Extraction, purification and identification of cytidine 3',5'-cyclic monophosphate from rat tissues

Pharmacological potential of cyclic nucleotide signaling in immunity

Cyclic nucleotides are important signaling molecules that play many critical physiological roles including controlling cell fate and development, regulation of metabolic processes, and responding to changes in the environment. Cyclic nucleotides are also pivotal regulators in immune signaling, orchestrating intricate processes that maintain homeostasis and defend against pathogenic threats. This review provides a comprehensive examination of the pharmacological potential of cyclic nucleotide signaling pathways within the realm of immunity. Beginning with an overview of the fundamental roles of cAMP and cGMP as ubiquitous second messengers, this review delves into the complexities of their involvement in immune responses. Special attention is given to the challenges associated with modulating these signaling pathways for therapeutic purposes, emphasizing the necessity for achieving cell-type specificity to avert unintended consequences. A major focus of the review is on the recent paradigm-shifting discoveries regarding specialized cyclic nucleotide signals in the innate immune system, notably the cGAS-STING pathway. The significance of cyclic dinucleotides, exemplified by 2'3'-cGAMP, in controlling immune responses against pathogens and cancer, is explored. The evolutionarily conserved nature of cyclic dinucleotides as antiviral agents, spanning across diverse organisms, underscores their potential as targets for innovative immunotherapies. Findings from the last several years have revealed a striking diversity of novel bacterial cyclic nucleotide second messengers which are involved in antiviral responses. Knowledge of the existence and precise identity of these molecules coupled with accurate descriptions of their associated immune defense pathways will be essential to the future development of novel antibacterial therapeutic strategies. The insights presented herein may help researchers navigate the evolving landscape of immunopharmacology as it pertains to cyclic nucleotides and point toward new avenues or lines of thinking about development of therapeutics against the pathways they regulate.

Cyclic CMP and cyclic UMP mediate bacterial immunity against phages

The cyclic pyrimidines 3',5'-cyclic cytidine monophosphate (cCMP) and 3',5'-cyclic uridine monophosphate (cUMP) have been reported in multiple organisms and cell types. As opposed to the cyclic nucleotides 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP), which are second messenger molecules with well-established regulatory roles across all domains of life, the biological role of cyclic pyrimidines has remained unclear. Here we report that cCMP and cUMP are second messengers functioning in bacterial immunity against viruses. We discovered a family of bacterial pyrimidine cyclase enzymes that specifically synthesize cCMP and cUMP following phage infection and demonstrate that these molecules activate immune effectors that execute an antiviral response. A crystal structure of a uridylate cyclase enzyme from this family explains the molecular mechanism of selectivity for pyrimidines as cyclization substrates. Defense systems encoding pyrimidine cyclases, denoted here Pycsar (pyrimidine cyclase system for antiphage resistance), are widespread in prokaryotes. Our results assign clear biological function to cCMP and cUMP as immunity signaling molecules in bacteria.

Identification of cCMP and cUMP Substrate Proteins and Cross Talk Between cNMPs

cCMP and cUMP are pyrimidine cyclic nucleotides which are present in several types of cells. These molecules could exert diverse cellular functions and might act as second messengers. In the last years, diverse approaches were performed to analyze possible cellular substrates and signaling pathways of cCMP and cUMP. In this review these approaches are summarized, and probable cross talk of these signaling molecules is described. These analyses might lead to the (patho)physiological and pharmacological relevance of these noncanonical cyclic nucleotides.

cCMP and cUMP Across the Tree of Life: From cCMP and cUMP Generators to cCMP- and cUMP-Regulated Cell Functions

The cyclic purine nucleotides cAMP and cGMP are well-established second messenger molecules that are generated by distinct nucleotidyl cyclases (NCs) and regulate numerous cell functions via specific effector molecules. In contrast, the existence of the cyclic pyrimidine nucleotides cCMP and cUMP has been controversial for many years. The development of highly specific and sensitive mass spectrometry methods has enabled the unequivocal detection and quantitation of cCMP and cUMP in biological systems. These cNMPs occur broadly in numerous mammalian cell lines and primary cells. cCMP has also been detected in mouse organs, and both cCMP and cUMP occur in various developmental stages of the zebrafish Danio rerio. So far, the soluble guanylyl cyclase (sGC) and soluble adenylyl cyclase (sAC) have been identified as cCMP and cUMP generators. Dissociations in the expression patterns of sAC and sGC relative to cCMP and cUMP abundance may point to the existence of hitherto unidentified cCMP- and cUMP-generating NCs. The broad occurrence of cCMP and cUMP in vertebrates and the distinct cNMP patterns suggest specific roles of these cNMPs in the regulation of numerous cell functions.

Holistic Methods for the Analysis of cNMP Effects

Cyclic nucleotide monophosphates (cNMPs) typify the archetype second messenger in living cells and serve as molecular switches with broad functionality. cAMP and cGMP are the best-described cNMPs; however, there is a growing body of evidence indicating that also cCMP and cUMP play a substantial role in signal transduction. Despite research efforts, to date, relatively little is known about the biology of these noncanonical cNMPs, which is due, at least in part, to methodological issues in the past entailing setbacks of the entire field. Only recently, with the use of state-of-the-art techniques, it was possible to revive noncanonical cNMP research. While high-sensitive detection methods disclosed relevant levels of cCMP and cUMP in mammalian cells, knowledge about the biological effectors and their physiological interplay is still incomplete. Holistic biophysical readouts capture cell responses label-free and in an unbiased fashion with the advantage to detect concealed aspects of cell signaling that are arduous to access via traditional biochemical assay approaches. In this chapter, we introduce the dynamic mass redistribution (DMR) technology to explore cell signaling beyond established receptor-controlled mechanisms. Both common and distinctive features in the signaling structure of cCMP and cUMP were identified. Moreover, the integrated response of whole live cells revealed a hitherto undisclosed additional effector of the noncanonical cNMPs. Future studies will show how holistic methods will become integrated into the methodological arsenal of contemporary cNMP research.

Metabolite Regulation of Nuclear Localization of Carbohydrate-response Element-binding Protein (ChREBP): ROLE OF AMP AS AN ALLOSTERIC INHIBITOR

The carbohydrate-response element-binding protein (ChREBP) is a glucose-responsive transcription factor that plays an essential role in converting excess carbohydrate to fat storage in the liver. In response to glucose levels, ChREBP is regulated by nuclear/cytosol trafficking via interaction with 14-3-3 proteins, CRM-1 (exportin-1 or XPO-1), or importins. Nuclear localization of ChREBP was rapidly inhibited when incubated in branched-chain α-ketoacids, saturated and unsaturated fatty acids, or 5-aminoimidazole-4-carboxamide ribonucleotide. Here, we discovered that protein-free extracts of high fat-fed livers contained, in addition to ketone bodies, a new metabolite, identified as AMP, which specifically activates the interaction between ChREBP and 14-3-3. The crystal structure showed that AMP binds directly to the N terminus of ChREBP-α2 helix. Our results suggest that AMP inhibits the nuclear localization of ChREBP through an allosteric activation of ChREBP/14-3-3 interactions and not by activation of AMPK. AMP and ketone bodies together can therefore inhibit lipogenesis by restricting localization of ChREBP to the cytoplasm during periods of ketosis.

cCMP and cUMP occur in vivo

Mammalian cells contain the cyclic pyrimidine nucleotides cCMP and cUMP. It is unknown whether these tentative new second messenger molecules occur in vivo. We used high performance liquid chromatography quadrupole tandem mass spectrometry to quantitate nucleoside 3',5'-cyclic monophosphates. cCMP was detected in all organs studied, most notably pancreas, spleen and the female reproductive system. cUMP was not detected in organs, probably due to the intrinsically low sensitivity of mass spectrometry to detect this molecule and organ matrix effects. Intratracheal infection of mice with recombinant Pseudomonas aeruginosa harboring the nucleotidyl cyclase toxin ExoY massively increased cUMP in lung. The identity of cCMP and cUMP in organs was confirmed by high performance liquid chromatography quadrupole time of flight mass spectrometry. cUMP also appeared in serum, urine and faeces following infection. Taken together, this report unequivocally shows for the first time that cCMP and cUMP occur in vivo.

From canonical to non-canonical cyclic nucleotides as second messengers: pharmacological implications

This review summarizes our knowledge on the non-canonical cyclic nucleotides cCMP, cUMP, cIMP, cXMP and cTMP. We place the field into a historic context and discuss unresolved questions and future directions of research. We discuss the implications of non-canonical cyclic nucleotides for experimental and clinical pharmacology, focusing on bacterial infections, cardiovascular and neuropsychiatric disorders and reproduction medicine. The canonical cyclic purine nucleotides cAMP and cGMP fulfill the criteria of second messengers. (i) cAMP and cGMP are synthesized by specific generators, i.e. adenylyl and guanylyl cyclases, respectively. (ii) cAMP and cGMP activate specific effector proteins, e.g. protein kinases. (iii) cAMP and cGMP exert specific biological effects. (iv) The biological effects of cAMP and cGMP are terminated by phosphodiesterases and export. The effects of cAMP and cGMP are mimicked by (v) membrane-permeable cyclic nucleotide analogs and (vi) bacterial toxins. For decades, the existence and relevance of cCMP and cUMP have been controversial. Modern mass-spectrometric methods have unequivocally demonstrated the existence of cCMP and cUMP in mammalian cells. For both, cCMP and cUMP, the criteria for second messenger molecules are now fulfilled as well. There are specific patterns by which nucleotidyl cyclases generate cNMPs and how they are degraded and exported, resulting in unique cNMP signatures in biological systems. cNMP signaling systems, specifically at the level of soluble guanylyl cyclase, soluble adenylyl cyclase and ExoY from Pseudomonas aeruginosa are more promiscuous than previously appreciated. cUMP and cCMP are evolutionary new molecules, probably reflecting an adaption to signaling requirements in higher organisms.

A facile and sensitive method for quantification of cyclic nucleotide monophosphates in mammalian organs: basal levels of eight cNMPs and identification of 2',3'-cIMP

A sensitive, versatile and economical method to extract and quantify cyclic nucleotide monophosphates (cNMPs) using LC-MS/MS, including both 3',5'-cNMPs and 2',3'-cNMPs, in mammalian tissues and cellular systems has been developed. Problems, such as matrix effects from complex biological samples, are addressed and have been optimized. This protocol allows for comparison of multiple cNMPs in the same system and was used to examine the relationship between tissue levels of cNMPs in a panel of rat organs. In addition, the study reports the first identification and quantification of 2',3'-cIMP. The developed method will allow for quantification of cNMPs levels in cells and tissues with varying disease states, which will provide insight into the role(s) and interplay of cNMP signalling pathways.

cCMP and cUMP: emerging second messengers

The cyclic purine nucleotides cAMP and cGMP are established second messengers. By contrast, the existence of the cyclic pyrimidine nucleotides cytidine 3',5'-cyclic monophosphate (cCMP) and uridine 3',5'-cyclic monophosphate (cUMP) has been controversial for decades. The recent development of highly sensitive mass spectrometry (MS) methods allowed precise quantitation and unequivocal identification of cCMP and cUMP in cells. Importantly, cCMP and cUMP generators, effectors, cleaving enzymes, and transporters have now been identified. Here, I discuss evidence in support of cCMP and cUMP as bona fide second messengers, the emerging therapeutic implications of cCMP and cUMP signaling, and important unresolved questions for this field.

Nucleotidyl cyclase activity of soluble guanylyl cyclase in intact cells

Soluble guanylyl cyclase (sGC) is activated by nitric oxide (NO) and generates the second messenger cyclic GMP (cGMP). Recently, purified sGC α1β1 has been shown to additionally generate the cyclic pyrimidine nucleotides cCMP and cUMP. However, since cyclic pyrimidine nucleotide formation occurred only the presence of Mn(2+) but not Mg(2+), the physiological relevance of these in vitro findings remained unclear. Therefore, we studied cyclic nucleotide formation in intact cells. We observed NO-dependent cCMP- and cUMP formation in intact HEK293 cells overexpressing sGC α1β1 and in RFL-6 rat fibroblasts endogenously expressing sGC, using HPLC-tandem mass spectrometry. The identity of cCMP and cUMP was unambiguously confirmed by HPLC-time-of-flight mass spectrometry. Our data indicate that cCMP and cUMP play second messenger roles and that Mn(2+) is a physiological sGC cofactor.

Metabolite regulation of nucleo-cytosolic trafficking of carbohydrate response element-binding protein (ChREBP): role of ketone bodies

The carbohydrate response element-binding protein (ChREBP) is a glucose-responsive transcription factor that plays a critical role in converting excess carbohydrate to storage fat in liver. In response to changing glucose levels, ChREBP activity is regulated by nucleo-cytoplasmic shuttling of ChREBP via interactions with 14-3-3 proteins and importins. The nuclear/cytosol trafficking is regulated partly by phosphorylation/dephosphorylation of serine 196 mediated by cAMP-dependent protein kinase and protein phosphatase. We show here that protein-free extracts of starved and high fat-fed livers contain metabolites that activate interaction of ChREBP·14-3-3 and inhibit the ChREBP/importin α interaction, resulting in cytosolic localization. These metabolites were identified as β-hydroxybutyrate and acetoacetate. Nuclear localization of GFP-ChREBP is rapidly inhibited in hepatocytes incubated in β-hydroxybutyrate or fatty acids, and the observed inhibition is closely correlated with the production of ketone bodies. These observations show that ketone bodies play an important role in the regulation of ChREBP activity by restricting ChREBP localization to the cytoplasm, thus inhibiting fat synthesis during periods of ketosis.

Analysis of substrate specificity and kinetics of cyclic nucleotide phosphodiesterases with N'-methylanthraniloyl-substituted purine and pyrimidine 3',5'-cyclic nucleotides by fluorescence spectrometry

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.

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

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

Regulation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channel activity by cCMP

Activation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is facilitated in vivo by direct binding of the second messenger cAMP. This process plays a fundamental role in the fine-tuning of HCN channel activity and is critical for the modulation of cardiac and neuronal rhythmicity. Here, we identify the pyrimidine cyclic nucleotide cCMP as another regulator of HCN channels. We demonstrate that cCMP shifts the activation curves of two members of the HCN channel family, HCN2 and HCN4, to more depolarized voltages. Moreover, cCMP speeds up activation and slows down deactivation kinetics of these channels. The two other members of the HCN channel family, HCN1 and HCN3, are not sensitive to cCMP. The modulatory effect of cCMP is reversible and requires the presence of a functional cyclic nucleotide-binding domain. We determined an EC(50) value of ∼30 μm for cCMP compared with 1 μm for cAMP. Notably, cCMP is a partial agonist of HCN channels, displaying an efficacy of ∼0.6. cCMP increases the frequency of pacemaker potentials from isolated sinoatrial pacemaker cells in the presence of endogenous cAMP concentrations. Electrophysiological recordings indicated that this increase is caused by a depolarizing shift in the activation curve of the native HCN current, which in turn leads to an enhancement of the slope of the diastolic depolarization of sinoatrial node cells. In conclusion, our findings establish cCMP as a gating regulator of HCN channels and indicate that this cyclic nucleotide has to be considered in HCN channel-regulated processes.

Nucleotidyl cyclase activity of soluble guanylyl cyclase α1β1

Soluble guanylyl cyclase (sGC) regulates several important physiological processes by converting GTP into the second-messenger cGMP. sGC has several structural and functional properties in common with adenylyl cyclases (ACs). Recently, we reported that membranous ACs and sGC are potently inhibited by 2',3'-O-(2,4,6-trinitrophenyl)-substituted purine and pyrimidine nucleoside 5'-triphosphates. Using a highly sensitive high-performance liquid chromatography-tandem mass spectrometry method, we report that highly purified recombinant sGC of rat possesses nucleotidyl cyclase activity. As opposed to GTP, ITP, XTP and ATP, the pyrimidine nucleotides UTP and CTP were found to be sGC substrates in the presence of Mn(2+). When Mg(2+) is used, sGC generates cGMP, cAMP, cIMP, and cXMP. In conclusion, soluble "guanylyl" cyclase possesses much broader substrate specificity than previously assumed. Our data have important implications for cyclic nucleotide-mediated signal transduction.

Human cyclic nucleotide phosphodiesterases possess a much broader substrate-specificity than previously appreciated

Phosphodiesterases (PDEs) capable of degrading cAMP and cGMP are indispensable for the regulation of cyclic nucleotide-mediated signals. The existence of other cyclic nucleotides such as cCMP and cUMP has been discussed controversially in the literature. Despite publications on PDEs hydrolyzing cCMP or cUMP, the molecular identity of such enzymes remained elusive. Recently, we have provided evidence for a role of cCMP as second messenger in vascular relaxation and inhibition of platelet aggregation. Using an HPLC-MS based assay, here, we show that human PDEs belonging to various families hydrolyze not only cAMP and cGMP but also other cyclic nucleotides.

Cyclic cytidine 3',5'-monophosphate (cCMP) signals via cGMP kinase I

We analysed the function and intracellular signalling of the cyclic pyrimidinic nucleotide cCMP. The membrane-permeable cCMP analogue dibutyryl-cCMP mediated mouse aorta relaxation. cCMP activated purified cGMP-dependent protein kinase (cGK) Iα and Iβ and stimulated cGK in aorta lysates. cCMP-induced relaxation was abolished in cGKI-knockout tissue. Additionally, deletion of inositol-trisphosphate receptor associated cGKI substrate (IRAG) suppressed cCMP-mediated relaxation. Signalling of cCMP via cGKI/IRAG appears to be of broader physiological importance because cCMP-mediated inhibition of platelet aggregation was absent in cGKI- and IRAG-deficient platelets. These results demonstrate that cCMP acts as intracellular messenger molecule, most unexpectedly utilizing the cGMP signal transduction pathway.

Cytidylyl and uridylyl cyclase activity of bacillus anthracis edema factor and Bordetella pertussis CyaA

Cyclic adenosine 3',5'-monophosphate (cAMP) and cyclic guanosine 3',5'-monophosphate (cGMP) are second messengers for numerous mammalian cell functions. The natural occurrence and synthesis of a third cyclic nucleotide (cNMP), cyclic cytidine 3',5'-monophosphate (cCMP), is a matter of controversy, and almost nothing is known about cyclic uridine 3',5'-monophosphate (cUMP). Bacillus anthracis and Bordetella pertussis secrete the adenylyl cyclase (AC) toxins edema factor (EF) and CyaA, respectively, weakening immune responses and facilitating bacterial proliferation. A cell-permeable cCMP analogue inhibits human neutrophil superoxide production. Here, we report that EF and CyaA also possess cytidylyl cyclase (CC) and uridylyl cyclase (UC) activity. CC and UC activity was determined by a radiometric assay, using [alpha-(32)P]CTP and [alpha-(32)P]UTP as substrates, respectively, and by a high-performance liquid chromatography method. The identity of cNMPs was confirmed by mass spectrometry. On the basis of available crystal structures, we developed a model illustrating conversion of CTP to cCMP by bacterial toxins. In conclusion, we have shown both EF and CyaA have a rather broad substrate specificity and exhibit cytidylyl and uridylyl cyclase activity. Both cCMP and cUMP may contribute to toxin actions.

Differential inhibition of various adenylyl cyclase isoforms and soluble guanylyl cyclase by 2',3'-O-(2,4,6-trinitrophenyl)-substituted nucleoside 5'-triphosphates

Adenylyl cyclases (ACs) catalyze the conversion of ATP into the second messenger cAMP and play a key role in signal transduction. In a recent study (Mol Pharmacol 70:878-886, 2006), we reported that 2',3'-O-(2,4,6-trinitrophenyl)-substituted nucleoside 5'-triphosphates (TNP-NTPs) are potent inhibitors (K(i) values in the 10 nM range) of the purified catalytic subunits VC1 and IIC2 of membranous AC (mAC). The crystal structure of VC1:IIC2 in complex with TNP-ATP revealed that the nucleotide binds to the catalytic site with the TNP-group projecting into a hydrophobic pocket. The aims of this study were to analyze the interaction of TNP-nucleotides with VC1:IIC2 by fluorescence spectroscopy and to analyze inhibition of mAC isoforms, soluble AC (sAC), soluble guanylyl cyclase (sGC), and G-proteins by TNP-nucleotides. Interaction of VC1:IIC2 with TNP-NDPs and TNP-NTPs resulted in large fluorescence increases that were differentially reduced by a water-soluble forskolin analog. TNP-ATP turned out to be the most potent inhibitor for ACV (K(i), 3.7 nM) and sGC (K(i), 7.3 nM). TNP-UTP was identified as the most potent inhibitor for ACI (K(i), 7.1 nM) and ACII (K(i), 24 nM). TNP-NTPs inhibited sAC and GTP hydrolysis by G(s)- and G(i)-proteins only with low potencies. Molecular modeling revealed that TNP-GTP and TNP-ATP interact very similarly, but not identically, with VC1:IIC2. Collectively, our data show that TNP-nucleotides are useful fluorescent probes to monitor conformational changes in VC1:IIC2 and that TNP-NTPs are a promising starting point to develop isoform-selective AC and sGC inhibitors. TNP-ATP is the most potent sGC inhibitor known so far.

Online immobilized metal affinity chromatography/mass spectrometric analysis of changes elicited by cCMP in the murine brain phosphoproteome

An automated online immobilized metal affinity chromatography/high-performance liquid chromatography mass spectrometric (IMAC-HPLC/MS/MS) method was developed to study cytidine 3',5'-cyclic monophosphate (cCMP)-specific protein phosphorylation, analogous to a previously successful offline IMAC method using microvolume IMAC pipette tips. The optimized method identified murine brain phosphoproteins selectively modified by challenge with cCMP, using manual interpretation of the results to confirm both phosphorylation and selectivity of response to cCMP. A number of proteins identified by this strategy have potential roles in hyperproliferation, a previously reported response to elevated levels of cCMP.

Mass spectrometric identification of Rab23 phosphorylation as a response to challenge by cytidine 3',5'-cyclic monophosphate in mouse brain

While the functions and mechanisms of action of adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) are well established and are the basis of the action of a large number of successful pharmaceuticals, the role of a third naturally occurring cyclic nucleotide, cytidine 3',5'-cyclic monophosphate (cCMP), remains to be elucidated. Immobilized metal affinity chromatography (IMAC) was used to selectively extract proteins phosphorylated in mouse brain in response to challenge by cAMP, cGMP and cCMP, followed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-ToFMS) and liquid chromatography/electrospray ionization tandem mass spectrometry (LC/ESI-MS/MS) of tryptic digests to identify Rab23 as the first protein reported to be phosphorylated only in response to cCMP.

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.

Nucleotide profile of mouse liver: response to 2'-O-dibutyryl cytidene 3',5'-cyclic monophosphate

The effect of 2'-O-dibutyryl cytidene 3',5'-cyclic monophosphate (dibutyryl cCMP) on the nucleotide profile of mouse liver was examined. Dibutyryl cCMP caused an increase in the amount of CTP in mouse liver. Perchloric acid extracts of liver tissue were neutralized with tri-N-octylamine in trichlorotriflouroethene and, after removal of CLO(4-), subjected to preliminary purification on a Cu2+-loaded column of Chelex 100. A high-pressure liquid chromatographic anion-exchange procedure was used and gave good resolution of the free nucleotides on a single column.

Kinetic analysis of cyclic CMP-specific and multifunctional phosphodiesterases by quantitative positive-ion fast-atom bombardment mass spectrometry

Two enzymes, cyclic CMP-specific phosphodiesterase and multifunctional phosphodiesterase, are responsible for the hydrolysis of cytidine 3',5'-cyclic monophosphate in living cells. Quantitation of both enzymes has been carried out by positive-ion fast-atom bombardment mass spectrometric analysis of the enzyme incubates after termination of the reaction. The kinetic data obtained are in close agreement with parallel data obtained by the conventional radiometric assay. The extra facility of the mass spectrometry based assay to monitor several incubation components simultaneously has been exploited to study the concurrent hydrolysis of alternate cyclic nucleotide substrates and provides kinetic parameters of significance in interpreting substrate-enzyme interactions. This is extended by the use of collisionally-induced dissociation of the protonated molecules of the liberated products to identify the mononucleotide isomers resulting from the cyclic nucleotide hydrolysis.

Identification of novel nucleotides found in the red seaweed Porphyra umbilicalis

Extracts of the red seaweed Porphyra umbilicalis are a valuable constituent of herbal medicines. The nucleotide complement of such an extract was purified and compounds identified by chromatographic behaviour, UV absorbance/pH profile, base-to-phosphate ratio, acid hydrolysis, and by fast-atom bombardment mass spectrometry with mass-analysed ion kinetic energy spectrum scanning. In addition to eighteen common nucleotides, and two cyclic nucleotides, fifteen novel nucleotides were identified, comprising eight deoxynucleotides and two cyclic deoxynucleotides, ten aminoacylnucleotides and two nucleoside trisphosphates together with 2'-phosphoadenosine-3',5'-cyclic pyrophosphate, 5'-phosphoadenosine-2',3'-cyclic monophosphate and N6,N6-dimethyladenosine-5'-monophosphate. The possible origin and potential actions of these novel compounds are discussed.

Radioimmunoassay of cytidine 3',5'-cyclic monophosphate: unambiguous assay by means of an optimized protocol incorporating a trilayer column separation to obviate cross-reactivity problems

Previous assays for cytidine 3', 5'-cyclic monophosphate (cyclic CMP) have been criticized as being ambiguous. Here a modified RIA protocol, in which the production of assay components has been optimized and a novel trilayer chromatography column separation introduced which successfully separates cyclic CMP from compounds, endogenous to living tissues, which cross-react with anti-cyclic CMP sera, is described. The assay is capable of assaying cyclic CMP between 0.1 and 5 pmol, can be increased in sensitivity by means of an additional acetylation step, and enables the separation of cyclic CMP, cyclic AMP and cyclic GMP so that all three can be estimated in a single sample.

Analysis of cyclic nucleotide-related enzymes by continuous-flow fast-atom bombardment mass spectrometry

Continuous-flow fast-atom bombardment mass spectrometry has been developed to directly monitor cyclic nucleotide (substrate) and its product levels from an on-going phosphodiesterase reaction. Analysis of cAMP and cCMP phosphodiesterase incubates have been performed where the temporal evolution of the enzymic reaction is monitored and the effect of enzyme concentration upon the rate of reaction determined. Quantitative data on the enzyme kinetics have been obtained, in the form of Lineweaver-Burke plots, that are shown to correlate well with well-established radiometric methods.

Differential modulation by N4, 2'-O-dibutyryl cytidine 3':5'-cyclic monophosphate of neutrophil activation

The cyclic pyrimidine nucleotide, cCMP, is an endogenous substance in mammalian cells but little is known on its functional role. We studied the effects of cCMP, its cell-permeant analogue, N4,2'-O-dibutyryl cytidine 3':5'-cyclic monophosphate (Bt2cCMP), and of butyrate on superoxide (O2-) formation and cytosolic Ca2+ [( Ca2+]i) in human neutrophils. Bt2cCMP inhibited O2- formation and the rise in [Ca2+]i induced by a chemotactic peptide at submaximally effective concentrations. O2- formation induced by platelet-activating factor was potentiated by Bt2cCMP, whereas the cyclic nucleotide had no effect on the rise in [Ca2+]i induced by this agonist. Bt2cCMP enhanced O2- formation induced by tau-hexachlorocyclohexane at a submaximally effective concentration. O2- formation stimulated by complement C5a, concanavalin A, NaF, A 23187, phorbol myristate acetate and arachidonic acid was not affected by Bt2cCMP. cCMP was less effective than Bt2cCMP to inhibit fMet-Leu-Phe-induced O2- formation, and butyrate was without effect on any of the functional parameters studied. Our data show that a cell-permeant analogue of cCMP differentially inhibits and potentiates activation of human neutrophils.

Cyclic CMP (cytidine 3',5'-monophosphate) suppresses changes in human sperm amplitude of lateral head displacement and hyperactivation

The dibutyryl analog of cCMP suppressed sperm amplitude of lateral head displacement and hyperactivation. Sperm motility was inhibited by dibutyryl cCMP with a shift toward less linear trajectory sperm movements. The results suggest a role of cCMP as an inhibitory signal on sperm motility patterns related to sperm capacitation.

Dibutyrylcytidine 3',5'-cyclic monophosphate stimulates neurite outgrowth in rat pheochromocytoma PC12

Dibutyrylcytidine 3',5'-cyclic monophosphate (Bt2cCMP) stimulated neurite outgrowth in rat pheochromocytoma PC12 cells in a dose-dependent manner at the dose range from 0.5 mM to 6 mM. About 25% of the cells had neurites in response to 6 mM Bt2cCMP. In contrast with the effect of nerve growth factor (NGF) which increased the percentage of cells with neurites gradually during a 6-day culture period, the stimulating effect of Bt2cCMP reached a plateau 2 days after plating. Staurosporine inhibited the neurite outgrowth induced by NGF, but not that by Bt2cCMP. These results suggest that Bt2cCMP stimulates neuronal differentiation by a mechanism different from that by NGF.

Cytidylate cyclase: development of assay and determination of kinetic properties of a cytidine 3',5'-cyclic monophosphate-synthesizing enzyme

A method is described for the separation of cytidine 3',5'-cyclic monophosphate (cyclic CMP) from cytidine tri-, di- and mono-phosphates and from cytidine 3',5'-cyclic pyrophosphate, cytidine 2'-monophosphate-3',5'-cyclic monophosphate, cytidine 2'-O-aspartyl-3',5'-cyclic monophosphate and cytidine monophosphate, compounds previously shown to be the result of putative cytidylate cyclase activity. This separation, involving elution of a novel bilayer column of QAE-Sephadex and alumina with 0.03 M-HCl, has been incorporated into an assay protocol to determine the enzyme-catalysed conversion of radiolabelled CTP to cyclic CMP. By this assay, cytidylate cyclase activity has been shown to be present in rat lung, spleen, ovary, testes, brain, stomach, liver, heart and kidney preparations; the activity was of a similar order in each tissue and had a sharp pH optimum of 7.0-7.5. The liver preparation had a Vmax. of 1.2 nmol of cyclic CMP formed/min per mg, and a Km of 220 microM-CTP, and although active in the absence of added cations, it was stimulated by Fe2+ and Mn2+ ions. In several of the tissues examined, the cytidylate cyclase activity was inversely proportional to age of the animals.

Cytidylate cyclase activity in mouse tissues: the enzymatic conversion of cytidine 5'-triphosphate to cytidine 3',5'-cyclic monophosphate (cyclic CMP)

Cytidylate cyclase activity, which enzymatically converts cytidine 5'-triphosphate (CTP) to cytidine 3',5'-cyclic monophosphate (cyclic CMP), has been demonstrated in mouse tissue homogenates by use of a highly sensitive enzyme immunoassay (EIA) specific for cyclic CMP. Cyclic CMP formation is dependent on the amount of homogenate and on the incubation time. Although the enzyme activity was detected at wide ranges of pH from 6.8 to 11.5, the maximal activity was observed at around pH 9.4. The optimal temperature was 37 degrees C. Cytidylate cyclase activity was almost completely lost if the homogenates were heated at 90 degrees C for 3 min prior to use. The enzyme reaction exhibited typical Michaelis-Menten kinetics with an apparent Km for CTP of approx. 0.31 mM. Cyclic CMP formation was greatly enhanced with 4 mM Mn2+, Mg2+, Co2+; Mn2+ was the most effective. Fe2+ and Ca2+ were without effect. Cu2+ and Zn2+ at a concentration of 0.1 to 0.5 mM were inhibitory to Mn2+-dependent activity. Moreover, the enzyme activity was inhibited by several nucleotides including ATP, ADP, 5'-AMP, and GTP. Cytidylate cyclase activity was found to be present in all homogenates from a variety of mouse tissues examined except heart, with the highest level found in brain, and the lowest in liver.

Regulation of mouse trophoblast giant cell nucleus development in hatched mouse blastocysts by cyclic cytidine 3',5'-monophosphate (cCMP)

The dibutyryl analog of cCMP enlarged the nuclei of trophoblast giant cells and promoted blastocyst development. The result suggests that cCMP has a trophic effect on embryonic development, specifically by altering the size of the trophoblast cell nucleus but does not enhance trophoblast cell proliferation processes.

Extraction, purification, identification and metabolism of 3',5'-cyclic UMP, 3',5'-cyclic IMP and 3',5'-cyclic dTMP from rat tissues

The large-scale extraction and partial purification of endogenous 3',5'-cyclic UMP, 3',5'-cyclic IMP and 3',5'-cyclic dTMP are described. Rat liver, kidney, heart, spleen and lung tissues were subjected to a sequential purification procedure involving freeze-clamping, perchlorate extraction, alumina and Sephadex ion-exchange chromatography and preparative electrophoresis. The samples thus obtained co-chromatographed with authentic cyclic UMP, cyclic IMP and cyclic dTMP on t.l.c. and h.p.l.c. and the u.v. spectra of the extracted samples were identical with those of the standards. Fast atom bombardment of the three cyclic nucleotide standards yielded mass spectra containing a molecular protonated ion in each case; mass-analysed ion kinetic-energy spectrometry ('m.i.k.e.s') of these ions produced a spectrum unique to the parent cyclic nucleotide. The extracted putative cyclic UMP, cyclic IMP and cyclic dTMP each produced a m.i.k.e.s. identical with that obtained with the corresponding cyclic nucleotide standard. Rat liver, heart, kidney, brain, intestine, spleen, testis and lung protein preparations were each found capable of the synthesis of cyclic UMP, cyclic IMP and cyclic dTMP from the corresponding nucleoside triphosphate, of the hydrolysis of these cyclic nucleotides and of their binding, with the exception that cyclic dTMP was not synthesized by the kidney preparation.

Cyclic CMP phosphodiesterase: isolation, specificity and kinetic properties

Cyclic CMP phosphodiesterase activity was demonstrated in rat liver, heart, brain, kidney, intestine, skeletal muscle, blood, testes, ovaries, spleen and lung; that present in the liver was purified to homogeneity by a sequential process of ammonium sulphate fractionation, gel filtration, two ion-exchange chromatographic steps, preparative electrophoresis and two affinity chromatographic stages with selection at each stage for maximum specificity. The final enzyme preparation was confirmed as a single protein by HPLC and isoelectric focussing; the total yield obtained was 1.5% and the final specific activity of 48.6 mumol cyclic CMP hydrolysed/min/mg reflected a 88,000 fold purification. The phosphodiesterase had a Mr of 2.8 X 10(4), pH optimum 7.2-7.4, isoelectric point between 4.2 and 4.4 and a Km of 9.0 mM cyclic CMP. This enzyme differs from a previously isolated cyclic CMP phosphodiesterase in its amino acid composition and specificity. The absolute specificity for 3',5'-cyclic CMP as substrate distinguishes this cyclic CMP phosphodiesterase from all other reported phosphodiesterases and shows it to be a novel enzyme. Its potential as a research tool and the significance of its occurrence are discussed.

The differentiation of isomeric biological compounds using collision-induced dissociation of ions generated by fast atom bombardment

Though fast atom bombardment ionization makes possible the ionization and molecular weight determination of polar or thermally labile biological compounds, the resulting mass spectra commonly give few or no fragment ions which would allow detailed structural analysis. In particular, isomeric compounds often give identical spectra. Collision-induced dissociation of ions resulting from fast atom bombardment ionization is shown to be a powerful combination which can differentiate isomeric substances. The technique is applied to isomeric bile acid salts and steroid conjugates and is capable of differentiating structural isomers which have similar fast atom bombardment mass spectra. A range of isomeric cyclic nucleotides is also shown to be amenable to the method. Sensitivity limits are examined and the unequivocal identification of two 3',5'-cyclic nucleotides isolated from living systems is demonstrated.