Molecular determinants of the interaction between the inositol 1,4,5-trisphosphate receptor-associated cGMP kinase substrate (IRAG) and cGMP kinase Ibeta

Soluble guanylyl cyclase: A novel target for the treatment of vascular cognitive impairment?

Vascular cognitive impairment (VCI) describes neurodegenerative disorders characterized by a vascular component. Pathologically, it involves decreased cerebral blood flow (CBF), white matter lesions, endothelial dysfunction, and blood-brain barrier (BBB) impairments. Molecularly, oxidative stress and inflammation are two of the major underlying mechanisms. Nitric oxide (NO) physiologically stimulates soluble guanylate cyclase (sGC) to induce cGMP production. However, under pathological conditions, NO seems to be at the basis of oxidative stress and inflammation, leading to a decrease in sGC activity and expression. The native form of sGC needs a ferrous heme group bound in order to be sensitive to NO (Fe(II)sGC). Oxidation of sGC leads to the conversion of ferrous to ferric heme (Fe(III)sGC) and even heme-loss (apo-sGC). Both Fe(III)sGC and apo-sGC are insensitive to NO, and the enzyme is therefore inactive. sGC activity can be enhanced either by targeting the NO-sensitive native sGC (Fe(II)sGC), or the inactive, oxidized sGC (Fe(III)sGC) and the heme-free apo-sGC. For this purpose, sGC stimulators acting on Fe(II)sGC and sGC activators acting on Fe(III)sGC/apo-sGC have been developed. These sGC agonists have shown their efficacy in cardiovascular diseases by restoring the physiological and protective functions of the NO-sGC-cGMP pathway, including the reduction of oxidative stress and inflammation, and improvement of vascular functioning. Yet, only very little research has been performed within the cerebrovascular system and VCI pathology when focusing on sGC modulation and its potential protective mechanisms on vascular and neural function. Therefore, within this review, the potential of sGC as a target for treating VCI is highlighted.

Novel Functional Features of cGMP Substrate Proteins IRAG1 and IRAG2

The inositol triphosphate-associated proteins IRAG1 and IRAG2 are cGMP kinase substrate proteins that regulate intracellular Ca²⁺. Previously, IRAG1 was discovered as a 125 kDa membrane protein at the endoplasmic reticulum, which is associated with the intracellular Ca²⁺ channel IP₃R-I and the PKGIβ and inhibits IP₃R-I upon PKGIβ-mediated phosphorylation. IRAG2 is a 75 kDa membrane protein homolog of IRAG1 and was recently also determined as a PKGI substrate. Several (patho-)physiological functions of IRAG1 and IRAG2 were meanwhile elucidated in a variety of human and murine tissues, e.g., of IRAG1 in various smooth muscles, heart, platelets, and other blood cells, of IRAG2 in the pancreas, heart, platelets, and taste cells. Hence, lack of IRAG1 or IRAG2 leads to diverse phenotypes in these organs, e.g., smooth muscle and platelet disorders or secretory deficiency, respectively. This review aims to highlight the recent research regarding these two regulatory proteins to envision their molecular and (patho-)physiological tasks and to unravel their functional interplay as possible (patho-)physiological counterparts.

Function of IRAG2 Is Modulated by NO/cGMP in Murine Platelets

Inositol 1,4,5-triphosphate receptor-associated 2 (IRAG2) is a type II membrane protein located at the endoplasmic reticulum. It is a homologue of inositol 1,4,5-triphosphate receptor-associated cGMP kinase substrate 1 (IRAG1), a substrate protein of cGMP-dependent protein kinase I (PKGI), and is among others expressed in platelets. Here, we studied if IRAG2 is also located in platelets and might be a substrate protein of PKGI. IRAG2 was detected in platelets of IRAG2-WT animals but not in those of IRAG2-KO animals. Next, we validated by co-immunoprecipitation studies that IRAG2 is associated with IP₃R1-3. No direct stable interaction with PKGIβ or with IRAG1 was observed. Phosphorylation of IRAG2 in murine platelets using a Ser/Thr-specific phospho-antibody was found in vitro and ex vivo upon cGMP stimulation. To gain insight into the function of IRAG2, platelet aggregation studies were performed using thrombin and collagen as agonists for treatment of isolated IRAG2-WT or IRAG2-KO platelets. Interestingly, platelet aggregation was reduced in the absence of IRAG2. Pretreatment of wild type or IRAG2-KO platelets with sodium nitroprusside (SNP) or 8-pCPT-cGMP revealed a further reduction in platelet aggregation in the absence of IRAG2. These results show that IRAG2 is a substrate of PKGI in murine platelets. Furthermore, our results indicate that IRAG2 is involved in the induction of thrombin- or collagen-induced platelet aggregation and that this effect is enhanced by cGMP-dependent phosphorylation of IRAG2. As IRAG1 was previously shown to inhibit platelet aggregation in a cGMP-dependent manner, it can be speculated that IRAG2 exerts an opposing function and might be an IRAG1 counterpart in murine platelets.

Nitric Oxide Signals Through IRAG to Inhibit TRPM4 Channels and Dilate Cerebral Arteries

Nitric oxide (NO) relaxes vascular smooth muscle cells (SMCs) and dilates blood vessels by increasing intracellular levels of cyclic guanosine monophosphate (cGMP), which stimulates the activity of cGMP-dependent protein kinase (PKG). However, the vasodilator mechanisms downstream of PKG remain incompletely understood. Here, we found that transient receptor potential melastatin 4 (TRPM4) cation channels, which are activated by Ca²⁺ released from the sarcoplasmic reticulum (SR) through inositol triphosphate receptors (IP₃Rs) under native conditions, are essential for SMC membrane depolarization and vasoconstriction. We hypothesized that signaling via the NO/cGMP/PKG pathway causes vasodilation by inhibiting TRPM4. We found that TRPM4 currents activated by stretching the plasma membrane or directly activating IP₃Rs were suppressed by exogenous NO or a membrane-permeable cGMP analog, the latter of which also impaired IP₃R-mediated release of Ca²⁺ from the SR. The effects of NO on TRPM4 activity were blocked by inhibition of soluble guanylyl cyclase or PKG. Notably, upon phosphorylation by PKG, IRAG (IP₃R-associated PKG substrate) inhibited IP₃R-mediated Ca²⁺ release, and knockdown of IRAG expression diminished NO-mediated inhibition of TRPM4 activity and vasodilation. Using superresolution microscopy, we found that IRAG, PKG, and IP₃Rs form a nanoscale signaling complex on the SR of SMCs. We conclude that NO/cGMP/PKG signaling through IRAG inhibits IP₃R-dependent activation of TRPM4 channels in SMCs to dilate arteries.

SIGNIFICANCE STATEMENT

Nitric oxide is a gaseous vasodilator produced by endothelial cells that is essential for cardiovascular function. Although NO-mediated signaling pathways have been intensively studied, the mechanisms by which they relax SMCs to dilate blood vessels remain incompletely understood. In this study, we show that NO causes vasodilation by inhibiting the activity of Ca²⁺-dependent TRPM4 cation channels. Probing further, we found that NO does not act directly on TRPM4 but instead initiates a signaling cascade that inhibits its activation by blocking the release of Ca²⁺ from the SR. Thus, our findings reveal the essential molecular pathways of NO-induced vasodilation-a fundamental unresolved concept in cardiovascular physiology.

The cGMP system: components and function

The cyclic guanosine monophosphate (cGMP) signaling system is one of the most prominent regulators of a variety of physiological and pathophysiological processes in many mammalian and non-mammalian tissues. Targeting this pathway by increasing cGMP levels has been a very successful approach in pharmacology as shown for nitrates, phosphodiesterase (PDE) inhibitors and stimulators of nitric oxide-guanylyl cyclase (NO-GC) and particulate GC (pGC). This is an introductory review to the cGMP signaling system intended to introduce those readers to this system, who do not work in this area. This article does not intend an in-depth review of this system. Signal transduction by cGMP is controlled by the generating enzymes GCs, the degrading enzymes PDEs and the cGMP-regulated enzymes cyclic nucleotide-gated ion channels, cGMP-dependent protein kinases and cGMP-regulated PDEs. Part A gives a very concise introduction to the components. Part B gives a very concise introduction to the functions modulated by cGMP. The article cites many recent reviews for those who want a deeper insight.

Loss of PKGIβ/IRAG1 Signaling Causes Anemia-Associated Splenomegaly

Inositol 1,4,5-triphosphate receptor-associated cGMP kinase substrate 1 (IRAG1) is a substrate protein of the NO/cGMP-signaling pathway and forms a ternary complex with the cGMP-dependent protein kinase Iβ (PKGIβ) and the inositol triphosphate receptor I (IP₃R-I). Functional studies about IRAG1 exhibited that IRAG1 is specifically phosphorylated by the PKGIβ, regulating cGMP-mediated IP₃-dependent Ca²⁺-release. IRAG1 is widely distributed in murine tissues, e.g., in large amounts in smooth muscle-containing tissues and platelets, but also in lower amounts, e.g., in the spleen. The NO/cGMP/PKGI signaling pathway is important in several organ systems. A loss of PKGI causes gastrointestinal disorders, anemia and splenomegaly. Due to the similar tissue distribution of the PKGIβ to IRAG1, we investigated the pathophysiological functions of IRAG1 in this context. Global IRAG1-KO mice developed gastrointestinal bleeding, anemia-associated splenomegaly and iron deficiency. Additionally, Irag1-deficiency altered the protein levels of some cGMP/PKGI signaling proteins—particularly a strong decrease in the PKGIβ—in the colon, spleen and stomach but did not change mRNA-expression of the corresponding genes. The present work showed that a loss of IRAG1 and the PKGIβ/IRAG1 signaling has a crucial function in the development of gastrointestinal disorders and anemia-associated splenomegaly. Furthermore, global Irag1-deficient mice are possible in vivo model to investigate PKGIβ protein functions.

cGMP Signaling in Cardiovascular Diseases: Linking Genotype and Phenotype

Cyclic guanosine 3',5'-monophosphate (cGMP) is the key second messenger molecule in nitric oxide signaling. Its rapid generation and fate, but also its role in mediating acute cellular functions has been extensively studied. In the past years, genetic studies suggested an important role for cGMP in affecting the risk of chronic cardiovascular diseases, for example, coronary artery disease and myocardial infarction. Here, we review the role of cGMP in atherosclerosis and other cardiovascular diseases and discuss recent genetic findings and identified mechanisms. Finally, we highlight open questions and promising research topics.

Homozygous mutation in murine retrovirus integration site 1 gene associated with a non-syndromic form of isolated familial achalasia

BACKGROUND

Achalasia is a condition characterized by impaired function of esophageal motility and incomplete relaxation of the lower esophagus sphincter, causing dysphagia and regurgitation. Rare cases of early-onset achalasia appear often in combination with further symptoms in a syndromic form as an inherited disease.

METHODS

Whole genome sequencing was used to investigate the genetic basis of isolated achalasia in a family of Tunisian origin. We analyzed the function of the affected protein with immunofluorescence and affinity chromatography study.

KEY RESULTS

A homozygous nonsense mutation was detected in murine retrovirus integration site 1 (MRVI1) gene (Human Genome Organisation Gene Nomenclature Committee (HGNC) approved gene symbol: IRAG1) encoding the inositol 1,4,5-trisphosphate receptor 1 (IP₃ R1)-associated cyclic guanosine monophosphate (cGMP) kinase substrate (IRAG). Sanger sequencing confirmed co-segregation of the mutation with the disease. Sequencing of the entire MRVI1 gene in 35 additional patients with a syndromic form of achalasia did not uncover further cases with MRVI1 mutations. Immunofluorescence analysis of transfected COS7 cells revealed GFP-IRAG with the truncating mutation p.Arg112* (transcript variant 1) or p.Arg121* (transcript variant 2) to be mislocalized in the cytoplasm and the nucleus. Co-transfection with cGMP-dependent protein kinase 1 isoform β (cGK1β) depicted a partial mislocalization of cGK1β due to mislocalized truncated IRAG. Isolation of protein complexes revealed that the truncation of this protein causes the loss of the interaction domain of IRAG with cGK1β.

CONCLUSIONS & INFERENCES

In individuals with an early onset of achalasia without further accompanying symptoms, MRVI1 mutations should be considered as the disease-causing defect.

IRAG1 Deficient Mice Develop PKG1β Dependent Pulmonary Hypertension

PKGs are serine/threonine kinases. PKG1 has two isoforms-PKG1α and β. Inositol trisphosphate receptor (IP₃R)-associated cGMP-kinase substrate 1 (IRAG1) is a substrate for PKG1β. IRAG1 is also known to further interact with IP₃RI, which mediates intracellular Ca²⁺ release. However, the role of IRAG1 in PH is not known. Herein, WT and IRAG1 KO mice were kept under normoxic or hypoxic (10% O₂) conditions for five weeks. Animals were evaluated for echocardiographic variables and went through right heart catheterization. Animals were further sacrificed to prepare lungs and right ventricular (RV) for immunostaining, western blotting, and pulmonary artery smooth muscle cell (PASMC) isolation. IRAG1 is expressed in PASMCs and downregulated under hypoxic conditions. Genetic deletion of IRAG1 leads to RV hypertrophy, increase in RV systolic pressure, and RV dysfunction in mice. Absence of IRAG1 in lung and RV have direct impacts on PKG1β expression. Attenuated PKG1β expression in IRAG1 KO mice further dysregulates other downstream candidates of PKG1β in RV. IRAG1 KO mice develop PH spontaneously. Our results indicate that PKG1β signaling via IRAG1 is essential for the homeostasis of PASMCs and RV. Disturbing this signaling complex by deleting IRAG1 can lead to RV dysfunction and development of PH in mice.

cGMP-dependent protein kinase I in vascular smooth muscle cells improves ischemic stroke outcome in mice

Recent works highlight the therapeutic potential of targeting cyclic guanosine monophosphate (cGMP)-dependent pathways in the context of brain ischemia/reperfusion injury (IRI). Although cGMP-dependent protein kinase I (cGKI) has emerged as a key mediator of the protective effects of nitric oxide (NO) and cGMP, the mechanisms by which cGKI attenuates IRI remain poorly understood. We used a novel, conditional cGKI knockout mouse model to study its role in cerebral IRI. We assessed neurological deficit, infarct volume, and cerebral perfusion in tamoxifen-inducible vascular smooth muscle cell-specific cGKI knockout mice and control animals. Stroke experiments revealed greater cerebral infarct volume in smooth muscle cell specific cGKI knockout mice (males: 96 ± 16 mm3; females: 93 ± 12 mm3, mean±SD) than in all control groups: wild type (males: 66 ± 19; females: 64 ± 14), cGKI control (males: 65 ± 18; females: 62 ± 14), cGKI control with tamoxifen (males: 70 ± 8; females: 68 ± 10). Our results identify, for the first time, a protective role of cGKI in vascular smooth muscle cells during ischemic stroke injury. Moreover, this protective effect of cGKI was found to be independent of gender and was mediated via improved reperfusion. These results suggest that cGKI in vascular smooth muscle cells should be targeted by therapies designed to protect brain tissue against ischemic stroke.

Ca2+ Signalling in Pericytes

Microcirculation is the generic name for the finest level of the circulatory system and consists of arteriolar and venular networks located upstream and downstream of capillaries, respectively. Anatomically arterioles are surrounded by a monolayer of spindle-shaped smooth muscle cells (myocytes), while terminal branches of precapillary arterioles, capillaries and all sections of postcapillary venules are surrounded by a monolayer of morphologically different perivascular cells (pericytes). Pericytes are essential components of the microvascular vessel wall. Wrapped around endothelial cells, they occupy a strategic position at the interface between the circulating blood and the interstitial space. There are physiological differences in the responses of pericytes and myocytes to vasoactive molecules, which suggest that these two types of vascular cells could have different functional roles in the regulation of local blood flow within the same microvascular bed. Also, pericytes may play different roles in different microcirculatory beds to meet the characteristics of individual organs. Contractile activity of pericytes and myocytes is controlled by changes of cytosolic free Ca²⁺concentration. In this chapter, we attempt to summarize the results in the field of Ca²⁺ signalling in pericytes especially in light of their contractile roles in different tissues and organs. We investigate the literature and describe our results regarding sources of Ca²⁺, relative importance and mechanisms of Ca²⁺ release and Ca²⁺ entry in control of the spatio-temporal characteristics of the Ca²⁺ signals in pericytes, where possible Ca²⁺ signalling and contractile responses in pericytes are compared to those of myocytes.

Profound analgesia is associated with a truncated peptide resulting from tissue specific alternative splicing of DRG CA8-204 regulated by an exon-level cis-eQTL

Carbonic anhydrase-8 (CA8) is an intracellular protein that functions as an allosteric inhibitor of inositol trisphosphate receptor-1 (ITPR1) critical to intracellular Ca⁺⁺ release, synaptic functions and neuronal excitability. We showed previously that murine nociception and analgesic responses are regulated by the expression of this gene in dorsal root ganglion (DRG) associated with a cis-eQTL. In this report, we identify an exon-level cis-eQTL (rs6471859) that regulates human DRG CA8 alternative splicing, producing a truncated 1,697bp transcript (e.g., CA8-204). Our functional genomic studies show the “G” allele at rs6471859 produces a cryptic 3’UTR splice site regulating expression of CA8-204. We developed constructs to study the expression and function of the naturally occurring CA8-204^G transcript (G allele at rs6471859), CA8-204^C (C allele at rs6471859 reversion mutation) and CA8-201 (full length transcript). CA8-204^G transcript expression occurred predominantly in non-neuronal cells (HEK293), while CA8-204^C expression was restricted to neuronal derived cells (NBL) in vitro. CA8-204^G produced a stable truncated transcript in HEK293 cells that was barely detectable in NBL cells. We also show CA8-204 produces a stable peptide that inhibits pITPR1 and Ca⁺⁺ release in HEK293 cells. These results imply homozygous G/G individuals at rs6471859, which are common in the general population, produce exclusively CA8-204^G that is barely detectable in neuronal cells. CA8 null mutations that greatly impact neuronal functions are associated with severe forms of spinal cerebellar ataxia, and our data suggest G/G homozygotes should display a similar phenotype. To address this question, we show in vivo using AAV8-FLAG-CA8-204^G and AAV8-V5-CA8-201 gene transfer delivered via intra-neural sciatic nerve injection (SN), that these viral constructs are able to transduce DRG cells and produce similar analgesic and anti-hyperalgesic responses to inflammatory pain. Immunohistochemistry (IHC) examinations of DRG tissues further show CA8-204^G peptide is expressed in advillin expressing neuronal cells, but to a lesser extent compared to glial cells. These findings explain why G/G homozygotes that exclusively produce this truncated functional peptide in DRG evade a severe phenotype. These genomic studies significantly advance the literature regarding structure-function studies on CA8-ITPR1 critical to calcium signaling pathways, synaptic functioning, neuronal excitability and analgesic responses.

Carbonic anhydrase-8 (CA8) inhibits IP3 binding to the inositol trisphosphate receptor-1(ITPR1), which regulates intracellular calcium signaling critical to neuronal functions. Recessive CA8-null mutants are associated with spinocerebellar ataxia and neurodegenerative disorders. We have previously demonstrated that nociception and analgesic responses are associated with a DRG cis-eQTL that regulates murine expression of this gene. This study focuses on a human DRG exon-level cis-eQTL (rs6471859) that regulates CA8-204 alternative splicing producing a truncated 1,697bp transcript. Herein, we demonstrate the “G” allele at rs6471859 produces a cryptic 3’UTR splice site regulating tissue-specific CA8-204 expression. In vitro studies show the “G” allele (CA8-204^G) produces a stable peptide that inhibits ITPR1 activation and Ca⁺⁺ release in non-neuronal cells, but not in neuronal cells. However, using AAV8 gene transfer in vivo we show CA8-204^G peptide is expressed in both glial and to a lesser extent in neuronal cells, producing profound analgesia and anti-hyperalgesia using inflammatory pain models, similar to the full length CA8-201 positive control. These data significantly extend our understanding of CA8 structure-function, demonstrating the truncated peptide may represent a novel therapeutic candidate.

The involvement of phosphorylation of myosin phosphatase targeting subunit 1 (MYPT1) and MYPT1 isoform expression in NO/cGMP mediated differential vasoregulation of cerebral arteries compared to systemic arteries

AIM

Constitutive release of NO blunts intrinsic and stimulated contractile activity in cerebral arteries (CA). Here, we explored whether phosphorylation and expression levels of the PKG-sensitive, leucine zipper positive (LZ⁺ ) splice variants of the regulatory subunit of myosin phosphatase (MYPT1) are involved and whether its expression is associated with higher cGMP sensitivity.

METHODS

Vascular contractility was investigated by wire myography. Phosphorylation of MYPT1 was determined by Western blotting.

RESULTS

Constitutive phosphorylation of MYPT1-T696 and T853 was lower and that of S695 and S668 was higher in cerebral arteries from the circulus arteriosus (CA-w) than in femoral arteries (FA), while total MYPT1 expression was not different. In CA-w but not in FA, L-NAME lowered phosphorylation of S695/S668 and increased phosphorylation of T696/T853 and of MLC₂₀ -S19, plus basal tone. The increase in basal tone was attenuated in CA-w and basilar arteries (BA) from heterozygous MYPT1-T696A/+ mice. Compared to FA, expression of the LZ⁺ -isoform was ~2-fold higher in CA-w coincident with a higher sensitivity to DEA-NONOate, cinaciguat and Y27632 in BA and 8-Br-cGMP (1 μmol/L) in pre-constricted (pCa 6.1) α-toxin permeabilized CAs. In contrast, 6-Bnz-cAMP (10 μmol/L) relaxed BA and FA similarly by ~80%.

CONCLUSION

Our results indicate that (i) regulation of the intrinsic contractile activity in CA involves phosphorylation of MYPT1 at T696 and S695/S668, (ii) the higher NO/cGMP/PKG sensitivity of CAs can be ascribed to the higher expression level of the LZ⁺ -MYPT1 isoform and (iii) relaxation by cAMP/PKA pathway is less dependent on the expression level of the LZ⁺ splice variants of MYPT1.

Whole exome sequencing identifies MRVI1 as a susceptibility gene for moyamoya syndrome in neurofibromatosis type 1

Background and purpose

Moyamoya angiopathy is a progressive cerebral vasculopathy. The p.R4810K substitution in RNF213 has previously been linked to moyamoya disease in Asian populations. When associated with other medical conditions, such as neurofibromatosis type 1, this vasculopathy is frequently reported as moyamoya syndrome. Intriguingly, most cases of moyamoya-complicated neurofibromatosis type 1 have been described in Caucasians, inverting the population ratio observed in Asians, although prevalence of neurofibromatosis type 1 is constant worldwide. Our aim was to investigate whether, among Caucasians, additive genetic factors may contribute to the occurrence of moyamoya in neurofibromatosis type 1.

Methods

Whole exome sequencing was carried out on an Italian family with moyamoya-complicated neurofibromatosis type 1 to identify putative genetic modifiers independent of the NF1 locus and potentially involved in moyamoya pathogenesis. Results were validated in an unrelated family of German ancestry.

Results

We identified the p.P186S substitution (rs35857561) in MRVI1 that segregated with moyamoya syndrome in both the Italian and German family.

Conclusions

The rs35857561 polymorphism in MRVI1 may be a genetic susceptibility factor for moyamoya in European patients with neurofibromatosis type 1. MRVI1 is a functional partner of ITPR1, PRKG1 and GUCY1A3, which are involved in response to nitric oxide. Mutations in GUCY1A3 have been recently linked to a recessive syndromic form of moyamoya with esophageal achalasia.

A concise discussion of the regulatory role of cGMP kinase I in cardiac physiology and pathology

The underlying cause of cardiac hypertrophy, fibrosis, and heart failure has been investigated in great detail using different mouse models. These studies indicated that cGMP and cGMP-dependent protein kinase type I (cGKI) may ameliorate these negative phenotypes in the adult heart. Recently, evidence has been published that cardiac mitochondrial BKCa channels are a target for cGKI and that activation of mitoBKCa channels may cause some of the positive effects of conditioning in ischemia/reperfusion injury. It will be pointed out that most studies could not present convincing evidence that it is the cGMP level and the activity cGKI in specific cardiac cells that reduces hypertrophy or heart failure. However, anti-fibrotic compounds stimulating nitric oxide-sensitive guanylyl cyclase may be an upcoming therapy for abnormal cardiac remodeling.

An N-terminally truncated form of cyclic GMP-dependent protein kinase Iα (PKG Iα) is monomeric and autoinhibited and provides a model for activation

The type I cGMP-dependent protein kinases (PKG I) serve essential physiological functions, including smooth muscle relaxation, cardiac remodeling, and platelet aggregation. These enzymes form homodimers through their N-terminal dimerization domains, a feature implicated in regulating their cooperative activation. Previous investigations into the activation mechanisms of PKG I isoforms have been largely influenced by structures of the cAMP-dependent protein kinase (PKA). Here, we examined PKG Iα activation by cGMP and cAMP by engineering a monomeric form that lacks N-terminal residues 1-53 (Δ53). We found that the construct exists as a monomer as assessed by whole-protein MS, size-exclusion chromatography, and small-angle X-ray scattering (SAXS). Reconstruction of the SAXS 3D envelope indicates that Δ53 has a similar shape to the heterodimeric RIα-C complex of PKA. Moreover, we found that the Δ53 construct is autoinhibited in its cGMP-free state and can bind to and be activated by cGMP in a manner similar to full-length PKG Iα as assessed by surface plasmon resonance (SPR) spectroscopy. However, we found that the Δ53 variant does not exhibit cooperative activation, and its cyclic nucleotide selectivity is diminished. These findings support a model in which, despite structural similarities, PKG Iα activation is distinct from that of PKA, and its cooperativity is driven by in trans interactions between protomers.

Do Evolutionary Changes in Astrocytes Contribute to the Computational Power of the Hominid Brain?

It is now well accepted that astrocytes are essential in all major nervous system functions of the rodent brain, including neurotransmission, energy metabolism, modulation of blood flow, ion and water homeostasis, and, indeed, higher cognitive functions, although the contribution of astrocytes in cognition is still in early stages of study. Here we review the most current research findings on human astrocytes, including their structure, molecular characterization, and functional properties. We also highlight novel tools that have been established for translational approaches to the comparative study of astrocytes from humans and experimental animals. Understanding the differences in astrocytes is essential to elucidate the contribution of astrocytes to normal physiology, cognitive processing and diverse pathologies of the central nervous system.

Human astrocytes: structure and functions in the healthy brain

Data collected on astrocytes’ physiology in the rodent have placed them as key regulators of synaptic, neuronal, network, and cognitive functions. While these findings proved highly valuable for our awareness and appreciation of non-neuronal cell significance in brain physiology, early structural and phylogenic investigations of human astrocytes hinted at potentially different astrocytic properties. This idea sparked interest to replicate rodent-based studies on human samples, which have revealed an analogous but enhanced involvement of astrocytes in neuronal function of the human brain. Such evidence pointed to a central role of human astrocytes in sustaining more complex information processing. Here, we review the current state of our knowledge of human astrocytes regarding their structure, gene profile, and functions, highlighting the differences with rodent astrocytes. This recent insight is essential for assessment of the relevance of findings using animal models and for comprehending the functional significance of species-specific properties of astrocytes. Moreover, since dysfunctional astrocytes have been described in many brain disorders, a more thorough understanding of human-specific astrocytic properties is crucial for better-adapted translational applications.

The role of cGMP and its signaling pathways in kidney disease

Cyclic nucleotide signal transduction pathways are an emerging research field in kidney disease. Activated cell surface receptors transduce their signals via intracellular second messengers such as cAMP and cGMP. There is increasing evidence that regulation of the cGMP-cGMP-dependent protein kinase 1-phosphodiesterase (cGMP-cGK1-PDE) signaling pathway may be renoprotective. Selective PDE5 inhibitors have shown potential in treating kidney fibrosis in patients with chronic kidney disease (CKD), via their downstream signaling, and these inhibitors also have known activity as antithrombotic and anticancer agents. This review gives an outline of the cGMP-cGK1-PDE signaling pathways and details the downstream signaling and regulatory functions that are modulated by cGK1 and PDE inhibitors with regard to antifibrotic, antithrombotic, and antitumor activity. Current evidence that supports the renoprotective effects of regulating cGMP-cGK1-PDE signaling is also summarized. Finally, the effects of icariin, a natural plant extract with PDE5 inhibitory function, are discussed. We conclude that regulation of cGMP-cGK1-PDE signaling might provide novel, therapeutic strategies for the worsening global public health problem of CKD.

Evidence that NO/cGMP/PKG signalling cascade mediates endothelium dependent inhibition of IP₃R mediated Ca²⁺ oscillations in myocytes and pericytes of ureteric microvascular network in situ

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Endothelium-dependent inhibition of Ca²⁺ oscillations in myocytes and pericytes was reversed by ODQ, an inhibitor of soluble guanylyl cyclase (sGC).

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Selective PKG inhibitor Rp-8-pCPT-cGMPS, reversed endothelium- dependent termination of agonist-induced Ca²⁺ oscillations in myocytes and pericytes.

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Selective PKG activator 8pCPT-cGMP induced inhibition of the agonist-induced Ca²⁺ oscillations in myocytes and pericytes.

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Inhibitory effect of SNAP was markedly enhanced by zaprinast.

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Inhibitory effect of NO/cGMP/PKG cascade is associated with suppressed Ca²⁺ release via IP₃Rs of myocytes and pericytes.

In ureteric microvessels the antagonistic relationship between Ca²⁺ signalling in endothelium and Ca²⁺ oscillations in myocytes and pericytes of arterioles and venules involves nitric oxide (NO), but the underlying mechanisms are not well understood. In the present study we investigated the effects of carbachol and NO donor SNAP on Ca²⁺ signalling and vasomotor responses of arterioles and venules in intact urteric microvascular network in situ using confocal microscopy. Vasomotor responses of arterioles and venules induced by AVP correlated with the occurrence of Ca²⁺ oscillations in the myocytes and pericytes and were not abolished by the removal of Ca²⁺ from extracellular fluid. Carbachol-induced rise of intracellular Ca²⁺ in endothelium was accompanied by the termination of the Ca²⁺ oscillations in myocytes and pericytes. This carbachol-induced inhibitory effect on Ca²⁺ oscillations in myocytes and pericytes was reversed by ODQ, an inhibitor of soluble guanylyl cyclase (sGC) and by Rp-8-pCPT-cGMPS, an inhibitor of protein kinase G (PKG). Ca²⁺ oscillations in myocytes and pericytes were also effectively blocked by NO donor SNAP. An Inhibitory effect of SNAP was markedly enhanced by zaprinast, a selective inhibitor of cGMP-specific phosphodiesterase-5, and reversed by sGC inhibitor, ODQ and PKG inhibitor, Rp-8-pCPT-cGMPS. The cGMP analogue and selective PKG activator 8pCPT-cGMP also induced inhibition of the AVP-induced Ca²⁺ oscillations in myocytes and pericytes. SNAP had no effects on Ca²⁺ oscillations induced by caffeine in distributing arcade arterioles. Consequently, we conclude that NO- mediated inhibition of Ca²⁺ oscillations in myocytes and pericytes predominantly recruits the cGMP/PKG dependent pathway. The inhibitory effect of NO/cGMP/PKG cascade is associated with suppressed Ca²⁺ release from the SR of myocytes and pericytes selectively via the inositol triphosphate receptor (IP₃R) channels.

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.

Interaction of cCMP with the cGK, cAK and MAPK Kinases in Murine Tissues

cAMP and cGMP are well established second messengers that are essential for numerous (patho)physiological processes. These purine cyclic nucleotides activate cAK and cGK, respectively. Recently, the existence of cCMP was described, and a possible function for this cyclic nucleotide was investigated. It was postulated that cCMP plays a role as a second messenger. However, the functions regulated by cCMP are mostly unknown. To elucidate probable functions, cCMP-binding and -activated proteins were identified using different methods. We investigated the effect of cCMP on purified cyclic nucleotide-dependent protein kinases and lung and jejunum tissues of wild type (WT), cGKI-knockout (cGKI KO) and cGKII-knockout (cGKII KO) mice. The catalytic activity of protein kinases was measured by a (γ-³²P) ATP kinase assay. Cyclic nucleotide-dependent protein kinases (cAK, cGKI and cGKII) in WT tissue lysates were stimulated by cCMP. In contrast, there was no stimulation of phosphorylation in KO tissue lysates. Competitive binding assays identified cAK, cGKI, and cGKII as cCMP-binding proteins. An interaction between cCMP/MAPK and a protein-protein complex of MAPK/cGK were detected via cCMP affinity chromatography and co-immunoprecipitation, respectively. These complexes were abolished or reduced in jejunum tissues from cGKI KO or cGKII KO mice. In contrast, these complexes were observed in the lung tissues from WT, cGKI KO and cGKII KO mice. Moreover, cCMP was also able to stimulate the phosphorylation of MAPK. These results suggest that MAPK signaling is regulated by cGMP-dependent protein kinases upon activation by cCMP. Based on these results, we propose that additional cCMP-dependent protein kinases that are capable of modulating MAPK signaling could exist. Hence, cCMP could potentially act as a second messenger in the cAK/cGK and MAPK signaling pathways and play an important role in physiological processes of the jejunum and lung.

The Golgi apparatus regulates cGMP-dependent protein kinase I compartmentation and proteolysis

cGMP-dependent protein kinase I (PKGI) is an important effector of cGMP signaling that regulates vascular smooth muscle cell (SMC) phenotype and proliferation. PKGI has been detected in the perinuclear region of cells, and recent data indicate that proprotein convertases (PCs) typically resident in the Golgi apparatus (GA) can stimulate PKGI proteolysis and generate a kinase fragment that localizes to the nucleus and regulates gene expression. However, the role of the endomembrane system in PKGI compartmentation and processing is unknown. Here, we demonstrate that PKGI colocalizes with endoplasmic reticulum (ER), ER-Golgi intermediate compartment, GA cisterna, and trans-Golgi network proteins in pulmonary artery SMC and cell lines. Moreover, PKGI localizes with furin, a trans-Golgi network-resident PC known to cleave PKGI. ER protein transport influences PKGI localization because overexpression of a constitutively inactive Sar1 transgene caused PKGI retention in the ER. Additionally, PKGI appears to reside within the GA because PKGI immunoreactivity was determined to be resistant to cytosolic proteinase K treatment in live cells. The GA appears to play a role in PKGI proteolysis because overexpression of inositol 1,4,5-trisphosphate receptor-associated cGMP kinase substrate, not only tethered heterologous PKGI-β to the ER and decreased its localization to the GA, but also diminished PKGI proteolysis and nuclear translocation. Also, inhibiting intra-GA protein transport with monensin was observed to decrease PKGI cleavage. These studies detail a role for the endomembrane system in regulating PKGI compartmentation and proteolysis. Moreover, they support the investigation of mechanisms regulating PKGI-dependent nuclear cGMP signaling in the pulmonary vasculature with Golgi dysfunction.

Huntingtin-associated protein 1 (HAP1) is a cGMP-dependent kinase anchoring protein (GKAP) specific for the cGMP-dependent protein kinase Iβ isoform

Protein-protein interactions are important in providing compartmentalization and specificity in cellular signal transduction. Many studies have hallmarked the well designed compartmentalization of the cAMP-dependent protein kinase (PKA) through its anchoring proteins. Much less data are available on the compartmentalization of its closest homolog, cGMP-dependent protein kinase (PKG), via its own PKG anchoring proteins (GKAPs). For the enrichment, screening, and discovery of (novel) PKA anchoring proteins, a plethora of methodologies is available, including our previously described chemical proteomics approach based on immobilized cAMP or cGMP. Although this method was demonstrated to be effective, each immobilized cyclic nucleotide did not discriminate in the enrichment for either PKA or PKG and their secondary interactors. Hence, with PKG signaling components being less abundant in most tissues, it turned out to be challenging to enrich and identify GKAPs. Here we extend this cAMP-based chemical proteomics approach using competitive concentrations of free cyclic nucleotides to isolate each kinase and its secondary interactors. Using this approach, we identified Huntingtin-associated protein 1 (HAP1) as a putative novel GKAP. Through sequence alignment with known GKAPs and secondary structure prediction analysis, we defined a small sequence domain mediating the interaction with PKG Iβ but not PKG Iα. In vitro binding studies and site-directed mutagenesis further confirmed the specificity and affinity of HAP1 binding to the PKG Iβ N terminus. These data fully support that HAP1 is a GKAP, anchoring specifically to the cGMP-dependent protein kinase isoform Iβ, and provide further evidence that also PKG spatiotemporal signaling is largely controlled by anchoring proteins.

Calcium signalling in pericytes

Recent advances in pericyte research have contributed to our understanding of the physiology and pathophysiology of microvessels. The microvasculature consists of arteriolar and venular networks located upstream and downstream of the capillaries. Arterioles are surrounded by a monolayer of spindle-shaped myocytes, while terminal branches of precapillary arterioles, capillaries and all sections of postcapillary venules are encircled by a monolayer of morphologically diverse pericytes. There are physiological differences in the response of pericytes and myocytes to vasoactive molecules, suggesting that these two vascular cell types could have different functional roles in the regulation of local blood flow. The contractile activity of pericytes and myocytes is controlled by changes of cytosolic free Ca(2+) concentration. In this short review, we summarize our results and those of other authors on the contractility of pericytes and their Ca(2+) signalling. We describe results regarding sources of Ca(2+) and mechanisms of Ca(2+) release and Ca(2+) entry in control of the spatiotemporal characteristics of the Ca(2+) signals in pericytes.

Ca2+ handling and sensitivity in airway smooth muscle: emerging concepts for mechanistic understanding and therapeutic targeting

Free calcium ions within the cytosol serve as a key secondary messenger system for a diverse range of cellular processes. Dysregulation of cytosolic Ca(2+) handling in airway smooth muscle (ASM) has been implicated in asthma, and it has been hypothesised that this leads, at least in part, to associated changes in both the architecture and function of the lung. Significant research is therefore directed towards furthering our understanding of the mechanisms which control ASM cytosolic calcium, in addition to those regulating the sensitivity of its downstream effector targets to calcium. Key aspects of the recent developments in this field were discussed at the 8th Young Investigators' Symposium on Smooth Muscle (2013, Groningen, The Netherlands), and are outlined in this review.

How calcium signals in myocytes and pericytes are integrated across in situ microvascular networks and control microvascular tone

The microcirculation is the site of gas and nutrient exchange. Control of central or local signals acting on the myocytes, pericytes and endothelial cells within it, is essential for health. Due to technical problems of accessibility, the mechanisms controlling Ca2+ signalling and contractility of myocytes and pericytes in different sections of microvascular networks in situ have not been investigated. We aimed to investigate Ca2+ signalling and functional responses, in a microcirculatory network in situ. Using live confocal imaging of ureteric microvascular networks, we have studied the architecture, morphology, Ca2+ signalling and contractility of myocytes and pericytes. Ca2+ signals vary between distributing arcade and downstream transverse and precapillary arterioles, are modified by agonists, with sympathetic agonists being ineffective beyond transverse arterioles. In myocytes and pericytes, Ca2+ signals arise from Ca2+ release from the sarcoplasmic reticulum through inositol 1,4,5-trisphosphate-induced Ca2+ release and not via ryanodine receptors or Ca2+ entry into the cell. The responses in pericytes are less oscillatory, slower and longer-lasting than those in myocytes. Myocytes and pericytes are electrically coupled, transmitting Ca2+ signals between arteriolar and venular networks dependent on gap junctions and Ca2+ entry via L-type Ca2+ channels. Endothelial Ca2+ signalling inhibits intracellular Ca2+ oscillations in myocytes and pericytes via L-arginine/nitric oxide pathway and intercellular propagating Ca2+ signals via EDHF. Increases of Ca2+ in pericytes and myocytes constrict all vessels except capillaries. These data reveal the structural and signalling specializations allowing blood flow to be regulated by myocytes and pericytes.

Cyclic guanosine monophosphate and the dependent protein kinase regulate lymphatic contractility in rat thoracic duct

  We have previously demonstrated a principal role for nitric oxide (NO) in the endothelium/shear-dependent regulation of contractility in rat thoracic duct (TD). In this study we tested the hypothesis that cyclic guanosine monophosphate (cGMP) and the dependent protein kinase (PKG) are central to the intrinsic and extrinsic flow-dependent modulation of lymphatic contractility. Lymphatic diameters and indices of pumping in isolated, cannulated and pressurized segments of rat TD were measured. The influences of increased transmural pressure (1-5 cmH2O) and imposed flow (1-5 cm H2O transaxial pressure gradients) on lymphatic function were studied before and after: (1) inhibition of guanylate cyclase (GC) with and without a NO donor; (2) application of stable cGMP analogue; and (3) inhibition of the cGMP activation of PKG. Additionally, Western blotting and immunofluorescent tissue staining were used to analyse the PKG isoforms expressed in TD. We found that the GC inhibitor ODQ induced changes in TD contractility similar to NO synthase blockade and prevented the relaxation induced by the NO donor S-nitroso-N-acetylpenicillamine. The cGMP analogue, 8-(4-Chlorophenylthio)-guanosine 3,5-cyclic monophosphate sodium salt (8pCPTcGMP), mimicked the extrinsic flow-induced relaxation in a dose-dependent manner, whereas treatment with the cGMP/PKG inhibitor, guanosine 3,5-cyclic monophosphorothioate, 8-(4-chlorophenylthio)-, Rp-isomer, triethylammonium salt (Rp-8-Br-PETcGMPS), eliminated intrinsic flow-dependent relaxation, and largely inhibited extrinsic flow-dependent relaxation. Western blotting demonstrated that both PKG-Iα and -Iβ isoforms are found in TD, with ∼10 times greater expression of the PKG-Iα protein in TD compared with the aorta and vena cava. The PKG-Iβ isoform expressed equally in TD and vena cava, both being ∼2 times higher than that in the aorta. Immunofluorescent labelling of PKG-Iα protein in the wall of rat thoracic duct confirmed its localization inside TD muscle cells. These findings demonstrate that cGMP is critical to the flow-dependent regulation of TD contractility; they also indicate an important involvement of PKG, especially PKG-Iα in these processes and identifies PKG protein as a potential therapeutic target.

Steroid-sensitive gene 1 is a novel cyclic GMP-dependent protein kinase I substrate in vascular smooth muscle cells

NO, via its second messenger cGMP, activates protein kinase GI (PKGI) to induce vascular smooth muscle cell relaxation. The mechanisms by which PKGI kinase activity regulates cardiovascular function remain incompletely understood. Therefore, to identify novel protein kinase G substrates in vascular cells, a λ phage coronary artery smooth muscle cell library was constructed and screened for phosphorylation by PKGI. The screen identified steroid-sensitive gene 1 (SSG1), which harbors several predicted PKGI phosphorylation sites. We observed direct and cGMP-regulated interaction between PKGI and SSG1. In cultured vascular smooth muscle cells, both the NO donor S-nitrosocysteine and atrial natriuretic peptide induced SSG1 phosphorylation, and mutation of SSG1 at each of the two predicted PKGI phosphorylation sites completely abolished its basal phosphorylation by PKGI. We detected high SSG1 expression in cardiovascular tissues. Finally, we found that activation of PKGI with cGMP regulated SSG1 intracellular distribution.

Methods for identification of cGKI substrates

The cGMP-dependent protein kinases (cGK), which belong to the family of serine/threonine kinases, exhibit their diverse functions in cells through interaction with a variety of substrate proteins. Several substrates were identified and the interactions studied using different methods inter alia co-immunoprecipitation (Co-IP) and cGMP-agarose affinity purification. In the following chapter, we will describe the preparation of cell or tissue lysates, the procedures of cGMP-agarose affinity purification and co-immunoprecipitation, and finally the separation and analysis of the protein complexes by SDS-PAGE or mass spectrometry.

cGMP-dependent 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, cGKIα and cGKIβ, 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 juxtaglomerular 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 endochondral bone growth. This chapter focuses on the involvement of cGKs in cardiovascular and non-cardiovascular processes including cell growth and metabolism.

Endogenous cGMP-dependent protein kinase reverses EGF-induced MAPK/ERK signal transduction through phosphorylation of VASP at Ser239

In our previous study, we demonstrated that type II cGMP-dependent protein kinase (PKG II) was expressed at lower levels in different human cancer cell lines and that exogenous PKG II inhibited epidermal growth factor (EGF)-induced MAPK/ERK signaling. In order to investigate its functions further in this signaling pathway, it is necessary to elucidate whether endogenous PKG has the same effect or not. This study aimed to investigate the possible inhibitory effect of endogenous PKG activity on EGF-induced MAPK/ERK signal transduction in human lung cancer cells and its mechanism. Human small cell lung carcinoma cells (SCLCs) were treated with the PKG-selective cGMP analog 8-pCPT-cGMP to activate endogenous PKG, EGF and cGMP followed by EGF, respectively. The results showed that increased endogenous PKG activity inhibited the EGF-induced phosphorylation of the epidermal growth factor receptor (EGFR) and the binding between Sos1 and Grb2. In addition, EGF-triggered Ras activation was reversed by increased endogenous PKG activity. While the EGF-induced phosphorylation of MEK and ERK were inhibited by increased endogenous PKG activity, there was a significant increase of phosphorylated vasodilator-stimulated phosphoprotein (p-VASP) at Ser239. Furthermore, we investigated whether endogenous PKG exerted its effects on EGF-induced MAPK/ERK signaling through phosphorylation of VASP at Ser239. Downregulation of the levels of p-VASP Ser239 by point mutation blocked the effects of endogenous PKG on EGF-induced MAPK/ERK signal transduction. The data shown here suggest that endogenous PKG reverses the EGF-induced MAPK/ERK signaling pathway by phosphorylating VASP at Ser239.

Gomisin J from Schisandra chinensis induces vascular relaxation via activation of endothelial nitric oxide synthase

Gomisin J (GJ) is a lignan contained in Schisandra chinensis (SC) which is a well-known medicinal herb for improvement of cardiovascular symptoms in Korean. Thus, the present study examined the vascular effects of GJ, and also determined the mechanisms involved. Exposure of rat thoracic aorta to GJ (1-30μg/ml) resulted in a concentration-dependent vasorelaxation, which was more prominent in the endothelium (ED)-intact aorta. ED-dependent relaxation induced by GJ was markedly attenuated by pretreatment with L-NAME, a nitric oxide synthase (NOS) inhibitor. In the intact endothelial cells of rat thoracic aorta, GJ also enhanced nitric oxide (NO) production. In studies using human coronary artery endothelial cells, GJ enhanced phosphorylation of endothelial NOS (eNOS) at Ser(1177) with increased cytosolic translocation of eNOS, and subsequently increased NO production. These effects of GJ were attenuated not only by calcium chelators including EGTA and BAPTA-AM, but also by LY294002, a PI3K/Akt inhibitor, indicating calcium- and PI3K/Akt-dependent activation of eNOS by GJ. Moreover, the levels of intracellular calcium were increased immediately after GJ administration, but Akt phosphorylation was started to increase at 20min of GJ treatment. Based on these results with the facts that ED-dependent relaxation occurred rapidly after GJ treatment, it was suggested that the ED-dependent vasorelaxant effects of GJ were mediated mainly by calcium-dependent activation of eNOS with subsequent production of endothelial NO.

Amplified NO/cGMP-mediated relaxation and ryanodine receptor-to-BKCa channel signalling in corpus cavernosum smooth muscle from phospholamban knockout mice

BACKGROUND AND PURPOSE

Relaxation of corpus cavernosum smooth muscle (CCSM) is induced by NO. NO promotes the formation of cGMP, which activates cGMP-dependent protein kinase I (PKGI). The large conductance calcium-activated potassium (BK(Ca) ) channel is regarded as a major target of NO/cGMP signalling; however, the mechanism of BK(Ca) activation remains unclear. The aim of the present study was to determine whether sarcoplasmic reticulum (SR) Ca(2+) load and Ca(2+) release from the SR via ryanodine receptors (RyRs) is important for BK(Ca) channel activation in response to NO/cGMP.

EXPERIMENTAL APPROACH

In vitro myography was performed on CCSM strips from wild-type and PLB knockout (PLB(-/-)) mice to evaluate contraction and relaxation in response to pharmacological agents and electrical field stimulation (EFS).

KEY RESULTS

In CCSM strips from PLB(-/-) mice, a model of increased SR Ca(2+) load, contractile force in response to EFS or phenylephrine (PE) was increased by nearly 100%. EFS of strips precontracted with PE induced transient relaxation in CCSM, an effect that was significantly larger in PLB(-/-) strips. Likewise, the relaxation of PE-induced contraction in response to SNP and cGMP was greater in PLB(-/-) , as demonstrated by a shift in the concentration-response curve towards lower concentrations. Blocking RyRs and BK(Ca) channels diminished the induced relaxations and eliminated the difference between wild-type and PLB(-/-).

CONCLUSIONS AND IMPLICATIONS

NO/cGMP activates BK(Ca) channels through RyR-mediated Ca(2+) release. This signalling pathway is responsible for approximately 40% of the NO/cGMP effects and is amplified by increased SR Ca(2+) concentrations.

Truncated IRAG variants modulate cGMP-mediated inhibition of human colonic smooth muscle cell contraction

Nitric oxide (NO) induces relaxation of colonic smooth muscle cells predominantly by cGMP/cGMP-dependent protein kinase I (cGKI)-induced phosphorylation of the inositol 1,4,5-trisphosphate receptor (IP(3)R)-associated cGMP kinase substrate (IRAG), to block store-dependent calcium signaling. In the present study we analyzed the structure and function of the human IRAG/MRVI1 gene. We describe four unique first exon variants transcribed from individual promoters in diverse human tissues. Tissue-specific alternative splicing with exon skipping and alternative splice donor and acceptor site usage further increases diversity of IRAG mRNA variants that encode for NH(2)- and COOH-terminally truncated proteins. At the functional level, COOH-terminally truncated IRAG variants lacking both the cGKI phosphorylation and the IP(3)RI interaction site counteract cGMP-mediated inhibition of calcium transients and relaxation of human colonic smooth muscle cells. Since COOH-terminally truncated IRAG mRNA isoforms are widely expressed in human tissues, our results point to an important role of IRAG variants as negative modulators of nitric oxide/cGKI-dependent signaling. The complexity of alternative splicing of the IRAG gene impressively demonstrates how posttranscriptional processing generates functionally distinct proteins from a single gene.

IRAG and novel PKG targeting in the cardiovascular system

Signaling by nitric oxide (NO) determines several cardiovascular functions including blood pressure regulation, cardiac and smooth muscle hypertrophy, and platelet function. NO stimulates the synthesis of cGMP by soluble guanylyl cyclases and thereby activates cGMP-dependent protein kinases (PKGs), mediating most of the cGMP functions. Hence, an elucidation of the PKG signaling cascade is essential for the understanding of the (patho)physiological aspects of NO. Several PKG signaling pathways were identified, meanwhile regulating the intracellular calcium concentration, mediating calcium desensitization or cytoskeletal rearrangement. During the last decade it emerged that the inositol trisphosphate receptor-associated cGMP-kinase substrate (IRAG), an endoplasmic reticulum-anchored 125-kDa membrane protein, is a main signal transducer of PKG activity in the cardiovascular system. IRAG interacts specifically in a trimeric complex with the PKG1β isoform and the inositol 1,4,5-trisphosphate receptor I and, upon phosphorylation, reduces the intracellular calcium release from the intracellular stores. IRAG motifs for phosphorylation and for targeting to PKG1β and 1,4,5-trisphosphate receptor I were identified by several approaches. The (patho)physiological functions for the regulation of smooth muscle contractility and the inhibition of platelet activation were perceived. In this review, the IRAG recognition, targeting, and function are summarized compared with PKG and several PKG substrates in the cardiovascular system.

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.

The cGMP-dependent protein kinase II Is an inhibitory modulator of the hyperpolarization-activated HCN2 channel

Opening of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is facilitated by direct binding of cyclic nucleotides to a cyclic nucleotide-binding domain (CNBD) in the C-terminus. Here, we show for the first time that in the HCN2 channel cGMP can also exert an inhibitory effect on gating via cGMP-dependent protein kinase II (cGKII)-mediated phosphorylation. Using coimmunoprecipitation and immunohistochemistry we demonstrate that cGKII and HCN2 interact and colocalize with each other upon heterologous expression as well as in native mouse brain. We identify the proximal C-terminus of HCN2 as binding region of cGKII and show that cGKII phosphorylates HCN2 at a specific serine residue (S641) in the C-terminal end of the CNBD. The cGKII shifts the voltage-dependence of HCN2 activation to 2–5 mV more negative voltages and, hence, counteracts the stimulatory effect of cGMP on gating. The inhibitory cGMP effect can be either abolished by mutation of the phosphorylation site in HCN2 or by impairing the catalytic domain of cGKII. By contrast, the inhibitory effect is preserved in a HCN2 mutant carrying a CNBD deficient for cGMP binding. Our data suggest that bidirectional regulation of HCN2 gating by cGMP contributes to cellular fine-tuning of HCN channel activity.

Redox signaling and cardiac sarcomeres

Oxidative stress is common in many clinically important cardiac disorders, including ischemia/reperfusion, diabetes, and hypertensive heart disease. Oxidative stress leads to derangements in pump function due to changes in the expression or function of proteins that regulate intracellular Ca(2+) homeostasis. There is growing evidence that the cardiodepressant actions of reactive oxygen species (ROS) also are attributable to ROS-dependent signaling events in the sarcomere. This minireview focuses on myofilament protein post-translational modifications induced by ROS or ROS-activated signaling enzymes that regulate cardiac contractility.

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.

Functional osteoclast attachment requires inositol-1,4,5-trisphosphate receptor-associated cGMP-dependent kinase substrate

Osteoclast activity is central to balanced bone turnover to maintain normal bone mass. A specialized osteoclast attachment to bone localizes acid secretion to remove bone mineral; in some cases, attachment is functionally impaired despite normal attachment proteins. The inositol-1,4,5-trisphosphate receptor-1 (IP3R1) is an intracellular calcium channel required for regulation of reversible osteoclast attachment by nitric oxide (NO), an important regulator of both normal and pathological bone degradation. In studies using human osteoclasts produced in vitro, we found that IP3R1 binds an endosomal isoform of the IP3R-associated cGMP-dependent kinase substrate (IRAG). IRAG is a substrate of cGMP-dependent kinase-1 (PKG1) and binds the PKG1 isoform PKG1β, which was the predominant form of PKG1 in human osteoclasts. Western blots of IRAG were consistent with NO-dependent serine phosphorylation of IRAG. An additional effect of PKG1β activity in osteoclasts was disassociation of IP3R1-IRAG complexes, as shown by analysis of IP3R1 complexes and by localization of the proteins within cells. IP3R1-IRAG complexes were stabilized by PKG or Src antagonists, Src activity being a requirement for IP3R1 calcium release downstream of PKG. IP3R1-mediated calcium release regulates cellular detachment in part through the calcium-dependent proteinase μ-calpain. In osteoclasts with IRAG suppressed by siRNA, activity of μ-calpain was increased relative to cells with normal IRAG, and regulation of μ-calpain by NO was lost. Furthermore, cells deficient in IRAG detached easily from substrate and had smaller attached diameters and randomly distributed podosomes, although IRAG knockdown did not affect cell viability. Our results indicate that IRAG is required for PKG1β-regulated cyclic calcium release during motility, and that disruption of the IP3R1-IRAG calcium regulation system is a novel cause of dysfunctional osteoclasts unrelated to defects in attachment proteins or acid secretion.

InsP3R-associated cGMP kinase substrate determines inositol 1,4,5-trisphosphate receptor susceptibility to phosphoregulation by cyclic nucleotide-dependent kinases

Ca(2+) release through inositol 1,4,5-trisphosphate receptors (InsP(3)R) can be modulated by numerous factors, including input from other signal transduction cascades. These events shape the spatio-temporal characteristics of the Ca(2+) signal and provide fidelity essential for the appropriate activation of effectors. In this study, we investigate the regulation of Ca(2+) release via InsP(3)R following activation of cyclic nucleotide-dependent kinases in the presence and absence of expression of a binding partner InsP(3)R-associated cGMP kinase substrate (IRAG). cGMP-dependent kinase (PKG) phosphorylation of only the S2+ InsP(3)R-1 subtype resulted in enhanced Ca(2+) release in the absence of IRAG expression. In contrast, IRAG bound to each InsP(3)R subtype, and phosphorylation of IRAG by PKG attenuated Ca(2+) release through all InsP(3)R subtypes. Surprisingly, simply the expression of IRAG attenuated phosphorylation and inhibited the enhanced Ca(2+) release through InsP(3)R-1 following cAMP-dependent protein kinase (PKA) activation. In contrast, IRAG expression did not influence the PKA-enhanced activity of the InsP(3)R-2. Phosphorylation of IRAG resulted in reduced Ca(2+) release through all InsP(3)R subtypes during concurrent activation of PKA and PKG, indicating that IRAG modulation is dominant under these conditions. These studies yield mechanistic insight into how cells with various complements of proteins integrate and prioritize signals from ubiquitous signaling pathways.

Nitric oxide induces airway smooth muscle cell relaxation by decreasing the frequency of agonist-induced Ca2+ oscillations

Nitric oxide (NO) induces airway smooth muscle cell (SMC) relaxation, but the underlying mechanism is not well understood. Consequently, we investigated the effects of NO on airway SMC contraction, Ca²⁺ signaling, and Ca²⁺ sensitivity in mouse lung slices with phase-contrast and confocal microscopy. Airways that were contracted in response to the agonist 5-hydroxytryptamine (5-HT) transiently relaxed in response to the NO donor, NOC-5. This NO-induced relaxation was enhanced by zaprinast or vardenafil, two selective inhibitors of cGMP-specific phosphodiesterase-5, but blocked by ODQ, an inhibitor of soluble guanylyl cyclase, and by Rp-8-pCPT-cGMPS, an inhibitor of protein kinase G (PKG). Simultaneous measurements of airway caliber and SMC [Ca²⁺]_i revealed that airway contraction induced by 5-HT correlated with the occurrence of Ca²⁺ oscillations in the airway SMCs. Airway relaxation induced by NOC-5 was accompanied by a decrease in the frequency of these Ca²⁺ oscillations. The cGMP analogues and selective PKG activators 8Br-cGMP and 8pCPT-cGMP also induced airway relaxation and decreased the frequency of the Ca²⁺ oscillations. NOC-5 inhibited the increase of [Ca²⁺]_i and contraction induced by the photolytic release of inositol 1,4,5-trisphosphate (IP₃) in airway SMCs. The effect of NO on the Ca²⁺ sensitivity of the airway SMCs was examined in lung slices permeabilized to Ca²⁺ by treatment with caffeine and ryanodine. Neither NOC-5 nor 8pCPT-cGMP induced relaxation in agonist-contracted Ca²⁺-permeabilized airways. Consequently, we conclude that NO, acting via the cGMP–PKG pathway, induced airway SMC relaxation by predominately inhibiting the release of Ca²⁺ via the IP₃ receptor to decrease the frequency of agonist-induced Ca²⁺ oscillations.

Regulation of bone turnover by calcium-regulated calcium channels

Calcium plays multiple roles in osteoclast formation, survival, and activity. Intracellular calcium is determined both by the release of intracellular stores and the influx of extracellular calcium through a variety of calcium channels. Osteoclasts express several classes of calcium channels, including ryanodine receptors (RyRs), inositol-1,4,5-trisphosphate receptors (IP(3)Rs), and calcium release-activated calcium channels (CRACs), which respond to depletion of intracellular stores. IP(3)R2 is expressed in osteoclast precursors and activated by cytokines that stimulate osteoclast differentiation. In mature osteoclasts, the IP(3)R1 isoform is highly expressed and is implicated in nitric oxide-cGMP-stimulated processes. RyR calcium channels may contribute to the release of intracellular calcium stores, while RyR2 in the plasma membrane may act to limit osteoclast activity based on extracellular calcium concentration. Orai, through regulation by endoplasmic reticular store-sensing proteins, including Stim-1, may also mediate calcium influx and act as a signal amplifier for calcium release by other calcium channels. Together, these receptors allow intracellular Ca(2+) signals to modulate bone turnover and, through calcium-sensing functions, allow coupling of osteoclast activity to extracellular conditions and integrating additional cytokine and nitric oxide signals via transient intracellular calcium signals.

IRAG determines nitric oxide- and atrial natriuretic peptide-mediated smooth muscle relaxation

AIMS

Nitric oxide (NO) and atrial natriuretic peptide (ANP) signalling via cGMP controls smooth muscle tone. One important signalling pathway of cGMP-dependent protein kinase type I (cGKI) is mediated by IRAG (IP(3) receptor associated cGKI substrate) which is highly expressed in smooth muscle tissues. To elucidate the role of IRAG for NO- and ANP-mediated smooth muscle tone regulation, cGKI localization, and for its possible function in blood pressure adjustment, we generated IRAG-knockout mice by targeted deletion of exon 3.

METHODS AND RESULTS

IRAG deletion prevented stable interaction of IP(3) receptor type I (IP(3)RI) with cGKIbeta determined by cGMP affinity chromatography. Confocal microscopy in vascular smooth muscle cells (VSMCs) showed that localization of cGKIbeta and cGKIalpha did not change in absence of IRAG. NO-, ANP-, and cGMP-dependent relaxation of hormone-contracted aortic vessels and colon was significantly affected in IRAG-knockout mice. The suppression of cGMP-induced relaxation was not rescued by selective expression of cGKIbeta in smooth muscle from cGKIbeta-transgenic mice. NO-, ANP-, and cGMP-mediated inhibition of the hormone-induced increase in intracellular calcium concentration measured by Fura2 was suppressed in IRAG-deficient VSMC. Telemetric measurements revealed that IRAG-deficient animals exhibited normal basal tone, but were resistant to blood pressure reduction induced by lipopolysaccharide-treatment.

CONCLUSION

These findings indicate that signalling of cGKIbeta via IRAG is an essential functional part for regulation of smooth muscle tone and of intracellular calcium by NO (exogenously applicated or endogenously synthesized) and by ANP. IRAG signalling does not modulate basal tone but might be important for blood pressure regulation under pathophysiological conditions.

Cyclic GMP signaling in cardiovascular pathophysiology and therapeutics

Cyclic guanosine 3',5'-monophosphate (cGMP) mediates a wide spectrum of physiologic processes in multiple cell types within the cardiovascular system. Dysfunctional signaling at any step of the cascade - cGMP synthesis, effector activation, or catabolism - have been implicated in numerous cardiovascular diseases, ranging from hypertension to atherosclerosis to cardiac hypertrophy and heart failure. In this review, we outline each step of the cGMP signaling cascade and discuss its regulation and physiologic effects within the cardiovascular system. In addition, we illustrate how cGMP signaling becomes dysregulated in specific cardiovascular disease states. The ubiquitous role cGMP plays in cardiac physiology and pathophysiology presents great opportunities for pharmacologic modulation of the cGMP signal in the treatment of cardiovascular diseases. We detail the various therapeutic interventional strategies that have been developed or are in development, summarizing relevant preclinical and clinical studies.

cGK substrates

Signalling of cGK (cGMP-dependent protein kinases) are mediated through phosphorylation of specific substrates. Several substrates of cGKI and cGKII were identified meanwhile. Some cGKI substrates are specifically regulated by the cGKIalpha or the cGKIbeta isozyme. In various cells and tissues, different cGK substrates exist that are essential for the regulation of diverse functions comprising tissue contractility, cell motility, cell contact, cellular secretion, cell proliferation, and cell differentiation. On the molecular level, cGKI substrates fulfill various cellular functions regulating e.g. the intracellular calcium and potassium concentration, the calcium sensitivity, and the organisation of the intracellular cytoskeleton. cGKII substrates are involved e.g. in chloride transport, sodium/proton transport and transcriptional regulation. The understanding of cGK signalling and function depends strongly on the identification of further specific substrates. In the last years, diverse approaches ranging from biochemistry to genetic deletion lead to the identification and establishment of several substrates, which raised new insights in the molecular mechanisms of cGK functions and elucidated new cellular cGK functions. However, the analysis of the dynamic signalling of cGK in tissues and cells will be necessary to discover new signalling pathways and substrates.

Selectivity in enrichment of cAMP-dependent protein kinase regulatory subunits type I and type II and their interactors using modified cAMP affinity resins

cAMP regulates cellular functions primarily by activating PKA. The involvement of PKAs in various signaling pathways occurring simultaneously in different cellular compartments necessitates stringent spatial and temporal regulation. This specificity is largely achieved by binding of PKA to protein scaffolds, whereby a distinct group of proteins called A kinase anchoring proteins (AKAPs) play a dominant role. AKAPs are a diverse family of proteins that all bind via a small PKA binding domain to the regulatory subunits of PKA. The binding affinities between PKA and several AKAPs can be different for different isoforms of the regulatory subunits of PKA. Here we employ a combination of affinity chromatography and mass spectrometry-based quantitative proteomics to investigate specificity in PKA-AKAP interactions. Three different immobilized cAMP analogs were used to enrich for PKA and its interacting proteins from several systems; HEK293 and RCC10 cells and rat lung and testis tissues. Stable isotope labeling was used to confidently identify and differentially quantify target proteins and their preferential binding affinity for the three different cAMP analogs. We were able to enrich all four isoforms of the regulatory subunits of PKA and concomitantly identify more than 10 AKAPs. A selective enrichment of the PKA RI isoforms could be achieved; which allowed us to unravel which AKAPs bind preferentially to the RI or RII regulatory domains of PKA. Of the twelve AKAPs detected, seven preferentially bound to RII, whereas the remaining five displayed at least dual specificity with a potential preference for RI. For some of these AKAPs our data provide the first insights into their specificity.

Thromboxane A2-induced bi-directional regulation of cerebral arterial tone

Myosin light chain phosphatase plays a critical role in modulating smooth muscle contraction in response to a variety of physiologic stimuli. A downstream target of the RhoA/Rho-kinase and nitric oxide (NO)/cGMP/cyclic GMP-dependent kinase (cGKI) pathways, myosin light chain phosphatase activity reflects the sum of both calcium sensitization and desensitization pathways through phosphorylation and dephosphorylation of the myosin phosphatase targeting subunit (MYPT1). As cerebral blood flow is highly spatio-temporally modulated under normal physiologic conditions, severe perturbations in normal cerebral blood flow, such as in cerebral vasospasm, can induce neurological deficits. In nonpermeabilized cerebral vessels stimulated with U-46619, a stable mimetic of endogenous thromboxane A2 implicated in the etiology of cerebral vasospasm, we observed significant increases in contractile force, RhoA activation, regulatory light chain phosphorylation, as well as phosphorylation of MYPT1 at Thr-696, Thr-853, and surprisingly Ser-695. Inhibition of nitric oxide signaling completely abrogated basal MYPT1 Ser-695 phosphorylation and significantly increased and potentiated U-46619-induced MYPT1 Thr-853 phosphorylation and contractile force, indicating that NO/cGMP/cGKI signaling maintains basal vascular tone through active inhibition of calcium sensitization. Surprisingly, a fall in Ser-695 phosphorylation did not result in an increase in phosphorylation of the Thr-696 site. Although activation of cGKI with exogenous cyclic nucleotides inhibited thromboxane A2-induced MYPT1 membrane association, RhoA activation, contractile force, and regulatory light chain phosphorylation, the anticipated decreases in MYPT1 phosphorylation at Thr-696/Thr-853 were not observed, indicating that the vasorelaxant effects of cGKI are not through dephosphorylation of MYPT1. Thus, thromboxane A2 signaling within the intact cerebral vasculature induces "buffered" vasoconstrictions, in which both the RhoA/Rho-kinase calcium-sensitizing and the NO/cGMP/cGKI calcium-desensitizing pathways are activated.

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.