Catalytic mechanism and regulation of mammalian adenylyl cyclases

The impact of signaling pathways on the desmosome ultrastructure in pemphigus

Introduction

The autoantibody-driven disease pemphigus vulgaris (PV) impairs desmosome adhesion in the epidermis. In desmosomes, the pemphigus autoantigens desmoglein 1 (Dsg1) and Dsg3 link adjacent cells. Dsgs are clustered by plaque proteins and linked to the keratin cytoskeleton by desmoplakin (Dp). The aim of this study was to identify the impact of several PV-related signaling pathways on desmosome ultrastructure.

Methods

STED microscopy, Dispase-based dissociation assay.

Results

As observed using STED microscopy, pemphigus autoantibodies (PV-IgG) reduced desmosome number, decreased desmosome size, increased plaque distance and thickness and caused loss of adhesion. Decreased desmosome number, increased plaque distance and thickness and loss of adhesion correlate with features found for newly assembled immature desmosomes, observed after Ca2+ depletion and repletion. This was paralleled by plaque asymmetry, keratin filament retraction and fragmentation of Dsg1 and Dsg3 immunostaining. Inhibition of each individual signaling pathway investigated here prevented the loss of adhesion and ameliorated keratin retraction. In addition, inhibition of p38MAPK or PLC completely rescued all parameters of desmosomes ultrastructure and increased desmosome number under basal conditions. In contrast, inhibition of MEK1/2 was only partially protective for desmosome size and plaque thickness, whereas inhibition of Src or increase of cAMP decreased desmosome size but increased the desmosome number even in the presence of PV-IgG.

Discussion

Alterations of the desmosomal plaque ultrastructure are closely related to loss of adhesion and regulated differently by signaling pathways involved in pemphigus pathogenesis. This insight may allow identification of novel treatment options targeting specific steps of desmosome turn-over in the future.

Moonlighting Crypto-Enzymes and Domains as Ancient and Versatile Signaling Devices

Increasing numbers of reports have revealed novel catalytically active cryptic guanylate cyclases (GCs) and adenylate cyclases (ACs) operating within complex proteins in prokaryotes and eukaryotes. Here we review the structural and functional aspects of some of these cyclases and provide examples that illustrate their roles in the regulation of the intramolecular functions of complex proteins, such as the phytosulfokine receptor (PSKR), and reassess their contribution to signal generation and tuning. Another multidomain protein, Arabidopsis thaliana K+ uptake permease (AtKUP5), also harbors multiple catalytically active sites including an N-terminal AC and C-terminal phosphodiesterase (PDE) with an abscisic acid-binding site. We argue that this architecture may enable the fine-tuning and/or sensing of K+ flux and integrate hormone responses to cAMP homeostasis. We also discuss how searches with motifs based on conserved amino acids in catalytic centers led to the discovery of GCs and ACs and propose how this approach can be applied to discover hitherto masked active sites in bacterial, fungal, and animal proteomes. Finally, we show that motif searches are a promising approach to discover ancient biological functions such as hormone or gas binding.

OR2H2 Activates CAMKKβ-AMPK-Autophagy Signaling Axis and Suppresses Senescence in VK2/E6E7 Cells

Olfactory receptors are expressed in multiple extra-nasal tissues and these ectopic olfactory receptors mediate tissue-specific functions and regulate cellular physiology. Ectopic olfactory receptors may play key roles in tissues constantly exposed to odorants, thus the functionality of these receptors in genital tissues is of particular interest. The functionality of ectopic olfactory receptors expressed in VK2/E6E7 human vaginal epithelial cells was investigated. OR2H2 was the most highly expressed olfactory receptor expressed in VK2/E6E7 cells, and activation of OR2H2 by aldehyde 13-13, a ligand of OR2H2, increased the intracellular calcium and cAMP concentrations. Immunoblotting demonstrated that activation of OR2H2 by aldehyde 13-13 stimulated the CAMKKβ-AMPK-mTORC1-autophagy signaling axis, and that these effects were negated by OR2H2 knockdown. AMPK is known to regulate senescence; consequently, we investigated further the effect of aldehyde 13-13 on senescence. In H2O2-induced senescent cells, activation of OR2H2 by aldehyde 13-13 restored proliferation, and reduced the expression of senescence markers, P16 and P19. Additionally, aldehyde 13-13 induced apoptosis of H2O2-induced senescent cells, compared with non-senescent normal cells. In vivo, aldehyde 13-13 increased the lifespan of Caenorhabditis elegans and budding yeast. These findings demonstrate that OR2H2 is a functional receptor in VK2/E6E7 cells, and that activation of OR2H2 activates the AMPK-autophagy axis, and suppresses cellular aging and senescence, which may increase cellular health.

Characterization of Adenylyl Cyclase Isoform 6 Residues Interacting with Forskolin

BACKGROUND

The adenylyl cyclase (AC) pathway, crucial for pulmonary vasodilation, is inhibited by hypoxia. Forskolin (FSK) binds allosterically to AC, stimulating ATP catalysis. As AC6 is the primary AC isoform in the pulmonary artery, selective reactivation of AC6 could provide targeted reinstatement of hypoxic AC activity. This requires elucidation of the FSK binding site in AC6.

METHODS

HEK293T cells stably overexpressing AC 5, 6, or 7 were incubated in normoxia (21% O₂) or hypoxia (10% O₂) or exposed to s-nitrosocysteine (CSNO). AC activity was measured using terbium norfloxacin assay; AC6 structure built by homology modeling; ligand docking to examine FSK-interacting amino acids; roles of selected residues determined by site-directed mutagenesis; FSK-dependent cAMP generation measured in wild-type and FSK-site mutants by biosensor-based live cell assay.

RESULTS

Only AC6 is inhibited by hypoxia and nitrosylation. Homology modeling and docking revealed residues T500, N503, and S1035 interacting with FSK. Mutation of T500, N503, or S1035 decreased FSK-stimulated AC activity. FSK site mutants were not further inhibited by hypoxia or CSNO; however, mutation of any of these residues prevented AC6 activation by FSK following hypoxia or CSNO treatment.

CONCLUSIONS

FSK-interacting amino acids are not involved in the hypoxic inhibition mechanism. This study provides direction to design FSK derivatives for selective activation of hypoxic AC6.

Human adenylyl cyclase 9 is auto-stimulated by its isoform-specific C-terminal domain

Human transmembrane adenylyl cyclase 9 (AC9) is not regulated by heterotrimeric G proteins. Key to the resistance to stimulation by Gs-coupled receptors (GsRs) is auto-inhibition by the COOH-terminal domain (C2b). The present study investigated the role of the C2b domain in the regulation of cyclic AMP production by AC9 in HEK293FT cells expressing the GloSensor22F cyclic AMP-reporter protein. Surprisingly, we found C2b to be essential for sustaining the basal output of cyclic AMP by AC9. A human mutation (E326D) in the parallel coiled-coil formed by the signalling helices of AC9 dramatically increased basal activity, which was also dependent on the C2b domain. Intriguingly, the same mutation enabled stimulation of AC9 by GsRs. In summary, auto-regulation by the C2b domain of AC9 sustains its basal activity and quenches activation by GsR. Thus, AC9 appears to be tailored to support constitutive activation of cyclic AMP effector systems. A switch from this paradigm to stimulation by GsRs may be occasioned by conformational changes at the coiled-coil or removal of the C2b domain.

Strategies to safely target widely expressed soluble adenylyl cyclase for contraception

In humans, the prototypical second messenger cyclic AMP is produced by 10 adenylyl cyclase isoforms, which are divided into two classes. Nine isoforms are G protein coupled transmembrane adenylyl cyclases (tmACs; ADCY1-9) and the 10th is the bicarbonate regulated soluble adenylyl cyclase (sAC; ADCY10). This review details why sAC is uniquely druggable and outlines ways to target sAC for novel forms of male and female contraception.

Strategies towards Targeting Gαi/s Proteins: Scanning of Protein-Protein Interaction Sites To Overcome Inaccessibility

Heterotrimeric G proteins are classified into four subfamilies and play a key role in signal transduction. They transmit extracellular signals to intracellular effectors subsequent to the activation of G protein-coupled receptors (GPCRs), which are targeted by over 30 % of FDA-approved drugs. However, addressing G proteins as drug targets represents a compelling alternative, for example, when G proteins act independently of the corresponding GPCRs, or in cases of complex multifunctional diseases, when a large number of different GPCRs are involved. In contrast to Gαq, efforts to target Gαi/s by suitable chemical compounds has not been successful so far. Here, a comprehensive analysis was conducted examining the most important interface regions of Gαi/s with its upstream and downstream interaction partners. By assigning the existing compounds and the performed approaches to the respective interfaces, the druggability of the individual interfaces was ranked to provide perspectives for selective targeting of Gαi/s in the future.

Adenylyl Cyclase (AC) Mediates the Antidepressant-Like Effects of Tropisetron on a Mouse Model of Maternal Separation Stress

The adenylyl cyclase (AC) pathway is involved in the pathophysiology of depression. Finding new antidepressants with high medicinal properties and low side effects is warranted. Therefore, this study was designed to determine the antidepressant-like effect of tropisetron on a maternal separation (MS) model in mice, considering the possible role of AC. NMRI male mice were divided into eleven groups. The control group was treated with saline and MS groups were treated with saline, tropisetron (a 5-HT3 receptor antagonist) at doses of 1, 3, and 5 mg/kg; forskolin (an activator of AC) at doses of 5, 10, and 25 mg/kg; a subeffective dose of forskolin with a subeffective dose of tropisetron; and an effective dose of tropisetron plus an effective dose of NB001 (3 mg/kg) (an AC inhibitor). After treatment, animals were subjected to behavioral tests including the forced swimming test (FST), splash test, and open field test (OFT). We showed that MS caused depressive-like behaviors determined as an increase in the immobility time in the forced swimming test (FST) and decreased grooming time in the splash test. Our results showed that administration of tropisetron, as well as forskolin, mitigated the depressive-like behaviors in MS mice. We found that coadministration of a subeffective dose of tropisetron plus a subeffective dose of forskolin potentiated the antidepressant-like effect of tropisetron. However, coadministration of an effective dose of NB001 with an effective dose of tropisetron did not significantly affect the antidepressant-like effect of tropisetron. We concluded that the antidepressant-like effects of tropisetron on MS mice are partially mediated through the adenylyl cyclase pathway.

CRISPR/Cas9 ADCY7 Knockout Stimulates the Insulin Secretion Pathway Leading to Excessive Insulin Secretion

AIM

Despite the enormous efforts to understand Congenital hyperinsulinism (CHI), up to 50% of the patients are genetically unexplained. We aimed to functionally characterize a novel candidate gene in CHI.

PATIENT

A 4-month-old boy presented severe hyperinsulinemic hypoglycemia. A routine CHI genetic panel was negative.

METHODS

A trio-based whole-exome sequencing (WES) was performed. Gene knockout in the RIN-m cell line was established by CRISPR/Cas9. Gene expression was performed using real-time PCR.

RESULTS

Hyperinsulinemic hypoglycemia with diffuse beta-cell involvement was demonstrated in the patient, who was diazoxide-responsive. By WES, compound heterozygous variants were identified in the adenylyl cyclase 7, ADCY7 gene p.(Asp439Glu) and p.(Gly1045Arg). ADCY7 is calcium-sensitive, expressed in beta-cells and converts ATP to cAMP. The variants located in the cytoplasmic domains C1 and C2 in a highly conserved and functional amino acid region. RIN-m^(-/-Adcy7) cells showed a significant increase in insulin secretion reaching 54% at low, and 49% at high glucose concentrations, compared to wild-type. In genetic expression analysis Adcy7 loss of function led to a 34.1-fold to 362.8-fold increase in mRNA levels of the insulin regulator genes Ins1 and Ins2 (p ≤ 0.0002), as well as increased glucose uptake and sensing indicated by higher mRNA levels of Scl2a2 and Gck via upregulation of Pdx1, and Foxa2 leading to the activation of the glucose stimulated-insulin secretion (GSIS) pathway.

CONCLUSION

This study identified a novel candidate gene, ADCY7, to cause CHI via activation of the GSIS pathway.

In vitro and in silico studies of 8(17),12E,14-labdatrien-18-oic acid in airways smooth muscle relaxation: new molecular insights about its mechanism of action

In the field of experimental pharmacology, researchers continuously investigate new relaxant agents of the airway smooth muscle cells (ASMCs), since the pathophysiology of respiratory illnesses, such as asthma, involves hyperresponsiveness and changes in ASMC homeostasis. In this scenario, labdane-type diterpenes, like forskolin (FSK), are a class of compounds known for their relaxing action on smooth muscle cells (SMCs), being this phenomenon related to the direct activation of AC-cAMP-PKA pathway. Considering the continuous effort of our group to study the mechanism of action and prospecting for compounds isolated from natural sources, in this paper, we presented how the diterpene 8(17),12E,14-labdatrien-18-oic acid (LBD) promotes relaxant effect on ASMC, performing in vitro experiments using isolated guinea pig trachea and in silico molecular docking/dynamics simulations. In vitro experiments showed that in the presence of aminophylline, FSK and LBD had their relaxant effect potentiated (EC₅₀ from 1.4 ± 0.2 × 10^-5 M to 1.5 ± 0.3 × 10^-6 M for LBD and from 2.0 ± 0.2 × 10^-7 M to 6.4 ± 0.4 × 10^-8 M for FSK) while in the presence of Rp-cAMPS this effect was attenuated (EC₅₀ from 1.4 ± 0.2 × 10^-5 M to 3 × 10^-4 M for LBD and from 2.0 ± 0.2 × 10^-7 to 3.1 ± 1.0 × 10^-6 M for FSK). Additionally, in silico simulations evidenced that the lipophilic character of LBD is probably responsible for its stability on AC binding site. LBD presented two preferential orientations, where the double bonds of the isoprene moiety as well as the unique polar group (carboxylic acid) in this compound form important anchoring points. In this sense, we consider that the LBD can interact stabilizing the catalytic dimmer of AC as the FSK, although less efficiently.

Refining a steroidogenic model: an analysis of RNA-seq datasets from insect prothoracic glands

BACKGROUND

The prothoracic gland (PG), the principal steroidogenic organ of insects, has been proposed as a model for steroid hormone biosynthesis and regulation.

RESULTS

To validate the robustness of the model, we present an analysis of accumulated transcriptomic data from PGs of two model species, Drosophila melanogaster and Bombyx mori. We identify that the common core components of the model in both species are encoded by nine genes. Five of these are Halloween genes whose expression differs substantially between the PGs of these species.

CONCLUSIONS

We conclude that the PGs can be a model for steroid hormone synthesis and regulation within the context of mitochondrial cholesterol transport and steroid biosynthesis but beyond these core mechanisms, gene expression in insect PGs is too diverse to fit in a context-specific model and should be analysed within a species-specific framework.

Dop1 enhances conspecific olfactory attraction by inhibiting miR-9a maturation in locusts

Dopamine receptor 1 (Dop1) mediates locust attraction behaviors, however, the mechanism by which Dop1 modulates this process remains unknown to date. Here, we identify differentially expressed small RNAs associated with locust olfactory attraction after activating and inhibiting Dop1. Small RNA transcriptome analysis and qPCR validation reveal that Dop1 activation and inhibition downregulates and upregulates microRNA-9a (miR-9a) expression, respectively. miR-9a knockdown in solitarious locusts increases their attraction to gregarious volatiles, whereas miR-9a overexpression in gregarious locusts reduces olfactory attraction. Moreover, miR-9a directly targets adenylyl cyclase 2 (ac2), causing its downregulation at the mRNA and protein levels. ac2 responds to Dop1 and mediates locust olfactory attraction. Mechanistically, Dop1 inhibits miR-9a expression through inducing the dissociation of La protein from pre-miR-9a and resulting in miR-9a maturation inhibition. Our results reveal a Dop1–miR-9a–AC2 circuit that modulates locust olfactory attraction underlying aggregation. This study suggests that miRNAs act as key messengers in the GPCR signaling.

Migratory locusts shift between aggregating together during gregarious phases and living individually during solitary phases. Here, the authors find that the D1-like dopamine receptor regulates the olfactory attraction underlying this behavioral switch via microRNA-9a and adenylyl cyclase.

International Union of Basic and Clinical Pharmacology. CI. Structures and Small Molecule Modulators of Mammalian Adenylyl Cyclases

Adenylyl cyclases (ACs) generate the second messenger cAMP from ATP. Mammalian cells express nine transmembrane AC (mAC) isoforms (AC1-9) and a soluble AC (sAC, also referred to as AC10). This review will largely focus on mACs. mACs are activated by the G-protein Gαs and regulated by multiple mechanisms. mACs are differentially expressed in tissues and regulate numerous and diverse cell functions. mACs localize in distinct membrane compartments and form signaling complexes. sAC is activated by bicarbonate with physiologic roles first described in testis. Crystal structures of the catalytic core of a hybrid mAC and sAC are available. These structures provide detailed insights into the catalytic mechanism and constitute the basis for the development of isoform-selective activators and inhibitors. Although potent competitive and noncompetitive mAC inhibitors are available, it is challenging to obtain compounds with high isoform selectivity due to the conservation of the catalytic core. Accordingly, caution must be exerted with the interpretation of intact-cell studies. The development of isoform-selective activators, the plant diterpene forskolin being the starting compound, has been equally challenging. There is no known endogenous ligand for the forskolin binding site. Recently, development of selective sAC inhibitors was reported. An emerging field is the association of AC gene polymorphisms with human diseases. For example, mutations in the AC5 gene (ADCY5) cause hyperkinetic extrapyramidal motor disorders. Overall, in contrast to the guanylyl cyclase field, our understanding of the (patho)physiology of AC isoforms and the development of clinically useful drugs targeting ACs is still in its infancy.

Serum Amyloid A3 Secreted by Preosteoclasts Inhibits Parathyroid Hormone-stimulated cAMP Signaling in Murine Osteoblasts

Continuous parathyroid hormone (PTH) blocks its own osteogenic actions in marrow stromal cell cultures by inducing Cox2 and receptor activator of nuclear factor κB ligand (RANKL) in the osteoblastic lineage cells, which then cause the hematopoietic lineage cells to secrete an inhibitor of PTH-stimulated osteoblast differentiation. To identify this inhibitor, we used bone marrow macrophages (BMMs) and primary osteoblasts (POBs) from WT and Cox2 knock-out (KO) mice. Conditioned medium (CM) from RANKL-treated WT, but not KO, BMMs blocked PTH-stimulated cAMP production in POBs. Inhibition was reversed by pertussis toxin (PTX), which blocks Gαi/o activation. Saa3 was the most highly differentially expressed gene in a microarray comparison of RANKL-treated WT versus Cox2 KO BMMs, and RANKL induced Saa3 protein secretion only from WT BMMs. CM from RANKL-stimulated BMMs with Saa3 knockdown did not inhibit PTH-stimulated responses in POBs. SAA added to POBs inhibited PTH-stimulated cAMP responses, which was reversed by PTX. Selective agonists and antagonists of formyl peptide receptor 2 (Fpr2) suggested that Fpr2 mediated the inhibitory actions of Saa3 on osteoblasts. In BMMs committed to become osteoclasts by RANKL treatment, Saa3 expression peaked prior to appearance of multinucleated cells. Flow sorting of WT marrow revealed that Saa3 was secreted only from the RANKL-stimulated B220(-) CD3(-)CD11b(-/low) CD115(+) preosteoclast population. We conclude that Saa3 secretion from preosteoclasts, induced by RANKL in a Cox2-dependent manner, inhibits PTH-stimulated cAMP signaling and osteoblast differentiation via Gαi/o signaling. The induction of Saa3 by PTH may explain the suppression of bone formation when PTH is applied continuously and may be a new therapeutic target for osteoporosis.

Functional non-nucleoside adenylyl cyclase inhibitors

In this study, we describe the synthesis of novel functional non-nucleoside adenylyl cyclase inhibitors, which can be easily modified with thiol containing biomolecules such as tumour targeting structures. The linkage between inhibitor and biomolecule contains cleavable bonds to enable efficient intracellular delivery in the reductive milieu of the cytosol as well as in the acidic environment within endosomes and lysosomes. The suitability of this synthetic approach was shown by the successful bioconjugation of a poor cell-permeable inhibitor with a cell-penetrating peptide. Additionally, we have demonstrated the excellent inhibitory effect of the compounds presented here in a live-cell Förster resonance energy transfer-based assay in human embryonic kidney cells.

TD-19, an erlotinib derivative, induces epidermal growth factor receptor wild-type nonsmall-cell lung cancer apoptosis through CIP2A-mediated pathway

Some patients with nonsmall-cell lung cancer (NSCLC) without epidermal growth factor receptor (EGFR) mutations still respond to gefitinib and erlotinib, suggesting that there may be a mechanism(s) other than the EGFR pathway that mediates the tumoricidal effects. In the current study, we tested the efficacy of TD-19, a novel compound chemically modified from erlotinib, which has more potent apoptotic effects than erlotinib in EGFR wild-type NSCLC cell lines. TD-19 induced significant cell death and apoptosis in H358, H441, H460, and A549 cells, as evidenced by increased caspase-3 activity and cleavage of procaspase-9 and poly (ADP-ribose) polymerase. The apoptotic effect of TD-19 in H460 cells, which were resistant to erlotinib, was associated with downregulation of cancerous inhibitor of protein phosphatase 2A (CIP2A), increased protein phosphatase 2A (PP2A) activity, and decreased AKT phosphorylation, but minimal effects on EGFR phosphorylation. Overexpression of CIP2A partially protected the H460 cells from TD-19-induced apoptosis. Okadaic acid, a known PP2A inhibitor, significantly reduced TD-19-induced apoptosis, while forskolin, which increased PP2A activity, increased the apoptotic effect of TD-19. TD-19 inhibited the growth of H460 xenograft tumors by ∼80%. We conclude that TD-19 exerted tumoricidal effects on NSCLC cells. TD-19 provides proof that the CIP2A pathway may be a novel target for the treatment of EGFR wild-type NSCLC.

Distinct roles of soluble and transmembrane adenylyl cyclases in the regulation of flagellar motility in Ciona sperm

Adenylyl cyclase (AC) is a key enzyme that synthesizes cyclic AMP (cAMP) at the onset of the signaling pathway to activate sperm motility. Here, we showed that both transmembrane AC (tmAC) and soluble AC (sAC) are distinctly involved in the regulation of sperm motility in the ascidian Ciona intestinalis. A tmAC inhibitor blocked both cAMP synthesis and the activation of sperm motility induced by the egg factor sperm activating and attracting factor (SAAF), as well as those induced by theophylline, an inhibitor of phoshodiesterase. It also significantly inhibited cAMP-dependent phosphorylation of a set of proteins at motility activation. On the other hand, a sAC inhibitor does not affect on SAAF-induced transient increase of cAMP, motility activation or protein phosphorylation, but it reduced swimming velocity to half in theophylline-induced sperm. A sAC inhibitor KH-7 induced circular swimming trajectory with smaller diameter and significantly suppressed chemotaxis of sperm to SAAF. These results suggest that tmAC is involved in the basic mechanism for motility activation through cAMP-dependent protein phosphorylation, whereas sAC plays distinct roles in increase of flagellar beat frequency and in the Ca2+-dependent chemotactic movement of sperm.

Differential dopamine receptor subtype regulation of adenylyl cyclases in lipid rafts in human embryonic kidney and renal proximal tubule cells

Dopamine D1-like receptors (D1R and D5R) stimulate adenylyl cyclase (AC) activity, whereas the D2-like receptors (D2, D3 and D4) inhibit AC activity. D1R, but not the D5R, has been reported to regulate AC activity in lipid rafts (LRs). We tested the hypothesis that D1R and D5R differentially regulate AC activity in LRs using human embryonic kidney (HEK) 293 cells heterologously expressing human D1 or D5 receptor (HEK-hD1R or HEK-hD5R) and human renal proximal tubule (hRPT) cells that endogenously express D1R and D5R. Of the AC isoforms expressed in HEK and hRPT cells (AC3, AC5, AC6, AC7, and AC9), AC5/6 was distributed to a greater extent in LRs than non-LRs in HEK-hD1R (84.5±2.3% of total), HEK-hD5R (68.9±3.1% of total), and hRPT cells (66.6 ± 2.2% of total) (P<0.05, n=4/group). In HEK-hD1R cells, the D1-like receptor agonist fenoldopam (1 μM/15 min) increased AC5/6 protein (+17.2 ± 3.9% of control) in LRs but decreased it in non-LRs (-47.3±5.3% of control) (P<0.05, vs. control, n=4/group). By contrast, in HEK-hD5R cells, fenoldopam increased AC5/6 protein in non-LRs (+67.1 ± 5.3% of control, P<0.006, vs. control, n=4) but had no effect in LRs. In hRPT cells, fenoldopam increased AC5/6 in LRs but had little effect in non-LRs. Disruption of LRs with methyl-β-cyclodextrin decreased basal AC activity in HEK-D1R (-94.5 ± 2.0% of control) and HEK-D5R cells (-87.1 ± 4.6% of control) but increased it in hRPT cells (6.8±0.5-fold). AC6 activity was stimulated to a greater extent by D1R than D5R, in agreement with the greater colocalization of AC5/6 with D1R than D5R in LRs. We conclude that LRs are essential not only for the proper membrane distribution and maintenance of AC5/6 activity but also for the regulation of D1R- and D5R-mediated AC signaling.

Soluble adenylyl cyclase accounts for high basal cCMP and cUMP concentrations in HEK293 and B103 cells

Intact HEK293 cells and B103 neuroblastoma cells possess high basal concentrations of the established second messengers cAMP and cGMP and of the emerging second messengers cCMP and cUMP. We asked the question which nucleotidyl cyclase accounts for the high basal cNMP concentrations. Activators and inhibitors of soluble guanylyl cyclase had no major effects on cNMPs, and the activator of membranous adenylyl cyclase forskolin increased only cAMP. Addition of bicarbonate to medium increased, whereas removal of bicarbonate decreased levels of all four cNMPs. The inhibitor of soluble adenylyl cyclase, 2-(1H-benzo[d]imidazol-2-ylthio)-N'-(5-bromo-2-hydroxybenzylidene) propanehydrazide (KH7), reduced bicarbonate-stimulated cNMPs. In conclusion, bicarbonate-stimulated soluble adenylyl cyclase plays an important role in the regulation of basal cellular cNMP levels, most notably cCMP and cUMP.

The role of transcriptional regulation in maintaining the availability of mycobacterial adenylate cyclases

Mycobacterium species have a complex cAMP regulatory network indicated by the high number of adenylate cyclases annotated in their genomes. However the need for a high level of redundancy in adenylate cyclase genes remains unknown. We have used semiquantitiative RT-PCR to examine the expression of eight Mycobacterium smegmatis cyclases with orthologs in the human pathogen Mycobacterium tuberculosis, where cAMP has recently been shown to be important for virulence. All eight cyclases were transcribed in all environments tested, and only four demonstrated environmental-mediated changes in transcription. M. smegmatis genes MSMEG_0545 and MSMEG_4279 were upregulated during starvation conditions while MSMEG_0545 and MSMEG_4924 were downregulated in H2O2 and MSMEG_3780 was downregulated in low pH and starvation. Promoter fusion constructs containing M. tuberculosis H37Rv promoters showed consistent regulation compared to their M. smegmatis orthologs. Overall our findings indicate that while low levels of transcriptional regulation occur, regulation at the mRNA level does not play a major role in controlling cellular cyclase availability in a given environment.

The effects of forskolin and rolipram on cAMP, cGMP and free fatty acid levels in diet induced obesity

Obesity is a major health problem. We investigated the effects of forskolin and rolipram in the diet of animals in which obesity had been induced. We used 50 female albino Wistar rats that were assigned randomly into five groups as follows: group 1, control; group 2, high fat diet; group 3, high fat diet + forskolin; group 4, high fat diet + rolipram; and group 5, high fat diet + rolipram + forskolin. The rats were fed for 10 weeks and rolipram and forskolin were administered during last two weeks. The animals were sacrificed and blood samples were obtained. Serum cAMP, cGMP and free fatty acids (FFA) levels were measured using ELISA assays. We also measured weight gain during the 10 week period. cAMP and FFA levels of groups 3, 4 and 5 were significantly higher than those of groups 1 and 2. We found no significant differences in serum cGMP levels among the groups. The weight gain in groups 3, 4 and 5 was significantly less than for group 2. We also found that the weight gain in group 5 was significantly less than in groups 3 and 4. We found that both forskolin and rolipram stimulated lipolysis and inhibited body weight increase by increasing cAMP levels. Also, combination therapy using the two agents may be more effective in preventing diet induced obesity than either agent alone. We found also that these agents did not effect cellular cGMP levels in diet induced obesity.

Localization of angiotensin-II type 1(AT1) receptors on buffalo spermatozoa: AT1 receptor activation during capacitation triggers rise in cyclic AMP and calcium

The purpose of this study was to determine the role of Ang-II in buffalo spermatozoa; localize angiotensin type 1 (AT1) receptors on the sperm surface and understand the signaling mechanisms involved therein. Immunoblotting and immunocytochemistry using polyclonal Rabbit anti-AT1 (N-10) IgG were performed to confirm the presence of AT1 receptors. Intracellular levels of cyclic adenosine monophosphate (cAMP) were determined by non-radioactive enzyme immunoassay, while that of Calcium [Ca(2+)] were estimated by fluorimetry using Fura2AM dye. The results obtained showed that AT1 receptors were found on the post-acrosomal region, neck and tail regions. Immunoblotting revealed a single protein band with molecular weight of 40 kDa. Ang-II treated cells produced significantly higher level of cAMP compared to untreated cells (22.66 ± 2.4 vs. 10.8 ± 0.98 pmol/10(8) cells, p < 0.01). The mean levels of Ca(2+) were also higher in Ang-II treated cells compared to control (117.4 ± 6.1 vs. 61.15 ± 4.2 nmol/10(8) cells; p < 0.01). The stimulatory effect of Ang-II in both the cases was significantly inhibited in the presence of Losartan (AT1 antagonist; p < 0.05) indicating the involvement of AT1 receptors. Further, presence of neomycin (protein kinase C inhibitor) inhibited significantly the Ang-II mediated rise in Ca(2+) indicating the involvement of PKC pathway. These findings confirm the presence of AT1 receptors in buffalo spermatozoa and that Ang-II mediates its actions via the activation of these receptors. Ang-II stimulates the rise in intracellular levels of cAMP and Ca(2+) during capacitation.

Biological roles of cAMP: variations on a theme in the different kingdoms of life

Cyclic AMP (cAMP) plays a key regulatory role in most types of cells; however, the pathways controlled by cAMP may present important differences between organisms and between tissues within a specific organism. Changes in cAMP levels are caused by multiple triggers, most affecting adenylyl cyclases, the enzymes that synthesize cAMP. Adenylyl cyclases form a large and diverse family including soluble forms and others with one or more transmembrane domains. Regulatory mechanisms for the soluble adenylyl cyclases involve either interaction with diverse proteins, as happens in Escherichia coli or yeasts, or with calcium or bicarbonate ions, as occurs in mammalian cells. The transmembrane cyclases can be regulated by a variety of proteins, among which the α subunit and the βγ complex from G proteins coupled to membrane receptors are prominent. cAMP levels also are controlled by the activity of phosphodiesterases, enzymes that hydrolyze cAMP. Phosphodiesterases can be regulated by cAMP, cGMP or calcium-calmodulin or by phosphorylation by different protein kinases. Regulation through cAMP depends on its binding to diverse proteins, its proximal targets, this in turn causing changes in a variety of distal targets. Specifically, binding of cAMP to regulatory subunits of cAMP-dependent protein kinases (PKAs) affects the activity of substrates of PKA, binding to exchange proteins directly activated by cAMP (Epac) regulates small GTPases, binding to transcription factors such as the cAMP receptor protein (CRP) or the virulence factor regulator (Vfr) modifies the rate of transcription of certain genes, while cAMP binding to ion channels modulates their activity directly. Further studies on cAMP signalling will have important implications, not only for advancing fundamental knowledge but also for identifying targets for the development of new therapeutic agents.

A bioassay for the detection of neutralizing antibodies against the α-melanocyte stimulating hormone analog afamelanotide in patients with erythropoietic protoporphyria

The tridecapeptide afamelanotide (Scenesse®) is a congener of α-melanocyte stimulating hormone (α-MSH). Upon binding to the melanocortin 1 receptor (MC1R) on the surface of pigment cells of the skin, the melanocytes, α-MSH or afamelanotide trigger the synthesis of cAMP, which stimulates the synthesis of melanin and therefore induces skin tanning. In a recent trial, afamelanotide administered as controlled release implants protected erythropoietic protoporphyria (EPP) patients from sunlight induced phototoxic skin reactions. Administration of biological therapeutic peptides may elicit unwanted immunogenic responses in recipients of these products. Although in a previous study using ELISA technique we excluded any newly developed immunogenicity during prolonged exposure to afamelanotide, we confirmed the previously published existence of low titers of antibodies against α-MSH in drug-naïve individuals that cross-reacted with afamelanotide. In order to investigate whether such antibodies are neutralizing, i.e. could block the biological effect of afamelanotide, we developed a cell culture-based bioassay. The basis of our assay was the measurement of afamelanotide-induced cAMP formation in a strain of the B16 mouse melanoma cell line, G4F-7, expressing the transfected human MC1R. Average half-effective concentrations of the natural hormone α-MSH and its congener afamelanotide were 38.8 ± 10.6 and 10.9 ± 7.17 nM (n=5), respectively. Neutralizing antibodies would reduce the cAMP formation. Two neutralizing anti-α-MSH antibodies served as positive controls. cAMP formation in the G4F-7 cells after addition of sera of drug-naïve (n=6) and of drug-exposed EPP patients (n=17) was significantly lower than after that from healthy volunteers (n=13). There was no difference between drug-naïve and drug-exposed patients. Using forskolin as a hormone-independent stimulator of cAMP formation, we excluded an unspecific interference of EPP sera with cAMP formation. We conclude that afamelanotide even after prolonged application to EPP patients did not elicit neutralizing antibodies. Further, the low titer immunoreactivity observed in sera of some drug-naïve individuals had no effect on the biological activity of afamelanotide.

Levobupivacaine differentially suppresses platelet aggregation by modulating calcium release in a dose-dependent manner

OBJECTIVE

Levobupivacaine, an amide local anesthetic widely used in regional anesthesia, is reported in recent studies that it is a potent inhibitor of platelet functions. However, the concentrations of levobupivacaine were limitedly estimated in these reports. Additionally, the mechanisms by which it affects platelet function and blood coagulation is still not entirely known. The purpose of this study was to further investigate its effects on platelet function and the possible signaling mechanisms under various concentrations of levobupivacaine.

METHODS

Blood samples collected from healthy volunteers were separated into whole blood, platelet-rich-plasma and washed platelets. The effect of levobupivacaine on platelet aggregation was studied using platelet function analyzer (PFA-100) and platelet aggregometer. Agonist-induced platelet adenosine triphosphate (ATP) release, cytosolic calcium mobilization, thromboxane B2 (TxB2) secretion and platelet P-selectin translocation under various concentrations of levobupivacaine were investigated.

RESULTS

Our results indicated that levobupivacaine possessed negative effect on platelet aggregation. The closure times of (PFA-100) were lengthened and the agonist-induced platelet aggregation was significantly attenuated by levobupivacaine even at a low dose (50 μgml(-1)). Pretreatment with levobupivacaine produced significant changes in agonist-induced platelet P-selectin translocation, ATP release, thromboxane A2 (TxA2) production, and calcium mobilization in a dose-dependent manner. The p38 mitogen-activated protein kinases (MAPK), protein kinase C (PKC) δ subtype, cytosolic phospholipase A2 (cPLA2), and protein kinase B (PKB or Akt) were involved in collagen-induced platelet signaling, which would be responsible for antiplatelet effects of levobupivacaine.

CONCLUSION

We explored possible targets of levobupivacaine on platelets aggregation signaling mechanisms. Our data revealed that p38 MAPK, PKC δ subtype, cPLA2, and Akt were pathways involved in collagen-induced platelet signaling, which might be responsible for antiplatelet effects of levobupivacaine. Our study did provide direct evidence bolstering the critical mechanisms of levobupivacaine within different contexts. Additionally, levobupivacaine imposed a negative effect on platelet aggregation through multiple signaling pathways.

Opioid receptor trafficking and signaling: what happens after opioid receptor activation?

Prolonged opioid treatment leads to a comprehensive cellular adaptation mediated by opioid receptors, a basis to understand the development of opioid tolerance and dependence. However, the molecular mechanisms underlying opioid-induced cellular adaptation remain obscure. Recent advances in opioid receptor trafficking and signaling in cells have extensively increased our insight into the network of intracellular signal integration. This review focuses on those important intracellular biochemical processes that play critical roles in the development of opioid tolerance and dependence after opioid receptor activation, and tries to explain what happens after opioid receptor activation, and how the cellular adaptation develops from cell membrane to nucleus. Decades of research have delineated a network on opioid receptor trafficking and signaling, but the challenge remains to explain opioid tolerance and dependence from a single cellular signal network.

Adenylyl and guanylyl cyclase assays

This unit presents two basic protocols to determine adenylyl cyclase and guanylyl cyclase activity in tissue and cell homogenates, permeabilized cells, or subcellular fractions. Each method is divided into two parts: the enzyme reaction that causes the formation of the labeled cyclic nucleotide, and the separation of cyclic nucleotide products from unreacted nucleotide triphosphates and metabolites using Dowex 50 resin and aluminum oxide chromatographies. In the case of guanylyl cyclase, alternative separation protocols are also provided. Additionally, protocols are provided that describe preparation of both the columns used in the assays and the tissue or cells to be assayed.

Structure-activity relationships for the interactions of 2'- and 3'-(O)-(N-methyl)anthraniloyl-substituted purine and pyrimidine nucleotides with mammalian adenylyl cyclases

Membranous adenylyl cyclases (ACs) play a key role in signal transduction and are promising drug targets. In previous studies we showed that 2',3'-(O)-(N-methylanthraniloyl) (MANT)-substituted nucleotides are potent AC inhibitors. The aim of this study was to provide systematic structure-activity relationships for 21 (M)ANT-substituted nucleotides at the purified catalytic AC subunit heterodimer VC1:IIC2, the VC1:VC1 homodimer and recombinant ACs 1, 2 and 5. (M)ANT-nucleotides inhibited fully activated VC1:IIC2 in the order of affinity for bases hypoxanthine>uracil>cytosine>adenine∼guanine≫xanthine. Omission of a hydroxyl group at the 2' or 3'-position reduced inhibitor potency as did introduction of a γ-thiophosphate group or omission of the γ-phosphate group. Substitution of the MANT-group by an ANT-group had little effect on affinity. Although all nucleotides bound to VC1:IIC2 similarly according to the tripartite pharmacophore model with a site for the base, the ribose, and the phosphate chain, nucleotides exhibited subtle differences in their binding modes as revealed by fluorescence spectroscopy and molecular modelling. MANT-nucleotides also differentially interacted with the VC1:VC1 homodimer as assessed by fluorescence spectroscopy and modelling. Similar structure-activity relationships as for VC1:IIC2 were obtained for recombinant ACs 1, 2 and 5, with AC2 being the least sensitive AC isoform in terms of inhibition. Overall, ACs possess a broad base-specificity with no preference for the "cognate" base adenine as verified by enzyme inhibition, fluorescence spectroscopy and molecular modelling. These properties of ACs are indicative for ligand-specific conformational landscapes that extend to the VC1:VC1 homodimer and should facilitate development of non-nucleotide inhibitors.

Impact of divalent metal ions on regulation of adenylyl cyclase isoforms by forskolin analogs

Mammalian membranous adenylyl cyclases (mACs) play an important role in transmembrane signalling events in almost every cell and represent an interesting drug target. Forskolin (FS) is an invaluable research tool, activating AC isoforms 1-8. However, there is a paucity of AC isoform-selective FS analogs. Therefore, we examined the effects of FS and six FS derivatives on recombinant ACs 1, 2 and 5, representing members of different mAC families. Correlations of the pharmacological properties of the different AC isoforms revealed pronounced differences between ACs 1, 2 and 5. Additionally, potencies and efficacies of FS derivatives changed for any given AC isoform, depending on the metal ion, Mg(2+) or Mn(2+). The most striking effects of Mg(2+) and Mn(2+) on the diterpene profile were observed for AC2 where the large inhibitory effect of BODIPY-FS in the presence of Mg(2+) was considerably reduced in the presence of Mn(2+). Sequence alignment and docking experiments confirmed an exceptional position of AC2 compared to ACs 1 and 5 with respect to the structural environment of the catalytic core and cation-dependent diterpene effects. In conclusion, mAC isoforms 1, 2 and 5 exhibit a distinct pharmacological diterpene profile, depending on the divalent cation present. mAC crystal structures and modelling/docking studies provided an explanation for the pharmacological differences between the AC isoforms. Our study constitutes an important step towards the development of isoform-specific diterpenes exhibiting stimulatory or inhibitory effects.

Structural basis for the high-affinity inhibition of mammalian membranous adenylyl cyclase by 2',3'-o-(N-methylanthraniloyl)-inosine 5'-triphosphate

2',3'-O-(N-Methylanthraniloyl)-ITP (MANT-ITP) is the most potent inhibitor of mammalian membranous adenylyl cyclase (mAC) 5 (AC5, K(i), 1 nM) yet discovered and surpasses the potency of MANT-GTP by 55-fold (J Pharmacol Exp Ther 329:1156-1165, 2009). AC5 inhibitors may be valuable drugs for treatment of heart failure. The aim of this study was to elucidate the structural basis for the high-affinity inhibition of mAC by MANT-ITP. MANT-ITP was a considerably more potent inhibitor of the purified catalytic domains VC1 and IIC2 of mAC than MANT-GTP (K(i), 0.7 versus 18 nM). Moreover, there was considerably more efficient fluorescence resonance energy transfer between Trp1020 of IIC2 and the MANT group of MANT-ITP compared with MANT-GTP, indicating optimal interaction of the MANT group of MANT-ITP with the hydrophobic pocket. The crystal structure of MANT-ITP in complex with the G(s)α- and forskolin-activated catalytic domains VC1:IIC2 compared with the existing MANT-GTP crystal structure revealed only subtle differences in binding mode. The higher affinity of MANT-ITP to mAC compared with MANT-GTP is probably due to fewer stereochemical constraints upon the nucleotide base in the purine binding pocket, allowing a stronger interaction with the hydrophobic regions of IIC2 domain, as assessed by fluorescence spectroscopy. Stronger interaction is also achieved in the phosphate-binding site. The triphosphate group of MANT-ITP exhibits better metal coordination than the triphosphate group of MANT-GTP, as confirmed by molecular dynamics simulations. Collectively, the subtle differences in ligand structure have profound effects on affinity for mAC.

Acute Ca(2+)-dependent desensitization of 5-HT(1A) receptors is mediated by activation of protein kinase A (PKA) in rat serotonergic neurons

This report investigates acute changes in the sensitivity of 5-HT(1A) receptors in dorsal raphe (dr) neurons in response to elevated serotonin. DR neurons were isolated from adult rats and measurements of inhibition of Ca(2+) current by 5-HT were obtained using the whole cell patch clamp technique. During a 10-min application of 5-HT (with normal [Ca(2+)](i) approximately 100 nM) a desensitization occurred. The response to 20 nM 5-HT decreased by 66% relative to control and remained depressed for about 30 min. When the internal [Ca(2+)] was buffered to <1 nM only a weak transient desensitization occurred that was surmountable with higher [5-HT]. Adenylyl cyclase activation with forskolin mimicked the desensitization and selective inhibition of protein kinase A (PKA), but not protein kinase C (PKC), partially antagonized the desensitization induced by 5-HT. To measure the activity of PKA and phosphatase enzymes, dr slices were incubated with the selective agonist dipropyl-5-carboxamidotryptamine (DP-5-CT, 1 microM) for 10 min and the phosphorylation of the PKA substrate Kemptide was followed using ATP-gamma(32)P. DP-5-CT inhibited the cAMP stimulated maximal activity of PKA but raised basal PKA activity, thus increasing the percentage of PKA in the active state (activity ratio), an effect that was prevented by the selective 5-HT(1A) antagonist WAY100635. DP-5-CT also caused a significant inhibition of phosphatase activity. These data support a model in the dr where 5-HT(1A)-receptor stimulation of PKA promotes phosphorylation of a target and phosphatase inhibition leading to heterologous desensitization. The effect would be expected to have physiological consequences for 5-HT-mediated inhibitory post synaptic potentials and the Ca(2+) component of the action potentials of dr neurons.

Cyclic AMP signaling in bivalve molluscs: an overview

The cyclic AMP (cAMP)-dependent signaling accounts for the control of cellular cascades involved in many physiological functions, and a wealth of information is available on the cAMP system that operates in mammalian cells. Nevertheless, cAMP has a central role also in nonmammalian vertebrates and invertebrates. The present review aims at examining the information available on bivalve molluscs, from the first studies carried out in the early 1980s to the last progresses made in the present days. The major focus is on the structural and operational characteristics of the main actors of the signaling pathway, i.e., adenylyl cyclase, G proteins, and protein kinase A, and on the role played by the cyclic nucleotide on smooth muscle, heart, gills, gonads, and metabolism regulation. Moreover, recent evidence regarding the cAMP system as a target of environmental stress factors are discussed. It will become clear that cAMP does play a wide and important role in bivalve physiology. Several issues have been sufficiently clarified, although investigated only in a few model species. However, further fundamental aspects remain unknown, mainly regarding molecular features and interactions with other signaling pathways, thus requiring further elucidation.

A survey of nucleotide cyclases in actinobacteria: unique domain organization and expansion of the class III cyclase family in Mycobacterium tuberculosis

Cyclic nucleotides are well-known second messengers involved in the regulation of important metabolic pathways or virulence factors. There are six different classes of nucleotide cyclases that can accomplish the task of generating cAMP, and four of these are restricted to the prokaryotes. The role of cAMP has been implicated in the virulence and regulation of secondary metabolites in the phylum Actinobacteria, which contains important pathogens, such as Mycobacterium tuberculosis, M. leprae, M. bovis and Corynebacterium, and industrial organisms from the genus Streptomyces. We have analysed the actinobacterial genome sequences found in current databases for the presence of different classes of nucleotide cyclases, and find that only class III cyclases are present in these organisms. Importantly, prominent members such as M. tuberculosis and M. leprae have 17 and 4 class III cyclases, respectively, encoded in their genomes, some of which display interesting domain fusions seen for the first time. In addition, a pseudogene corresponding to a cyclase from M. avium has been identified as the only cyclase pseudogene in M. tuberculosis and M. bovis. The Corynebacterium and Streptomyces genomes encode only a single adenylyl cyclase each, both of which have corresponding orthologues in M. tuberculosis. A clustering of the cyclase domains in Actinobacteria reveals the presence of typical eukaryote-like, fungi-like and other bacteria-like class III cyclase sequences within this phylum, suggesting that these proteins may have significant roles to play in this important group of organisms.

The adenylyl cyclase activity of anthrax edema factor

Bacillus anthracis, the etiologic agent for anthrax, secretes edema factor (EF) to disrupt intracellular signaling pathways. Upon translocation into host cells and association with a calcium sensor, calmodulin (CaM), EF becomes a highly active adenylyl cyclase (AC) that raises the intracellular concentration of cyclic AMP (cAMP). Growing evidence shows that EF plays a key role in anthrax pathogenesis by affecting cellular functions vital for host defense. This strategy is also used by Bordetella pertussis, a bacterium that causes whooping cough. Pertussis bacteria secrete the bifunctional toxin CyaA which raises the intracellular cAMP. Here, we discuss recent advances from structural analyses that reveal the molecular basis of the conserved mechanism of activation and catalysis of EF and CyaA by CaM even though these two toxins use the completely different sequences to bind CaM. Comparison of the biochemical and structural characteristics of these two AC toxins with host ACs reveal that they have diverse strategies of catalytic activation, yet use the same two-metal-ion catalytic mechanism.

Role of VASP phosphorylation for the regulation of microglia chemotaxis via the regulation of focal adhesion formation/maturation

Microglia activation and migration are known to play crucial roles for the response to brain injuries. Extracellular ADP was reported to induce microglia chemotaxis and membrane ruffles through P2Y12 receptor. In this study, we examined the role of VASP phosphorylation in ADP-induced microglia chemotaxis and membrane ruffle formation. ADP stimulation transiently increased intracellular cAMP level, VASP phosphorylation at Ser153, membrane ruffle formation, and chemotaxis. PKA inhibitor effectively inhibited VASP phosphorylation and chemotaxis, indicating that P2Y12-mediated activation of PKA and subsequent VASP phosphorylation are involved in the regulation of microglia chemotaxis. Forskolin and okadaic acid induced sustained VASP phosphorylation at a high level, causing a significant reduction of the retraction of membrane ruffles and chemotaxis. In forskolin- or okadaic acid-treated cells, phosphorylated VASP remained at the membrane cortex, and size and number of mature focal adhesions were not increased, indicating that prolonged phosphorylation of VASP could inhibit transformation of focal complexes into focal adhesions. VASP knockdown cells showed markedly reduced frequency and distance of membrane ruffling upon ADP stimulation, reinforcing the idea that VASP is required for the ruffle formation. Cells expressing GFP-VASP(S153A) also showed a significant reduction of protrusion distance during ruffle formation, but the frequency and the distance of retraction were not affected by FSK at all. This result suggests that dephosphorylation of VASP might be required for the growth of adhesion strength during membrane retraction. Our results suggest that VASP phosphorylation by PKA plays an important role in membrane ruffle formation and chemotaxis via the regulation of focal adhesion formation/maturation.

Phosphodiesterase inhibition by naloxone augments the inotropic actions of beta-adrenergic stimulation

BACKGROUND

In a shock state, naloxone generates the cardiovascular pressor effect by displacing the endogenous opiate-like peptide beta-endorphin, resulting in restoration of the normal response to catecholamines. In addition to this opioid antagonistic effect, the non-opiate receptor-mediated effect has also been proposed. The aim of this study was to define the mechanism of non-opiate receptor-mediated action of naloxone.

METHODS

In guinea-pig ventricular tissues, cumulative concentration-response curves for isoproterenol as well as for forskolin and 3-isobutylmethylxanthine (IBMX) were obtained by increasing the concentration stepwise. To assess the effect on the phosphodiesterase (PDE), the effects of naloxone on contractile forces induced by isoproterenol (0.05 microM) in the presence of IBMX, cilostamide (a PDE III inhibitor), or rolipram (a PDE IV inhibitor) were observed. Naloxone-induced changes in cAMP production by isoproterenol both in the absence and in the presence of IBMX were measured. Naloxone-induced changes in cAMP production by forskolin in the presence of IBMX were also measured.

RESULTS

Naloxone (30 microM) produced a leftward shift of the isoproterenol concentration-response curve (0.01-2 microM) without changing the maximal response. Forskolin (0.5-10 microM) produced a concentration-dependent increase in contractile forces. Naloxone increased the maximal inotropic response of forskolin. Naloxone showed no effect on the IBMX concentration-response curve. In the presence of IBMX (200 microM), naloxone did not alter the contractions evoked by isoproterenol or forskolin. Whereas naloxone increased contractile forces significantly (approximately 25%) more than that of isoproterenol in the presence of rolipram, no alteration of contractile forces in the cilostamide-incubated muscles was observed. Naloxone caused a concentration-related increase of cAMP in the absence of IBMX, but caused no change in its presence.

CONCLUSIONS

The enhancement of myocardial contractility by naloxone in the presence of stimulation of adenylyl cyclase activity appears to be mediated by inhibition of PDE, specifically PDE III.

cAMP-dependent signal pathways in unicellular eukaryotes

The review summarizes current data about mechanisms of signal transduction with participation of cAMP (cyclic adenosine monophosphate) and elements of the complex cAMP-protein kinase A (PKA) signal pathway in unicellular eukaryotes. Conceptions of evolutionary origin of eukaryotic signal transduction systems are developed.

Adenylyl cyclases as innovative therapeutic goals

Pharmacological modulation of intracellular cyclic AMP (cAMP) signalling could provide new therapeutic and experimental tools. Although drugs interfering with this pathway have traditionally targeted membrane receptors, the effector enzyme adenylyl cyclase (AC), which functions as a signalling catalyst, also presents an interesting target. Thus, development of isoform-selective stimulator and/or inhibitor compounds for AC could lead to organ-specific pharmacotherapeutics for treating heart failure, cancer and neurodegenerative diseases. In this review, the potential of AC as the object of drug therapy is discussed.

Differential interactions of the catalytic subunits of adenylyl cyclase with forskolin analogs

The diterpene forskolin (FS) binds to, and activates, mammalian membranous adenylyl cyclase (AC) isoforms I-VIII. Diterpenes without C(1)-OH group do not activate ACs. The C(1)-OH group forms a hydrogen bond with the backbone oxygen of Val506 of the C1 catalytic subunit of AC (isoform V numbering). To better understand the mechanism of AC activation we examined the interactions of FS and eight FS analogs with purified catalytic AC subunits C1 (AC V) and C2 (AC II) by fluorescence spectroscopy, using 2',3'-O-(N-methylanthraniloyl)-guanosine 5'-triphosphate (MANT-GTP) as fluorescent reporter probe, and by enzymatic activity. FS analogs induced C1/C2 assembly as assessed by fluorescence resonance energy transfer from Trp1020 of C2 to MANT-GTP and by increased direct MANT-GTP fluorescence in the order of efficacy FS approximately 7-deacetyl-FS approximately 6-acetyl-7-deacetyl-FS approximately 9-deoxy-FS>7-deacetyl-7-(N-methylpiperazino-gamma-butyryloxy)-FS>1-deoxy-FS approximately 1,9-dideoxy-FS approximately 7-deacetyl-1-deoxy-FS approximately 7-deacetyl-1,9-dideoxy-FS. In contrast, FS analogs activated catalysis in the order of efficacy FS>7-deacety-FS approximately 6-acetyl-7-deacetyl-FS approximately 9-deoxy-FS>7-deacetyl-7-(N-methylpiperazino-gamma-butyryloxy)-FS>>1-deoxy-FS, 1,9-dideoxy-FS, 7-deacetyl-1-deoxy-FS and 7-deacetyl-1,9-dideoxy-FS (all ineffective). 1-Deoxy-FS analogs inhibited FS-stimulated catalysis by an apparently non-competitive mechanism. Our data suggest a two-step mechanism of AC activation by diterpenes. In the first step, diterpenes, regardless of their substitution pattern, promote C1/C2 assembly. In the second and yet poorly understood step, diterpenes that form a hydrogen bond between C(1)-OH and Val506 promote a conformational switch that results in activation of catalysis. The apparent non-competitive interaction of FS with 1-deoxy-FS analogs is explained by impaired ligand exchange due to strong hydrophobic interactions with C1/C2.

The evolution of guanylyl cyclases as multidomain proteins: conserved features of kinase-cyclase domain fusions

Guanylyl cyclases (GCs) are enzymes that generate cyclic GMP and regulate different physiologic and developmental processes in a number of organisms. GCs possess sequence similarity to class III adenylyl cyclases (ACs) and are present as either membrane-bound receptor GCs or cytosolic soluble GCs. We sought to determine the evolution of GCs using a large-scale bioinformatic analysis and found multiple lineage-specific expansions of GC genes in the genomes of many eukaryotes. Moreover, a few GC-like proteins were identified in prokaryotes, which come fused to a number of different domains, suggesting allosteric regulation of nucleotide cyclase activity. Eukaryotic receptor GCs are associated with a kinase homology domain (KHD), and phylogenetic analysis of these proteins suggest coevolution of the KHD and the associated cyclase domain as well as a conservation of the sequence and the size of the linker region between the KHD and the associated cyclase domain. Finally, we also report the existence of mimiviral proteins that contain putative active kinase domains associated with a cyclase domain, which could suggest early evolution of the fusion of these two important domains involved in signal transduction.

The C1 and C2 domains target human type 6 adenylyl cyclase to lipid rafts and caveolae

Previous data has shown that adenylyl cyclase type 6 (AC6) is expressed principally in lipid rafts or caveolae of cardiac myocytes and other cell types while certain other isoforms of AC are excluded from these microdomains. The mechanism by which AC6 is localized to lipid rafts or caveolae is unknown. In this study, we show AC6 is localized in lipid rafts of COS-7 cells (expressing caveolin-1) and in HEK-293 cells or cardiac fibroblasts isolated from caveolin-1 knock-out mice (both of which lack prototypical caveolins). To determine the region of AC6 that confers raft localization, we independently expressed each of the major intracellular domains, the N-terminus, C1 and C2 domains, and examined their localization with various approaches. The N-terminus did not associate with lipid rafts or caveolae of either COS-7 or HEK-293 cells nor did it immunoprecipitate with caveolin-1 when expressed in COS-7 cells. By contrast, the C1 and C2 domains each associated with lipid rafts to varying degrees and were present in caveolin-1 immunoprecipitates. There were no differences in the pattern of localization of either the C1 or C2 domains between COS-7 and HEK-293 cells. Further dissection of the C1 domain into four individual proteins indicated that the N-terminal half of this domain is responsible for its raft localization. To probe for a role of a putative palmitoylation motif in the C-terminal portion of the C2 domain, we expressed various truncated forms of AC6 lacking most or all of the C-terminal 41 amino acids. These truncated AC6 proteins were not altered in terms of their localization in lipid rafts or their catalytic activity, implying that this C-terminal region is not required for lipid raft targeting of AC6. We conclude that while the C1 domain may be most important, both the C1 and C2 domains of AC6 play a role in targeting AC6 to lipid rafts.

Enzymatic characteristics of a Ser/Thr protein kinase, SpkA, from Myxococcus xanthus

Two Ser/Thr protein kinases, SpkA and SpkB, selected from Myxococcus xanthus based on amino acid sequence similarities with the catalytic subunits of cAMP-dependent protein kinases (PKA) were synthesized using a cell-free protein synthesis system. In various protein kinase assays, purified StkA and StkB showed their highest protein kinase activities in a PKA assay using the selective PKA substrate Kemptide and in a protein kinase C (PKC) assay using the selective PKC substrate neurogranin((28-43)), respectively. SpkA had apparent K(m) values of 45 microM and 37 microM for Kemptide and ATP, respectively. Phosphorylation of Kemptide was inhibited by a specific PKA inhibitor peptide, PKI(5-24), and the IC(50) and K(i) values for inhibition of the SpkA activity were 117 nM and 36 nM, respectively.

Rv1675c (cmr) regulates intramacrophage and cyclic AMP-induced gene expression in Mycobacterium tuberculosis-complex mycobacteria

Cyclic AMP (cAMP) has recently been shown to be a global regulator of gene expression in Mycobacterium tuberculosis (Mtb). In this study we identified a new cAMP-associated regulon in Mtb and Mycobacterium bovis BCG, which is distinct from the previously described CRP(Mt) regulon. Proteomic comparison of wild-type M. bovis BCG with a Rv1675c (cmr) knockout strain showed dysregulated expression of four previously identified proteins encoded by the cAMP-induced genes (cAIGs) mdh, groEL2, Rv1265 and PE_PGRS6a. Regulated expression of these four cAIGs also occurred during macrophage infection, and this regulation required cmr in both Mtb and M. bovis BCG. Purified His-Cmr bound to the DNA sequences upstream of three cAIGs (mdh, groEL2, Rv1265) in electrophoretic mobility shift assays, suggesting direct regulation of these genes by Cmr. We also found that low pH stimulated cAMP production in both Mtb and M. bovis BCG, but broadly affected cAIG regulation only in M. bovis BCG. These studies identify Cmr as a transcription factor that regulates cAIGs within macrophages, and suggest that multiple factors affect cAMP-associated gene regulation in tuberculosis-complex mycobacteria. cAMP signalling and Cmr-mediated gene regulation during Mtb infection of macrophages may have implications for TB pathogenesis.

The cyclic AMP phenotype of fragile X and autism

Cyclic AMP (cAMP) is a second messenger involved in many processes including mnemonic processing and anxiety. Memory deficits and anxiety are noted in the phenotype of fragile X (FX), the most common heritable cause of mental retardation and autism. Here we review reported observations of altered cAMP cascade function in FX and autism. Cyclic AMP is a potentially useful biochemical marker to distinguish autism comorbid with FX from autism per se and the cAMP cascade may be a viable therapeutic target for both FX and autism.

Activation and inhibition of adenylyl cyclase isoforms by forskolin analogs

Adenylyl cyclase (AC) isoforms 1 to 9 are differentially expressed in tissues and constitute an interesting drug target. ACs 1 to 8 are activated by the diterpene, forskolin (FS). It is unfortunate that there is a paucity of AC isoform-selective activators. To develop such compounds, an understanding of the structure/activity relationships of diterpenes is necessary. Therefore, we examined the effects of FS and nine FS analogs on ACs 1, 2, and 5 expressed in Spodoptera frugiperda insect cells. Diterpenes showed the highest potencies at AC1 and the lowest potencies at AC2. We identified full agonists, partial agonists, antagonists, and inverse agonists, i.e., diterpenes that reduced basal AC activity. Each AC isoform exhibited a distinct pharmacological profile. AC2 showed the highest basal activity of all AC isoforms and highest sensitivity to inverse agonistic effects of 1-deoxy-forskolin, 7-deacetyl-1,9-dideoxy-forskolin, and, particularly, BODIPY-forskolin. In contrast, BODIPY-forskolin acted as partial agonist at the other ACs. 1-Deoxy-forskolin analogs were devoid of agonistic activity at ACs but antagonized the effects of FS in a mixed competitive/noncompetitive manner. At purified catalytic AC subunits, BODIPY-forskolin acted as weak partial agonist/strong partial antagonist. Molecular modeling revealed that the BODIPY group rotates promiscuously outside of the FS-binding site. Collectively, ACs are not uniformly activated and inhibited by FS and FS analogs, demonstrating the feasibility to design isoform-selective FS analogs. The two- and multiple-state models, originally developed to conceptualize ligand effects at G-protein-coupled receptors, can be applied to ACs to explain certain experimental data.

Heterodimers of adenylyl cyclases 2 and 5 show enhanced functional responses in the presence of Galpha s

Recent studies have demonstrated that adenylyl cyclase isoforms can form both homo- and heterodimers and that this may be the basic functional unit of these enzymes (see Cooper, D.M.F. and Crossthwaite, A.J. (2006) Trends. Pharmacol. Sci. 8:426-431). Here, we show that adenylyl cyclases 2 and 5 can form a functional heterodimeric complex in HEK293 cells using a combination of BRET, confocal imaging, co-immunoprecipitation and assays of adenylyl cyclase activity. The AC2/5 complex is formed constitutively and is stable in the presence of receptor or forskolin stimulation. The complex formed by AC2/5 is also much more sensitive to the presence of Galpha(s) and forskolin than either of the parent AC isoforms themselves. Finally, we also show that this complex can be detected in native tissues as AC2 and AC5 were localized to the same structures in adult mouse ventricular myocytes and neonatal mouse cardiac fibroblasts and could be co-immunoprecipitated from lysates of mouse heart.

Introduction of bisecting GlcNAc in N-glycans of adenylyl cyclase III enhances its activity

Adenylyl cyclases (ACs) catalyze the synthesis of cAMP in response to extracellular and intracellular signals and are responsible for a wide variety of biological activities including cell growth, differentiation, and metabolism. There are nine, currently known, isoforms of transmembrane ACs, and the primary structure of the catalytic unit and the potential N-glycosylation sites are highly conserved among them. The enzyme beta1,4-N-acetylglucosaminyltransferase III (GnT-III) catalyzes the addition of a bisecting N-acetylglucosamine (GlcNAc) to N-glycans. We have been studying the function of GnT-III on signaling molecules. In this study, we report on the effects of a bisecting GlcNAc on AC signaling. We established GnT-III stable expressing cell lines of Neuro-2a mouse neuroblastoma cells and B16 mouse melanoma cells. Forskolin-induced AC activation and downstream signaling, such as the synthesis of cAMP and the phosphorylation of transcriptional factor CRE-binding protein were upregulated in the GnT-III transfectants compared with mock transfectants or a dominant negative mutant of GnT-III-transfected cells. Since endogenous AC expression levels in Neuro-2a and B16 cells were too low to permit the glycosylation status to be examined, AC type III (ACIII) was overexpressed in a stable expression system using Flp-In-293 cells. The N-glycans of ACIII in the GnT-III transfectants were confirmed to be modified by the introduction of a bisecting GlcNAc, and AC activity was found to be significantly up-regulated in the GnT-III transfectants. Thus, the structure of N-glycans of ACIII regulates its enzymatic activity and downstream signaling.

The Arabidopsis thaliana brassinosteroid receptor (AtBRI1) contains a domain that functions as a guanylyl cyclase in vitro

BACKGROUND

Guanylyl cyclases (GCs) catalyze the formation of the second messenger guanosine 3',5'-cyclic monophosphate (cGMP) from guanosine 5'-triphosphate (GTP). Cyclic GMP has been implicated in an increasing number of plant processes, including responses to abiotic stresses such as dehydration and salt, as well as hormones.

PRINCIPLE FINDINGS

Here we used a rational search strategy based on conserved and functionally assigned residues in the catalytic centre of annotated GCs to identify candidate GCs in Arabidopsis thaliana and show that one of the candidates is the brassinosteroid receptor AtBR1, a leucine rich repeat receptor like kinase. We have cloned and expressed a 114 amino acid recombinant protein (AtBR1-GC) that harbours the putative catalytic domain, and demonstrate that this molecule can convert GTP to cGMP in vitro.

CONCLUSIONS

Our results suggest that AtBR1 may belong to a novel class of GCs that contains both a cytosolic kinase and GC domain, and thus have a domain organisation that is not dissimilar to that of atrial natriuretic peptide receptors, NPR1 and NPR2. The findings also suggest that cGMP may have a role as a second messenger in brassinosteroid signalling. In addition, it is conceivable that other proteins containing the extended GC search motif may also have catalytic activity, thus implying that a significant number of GCs, both in plants and animals, remain to be discovered, and this is in keeping with the fact that the single cellular green alga Chlamydomonas reinhardtii contains over 90 annotated putative CGs.