Collectively, these data show that native and expressed HCN4 channels can be regulated by PKA, and raise the possibility that this mechanism could contribute to sympathetic regulation of heart rate.

The N terminus of phosphodiesterase TbrPDEB1 of Trypanosoma brucei contains the signal for integration into the flagellar skeleton

The precise subcellular localization of the components of the cyclic AMP (cAMP) signaling pathways is a crucial aspect of eukaryotic intracellular signaling. In the human pathogen Trypanosoma brucei, the strict control of cAMP levels by cAMP-specific phosphodiesterases is essential for parasite survival, both in cell culture and in the infected host. Among the five cyclic nucleotide phosphodiesterases identified in this organism, two closely related isoenzymes, T. brucei PDEB1 (TbrPDEB1) (PDEB1) and TbrPDEB2 (PDEB2) are predominantly responsible for the maintenance of cAMP levels. Despite their close sequence similarity, they are distinctly localized in the cell. PDEB1 is mostly located in the flagellum, where it forms an integral part of the flagellar skeleton. PDEB2 is mainly located in the cell body, and only a minor part of the protein localizes to the flagellum. The current study, using transfection of procyclic trypanosomes with green fluorescent protein (GFP) reporters, demonstrates that the N termini of the two enzymes are essential for determining their final subcellular localization. The first 70 amino acids of PDEB1 are sufficient to specifically direct a GFP reporter to the flagellum and to lead to its detergent-resistant integration into the flagellar skeleton. In contrast, the analogous region of PDEB2 causes the GFP reporter to reside predominantly in the cell body. Mutagenesis of selected residues in the N-terminal region of PDEB2 demonstrated that single amino acid changes are sufficient to redirect the reporter from a cell body location to stable integration into the flagellar skeleton.

The cyclic nucleotide-gated ion channel CNGA3 contributes to coolness-induced responses of Grueneberg ganglion neurons

Localized to the vestibule of the nasal cavity, neurons of the Grueneberg ganglion (GG) respond to cool ambient temperatures. The molecular mechanisms underlying this thermal response are still elusive. Recently, it has been suggested that cool temperatures may activate a cyclic guanosine monophosphate (cGMP) pathway in the GG, which would be reminiscent of thermosensory neurons in Caenorhabditis elegans. In search for other elements of such a cascade, we have found that the cyclic nucleotide-gated ion channel CNGA3 was strongly expressed in the GG and that expression of CNGA3 was confined to those cells that are responsive to coolness. Further experiments revealed that the response of GG neurons to cool temperatures was significantly reduced in CNGA3-deficient mice compared to wild-type conspecifics. The observation that a cGMP-activated non-selective cation channel significantly contributes to the coolness-evoked response in GG neurons strongly suggests that a cGMP cascade is part of the transduction process.

Molecular remodeling of ion channels, exchangers and pumps in atrial and ventricular myocytes in ischemic cardiomyopathy

Existing molecular knowledge base of cardiovascular diseases is rudimentary because of lack of specific attribution to cell type and function. The aim of this study was to investigate cell-specific molecular remodeling in human atrial and ventricular myocytes associated with ischemic cardiomyopathy. Our strategy combines two technological innovations, laser-capture microdissection of identified cardiac cells in selected anatomical regions of the heart and splice microarray of a narrow catalog of the functionally most important genes regulating ion homeostasis. We focused on expression of a principal family of genes coding for ion channels, exchangers and pumps (CE&P genes) that are involved in electrical, mechanical and signaling functions of the heart and constitute the most utilized drug targets. We found that (1) CE&P genes remodel in a cell-specific manner: ischemic cardiomyopathy affected 63 CE&P genes in ventricular myocytes and 12 essentially different genes in atrial myocytes. (2) Only few of the identified CE&P genes were previously linked to human cardiac disfunctions. (3) The ischemia-affected CE&P genes include nuclear chloride channels, adrenoceptors, cyclic nucleotide-gated channels, auxiliary subunits of Na(+), K(+) and Ca(2+) channels, and cell-surface CE&Ps. (4) In both atrial and ventricular myocytes ischemic cardiomyopathy reduced expression of CACNG7 and induced overexpression of FXYD1, the gene crucial for Na(+) and K(+) homeostasis. Thus, our cell-specific molecular profiling defined new landmarks for correct molecular modeling of ischemic cardiomyopathy and development of underlying targeted therapies.

Progressive maturation in contracting cardiomyocytes derived from human embryonic stem cells: Qualitative effects on electrophysiological responses to drugs

The field of drug testing currently needs a new integrated assay system, as accurate as systems using native tissues, that will allow us to predict arrhythmia risks of candidate drugs and the relationship between genetic mutations and acquired electrophysiological phenotypes. This could be accomplished by combining the microelectrode array (MEA) system with cardiomyocytes (CMs) derived from human embryonic stem cells (hESC) and induced pluripotential stem cells. CMs have been successfully induced from both types, but their maturation process is not systematically controlled; this results in loss of beating potency and insufficient ion channel function. We generated a transgenic hESC line that facilitates maintenance of hESC-CM clusters every 2 weeks by expressing GFP driven by a cardiac-specific alphaMHC promoter, thereby producing a compact pacemaker lineage within a ventricular population over a year. Further analyses, including quantitative RT-PCR, patch-clamp, and MEA-mediated QT tests, demonstrated that replating culturing continuously enhanced gene expression, ionic current amplitudes, and resistance to K(+) channel blockades in hESC-CMs. Moreover, temporal three-dimensional (3D) culturing accelerated maturation by restoring the global gene repressive status established in the adhesive status. Replating/3D culturing thus produces hESC-CMs that act as functional syncytia suitable for use in regenerative medicine and accurate drug tests.

The role of Epac proteins, novel cAMP mediators, in the regulation of immune, lung and neuronal function

Chronic degenerative inflammatory diseases, such as chronic obstructive pulmonary disease and Alzheimer's dementia, afflict millions of people around the world, causing death and debilitation. Despite the global impact of these diseases, there have been few innovative breakthroughs into their cause, treatment or cure. As with many debilitating disorders, chronic degenerative inflammatory diseases may be associated with defective or dysfunctional responses to second messengers, such as cyclic adenosinemonophosphate (cAMP). The identification of the cAMP-activated guanine nucleotide exchange factors for Ras-like GTPases, Epac1 (also known as cAMP-GEF-I) and Epac2 (also known as cAMP-GEF-II), profoundly altered the prevailing assumptions concerning cAMP signalling, which until then had been solely associated with protein kinase A (PKA). Studies of the molecular mechanisms of Epac-related signalling have demonstrated that these novel cAMP sensors regulate many physiological processes either alone and/or in concert with PKA. These include calcium handling, cardiac and smooth muscle contraction, learning and memory, cell proliferation and differentiation, apoptosis, and inflammation. The diverse signalling properties of cAMP might be explained by spatio-temporal compartmentalization, as well as A-kinase anchoring proteins, which seem to coordinate Epac signalling networks. Future research should focus on the Epac-regulated dynamics of cAMP, and, hopefully, the development of compounds that specifically interfere with the Epac signalling system in order to determine the precise significance of Epac proteins in chronic degenerative inflammatory disorders.

Structural genomics target selection for the New York consortium on membrane protein structure

The New York Consortium on Membrane Protein Structure (NYCOMPS), a part of the Protein Structure Initiative (PSI) in the USA, has as its mission to establish a high-throughput pipeline for determination of novel integral membrane protein structures. Here we describe our current target selection protocol, which applies structural genomics approaches informed by the collective experience of our team of investigators. We first extract all annotated proteins from our reagent genomes, i.e. the 96 fully sequenced prokaryotic genomes from which we clone DNA. We filter this initial pool of sequences and obtain a list of valid targets. NYCOMPS defines valid targets as those that, among other features, have at least two predicted transmembrane helices, no predicted long disordered regions and, except for community nominated targets, no significant sequence similarity in the predicted transmembrane region to any known protein structure. Proteins that feed our experimental pipeline are selected by defining a protein seed and searching the set of all valid targets for proteins that are likely to have a transmembrane region structurally similar to that of the seed. We require sequence similarity aligning at least half of the predicted transmembrane region of seed and target. Seeds are selected according to their feasibility and/or biological interest, and they include both centrally selected targets and community nominated targets. As of December 2008, over 6,000 targets have been selected and are currently being processed by the experimental pipeline. We discuss how our target list may impact structural coverage of the membrane protein space.