Autophosphorylation of type Ibeta cGMP-dependent protein kinase increases basal catalytic activity and enhances allosteric activation by cGMP or cAMP

Characterizing the Protein Isoforms of foraging (for), the PKGI Ortholog in Drosophila melanogaster

The foraging (for) gene of Drosophila melanogaster encodes a cGMP-dependent protein kinase (PKG), which is a major effector of the cGMP signaling pathway involved in the regulation of behaviour and metabolic traits. Despite being well studied at the transcript level, little is known about the for gene at the protein level. Here, we provide a detailed characterization of the for gene protein (FOR) products and present new tools for their study, including five isoform-specific antibodies and a transgenic strain that carries an HA-labelled for allele (for^BAC::HA). Our results showed that multiple FOR isoforms were expressed in the larval and adult stages of D. melanogaster and that the majority of whole-body FOR expression arises from three (P1, P1α, and P3) of eight putative protein isoforms. We found that FOR expression differed between the larval and adult stages and between the dissected larval organs we analyzed, which included the central nervous system (CNS), fat body, carcass, and intestine. Moreover, we showed that the FOR expression differed between two allelic variants of the for gene, namely, for^s (sitter) and for^R (rover), that are known to differ in many food-related traits. Together, our in vivo identification of FOR isoforms and the existence of temporal, spatial, and genetic differences in their expression lay the groundwork for determining their functional significance.

An auto-inhibited state of protein kinase G and implications for selective activation

Cyclic GMP-dependent protein kinases (PKGs) are key mediators of the nitric oxide/cyclic guanosine monophosphate (cGMP) signaling pathway that regulates biological functions as diverse as smooth muscle contraction, cardiac function, and axon guidance. Understanding how cGMP differentially triggers mammalian PKG isoforms could lead to new therapeutics that inhibit or activate PKGs, complementing drugs that target nitric oxide synthases and cyclic nucleotide phosphodiesterases in this signaling axis. Alternate splicing of PRKG1 transcripts confers distinct leucine zippers, linkers, and auto-inhibitory (AI) pseudo-substrate sequences to PKG Iα and Iβ that result in isoform-specific activation properties, but the mechanism of enzyme auto-inhibition and its alleviation by cGMP is not well understood. Here, we present a crystal structure of PKG Iβ in which the AI sequence and the cyclic nucleotide-binding (CNB) domains are bound to the catalytic domain, providing a snapshot of the auto-inhibited state. Specific contacts between the PKG Iβ AI sequence and the enzyme active site help explain isoform-specific activation constants and the effects of phosphorylation in the linker. We also present a crystal structure of a PKG I CNB domain with an activating mutation linked to Thoracic Aortic Aneurysms and Dissections. Similarity of this structure to wildtype cGMP-bound domains and differences with the auto-inhibited enzyme provide a mechanistic basis for constitutive activation. We show that PKG Iβ auto-inhibition is mediated by contacts within each monomer of the native full-length dimeric protein, and using the available structural and biochemical data we develop a model for the regulation and cooperative activation of PKGs.

A substitution in cGMP-dependent protein kinase 1 associated with aortic disease induces an active conformation in the absence of cGMP

Type 1 cGMP-dependent protein kinases (PKGs) play important roles in human cardiovascular physiology, regulating vascular tone and smooth-muscle cell phenotype. A mutation in the human PRKG1 gene encoding cGMP-dependent protein kinase 1 (PKG1) leads to thoracic aortic aneurysms and dissections. The mutation causes an arginine-to-glutamine (RQ) substitution within the first cGMP-binding pocket in PKG1. This substitution disrupts cGMP binding to the pocket, but it also unexpectedly causes PKG1 to have high activity in the absence of cGMP via an unknown mechanism. Here, we identified the molecular mechanism whereby the RQ mutation increases basal kinase activity in the human PKG1α and PKG1β isoforms. Although we found that the RQ substitution (R177Q in PKG1α and R192Q in PKG1β) increases PKG1α and PKG1β autophosphorylation in vitro, we did not detect increased autophosphorylation of the PKG1α or PKG1β RQ variant isolated from transiently transfected 293T cells, indicating that increased basal activity of the RQ variants in cells was not driven by PKG1 autophosphorylation. Replacement of Arg-177 in PKG1α with alanine or methionine also increased basal activity. PKG1 exists as a parallel homodimer linked by an N-terminal leucine zipper, and we show that the WT chain in WT-RQ heterodimers partly reduces basal activity of the RQ chain. Using hydrogen/deuterium-exchange MS, we found that the RQ substitution causes PKG1β to adopt an active conformation in the absence of cGMP, similar to that of cGMP-bound WT enzyme. We conclude that the RQ substitution in PKG1 increases its basal activity by disrupting the formation of an inactive conformation.

Pleiotropy of the Drosophila melanogaster foraging gene on larval feeding-related traits

Little is known about the molecular underpinning of behavioral pleiotropy. The Drosophila melanogaster foraging gene is highly pleiotropic, affecting many independent larval and adult phenotypes. Included in foraging's multiple phenotypes are larval foraging path length, triglyceride levels, and food intake. foraging has a complex structure with four promoters and 21 transcripts that encode nine protein isoforms of a cGMP dependent protein kinase (PKG). We examined if foraging's complex molecular structure underlies the behavioral pleiotropy associated with this gene. Using a promotor analysis strategy, we cloned DNA fragments upstream of each of foraging's transcription start sites and generated four separate forpr-Gal4s. Supporting our hypothesis of modular function, they had discrete, restricted expression patterns throughout the larva. In the CNS, forpr1-Gal4 and forpr4-Gal4 were expressed in neurons while forpr2-Gal4 and forpr3-Gal4 were expressed in glia cells. In the gastric system, forpr1-Gal4 and forpr3-Gal4 were expressed in enteroendocrine cells of the midgut while forpr2-Gal4 was expressed in the stem cells of the midgut. forpr3-Gal4 was expressed in the midgut enterocytes, and midgut and hindgut visceral muscle. forpr4-Gal4's gastric system expression was restricted to the hindgut. We also found promoter specific expression in the larval fat body, salivary glands, and body muscle. The modularity of foraging's molecular structure was also apparent in the phenotypic rescues. We rescued larval path length, triglyceride levels (bordered on significance), and food intake of for0 null larvae using different forpr-Gal4s to drive UAS-forcDNA. In a foraging null genetic background, forpr1-Gal4 was the only promoter driven Gal4 to rescue larval path length, forpr3-Gal4 altered triglyceride levels, and forpr4-Gal4 rescued food intake. Our results refine the spatial expression responsible for foraging's associated phenotypes, as well as the sub-regions of the locus responsible for their expression. foraging's pleiotropy arises at least in part from the individual contributions of its four promoters.

Redox regulation of a guard cell SNF1-related protein kinase in Brassica napus, an oilseed crop

Kinase-mediated phosphorylation is a pivotal regulatory process in stomatal responses to stresses. Through a redox proteomics study, a sucrose non-fermenting 1-related protein kinase (SnRK2.4) was identified to be redox-regulated in Brassica napus guard cells upon abscisic acid treatment. There are six genes encoding SnRK2.4 paralogs in B. napus Here, we show that recombinant BnSnRK2.4-1C exhibited autophosphorylation activity and preferentially phosphorylated the N-terminal region of B. napus slow anion channel (BnSLAC1-NT) over generic substrates. The in vitro activity of BnSnRK2.4-1C requires the presence of manganese (Mn²⁺). Phosphorylation sites of autophosphorylated BnSnRK2.4-1C were mapped, including serine and threonine residues in the activation loop. In vitro BnSnRK2.4-1C autophosphorylation activity was inhibited by oxidants such as H₂O₂ and recovered by active thioredoxin isoforms, indicating redox regulation of BnSnRK2.4-1C. Thiol-specific isotope tagging followed by mass spectrometry analysis revealed specific cysteine residues responsive to oxidant treatments. The in vivo activity of BnSnRK2.4-1C is inhibited by 15 min of H₂O₂ treatment. Taken together, these data indicate that BnSnRK2.4-1C, an SnRK preferentially expressed in guard cells, is redox-regulated with potential roles in guard cell signal transduction.

The VO2-on Kinetics in Constant Load Exercise Sub-Anaerobic Threshold Reflects Endothelial Function and Dysfunction in Muscle Microcirculation

To propose a test to evaluate endothelial function, based on VO2 on-transition kinetics in sub-anaerobic threshold (AT) constant load exercise, we tested healthy subjects and patients with ischemic-hypertensive cardiopathy by two cardiopulmonary tests on a cycle ergometer endowed with an electric motor to overcome initial inertia: a pre-test and, after at least 24 h, one 6 min constant load exercise at 90 % AT. We measured net phase 3 VO2-on kinetics and, by phase 2 time constant (τ), valued endothelial dysfunction. We found shorter τ in repeated tests, shorter time between first and second test, by persisting endothelium-dependent arteriolar vasodilatation and/or several other mechanisms. Reducing load to 80 % and 90 % AT did not produce significant changes in τ of healthy volunteers, while in heart patients an AT load of 70 %, compared to 80 % AT, shortened τ (=4.38±1.65 s, p=0.013). In heart patients, no correlation was found between NYHA class, ejection fraction (EF), and the two variables derived from incremental cycle cardio-pulmonary exercise, as well as between EF and τ; while NYHA class groups were well correlated with τ duration (r=0.92, p=0.0001). Doxazosin and tadalafil also significantly reduced τ. In conclusion, the O2 consumption kinetics during the on-transition of constant load exercise below the anaerobic threshold are highly sensitive to endothelial function in muscular microcirculation, and constitute a marker for the evaluation of endothelial dysfunction.

Structures of cGMP-Dependent Protein Kinase (PKG) Iα Leucine Zippers Reveal an Interchain Disulfide Bond Important for Dimer Stability

cGMP-dependent protein kinase (PKG) Iα is a central regulator of smooth muscle tone and vasorelaxation. The N-terminal leucine zipper (LZ) domain dimerizes and targets PKG Iα by interacting with G-kinase-anchoring proteins. The PKG Iα LZ contains C42 that is known to form a disulfide bond upon oxidation and to activate PKG Iα. To understand the molecular details of the PKG Iα LZ and C42-C42' disulfide bond, we determined crystal structures of the PKG Iα wild-type (WT) LZ and C42L LZ. Our data demonstrate that the C42-C42' disulfide bond dramatically stabilizes PKG Iα and that the C42L mutant mimics the oxidized WT LZ structurally.

Catalytic activity of cGMP-dependent protein kinase type I in intact cells is independent of N-terminal autophosphorylation

Although cGMP-dependent protein kinase type I (cGKI) is an important mediator of cGMP signaling and upcoming drug target, its in vivo-biochemistry is not well understood. Many studies showed that purified cGKI autophosphorylates multiple sites at its N-terminus. Autophosphorylation might be involved in kinase activation, but it is unclear whether this happens also in intact cells. To study cGKI autophosphorylation in vitro and in vivo, we have generated phospho-specific antisera against major in vitro-autophosphorylation sites of the cGKI isoforms, cGKIα and cGKIβ. These antisera detected specifically and with high sensitivity phospho-cGKIα (Thr58), phospho-cGKIα (Thr84), or phospho-cGKIβ (Thr56/Ser63/Ser79). Using these antisera, we show that ATP-induced autophosphorylation of cGKI in purified preparations and cell extracts did neither require nor induce an enzyme conformation capable of substrate heterophosphorylation; it was even inhibited by pre-incubation with cGMP. Interestingly, phospho-cGKI species were not detectable in intact murine cells and tissues, both under basal conditions and after induction of cGKI catalytic activity. We conclude that N-terminal phosphorylation, although readily induced in vitro, is not required for the catalytic activity of cGKIα and cGKIβ in vivo. These results will also inform screening strategies to identify novel cGKI modulators.

Interleukin 1/Toll-like receptor-induced autophosphorylation activates interleukin 1 receptor-associated kinase 4 and controls cytokine induction in a cell type-specific manner

IRAK4 is a central kinase in innate immunity, but the role of its kinase activity is controversial. The mechanism of activation for IRAK4 is currently unknown, and little is known about the role of IRAK4 kinase in cytokine production, particularly in different human cell types. We show IRAK4 autophosphorylation occurs by an intermolecular reaction and that autophosphorylation is required for full catalytic activity of the kinase. Phosphorylation of any two of the residues Thr-342, Thr-345, and Ser-346 is required for full activity, and the death domain regulates the activation of IRAK4. Using antibodies against activated IRAK4, we demonstrate that IRAK4 becomes phosphorylated in human cells following stimulation by IL-1R and Toll-like receptor agonists, which can be blocked pharmacologically by a dual inhibitor of IRAK4 and IRAK1. Interestingly, in dermal fibroblasts, although complete inhibition of IRAK4 kinase activity does not inhibit IL-1-induced IL-6 production, NF-κB, or MAPK activation, there is complete ablation of these processes in IRAK4-deficient cells. In contrast, the inhibition of IRAK kinase activity in primary human monocytes reduces R848-induced IL-6 production with minimal effect on NF-κB or MAPK activation. Taken together, these studies define the mechanism of IRAK4 activation and highlight the differential role of IRAK4 kinase activity in different human cell types as well as the distinct roles IRAK4 scaffolding and kinase functions play.

Nitration of tyrosine 247 inhibits protein kinase G-1α activity by attenuating cyclic guanosine monophosphate binding

The cGMP-dependent protein kinase G-1α (PKG-1α) is a downstream mediator of nitric oxide and natriuretic peptide signaling. Alterations in this pathway play a key role in the pathogenesis and progression of vascular diseases associated with increased vascular tone and thickness, such as pulmonary hypertension. Previous studies have shown that tyrosine nitration attenuates PKG-1α activity. However, little is known about the mechanisms involved in this event. Utilizing mass spectrometry, we found that PKG-1α is susceptible to nitration at tyrosine 247 and 425. Tyrosine to phenylalanine mutants, Y247F- and Y425F-PKG-1α, were both less susceptible to nitration than WT PKG-1α, but only Y247F-PKG-1α exhibited preserved activity, suggesting that the nitration of Tyr(247) is critical in attenuating PKG-1α activity. The overexpression of WT- or Y247F-PKG-1α decreased the proliferation of pulmonary artery smooth muscle cells (SMC), increased the expression of SMC contractile markers, and decreased the expression of proliferative markers. Nitrosative stress induced a switch from a contractile to a synthetic phenotype in cells expressing WT- but not Y247F-PKG-1α. An antibody generated against 3-NT-Y247 identified increased levels of nitrated PKG-1α in humans with pulmonary hypertension. Finally, to gain a more mechanistic understanding of how nitration attenuates PKG activity, we developed a homology model of PKG-1α. This model predicted that the nitration of Tyr(247) would decrease the affinity of PKG-1α for cGMP, which we confirmed using a [(3)H]cGMP binding assay. Our study shows that the nitration of Tyr(247) and the attenuation of cGMP binding is an important mechanism regulating in PKG-1α activity and SMC proliferation/differentiation.

West Nile virus methyltransferase domain interacts with protein kinase G

Background

The flaviviral nonstructural protein 5 (NS5) is a phosphoprotein, though the precise identities and roles of many specific phosphorylations remain unknown. Protein kinase G (PKG), a cGMP-dependent protein kinase, has previously been shown to phosphorylate dengue virus NS5.

Methods

We used mass spectrometry to specifically identify NS5 phosphosites. Co-immunoprecipitation assays were used to study protein-protein interactions. Effects on viral replication were measured via replicon system and plaque assay titering.

Results

We identified multiple sites in West Nile virus (WNV) NS5 that are phosphorylated during a WNV infection, and showed that the N-terminal methyltransferase domain of WNV NS5 can be specifically phosphorylated by PKG in vitro. Expressing PKG in cell culture led to an enhancement of WNV viral production. We hypothesized this effect on replication could be caused by factors beyond the specific phosphorylations of NS5. Here we show for the first time that PKG is also able to stably interact with a viral substrate, WNV NS5, in cell culture and in vitro. While the mosquito-borne WNV NS5 interacted with PKG, tick-borne Langat virus NS5 did not. The methyltransferase domain of NS5 is able to mediate the interaction between NS5 and PKG, and mutating positive residues in the αE region of the methyltransferase interrupts the interaction. These same mutations completely inhibited WNV replication.

Conclusions

PKG is not required for WNV replication, but does make a stable interaction with NS5. While the consequence of the NS5:PKG interaction when it occurs is unclear, mutational data demonstrates that this interaction occurs in a region of NS5 that is otherwise necessary for replication. Overall, the results identify an interaction between virus and a cellular kinase and suggest a role for a host kinase in enhancing flaviviral replication.

Mosquito protein kinase G phosphorylates flavivirus NS5 and alters flight behavior in Aedes aegypti and Anopheles gambiae

Many arboviral proteins are phosphorylated in infected mammalian cells, but it is unknown if the same phosphorylation events occur when insects are similarly infected. One of the mammalian kinases responsible for phosphorylation, protein kinase G (PKG), has been implicated in the behavior of multiple nonvector insects, but is unstudied in mosquitoes. PKG from Aedes aegypti was cloned, and phosphorylation of specific viral sites was monitored by mass spectrometry from biochemical and cell culture experiments. PKG from Aedes mosquitoes is able to phosphorylate dengue nonstructural protein 5 (NS5) at specific sites in cell culture and cell-free systems and autophosphorylates its own regulatory domain in a cell-free system. Injecting Aedes aegypti and Anopheles gambiae mosquitoes with a pharmacological PKG activator resulted in increased Aedes wing activity during periods of their natural diurnal/crepuscular activity and increased Anopheles nocturnal locomotor/flight activity. Thus, perturbation of the PKG signaling pathway in mosquitoes alters flight behavior. The demonstrated effect of PKG alterations is consistent with a viral PKG substrate triggering increased PKG activity. This increased PKG activity could be the mechanism by which dengue virus increases flight behavior and possibly facilitates transmission. Whether or not PKG is part of the mechanism by which dengue increases flight behavior, this report is the first to show PKG can modulate behavior in hematophagous disease vectors.

Transcriptional and post-transcriptional regulation of cGMP-dependent protein kinase (PKG-I): pathophysiological significance

The ability of the endothelium to produce nitric oxide, which induces generation of cyclic guanosine monophosphate (cGMP) that activates cGMP-dependent protein kinase (PKG-I), in vascular smooth muscle cells (VSMCs), is essential for the maintenance of vascular homeostasis. Yet, disturbance of this nitric oxide/cGMP/PKG-I pathway has been shown to play an important role in many cardiovascular diseases. In the last two decades, in vitro and in vivo models of vascular injury have shown that PKG-I is suppressed following nitric oxide, cGMP, cytokine, and growth factor stimulation. The molecular basis for these changes in PKG-I expression is still poorly understood, and they are likely to be mediated by a number of processes, including changes in gene transcription, mRNA stability, protein synthesis, or protein degradation. Emerging studies have begun to define mechanisms responsible for changes in PKG-I expression and have identified cis- and trans-acting regulatory elements, with a plausible role being attributed to post-translational control of PKG-I protein levels. This review will focus mainly on recent advances in understanding of the regulation of PKG-I expression in VSMCs, with an emphasis on the physiological and pathological significance of PKG-I down-regulation in VSMCs in certain circumstances.

cGMP-dependent protein kinase I is involved in neurite outgrowth via a Rho effector, rhotekin, in Neuro2A neuroblastoma cells

Although the cGMP/cGMP-dependent protein kinase (cGK) signaling is involved in the regulation of neurite outgrowth, its mechanism remains to be clarified. In this study, we identified a Rho effector, rhotekin, as a cGK-I-interacting protein. Rhotekin was also a substrate for cGK-Iα. In neurite-extended Neuro2A neuroblastoma cells, cGK-Iα and rhotekin were colocalized in the plasma membrane and extended neurites, while treatment with cGMP resulted in translocation of rhotekin to the cytoplasm. In addition, we found that cGK-Iα and rhotekin synergistically suppressed Rho-induced neurite retraction. Our findings suggest that cGK-Iα interacts with and phosphorylates rhotekin, thereby contributing to neurite outgrowth regulation.

Phosphorylation events during viral infections provide potential therapeutic targets

For many medically relevant viruses, there is now considerable evidence that both viral and cellular kinases play important roles in viral infection. Ultimately, these kinases, and the cellular signaling pathways that they exploit, may serve as therapeutic targets for treating patients. Currently, small molecule inhibitors of kinases are under investigation as therapy for herpes viral infections. Additionally, a number of cellular or host-directed tyrosine kinase inhibitors that have been previously FDA approved for cancer treatment are under study in animal models and clinical trials, as they have shown promise for the treatment of various viral infections as well. This review will highlight the wide range of viral proteins phosphorylated by viral and cellular kinases, and the potential for variability of kinase recognition sites within viral substrates to impact phosphorylation and kinase prediction. Research studying kinase-targeting prophylactic and therapeutic treatments for a number of viral infections will also be discussed.

Collagen stimulation of platelets induces a rapid spatial response of cAMP and cGMP signaling scaffolds

Intracellular communication is tightly regulated in both space and time. Spatiotemporal control is important to achieve a high level of specificity in both dimensions. For instance, cAMP-dependent kinase (PKA) attains spatial resolution by interacting with distinct members of the family of A-kinase anchoring proteins (AKAPs) that position PKA at specific loci within the cell. To control the cAMP induced signal in time, distinct signal terminators such as phosphodiesterases and phosphatases are often co-localized at the AKAP scaffold. In platelets, high levels of cAMP/cGMP maintain the resting state to allow free circulation. Exposure to collagen, for instance when the vessel is damaged, triggers platelet activation through initiation of the GPVI (glycoprotein VI)/FcRγ-chain forming the onset of a plethora of signaling pathways. Consequently overall intra-platelet cAMP and cGMP levels drop, however detail on how PKA, but also cGMP-dependent protein kinase (PKG) respond in relation to their localized signaling scaffolds is currently missing. To investigate this, we employed a quantitative chemical proteomics approach in activated human platelets enabling the specific enrichment of cAMP/cGMP signaling nodes. Our data reveal that within a few minutes several specific PKA and PKG signaling nodes respond significantly to the activating signal, whereas others do not, suggesting a rapid adaption of specific localized cAMP and cGMP pools to the stimulus. Using protein phosphorylation data gathered we touch upon the potential cross-talk between protein phosphorylation and signaling scaffold function as a general theme in platelet spatiotemporal control.

The amino terminus of cGMP-dependent protein kinase Iβ increases the dynamics of the protein's cGMP-binding pockets

The type I cGMP-dependent protein kinases play critical roles in regulating vascular tone, platelet activation and synaptic plasticity. PKG I α and PKG Iβ differ in their first ~100 amino acids giving each isoform unique dimerization and autoinhibitory domains with identical cGMP-binding pockets and catalytic domains. The N-terminal leucine zipper and autoinhibitory domains have been shown to mediate isoform specific affinity for cGMP. PKG Iα has a >10 fold higher affinity for cGMP than PKG Iβ, and PKG Iβ that is missing its leucine zipper has a three-fold decreased affinity for cGMP. The exact mechanism through which the N-terminus of PKG alters cGMP-affinity is unknown. In the present study, we have used deuterium exchange mass spectrometry to study how PKG Iβ's N-terminus affects the conformation and dynamics of its cGMP-binding pockets. We found that the N-terminus increases the rate of deuterium exchange throughout the cGMP-binding domain. Our results suggest that the N-terminus shifts the conformational dynamics of the binding pockets, leading to an "open" conformation that has an increased affinity for cGMP.

cGMP-dependent protein kinases and cGMP phosphodiesterases in nitric oxide and cGMP action

To date, studies suggest that biological signaling by nitric oxide (NO) is primarily mediated by cGMP, which is synthesized by NO-activated guanylyl cyclases and broken down by cyclic nucleotide phosphodiesterases (PDEs). Effects of cGMP occur through three main groups of cellular targets: cGMP-dependent protein kinases (PKGs), cGMP-gated cation channels, and PDEs. cGMP binding activates PKG, which phosphorylates serines and threonines on many cellular proteins, frequently resulting in changes in activity or function, subcellular localization, or regulatory features. The proteins that are so modified by PKG commonly regulate calcium homeostasis, calcium sensitivity of cellular proteins, platelet activation and adhesion, smooth muscle contraction, cardiac function, gene expression, feedback of the NO-signaling pathway, and other processes. Current therapies that have successfully targeted the NO-signaling pathway include nitrovasodilators (nitroglycerin), PDE5 inhibitors [sildenafil (Viagra and Revatio), vardenafil (Levitra), and tadalafil (Cialis and Adcirca)] for treatment of a number of vascular diseases including angina pectoris, erectile dysfunction, and pulmonary hypertension; the PDE3 inhibitors [cilostazol (Pletal) and milrinone (Primacor)] are used for treatment of intermittent claudication and acute heart failure, respectively. Potential for use of these medications in the treatment of other maladies continues to emerge.

Pericyte-mediated regulation of capillary diameter: a component of neurovascular coupling in health and disease

Because regional blood flow increases in association with the increased metabolic demand generated by localized increases in neural activity, functional imaging researchers often assume that changes in blood flow are an accurate read-out of changes in underlying neural activity. An understanding of the mechanisms that link changes in neural activity to changes in blood flow is crucial for assessing the validity of this assumption, and for understanding the processes that can go wrong during disease states such as ischaemic stroke. Many studies have investigated the mechanisms of neurovascular regulation in arterioles but other evidence suggests that blood flow regulation can also occur in capillaries, because of the presence of contractile cells, pericytes, on the capillary wall. Here we review the evidence that pericytes can modulate capillary diameter in response to neuronal activity and assess the likely importance of neurovascular regulation at the capillary level for functional imaging experiments. We also discuss evidence suggesting that pericytes are particularly sensitive to damage during pathological insults such as ischaemia, Alzheimer's disease and diabetic retinopathy, and consider the potential impact that pericyte dysfunction might have on the development of therapeutic interventions and on the interpretation of functional imaging data in these disorders.

Novel crosstalk to BMP signalling: cGMP-dependent kinase I modulates BMP receptor and Smad activity

Integration of multiple signals into the canonical BMP/Smad pathway poses a big challenge during the course of embryogenesis and tissue homeostasis. Here, we show that cyclic guanosine 3',5'-monophosphate (cGMP)-dependent kinase I (cGKI) modulates BMP receptors and Smads, providing a novel mechanism enhancing BMP signalling. cGKI, a key mediator of vasodilation and hypertension diseases, interacts with and phosphorylates the BMP type II receptor (BMPRII). In response to BMP-2, cGKI then dissociates from the receptors, associates with activated Smads, and undergoes nuclear translocation. In the nucleus, cGKI binds with Smad1 and the general transcription factor TFII-I to promoters of BMP target genes such as Id1 to enhance transcriptional activation. Accordingly, cGKI has a dual function in BMP signalling: (1) it modulates BMP receptor/Smad activity at the plasma membrane and (2) after redistribution to the nucleus, it further regulates transcription as a nuclear co-factor for Smads. Consequently, cellular defects caused by mutations in BMPRII, found in pulmonary arterial hypertension patients, were compensated through cGKI, supporting the positive action of cGKI on BMP-induced Smad signalling downstream of the receptors.

Cyclic GMP specifically suppresses Type-Ialpha cGMP-dependent protein kinase expression by ubiquitination

Type I cGMP-dependent protein kinase (PKG-I) mediates nitric oxide (NO) and hormone dependent smooth muscle relaxation and stimulates smooth muscle cell-specific gene expression. Expression of PKG-I in cultured smooth muscle cells depends on culture conditions and is inhibited by inflammatory cytokines such as interleukin-I and tumor necrosis factor-alpha, which are known to stimulate Type II NO synthase (iNOS) expression. We report here that the suppression of PKG-I protein levels in smooth muscle cells is triggered by the ubiquitin/26S proteasome pathway. Incubation of vascular smooth muscle cells with phosphodiesterase-resistant cyclic GMP analogs (e.g., 8-bromo-cGMP) decreases PKG-I protein level in a time- and concentration-dependent manner. To study this process, we tested the effects of 8-Br-cGMP on PKG-I protein level in Cos7 cells, which do not express endogenous type I PKG mRNA. 8-Br-cGMP induced the ubiquitination and down-regulation of PKG-Ialpha, but not PKG-Ibeta. Treatment of cells with the 26S proteasome inhibitor, MG-132, increased ubiquitination of PKG. Blocking PKG-I catalytic activity using the cell-permeant specific PKG-I inhibitor, DT-2, inhibited cGMP-induced PKG-I ubiquitination and down-regulation, suggesting that PKG catalytic activity and autophosphorylation were required for suppression of PKG-I level. Mutation of the known autophosphorylation sites of PKG-Ialpha to alanine uncovered a specific role for autophosphorylation of serine-64 in cGMP-dependent ubiquitination and suppression of PKG-I level. The results suggest that chronic elevation of cGMP, as seen in inflammatory conditions, triggers ubiquitination and degradation of PKG-Ialpha in smooth muscle.

cGMP regulated protein kinases (cGK)

cGMP-dependent protein kinases (cGK) are serine/threonine kinases that are widely distributed in eukaryotes. Two genes--prkg1 and prkg2--code for cGKs, namely cGKI and cGKII. In mammals, two isozymes, cGKIalpha and cGKIbeta, are generated from the prkg1 gene. The cGKI isozymes are prominent in all types of smooth muscle, platelets, and specific neuronal areas such as cerebellar Purkinje cells, hippocampal neurons, and the lateral amygdala. The cGKII prevails in the secretory epithelium of the small intestine, the juxta-glomerular cells, the adrenal cortex, the chondrocytes, and in the nucleus suprachiasmaticus. Both cGKs are major downstream effectors of many, but not all signalling events of the NO/cGMP and the ANP/cGMP pathways. cGKI relaxes smooth muscle tone and prevents platelet aggregation, whereas cGKII inhibits renin secretion, chloride/water secretion in the small intestine, the resetting of the clock during early night, and endochondreal bone growth. cGKs are also modulators of cell growth and many other functions.

Prolonged treatment of porcine pulmonary artery with nitric oxide decreases cGMP sensitivity and cGMP-dependent protein kinase specific activity

A cultured porcine pulmonary artery (PA) model was used to examine the effects of prolonged nitric oxide (NO) treatment on the response to acutely applied NO, cGMP analog, or atrial natriuretic peptide (ANP). Twenty-four-hour treatment with the NO donor (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA-NO) resulted in >10-fold decrease in the response to acutely applied DETA-NO. In parallel with this, the relaxant response to acutely applied cGMP analog, beta-phenyl-1,N(2)-etheno-8-bromoguanosine-3',5'-cyclic monophosphorothioate, Sp isomer (Sp-8-Br-PET-cGMPS), and ANP decreased. The reduction in ANP responsiveness in PA was not associated with a reduction in cGMP levels evoked by 10(-6) M ANP. Twenty-four hours in culture and treatment with DETA-NO decreased total cGMP-dependent protein kinase (cGKI) mRNA level compared with that in freshly prepared PA (1.05 +/- 0.12, 0.42 +/- 0.08, and 0.11 +/- 0.01 amol/mug, respectively). Total cGKI protein levels were decreased to a lesser extent by 24 h in culture and further decreased by 24-h DETA-NO treatment compared with that in freshly prepared PA (361 +/- 33, 272 +/- 20, and 238 +/- 25 ng/mg total protein, respectively). Maximal cGMP-stimulated phosphotransferase activity was reduced in 24-h cultured and DETA-NO-treated PA (986 +/- 84, 815 +/- 81, and 549 +/- 78 pmol P(i).min(-1).mg soluble protein(-1)), but the cGMP concentration resulting in 50% of maximal phosphotransferase activity was not. cGKI specific activity (maximal cGMP-activated phosphotransferase activity/ng cGKI) was significantly reduced in PA treated with DETA-NO for 24 h compared with freshly prepared and 24-h cultured PA (1.95 +/- 0.22, 2.64 +/- 0.25, and 2.85 +/- 0.28 pmol P(i).min(-1).ng cGKI(-1), respectively). We conclude that prolonged NO treatment induces decreased acute NO responsiveness in PA in part by decreasing cGMP sensitivity. It does so by decreasing both cGKI expression and cGKI specific activity.

A multi-angular mass spectrometric view at cyclic nucleotide dependent protein kinases: in vivo characterization and structure/function relationships

Mass spectrometry has evolved in recent years to a well-accepted and increasingly important complementary technique in molecular and structural biology. Here we review the many contributions mass spectrometry based studies have made in recent years in our understanding of the important cyclic nucleotide activated protein kinase A (PKA) and protein kinase G (PKG). We both describe the characterization of kinase isozymes, substrate phosphorylation, binding partners and post-translational modifications by proteomics based methodologies as well as their structural and functional properties as revealed by native mass spectrometry, H/D exchange MS and ion mobility. Combining all these mass spectrometry based data with other biophysical and biochemical data has been of great help to unravel the intricate regulation of kinase function in the cell in all its magnificent complexity.

Evidence for amylase release by cGMP via cAMP-dependent protein kinase in rat parotid acinar cells

Amylase release from the rat parotid gland is primarily mediated by a cAMP-dependent protein kinase (PKA). We previously reported that cGMP/cGMP-dependent protein kinase (PKG) signaling evokes amylase release. In the present study, we investigated whether cGMP-mediated amylase release might be due to cGMP/PKA signaling, as well as cGMP/PKG pathway. Activation of PKA by cGMP was required 100-1000-fold greater concentration than activation by cAMP in a parotid cytosol fraction. Synergistic activation of PKA by the combination of physiological cAMP and low concentration of cGMP was observed. Amylase release from intact acinar cells was synergistically stimulated by the combination of diBu-cAMP and 8-pCPT-cGMP. cGMP dose-dependently stimulated amylase release from saponin-permeabilized parotid acinar cells. Phosphorylation by cGMP produced phosphorylated proteins of the same size as those produced by cAMP. Phosphorylation by cGMP was inhibited by the addition of PKA inhibitor, H-89. These results suggest that cGMP activates both PKG and PKA. Thus, it appears that both cGMP/PKG and cGMP/PKA pathways mediate amylase release from rat parotid acinar cells.

Function of cGMP-Dependent Protein Kinases as Revealed by Gene Deletion

Over the past few years, a wealth of biochemical and functional data have been gathered on mammalian cGMP-dependent protein kinases (cGKs). In mammals, three different kinases are encoded by two genes. Mutant and chimeric cGK proteins generated by molecular biology techniques yielded important biochemical knowledge, such as the function of the NH2-terminal domains of cGKI and cGKII, the identity of the cGMP-binding sites of cGKI, and the substrate specificity of the enzymes. Genetic approaches have proven especially useful for the analysis of the biological functions of cGKs. Recently, some of the in vivo targets and mechanisms leading to changes in neuronal adaptation, smooth muscle relaxation and growth, intestinal water secretion, bone growth, renin secretion, and other important functions have been identified. These data show that cGKs are signaling molecules involved in many biological functions.

Multigene family encoding 3',5'-cyclic-GMP-dependent protein kinases in Paramecium tetraurelia cells

In the ciliate Paramecium tetraurelia, 3',5'-cyclic GMP (cGMP) is one of the second messengers involved in several signal transduction pathways. The enzymes for its production and degradation are well established for these cells, whereas less is known about the potential effector proteins. On the basis of a current Paramecium genome project, we have identified a multigene family with at least 35 members, all of which encode cGMP-dependent protein kinases (PKGs). They can be classified into 16 subfamilies with several members each. Two of the genes, PKG1-1 and PKG2-1, were analyzed in more detail after molecular cloning. They encode monomeric enzymes of 770 and 819 amino acids, respectively, whose overall domain organization resembles that in higher eukaryotes. The enzymes contain a regulatory domain of two tandem cyclic nucleotide-binding sites flanked by an amino-terminal region for intracellular localization and a catalytic domain with highly conserved regions for ATP binding and catalysis. However, some Paramecium PKGs show a different structure. In Western blots, PKGs are detected both as cytosolic and as structure-bound forms. Immunofluorescence labeling shows enrichment in the cell cortex, notably around the dense-core secretory vesicles (trichocysts), as well as in cilia. Immunogold electron microscopy analysis reveals consistent labeling of ciliary membranes, of the membrane complex composed of cell membrane and cortical Ca2+ stores, and of regions adjacent to ciliary basal bodies, trichocysts, and trafficking vesicles. Since PKGs (re)phosphorylate the exocytosis-sensitive phosphoprotein pp63/pf upon stimulation, the role of PKGs during stimulated exocytosis is discussed, in addition to a role in ciliary beat regulation.

Nitric oxide sensitivity in pulmonary artery and airway smooth muscle: a possible role for cGMP responsiveness

We aimed to assess intrinsic smooth muscle mechanisms contributing to greater nitric oxide (NO) responsiveness in pulmonary vascular vs. airway smooth muscle. Porcine pulmonary artery smooth muscle (PASM) and tracheal smooth muscle (TSM) strips were used in concentration-response studies to the NO donor (Z)-1-[N-2-aminoethyl-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA-NO). PASM consistently exhibited greater relaxation at a given DETA-NO concentration (NO responsiveness) than TSM NO responsiveness, with DETA-NO log EC(50) being -6.55 +/- 0.11 and -5.37 +/- 0.13 for PASM and TSM, respectively (P < 0.01). We determined relationships between tissue cGMP concentration ([cGMP](i)) and relaxation using the particulate guanylyl cyclase agonist atrial natriuretic peptide. Atrial natriuretic peptide resulted in nearly complete relaxation, with no detectable increase in [cGMP](i) in PASM and only 20% relaxation (10-fold increase in [cGMP](i)) in TSM, indicating that TSM is less cGMP responsive than PASM. Total cGMP-dependent protein kinase I (cGKI) mRNA expression was greater in PASM than in TSM (2.23 +/- 0.36 vs. 0.93 +/- 0.31 amol mRNA/mug total RNA, respectively; P < 0.01), but total cGKI protein expression was not significantly different (0.56 +/- 0.07 and 0.49 +/- 0.04 ng cGKI/mug protein, respectively). The phosphotransferase assay for the soluble fraction of tissue homogenates demonstrated no difference in the cGMP EC(50) between PASM and TSM. The maximal phosphotransferase activity indexed to the amount of total cGKI in the homogenate differed significantly between PASM and TSM (1.61 +/- 0.15 and 1.04 +/- pmol.min(-1).ng cGKI(-1), respectively; P < 0.05), suggesting that cGKI may be regulated differently in the two tissues. A novel intrinsic smooth muscle mechanism accounting for greater NO responsiveness in PASM vs. TSM is thus greater cGMP responsiveness from increased cGKI-specific activity in PASM.

Chronic hyperammonemia in vivo impairs long-term potentiation in hippocampus by altering activation of cyclic GMP-dependent-protein kinase and of phosphodiesterase 5

Long-term potentiation (LTP) is impaired in the CA1 area of hippocampal slices from rats with chronic moderate hyperammonemia. We studied the mechanisms by which hyperammonemia in vivo impairs LTP. This process requires sequential activation of soluble guanylate cyclase, cyclic GMP-dependent protein kinase (PKG) and cyclic GMP-degrading phosphodiesterase. Application of the tetanus induced a rapid increase of cyclic GMP in slices from control or hyperammonemic rats, which is followed in control slices by a sustained decrease in cyclic GMP due to sustained activation of cyclic GMP-degrading phosphodiesterase, which in turn is due to sustained activation of PKG. In slices from rats with chronic hyperammonemia tetanus-induced decrease in cyclic GMP was delayed and transient due to lower and transient activation of PKG and of the phosphodiesterase. Hyperammonemia-induced impairment of LTP may be involved in the alterations of cognitive function in patients with hepatic encephalopathy.

Inhibition of cGMP-dependent protein kinase by the cell-permeable peptide DT-2 reveals a novel mechanism of vasoregulation

Cyclic GMP-dependent protein kinase (PKG) serves as an important physiological regulator of vascular reactivity and tone. However, available inhibitors of PKG have exhibited variable effects in intact tissue, hindering the elucidation of the functional role of PKG in blood vessels. In this study, we have determined the effects of our previously engineered potent and selective PKG Ialpha inhibitor DT-2 on basal and cGMP-stimulated purified recombinant PKG, and compared DT-2 with commonly used PKG inhibitors (8R,9S,11S)-(-)-9-methoxy-carbamyl-8-methyl-2,3,9,10-tetrahydro-8,11-epoxy-1H,8H,11H-2,7b,11a-trizadibenzo-(a,g)-cycloocta-(c,d,e)-trinden-1-one (KT-5823), Rp-8-(4-chlorophenylthio)-guanosine-3',5'-cyclic monophosphorothioate (Rp-8-pCPT-cGMPS), and (beta-phenyl-1,N2-etheno-8-bromoguanosine-3',5'-cyclic monophosphorothioate, Rp-isomer (Rp-8-Br-PET-cGMPS). As expected, all inhibitors reduced cGMP-stimulated PKG activity. However, only DT-2 decreased cGMP-independent or basal PKG activity, whereas KT5823 showed no effect and the Rp-compounds actually had partial agonist activity. To evaluate the potential functional impact of this unique inhibition by DT-2 under physiologically relevant conditions, we analyzed the inhibitors in isolated pressurized cerebral arteries. KT-5823 and Rp-8-pCPT-cGMPS demonstrated marginal reversal of vasodilation induced by 8-Br-cGMP. By comparison, DT-2 completely reversed 8-Br-cGMP induced dilations with comparable potency to Rp-8-Br-PET-cGMPS. In fact, DT-2 constricted arteries beyond their starting (pre-8-Br-cGMP) diameters and caused constriction even in the absence of exogenous 8-Br-cGMP, an effect that was not observed with any other inhibitor. The direct constricting effect of DT-2 was essentially abolished in cultured arteries, where PKG expression was reduced by approximately 90%. These findings indicate that DT-2 not only effectively inhibits cGMP-stimulated PKG activity but also reduces basal PKG activity both in vitro and in vivo. Moreover, these distinctive inhibitory properties of DT-2 suggest an important role for constitutive PKG activity in the continuous regulation of cerebral artery tone.

NRSF regulates the fetal cardiac gene program and maintains normal cardiac structure and function

Reactivation of the fetal cardiac gene program is a characteristic feature of hypertrophied and failing hearts that correlates with impaired cardiac function and poor prognosis. However, the mechanism governing the reversible expression of fetal cardiac genes remains unresolved. Here we show that neuron-restrictive silencer factor (NRSF), a transcriptional repressor, selectively regulates expression of multiple fetal cardiac genes, including those for atrial natriuretic peptide, brain natriuretic peptide and alpha-skeletal actin, and plays a role in molecular pathways leading to the re-expression of those genes in ventricular myocytes. Moreover, transgenic mice expressing a dominant-negative mutant of NRSF in their hearts exhibit dilated cardiomyopathy, high susceptibility to arrhythmias and sudden death. We demonstrate that genes encoding two ion channels that carry the fetal cardiac currents I(f) and I(Ca,T), which are induced in these mice and are potentially responsible for both the cardiac dysfunction and the arrhythmogenesis, are regulated by NRSF. Our results indicate NRSF to be a key transcriptional regulator of the fetal cardiac gene program and suggest an important role for NRSF in maintaining normal cardiac structure and function.

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.

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.

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.

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.

Autoinhibition and isoform-specific dominant negative inhibition of the type II cGMP-dependent protein kinase

In the absence of cyclic nucleotides, the cAMP-dependent protein kinase and cGMP-dependent protein kinases (cGKs) suppress phosphotransfer activity at the catalytic cleft by competitive inhibition of substrate binding with a pseudosubstrate sequence within the holoenzyme. The magnitude of inhibition can be diminished by autophosphorylation near this pseudosubstrate sequence. Activation of type I cGK (cGKI) and type II cGK (cGKII) are differentially regulated by their cyclic nucleotide-binding sites. To address the possibility that the distinct activation mechanisms of cGKII and cGKI result from differences in the autophosphorylation of the inhibitory domain, we investigated the effects of autophosphorylation on the kinetics of activation. Unlike the type I cGKs (cGKIalpha and Ibeta), cGKII autophosphorylation did not alter the basal activity, nor the sensitivity of the enzyme to cyclic nucleotide activation. To determine residues responsible for autoinhibition of cGKII, Ala was substituted for basic residues (Lys(122), Arg(118), and Arg(119)) or a hydrophobic residue (Val(125)) within the putative pseudosubstrate domain of cGKII. The integrity of these residues was essential for full cGKII autoinhibition. Furthermore, a cGKII truncation mutant containing this autoinhibitory region demonstrated a nanomolar IC(50) toward a constitutively active form of cGKII. Finally, we present evidence that the dominant negative properties of this truncation mutant are specific to cGKII when compared with cAMP-dependent protein kinase Calpha and cGKIbeta. These findings extend the known differences in the activation mechanisms among cGK isoforms and allow the design of an isoform-specific cGKII inhibitor.

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.

Stimulation of alpha2-adrenergic receptor inhibits cholangiocarcinoma growth through modulation of Raf-1 and B-Raf activities

Growth factor signaling, mediated by the mitogen-activated protein kinase (MAPK) cascade, induces cell mitosis. Adenosine 3',5'-monophosphate (cAMP) may inhibit or stimulate mitosis (depending on the cell type) through the activation of MAPK and Raf proteins. Among Raf proteins, Raf-1 and B-Raf differentially regulate mitosis. Our aims were to evaluate the role and mechanisms of action of the alpha(2)-adrenergic agonist UK14,304 in the regulation of growth of the human cholangiocarcinoma cell line Mz-ChA-1. Immunocytochemistry and immunoblotting for alpha(2A)-, alpha(2B)-, or alpha(2C)-adrenergic receptor subtypes showed positive reaction in Mz-ChA-1 cells. We found that physiological concentrations of UK14,304 increased cAMP levels and inhibited proliferation and MAPK activity in Mz-ChA-1 cells. Mz-ChA-1 cells expressed Raf-1 and B-Raf. Epidermal growth factor (EGF) immediately and transiently stimulated Raf-1 activity, whereas B-Raf activity was increased with prolonged EGF stimulation. EGF-stimulated Raf-1 and B-Raf activities were both inhibited by UK14,304. UK14,304 did not affect Ras activity. In Mz-ChA-1 cells, alpha(2)-adrenoreceptor stimulation causes up-regulation of cAMP, which inhibits EGF-induced MAPK activity through an acute increase of Raf-1 and sustained activation of B-Raf. In conclusion, because alpha(2)-AR inhibition of growth occurred downstream of Ras, adrenergic stimulation or other stimulants of cAMP may overcome the Ras mutations and offer a new therapeutic approach for patients with cholangiocarcinoma.

Roles for both cyclic GMP and cyclic AMP in the inhibition of collagen-induced platelet aggregation by nitroprusside

In studies on human platelets, nitroprusside (NP) alone at 1-10 micromol/l increased platelet cyclic AMP (cAMP) by 40-70%, whereas increases in cyclic GMP (cGMP) were much larger in percentage though not in concentration terms. Collagen enhanced these increases in cAMP up to fourfold, without affecting cGMP. This effect was partly prevented by indomethacin or aspirin, indicating that platelet cyclo-oxygenase products acted synergistically with NP to increase cAMP. ADP released from the platelets by collagen tended to restrict this cAMP accumulation. Addition of 2',5'-dideoxyadenosine (DDA), an inhibitor of adenylyl cyclase, decreased both the inhibition of collagen-induced platelet aggregation by NP and the associated accumulation of cAMP without affecting cGMP, indicating that cAMP mediates part of the inhibitory effect of NP. Unlike DDA, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), an inhibitor of guanylyl cyclase, blocked all increases in both cGMP and cAMP caused by NP, as well as the inhibition of platelet aggregation, suggesting that cAMP accumulation was secondary to that of cGMP. Human platelet cGMP-dependent protein kinase (PKG) coelectrophoresed with the purified bovine type Ibeta isoenzyme. An inhibitor of this enzyme (Rp)-beta-phenyl-1,N2-etheno-8-bromoguanosine 3',5'-cyclic-monophosphorothioate, diminished the inhibition of collagen-induced platelet aggregation by NP, but had little additional effect when DDA was present. This showed that both PKG and cAMP participate in the inhibition of collagen-induced platelet aggregation by NP. Moreover, selective activators of PKG and cAMP-dependent protein kinases had supra-additive inhibitory effects, suggesting that an optimal inhibitory effect of NP requires simultaneous activation of both enzymes.

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.

Activation of phosphodiesterase 5 and inhibition of guanylate cyclase by cGMP-dependent protein kinase in smooth muscle

The regulation of cGMP-specific phosphodiesterase (PDE) 5 and soluble guanylate cyclase (GC) by cGMP- and cAMP-dependent protein kinases (PKG and PKA respectively) was examined in gastric smooth muscle. The NO donor, sodium nitroprusside (SNP), stimulated PDE5 phosphorylation and activity, which was blocked by the selective PKG inhibitor, KT5823, resulting in an elevation of cGMP levels. Activation of PKA either directly by Sp-5,6-dichloro-1-beta-d-ribofuranosyl benzimidazole 3',5'-cyclic monophosphothioate, or via isoproterenol- and forskolin-dependent increase in cAMP, also caused an increase in PDE5 phosphorylation and activity, but only in the presence of cGMP; consistent with the dependence of PDE5 phosphorylation and activity on cGMP binding to allosteric sites in the regulatory domain of PDE5. The selective PKA inhibitors, myristoylated protein kinase inhibitor and H-89, blocked the increase in PDE5 phosphorylation and activity induced by PKA. SNP also stimulated soluble GC phosphorylation and activity. KT5823 abolished phosphorylation and augmented soluble GC activity, implying feedback inhibition of soluble GC by PKG-dependent phosphorylation. Phosphorylation by PKG was direct and could be induced in vitro. Activation of PKA had no effect on soluble GC. Thus cGMP levels are regulated by PKG- and PKA-dependent activation of PDE5 and PKG-specific inhibition of soluble GC.

BK(Ca) channel activation by membrane-associated cGMP kinase may contribute to uterine quiescence in pregnancy

We investigated the influence of pregnancy on large-conductance calcium-activated potassium channel (BK(Ca)) activity (NP(o)) and on channel expression in membranes of isolated human myometrial smooth muscle cells. NP(o) in inside-out patches was higher in pregnant myometria (PM) compared with nonpregnant myometria (NPM), and the half-maximal activation potential was shifted by 39 mV to more negative potentials. This effect was not due to an enhanced BK(Ca) channel expression. In the presence of cAMP kinase (PKA) or cGMP kinase (PKG), NP(o) increased in patches from PM but decreased in those from NPM. Western blot analysis and use of a specific PKG inhibitor (1 microM KT-5823) verified the existence of a partially active membrane-associated PKG. Inhibition of PKA by 100 nM PKI, the inhibitory peptide of PKA, had no effect on NP(o). 8-p-Chlorophenylthio-cGMP (8-pCPT-cGMP) hyperpolarized cells from PM. This effect was abolished by iberiotoxin, a specific blocker of BK(Ca) channels. It is concluded that an endogenous, membrane-bound PKG in myometrial cells specifically enhances BK(Ca) channel activity during pregnancy and thus may contribute to uterine quiescence during pregnancy.

cAMP-dependent vasodilators cross-activate the cGMP-dependent protein kinase to stimulate BK(Ca) channel activity in coronary artery smooth muscle cells

cAMP-dependent vasodilators are used to treat a variety of cardiovascular disorders; however, the signal transduction pathways and effector mechanisms stimulated by these agents are not fully understood. In the present study we demonstrate that cAMP-stimulating agents enhance the activity of the large-conductance, calcium-activated potassium (BK(Ca)) channel in single myocytes from coronary arteries by "cross-activation" of the cGMP-dependent protein kinase (protein kinase G, PKG). Single-channel patch-clamp data revealed that 10 micromol/L isoproterenol, forskolin, or dopamine opens BK(Ca) channels in coronary myocytes and that this effect is attenuated by inhibitors of PKG (KT5823; Rp-8-pCPT-cGMPS), but not by inhibiting the cAMP-dependent protein kinase (protein kinase A, PKA). In addition, a membrane-permeable analog, CPT-cAMP, also opened BK(Ca) channels in these myocytes, and this effect was reversed by KT5823. Direct biochemical measurement confirmed that dopamine or forskolin stimulates PKG activity in coronary arteries but does not elevate cGMP. Finally, the stimulatory effect of cAMP on BK(Ca) channels was reconstituted in a cell-free, inside-out patch by addition of purified PKG activated by either cGMP or cAMP. In contrast, channel gating was unaffected by exposure to the purified catalytic subunit of PKA. In summary, findings from on-cell and cell-free patch-clamp experiments provide direct evidence that cAMP-dependent vasodilators open BK(Ca) channels in coronary myocytes by cross-activation of PKG (but not via PKA). Biochemical assay confirmed this cross-activation mechanism of cAMP action in these arteries. This signaling pathway is a novel mechanism for regulation of potassium channel activity in vascular smooth muscle and other cells.

Identification of a conserved residue responsible for the autoinhibition of cGMP-dependent protein kinase Ialpha and beta

We isolated a constitutively active form of cGMP-dependent protein kinase Ialpha (cGK Ialpha) by PCR-driven random mutagenesis. The replacement of Ile-63 by Thr in the autoinhibitory domain results in the enhancement of autophosphorylation and the basal kinase activity in the absence of cGMP. The hydrophobicity at position 63 is essential for the inactive state of cGK Ialpha, and Ile-78 of cGK Ibeta is also required for the autoinhibitory property. Furthermore, cGK Ialpha (Ile-63-Thr) is constitutively active in vivo. These findings suggest that a conserved residue in the autoinhibitory domain was involved in the autoinhibition of both cGK Is.

Distinguishing the roles of the two different cGMP-binding sites for modulating phosphorylation of exogenous substrate (heterophosphorylation) and autophosphorylation of cGMP-dependent protein kinase

The role of each of the two different cGMP-binding sites (referred to as slow and fast sites) of type I cGMP-dependent protein kinase (PKG) in altering the rate of catalysis of phosphorylation of exogenous substrates (heterophosphorylation) or the rate of autophosphorylation has not been resolved. In the present study, the cGMP concentration required for half-maximal activation (A(50)) of wild-type PKG type Ibeta (WT) was 5-fold higher for heterophosphorylation than for autophosphorylation. cGMP occupation of the slow site was associated with an increase in the autophosphorylation rate, whereas occupation of the fast and slow site together was associated with a decrease in the autophosphorylation rate compared with the rate observed with occupation of the slow site alone. The contributions of each cGMP-binding site were investigated using PKG mutants containing substitutions of an invariant threonine residue that is critical for high affinity cGMP-binding in each site. Site-directed mutagenesis of Thr-317 of the fast site (T317A) increased the cGMP A(50) for heterophosphorylation 4-fold at 30 degrees C, with nominal effect on cGMP A(50) for autophosphorylation compared with WT. The analogous slow site mutation (T193A) increased the cGMP A(50) for heterophosphorylation and autophosphorylation 32- and 64-fold, respectively. Compared with WT, the cGMP A(50) of the double mutant (T193A/T317A) for heterophosphorylation was increased 300-fold, whereas the cGMP A(50) for autophosphorylation was similar to that of T193A. Thus, occupation of both cGMP-binding sites of PKG is required for maximal stimulation of heterophosphorylation, whereas occupation of the slow site alone is sufficient for stimulation of the rate of autophosphorylation, and additional occupation of the fast site reduces this rate.

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

Intracellular cAMP and cGMP levels are increased in response to a variety of hormonal and chemical stimuli; these nucleotides play key roles as second messenger signals in modulating myriad physiological processes. The cAMP-dependent protein kinase and cGMP-dependent protein kinase are major intracellular receptors for these nucleotides, and the actions of these enzymes account for much of the cellular responses to increased levels of cAMP or cGMP. This review summarizes many studies that have contributed significantly to an improved understanding of the catalytic, regulatory, and structural properties of these protein kinases. These accumulated findings provide insights into the mechanisms by which these enzymes produce their specific physiological effects and are helpful in considering the actions of other protein kinases as well.

Cyclic AMP- and cyclic GMP-dependent protein kinases differ in their regulation of cyclic AMP response element-dependent gene transcription

The ability of cGMP-dependent protein kinases (cGKs) to activate cAMP response element (CRE)-dependent gene transcription was compared with that of cAMP-dependent protein kinases (cAKs). Although both the type Ibeta cGMP-dependent protein kinase (cGKIbeta) and the type II cAMP-dependent protein kinase (cAKII) phosphorylated the cytoplasmic substrate VASP (vasodilator- and A kinase-stimulated phosphoprotein) to a similar extent, cyclic nucleotide regulation of CRE-dependent transcription was at least 10-fold higher in cAKII-transfected cells than in cGKIbeta-transfected cells. Overexpression of each kinase in mammalian cells resulted in a cytoplasmic localization of the unactivated enzyme. As reported previously, the catalytic (C) subunit of cAKII translocated to the nucleus following activation by 8-bromo-cyclic AMP. However, cGKIbeta did not translocate to the nucleus upon activation by 8-bromo-cyclic GMP. Replacement of an autophosphorylated serine (Ser79) of cGKIbeta with an aspartic acid resulted in a mutant kinase with constitutive kinase activity in vitro and in vivo. The cGKIbetaS79D mutant localized to the cytoplasm and was only a weak activator of CRE-dependent gene transcription. However, an amino-terminal deletion mutant of cGKIbeta was found in the nucleus as well as the cytoplasm and was a strong activator of CRE-dependent gene transcription. These data suggest that the inability of cGKs to translocate to the nucleus is responsible for the differential ability of cAKs and cGKs to activate CRE-dependent gene transcription and that nuclear redistribution of cGKs is not required for NO/cGMP regulation of gene transcription.

Phosphorylation of the alpha-subunits of the Na+/K+-ATPase from mammalian kidneys and Xenopus oocytes by cGMP-dependent protein kinase results in stimulation of ATPase activity

Phosphorylation of Na+/K+-ATPase by cGMP-dependent protein kinase (PKG) has been studied in enzymes purified from pig, dog, sheep and rat kidneys, and in Xenopus oocytes. PKG phosphorylates the alpha-subunits of all animal species investigated. Phosphorylation of the beta-subunit was not observed. The stoichiometry of phosphorylation estimated for pig, sheep and dog renal Na+/K+-ATPase is 3.5, 2.2 and 2.1 mol Pi per mol alpha-subunit, respectively. Proteolytic fingerprinting of the pig alpha1-subunits phosphorylated by PKG using specific antibodies raised against N-terminus or C-terminus reveals that phosphorylation sites are located within the intracellular loop of the alpha-subunit between the 35 kDa N-terminal and 27 kDa C-terminal fragments. Phosphorylation sites within the alpha1-subunit of the purified Na+/K+-ATPase do not appear to be easily accessible for PKG since incorporation of Pi requires 0.2% of Triton X-100. Administration of cGMP and PKG in the presence of 5 mm ATP, which prevents inactivation of the Na+/K+-ATPase by detergent, leads to stimulation of hydrolytic activity by 61%. Administration of 50 microm of cGMP or dbcGMP in yolk-free homogenates of Xenopus oocytes leads to stimulation of ouabain-dependent ATPase activity by 130-198% and to incorporation of 33P into the alpha-subunit without the detergent. Hence, PKG plays regulatory role in active transmembraneous transport of Na+ and K+ via phosphorylation of the catalytic subunit of the Na+/K+-ATPase.