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2.  Analysis of Substrate Specificity and Kinetics of Cyclic Nucleotide Phosphodiesterases with N’-Methylanthraniloyl-Substituted Purine and Pyrimidine 3′,5′-Cyclic Nucleotides by Fluorescence Spectrometry 
PLoS ONE  2013;8(1):e54158.
As second messengers, the cyclic purine nucleotides adenosine 3′,5′-cyclic monophosphate (cAMP) and guanosine 3′,5′-cyclic monophosphate (cGMP) play an essential role in intracellular signaling. Recent data suggest that the cyclic pyrimidine nucleotides cytidine 3′,5′-cyclic monophosphate (cCMP) and uridine 3′,5′-cyclic monophosphate (cUMP) also act as second messengers. Hydrolysis by phosphodiesterases (PDEs) is the most important degradation mechanism for cAMP and cGMP. Elimination of cUMP and cCMP is not completely understood, though. We have shown that human PDEs hydrolyze not only cAMP and cGMP but also cyclic pyrimidine nucleotides, indicating that these enzymes may be important for termination of cCMP- and cUMP effects as well. However, these findings were acquired using a rather expensive HPLC/mass spectrometry assay, the technical requirements of which are available only to few laboratories. N’-Methylanthraniloyl-(MANT-)labeled nucleotides are endogenously fluorescent and suitable tools to study diverse protein/nucleotide interactions. In the present study, we report the synthesis of new MANT-substituted cyclic purine- and pyrimidine nucleotides that are appropriate to analyze substrate specificity and kinetics of PDEs with more moderate technical requirements. MANT-labeled nucleoside 3′,5′-cyclic monophosphates (MANT-cNMPs) are shown to be substrates of various human PDEs and to undergo a significant change in fluorescence upon cleavage, thus allowing direct, quantitative and continuous determination of hydrolysis via fluorescence detection. As substrates of several PDEs, MANT-cNMPs show similar kinetics to native nucleotides, with some exceptions. Finally, they are shown to be also appropriate tools for PDE inhibitor studies.
doi:10.1371/journal.pone.0054158
PMCID: PMC3544816  PMID: 23342095
5.  Quantification of cAMP and cGMP analogs in intact cells: pitfalls in enzyme immunoassays for cyclic nucleotides 
Immunoassays are routinely used as research tools to measure intracellular cAMP and cGMP concentrations. Ideally, this application requires antibodies with high sensitivity and specificity. The present work evaluates the cross-reactivity of commercially available cyclic nucleotide analogs with two non-radioactive and one radioactive cAMP and cGMP immunoassay. Most of the tested cyclic nucleotide analogs showed low degree competition with the antibodies; however, with Rp-cAMPS, 8-Br-cGMP and 8-pCPT-cGMP, a strong cross-reactivity with the corresponding cAMP and cGMP, respectively, immunoassays was observed. The determined EIA-binding constants enabled the measurement of the intracellular cyclic nucleotide concentrations and revealed a time- and lipophilicity-dependent cell membrane permeability of the compounds in the range of 10–30% of the extracellular applied concentration, thus allowing a more accurate prediction of the intracellular analog levels in a given experiment.
Electronic supplementary material
The online version of this article (doi:10.1007/s00210-011-0662-6) contains supplementary material, which is available to authorized users.
doi:10.1007/s00210-011-0662-6
PMCID: PMC3145891  PMID: 21713381
Cyclic nucleotides; Enzyme immunoassay; Lipophilicity; Cell permeability
6.  Chemical tools selectively target components of the PKA system 
Background
In the eukaryotic cell the cAMP-dependent protein kinase (PKA) is a key enzyme in signal transduction and represents the main target of the second messenger cAMP. Here we describe the design, synthesis and characterisation of specifically tailored cAMP analogs which can be utilised as a tool for affinity enrichment and purification as well as for proteomics based analyses of cAMP binding proteins.
Results
Two sets of chemical binders were developed based on the phosphorothioate derivatives of cAMP, Sp-cAMPS and Rp-cAMPS acting as cAMP-agonists and -antagonists, respectively. These compounds were tested via direct surface plasmon resonance (SPR) analyses for their binding properties to PKA R-subunits and holoenzyme. Furthermore, these analogs were used in an affinity purification approach to analyse their binding and elution properties for the enrichment and improvement of cAMP binding proteins exemplified by the PKA R-subunits. As determined by SPR, all tested Sp-analogs provide valuable tools for affinity chromatography. However, Sp-8-AEA-cAMPS displayed (i) superior enrichment properties while maintaining low unspecific binding to other proteins in crude cell lysates, (ii) allowing mild elution conditions and (iii) providing the capability to efficiently purify all four isoforms of active PKA R-subunit in milligram quantities within 8 h. In a chemical proteomics approach both sets of binders, Rp- and Sp-cAMPS derivatives, can be employed. Whereas Sp-8-AEA-cAMPS preferentially binds free R-subunit, Rp-AHDAA-cAMPS, displaying antagonist properties, not only binds to the free PKA R-subunits but also to the intact PKA holoenzyme both from recombinant and endogenous sources.
Conclusion
In summary, all tested cAMP analogs were useful for their respective application as an affinity reagent which can enhance purification of cAMP binding proteins. Sp-8-AEA-cAMPS was considered the most efficient analog since Sp-8-AHA-cAMPS and Sp-2-AHA-cAMPS, demonstrated incomplete elution from the matrix, as well as retaining notable amounts of bound protein contaminants. Furthermore it could be demonstrated that an affinity resin based on Rp-8-AHDAA-cAMPS provides a valuable tool for chemical proteomics approaches.
doi:10.1186/1472-6769-9-3
PMCID: PMC2660902  PMID: 19216744
7.  Biochemical characterization and cellular imaging of a novel, membrane permeable fluorescent cAMP analog 
BMC Biochemistry  2008;9:18.
Background
A novel fluorescent cAMP analog (8-[Pharos-575]- adenosine-3', 5'-cyclic monophosphate) was characterized with respect to its spectral properties, its ability to bind to and activate three main isoenzymes of the cAMP-dependent protein kinase (PKA-Iα, PKA-IIα, PKA-IIβ) in vitro, its stability towards phosphodiesterase and its ability to permeate into cultured eukaryotic cells using resonance energy transfer based indicators, and conventional fluorescence imaging.
Results
The Pharos fluorophore is characterized by a Stokes shift of 42 nm with an absorption maximum at 575 nm and the emission peaking at 617 nm. The quantum yield is 30%. Incubation of the compound to RIIα and RIIβ subunits increases the amplitude of excitation and absorption maxima significantly; no major change was observed with RIα. In vitro binding of the compound to RIα subunit and activation of the PKA-Iα holoenzyme was essentially equivalent to cAMP; RII subunits bound the fluorescent analog up to ten times less efficiently, resulting in about two times reduced apparent activation constants of the holoenzymes compared to cAMP. The cellular uptake of the fluorescent analog was investigated by cAMP indicators. It was estimated that about 7 μM of the fluorescent cAMP analog is available to the indicator after one hour of incubation and that about 600 μM of the compound had to be added to intact cells to half-maximally dissociate a PKA type IIα sensor.
Conclusion
The novel analog combines good membrane permeability- comparable to 8-Br-cAMP – with superior spectral properties of a modern, red-shifted fluorophore. GFP-tagged regulatory subunits of PKA and the analog co-localized. Furthermore, it is a potent, PDE-resistant activator of PKA-I and -II, suitable for in vitro applications and spatial distribution evaluations in living cells.
doi:10.1186/1471-2091-9-18
PMCID: PMC2443153  PMID: 18578870
8.  Binding of Regulatory Subunits of Cyclic AMP-Dependent Protein Kinase to Cyclic CMP Agarose 
PLoS ONE  2012;7(7):e39848.
The bacterial adenylyl cyclase toxins CyaA from Bordetella pertussis and edema factor from Bacillus anthracis as well as soluble guanylyl cyclase α1β1 synthesize the cyclic pyrimidine nucleotide cCMP. These data raise the question to which effector proteins cCMP binds. Recently, we reported that cCMP activates the regulatory subunits RIα and RIIα of cAMP-dependent protein kinase. In this study, we used two cCMP agarose matrices as novel tools in combination with immunoblotting and mass spectrometry to identify cCMP-binding proteins. In agreement with our functional data, RIα and RIIα were identified as cCMP-binding proteins. These data corroborate the notion that cAMP-dependent protein kinase may serve as a cCMP target.
doi:10.1371/journal.pone.0039848
PMCID: PMC3392273  PMID: 22808067
9.  Small Molecule AKAP-Protein Kinase A (PKA) Interaction Disruptors That Activate PKA Interfere with Compartmentalized cAMP Signaling in Cardiac Myocytes* 
The Journal of Biological Chemistry  2010;286(11):9079-9096.
A-kinase anchoring proteins (AKAPs) tether protein kinase A (PKA) and other signaling proteins to defined intracellular sites, thereby establishing compartmentalized cAMP signaling. AKAP-PKA interactions play key roles in various cellular processes, including the regulation of cardiac myocyte contractility. We discovered small molecules, 3,3′-diamino-4,4′-dihydroxydiphenylmethane (FMP-API-1) and its derivatives, which inhibit AKAP-PKA interactions in vitro and in cultured cardiac myocytes. The molecules bind to an allosteric site of regulatory subunits of PKA identifying a hitherto unrecognized region that controls AKAP-PKA interactions. FMP-API-1 also activates PKA. The net effect of FMP-API-1 is a selective interference with compartmentalized cAMP signaling. In cardiac myocytes, FMP-API-1 reveals a novel mechanism involved in terminating β-adrenoreceptor-induced cAMP synthesis. In addition, FMP-API-1 leads to an increase in contractility of cultured rat cardiac myocytes and intact hearts. Thus, FMP-API-1 represents not only a novel means to study compartmentalized cAMP/PKA signaling but, due to its effects on cardiac myocytes and intact hearts, provides the basis for a new concept in the treatment of chronic heart failure.
doi:10.1074/jbc.M110.160614
PMCID: PMC3058960  PMID: 21177871
Adenylate Cyclase (Adenylyl Cyclase); Cyclic AMP (cAMP); Protein Kinase A (PKA); Protein Phosphorylation; Protein-Protein Interactions; Signal Transduction; AKAP; Compartmentalized Signaling; AKAP18; Yotiao; AKAP150; Compartmentalization

Results 1-9 (9)