Cyclic nucleotide-dependent protein kinases: intracellular receptors for cAMP and cGMP action

Isoforms of cAMP-dependent protein kinase in the bivalve mollusk Mytilus galloprovincialis: activation by cyclic nucleotides and effect of temperature

Two different isoforms of cAMP-dependent protein kinase (PKA) have been partially purified from the posterior adductor muscle and the mantle tissue of the sea mussel Mytilus galloprovincialis. The holoenzymes contain as regulatory subunit (R) the previously identified isoforms Rmyt1 and Rmyt2, and were named PKAmyt1 and PKAmyt2, respectively. Both cAMP and cGMP can activate these PKA isoforms completely, although they exhibit a sensitivity approximately 100-fold higher for cAMP than for cGMP. When compared to PKAmyt2, the affinity of PKAmyt1 for cAMP and cGMP is 2- and 3.5-fold higher, respectively. The effect of temperature on the protein kinase activity of both PKA isoforms was examined. Temperature changes did not affect significantly the apparent activation constants (Ka) for cAMP. However, the protein kinase activity was clearly modified and a remarkable difference was observed between both PKA isoforms. PKAmyt1 showed a linear Arrhenius plot over the full range of temperature tested, with an activation energy of 15.3+/-1.5 kJ/mol. By contrast, PKAmyt2 showed a distinct break in the Arrhenius plot at 15 degrees C; the activation energy when temperature was above 15 degrees C was 7-fold higher than that of lower temperatures (70.9+/-8.1 kJ/mol vs 10.6+/-6.5 kJ/mol). These data indicate that, above 15 degrees C, PKAmyt2 activity is much more temperature-dependent than that of PKAmyt1. This different behavior would be related to the different role that these isoforms may play in the tissues where they are located.

Regulation of actin-based cell migration by cAMP/PKA

A wide variety of soluble signaling substances utilize the cyclic AMP-dependent protein kinase (PKA) pathway to regulate cellular behaviors including intermediary metabolism, ion channel conductivity, and transcription. A growing literature suggests that integrin-mediated cell adhesion may also utilize PKA to modulate adhesion-associated events such as actin cytoskeletal dynamics and migration. PKA is dynamically regulated by integrin-mediated cell adhesion to extracellular matrix (ECM). Furthermore, while some hallmarks of cell migration and cytoskeletal organization require PKA activity (e.g. activation of Rac and Cdc42; actin filament assembly), others are inhibited by it (e.g. activation of Rho and PAK; interaction of VASP with the c-Abl tyrosine kinase). Also, cell migration and invasion can be impeded by either inhibition or hyper-activation of PKA. Finally, a number of A-kinase anchoring proteins (AKAPs) serve to associate PKA with various components of the actin cytoskeleton, thereby enhancing and/or specifying cAMP/PKA signaling in those regions. This review discusses the growing literature that supports the hypothesis that PKA plays a central role in cytoskeletal regulation and cell migration.

Phosphorylation of septin 3 on Ser-91 by cGMP-dependent protein kinase-I in nerve terminals

The septins are a family of GTPase enzymes required for cytokinesis and play a role in exocytosis. Among the ten vertebrate septins, Sept5 (CDCrel-1) and Sept3 (G-septin) are primarily concentrated in the brain, wherein Sept3 is a substrate for PKG-I (cGMP-dependent protein kinase-I) in nerve terminals. There are two motifs for potential PKG-I phosphorylation in Sept3, Thr-55 and Ser-91, but phosphoamino acid analysis revealed that the primary site is a serine. Derivatization of phosphoserine to S-propylcysteine followed by N-terminal sequence analysis revealed Ser-91 as a major phosphorylation site. Tandem MS revealed a single phosphorylation site at Ser-91. Substitution of Ser-91 with Ala in a synthetic peptide abolished phosphorylation. Mutation of Ser-91 to Ala in recombinant Sept3 also abolished PKG phosphorylation, confirming that Ser-91 is the major site in vitro. Antibodies raised against a peptide containing phospho-Ser-91 detected phospho-Sept3 only in the cytosol of nerve terminals, whereas Sept3 was located in a peripheral membrane extract. Therefore Sept3 is phosphorylated on Ser-91 in nerve terminals and its phosphorylation may contribute to the regulation of its subcellular localization in neurons.

Minireview: PRKAR1A: normal and abnormal functions

The type 1alpha regulatory subunit (RIalpha) of cAMP-dependent protein kinase (PKA) (coded by the PRKAR1A gene) is the main component of type I PKA, which regulates most of the serine-threonine kinase activity catalyzed by the PKA holoenzyme in response to cAMP. Carney complex (CNC), or the complex of spotty skin pigmentation, myxomas, and endocrine overactivity, is a multiple endocrine (and not only) neoplasia syndrome that is due to PRKAR1A-inactivating mutations. The R1alpha protein and PRKAR1A mRNA have been found to be up-regulated in a series of cell lines and human and rodent neoplasms, suggesting this molecule's involvement in tumorigenesis and its potential role in cell cycle regulation, growth, and/or proliferation. Alterations in PKA activity elicit a variety of effects depending on the tissue, developmental stage, degree of differentiation, and cAMP levels. In addition, RIalpha may have functions independent of PKA. The presence of inactivating germline mutations and the loss of its wild-type allele in some CNC lesions indicate that PRKAR1A might function as a tumor suppressor gene in these tissues, but could PRKAR1A be a classic tumor suppressor gene? Probably not, and this review explains why.

Synergistic activation of insect cAMP-dependent protein kinase A (type II) by cyclicAMP and cyclicGMP

The high cGMP sensitivity of cAMP-dependent protein kinase A (type II) (PKAII) from invertebrates led to the hypothesis that cGMP directly activates PKAII under physiological conditions. We tested this idea using PKAII holoenzyme purified from the honeybee brain in an assay with short stimulation times. In the presence of very low cAMP concentrations, we found a synergistic increase in PKAII activation by physiological cGMP concentrations. Cloning honeybee regulatory subunit RII and phylogenetic comparison of the two cyclic nucleotide-binding sites of RII reveal a high relation of domain A of insect RII with cGMP-binding domains of cGMP-dependent protein kinases.

Severe impairment of growth and differentiation in a Neurospora crassa mutant lacking all heterotrimeric G alpha proteins

Heterotrimeric G alpha proteins play a critical role in regulating growth and differentiation in filamentous fungi. No systematic analysis of functional relationships between subunits has been investigated. This study explores the relative contributions of Neurospora crassa G alpha subunits, gna-1, gna-2, and gna-3, in directing development by analyzing strains deleted for various combinations of these genes. Although viable, mutants lacking all G alpha subunits or gna-1 and gna-3 are severely restricted in apical growth, forming small colonies. These strains form little aerial hyphae during asexual development on solid medium and exhibit inappropriate sporulation in submerged cultures. Similar to all strains carrying the Delta gna-1 mutation, these mutants are female sterile. Defects attributed to gna-2 are observed only in conjunction with the loss of gna-1 or gna-3, suggesting a minor role for this G alpha in N. crassa biology. Results from analysis of adenylyl cyclase and epistatic studies with the cAMP-dependent protein kinase regulatory subunit (mcb) indicate separate functions for GNA-1 and GNA-3 in cAMP metabolism and additional cAMP-independent roles for GNA-1. These studies indicate that although G alpha subunits are not essential for viability in filamentous fungi, their loss results in an organism that cannot effectively forage for nutrients or undergo asexual or sexual reproduction.

Probing noncovalent protein-ligand interactions of the cGMP-dependent protein kinase using electrospray ionization time of flight mass spectrometry

Nanoflow electrospray ionization time of flight mass spectrometry (ESI-TOF-MS) was used to study activation properties of the cGMP-dependent protein kinase (PKG). Our nanoflow ESI-TOF-MS analysis confirms that PKG mainly occurs as a 153 kDa homodimer and is able to bind four cGMP molecules, which is in agreement with the known stoichiometry. Binding order and stoichiometry of cGMP, the non-hydrolysable ATP analog beta,gamma-imidoadenosine 5'-triphosphate (AMPPNP) and Mn2+ for PKG were characterized as model for the active PKG-cGMP-ATP/Mg2+ complex. Already in the absence of cGMP, a noncovalent complex between PKG and two molecules of AMPPNP could be observed by ESI-TOF-MS. Binding of AMPPNP to PKG was strongly enhanced by the addition of MnCl2 to the spray solution. This is in agreement with binding of AMPPNP/Mn2+ in the ATP binding pocket of PKG since all protein kinases require a metal ion to accompany ATP in the ATP-binding pocket for proper positioning of the beta and gamma phosphates. Additionally, this finding could imply that within the inactive conformation of PKG, the autoinhibition-domain, when in contact with the substrate-docking domain, does not block the entrance to the ATP-binding site. In the presence of cGMP, less of the fully saturated PKG-(cGMP)4(AMPPNP/Mn2+)2 complex was observed, suggesting that the PKG-ATP interaction is weakened in the active conformation of PKG. Additionally, limited proteolysis in combination with native-ESI MS showed to be a useful tool to study the contact regions on the PKG-dimer and also allowed the rapid determination of the overall autophosphorylation status of the protein. These measurements indicated that autophosphorylation mainly occurs within the first 80 aminoterminal residues and involves in total 3-4 phosphates per subunit.

Protein kinase A dependent signalling mediates anti-apoptotic effects of the atrial natriuretic peptide in ischemic livers

BACKGROUND/AIMS

Preconditioning of livers with atrial natriuretic peptide (ANP) attenuates ischemia-reperfusion injury (IRI) via the particulate guanylate cyclase. Recently, we have shown that ANP affects the p38 MAPK signalling cascade in the liver. Thus, aim of the present study was to elucidate the role of cGMP- and p38 MAPK-dependent signalling pathways in ANP-mediated anti-apoptotic effects.

METHODS

Rat livers were perfused with KH-buffer with or without ANP, 8-Br-cGMP (+/-kinase inhibitors) and kept in UW solution (4 degrees C, 24h). Caspase-3-like activity was measured by a fluorometric assay. Expression of cGMP-dependent protein kinases (PKG) in liver tissue was determined by RT-PCR, BAD phosphorylation by Western blot, and cAMP-dependent protein kinase (protein kinase A, PKA) activity by in vitro phosphorylation.

RESULTS

Compared to control organs, ANP-preconditioning reduced post-ischemic caspase-3-like activity. Neither perfusion with a p38 MAPK inhibitor nor with a PKG inhibitor abolished the ANP-mediated anti-apoptotic action. The two PKG isoforms were demonstrated not to be expressed in the liver. In contrast, liver perfusion with a selective PKA inhibitor abrogated the anti-apoptotic effect of ANP. Phosphorylation of pro-apoptotic BAD by ANP-activated PKA might inhibit liver cell apoptosis.

CONCLUSIONS

ANP mediates its anti-apoptotic action during ischemic injury via a crosstalk with the PKA pathway.

Crystal structures of RIalpha subunit of cyclic adenosine 5'-monophosphate (cAMP)-dependent protein kinase complexed with (Rp)-adenosine 3',5'-cyclic monophosphothioate and (Sp)-adenosine 3',5'-cyclic monophosphothioate, the phosphothioate analogues of cAMP

Cyclic adenosine 5'-monophosphate (cAMP) is an ancient signaling molecule, and in vertebrates, a primary target for cAMP is cAMP-dependent protein kinase (PKA). (R(p))-adenosine 3',5'-cyclic monophosphothioate ((R(p))-cAMPS) and its analogues are the only known competitive inhibitors and antagonists for cAMP activation of PKA, while (S(p))-adenosine 3',5'-cyclic monophosphothioate ((S(p))-cAMPS) functions as an agonist. The crystal structures of a Delta(1-91) deletion mutant of the RIalpha regulatory subunit of PKA bound to (R(p))-cAMPS and (S(p))-cAMPS were determined at 2.4 and 2.3 A resolution, respectively. While the structures are similar to each other and to the crystal structure of RIalpha bound to cAMP, differences in the dynamical properties of the protein when (R(p))-cAMPS is bound are apparent. The structures highlight the critical importance of the exocyclic oxygen's interaction with the invariant arginine in the phosphate binding cassette (PBC) and the importance of this interaction for the dynamical properties of the interactions that radiate out from the PBC. The conformations of the phosphate binding cassettes containing two invariant arginine residues (Arg209 on domain A, and Arg333 on domain B) are somewhat different due to the sulfur interacting with this arginine. Furthermore, the B-site ligand together with the entire domain B show significant differences in their overall dynamic properties in the crystal structure of Delta(1-91) RIalpha complexed with (R(p))-cAMPS phosphothioate analogue ((R(p))-RIalpha) compared to the cAMP- and (S(p))-cAMPS-bound type I and II regulatory subunits, based on the temperature factors. In all structures, two structural solvent molecules exist within the A-site ligand binding pocket; both mediate water-bridged interactions between the ligand and the protein. No structured waters are in the B-site pocket. Owing to the higher resolution data, the N-terminal segment (109-117) of the RIalpha subunit can also be traced. This strand forms an intermolecular antiparallel beta-sheet with the same strand in an adjacent molecule and implies that the RIalpha subunit can form a weak homodimer even in the absence of its dimerization domain.

Downregulation of cGMP-dependent protein kinase-1 activity in the corpus cavernosum smooth muscle of diabetic rabbits

Increased guanosine 3',5'-cyclic monophosphate (cGMP), induced by nitric oxide release, is crucial for corpus cavernosum smooth muscle (CCSM) relaxation within the penis. This CCSM relaxation (necessary for penile erection) is impaired in men with erectile dysfunction (ED), especially those men with diabetes. One of the effector proteins for cGMP is cGMP-dependent protein kinase-1 (PKG-1). PKG-1 knockout mice exhibit detrusor overactivity (Am J Physiol Regul Integr Comp Physiol 279: R1112-R1120, 2000) and, more relevant to this study, ED (Proc Natl Acad Sci USA 97: 2349-2354, 2000), suggesting an in vivo role for PKG-1 in urogenital smooth muscle relaxation. In the current study, using normal rabbit CCSM, Western blot analysis revealed high expression of PKG-1 at levels almost equivalent to aorta (previously shown to have high PKG-1 expression) and that the two known alternatively spliced isoforms of PKG-1 (alpha and beta) are expressed in nearly equal amounts in the CCSM. However, in response to alloxan-induced diabetes, there was a decrease in expression of both PKG-1 isoforms at the mRNA and protein levels as determined by real-time RT-PCR and Western blotting, respectively, but with the PKG-1alpha isoform expression decreased to a greater extent. Moreover, diabetes was associated with significantly decreased PKG-1 activity of CCSM in vitro, correlating with decreased CCSM relaxation. Immunofluorescence microscopy revealed a diabetes-associated decrease in PKG-1 in the CCSM cells. In conclusion, our results demonstrate for the first time a significant downregulation of PKG-1 expression associated with decreased PKG-1 activity in the CCSM in response to diabetes. Furthermore, these results suggest a mechanistic basis for the decreased efficacy of phosphodiesterase V inhibitors in treating diabetic patients with ED.

Down-Regulation of IL-2 Production in T Lymphocytes by Phosphorylated Protein Kinase A-RIIβ

The beta isoform of the type II regulatory subunit (RIIbeta) of protein kinase A suppresses CREB transcriptional activity and c-Fos production in T cells following activation via the TCR. Because CREB is an integral nuclear transcription factor for IL-2 production by T cells, we tested the hypothesis that RIIbeta down-regulates IL-2 expression and IL-2 production in T cells. Stable transfection of RIIbeta in Jurkat T cells led to an approximately 90% reduction in IL-2 mRNA and IL-2 protein following T cell activation. The inhibition of IL-2 production was associated with phosphorylation of the RIIbeta subunit at serine 114 (pRIIbeta) and localization of pRIIbeta in intranuclear clusters. A serine 114 phosphorylation-defective mutant, RIIbeta(S114A), did not form these intranuclear clusters as well as wild-type RIIbeta, and did not inhibit IL-2 mRNA and protein synthesis, indicating that serine 114 phosphorylation is required for both nuclear localization and down-regulation of IL-2 production by RIIbeta. In contrast to its effect on IL-2, RIIbeta induced constitutive up-regulation of CD154 mRNA and cell surface expression. Thus, pRIIbeta differentially regulates gene expression following T cell activation. Unexpectedly, we also found that stable overexpression of another protein kinase A regulatory subunit, RIalpha, had the opposite effect on IL-2 expression, causing a 3- to 4-fold increase in IL-2 production following stimulation. In summary, our data demonstrate a novel mechanism by which serine 114 phosphorylation and nuclear localization of RIIbeta controls the regulation of gene expression in T cells.

cAMP protects neutrophils against TNF-alpha-induced apoptosis by activation of cAMP-dependent protein kinase, independently of exchange protein directly activated by cAMP (Epac)

It is unclear by which receptor cyclic adenosine monophosphate (cAMP) acts to promote neutrophil survival. We found that 8-(4-chlorophenylthio)-2'-O-methyl-cAMP, a specific activator of the recently discovered cAMP receptor, cAMP-regulated guanosine 5'-triphosphate exchange protein directly activated by cAMP, failed to protect human neutrophils from cell death. In contrast, specific activators of cAMP-dependent protein kinase type I (cA-PKI) could protect against death receptor [tumor necrosis factor receptor 1 (TNFR-1), Fas]-mediated apoptosis as well as cycloheximide-accelerated "spontaneous" apoptosis. A novel "caged" cA-PK-activating analog, 8-bromo (8-Br)-acetoxymethyl-cAMP, was more than 20-fold more potent than 8-Br-cAMP to protect neutrophils challenged with TNF-alpha against apoptosis. This analog acted more rapidly than forskolin (which increases the endogenous cAMP production) and allowed us to demonstrate that cA-PK must be activated during the first 10 min after TNF-alpha challenge to protect against apoptosis. The protective effect was mediated solely through cA-PK activation, as it was abolished by the cA-PKI-directed inhibitor Rp-8-Br-cAMPS and the general cA-PK inhibitor H-89. Neutrophils not stimulated by cAMP-elevating agents showed increased apoptosis when exposed to the cA-PK inhibitors Rp-8-Br-cAMPS and H-89, suggesting that even moderate activation of cA-PK is sufficient to enhance neutrophil longevity and thereby contribute to neutrophil accumulation in chronic inflammation.

Adenovirus-mediated delivery and expression of a cAMP-dependent protein kinase inhibitor gene to BEAS-2B epithelial cells abolishes the anti-inflammatory effects of rolipram, salbutamol, and prostaglandin E2: a comparison with H-89

cAMP-elevating drugs are thought to mediate their biological effects by activating the cAMP/cAMP-dependent protein kinase (PKA) cascade. However, this hypothesis is difficult to confirm due to a lack of selective inhibitors. Here, we have probed the role of PKA in mediating inhibitory effects of several cAMP-elevating drugs in BEAS-2B epithelial cells using an adenovirus vector encoding a PKA inhibitor protein (PKIalpha) and have compared it to H-89, a commonly used small molecule PKA inhibitor. Initial studies established efficient gene transfer and confirmed functionality of PKIalpha 48 h after virus infection. All cAMP-elevating drugs tested promoted the phosphorylation of cAMP response element-binding protein (CREB), activated a cAMP response element (CRE)-driven luciferase reporter gene, and suppressed both granulocyte/macrophage colony-stimulating factor (GM-CSF) generation and [(3)H]arachidonic acid (AA) release in response to interleukin-1beta and monocyte chemotactic protein (MCP)-1, respectively. These effects were abolished by PKIalpha. In contrast, H-89 behaved unpredictably under the same conditions. Thus, although CREB phosphorylation evoked by a range of cAMP-elevating drugs was abolished by H-89, neither activation of the CRE-dependent luciferase reporter gene construct nor the inhibition of GM-CSF generation was inhibited. Paradoxically, H-89 antagonized MCP-1-induced [(3)H]AA release and enhanced the inhibitory effect of submaximal concentrations of rolipram and 8-bromo-cAMP. We suggest that expression of PKIalpha in susceptible cells provides a simple and unambiguous way to assess the role of PKA in cAMP signaling and to probe the mechanism of action of other drugs and cAMP-dependent responses where the participation of PKA is equivocal. Furthermore, these data suggest that H-89 is not a selective inhibitor of PKA and should be avoided.

Kinetics of a cellular nitric oxide/cGMP/phosphodiesterase-5 pathway

Rat platelets served as a model to evaluate quantitatively how guanylate cyclase (GC)-coupled nitric oxide (NO) receptors and phosphodiesterases (here phosphodiesterase-5) interact to transduce NO signals in cells. The platelets expressed mRNA only for the alpha(1) and beta(1) GC-coupled receptor subunits. In intact platelets, the potency of NO for elevating cGMP (EC(50) = 10 nm) was lower than in lysed platelets (EC(50) = 1.7 nm). The limiting activities of GC and phosphodiesterase in intact platelets were both very high, being equivalent to about 100 microm/s. With low phosphodiesterase activity (imposed by 100 microm sildenafil), the cGMP response over time was hyperbolic in shape for a range of NO concentrations or GC activities due to GC desensitization. Without a phosphodiesterase inhibitor, NO generated only brief cGMP transients, peaking after 2-5 s but amounting maximally to about 150 microm cGMP. The transients were caused partly by GC desensitization, which varied in rate (half-time up to 3 s) and extent (up to 80%) depending on the NO concentration, and partly by an enhancement of the phosphodiesterase catalytic activity with time, which was deduced to be up to 30-fold and to occur with a half-time of up to 5 s. The results were simulated by a quantitative model, which also explains the varied shapes of cGMP responses to NO found in other cells. Downstream phosphorylation in platelets was detectable within 2 s, and, with continuous exposure (1 min), this pathway could be engaged by subnanomolar NO concentrations (EC(50) = 0.5 nm).

Drug-induced decrease of protein kinase a activity reveals alteration in BDNF expression of bipolar affective disorder

Bipolar affective disorder (BAD) is a severe disease whose molecular and cellular bases are not well known. The aim of the present study was to probe the cAMP signaling downstream targets by pharmacologically manipulating the protein kinase A (PKA) enzyme, along with the assessment of brain-derived neurotrophic factor (BDNF) expression in lymphoblasts. The time course of lymphoblast PKA activity (up to 72 h) revealed optimal activity at 24 h. Then, the enzyme activity and protein levels of PKA Calpha subunit and phopsho-cAMP responsive element binding (CREB) were assayed in lymphoblasts derived from 12 BAD and 12 control (CT) subjects and cultured for 24 h in the presence of cAMP analog drugs. The results indicated that basal PKA activity and PKA Calpha subunit immunolabeling are increased in cells from BAD compared with controls. Enzyme activity was increased by Sp-isomer in BAD and in CT's cells, without change in protein levels. In contrast, the Rp-isomer decreased enzyme activity and protein levels. In drug-naive conditions, there was no change in BDNF expression of BAD cells compared with CT cells. Treatment with Sp-isomer induced increased BDNF in both groups, while treatment with Rp-isomer induced a significant decrease in BDNF expression of BAD compared with CT. The p-CREB changes followed changes in BDNF levels, with increased and decreased Sp-isomer and Rp-isomer treatment, respectively. Our results suggest that mood disorder is associated with PKA upregulation and this could mask alteration in BDNF expression, because slowing down of PKA signaling results in a decrease of BDNF expression. These findings, combined with previous reports, provide a new insight to explain pharmacological features in different diagnostic groups.

Molecular mechanisms and pharmacokinetics of phosphodiesterase-5 antagonists

The prominence of phosphodiesterase-5 (PDE-5) inhibitors in the treatment of male erectile dysfunction and other diseases related to vascular dysfunction mandates a comprehensive understanding of the properties and effects of these compounds. Three potent and selective PDE-5 inhibitors (sildenafil, tadalafil, and vardenafil) have been approved for clinical use. The clinical efficacy and safety profiles of these medications are related to their molecular mode of action, the selectivity for PDE-5, and the pharmacokinetic properties (absorption, bioavailability, time to onset of action, distribution, metabolism, and elimination). These PDE-5 inhibitors share some common properties with regard to mechanisms of action and selectivities for PDE-5. They also have distinctive characteristics that may impact their clinical use. This article focuses on the basic biochemistry of cyclic guanosine monophosphate signaling and the pharmacokinetic parameters that describe characteristics of drug action of these PDE-5 inhibitors in facilitating smooth muscle relaxation, leading to improved penile erectile response or causing side effects.

Protein kinase A regulatory subunit type II beta directly interacts with and suppresses CREB transcriptional activity in activated T cells

Levels of the type IIbeta regulatory subunit (RIIbeta) of protein kinase A are abnormally high in the nuclei of T cells of some subjects with the autoimmune disorder systemic lupus erythematosus (SLE). However, the role of nuclear RIIbeta in the regulation of T cell function is unknown. Based on previous studies demonstrating that nuclear protein kinase A-RII subunits can modify cAMP response element (CRE)-dependent transcription, we tested the hypothesis that nuclear RIIbeta can alter CRE-directed gene expression in T cells through interaction with the nuclear transcription factor CRE-binding protein CREB. To test this hypothesis, we used the RIIbeta-deficient S49 and the Jurkat T cell lines. In both cell lines, transient transfection of RIIbeta resulted in nuclear localization of a portion of the ectopically expressed RIIbeta. In vitro and in vivo analyses revealed a novel, specific interaction between RIIbeta and CREB that mapped to the N-terminal 135 aa of RIIbeta. In functional studies, RIIbeta inhibited the transcriptional activity of a GAL4-CREB fusion protein by 67% in Jurkat T cells following activation with anti-CD3 and anti-CD28 mAbs. Importantly, deletion of the CREB-binding region of RIIbeta completely abrogated inhibition. Additionally, RIIbeta suppressed CRE-directed reporter gene expression and substantially reduced induction of promoter activity and endogenous protein levels of the CREB-dependent gene, c-fos, in activated T cells. We conclude that nuclear RIIbeta can act as a repressor of CREB transcriptional activity in T cells, providing a potential functional significance for aberrant levels of nuclear RIIbeta in systemic lupus erythematosus T cells.

Dimerization of cGMP-dependent protein kinase Ibeta is mediated by an extensive amino-terminal leucine zipper motif, and dimerization modulates enzyme function

All mammalian cGMP-dependent protein kinases (PKGs) are dimeric. Dimerization of PKGs involves sequences located near the amino termini, which contain a conserved, extended leucine zipper motif. In PKG Ibeta this includes eight Leu/Ile heptad repeats, and in the present study, deletion and site-directed mutagenesis have been used to systematically delete these repeats or substitute individual Leu/Ile. The enzymatic properties and quaternary structures of these purified PKG mutants have been determined. All had specific enzyme activities comparable to wild type PKG. Simultaneous substitution of alanine at four or more of the Leu/Ile heptad repeats ((L3A/L10A/L17A/I24A), (L31A/I38A/L45A/I52A), (L17A/I24A/L31A/I38A/L45A/I52A), and (L3A/L10A/L45A/I52A)) of the motif produces a monomeric PKG Ibeta. Mutation of two Leu/Ile heptad repeats can produce either a dimeric (L3A/L10A) or monomeric (L17A/I24A and L31A/I38A) PKG. Point mutation of Leu-17 or Ile-24 (L17A or I24A) does not disrupt dimerization. These results suggest that all eight Leu/Ile heptad repeats are involved in dimerization of PKG Ibeta. Six of the eight repeats are sufficient to mediate dimerization, but substitutions at some positions (Leu-17, Ile-24, Leu-31, and Ile-38) appear to have greater impact than others on dimerization. The Ka of cGMP for activation of monomeric mutants (PKG Ibeta (delta1-52) and PKG Ibeta L17A/I24A/L31A/I38A/L45A/I52A) is 2- to 3-fold greater than that for wild type dimeric PKG Ibeta, and there is a corresponding 2- to 3-fold increase in cGMP-dissociation rate of the high affinity cGMP-binding site (site A) of these monomers. These results indicate that dimerization increases sensitivity for cGMP activation of the enzyme.

Regulation of hippocampal synaptic plasticity by cyclic AMP-dependent protein kinases

Protein kinases critically regulate synaptic plasticity in the mammalian hippocampus. Cyclic-AMP dependent protein kinase (PKA) is a serine-threonine kinase that has been strongly implicated in the expression of specific forms of long-term potentiation (LTP), long-term depression (LTD), and hippocampal long-term memory. We review the roles of PKA in activity-dependent forms of hippocampal synaptic plasticity by highlighting particular themes that have emerged in ongoing research. These include the participation of distinct isoforms of PKA in specific types of synaptic plasticity, modification of the PKA-dependence of LTP by multiple factors such as distinct patterns of imposed activity, environmental enrichment, and genetic manipulation of signalling molecules, and presynaptic versus postsynaptic mechanisms for PKA-dependent LTP. We also discuss many of the substrates that have been implicated as targets for PKA's actions in hippocampal synaptic plasticity, including CREB, protein phosphatases, and glutamatergic receptors. Future prospects for shedding light on the roles of PKA are also described from the perspective of specific aspects of synaptic physiology and brain function that are ripe for investigation using incisive genetic, cell biological, and electrophysiological approaches.

The Roles of Cyclic Adenosine Monophosphate- and Cyclic Guanosine Monophosphate-Dependent Protein Kinase Pathways in Hydrogen Peroxide-Induced Contractility of Microvascular Lung Pericytes

BACKGROUND

Sepsis and posttraumatic inflammatory processes are accompanied by definite changes in microvascular permeability, particularly in the lung. These permeability changes may occur because of damaged regulatory mechanisms at the level of the capillary wall. Pericytes are adventitial cells located within the basement membrane of capillaries. These cells contain multiple cytoplasmic processes that envelope endothelial cells, and are consequently thought to stabilize capillary walls and participate in microcirculation and endothelial cell permeability. Data from this laboratory and other laboratories have confirmed that pericytes are contractile cells, adding to the evidence that pericytes may influence or help regulate capillary permeability. We have already determined that hydrogen peroxide (H2O2) causes dose-dependent relaxation in microvascular lung pericytes (MLPs) at 10 minutes and, conversely, dose-dependent contraction at 30 minutes. It is the aim of this study to determine the mechanism of this biphasic contractile response. Specifically, we will determine whether cyclic adenosine monophosphate (cAMP)- or cyclic guanosine monophosphate (cGMP)-dependent protein kinase intracellular pathways are responsible for the hydrogen peroxide-induced contractility of MLPs.

METHODS

Rat MLPs were isolated by previously published protocol and cultured on collagen gel matrices. MLPs were pretreated with either ODQ, a soluble guanylate cyclase inhibitor (100 mumol/L), for 15 minutes; GKIP, a protein kinase G inhibitor (100 mumol/L), for 1 hour; SQ22536, an adenylate cyclase inhibitor (100 mumol/L), for 15 minutes; or H89, a protein kinase A inhibitor (10 mumol/L), for 1 hour. Hydrogen peroxide was then introduced to each MLP culture at 10 mumol/L, 100 mumol/L, and 1 mmol/L. After each of these treatments, the surface area of the collagen gels was digitally quantified at 10 and 30 minutes.

RESULTS

SQ22536 attenuated both relaxation at 10 minutes and the contraction seen at 30 minutes for all concentrations of H2O2. H89 caused a marked basal relaxation and prevented the cells from contracting at 30-minute exposures to all concentrations of H2O2. Both ODQ and GKIP attenuated the relaxation at 10 minutes but had no affect on the later contraction.

CONCLUSION

The cGMP-dependent protein kinase pathway is a mechanism for H2O2-induced relaxation of MLPs. Up-regulation of cAMP and cGMP is responsible for early H2O2-induced relaxation and late contraction. Protein kinase A (cAMP-dependent protein kinase pathway) may be an important intracellular signaling protein in the H2O2-induced contraction of MLPs or may be unable to down-regulate cAMP once inhibited. This evidence further supports the concept that there are separate intracellular pathways that regulate divergent cellular responses. This idea parallels the clinical concept of reversible and irreversible dysfunction of cellular processes in shock, and that the cellular dysfunction is initiated by separate intracellular pathways.

Biological chemistry of reactive oxygen and nitrogen and radiation-induced signal transduction mechanisms

In the past few years, nuclear DNA damage-sensing mechanisms activated by ionizing radiation have been identified, including ATM/ATR and the DNA-dependent protein kinase. Less is known about sensing mechanisms for cytoplasmic ionization events and how these events influence nuclear processes. Several studies have demonstrated the importance of cytoplasmic signaling pathways in cytoprotection and mutagenesis. For cytoplasmic signaling, radiation-stimulated reactive oxygen species (ROS) and reactive nitrogen species (RNS) are essential activators of these pathways. This review summarizes recent studies on the chemistry of radiation-induced ROS/RNS generation and emphasizes interactions between ROS and RNS and the relative roles of cellular ROS/RNS generators as amplifiers of the initial ionization events. Cellular mechanisms for regulating ROS/RNS levels are discussed. The mechanisms by which cells sense ROS/RNS are examined in terms of how ROS/RNS modify protein structure and function, for example, interactions with metal-thiol clusters, protein tyrosine nitration, protein cysteine oxidation, S-thiolation and S-nitrosylation. We propose that radiation-induced ROS are the initiators and that nitric oxide (NO*) or derivatives are the effectors activating these signal transduction pathways. In responding to cellular ionization events, the cell converts an oxidative signal to a nitrosative one because ROS are too reactive and unspecific in their reactions for regulatory purposes and the cell is equipped to precisely modulate NO* levels.

Ezrin distribution is abnormal in principal cells from a murine model of autosomal recessive polycystic kidney disease

Abnormalities in cell proliferation and intracellular signaling are features of inherited human and murine polycystic kidney diseases (PKD), regardless of the primary genetic defects. Loss of protein kinase A regulation of cell proliferation has been reported in the murine C57BL/6JCys1cpk-/- (cpk) model of autosomal recessive PKD. Qualitative differences in protein kinase A subunit distribution were observed between filter-grown cultures of noncystic- (C57BL/6J mice) and cystic cpk-derived principal cells. It was hypothesized that protein kinase A subunit distribution differences were mediated by differences in A-kinase anchoring protein (AKAP) expression, so expression of four AKAPs was examined in filter-grown cultures of primary murine cystic- and noncystic-derived principal cells. AKAP-KL expression was ambiguous, but mAKAP, AKAP95, and ezrin were expressed at expected molecular sizes and cellular locations in noncystic-derived cells. Perinuclear mAKAP and nuclear AKAP95 were distributed normally in cpk-derived cells. Expression of AKAP95 in cystic epithelium was diminished relative to controls, and ezrin expression was modestly decreased and abnormally distributed within a region near the apical surface. Qualitative differences were observed in ezrin location in response to medium change or stimulation with epidermal growth factor which suggested cell-specific differences may result from the cpk mutation or the abnormal epidermal growth factor receptor phenotype that characterizes PKD. Ezrin has been implicated in tubulogenesis, so altered ezrin expression or function could be disruptive. If PKD mutations that contribute to PKD pathogenesis are postulated to disrupt normal tubular development, perhaps the mechanism includes altered ezrin function and abnormal protein kinase A targeting.

Function of gastrointestinal smooth muscle: from signaling to contractile proteins

The action of smooth muscle in the intestinal wall produces tonic contractions that maintain organ dimension against an imposed load such as a bolus of food, as well as forceful contractions that produce muscle shortening to propel the bolus along the gastrointestinal tract. These functions are regulated by intrinsic electrical and mechanical properties of smooth muscle. The complex signaling process that underlies these functions is discussed in this article. We propose a model that describes the facilitation of sustained contraction of smooth muscle cells in the gut.

Autophosphorylation of cGMP-dependent protein kinase type II

Cyclic nucleotides are shown to stimulate the autophosphorylation of type II cGMP-dependent protein kinase (cGK) on multiple sites. Mass spectrometric based analyses, using a quadrupole time-of-flight-mass spectrometry instrument revealed that cGMP stimulated the in vitro phosphorylation of residues Ser110 and Ser114, and, at a slow rate, of Ser126 and Thr109 or Ser117, all located in the autoinhibitory region. In addition Ser445 was found to be phosphorylated in a cGMP-dependent manner, whereas Ser110 and Ser97 were already prephosphorylated to a large extent in Sf9 cells. cGMP-dependent phosphorylation of cGK II was also demonstrated in intact COS-1 cells and intestinal epithelium. Substitution of most of the potentially autophosphorylated residues for alanines largely abolished the cGMP stimulation of the autophosphorylation. Prolonged autophosphorylation of purified recombinant cGK II in vitro resulted in a 40-50% increase in basal kinase activity, but its maximal cGMP-stimulated activity and the EC50 for cGMP remained unaltered. Mutation of the major phosphorylatable serines 110, 114, and 445 into "phosphorylation-mimicking" glutamates had no effect on the kinetic parameters of cGK II. However, replacing the slowly autophosphorylated residue Ser126 by Glu rendered cGK II constitutively active. These results show that the fast phase of cyclic nucleotide-stimulated autophosphorylation of cGK II has a relatively small feed forward effect on its activity, whereas the secondary phase, presumably involving Ser126 phosphorylation, may generate a constitutively active form of the enzyme.

Characterization of a type I regulatory subunit of cAMP-dependent protein kinase from the bivalve mollusk Mytilus galloprovincialis

Two isoforms of the regulatory subunit (R) of cAMP-dependent protein kinase (PKA), named R(myt1) and R(myt2), had been purified in our laboratory from two different tissues of the sea mussel Mytilus galloprovincialis. In this paper, we report the sequences of several peptides obtained from tryptic digestion of R(myt1). As a whole, these sequences showed high homology with regions of type I R subunits from invertebrate and also from mammalian sources, but homology with those of fungal and type II R subunits was much lower, which indicates that R(myt1) can be considered as a type I R isoform. This conclusion is also supported by the following biochemical properties: (1) R(myt1) was proved to have interchain disulfide bonds stabilizing its dimeric structure; (2) it failed to be phosphorylated by the catalytic (C) subunit purified from mussel; (3) it has a higher pI value than that of the R(myt2) isoform; and (4) it showed cross-reactivity with mammalian anti-RIbeta antibody.

A STAT-regulated, stress-induced signalling pathway in Dictyostelium

The Dictyostelium stalk cell inducer differentiation-inducing factor (DIF) directs tyrosine phosphorylation and nuclear accumulation of the STAT (signal transducer and activator of transcription) protein Dd-STATc. We show that hyperosmotic stress, heat shock and oxidative stress also activate Dd-STATc. Hyperosmotic stress is known to elevate intracellular cGMP and cAMP levels, and the membrane-permeant analogue 8-bromo-cGMP rapidly activates Dd-STATc, whereas 8-bromo-cAMP is a much less effective inducer. Surprisingly, however, Dd-STATc remains stress activatable in null mutants for components of the known cGMP-mediated and cAMP-mediated stress-response pathways and in a double mutant affecting both pathways. Also, Dd-STATc null cells are not abnormally sensitive to hyperosmotic stress. Microarray analysis identified two genes, gapA and rtoA, that are induced by hyperosmotic stress. Osmotic stress induction of gapA and rtoA is entirely dependent on Dd-STATc. Neither gene is inducible by DIF but both are rapidly inducible with 8-bromo-cGMP. Again, 8-bromo-cAMP is a much less potent inducer than 8-bromo-cGMP. These data show that Dd-STATc functions as a transcriptional activator in a stress-response pathway and the pharmacological evidence, at least, is consistent with cGMP acting as a second messenger.

cAMP effector mechanisms. Novel twists for an 'old' signaling system

Cyclic AMP (cAMP) has traditionally been thought to act exclusively through cAMP-dependent protein kinase (cAPK, PKA), but a growing number of cAMP effects are not attributable to general activation of cAPK. At present, cAMP is known also to directly regulate ion channels and the ubiquitous Rap guanine exchange factors Epac 1 and 2. Adding to the sophistication of cAMP signaling is the fact that (1) the cAPK holoenzyme is incompletely dissociated even at saturating cAMP, the level of free R subunit of cAPK being able to regulate the maximal activity of cAPK, (2) cAPK activity can be modulated by oxidative glutathionylation, and (3) cAPK is anchored close to relevant substrates, other signaling enzymes, and local compartments of cAMP. Finally, we will demonstrate an example of fine-tuning of cAMP signaling through synergistic induction of neurite extensions by cAPK and Epac.

cGMP-dependent protein kinase protects cGMP from hydrolysis by phosphodiesterase-5

The physiological effects of cGMP are largely determined by the activities of intracellular receptors, including cGMP-dependent protein kinase (PKG) and cGMP-binding cyclic nucleotide phosphodiesterases (PDEs), and the distribution of cGMP among these receptors dictates activity of the signalling pathway. In the present study, the effects of PKG-Ialpha or PKG-Ibeta on the rate of cGMP hydrolysis by the isolated PDE5 catalytic domain were examined. PKG-Ialpha strongly inhibited cGMP hydrolysis with an IC(50) value of 217 nM, which is similar to the physiological concentration of PKG in pig coronary artery reported previously. By contrast, PKG-Ibeta, which has lower affinity for cGMP than does PKG-Ialpha, inhibited cGMP hydrolysis with an IC(50) of approx. 1 microM. Inhibition by PKG-Ialpha was more effective than that by PKG-Ibeta, consistent with their relative affinities for cGMP. Autophosphorylation of PKGs increased their cGMP-binding affinities and their inhibitory effects on PDE5 hydrolysis of cGMP. Autophosphorylation of PKG-Ibeta increased its inhibitory potency on PDE5 hydrolysis of cGMP by 10-fold compared with a 2-fold increase upon autophosphorylation of PKG-Ialpha. The results indicate that cGMP bound to allosteric cGMP-binding sites of PKG is protected from hydrolysis by PDE5 and that persistent protection of cGMP by either non-phosphorylated or autophosphorylated PKGs may be a positive-feedback control to sustain cGMP signalling.

Identification of novel target proteins of cyclic GMP signaling pathways using chemical proteomics

For deciphering the cyclic guanosine monophosphate (cGMP) signaling pathway, we employed chemical proteomics to identify the novel target molecules of cGMP. We used cGMP that was immobilized onto agarose beads with linkers directed at three different positions of cGMP. We performed a pull-down assay using the beads as baits on tissue lysates and identified 9 proteins by MALDI-TOF (Matrix-Assisted Laser Desorption/Ionization Time-of-Flight) mass spectrometry. Some of the identified proteins were previously known cGMP targets, including cGMP-dependent protein kinase and cGMP-stimulated phosphodiesterase. Surprisingly, some of the coprecipitated proteins were never formerly reported to associate with the cGMP signaling pathway. The competition binding assays showed that the interactions are not by nonspecific binding to either the linker or bead itself, but by specific binding to cGMP. Furthermore, we observed that the interactions are highly specific to cGMP against other nucleotides, such as cyclic adenosine monophosphate (cAMP) and 5\'-GMP, which are structurally similar to cGMP. As one of the identified targets, MAPK1 was confirmed by immunoblotting with an anti-MAPK1 antibody. For further proof, we observed that the membrane-permeable cGMP (8-bromo cyclic GMP) stimulated mitogen-activated protein kinase 1 signaling in the treated cells. Our present study suggests that chemical proteomics can be a very useful and powerful technique for identifying the target proteins of small bioactive molecules.

Activation of cGMP-dependent protein kinase by protein kinase C

The cGMP-dependent protein kinases (PKG) are emerging as important components of mainstream signal transduction pathways. Nitric oxide-induced cGMP formation by stimulation of soluble guanylate cyclase is generally accepted as being the most widespread mechanism underlying PKG activation. In the present study, PKG was found to be a target for phorbol 12-myristate 13-acetate (PMA)-responsive protein kinase C (PKC). PKG1alpha became phosphorylated in HEK-293 cells stimulated with PMA and also in vitro using purified components. PKC-dependent phosphorylation was found to activate PKG as measured by phosphorylation of vasodilator-stimulated phosphoprotein, and by in vitro kinase assays. Although there are 11 potential PKC substrate recognition sites in PKG1alpha, threonine 58 was examined due to its proximity to the pseudosubstrate domain. Antibodies generated against the phosphorylated form of this region were used to demonstrate phosphorylation in response to PMA treatment of the cells with kinetics similar to vasodilator-stimulated phosphoprotein phosphorylation. A phospho-mimetic mutation at this site (T58E) generated a partially activated PKG that was more sensitive to cGMP levels. A phospho-null mutation (T58A) revealed that this residue is important but not sufficient for PKG activation by PKC. Taken together, these findings outline a novel signal transduction pathway that links PKC stimulation with cyclic nucleotide-independent activation of PKG.

Characterization of a cAMP-stimulated cAMP phosphodiesterase in Dictyostelium discoideum

A cyclic nucleotide phosphodiesterase, PdeE, that harbors two cyclic nucleotide binding motifs and a binuclear Zn(2+)-binding domain was characterized in Dictyostelium. In other eukaryotes, the Dictyostelium domain shows greatest homology to the 73-kDa subunit of the pre-mRNA cleavage and polyadenylation specificity factor. The Dictyostelium PdeE gene is expressed at its highest levels during aggregation, and its disruption causes the loss of a cAMP-phosphodiesterase activity. The pdeE null mutants show a normal cAMP-induced cGMP response and a 1.5-fold increase of cAMP-induced cAMP relay. Overexpression of a PdeE-yellow fluorescent protein (YFP) fusion construct causes inhibition of aggregation and loss of the cAMP relay response, but the cells can aggregate in synergy with wild-type cells. The PdeE-YFP fusion protein was partially purified by immunoprecipitation and biochemically characterized. PdeE and its Dictyostelium ortholog, PdeD, are both maximally active at pH 7.0. Both enzymes require bivalent cations for activity. The common cofactors Zn(2+) and Mg(2+) activated PdeE and PdeD maximally at 10 mm, whereas Mn(2+) activated the enzymes to 4-fold higher levels, with half-maximal activation between 10 and 100 microm. PdeE is an allosteric enzyme, which is approximately 4-fold activated by cAMP, with half-maximal activation occurring at about 10 microm and an apparent K(m) of approximately 1 mm. cGMP is degraded at a 6-fold lower rate than cAMP. Neither cGMP nor 8-Br-cAMP are efficient activators of PdeE activity.

Mechanisms associated with cGMP binding and activation of cGMP-dependent protein kinase

Using small-angle x-ray scattering, we have observed the cGMP-induced elongation of an active, cGMP-dependent, monomeric deletion mutant of cGMP-dependent protein kinase (Delta(1-52)PKG-I beta). On saturation with cGMP, the radius of gyration of Delta(1-52)PKG-I beta increases from 29.4 +/- 0.1 A to 40.1 +/- 0.7 A, and the maximum linear dimension increases from 90 A +/- 10% to 130 A +/- 10%. The elongation is due to a change in the interaction between structured regulatory (R) and catalytic (C) domains. A model of cGMP binding to Delta(1-52)PKG-I beta indicates that elongation of Delta(1-52)PKG-I beta requires binding of cGMP to the low-affinity binding site of the R domain. A comparison with cAMP-dependent protein kinase suggests that both elongation and activation require cGMP binding to both sites; cGMP binding to the low-affinity site therefore seems to be a necessary, but not sufficient, condition for both elongation and activation of Delta(1-52)PKG-I beta. We also predict that there is little or no cooperativity in cGMP binding to the two sites of Delta(1-52)PKG-I beta under the conditions used here. Results obtained by using the Delta(1-52)PKG-I beta monomer indicate that a previously observed elongation of PKG-I alpha is consistent with a pure change in the interaction between the R domain and the C domain, without alteration of the dimerization interaction. This study has revealed important features of molecular mechanisms in the biochemical network describing PKG-I beta activation by cGMP, yielding new insight into ligand activation of cyclic nucleotide-dependent protein kinases, a class of regulatory proteins that is key to many cellular processes.

Altered cAMP-dependent protein kinase subunit immunolabeling in post-mortem brain from patients with bipolar affective disorder

Previous findings of reduced [3H]cAMP binding and increased activities of cAMP-dependent protein kinase (PKA) in discrete post-mortem brain regions from patients with bipolar affective disorder (BD) suggest that PKA, the major downstream target of cAMP, is also affected in this illness. As prolonged elevation of intracellular cAMP levels can modify PKA regulatory (R) and catalytic (C) subunit levels, we sought to determine whether these PKA abnormalities are related to changes in the abundance of PKA subunits in BD brain. Using immunoblotting techniques along with PKA subunit isoform-specific polyclonal antisera, levels of PKA RIalpha, RIbeta, RIIalpha, RIIbeta and Calpha subunits were measured in cytosolic and particulate fractions of temporal, frontal and parietal cortices of post-mortem brain from BD patients and matched, non-neurological, non-psychiatric controls. Immunoreactive levels of cytosolic Calpha in temporal and frontal cortices, as well as that of cytosolic RIIbeta in temporal cortex, were significantly higher in the BD compared with the matched control brains. These changes were independent of age, post-mortem interval or pH and unrelated to ante-mortem lithium treatment or suicide. These findings strengthen further the notion that the cAMP/PKA signaling system is up-regulated in discrete cerebral cortical regions in BD.

Selective phosphorylation of the IP3R-I in vivo by cGMP-dependent protein kinase in smooth muscle

This study examined the expression of inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R) types and PKG isoforms in isolated gastric smooth muscle cells and determined the ability of PKG and PKA to phosphorylate IP(3)Rs and inhibit IP(3)-dependent Ca(2+) release, which mediates the initial phase of agonist-induced contraction. PKG-Ialpha and PKG-Ibeta were expressed in gastric smooth muscle cells, together with IP(3)-R-associated cG-kinase substrate, a protein that couples PKG-Ibeta to IP(3)R-I. IP(3)R-I and IP(3)R-III were also expressed, but only IP(3)R-I was phosphorylated by PKA and PKG in vitro and exclusively by PKG in vivo. Sequential phosphorylation by PKA and by PKG-Ialpha in vitro showed that PKA phosphorylated the same site as PKG (presumably S(1755)) and an additional PKA-specific site (S(1589)). In intact muscle cells, agents that activated PKG or both PKG and PKA induced IP(3)R-I phosphorylation that was reversed by the PKG inhibitor (8R,9S,11s)-(-)-9-methoxy-carbamyl-8-methyl-2,3,9,10-tetrahydro-8,11-epoxy-1H,8H,1H,-2,7b,11a-trizadizo-benzo9(a,g)cycloocta(c,d,e)-trinden-1-one. Agents that activated PKA induced IP(3)R-I phosphorylation in permeabilized but not intact muscle cells, implying that PKA does not gain access to IP(3)R-I in intact muscle cells. The pattern of IP(3)R-I phosphorylation in vivo and in vitro was more consistent with phosphorylation by PKG-Ialpha. Phosphorylation of IP(3)R-I in microsomes by PKG, PKA, or a combination of PKG and PKA inhibited IP(3)-induced Ca(2+) release to the same extent, implying that inhibition was mediated by phosphorylation of the PKG-specific site. We conclude that IP(3)R-I is selectively phosphorylated by PKG-I in intact smooth muscle resulting in inhibition of IP(3)-dependent Ca(2+) release.

Specificity of the cyclic adenosine 3',5'-monophosphate signal in granulosa cell function

Cyclic AMP signaling is involved in most aspects of differentiation and maturation of the granulosa cells in the ovarian follicle. As the genetic programs activated at different stages of follicle growth maturation are being elucidated, it is becoming increasingly difficult to reconcile the simplicity of the cAMP cascade with the complexity and the divergent patterns of gene expression activated in these cells. To account for these divergent outcomes of the cAMP signal, three aspects of this signaling cascade in granulosa cells will be reviewed. We will discuss the evidence for gonadotropin receptors coupling to different G proteins and effectors. Next, we will explore the possibility that the temporal and spatial dimensions of the cAMP signal itself may contribute to the diverse outcomes. Finally, we will summarize available data showing that the cAMP signal is distributed through several cascades of kinase activation. It is hoped this compendium will provide a framework with which to understand how the initial signals activated by gonadotropins control the complex patterns of gene expression that are required for follicle maturation and ovulation.

The translational regulation of lipoprotein lipase by epinephrine involves an RNA binding complex including the catalytic subunit of protein kinase A

The balance of lipid flux in adipocytes is controlled by the opposing actions of lipolysis and lipogenesis, which are controlled primarily by hormone-sensitive lipase and lipoprotein lipase (LPL), respectively. Catecholamines stimulate adipocyte lipolysis through reversible phosphorylation of hormone-sensitive lipase, and simultaneously inhibit LPL activity. However, LPL regulation is complex and previous studies have described translational regulation of LPL in response to catecholamines because of an RNA-binding protein that interacts with the 3'-untranslated region of LPL mRNA. In this study, we identified several protein components of an LPL RNA binding complex. Using an LPL RNA affinity column, we identified two of the RNA-binding proteins as the catalytic (C) subunit of cAMP-dependent protein kinase (PKA), and A kinase anchoring protein (AKAP) 121/149, one of the PKA anchoring proteins, which has known RNA binding activity. To determine whether the C subunit was involved in LPL translation inhibition, the C subunit was depleted from the cytoplasmic extract of epinephrine-stimulated adipocytes by immunoprecipitation. This resulted in the loss of LPL translation inhibition activity of the extract, along with decreased RNA binding activity in a gel shift assay. To demonstrate the importance of the AKAPs, inhibition of PKA-AKAP binding with a peptide competitor (HT31) prevented epinephrine-mediated inhibition of LPL translation. C subunit kinase activity was necessary for LPL RNA binding and translation inhibition, suggesting that the phosphorylation of AKAP121/149 or other proteins was an important part of RNA binding complex formation. The hormonal activation of PKA results in the reversible phosphorylation of hormone-sensitive lipase, which is the primary mediator of adipocyte lipolysis. These studies demonstrate a dual role for PKA to simultaneously inhibit LPL-mediated lipogenesis through inhibition of LPL translation.

Identification of a novel type of cGMP phosphodiesterase that is defective in the chemotactic stmF mutants

StmF mutants are chemotactic mutants that are defective in a cGMP phosphodiesterase (PDE) activity. We identified a novel gene, PdeD, that harbors two cyclic nucleotide-binding domains and a metallo-beta-lactamase homology domain. Similar to stmF mutants, pdeD-null mutants displayed extensively streaming aggregates, prolonged elevation of cGMP levels after chemotactic stimulation, and reduced cGMP-PDE activity. PdeD transcripts were lacking in stmF mutant NP377, indicating that this mutant carries a PdeD lesion. Expression of a PdeD-YFP fusion protein in pdeD-null cells restored the normal cGMP response and showed that PdeD resides in the cytosol. When purified by immunoprecipitation, the PdeD-YFP fusion protein displayed cGMP-PDE activity, which was retained in a truncated construct that contained only the metallo-beta-lactamase domain.

A conserved serine juxtaposed to the pseudosubstrate site of type I cGMP-dependent protein kinase contributes strongly to autoinhibition and lower cGMP affinity

Serines 64 and 79 are homologous residues that are juxtaposed to the autoinhibitory pseudosubstrate site in cGMP-dependent protein kinase type Ialpha and type Ibeta (PKG-Ialpha and PKG-Ibeta), respectively. Autophosphorylation of this residue is associated with activation of type I PKGs. To determine the role of this conserved serine, point mutations have been made in PKG-Ialpha (S64A, S64T, S64D, and S64N) and PKG-Ibeta (S79A). In wild-type PKG-Ialpha, basal kinase activity ratio (-cGMP/+cGMP) is 0.11, autophosphorylation increases this ratio 3-fold, and the K(a) and K(D) values for cGMP are 127 and 36 nm, respectively. S64A PKG-Ialpha basal kinase activity ratio increases 2-fold, cGMP binding affinity increases approximately 10-fold in both K(a) and K(D), and activation by autophosphorylation is slight. S64D and S64N mutants are nearly constitutively active in the absence of cGMP, cGMP binding affinity in each increases 18-fold, and autophosphorylation does not affect the kinase activity of these mutants. Mutation of the homologous site in PKG-Ibeta (S79A) increases the basal kinase activity ratio 2-fold and cGMP binding affinity 5-fold over that of wild-type PKG-Ibeta. The combined results demonstrate that a conserved serine juxtaposed to the pseudosubstrate site in type I PKGs contributes importantly to enzyme function by increasing autoinhibition and decreasing cGMP binding affinity.

Toxoplasma gondii cyclic GMP-dependent kinase: chemotherapeutic targeting of an essential parasite protein kinase

The trisubstituted pyrrole 4-[2-(4-fluorophenyl)-5-(1-methylpiperidine-4-yl)-1H-pyrrol-3-yl]pyridine (compound 1) has in vivo activity against the apicomplexan parasites Toxoplasma gondii and Eimeria tenella in animal models. The presumptive molecular target of this compound in E. tenella is cyclic GMP-dependent protein kinase (PKG). Native PKG purified from T. gondii has kinetic and pharmacologic properties similar to those of the E. tenella homologue, and both have been functionally expressed as recombinant proteins in T. gondii. Computer modeling of parasite PKG was used to predict catalytic site amino acid residues that interact with compound 1. The recombinant laboratory-generated mutants T. gondii PKG T761Q or T761M and the analogous E. tenella T770 alleles have reduced binding affinity for, and are not inhibited by, compound 1. By all other criteria, PKG with this class of catalytic site substitution is indistinguishable from wild-type enzyme. A genetic disruption of T. gondii PKG can only be achieved if a complementing copy of PKG is provided in trans, arguing that PKG is an essential protein. Strains of T. gondii, disrupted at the genomic PKG locus and dependent upon the T. gondii T761-substituted PKGs, are as virulent as wild type in mice. However, unlike mice infected with wild-type T. gondii that are cured by compound 1, mice infected with the laboratory-generated strains of T. gondii do not respond to treatment. We conclude that PKG represents the primary molecular target responsible for the antiparasitic efficacy of compound 1.

Reinventing the wheel of cyclic AMP: novel mechanisms of cAMP signaling

Mechanisms of cAMP signal transduction have been thoroughly investigated for more than 40 years. From the binding of hormonal ligands to their receptors on the outer surface of the plasma membrane to the cytoplasmic activation of effectors, the ensuing cAMP signaling cascades and the nuclear gene regulatory functions, coupled with the structural elucidation of the cAMP-dependent protein kinase (PKA) and in vivo functional characterizations of each of the components of PKA by homologous recombination gene targeting, our understanding of cAMP-mediated signal transduction has reached its pinnacle. Despite this trove of knowledge, some recent findings have emerged that suggest hitherto novel and alternative mechanisms of cAMP action that could increase the signaling bandwidth of cAMP and PKA in cell growth and transcriptional regulation. This article attempts to review some of these novel and unconventional mechanisms of cAMP and PKA signaling, and to generate further enthusiasm in investigating and validating these new frontiers of the cAMP signal transduction pathway.

Purification and molecular characterization of cGMP-dependent protein kinase from Apicomplexan parasites. A novel chemotherapeutic target

The trisubstituted pyrrole 4-[2-(4-fluorophenyl)-5-(1-methylpiperidine-4-yl)-1H-pyrrol-3-yl]pyridine (Compound 1) inhibits the growth of Eimeria spp. both in vitro and in vivo. The molecular target of Compound 1 was identified as cGMP-dependent protein kinase (PKG) using a tritiated analogue to purify a approximately 120-kDa protein from lysates of Eimeria tenella. This represents the first example of a protozoal PKG. Cloning of PKG from several Apicomplexan parasites has identified a parasite signature sequence of nearly 300 amino acids that is not found in mammalian or Drosophila PKG and which contains an additional, third cGMP-binding site. Nucleotide cofactor regulation of parasite PKG is remarkably different from mammalian enzymes. The activity of both native and recombinant E. tenella PKG is stimulated 1000-fold by cGMP, with significant cooperativity. Two isoforms of the parasite enzyme are expressed from a single copy gene. NH(2)-terminal sequence of the soluble isoform of PKG is consistent with alternative translation initiation within the open reading frame of the enzyme. A larger, membrane-associated isoform corresponds to the deduced full-length protein sequence. Compound 1 is a potent inhibitor of both soluble and membrane-associated isoforms of native PKG, as well as recombinant enzyme, with an IC(50) of <1 nm.

Formation of inactive cAMP-saturated holoenzyme of cAMP-dependent protein kinase under physiological conditions

The complex of the subunits (RIalpha, Calpha) of cAMP-dependent protein kinase I (cA-PKI) was much more stable (K(d) = 0.25 microm) in the presence of excess cAMP than previously thought. The ternary complex of C subunit with cAMP-saturated RIalpha or RIIalpha was devoid of catalytic activity against either peptide or physiological protein substrates. The ternary complex was destabilized by protein kinase substrate. Extrapolation from the in vitro data suggested about one-fourth of the C subunit to be in ternary complex in maximally cAMP-stimulated cells. Cells overexpressing either RIalpha or RIIalpha showed decreased CRE-dependent gene induction in response to maximal cAMP stimulation. This could be explained by enhanced ternary complex formation. Modulation of ternary complex formation by the level of R subunit may represent a novel way of regulating the cAMP kinase activity in maximally cAMP-stimulated cells.

Protein phosphatase 2A is involved in the regulation of protein kinase A signaling pathway during in vitro chondrogenesis

We have evaluated the importance of the Ser/Thr protein phosphorylation and dephosphorylation for chondrogenesis in high-density chicken limb bud mesenchymal cell cultures (HDCs) by using H89, a cell-permeable protein kinase inhibitor, and okadaic acid (OA), a phosphoprotein phosphatase (PP)-specific inhibitor molecule. When 20 nM OA was applied to the HDCs on Days 2 and 3 of culturing, it significantly inhibited protein phosphatase 2A (PP2A), enhanced cartilage formation, and elevated the activity of cAMP-dependent protein kinase (PKA). Application of 20 microM H89 significantly decreased the activity of PKA and blocked the chondrogenesis in HDCs. Furthermore, OA enhanced cartilage formation and elevated the suppressed activity of PKA even in the H89-pretreated HDCs. cGMP-dependent protein kinase was not detected in HDCs, while protein kinase Cmu (PKCmu), which is also inhibited by nanomolar concentrations of H89, was present throughout the culturing period. Neither OA nor H89 influenced the expression of the catalytic subunit of PKA or the cAMP response element binding protein, CREB. However, a significantly elevated amount of Ser-133-phosphorylated-CREB (P-CREB) was detected following addition of OA, while H89 treatment resulted in a decrease of the amount of P-CREB. Our results demonstrate that PP2A plays a role in the regulation of the PKA signaling pathway and that the phosphorylation level of CREB is influenced by the activity of both enzymes during in vitro chondrogenesis.

Molecular characterization of a coccidian parasite cGMP dependent protein kinase

The cGMP-dependent protein kinase (PKG) of Eimeria tenella and Toxoplasma gondii is the target of a novel coccidiostat that is effective against coccidiosis and toxoplasmosis in animal models. Preparations of native PKG enzyme from Toxoplasma and Eimeria contain a membrane-associated polypeptide (isoform-I) of about 110 kDa and a slightly smaller soluble polypeptide (isoform-II). Expression of T. gondii and E. tenella PKG cDNA clones in Toxoplasma yield similarly sized recombinant polypeptides, which co-migrate on SDS-polyacrylamide gels with the corresponding native isoforms. Results of targeted mutagenesis of potential translational initiation sites suggest that parasite isoform-II is a product of alternative translational initiation from an internal initiator methionine codon. Exclusive expression of isoform-II or isoform-I can be achieved by preventing initiation at the respective primary or secondary sites. Immunofluorescence analysis indicates that recombinant isoform-I localizes primarily to the parasite plasma membrane, while isoform-II remains cytosolic. Mutagenesis and metabolic labeling studies reveal that the observed membrane-association of full-length recombinant PKG is mediated by N-terminal myristoylation and palmitoylation at amino acids G2 and C4. We also confirm the functional significance of a putative third PKG allosteric site, common to apicomplexan PKGs but absent from vertebrate or insect PKGs. In assays with transiently transfected parasites, constructs harboring a mutation at this site express markedly lower levels of cGMP-dependent PKG activity, while a triple mutant bearing mutations in all three sites reduces kinase activity to background levels.

The role of a parasite-specific allosteric site in the distinctive activation behavior of Eimeria tenella cGMP-dependent protein kinase

A cGMP-dependent protein kinase (PKG) was recently identified as an anticoccidial target for the apicomplexan parasite Eimeria tenella [Gurnett, A., Liberator, P. A., Dulski, P., Salowe, S., Donald, R. G. K., Anderson, J., Wiltsie, J., Diaz, C., Harris, G., Chang, B., Darkin-Rattray, S. J., Nare, B., Crumley, T., Blum, P., Misura, A., Tamas, T., Sardana, M., Yuan, J., Biftu, T., and Schmatz, D. (2002) J. Biol. Chem. (in press)]. Unlike the PKGs of higher organisms that have two cGMP binding sites in their regulatory domain, the PKG from Eimeria tenella (Et-PKG) contains three putative cGMP binding sites and has distinctive activation properties, including a very large stimulation by cGMP ( approximately 1000-fold) with significant cooperativity (Hill coefficient of 1.7). During our investigation of Et-PKG activation, we found that 8-substituted cGMP analogues are weak partial activators. For example, 8-NBD-cGMP provides a maximal stimulation of activity of only 20-fold with little evident cooperativity, although cGMP can synergize with the analogue to provide full activation. The results suggest that partial activation is a consequence of restricted binding of 8-NBD-cGMP to a subset of cGMP sites in the enzyme. Site-directed mutagenesis of conserved arginine and glutamate residues in the parasite-specific third cGMP site confirms that this site is an important functional participant in the allosteric regulation of the kinase and that it exhibits very high selectivity against 8-NBD-cGMP. Since the results are consistent with full activation of Et-PKG requiring cyclic nucleotide binding in all three allosteric sites, one role for the additional cGMP site may be to establish a stricter regulatory mechanism for the kinase activity than is present in the PKGs of higher organisms containing only two allosteric sites.

cAMP-dependent protein kinase and proliferation differ in normal and polycystic kidney epithelia

Developmental control of cell proliferation is crucial, and abnormal principal cell proliferation may contribute to cystogenesis in polycystic kidney disease. This study investigates roles of cAMP and its primary effector, cAMP-dependent protein kinase (protein kinase A; PKA), in control of cell proliferation in filter-grown noncystic (NC) and cystic (CY)-derived principal cell cultures. These cultures had similar cAMP pathway characteristics upstream of PKA subunit distribution but differed in predicted PKA subtype distribution. Functionally, cultures were proliferative before polarization, with constitutively higher proliferation in CY cultures. NC cultures achieved levels similar to those of CY cultures on pharmacological manipulation of cAMP production or PKA activation or inhibition of PKA subtype I activity. Inhibition of overall PKA activity, or of PKA subtype II anchoring, diminished cAMP/PKA-mediated proliferation in NC cultures but had no effect on CY cultures. Polarized CY monolayers remained proliferative, but NC monolayers lost responsiveness. No large proliferation changes resulted from treatments of polarized cultures; however, polarized NC and CY cultures differed in poststimulation handling of PKA catalytic and type IIalpha regulatory subunits. Our results support PKA subtype regulation of prepolarization proliferation in NC principal cells and altered regulation of PKA in CY cells and suggest that differences at or downstream of PKA can contribute to altered proliferation in a developmental renal disease.

PKA-dependent activation of PDE3A and PDE4 and inhibition of adenylyl cyclase V/VI in smooth muscle

Regulation of adenylyl cyclase type V/VI and cAMP-specific, cGMP-inhibited phosphodiesterase (PDE) 3 and cAMP-specific PDE4 by cAMP-dependent protein kinase (PKA) and cGMP-dependent protein kinase (PKG) was examined in gastric smooth muscle cells. Expression of PDE3A but not PDE3B was demonstrated by RT-PCR and Western blot. Basal PDE3 and PDE4 activities were present in a ratio of 2:1. Forskolin, isoproterenol, and the PKA activator 5,6-dichloro-1-beta-D-ribofuranosyl benzimidazole 3',5'-cyclic monophosphate, SP-isomer, stimulated PDE3A phosphorylation and both PDE3A and PDE4 activities. Phosphorylation of PDE3A and activation of PDE3A and PDE4 were blocked by the PKA inhibitors [protein kinase inhibitor (PKI) and H-89] but not by the PKG inhibitor (KT-5823). Sodium nitroprusside inhibited PDE3 activity and augmented forskolin- and isoproterenol-stimulated cAMP levels; PDE3 inhibition was reversed by blockade of cGMP synthesis. Forskolin stimulated adenylyl cyclase phosphorylation and activity; PKI blocked phosphorylation and enhanced activity. Stimulation of cAMP and inhibition of inositol 1,4,5-trisphosphate-induced Ca(2+) release and muscle contraction by isoproterenol were augmented additively by PDE3 and PDE4 inhibitors. The results indicate that PKA regulates cAMP levels in smooth muscle via stimulatory phosphorylation of PDE3A and PDE4 and inhibitory phosphorylation of adenylyl cyclase type V/VI. Concurrent generation of cGMP inhibits PDE3 activity and augments cAMP levels.

Zaprinast, an inhibitor of cGMP-selective phosphodiesterases, enhances the secretion of TNF-alpha and IL-1beta and the expression of iNOS and MHC class II molecules in rat microglial cells

Proinflammatory cytokines produced by activated glial cells may in turn augment the immune/inflammatory reactions of glial cells through autocrine and paracrine routes. The NO/cGMP signaling represents one of the reactions of activated glial cells. We investigated whether the production of proinflammatory cytokines by glial cells is affected by NO-dependent downstream cGMP signaling. In primary cultures of mixed astrocytes and microglial cells, zaprinast (0.1 mM), an inhibitor of cGMP-selective phosphodiesterases, enhanced the basal and LPS (1.0 microg/ml)-induced secretion of TNF-alpha and IL-1beta. Zaprinast also enhanced NO production induced by LPS or IFN-gamma (100 U/ml), and in microglial cell cultures, but not in astrocyte cultures, zaprinast enhanced the basal and the IFN-gamma-induced production of the cytokines, TNF-alpha and IL-1beta, and of NO. This upregulation by zaprinast was partially inhibited by KT5823 (1.0 microM), an inhibitor of protein kinase G. The LPS-induced production of TNF-alpha, IL-1beta, and NO was inhibited by ODQ (50 microM), an inhibitor of soluble guanylyl cyclase, and by KT5823. Immunohistochemical analysis of mixed glial cell cultures showed that LPS/IFN-gamma-induced iNOS expression and the enhanced expression of iNOS by zaprinast were restricted to microglial cells. Zaprinast enhanced the IFN-gamma (200 U/ml)-induced expression of MHC Class II molecules in astrocytes and microglial cells in mixed cultures, but did not enhance this IFN-gamma-induced expression in pure astrocytes, which lacked paracrine TNF-alpha from microglial cells. Summarizing, zaprinast, which is associated with cGMP/protein kinase G signaling, may augment central immune/inflammatory reactions, possibly via the increased production of TNF-alpha and IL-1beta by activated microglial cells.