Catalytic mechanism and regulation of mammalian adenylyl cyclases

A conformational transition in the adenylyl cyclase catalytic site yields different binding modes for ribosyl-modified and unmodified nucleotide inhibitors

Adenylyl cyclases (ACs) are promising pharmacological targets for treating heart failure, cancer, and psychosis. Ribose-substituted nucleotides have been reported as a potent family of AC inhibitors. In silico analysis of the docked conformers of such nucleotides in AC permits assembly of a consistent, intuitive QSAR model with strong correlation relative to measured pK(i) values. Energy decomposition suggests that the MANT group effects an AC conformational transition upon ligand binding.

Similar pattern in cardiac differentiation of human embryonic stem cell lines, BG01V and ReliCell®hES1, under low serum concentration supplemented with bone morphogenetic protein-2

Human embryonic stem cells (hESCs) can differentiate into cardiomyocytes, but the efficiency of this process is highly variable. So, developing generic differentiation protocols and their empirical testing on a range of independently derived hESC lines pose a daunting challenge due to considerable diversity in culture methods practiced between lines. Maintenance of BG01V and ReliCellhES1 has routinely been on mouse embryonic fibroblast (MEF) feeder layers using manual passaging. We assessed cardiac differentiation from both the cell lines via embryoid body (EB) formation. Subsequent culture in low fetal bovine serum (5%)-containing medium produced spontaneously contracting EBs, in the presence of bone morphogenetic protein-2 (BMP-2; 25 ng/ml). Derived cardiomyocytes expressed cardiac genes and proteins and responded to functional assays. Further, the activation of the Smad signaling machinery evoked by BMP-2 has been confirmed through inhibitor studies. Therefore, in our hands, the same differentiation conditions functioned in two independently derived hESC lines. Similar studies in other lines may facilitate development of universal protocols. The present data may also provide valuable insights for testing of other factors that might promote cardiomyocyte differentiation in low-serum formulations.

A chemoattractant-mediated Gi-coupled pathway activates adenylyl cyclase in human neutrophils

Neutrophils and Dictyostelium use conserved signal transduction pathways to decipher chemoattractant gradients and migrate directionally. In both cell types, addition of chemoattractants stimulates the production of cAMP, which has been suggested to regulate chemotaxis. We set out to define the mechanism by which chemoattractants increase cAMP levels in human neutrophils. We show that chemoattractants elicit a rapid and transient activation of adenylyl cyclase (AC). This activation is sensitive to pertussis toxin treatment but independent of phosphoinositide-3 kinase activity and an intact cytoskeleton. Remarkably, and in sharp contrast to Galpha(s)-mediated activation, chemoattractant-induced AC activation is lost in cell lysates. Of the nine, differentially regulated transmembrane AC isoforms in the human genome, we find that isoforms III, IV, VII, and IX are expressed in human neutrophils. We conclude that the signal transduction cascade used by chemoattractants to activate AC is conserved in Dictyostelium and human neutrophils and is markedly different from the canonical Galpha(s)-meditated pathway.

Fatty acid regulation of adenylyl cyclase Rv2212 from Mycobacterium tuberculosis H37Rv

Adenylyl cyclase Rv2212 from Mycobacterium tuberculosis has a domain composition identical to the pH-sensing isoform Rv1264, an N-terminal regulatory domain and a C-terminal catalytic domain. The maximal velocity of Rv2212 was the highest of all 10 mycobacterial cyclases investigated to date (3.9 micromol cAMP.mg(-1).min(-1)), whereas ATP substrate affinity was low (SC(50) = 2.1 mm ATP). Guanylyl cyclase side activity was absent. The activities and kinetics of the holoenzyme and of the catalytic domain alone were similar, i.e. in distinct contrast to the Rv1264 adenylyl cyclase, in which the N-terminal domain is autoinhibitory. Unsaturated fatty acids strongly stimulated Rv2212 activity by increasing substrate affinity. In addition, fatty acids greatly enhanced the pH sensitivity of the holoenzyme, thus converting Rv2212 to a pH sensor adenylyl cyclase. Fatty acid binding to Rv2212 was modelled by homology to a recent structure of the N-terminal domain of Rv1264, in which a fatty acid-binding pocket is defined. Rv2212 appears to integrate three cellular parameters: ATP concentration, presence of unsaturated fatty acids, and pH. These regulatory properties open the possibility that novel modes of cAMP-mediated signal transduction exist in the pathogen.

Structure of the class IV adenylyl cyclase reveals a novel fold

The crystal structure of the class IV adenylyl cyclase (AC) from Yersinia pestis (Yp) is reported at 1.9 A resolution. The class IV AC fold is distinct from the previously described folds for class II and class III ACs. The dimeric AC-IV folds into an antiparallel eight-stranded barrel whose connectivity has been seen in only three previous structures: yeast RNA triphosphatase and two proteins of unknown function from Pyrococcus furiosus and Vibrio parahaemolyticus. Eight highly conserved ionic residues E10, E12, K14, R63, K76, K111, D126, and E136 lie in the barrel core and form the likely binding sites for substrate and divalent cations. A phosphate ion is observed bound to R63, K76, K111, and R113 near the center of the conserved cluster. Unlike the AC-II and AC-III active sites that utilize two-Asp motifs for cation binding, the AC-IV active site is relatively enriched in glutamate and features an ExE motif as its most conserved element. Homologs of Y. pestis AC-IV, including human thiamine triphosphatase, span the three kingdoms of life and delineate an ancient family of phosphonucleotide processing enzymes.

Molecular details of cAMP generation in mammalian cells: a tale of two systems

The second messenger cAMP has been extensively studied for half a century, but the plethora of regulatory mechanisms controlling cAMP synthesis in mammalian cells is just beginning to be revealed. In mammalian cells, cAMP is produced by two evolutionary related families of adenylyl cyclases, soluble adenylyl cyclases (sAC) and transmembrane adenylyl cyclases (tmAC). These two enzyme families serve distinct physiological functions. They share a conserved overall architecture in their catalytic domains and a common catalytic mechanism, but they differ in their sub-cellular localizations and responses to various regulators. The major regulators of tmACs are heterotrimeric G proteins, which transduce extracellular signals via G protein-coupled receptors. sAC enzymes, in contrast, are regulated by the intracellular signaling molecules bicarbonate and calcium. Here, we discuss and compare the biochemical, structural and regulatory characteristics of the two mammalian AC families. This comparison reveals the mechanisms underlying their different properties but also illustrates many unifying themes for these evolutionary related signaling enzymes.

Mycobacterial adenylyl cyclases: biochemical diversity and structural plasticity

The conversion of adenine and guanine nucleoside triphosphates to cAMP and cGMP is carried out by nucleotide cyclases, which vary in their primary sequence and are therefore grouped into six classes. The class III enzymes encompass all eukaryotic adenylyl and guanylyl cyclase, and several bacterial and archaebacterial cyclases. Mycobacterial nucleotide cyclases show distinct biochemical properties and domain fusions, and we review here biochemical and structural studies on these enzymes from Mycobacterium tuberculosis and related bacteria. We also present an in silico analysis of nucleotide cyclases found in completely sequenced mycobacterial genomes. It is clear that this group of enzymes demonstrates the tinkering in the class III cyclase domain during evolution, involving subtle structural changes that retain the overall catalytic function and fine tune their activities.

Lysophosphatidic acid stimulates cAMP accumulation and cAMP response element-binding protein phosphorylation in immortalized hippocampal progenitor cells

cAMP response element-binding protein (CREB) has been known to play a pivotal role in neuronal differentiation and neuronal plasticity. Lysophosphatidic acid (LPA) was reported to activate CREB in Rat2 fibroblast cells. To study the roles of LPA in neuronal differentiation, we determined whether LPA activates CREB in H19-7, hippocampal progenitor cells. LPA induced three-fold increase in cAMP level in a pertussis toxin-independent manner. Moreover, LPA stimulated CREB phosphorylation, which was inhibited by not only H89 but also Rp-cAMP. In H19-7 cells, high-level expression of lpa1 and moderate-level expression of lpa4 were detected, whereas any detectible expression of lpa2 or lpa3 was not detected by reverse transcription polymerase chain reaction. Together, these data suggested that LPA potentiates cAMP accumulation through activating Gs, and thereby, LPA can stimulate cAMP-CREB signaling cascade.

Stimulation of cyclic AMP production by the Caenorhabditis elegans muscarinic acetylcholine receptor GAR-3 in Chinese hamster ovary cells

Among the three G-protein-linked acetylcholine receptors (GARs) in Caenorhabditis elegans (C. elegans), GAR-3 is structurally and pharmacologically most similar to mammalian muscarinic acetylcholine receptors (mAChRs). Using Chinese hamster ovary (CHO) cells stably expressing GAR-3b, the major alternatively spliced isoform of GAR-3, we observed that carbachol stimulated cyclic AMP (cAMP) production in a dose- and time-dependent manner. The stimulating effect of carbachol was abolished by atropine, a muscarinic antagonist, indicating that the cAMP production is specifically mediated by GAR-3b. When the cells were treated with BAPTA-AM and EGTA, which reduce the cytosolic Ca(2+) level, carbachol-stimulated cAMP accumulation was inhibited by approximately 56%. Inhibition of protein kinase C (PKC) by chronic treatment with phorbol 12-myristate 13-acetate (PMA) or by GF109203X decreased carbachol-stimulated cAMP production by as much as 68%. It thus appears that Ca(2+) and PKC are critically involved in GAR-3b-mediated cAMP formation. We also observed that carbachol-stimulated cAMP production was further enhanced by pertussis toxin (PTX) treatment. This observation indicates that GAR-3b couples to a PTX-sensitive G protein, presumably Gi, to attenuate the cAMP accumulation. Taken together, our data show that GAR-3b stimulates cAMP production in CHO cells and suggest that GAR-3b couples to both stimulatory and inhibitory pathways to modulate the intracellular cAMP level.

Adenylyl cyclase activity and its modulation in the gills of Mytilus galloprovincialis exposed to Cr6+ and Cu2+

The adenylyl cyclase (AC)/cAMP system regulates a large number of physiological functions in bivalve mussels, although its basal properties and the potential effects of environmental pollutants are scarcely studied. We characterized some properties of AC and measured both the enzyme activity and the cAMP levels in the gills of the filter-feeding sea mussel Mytilus galloprovincialis. Basal AC activity was 5.6+/-0.8 pmol cAMP 10 min(-1) mg protein(-1) and showed a Km value of 0.82+/-0.06 mM for ATP in the presence of 5mM Mg2+. It was stimulated up to 2.5- and 3.5-fold by 5-HT and GTPgammaS, respectively. Similarly to what was found in other bivalves, forskolin is a poor activator that reached significant stimulation only at 100 microM. Both basal and 5-HT-stimulated AC activity were significantly increased in the gills of mussels exposed for 7 days in aquaria to Cr6+ (10 ng/l) and Cu2+ (5 microg/l). The cAMP content of the gill under these conditions was also significantly higher than in control animals. In vitro exposure of gill membrane preparations to Cr6+ and Cu2+ induced a bimodal effect. Cu2+ significantly stimulated AC activity at nanomolar concentrations, but a strong inhibition was displayed in the micromolar range. A similar bell-shaped curve was obtained in the presence of Cr6+, with maximal AC stimulation at 10(-8)M and inhibition at 10(-5)M. Overall, these data suggest that the mussel AC/cAMP system can be affected with different patterns by heavy metals. AC activity is strongly affected by acute exposure to heavy metals in vitro, probably through a direct interaction of the pollutants with the enzyme molecule, while AC activity and cAMP content increase in organisms exposed for 7 days in vivo, probably as a defense response to acclimate the physiological functions to the environmental challenge.

Platelet adenylyl cyclase activity: a biological marker for major depression and recent drug use

BACKGROUND

Adenylyl cyclase (AC) is an enzyme that can regulate the physiologic effects of numerous drugs and hormones through the production of cyclic adenosine-3',5'-monophosphate (cAMP). Some studies suggest that certain measures of AC activity are lower among depressed subjects. We examined the relationship between various measures of AC activity and major depression, taking into account potential confounders, such as drug use and gender.

METHODS

We assessed the relationship between platelet levels of AC activity and lifetime diagnosis of major depression among 1481 participants (226 subjects with a history of major depression and 1255 control subjects) in an international, cross-sectional study initiated by the World Health Organization and the International Society on Biomedical Research on Alcoholism.

RESULTS

After accounting for recent drug use, subjects with a history of major depression had markedly lower mean levels for all measures of platelet AC activity compared with control subjects. The adjusted odds ratios for major depression comparing the bottom to the top quartile of AC activity were 2.69 for basal (95% confidence interval [CI] 1.30-5.56), 3.72 for cesium fluoride-stimulated (95% CI 1.54-8.98), 6.20 for forskolin-stimulated (95% CI 2.04-18.80), and 2.20 for Gpp(NH)p-stimulated (95% CI 1.03-4.70).

CONCLUSIONS

Subjects with major depression have lower platelet AC activity levels, and this relationship is dramatically attenuated by various types of drug use.

Alcoholism: the dissection for endophenotypes

Alcohol dependence (alcoholism) is a complex disorder attributed to the interaction of genetic and environmental factors that form a collage of "disease" predisposition, which is not identical for every alcohol-dependent individual. There is considerable evidence to demonstrate that genetic predisposition accounts for roughly half the risk in the development of alcohol dependence. Both family and population studies have identified a number of genomic regions with suggestive links to alcoholism, yet there have been relatively few definitive findings with regard to genetic determinants of alcoholism. This ambiguity can be attributed to a multitude of complications of studying complex mental disorders, such as clinical heterogeneity, polygenic determinants, reduced penetrance, and epistatic effects. Complex mental disorders are clinical manifestations described by combinations of various signs and symptoms. One approach to overcoming the ambiguity in studying the association between genetic risk factors and disease is to dissect the complex, heterogeneous disorder by using intermediate phenotypes--or endophenotypes--to generate more homogeneous diagnostic groupings than an all-encompassing definition, such as the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV)-derived term "alcohol dependence" or the commonly used term "alcoholism." The advantage of using endophenotypes is that the number of influential factors that contribute to these characteristics should be fewer and more easily identified than the number of factors affecting the heterogeneous entity of alcohol dependence (alcoholism). A variety of alcohol-related characteristics have been investigated in epidemiological, clinical, and basic research as potential endophenotypes of alcohol dependence. These include phenotypes related to alcohol metabolism, physiological and endocrine measures, neural imaging, electrophysiology, personality, drinking behavior, and responses to alcohol and alcohol-derived cues. This review summarizes the current literature, focused on human data, of promising endophenotypes for dissecting alcoholism.

PKA-dependent and PKA-independent pathways for cAMP-regulated exocytosis

Stimulus-secretion coupling is an essential process in secretory cells in which regulated exocytosis occurs, including neuronal, neuroendocrine, endocrine, and exocrine cells. While an increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) is the principal signal, other intracellular signals also are important in regulated exocytosis. In particular, the cAMP signaling system is well known to regulate and modulate exocytosis in a variety of secretory cells. Until recently, it was generally thought that the effects of cAMP in regulated exocytosis are mediated by activation of cAMP-dependent protein kinase (PKA), a major cAMP target, followed by phosphorylation of the relevant proteins. Although the involvement of PKA-independent mechanisms has been suggested in cAMP-regulated exocytosis by pharmacological approaches, the molecular mechanisms are unknown. Newly discovered cAMP-GEF/Epac, which belongs to the cAMP-binding protein family, exhibits guanine nucleotide exchange factor activities and exerts diverse effects on cellular functions including hormone/transmitter secretion, cell adhesion, and intracellular Ca(2+) mobilization. cAMP-GEF/Epac mediates the PKA-independent effects on cAMP-regulated exocytosis. Thus cAMP regulates and modulates exocytosis by coordinating both PKA-dependent and PKA-independent mechanisms. Localization of cAMP within intracellular compartments (cAMP compartmentation or compartmentalization) may be a key mechanism underlying the distinct effects of cAMP in different domains of the cell.

An adenylyl cyclase pseudogene in Mycobacterium tuberculosis has a functional ortholog in Mycobacterium avium

A number of genes similar to mammalian Class III nucleotide cyclases are found in mycobacteria, and biochemical characterization of some of these proteins has indicated that they code for adenylyl cyclases, with properties similar to the mammalian enzymes. Our earlier bioinformatic analysis had predicted that the Rv1120c gene in Mycobacterium tuberculosis is a pseudogene, while analysis of the genome of Mycobacterium avium indicated the presence of a functional ortholog. We therefore cloned and expressed Rv1120c and its ortholog from M. avium, Ma1120, in Escherichia coli, and find that while the protein from M. tuberculosis is misfolded and found in inclusion bodies, Ma1120 is expressed to high levels as a functional adenylyl cyclase. Sequence analysis of Ma1120 indicates interesting variations in critical amino acids that are known to be important for catalytic activity. Ma1120 is maximally active in the presence of MnATP as substrate ((app)Km approximately 400 microM), and is inhibited by P-site inhibitors (IC50 of 2',5'-dideoxy-3'-adenosine triphosphate approximately 730 nM) and tyrphostins (IC50 approximately 36 microM) in a manner similar to the mammalian enzymes. This therefore represents the first Class III cyclase biochemically characterized from M. avium, and the absence of a functional ortholog in M. tuberculosis suggests a unique role for this enzyme in M. avium.

Characterization of a D2-like dopamine receptor (AmDOP3) in honey bee, Apis mellifera

Dopamine is an important neurotransmitter in vertebrate and invertebrate nervous systems and is widely distributed in the brain of the honey bee, Apis mellifera. We report here the functional characterization and cellular localization of the putative dopamine receptor gene, Amdop3, a cDNA clone isolated and identified in previous studies as AmBAR3 (Apis mellifera Biogenic Amine Receptor 3). The Amdop3 cDNA encodes a 694 amino acid protein, AmDOP3. Comparison of AmDOP3 to Drosophila melanogaster sequences indicates that it is orthologous to the D2-like dopamine receptor, DD2R. Using AmDOP3 receptors expressed in HEK293 cells we show that of the endogenous biogenic amines, dopamine is the most potent AmDOP3 agonist, and that activation of AmDOP3 receptors results in down regulation of intracellular levels of cAMP, a property characteristic of D2-like dopamine receptors. In situ hybridization reveals that Amdop3 is widely expressed in the brain but shows a pattern of expression that differs from that of either Amdop1 or Amdop2, both of which encode D1-like dopamine receptors. Nonetheless, overlaps in the distribution of cells expressing Amdop1, Amdop2 and Amdop3 mRNAs suggest the likelihood of D1:D2 receptor interactions in some cells, including subpopulations of mushroom body neurons.

A novel mechanism for adenylyl cyclase inhibition from the crystal structure of its complex with catechol estrogen

Catechol estrogens are steroid metabolites that elicit physiological responses through binding to a variety of cellular targets. We show here that catechol estrogens directly inhibit soluble adenylyl cyclases and the abundant trans-membrane adenylyl cyclases. Catechol estrogen inhibition is non-competitive with respect to the substrate ATP, and we solved the crystal structure of a catechol estrogen bound to a soluble adenylyl cyclase from Spirulina platensis in complex with a substrate analog. The catechol estrogen is bound to a newly identified, conserved hydrophobic patch near the active center but distinct from the ATP-binding cleft. Inhibitor binding leads to a chelating interaction between the catechol estrogen hydroxyl groups and the catalytic magnesium ion, distorting the active site and trapping the enzyme substrate complex in a non-productive conformation. This novel inhibition mechanism likely applies to other adenylyl cyclase inhibitors, and the identified ligand-binding site has important implications for the development of specific adenylyl cyclase inhibitors.

Neurogranin/RC3 enhances long-term potentiation and learning by promoting calcium-mediated signaling

In neurons, neurogranin (Ng) binds calmodulin (CaM), and its binding affinity is reduced by increasing Ca2+, phosphorylation by PKC, or oxidation by oxidants. Ng concentration in the hippocampus of adult mice varied broadly (Ng+/+, 160-370 and Ng+/-, approximately 70-230 pmol/mg); the level in Ng+/+ mice is one of the highest among all neuronal CaM-binding proteins. Among Ng+/- mice, but less apparent in Ng+/+, a significant relationship existed between their hippocampal levels of Ng and performances in the Morris water maze. Ng-/- mice performed poorly in this task; they also displayed deficits in high-frequency-induced long-term potentiation (LTP) in area CA1 of hippocampal slices, whereas low-frequency-induced long-term depression was enhanced. Thus, compared with Ng+/+ mice, the frequency-response curve of Ng-/- shifted to the right. Paired-pulse facilitation and synaptic fatigue during prolonged stimulation at 10 Hz (900 pulses) were unchanged in Ng-/- slices, indicating their normal presynaptic function. Measurements of Ca2+ transients in CA1 pyramidal neurons after weak and strong tetanic stimulations (100 Hz, 400 and 1000 msec, respectively) revealed a significantly greater intracellular Ca2+ ([Ca2+]i) response in Ng+/+ compared with Ng-/- mice, but the decay time constants did not differ. The diminished Ca2+ dynamics in Ng-/- mice are a likely cause of their decreased propensity to undergo LTP. Thus, Ng may promote a high [Ca2+]i by a "mass-action" mechanism; namely, the higher the Ng concentration, the more Ng-CaM complexes will be formed, which effectively raises [Ca2+]i at any given Ca2+ influx. This mechanism provides potent signal amplification in enhancing synaptic plasticity as well as learning and memory.

The structure of a pH-sensing mycobacterial adenylyl cyclase holoenzyme

Class III adenylyl cyclases contain catalytic and regulatory domains, yet structural insight into their interactions is missing. We show that the mycobacterial adenylyl cyclase Rv1264 is rendered a pH sensor by its N-terminal domain. In the structure of the inhibited state, catalytic and regulatory domains share a large interface involving catalytic residues. In the structure of the active state, the two catalytic domains rotate by 55 degrees to form two catalytic sites at their interface. Two alpha helices serve as molecular switches. Mutagenesis is consistent with a regulatory role of the structural transition, and we suggest that the transition is regulated by pH.

A Myxococcus xanthus CbpB containing two cAMP-binding domains is involved in temperature and osmotic tolerances

Our previous data indicated that a Myxococcus xanthus sensor-type adenylyl cyclase (CyaA) functions in signal transduction during osmotic stress. However, the cAMP-mediated signal transduction pathway in this bacterium was unknown. Here, we isolated a clone from a M. xanthus genomic DNA library using oligonucleotide probes designed based on the conserved cAMP-binding domains of the cAMP-dependent protein kinase (PKA) regulatory subunits. The clone contained two open-reading frames (ORFs), cbpA and cbpB, encoding hydrophilic proteins with one and two cAMP-binding domains, respectively. The CbpB exhibited partial primary structural similarity to PKA regulatory subunits. cbpA and cbpB mutants, generated by gene disruption, showed normal growth, development and spore germination. However, the cbpB mutant cultured under high- or low-temperature conditions exhibited a marked reduction in growth. cbpB mutant cells were also more sensitive to osmotic stress than wild-type cells. The cbpA mutant possessed normal resistance to such stress. The phenotype of cbpB mutant was similar to those of PKA regulatory subunit mutants of some eukaryotic microorganisms.

Characterization of phylogenetically distant members of the adenylate cyclase family from mycobacteria: Rv1647 from Mycobacterium tuberculosis and its orthologue ML1399 from M. leprae

Analysis of the genome sequence of Mycobacterium tuberculosis H37Rv has identified 16 genes that are similar to the mammalian adenylate and guanylate cyclases. Rv1647 was predicted to be an active adenylate cyclase but its position in a phylogenetically distant branch from the other enzymes characterized so far from M. tuberculosis makes it an interestingly divergent nucleotide cyclase to study. In agreement with its divergence at the sequence level from other nucleotide cyclases, the cloning, expression and purification of Rv1647 revealed differences in its biochemical properties from the previously characterized Rv1625c adenylate cyclase. Adenylate cyclase activity of Rv1647 was activated by detergents but was resistant to high concentrations of salt. Mutations of substrate-specifying residues to those present in guanylate cyclases failed to convert the enzyme into a guanylate cyclase, and did not alter its oligomeric status. Orthologues of Rv1647 could be found in M. leprae, M. avium and M. smegmatis. The orthologue from M. leprae (ML1399) was cloned, and the protein was expressed, purified and shown biochemically to be an adenylate cyclase, thus representing the first adenylate cyclase to be described from M. leprae. Importantly, Western-blot analysis of subcellular fractions from M. tuberculosis and M. leprae revealed that the Rv1647 and ML1399 gene products respectively were expressed in these bacteria. Additionally, M. tuberculosis was also found to express the Rv1625c adenylate cyclase, suggesting that multiple adenylate cyclase proteins may be expressed simultaneously in this organism. These results suggest that class III cyclase-like gene products probably have an important role to play in the physiology and perhaps the pathology of these medically important bacteria.

Nitric oxide-dependent allosteric inhibitory role of a second nucleotide binding site in soluble guanylyl cyclase

The mechanism of desensitization of the nitric oxide (NO) receptor (alpha1.beta1 isoform of soluble guanylyl cyclase, sGC) is not known. Models of the structure of alpha1.beta1, based on the x-ray crystal structure of adenylyl cyclase (AC) suggest the existence of a nucleotide-like binding site, in addition to the putative catalytic site. We have previously reported that mutating residues that coordinate Mg(2+)GTP (substrate) binding in alpha1.beta1 into those present in AC fully reverts GC activity to AC activity. The wild-type form of alpha1.beta1 (GC-wt) and the mutant form (AC-mut, alpha1R592Q.beta1E473K,C541D) were purified, and their sensitivities to various nucleotides were assessed. In using the AC-mut as well as other mutants that coordinate purine binding, we were able to distinguish allosteric inhibitory effects of guanine nucleotides from competitively inhibitory effects on catalytic activity. Here we report that several nucleotide analogs drastically alter sGC and AC-mut activity by acting at a second nucleotide site, likely pseudosymmetric to the catalytic site. In particular, Mg(2+)GTP gamma S and Mg(2+)ATP gamma S inhibited cyclase activity through a mixed, non-competitive mechanism that was only observable under NO stimulation and not under basal conditions. The non-competitive pattern of inhibition was not present in mutants carrying the substitution beta1D477A, the pseudosymmetric equivalent to alpha1D529 (located in the substrate-binding site and involved in substrate binding and catalysis), or with the double mutations alpha1E525K,C594D, the pseudosymmetric equivalent to beta1E473K,C541D. Taken together these data suggest that occupation of the second site by nucleotides may underlie part of the mechanism of desensitization of sGC.

Isoform-specific regulation of adenylyl cyclase function by disruption of membrane trafficking

Oligomerization plays an important role in endoplasmic reticulum processing and membrane insertion (and ultimately in regulation of function) of a number of transmembrane spanning proteins. Furthermore, it is known that adenylyl cyclases (ACs), critical regulators of cellular functions, associate into higher order (dimeric) forms. However, the importance of these higher order aggregates in regulating adenylyl cyclase activity or trafficking to the cell membrane is unclear. Therefore, we examined the potential role of oligomerization in the membrane trafficking of adenylyl cyclase. For this purpose, the ability of full-length adenylyl cyclase and various truncation mutants to self-assemble and to be targeted to the cell membrane was assessed. A truncation mutant comprised of the initial six transmembrane spanning domains and half of the C1 catalytic domain coimmunoprecipitated with full-length AC VI. Using both biotinylation assays and assessment of enzyme distribution using sucrose density gradients, we demonstrate that expression of this mutant in human embryonic kidney 293 cells impaired the ability of AC VI to traffic to the plasma membrane. Furthermore, mutant expression resulted in a significant reduction in adenylyl cyclase activity. The decrease in AC VI membrane expression was not caused by alterations in enzyme transcription. The effect of the mutant was specific for the AC V and VI isoforms and expression of the transmembrane M1 domain but not the C1a domain was required for the mutant to affect adenylyl cyclase activity. In aggregate, these data suggest that alterations in the ability of adenylyl cyclases to form higher order forms regulate both enzyme trafficking and enzyme activity.

A Gbetagamma stimulated adenylyl cyclase is involved in Xenopus laevis oocyte maturation

Xenopus laevis oocyte maturation is induced by the steroid hormone progesterone through a nongenomic mechanism that implicates the inhibition of the effector system adenylyl cyclase (AC). Recently, it has been shown that the G protein betagamma heterodimer is involved in oocyte maturation arrest. Since AC is the proposed target for Gbetagamma action, we considered of importance to identify and characterize the Gbetagamma regulated AC isoform(s) that are expressed in the Xenopus oocyte. Through biochemical studies, we found that stage VI plasma membrane oocyte AC activity showed attributes of an AC2 isoform. Furthermore, exogenous Gbetagamma was capable to activate oocyte AC only in the presence of the activated form of Galphas (Galphas-GTPgammaS), which is in agreement with the Ggammabeta conditional activation reported for the mammalian AC2 and AC4 isotypes. In order to study the functional role of AC in oocyte maturation we cloned from a Xenopus oocyte cDNA library a gene encoding an AC with high identity to AC7 (xAC7). Based on this sequence, we constructed a minigene encoding the AC-Gbetagamma interacting region (xAC7pep) to block, within the oocyte, this interaction. We found that microinjection of the xAC7pep potentiated progesterone-induced maturation, as did the AC2 minigene. From these results we can conclude that a Gbetagamma-activated AC is playing an important role in Xenopus oocyte meiotic arrest in a Galphas-GTP dependent manner.

Mitochondrial Ca 2+ -Activated K + Channels in Cardiac Myocytes

BACKGROUND

The large-conductance Ca2+-activated K+ (BK(Ca)) channel in the cardiac inner mitochondrial membrane (mitoK(Ca) channel) has been shown to protect the heart against ischemic injury. However, questions about the cardioprotective mechanism and the kinase-mediated regulation of mitoK(Ca) channels remain to be answered.

METHODS AND RESULTS

Flavoprotein fluorescence in guinea pig ventricular myocytes was measured to assay mitoK(Ca) channel activity. The mitochondrial Ca2+ concentration ([Ca2+]m) and membrane potential (DeltaPsi(m)) were measured by loading cells with rhod-2 and JC-1, respectively. Cell death was assessed by trypan blue permeability. The BK(Ca) channel opener NS1619 reversibly increased the flavoprotein oxidation in a concentration-dependent manner. NS1619 (30 micromol/L) attenuated the ouabain (1 mmol/L)-induced elevation of [Ca2+]m with accompanying depolarization of DeltaPsi(m). These effects of NS1619 were completely antagonized by the BK(Ca) channel blocker paxilline (2 micromol/L) but not by the mitochondrial ATP-sensitive K+ (mitoK(ATP)) channel blocker 5-hydroxydecanoate (500 micromol/L). Paxilline, however, failed to block the oxidative effect of diazoxide (100 micromol/L), a mitoK(ATP) channel opener. The combined application of submaximally effective concentrations of NS1619 (10 micromol/L) and diazoxide (30 micromol/L) produced additive effects. NS1619 (30 micromol/L) blunted the rate of cell death during exposure to ouabain; this cardioprotective effect was prevented by paxilline. Activation of cAMP-dependent protein kinase by 8-bromoadenosine 3'5'-cyclic monophosphate (0.5 mmol/L) and forskolin (10 micromol/L) potentiated the NS1619-induced flavoprotein oxidation.

CONCLUSIONS

Opening of mitoK(Ca) channels, which is modulated by cAMP-dependent protein kinase, depolarizes the DeltaPsi(m) and attenuates the mitochondrial Ca2+ overload. Our study further indicates that mitoK(Ca) channel activation confers cardioprotection in a manner similar to but independent of mitoK(ATP) channel activation.

Structural basis for the inhibition of mammalian membrane adenylyl cyclase by 2 '(3')-O-(N-Methylanthraniloyl)-guanosine 5 '-triphosphate

Membrane-bound mammalian adenylyl cyclase (mAC) catalyzes the synthesis of intracellular cyclic AMP from ATP and is activated by stimulatory G protein alpha subunits (Galpha(s)) and by forskolin (FSK). mACs are inhibited with high potency by 2 '(3')-O-(N-methylanthraniloyl) (MANT)-substituted nucleotides. In this study, the crystal structures of the complex between Galpha(s).GTPgammaS and the catalytic C1 and C2 domains from type V and type II mAC (VC1.IIC2), bound to FSK and either MANT-GTP.Mg(2+) or MANT-GTP.Mn(2+) have been determined. MANT-GTP coordinates two metal ions and occupies the same position in the catalytic site as P-site inhibitors and substrate analogs. However, the orientation of the guanine ring is reversed relative to that of the adenine ring. The MANT fluorophore resides in a hydrophobic pocket at the interface between the VC1 and IIC2 domains and prevents mAC from undergoing the "open" to "closed" domain rearrangement. The K(i) of MANT-GTP for inhibition of VC1.IIC2 is lower in the presence of mAC activators and lower in the presence of Mn(2+) compared with Mg(2+), indicating that the inhibitor binds more tightly to the catalytically most active form of the enzyme. Fluorescence resonance energy transfer-stimulated emission from the MANT fluorophore upon excitation of Trp-1020 in the MANT-binding pocket of IIC2 is also stronger in the presence of FSK. Mutational analysis of two non-conserved amino acids in the MANT-binding pocket suggests that residues outside of the binding site influence isoform selectivity toward MANT-GTP.

Receptor-mediated reversible translocation of the G protein betagamma complex from the plasma membrane to the Golgi complex

Heterotrimeric G proteins have been thought to function on the plasma membrane after activation by transmembrane receptors. Here we show that, after activation by receptors, the G protein betagamma complex selectively translocates to the Golgi. Receptor inactivation results in Gbetagamma translocating back to the plasma membrane. Both translocation processes occur rapidly within seconds. The efficiency of translocation is influenced by the type of gamma subunit present in the G protein. Distinctly different receptor types are capable of inducing the translocation. Receptor-mediated translocation of Gbetagamma can spatially segregate G protein signaling activity.

Glycosylation of the receptor guanylate cyclase C: role in ligand binding and catalytic activity

GC-C (guanylate cyclase C) is the receptor for heat-stable enterotoxins, guanylin and uroguanylin peptides. Ligand binding to the extracellular domain of GC-C activates the guanylate cyclase domain leading to accumulation of cGMP. GC-C is expressed as differentially glycosylated forms in HEK-293 cells (human embryonic kidney-293 cells). In the present study, we show that the 145 kDa form of GC-C contains sialic acid and galactose residues and is present on the PM (plasma membrane) of cells, whereas the 130 kDa form is a high mannose form that is resident in the endoplasmic reticulum and serves as the precursor for the PM-associated form. Ligand-binding affinities of the differentially glycosylated forms are similar, indicating that glycosylation of GC-C does not play a role in direct ligand interaction. However, ligand-stimulated guanylate cyclase activity was observed only for the fully mature form of the receptor present on the PM, suggesting that glycosylation had a role to play in imparting a conformation to the receptor that allows ligand stimulation. Treatment of cells at 20 degrees C led to intracellular accumulation of a mature glycosylated form of GC-C that now showed ligand-stimulated guanylate cyclase activity, indicating that localization of GC-C was not critical for its catalytic activity. To determine if complex glycosylation was required for ligand-stimulated activation of GC-C, the receptor was expressed in HEK-293 cells that were deficient in N -acetylglucosaminyltransferase 1. This minimally glycosylated form of the receptor was expressed on the cell surface and could bind a ligand with an affinity comparable with the 145 kDa form of the receptor. However, this form of the receptor was poorly activated by the ligand. Therefore our studies indicate a novel role for glycosidic modification of GC-C during its biosynthesis, in imparting subtle conformational changes in the receptor that allow for ligand-mediated activation and perhaps regulation of basal activity.

Cyclic nucleotides

The natural occurrence of cyclic nucleotides in higher plants, formerly a topic of fierce debate, is now established, as is the presence of nucleotidyl cyclases and cyclic nucleotide phosphodiesterases capable of their synthesis and breakdown. Here we describe the significant properties of cyclic nucleotides, also outlining their second messenger functions and the history of plant cyclic nucleotide research over its first three decades. Findings of the last five years are detailed within the context of the functional role of cyclic nucleotides in higher plants, with particular emphasis upon nucleotidyl cyclases and cyclic nucleotide-responsive protein kinases, -binding proteins and -gated ion channels, with future objectives and strategies discussed.

The class III adenylyl cyclases: multi-purpose signalling modules

cAMP serves as a second messenger in virtually all organisms. The most wide-spread class of cAMP-generating enzymes are the class III adenylyl cyclases. Most class III adenylyl cyclases are multi-domain proteins. The catalytic domains exclusively work as dimers, catalysis proceeds at the dimer interface, so that both monomers provide catalytic residues to each catalytic center. Inspection of amino acid sequence profiles suggests a division of the class III adenylyl cyclases in to four subclasses, class IIIa-IIId. Genome projects and postgenomic analysis have provided novel aspects in terms of catalysis and regulation. Alterations in the canonical catalytic residues occur in all four subclasses suggesting a plasticity of the catalytic mechanisms. The vast variety of additional, probably regulatory modules found in class III adenylyl cyclases obviously reflects a large collection of regulatory inputs the catalytic domains have adapted to. The large versatility of class III adenylyl cyclase catalytic domains remains a major scientific challenge.

Multisite phosphorylation mechanism for protein kinase A activation of the smooth muscle ATP-sensitive K+ channel

The activation of ATP-sensitive K+ channels by protein kinase A in vascular smooth muscle is an important component of the action of vasodilators. In this study, we examine the molecular mechanisms of regulation of the cloned equivalent of this channel comprising the sulfonylurea receptor 2B and the inward rectifier 6.1 subunit (SUR2B/Kir6.1). Specifically, we focus on whether the channel is directly phosphorylated and the sites at which this occurs in the protein complex. We identify one site in Kir6.1 (S385) and two sites in SUR2B (T633 and S1465) using a combination of biochemical and functional assays. Our work supports a model in which multiple sites in the channel complex have to be phosphorylated before activation occurs.

Differential inhibition of adenylyl cyclase isoforms and soluble guanylyl cyclase by purine and pyrimidine nucleotides

Mammals express nine membranous adenylyl cyclase isoforms (ACs 1-9), a structurally related soluble guanylyl cyclase (sGC) and a soluble AC (sAC). Moreover, Bacillus anthracis and Bacillus pertussis produce the AC toxins, edema factor (EF), and adenylyl cyclase toxin (ACT), respectively. 2'(3')-O-(N-methylanthraniloyl)-guanosine 5'-[gamma-thio]triphosphate is a potent competitive inhibitor of AC in S49 lymphoma cell membranes. These data prompted us to study systematically the effects of 24 nucleotides on AC in S49 and Sf9 insect cell membranes, ACs 1, 2, 5, and 6, expressed in Sf9 membranes and purified catalytic subunits of membranous ACs (C1 of AC5 and C2 of AC2), sAC, sGC, EF, and ACT in the presence of MnCl(2). N-Methylanthraniloyl (MANT)-GTP inhibited C1.C2 with a K(i) of 4.2 nm. Phe-889 and Ile-940 of C2 mediate hydrophobic interactions with the MANT group. MANT-inosine 5'-[gamma-thio]triphosphate potently inhibited C1.C2 and ACs 1, 5, and 6 but exhibited only low affinity for sGC, EF, ACT, and G-proteins. Inosine 5'-[gamma-thio]triphosphate and uridine 5'-[gamma-thio]triphosphate were mixed G-protein activators and AC inhibitors. AC5 was up to 15-fold more sensitive to inhibitors than AC2. EF and ACT exhibited unique inhibitor profiles. At sAC, 2',5'-dideoxyadenosine 3'-triphosphate was the most potent compound (IC(50), 690 nm). Several MANT-adenine and MANT-guanine nucleotides inhibited sGC with K(i) values in the 200-400 nm range. UTP and ATP exhibited similar affinities for sGC as GTP and were mixed sGC substrates and inhibitors. The exchange of MnCl(2) against MgCl(2) reduced inhibitor potencies at ACs and sGC 1.5-250-fold, depending on the nucleotide and cyclase studied. The omission of the NTP-regenerating system from cyclase reactions strongly reduced the potencies of MANT-ADP, indicative for phosphorylation to MANT-ATP by pyruvate kinase. Collectively, AC isoforms and sGC are differentially inhibited by purine and pyrimidine nucleotides.

Selective inhibition of anthrax edema factor by adefovir, a drug for chronic hepatitis B virus infection

Edema factor (EF), a key virulence factor in anthrax pathogenesis, has calmodulin (CaM)-activated adenylyl cyclase activity. We have found that adefovir dipivoxil, a drug approved to treat chronic infection of hepatitis B virus, effectively inhibits EF-induced cAMP accumulation and changes in cytokine production in mouse primary macrophages. Adefovir diphosphate (PMEApp), the active cellular metabolite of adefovir dipivoxil, inhibits the adenylyl cyclase activity of EF in vitro with high affinity (K(i) = 27 nM). A crystal structure of EF-CaM-PMEApp reveals that the catalytic site of EF forms better van der Waals contacts and more hydrogen bonds with PMEApp than with its endogenous substrate, ATP, providing an explanation for the approximately 10,000-fold higher affinity EF-CaM has for PMEApp versus ATP. Adefovir dipivoxil is a clinically approved drug that can block the action of an anthrax toxin. It can be used to address the role of EF in anthrax pathogenesis.

Class III nucleotide cyclases in bacteria and archaebacteria: lineage-specific expansion of adenylyl cyclases and a dearth of guanylyl cyclases

The Class III nucleotide cyclases are found in bacteria, eukaryotes and archaebacteria. Our survey of the bacterial and archaebacterial genome and plasmid sequences identified 193 Class III cyclase genes in only 29 species, of which we predict the majority to be adenylyl cyclases. Interestingly, several putative cyclase genes were found to have non-conserved substrate specifying residues. Ancestors of the eukaryotic C1-C2 domain containing soluble adenylyl cyclases as well as the protist guanylyl cyclases were found in bacteria. Diverse domains were fused to the cyclase domain and phylogenetic analysis indicated that most proteins within a single cluster have similar domain compositions, emphasising the ancient evolutionary origin and versatility of the cyclase domain.

Forskolin-resistant Y1 adrenal cell mutants are deficient in adenylyl cyclase type 4

Four mutant clones independently derived from the Y1 mouse adrenocortical tumor cell line have adenylyl cyclase (AC) activities that are resistant to forskolin, a direct activator of AC. In this study the AC isoform composition of the forskolin-resistant mutants was examined in order to explore the underlying basis for the resistance to forskolin. As determined by Western blot and RT-PCR analysis, the four forskolin-resistant mutants all were deficient in AC-4; the levels of other AC isoforms (AC-1, AC-3 and AC-5/6) were comparable to the levels in parent Y1 cells. Transfection of one of the mutant clones with an AC-4 expression vector increased forskolin-stimulated cAMP signaling, and restored forskolin-induced changes in cell morphology and growth. Taken together, these observations indicate that AC-4 deficiency is a hallmark of the forskolin-resistant phenotype of these mutants and suggest that AC-4 is an important target of forskolin action in the Y1 adrenal cell line.

In vitro effects of lead ions on peripheral benzodiazepine receptors and adenylyl cyclase activity in the mantle of Mytilus galloprovincialis

As an extension of our previous work, where the density of peripheral benzodiazepine receptors (PBR) increased in mantle mitochondria of the marine mollusk Mytilus galloprovincialis Lmk. under chronic exposure to lead, the present study investigates the in vitro effects of an exogenous source of lead ions on PBR and on adenylyl cyclase (AC) complex in mantle membranes of mussels collected from a non-polluted coastal area. PBR binding experiments used the specific isoquinoline carboxamide derivative [3H]PK 11195, and AC activity was measured using a modified procedure adapted to M. galloprovincialis. Lead ions (Pb2+) dose-dependently decreased either the [3H]PK 11195 specific binding in mitochondria or basal AC velocity in plasma membranes of mussel mantle. The IC50 values for lead ions were 10 microM with [3H]PK 11195 binding and 25 microM with AC activity, with maximal inhibition values of 60% and 70%, respectively. Moreover, lead behaved as a non-competitive inhibitor on [3H]PK 11195 binding and as a 'mixed' inhibitor on AC activity. The present results suggest that some of the early effects induced by lead in mussel cell metabolism consist in significant changes of the PBR density and cyclic AMP production in the mantle of M. galloprovincialis.

MANT-substituted guanine nucleotides: a novel class of potent adenylyl cyclase inhibitors

Mammals express nine membranous adenylyl cyclase (AC) isoforms (AC1-AC9), but the precise functions of AC isoforms are still incompletely understood. This situation is at least partially due to the paucity of potent and isoenzyme-specific AC inhibitors. The original aim of our research was to develop a fluorescence assay for the stimulatory G-protein of AC, G(s). 2'(3')-O-(N-methylanthraniloyl)-(MANT)-substituted nucleotides are fluorescent and were previously used for the fluorescence analysis of purified G(i)/G(o)-proteins. We studied the effects of MANT-guanosine 5'-[gamma-thio]triphosphate (MANT-GTPgammaS) and MANT-guanosine 5'-[beta,gamma-imido]triphosphate (MANT-GppNHp) on Galpha(s)- and Galpha(i)-mediated signaling. MANT-GTPgammaS and MANT-GppNHp had lower affinities for Galpha(s) and Galpha(i) than GTPgammaS and GppNHp. In contrast to guanosine 5'-[beta-thio]diphosphate, MANT-GTPgammaS noncompetitively inhibited GTPgammaS-stimulated AC in Galpha(s)-expressing Sf9 insect cell membranes. AC inhibition by MANT-GTPgammaS and MANT-GppNHp was not due to Galpha(s) inhibition since it was also observed in Galpha(s)-deficient S49 cyc(-) lymphoma cell membranes. Mn(2+) blocked Galpha(i)-mediated AC inhibition by GTPgammaS and GppNHp in S49 cyc(-) membranes but not AC inhibition by MANT-GTPgammaS and MANT-GppNHp. MANT-GTPgammaS and MANT-GppNHp competitively inhibited forskolin/Mn(2+)-stimulated AC in S49 cyc(-) membranes with K(i) values of 53 nM and 160 nM, respectively. Taken together, MANT-substituted guanine nucleotides constitute a novel class of potent competitive AC inhibitors. The availability of potent fluorescent AC inhibitors will help us study the kinetics of AC/nucleotide interactions as well as function, trafficking and localization of AC isoenzymes in intact cells. In future studies, we will examine the specificity of MANT-nucleotides for AC isoenzymes.

Regulation and organization of adenylyl cyclases and cAMP

Adenylyl cyclases are a critically important family of multiply regulated signalling molecules. Their susceptibility to many modes of regulation allows them to integrate the activities of a variety of signalling pathways. However, this property brings with it the problem of imparting specificity and discrimination. Recent studies are revealing the range of strategies utilized by the cyclases to solve this problem. Microdomains are a consequence of these solutions, in which cAMP dynamics may differ from the broad cytosol. Currently evolving methodologies are beginning to reveal cAMP fluctuations in these various compartments.

MD simulations of anthrax edema factor: calmodulin complexes with mutations in the edema factor "switch a" region and docking of 3'-deoxy ATP into the adenylyl cyclase active site of wild-type and mutant edema factor variants

Bacillus anthracis, a spore-forming infectious bacterium, produces an exotoxin, called the edema factor (EF), that functions in part by disrupting internal signalling pathways. When complexed with human host cell calmodulin (CaM), EF becomes an active adenylyl cyclase, producing the internal signal substance cyclic-AMP in an uncontrolled fashion. Recently, the crystal structures for uncomplexed EF and EF:CaM complexes in the presence and absence of a substrate analog (3'-deoxy-ATP), were reported. EF mutational studies have implicated a number of residues important in CaM binding and/or in the generation of the adenylyl cyclase active site, formed by the movements of the EF switch A, B and C regions upon CaM binding. Here we report on the results of molecular dynamics (MD) simulations on two EF:CaM complexes, one containing wild-type EF and the other containing EF in which a cluster of residues in the switch A region (L523, K525, Q526 and V529) have been mutated to alanine. The switch A mutations cause a large increase in the flexibility of the switch C region, the rupture of a number of EF-CaM interactions, an expansion of the carboxyl-terminal domain of CaM, and a change in the Ca(2+) ion binding abilities of the CaM that is in complex with EF. The results indicate the importance of the mutated switch A residues in maintaining a compact EF:CaM complex that appears to be a prerequisite for the generation of a fully-functional adenylyl cyclase active site. The effects of mutating key residues (K346, K353, H577, E588, D590 and N639) in the active site region of EF (to alanine) on the ability of EF to bind the 3'-deoxy-ATP substrate analog were also examined. Active-site residue substitutions at positions 583 (N583A) and 577 (H577A) were found to be particularly disruptive for the placement of the adenine ring moiety into the position found in the x-ray crystal structure of the ligand-protein complex.

Participation of NMDA-mediated phosphorylation and oxidation of neurogranin in the regulation of Ca2+- and Ca2+/calmodulin-dependent neuronal signaling in the hippocampus

Neurogranin/RC3 (Ng) is a postsynaptic protein kinase C (PKC) substrate and calmodulin (CaM)-binding protein whose CaM-binding affinity is modulated by Ca2+, phosphorylation and oxidation. Ng has been implicated in the modulation of postsynaptic signal transduction pathways and synaptic plasticity. Previously, we showed a severe deficit of spatial memory in Ng knockout (KO) mice. Activation of the NMDA receptor and its downstream signaling molecules are known to be involved in long-term memory formation. In the present study, using mouse hippocampal slices, we demonstrated that NMDA induced a rapid and transient phosphorylation and oxidation of Ng. NMDA also caused activation of PKC as evidenced by their phosphorylations, whereas, such activations were greatly reduced in the KO mice. A higher degree of phosphorylation of Ca2+/CaM-dependent kinase II and activation of cyclic AMP-dependent protein kinase were also evident in the WT compared to those of the KO mice. Phosphorylation of downstream targets, including mitogen-activated protein kinases and cAMP response element-binding protein, were significantly attenuated in the KO mice. These results suggest that by its Ca2+-sensitive CaM-binding feature, and through its phosphorylation and oxidation, Ng regulates the Ca2+- and Ca2+/CaM-dependent signaling pathways subsequent to the stimulation of NMDA receptor. These findings support the hypothesis that the derangement of hippocampal signal transduction cascades in Ng KO mice causes the deficits in synaptic plasticity, learning and memory that occur in these mice.

Type 5 adenylyl cyclase disruption alters not only sympathetic but also parasympathetic and calcium-mediated cardiac regulation

In a genetically engineered mouse line with disruption of type 5 adenylyl cyclase (AC5-/-), a major cardiac isoform, there was no compensatory increase in other isoforms of AC in the heart. Both basal and isoproterenol (ISO)-stimulated AC activities were decreased by 30% to 40% in cardiac membranes. The reduced AC activity did not affect cardiac function (left ventricular ejection fraction [LVEF]) at baseline. However, increases in LVEF after ISO were significantly attenuated in AC5-/- (P<0.05, n=11). Paradoxically, conscious AC5-/- mice had a higher heart rate compared with wild-type (WT) mice (613+/-8 versus 523+/-11 bpm, P<0.01, n=14 to 15). Muscarinic agonists decreased AC activity, LVEF, and heart rate more in WT than in AC5-/-. In addition, baroreflex-mediated, ie, neuronally regulated, bradycardia after phenylephrine was also attenuated in AC5-/-. The carbachol-activated outward potassium current (at -40 mV) normalized to cell capacitance in AC5-/- (2.6+/-0.4 pA/pF, n=16) was similar to WT (2.9+/-0.3 pA/pF, n=27), but calcium (Ca2+)-mediated inhibition of AC activity and Ca2+ channel function were diminished in AC5-/-. Thus, AC5-/- attenuates sympathetic responsiveness and also impairs parasympathetic and Ca2+-mediated regulation of the heart, indicating that those actions are not only regulated at the level of the receptor and G-protein but also at the level of type 5 AC.

Tissue transglutaminase directly regulates adenylyl cyclase resulting in enhanced cAMP-response element-binding protein (CREB) activation

Tissue transglutaminase (tTG) is present in the human nervous system and is predominantly localized to neurons. Treatment of human neuroblastoma SH-SY5Y cells with retinoic acid results in increased tTG expression, which is both necessary and sufficient for differentiation. The goal of the present study was to determine whether tTG modulates the activation of the cyclic AMP-response element (CRE)-binding protein, CREB, an event that likely plays a central role in the differentiation of SH-SY5Y cells. SH-SY5Y cells stably transfected with active wild type tTG, tTG without transamidating activity (C277S), an antisense tTG construct that depleted the endogenous levels of tTG, or vector only were used for the study. Treatment with forskolin, an adenylyl cyclase activator, increased that activation-associated phosphorylation of CREB, which was prolonged by tTG overexpression. CRE-reporter gene activity was also significantly elevated in the tTG cells compared with the other cells. The enhancement of CREB phosphorylation/activation in the tTG cells is likely due to the fact that tTG significantly potentiates cAMP production, and our findings indicate that tTG enhances adenylyl cyclase activity by modulating the conformation state of adenylyl cyclase. This is the first study to provide evidence of the mechanism by which tTG may contribute to neuronal differentiation.

Structure-based inhibitor discovery against adenylyl cyclase toxins from pathogenic bacteria that cause anthrax and whooping cough

Edema factor (EF) and CyaA are adenylyl cyclase toxins secreted by pathogenic bacteria that cause anthrax and whooping cough, respectively. Using the structure of the catalytic site of EF, we screened a data base of commercially available, small molecular weight chemicals for those that could specifically inhibit adenylyl cyclase activity of EF. From 24 compounds tested, we have identified one quinazoline compound, ethyl 5-aminopyrazolo[1,5-a]quinazoline-3-carboxylate, that specifically inhibits adenylyl cyclase activity of EF and CyaA with approximately 20 microm Ki. This compound neither affects the activity of host resident adenylyl cyclases type I, II, and V nor exhibits promiscuous inhibition. The compound is a competitive inhibitor, consistent with the prediction that it binds to the adenine portion of the ATP binding site on EF. EF is activated by the host calcium sensor, calmodulin. Surface plasmon resonance spectroscopic analysis shows that this compound does not affect the binding of calmodulin to EF. This compound is dissimilar from a previously described, non-nucleoside inhibitor of host adenylyl cyclase. It may serve as a lead to design antitoxins to address the role of adenylyl cyclase toxins in bacterial pathogenesis and to fight against anthrax and whooping cough.

Differential expression of membrane and soluble adenylyl cyclase isoforms in cytotrophoblast cells and syncytiotrophoblasts of human placenta

Adenylyl cyclase (AC) activity is ubiquitous in mammalian cells, and various forms of this enzyme exist that widely differ with regard to tissue distribution, abundance, and modes of regulation. Human placenta is made, among others, of cytotrophoblast cells and syncytiotrophoblasts. This latter is a polynucleate structure that originates from the differentiation of proliferative mononucleated cytotrophoblast cells, the placental stem cell, into syncytiotrophoblasts. In vitro, this differentiation process is associated with a concomitant increase in cellular levels of cAMP and enhanced expression of genes representative of syncytiotrophoblasts endocrine activity. Thus, in this study we evaluated the differential distribution of AC isoforms in cytotrophoblast cells and syncytiotrophoblasts by reverse transcription-polymerase chain reaction (RT-PCR) using total RNA or purified mRNA. Our results demonstrate that all membrane and soluble AC mRNA isoforms are present in both cell types. Interestingly in syncytiotrophoblasts, AC4 and AC8 mRNA are highly expressed, while AC1, AC2 mRNA are less abundant when compared to cytotrophoblast cells. Additionally, the soluble AC is expressed in both trophoblast cells, but its expression is greatly reduced in differentiated cells, syncytiotrophoblasts. The presence of these AC proteins in both cells was confirmed by Western blotting. Taken together, these data help us to characterize the different AC isoforms in human cytotrophoblast cells and syncytiotrophoblasts, and demonstrate that the AC isoforms expression seems to be mainly modulated in groups 1 and 2. Moreover, the important decrease of the soluble AC isoform in syncytiotrophoblasts as compared to cytotrophoblast cells could suggest an important role of this AC in the extravillous trophoblast formation.

Mutational analysis of the Mycobacterium tuberculosis Rv1625c adenylyl cyclase: residues that confer nucleotide specificity contribute to dimerization

The mycobacterial Rv1625c gene product is an adenylyl cyclase with sequence similarity to the mammalian enzymes. The catalytic domain of the enzyme forms a homodimer and residues specifying adenosine triphosphate (ATP) specificity lie at the dimer interface. Mutation of these residues to those present in guanylyl cyclases failed to convert the enzyme to a guanylyl cyclase, but dramatically reduced its adenylyl cyclase activity and altered its oligomeric state. Computational modeling revealed subtle differences in the dimer interface that could explain the biochemical data, suggesting that the structural and catalytic features of this homodimeric adenylyl cyclase are in contrast to those of the heterodimeric mammalian enzymes.

Isoform-specific regulation of adenylyl cyclase: a potential target in future pharmacotherapy

Adenylyl cyclase (AC) is a target enzyme of multiple G-protein-coupled receptors (GPCRs). In the past decade, the cloning, structure and biochemical properties of nine AC isoforms were reported, and each isoform of AC shows distinct patterns of tissue distribution and biochemical/pharmacological properties. In addition to the conventional regulators of this enzyme, such as calmodulin (CaM) or PKC, novel regulators, for example, caveolin, have been identified. Most importantly, these regulators work on AC in an isoform dependent manner. Recent studies have demonstrated that certain classic AC inhibitors, i.e., P-site inhibitors, show an isoform-dependent inhibition of AC. The side chain modifications of forskolin, a diterpene extract from Coleus forskolii, markedly enhance its isoform selectivity. When taken together, these findings suggest that it is feasible to develop new pharmacotherapeutic agents that target AC isoforms to regulate various neurohormonal signals in a highly tissue-/organ-specific manner.

Regulation of ACTH levels in anterior pituitary cells during stimulated secretion: evidence for aspartyl and cysteine proteases in the cellular metabolism of ACTH

The regulation of cellular levels of adrenocorticotropin hormone (ACTH) in response to stimulated secretion was investigated to define the extent of cellular depletion of ACTH and subsequent increases to replenish ACTH levels in anterior pituitary cells (in primary culture). Treatment of cells with secretagogues for short-term incubation times (hours) resulted in extensive depletion of cellular ACTH. Corticotropin releasing factor (CRF) induced depletion of cellular levels of ACTH by 60-70% of control levels. The CRF-induced reduction of cellular ACTH was inhibited by the glucocorticoid dexamethasone. Phorbol myristate acetate (PMA), which stimulates protein kinase C (PKC), reduced ACTH levels by 50-60%. Forskolin, a stimulator of cAMP production, produced a moderate reduction in cellular ACTH. During prolonged incubation of cells (2 days) with these secretagogues, further reduction of ACTH levels by 70-80% was observed. However, increased cellular levels of ACTH occurred with continued treatment of cells with secretagogues, which provided nearly complete replenishment of cellular ACTH after 5 days treatment with secretagogues. Notably, the rising levels of cellular ACTH were inhibited by the aspartyl protease inhibitor acetyl-pepstatin A, and by the cysteine protease inhibitor E64d. These results demonstrate that depletion and recovery of ACTH levels are coordinately regulated, and that the increases in cellular levels of ACTH during the recovery phase involves participation of aspartyl and cysteine proteases. Thus, aspartyl and cysteine proteases may be involved in the cellular metabolism of ACTH.

2'(3')-O-(N-methylanthraniloyl)-substituted GTP analogs: a novel class of potent competitive adenylyl cyclase inhibitors

2'(3')-O-(N-Methylanthraniloyl)-(MANT)-substituted nucleotides are fluorescent and widely used for the kinetic analysis of enzymes and signaling proteins. We studied the effects of MANT-guanosine 5'-[gamma-thio]triphosphate (MANT-GTP gamma S) and MANT-guanosine 5'-[beta,gamma-imido]triphosphate (MANT-GppNHp) on G alpha(s)- and G alpha(i)-protein-mediated signaling. MANT-GTP gamma S/MANT-GppNHp had lower affinities for G alpha(s) and G alpha(i) than GTP gamma S/GppNHp as assessed by inhibition of GTP hydrolysis of receptor-G alpha fusion proteins. MANT-GTP gamma S was much less effective than GTP gamma S at disrupting the ternary complex between the formyl peptide receptor and G alpha(i2). MANT-GTP gamma S/MANT-GppNHp non-competitively inhibited GTP gamma S/GppNHp-, AlF(4)(-)-, beta(2)-adrenoceptor plus GTP-, cholera toxin plus GTP-, and forskolin-stimulated adenylyl cyclase (AC) in G alpha(s)-expressing Sf9 insect cell membranes and S49 wild-type lymphoma cell membranes. AC inhibition by MANT-GTP gamma S/MANT-GppNHp was not due to G alpha(s) inhibition because it was also observed in G alpha(s)-deficient S49 cyc(-) lymphoma cell membranes. Mn(2+) blocked AC inhibition by GTP gamma S/GppNHp in S49 cyc(-) membranes but enhanced the potency of MANT-GTP gamma S/MANT-GppNHp at inhibiting AC by approximately 4-8-fold. MANT-GTP gamma S and MANT-GppNHp competitively inhibited forskolin/Mn(2+)-stimulated AC in S49 cyc(-) membranes with K(i) values of 53 and 160 nm, respectively. The K(i) value for MANT-GppNHp at insect cell AC was 155 nm. Collectively, MANT-GTP gamma S/MANT-GppNHp bind to G alpha(s)- and G alpha(i)-proteins with low affinity and are ineffective at activating G alpha. Instead, MANT-GTP gamma S/MANT-GppNHp constitute a novel class of potent competitive AC inhibitors.

Small ligands modulating the activity of mammalian adenylyl cyclases: a novel mode of inhibition by calmidazolium

Molecular cloning of membrane-spanning mammalian adenylyl cyclases (ACs) has led to the discovery of nine different isotypes, making ACs potentially useful therapeutic targets. This study investigated the mechanism by which fungicidal nitroimidazole compounds modulate AC activity. Current evidence indicates that biological control of AC activity occurs through the cytosolic domains. Hence, full-length ACII, ACIX, and recombinant fusion proteins composed of the cytoplasmic loops of human ACIX or the first and second cytoplasmic loops of rat ACV and ACII, respectively, were expressed in human embryonic kidney 293 cells. The AC activities of the respective proteins were characterized, and their modulation by nitroimidazoles was investigated. Calmidazolium inhibited the activities of both full-length ACs and soluble fusion proteins (IC(50), approximately 10 microM). Inhibition of ACIX by calmidazolium was mediated by direct interaction with the catalytic core in a noncompetitive fashion. ACIX was essentially insensitive to 2'-deoxyadenosine 3'-monophosphate, a known blocker of AC activity. The ACV-ACII fusion protein was inhibited by calmidazolium (IC(50), approximately 20 microM) as well as by 2'-deoxyadenosine 3'-AMP (IC(50), approximately 2 microM), in a manner indicating independent mechanisms of action. Taken together, the data demonstrate that ACIX is insensitive to adenosine analogs and that calmidazolium inhibits AC activity by a novel, noncompetitive mechanism.

Identification of a novel protein with guanylyl cyclase activity in Arabidopsis thaliana

Guanylyl cyclases (GCs) catalyze the formation of the second messenger guanosine 3',5'-cyclic monophosphate (cGMP) from guanosine 5'-triphosphate (GTP). While many cGMP-mediated processes in plants have been reported, no plant molecule with GC activity has been identified. When the Arabidopsis thaliana genome is queried with GC sequences from cyanobacteria, lower and higher eukaryotes no unassigned proteins with significant similarity are found. However, a motif search of the A. thaliana genome based on conserved and functionally assigned amino acids in the catalytic center of annotated GCs returns one candidate that also contains the adjacent glycine-rich domain typical for GCs. In this molecule, termed AtGC1, the catalytic domain is in the N-terminal part. AtGC1 contains the arginine or lysine that participates in hydrogen bonding with guanine and the cysteine that confers substrate specificity for GTP. When AtGC1 is expressed in Escherichia coli, cell extracts yield >2.5 times more cGMP than control extracts and this increase is not nitric oxide dependent. Furthermore, purified recombinant AtGC1 has Mg(2+)-dependent GC activity in vitro and >3 times less adenylyl cyclase activity when assayed with ATP as substrate in the absence of GTP. Catalytic activity in vitro proves that AtGC1 can function either as a monomer or homo-oligomer. AtGC1 is thus not only the first functional plant GC but also, due to its unusual domain organization, a member of a new class of GCs.