Regulation of calcium entry by cyclic GMP signaling in Toxoplasma gondii

Ca2+ signaling impacts almost every aspect of cellular life. Ca2+ signals are generated through the opening of ion channels that permit the flow of Ca2+ down an electrochemical gradient. Cytosolic Ca2+ fluctuations can be generated through Ca2+ entry from the extracellular milieu or release from intracellular stores. In Toxoplasma gondii, Ca2+ ions play critical roles in several essential functions for the parasite, like invasion of host cells, motility, and egress. Plasma membrane Ca2+ entry in T. gondii was previously shown to be activated by cytosolic calcium and inhibited by the voltage-operated Ca2+ channel blocker nifedipine. However, Ca2+ entry in T. gondii did not show the classical characteristics of store regulation. In this work, we characterized the mechanism by which cytosolic Ca2+ regulates plasma membrane Ca2+ entry in extracellular T. gondii tachyzoites loaded with the Ca2+ indicator Fura-2. We compared the inhibition by nifedipine with the effect of the broad spectrum TRP channel inhibitor, anthranilic acid or ACA, and we find that both inhibitors act on different Ca2+ entry activities. We demonstrate, using pharmacological and genetic tools, that an intracellular signaling pathway engaging cyclic GMP, protein kinase G, Ca2+, and the phosphatidyl inositol phospholipase C affects Ca2+ entry and we present a model for crosstalk between cyclic GMP and cytosolic Ca2+ for the activation of T. gondii's lytic cycle traits.

DMHPpp1r17 neurons regulate aging and lifespan in mice through hypothalamic-adipose inter-tissue communication

Recent studies have shown that the hypothalamus functions as a control center of aging in mammals that counteracts age-associated physiological decline through inter-tissue communications. We have identified a key neuronal subpopulation in the dorsomedial hypothalamus (DMH), marked by Ppp1r17 expression (DMHPpp1r17 neurons), that regulates aging and longevity in mice. DMHPpp1r17 neurons regulate physical activity and WAT function, including the secretion of extracellular nicotinamide phosphoribosyltransferase (eNAMPT), through sympathetic nervous stimulation. Within DMHPpp1r17 neurons, the phosphorylation and subsequent nuclear-cytoplasmic translocation of Ppp1r17, regulated by cGMP-dependent protein kinase G (PKG; Prkg1), affect gene expression regulating synaptic function, causing synaptic transmission dysfunction and impaired WAT function. Both DMH-specific Prkg1 knockdown, which suppresses age-associated Ppp1r17 translocation, and the chemogenetic activation of DMHPpp1r17 neurons significantly ameliorate age-associated dysfunction in WAT, increase physical activity, and extend lifespan. Thus, these findings clearly demonstrate the importance of the inter-tissue communication between the hypothalamus and WAT in mammalian aging and longevity control.

The Antioxidant/Nitric Oxide-Quenching Agent Cobinamide Prevents Aortic Disease in a Mouse Model of Marfan Syndrome

Major pathologic changes in the proximal aorta underlie the life-threatening aortic aneurysms and dissections in Marfan Syndrome; current treatments delay aneurysm development without addressing the primary pathology. Because excess oxidative stress and nitric oxide/protein kinase G signaling likely contribute to the aortopathy, we hypothesized that cobinamide, a strong antioxidant that can attenuate nitric oxide signaling, could be uniquely suited to prevent aortic disease. In a well-characterized mouse model of Marfan Syndrome, cobinamide dramatically reduced elastin breaks, prevented excess collagen deposition and smooth muscle cell apoptosis, and blocked DNA, lipid, and protein oxidation and excess nitric oxide/protein kinase G signaling in the ascending aorta. Consistent with preventing pathologic changes, cobinamide diminished aortic root dilation without affecting blood pressure. Cobinamide exhibited excellent safety and pharmacokinetic profiles indicating it could be a practical treatment. We conclude that cobinamide deserves further study as a disease-modifying treatment of Marfan Syndrome.

Stimulation of Erythrocyte Soluble Guanylyl Cyclase Induces cGMP Export and Cardioprotection in Type 2 Diabetes

Reduced nitric oxide (NO) bioactivity in red blood cells (RBCs) is critical for augmented myocardial ischemia-reperfusion injury in type 2 diabetes. This study identified the nature of "NO bioactivity" by stimulating the intracellular NO receptor soluble guanylyl cyclase (sGC) in RBCs. sGC stimulation in RBCs from patients with type 2 diabetes increased export of cyclic guanosine monophosphate from RBCs and activated cardiac protein kinase G, thereby attenuating ischemia-reperfusion injury. These results provide novel insight into RBC signaling by identifying cyclic guanosine monophosphate from RBC as a mediator of protection against cardiac ischemia-reperfusion injury induced by sGC stimulation in RBCs.

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.

Targeting the nitric oxide/cGMP signaling pathway to treat chronic pain

Nitric oxide (NO)/cyclic guanosine 3',5'-monophosphate (cGMP) signaling has been shown to act as a mediator involved in pain transmission and processing. In this review, we summarize and discuss the mechanisms of the NO/cGMP signaling pathway involved in chronic pain, including neuropathic pain, bone cancer pain, inflammatory pain, and morphine tolerance. The main process in the NO/cGMP signaling pathway in cells involves NO activating soluble guanylate cyclase, which leads to subsequent production of cGMP. cGMP then activates cGMP-dependent protein kinase (PKG), resulting in the activation of multiple targets such as the opening of ATP-sensitive K⁺ channels. The activation of NO/cGMP signaling in the spinal cord evidently induces upregulation of downstream molecules, as well as reactive astrogliosis and microglial polarization which participate in the process of chronic pain. In dorsal root ganglion neurons, natriuretic peptide binds to particulate guanylyl cyclase, generating and further activating the cGMP/PKG pathway, and it also contributes to the development of chronic pain. Upregulation of multiple receptors is involved in activation of the NO/cGMP signaling pathway in various pain models. Notably the NO/cGMP signaling pathway induces expression of downstream effectors, exerting both algesic and analgesic effects in neuropathic pain and inflammatory pain. These findings suggest that activation of NO/cGMP signaling plays a constituent role in the development of chronic pain, and this signaling pathway with dual effects is an interesting and promising target for chronic pain therapy.

Plasmodium Egress Across the Parasite Life Cycle

Human malaria, caused by infection with Plasmodium parasites, remains one of the most important global public health problems, with the World Health Organization reporting more than 240 million cases and 600,000 deaths annually as of 2020 (World malaria report 2021). Our understanding of the biology of these parasites is critical for development of effective therapeutics and prophylactics, including both antimalarials and vaccines. Plasmodium is a protozoan organism that is intracellular for most of its life cycle. However, to complete its complex life cycle and to allow for both amplification and transmission, the parasite must egress out of the host cell in a highly regulated manner. This review discusses the major pathways and proteins involved in the egress events during the Plasmodium life cycle-merozoite and gametocyte egress out of red blood cells, sporozoite egress out of the oocyst, and merozoite egress out of the hepatocyte. The similarities, as well as the differences, between the various egress pathways of the parasite highlight both novel cell biology and potential therapeutic targets to arrest its life cycle.

PKGIα is activated by metal-dependent oxidation in vitro but not in intact cells

Type I cGMP-dependent protein kinases (PKGIs) are important components of various signaling pathways, and are canonically activated by nitric oxide- and natriuretic peptide-induced cGMP generation. However, some reports have shown that PKGIα can also be activated in vitro by oxidizing agents. Using in vitro kinase assays, here we found that purified PKGIα stored in phosphate-buffered saline with Flag peptide became oxidized and activated even in the absence of oxidizing agent; furthermore, once established, this activation could not be reversed by reduction with dithiothreitol. We demonstrate that activation was enhanced by addition of Cu²⁺ before storage, indicating it was driven by oxidation and mediated by trace metals present during storage. Previous reports suggested that PKGIα Cys⁴³, Cys¹¹⁸, and Cys¹⁹⁶ play key roles in oxidation-induced kinase activation; we show that activation was reduced by C118A or C196V mutations, although C43S PKGIα activation was not reduced. In contrast, under the same conditions, purified PKGIβ activity only slightly increased with storage. Using PKGIα/PKGIβ chimeras, we found that residues throughout the PKGIα-specific autoinhibitory loop were responsible for this activation. To explore whether oxidants activate PKGIα in H9c2 and C2C12 cells, we monitored vasodilator-stimulated phosphoprotein (VASP) phosphorylation downstream of PKGIα. While we observed PKGIα Cys⁴³ crosslinking in response to H₂O₂ (indicating an oxidizing environment in the cells), we were unable to detect increased VASP phosphorylation under these conditions. Taken together, we conclude that while PKGIα can be readily activated by oxidation in vitro, there is currently no direct evidence of oxidation-induced PKGIα activation in vivo.

Sodium Nitroprusside-Induced Activation of Vascular Smooth Muscle BK Channels Is Mediated by PKG Rather Than by a Direct Interaction with NO

Nitric oxide (NO) is a powerful vasodilator in different vascular beds and NO-donors are widely used in clinical practice. Early data suggested that NO and NO-donors activate vascular smooth muscle high-conductance, calcium-activated potassium channels (BK channels). There exist two hypotheses explaining the effect of NO and NO-donors on BK channels—one stating that protein kinase G (PKG) mediates the effect of NO, and the other one stating that NO acts directly on the channel. Thus, the degree of contribution of PKG to the NO-induced activation of the BK channel is still not completely clear. This study tested the hypothesis that the sodium nitroprusside (SNP)-induced activation of vascular smooth muscle BK channels is fully mediated by PKG. This hypothesis was investigated using the patch-clamp technique and freshly isolated smooth muscle cells from rat tail artery. In whole-cell experiments, SNP considerably increased the outward current compared with the addition of the bath solution. SNP did not alter the current in the presence of iberiotoxin, the specific blocker of BK channels, during co-application with hydroxocobalamin, an NO-scavenger, and in the presence of Rp-8-Br-PET-cGMPS, the specific PKG-inhibitor. In inside-out patches, the activity of BK channels was increased by SNP, SNAP, and DEA-NO. However, these effects did not differ from the effect of the application of drug-free bath solution. Furthermore, a similar increase in single BK channel activity was induced by Rp-8-Br-PET-cGMPS, Rp-8-Br-PET-cGMPS together with SNP, hydroxocobalamin, and hydroxocobalamin together with SNP or DEA-NO. Finally, the activity of excised BK channels did not change in the absence of any application but was considerably increased by PKG compared with the addition of drug-free bath solution. These results suggest that NO released from NO-donors stimulates the BK current only through activation of PKG.

Nicotinic-acid derivative BGP-15 improves diastolic function in a rabbit model of atherosclerotic cardiomyopathy

BACKGROUND AND PURPOSE

Small molecule BGP-15 has been reported to alleviate signs of heart failure and improve muscle function in murine models. Here, we investigated the acute and chronic effects of BGP-15 in a rabbit model of atherosclerotic cardiomyopathy.

EXPERIMENTAL APPROACH

Rabbits were maintained on standard chow (Control) or atherogenic diet (HC) for 16 weeks. BGP-15 was administered intravenously (once) or orally (for 16 weeks), to assess acute and chronic effects. Cardiac function was evaluated by echocardiography, endothelium-dependent vasorelaxation was assessed, and key molecules of the protein kinase G (PKG) axis were examined by ELISA and Western blot. Passive force generation was investigated in skinned cardiomyocytes.

KEY RESULTS

Both acute and chronic BGP-15 treatment improved the diastolic performance of the diseased heart, however, vasorelaxation and serum lipid markers were unaffected. Myocardial cGMP levels were elevated in the BGP-15-treated group, along with preserved PKG activity and increased phospholamban Ser16-phosphorylation. PDE5 expression decreased in the BGP-15-treated group, and the substance inhibited PDE1 enzyme. Cardiomyocyte passive tension reduced in BGP-15-treated rabbits, the ratio of titin N2BA/N2B isoforms increased, and PKG-dependent N2B-titin phosphorylation elevated in the BGP-15-treated group.

CONCLUSIONS AND IMPLICATIONS

Here we report that BGP-15-treatment improves diastolic function, reduces cardiomyocyte stiffness, and restores titin compliance in a rabbit model of atherosclerotic cardiomyopathy by increasing the activity of the cGMP-PKG axis. As BGP-15 is proven to be safe, it may have clinical value in the treatment of diastolic dysfunction.

Mediation of the Cardioprotective Effects of Mannitol Discovered, with Refutation of Common Protein Kinases

The osmodiuretic agent Mannitol exerts cardioprotection against ischemia and reperfusion (I/R) injury when applied as a pre- and/or postconditioning stimulus. Previously, we demonstrated that these properties are mediated via the activation of mitochondrial ATP-sensitive potassium (mK_ATP) channels. However, considering Mannitol remains in the extracellular compartment, the question arises as to which receptor and intracellular signaling cascades are involved in myocardial protection by the osmodiuretic substance. Protein kinase B (Akt) and G (PKG), as part of the reperfusion injury salvage kinase (RISK) and/or endothelial nitric oxide (eNOS)/PKG pathway, are two well-investigated intracellular targets conferring myocardial protection upstream of mitochondrial potassium channels. Adenosine receptor subtypes have been shown to trigger different cardioprotective pathways, for example, the reperfusion injury. Further, Mannitol induces an increased activation of the adenosine 1 receptor (A1R) in renal cells conferring its nephroprotective properties. Therefore, we investigated whether (1) Akt and PKG are possible signaling targets involved in Mannitol-induced conditioning upstream of the mK_ATP channel and/or whether (2) cardioprotection by Mannitol is mediated via activation of the A1R. All experiments were performed on male Wistar rats in vitro employing the Langendorff isolated heart perfusion technique with infarct size determination as the primary endpoint. To unravel possible protein kinase activation, Mannitol was applied in combination with the Akt (MK2206) or PKG (KT5823) inhibitor. In further groups, an A1R blocker (DPCPX) was given with or without Mannitol. Preconditioning with Mannitol (Man) significantly reduced the infarct size compared to the control group. Co-administration of the A1R blocker DPXPC fully abolished myocardial protection of Mannitol. Interestingly and in contrast to the initial hypothesis, neither administration of the Akt nor the PKG blocker had any impact on the cardioprotective properties of Mannitol-induced preconditioning. These results are quite unexpected and show that the protein kinases Akt and PKG—as possible targets of known protective signaling cascades—are not involved in Mannitol-induced preconditioning. However, the cardioprotective effects of Mannitol are mediated via the A1R.

M. tuberculosis PknG manipulates host autophagy flux to promote pathogen intracellular survival

The eukaryotic-type protein kinase G (PknG), one of the eleven eukaryotic type serine-threonine protein kinase (STPK) in Mycobacterium tuberculosis (Mtb), is involved in mycobacterial survival within macrophages, presumably by suppressing phagosome and autophagosome maturation, which makes PknG an attractive drug target. However, the exact mechanism by which PknG inhibits pathogen clearance during mycobacterial infection remains largely unknown. Here, we show that PknG promotes macroautophagy/autophagy induction but inhibits autophagosome maturation, causing an overall effect of blocked autophagy flux and enhanced pathogen intracellular survival. PknG prevents the activation of AKT (AKT serine/threonine kinase) via competitively binding to its pleckstrin homology (PH) domain, leading to autophagy induction. Remarkably, PknG could also inhibit autophagosome maturation to block autophagy flux via targeting host small GTPase RAB14. Specifically, PknG directly interacts with RAB14 to block RAB14-GTP hydrolysis. Furthermore, PknG phosphorylates TBC1D4/AS160 (TBC1 domain family member 4) to suppress its GTPase-activating protein (GAP) activity toward RAB14. In macrophages and in vivo, PknG promotes Mtb intracellular survival through blocking autophagy flux, which is dependent on RAB14. Taken together, our data unveil a dual-functional bacterial effector that tightly regulates host autophagy flux to benefit pathogen intracellular survival.Abbreviations: AKT: AKT serine/threonine kinase; ATG5: autophagy related 5; BMDMs: bone marrow-derived macrophages; DTT: dithiothreitol; FBS: fetal calf serum; GAP: GTPase-activating protein; MOI: multiplicity of infection; Mtb: Mycobacterium tuberculosis; MTOR: mechanistic target of rapamycin kinase; OADC: oleic acid-albumin-dextrose-catalase; PC, phosphatidylcholine; PH: pleckstrin homology; PI3K: phosphoinositide 3-kinase; PknG: protein kinase G; PtdIns(3,4,5)P₃: phosphatidylinositol(3,4,5)-trisphosphate; SQSTM1: sequestosome 1; STPK: serine-threonine protein kinase; TB: tuberculosis; TBC1D4: TBC1 domain family member 4; TPR: tetratricopeptide repeat; ULK1: unc-51 like autophagy activating kinase 1; WT: wild-type.

Mycobacterium tuberculosis protein kinase G acts as an unusual ubiquitinating enzyme to impair host immunity

Upon Mycobacterium tuberculosis (Mtb) infection, protein kinase G (PknG), a eukaryotic-type serine-threonine protein kinase (STPK), is secreted into host macrophages to promote intracellular survival of the pathogen. However, the mechanisms underlying this PknG-host interaction remain unclear. Here, we demonstrate that PknG serves both as a ubiquitin-activating enzyme (E1) and a ubiquitin ligase (E3) to trigger the ubiquitination and degradation of tumor necrosis factor receptor-associated factor 2 (TRAF2) and TGF-β-activated kinase 1 (TAK1), thereby inhibiting the activation of NF-κB signaling and host innate responses. PknG promotes the attachment of ubiquitin (Ub) to the ubiquitin-conjugating enzyme (E2) UbcH7 via an isopeptide bond (UbcH7 K82-Ub), rather than the usual C86-Ub thiol-ester bond. PknG induces the discharge of Ub from UbcH7 by acting as an isopeptidase, before attaching Ub to its substrates. These results demonstrate that PknG acts as an unusual ubiquitinating enzyme to remove key components of the innate immunity system, thus providing a potential target for tuberculosis treatment.

Gaseous transmitter regulation of hypoxia-evoked catecholamine secretion from murine adrenal chromaffin cells

Emerging evidence suggests that gaseous molecules, carbon monoxide (CO), and hydrogen sulfide (H₂S) generated by heme oxygenase (HO)-2 and cystathionine γ-lyase (CSE), respectively, function as transmitters in the nervous system. Present study examined the roles of CO and H₂S in hypoxia-induced catecholamine (CA) release from adrenal medullary chromaffin cells (AMCs). Studies were performed on AMCs from adult (≥6 wk of age) wild-type (WT), HO-2 null, CSE null, and HO-2/CSE double null mice of either gender. CA secretion was determined by carbon fiber amperometry and [Ca²⁺]_i by microflurometry using Fura-2. HO-2- and CSE immunoreactivities were seen in WT AMC, which were absent in HO-2 and CSE null mice. Hypoxia (medium Po₂ 30-38 mmHg) evoked CA release and elevated [Ca²⁺]_i. The magnitude of hypoxic response was greater in HO-2 null mice and in HO inhibitor-treated WT AMC compared with controls. H₂S levels were elevated in HO-2 null AMC. Either pharmacological inhibition or genetic deletion of CSE prevented the augmented hypoxic responses of HO-2 null AMC and H₂S donor rescued AMC responses to hypoxia in HO-2/CSE double null mice. CORM3, a CO donor, prevented the augmented hypoxic responses in WT and HO-2 null AMC. CO donor reduced H₂S levels in WT AMC. The effects of CO donor were blocked by either ODQ or 8pCT, inhibitors of soluble guanylyl cyclase (SGC) or protein kinase G, respectively. These results suggest that HO-2-derived CO inhibits hypoxia-evoked CA secretion from adult murine AMC involving soluble guanylyl cyclase (SGC)-protein kinase G (PKG)-dependent regulation of CSE-derived H₂S.NEW & NOTEWORTHY Catecholamine secretion from adrenal chromaffin cells is an important physiological mechanism for maintaining homeostasis during hypoxia. Here, we delineate carbon monoxide (CO)-sensitive hydrogen sulfide (H₂S) signaling as an important mediator of hypoxia-induced catecholamine secretion from murine adrenal chromaffin cells. Heme oxygenase-2 derived CO is a physiological inhibitor of catcholamince secretion by hypoxia and the effects of CO involve inhibition of cystathionine γ-lyase-derived H₂S production through soluble guanylyl cyclase-protein kinase G signaling cascade.

Downregulation of Soluble Guanylate Cyclase and Protein Kinase G With Upregulated ROCK2 in the Pulmonary Artery Leads to Thromboxane A2 Sensitization in Monocrotaline-Induced Pulmonary Hypertensive Rats

Thromboxane A2 (TXA₂) promotes various physiological responses including pulmonary artery (PA) contraction, and pathophysiological implications have been suggested in cardiovascular diseases including pulmonary hypertension. Here, we investigated the role of TXA₂ receptor (TP)-mediated signaling in the pathophysiology of pulmonary arterial hypertension (PAH). The sensitivity of PA to the contractile agonist could be set by relaxing signals such as the nitric oxide (NO), soluble guanylate cyclase (sGC), and cGMP-dependent kinase (PKG) pathways. Changes in the TP agonist (U46619)-induced PA contraction and its modulation by NO/cGMP signaling were analyzed in a monocrotaline-induced PAH rat model (PAH-MCT). In the myograph study, PA from PAH-MCT showed higher responsiveness to U46619, that is decreased EC₅₀. Immunoblot analysis revealed a lower expression of eNOS, sGC, and PKG, while there was a higher expression of RhoA-dependent kinase 2 (ROCK2) in the PA from PAH-MCT than in the control. In PAH-MCT, the higher sensitivity to U46619 was reversed by 8-Br-cGMP, a membrane-permeable cGMP analog, but not by the NO donor, sodium nitroprusside (SNP 30 μM). In contrast, in the control PA, inhibition of sGC by its inhibitor (1H- [1,2,4] oxadiazolo [4,3-a] quinoxalin-1-one (ODQ), 10 μM) lowered the threshold of U46619-induced contraction. In the presence of ODQ, SNP treatment had no effect whereas the addition of 8-Br-cGMP lowered the sensitivity to U46619. The inhibition of ROCK by Y-27632 attenuated the sensitivity to U46619 in both control and PAH-MCT. The study suggests that the attenuation of NO/cGMP signaling and the upregulation of ROCK2 increase the sensitivity to TXA₂ in the PAH animal, which might have pathophysiological implications in patients with PAH.

CRD-733, a Novel PDE9 (Phosphodiesterase 9) Inhibitor, Reverses Pressure Overload-Induced Heart Failure

BACKGROUND

Augmentation of NP (natriuretic peptide) receptor and cyclic guanosine monophosphate (cGMP) signaling has emerged as a therapeutic strategy in heart failure (HF). cGMP-specific PDE9 (phosphodiesterase 9) inhibition increases cGMP signaling and attenuates stress-induced hypertrophic heart disease in preclinical studies. A novel cGMP-specific PDE9 inhibitor, CRD-733, is currently being advanced in human clinical studies. Here, we explore the effects of chronic PDE9 inhibition with CRD-733 in the mouse transverse aortic constriction pressure overload HF model.

METHODS

Adult male C57BL/6J mice were subjected to transverse aortic constriction and developed significant left ventricular (LV) hypertrophy after 7 days (P<0.001). Mice then received daily treatment with CRD-733 (600 mg/kg per day; n=10) or vehicle (n=17), alongside sham-operated controls (n=10).

RESULTS

CRD-733 treatment reversed existing LV hypertrophy compared with vehicle (P<0.001), significantly improved LV ejection fraction (P=0.009), and attenuated left atrial dilation (P<0.001), as assessed by serial echocardiography. CRD-733 prevented elevations in LV end diastolic pressures (P=0.037) compared with vehicle, while lung weights, a surrogate for pulmonary edema, were reduced to sham levels. Chronic CRD-733 treatment increased plasma cGMP levels compared with vehicle (P<0.001), alongside increased phosphorylation of Ser²⁷³ of cardiac myosin binding protein-C, a cGMP-dependent protein kinase I phosphorylation site.

CONCLUSIONS

The PDE9 inhibitor, CRD-733, improves key hallmarks of HF including LV hypertrophy, LV dysfunction, left atrial dilation, and pulmonary edema after pressure overload in the mouse transverse aortic constriction HF model. Additionally, elevated plasma cGMP may be used as a biomarker of target engagement. These findings support future investigation into the therapeutic potential of CRD-733 in human HF.

Chlorogenic Acid Alleviates Hyperglycemia-Induced Cardiac Fibrosis through Activation of the NO/cGMP/PKG Pathway in Cardiac Fibroblasts

SCOPE

Hyperglycemia-induced cardiac fibrosis is one of the main causes of diabetic cardiomyopathy (DM). Chlorogenic acid (CGA) found in many foods has excellent hypoglycemic effectiveness, but it is not known whether CGA can improve DM by inhibiting cardiac fibrosis caused by hyperglycemia.

METHODS AND RESULTS

Type I diabetic mice are induced by streptozotocin, and after treatment with CGA for 12 weeks, cardiac functions and fibrosis are determined. CGA significantly attenuates hyperglycemia-induced cardiac fibrosis and improves cardiac functions. The mechanism of CGA on fibrotic inhibition is further studied by immunofluorescence, western blot and RNA interference technology in vivo and in vitro. The results show CGA exerted its anti-fibrotic effects through activating the cyclic GMP/protein kinase G pathway (cGMP/PKG) to block hyperglycemia-induced nuclear translocation of p-Smad2/3, and then inhibiting pro-fibrotic gene expression in cardiac fibroblasts without depending on its hypoglycemic function. Moreover, the data also revealed that CGA increased cGMP level and activated PKG in cardiac fibroblasts by enhancing endothelial nitric oxide synthase (eNOS) activity and NO production.

CONCLUSION

Besides lowering blood glucose, CGA also has an independent ability to inhibit cardiac fibrosis. Therefore, long-term consumption of foods rich in CGA for diabetic patients will have great benefits to improve diabetic cardiomyopathy.

cGMP via PKG activates 26S proteasomes and enhances degradation of proteins, including ones that cause neurodegenerative diseases

Most studies of the regulation of proteolysis by the ubiquitin proteasome system have focused on the control of ubiquitination. However, it is now clear that the activity of the 26S proteasome and rates of protein degradation in cells are also tightly regulated through proteasome phosphorylation. Here we demonstrate that agents that raise cGMP and activate cGMP-dependent protein kinase (e.g., widely used phosphodiesterase 5 inhibitors) stimulate proteasome activities and intracellular proteolysis without affecting autophagy. Furthermore, we showed that raising cGMP reduced the levels of the disease-causing mutant tau in a zebrafish model by increasing its degradation, and also decreased the associated morphological abnormalities. Thus, activating the proteasome via cGMP is a promising strategy to prevent the progression of neurodegenerative diseases.

Because raising cAMP enhances 26S proteasome activity and the degradation of cell proteins, including the selective breakdown of misfolded proteins, we investigated whether agents that raise cGMP may also regulate protein degradation. Treating various cell lines with inhibitors of phosphodiesterase 5 or stimulators of soluble guanylyl cyclase rapidly enhanced multiple proteasome activities and cellular levels of ubiquitinated proteins by activating protein kinase G (PKG). PKG stimulated purified 26S proteasomes by phosphorylating a different 26S component than is modified by protein kinase A. In cells and cell extracts, raising cGMP also enhanced within minutes ubiquitin conjugation to cell proteins. Raising cGMP, like raising cAMP, stimulated the degradation of short-lived cell proteins, but unlike cAMP, also markedly increased proteasomal degradation of long-lived proteins (the bulk of cell proteins) without affecting lysosomal proteolysis. We also tested if raising cGMP, like cAMP, can promote the degradation of mutant proteins that cause neurodegenerative diseases. Treating zebrafish models of tauopathies or Huntington’s disease with a PDE5 inhibitor reduced the levels of the mutant huntingtin and tau proteins, cell death, and the resulting morphological abnormalities. Thus, PKG rapidly activates cytosolic proteasomes, protein ubiquitination, and overall protein degradation, and agents that raise cGMP may help combat the progression of neurodegenerative diseases.

Elucidation of cGMP-dependent induction of mitochondrial biogenesis through PKG and p38 MAPK in the kidney

Previous studies have shown that cGMP increases mitochondrial biogenesis (MB). Our laboratory has determined that formoterol and LY344864, agonists of the β₂-adrenergic receptor and 5-HT_1F receptor, respectively, signal MB in a soluble guanylyl cyclase (sGC)-dependent manner. However, the pathway between cGMP and MB produced by these pharmacological agents in renal proximal tubule cells (RPTCs) and the kidney has not been determined. In the present study, we showed that treatment of RPTCs with formoterol, LY344864, or riociguat, a sGC stimulator, induces MB through protein kinase G (PKG), a target of cGMP, and p38, an associated downstream target of PKG and a regulator of peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α) expression in RPTCs. We also examined if p38 plays a role in PGC-1α phosphorylation in vivo. Administration of l-skepinone, a potent and specific inhibitor of p38α and p38β, to naïve mice inhibited phosphorylated PGC-1α localization in the nuclear fraction of the renal cortex. Taken together, we demonstrated a pathway, sGC/cGMP/PKG/p38/PGC-1α, for pharmacological induction of MB and the importance of p38 in this pathway.

Protein kinase inhibitors infused intraventricularly or into the ventromedial hypothalamus block short latency facilitation of lordosis by oestradiol

An injection of unesterified oestradiol (E₂ ) facilitates receptive behaviour in E₂ benzoate (EB)-primed, ovariectomised female rats when it is administered i.c.v. or systemically. The present study tested the hypothesis that inhibitors of protein kinase A (PKA), protein kinase G (PKG) or the Src/mitogen-activated protein kinase (MAPK) complex interfere with E₂ facilitation of receptive behaviour. In Experiment 1, lordosis induced by i.c.v. infusion of E₂ was significantly reduced by i.c.v. administration of Rp-cAMPS, a PKA inhibitor, KT5823, a PKG inhibitor, and PP2 and PD98059, Src and MAPK inhibitors, respectively, between 30 and 240 minutes after infusion. In Experiment 2, we determined whether the ventromedial hypothalamus (VMH) is one of the neural sites at which those intracellular pathways participate in lordosis behaviour induced by E₂ . Administration of each of the four protein kinase inhibitors into the VMH blocked facilitation of lordosis induced by infusion of E₂ also into the VMH. These data support the hypothesis that activation of several protein kinase pathways is involved in the facilitation of lordosis by E₂ in EB-primed rats.

The mitochondrial NO-synthase/guanylate cyclase/protein kinase G signaling system underpins the dual effects of nitric oxide on mitochondrial respiration and opening of the permeability transition pore

The available data on the involvement of nitric oxide (NO) and mitochondrial calcium-dependent NO synthase (mtNOS) in the control of mitochondrial respiration and the permeability transition pore (mPTP) are contradictory. We have proposed that the mitochondrial mtNOS/guanylate cyclase/protein kinase G signaling system (mtNOS-SS) is also implicated in the control of respiration and mPTP, providing the interplay between NO and mtNOS-SS, which, in turn, may result in inconsistent effects of NO. Therefore, using rat liver mitochondria, we applied specific inhibitors of the enzymes of this signaling system to evaluate its role in the control of respiration and mPTP opening. Steady-state respiration was supported by pyruvate, glutamate, or succinate in the presence of hexokinase, glucose, and ADP. When applied at low concentrations, l-arginine (to 500 µm) and NO donors (to 50 µm) activated the respiration and increased the threshold concentrations of calcium and d,l-palmitoylcarnitine required for the dissipation of the mitochondrial membrane potential and pore opening. Both effects were eliminated by the inhibitors of NO synthase, guanylate cyclase, and kinase G, which denotes the involvement of mtNOS-SS in the activation of respiration and deceleration of mPTP opening. At high concentrations, l-arginine and NO donors inhibited the respiration and promoted pore opening, indicating that adverse effects induced by an NO excess dominate over the protection provided by mtNOS-SS. Thus, these results demonstrate the opposite impact of NO and mtNOS-SS on the respiration and mPTP control, which can explain the dual effects of NO.

"A Step and a Ceiling": mechanical properties of Ca2+ spark vasoregulation in resistance arteries by pressure-induced oxidative activation of PKG

We investigated the biomechanical relationship between intraluminal pressure within small mesenteric resistance arteries, oxidant activation of PKG, Ca2+ sparks, and BK channel vasoregulation. Mesenteric resistance arteries from wild type (WT) and genetically modified mice with PKG resistance to oxidative activation were studied using wire and pressure myography. Ca2+ sparks and Ca2+ transients within vascular smooth muscle cells of intact arteries were characterized using high-speed confocal microscopy of intact arteries. Arteries were studied under conditions of varying intraluminal pressure and oxidation. Intraluminal pressure specifically, rather than the generic stretch of the artery, was necessary to activate the oxidative pathway. We demonstrated a graded step activation profile for the generation of Ca2+ sparks and also a functional "ceiling" for this pressure --sensitive oxidative pathway. During steady state pressure - induced constriction, any additional Ca2+ sensitive-K+ channel functional availability was independent of oxidant activated PKG. There was an increase in the amplitude, but not the Area under the Curve (AUC) of the caffeine-induced Ca2+ transient in pressurized arteries from mice with oxidant-resistant PKG compared with wild type. Overall, we surmise that intraluminal pressure within resistance arteries controls Ca2+ spark vasoregulation through a tightly controlled pathway with a graded onset switch. The pathway, underpinned by oxidant activation of PKG, cannot be further boosted by additional pressure or oxidation once active. We propose that these restrictive characteristics of pressure-induced Ca2+ spark vasoregulation confer stability for the artery in order to provide a constant flow independent of additional pressure fluctuations or exogenous oxidants.

Calmodulin and protein kinases A/G mediate ciliary beat response in the human nasal epithelium

BACKGROUND

Mucociliary clearance of the airway epithelium is an essential function for mucosal defense. We recently proposed a hypothetical mechanism of ciliary beat regulation, in which the pannexin-1 (Panx1)-P2X7 unit serves as an oscillator generating a periodic increase in intracellular Ca²⁺ ([Ca²⁺ ]_i ). In the present study, we examined the localization of Panx1 and P2X7 at the ultrastructural level, and investigated the regulatory pathway subsequent to [Ca²⁺ ]_i increase.

METHODS

The inferior turbinate mucosa was collected from patients with chronic hypertrophic rhinitis during endoscopic sinonasal surgery. The mucosa was examined by transmission immunoelectron microscopy for Panx1 and P2X7. Alternatively, the mucosa was cut into thin strips, and ciliary beat frequency (CBF) was measured under a phase-contrast light microscope with a high-speed digital video camera.

RESULTS

In immunoelectron microscopy, immunoreactivities for Panx1 and P2X7 were localized along the plasma membrane of the entire length of the cilia. CBF was significantly increased by stimulation with 100 µM acetylcholine (Ach). The Ach-induced CBF increase was significantly inhibited by calmidazolium (calmodulin antagonist), SQ22536 (adenylate cyclase inhibitor), ODQ (guanylate cyclase inhibitor), KT5720 (protein kinase A inhibitor), and KT5823 (protein kinase G inhibitor). Fluorodinitrobenzene (creatine kinase inhibitor) completely inhibited the ciliary beat in a time- and dose-dependent manner.

CONCLUSION

These results indicate that Panx1 and P2X7 coexist at the cilia of the human nasal epithelial cells and that the ciliary beat is regulated by calmodulin, adenylate/guanylate cyclases and protein kinases A/G, and crucially depends on creatine kinase.

Oxidation of PKGIα mediates an endogenous adaptation to pulmonary hypertension

Chronic hypoxia causes pulmonary hypertension (PH), vascular remodeling, right ventricular (RV) hypertrophy, and cardiac failure. Protein kinase G Iα (PKGIα) is susceptible to oxidation, forming an interprotein disulfide homodimer associated with kinase targeting involved in vasodilation. Here we report increased disulfide PKGIα in pulmonary arteries from mice with hypoxic PH or lungs from patients with pulmonary arterial hypertension. This oxidation is likely caused by oxidants derived from NADPH oxidase-4, superoxide dismutase 3, and cystathionine γ-lyase, enzymes that were concomitantly increased in these samples. Indeed, products that may arise from these enzymes, including hydrogen peroxide, glutathione disulfide, and protein-bound persulfides, were increased in the plasma of hypoxic mice. Furthermore, low-molecular-weight hydropersulfides, which can serve as "superreductants" were attenuated in hypoxic tissues, consistent with systemic oxidative stress and the oxidation of PKGIα observed. Inhibiting cystathionine γ-lyase resulted in decreased hypoxia-induced disulfide PKGIα and more severe PH phenotype in wild-type mice, but not in Cys42Ser PKGIα knock-in (KI) mice that are resistant to oxidation. In addition, KI mice also developed potentiated PH during hypoxia alone. Thus, oxidation of PKGIα is an adaptive mechanism that limits PH, a concept further supported by polysulfide treatment abrogating hypoxia-induced RV hypertrophy in wild-type, but not in the KI, mice. Unbiased transcriptomic analysis of hypoxic lungs before structural remodeling identified up-regulation of endothelial-to-mesenchymal transition pathways in the KI compared with wild-type mice. Thus, disulfide PKGIα is an intrinsic adaptive mechanism that attenuates PH progression not only by promoting vasodilation but also by limiting maladaptive growth and fibrosis signaling.

Cysteine-Based Redox Sensing and Its Role in Signaling by Cyclic Nucleotide-Dependent Kinases in the Cardiovascular System

Oxidant molecules are produced in biological systems and historically have been considered causal mediators of damage and disease. While oxidants may contribute to the pathogenesis of disease, evidence continues to emerge that shows these species also play important regulatory roles in health. A major mechanism of oxidant sensing and signaling involves their reaction with reactive cysteine thiols within proteins, inducing oxidative posttranslational modifications that can couple to altered function to enable homeostatic regulation. Protein kinase A and protein kinase G are regulated by oxidants in this way, and this review focuses on our molecular-level understanding of these events and their role in regulating cardiovascular physiology during health and disease.

Selective inhibition of intestinal guanosine 3',5'-cyclic monophosphate signaling by small-molecule protein kinase inhibitors

The guanosine 3',5'-cyclic monophosphate (cGMP)-dependent protein kinase II (cGKII) serine/threonine kinase relays signaling through guanylyl cyclase C (GCC) to control intestinal fluid homeostasis. Here, we report the discovery of small-molecule inhibitors of cGKII. These inhibitors were imidazole-aminopyrimidines, which blocked recombinant human cGKII at submicromolar concentrations but exhibited comparatively little activity toward the phylogenetically related protein kinases cGKI and cAMP-dependent protein kinase (PKA). Whereas aminopyrimidyl motifs are common in protein kinase inhibitors, molecular modeling of these imidazole-aminopyrimidines in the ATP-binding pocket of cGKII indicated an unconventional binding mode that directs their amine substituent into a narrow pocket delineated by hydrophobic residues of the hinge and the αC-helix. Crucially, this set of residues included the Leu-530 gatekeeper, which is not conserved in cGKI and PKA. In intestinal organoids, these compounds blocked cGKII-dependent phosphorylation of the vasodilator-stimulated phosphoprotein (VASP). In mouse small intestinal tissue, cGKII inhibition significantly attenuated the anion secretory response provoked by the GCC-activating bacterial heat-stable toxin (STa), a frequent cause of infectious secretory diarrhea. In contrast, both PKA-dependent VASP phosphorylation and intestinal anion secretion were unaffected by treatment with these compounds, whereas experiments with T84 cells indicated that they weakly inhibit the activity of cAMP-hydrolyzing phosphodiesterases. As these protein kinase inhibitors are the first to display selective inhibition of cGKII, they may expedite research on cGMP signaling and may aid future development of therapeutics for managing diarrheal disease and other pathogenic syndromes that involve cGKII.

Particulate Guanylyl Cyclase A/cGMP Signaling Pathway in the Kidney: Physiologic and Therapeutic Indications

The particulate guanylyl cyclase A (pGC-A)/cGMP pathway plays important roles in regulating renal physiological function and as well as in counteracting pathophysiological conditions. Naturally occurring peptide pGC-A activators consist of atrial natriuretic peptide (ANP), b-type NP (BNP), and urodilatin (URO). These activators bind and activate pGC-A, generating the second messenger cyclic 3′,5′ guanosine monophosphate (cGMP). Cyclic GMP binds to downstream pathway effector molecules including protein kinase G (PKG), cGMP-gated ion channels, and phosphodiesterases (PDEs). These mediators result in a variety of physiological actions in the kidney, including diuresis, natriuresis, increased glomerular filtration rate (GFR) and organ protection, thus, opposing renal cellular injury and remodeling. Downstream proteins regulated by PKG include collagen 1 (Col-1), transforming growth factor beta (TGF-β) and apoptosis-related proteins. In addition to their physiological regulatory effects, pGC-A/cGMP signaling is critical for preserving renal homeostasis in different renal diseases such as acute kidney injury (AKI). Regarding therapeutic options, native pGC-A activators have short half-lives and their activity can be further enhanced by advances in innovative peptide engineering. Thus, novel designer peptide pGC-A activators with enhanced renal activity are under development.

Effects of Nitric Oxide on Voltage-Gated K⁺ Currents in Human Cardiac Fibroblasts through the Protein Kinase G and Protein Kinase A Pathways but Not through S-Nitrosylation

This study investigated the expression of voltage-gated K⁺ (K_V) channels in human cardiac fibroblasts (HCFs), and the effect of nitric oxide (NO) on the K_V currents, and the underlying phosphorylation mechanisms. In reverse transcription polymerase chain reaction, two types of K_V channels were detected in HCFs: delayed rectifier K⁺ channel and transient outward K⁺ channel. In whole-cell patch-clamp technique, delayed rectifier K⁺ current (I_K) exhibited fast activation and slow inactivation, while transient outward K⁺ current (I_to) showed fast activation and inactivation kinetics. Both currents were blocked by 4-aminopyridine. An NO donor, S-nitroso-N-acetylpenicillamine (SNAP), increased the amplitude of I_K in a concentration-dependent manner with an EC₅₀ value of 26.4 µM, but did not affect I_to. The stimulating effect of SNAP on I_K was blocked by pretreatment with 1H-(1,2,4)oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) or by KT5823. 8-bromo-cyclic GMP stimulated the I_K. The stimulating effect of SNAP on I_K was also blocked by pretreatment with KT5720 or by SQ22536. Forskolin and 8-bromo-cyclic AMP each stimulated I_K. On the other hand, the stimulating effect of SNAP on I_K was not blocked by pretreatment of N-ethylmaleimide or by DL-dithiothreitol. Our data suggest that NO enhances I_K, but not I_to, among K_V currents of HCFs, and the stimulating effect of NO on I_K is through the PKG and PKA pathways, not through S-nitrosylation.

Mutations of PKA cyclic nucleotide-binding domains reveal novel aspects of cyclic nucleotide selectivity

Cyclic AMP and cyclic GMP are ubiquitous second messengers that regulate the activity of effector proteins in all forms of life. The main effector proteins, the 3',5'-cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) and the 3',5'-cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG), are preferentially activated by cAMP and cGMP, respectively. However, the molecular basis of this cyclic nucleotide selectivity is still not fully understood. Analysis of isolated cyclic nucleotide-binding (CNB) domains of PKA regulatory subunit type Iα (RIα) reveals that the C-terminal CNB-B has a higher cAMP affinity and selectivity than the N-terminal CNB-A. Here, we show that introducing cGMP-specific residues using site-directed mutagenesis reduces the selectivity of CNB-B, while the combination of two mutations (G316R/A336T) results in a cGMP-selective binding domain. Furthermore, introducing the corresponding mutations (T192R/A212T) into the PKA RIα CNB-A turns this domain into a highly cGMP-selective domain, underlining the importance of these contacts for achieving cGMP specificity. Binding data with the generic purine nucleotide 3',5'-cyclic inosine monophosphate (cIMP) reveal that introduced arginine residues interact with the position 6 oxygen of the nucleobase. Co-crystal structures of an isolated CNB-B G316R/A336T double mutant with either cAMP or cGMP reveal that the introduced threonine and arginine residues maintain their conserved contacts as seen in PKG I CNB-B. These results improve our understanding of cyclic nucleotide binding and the molecular basis of cyclic nucleotide specificity.

Vascular smooth muscle cell contractile protein expression is increased through protein kinase G-dependent and -independent pathways by glucose-6-phosphate dehydrogenase inhibition and deficiency

Homeostatic control of vascular smooth muscle cell (VSMC) differentiation is critical for contractile activity and regulation of blood flow. Recently, we reported that precontracted blood vessels are relaxed and the phenotype of VSMC is regulated from a synthetic to contractile state by glucose-6-phosphate dehydrogenase (G6PD) inhibition. In the current study, we investigated whether the increase in the expression of VSMC contractile proteins by inhibition and knockdown of G6PD is mediated through a protein kinase G (PKG)-dependent pathway and whether it regulates blood pressure. We found that the expression of VSMC-restricted contractile proteins, myocardin (MYOCD), and miR-1 and miR-143 are increased by G6PD inhibition or knockdown. Importantly, RNA-sequence analysis of aortic tissue from G6PD-deficient mice revealed uniform increases in VSMC-restricted genes, particularly those regulated by the MYOCD-serum response factor (SRF) switch. Conversely, expression of Krüppel-like factor 4 (KLF4) is decreased by G6PD inhibition. Interestingly, the G6PD inhibition-induced expression of miR-1 and contractile proteins was blocked by Rp-β-phenyl-1,N2-etheno-8-bromo-guanosine-3',5'-cyclic monophosphorothioate, a PKG inhibitor. On the other hand, MYOCD and miR-143 levels are increased by G6PD inhibition through a PKG-independent manner. Furthermore, blood pressure was lower in the G6PD-deficient compared with wild-type mice. Therefore, our results suggest that the expression of VSMC contractile proteins induced by G6PD inhibition occurs via PKG1α-dependent and -independent pathways.

Muscarinic Stimulation Facilitates Sarcoplasmic Reticulum Ca Release by Modulating Ryanodine Receptor 2 Phosphorylation Through Protein Kinase G and Ca/Calmodulin-Dependent Protein Kinase II

Although the effects and the underlying mechanism of sympathetic stimulation on cardiac Ca handling are relatively well established both in health and disease, the modes of action and mechanisms of parasympathetic modulation are poorly defined. Here, we demonstrate that parasympathetic stimulation initiates a novel mode of excitation-contraction coupling that enhances the efficiency of cardiac sarcoplasmic reticulum Ca store utilization. This efficient mode of excitation-contraction coupling involves reciprocal changes in the phosphorylation of ryanodine receptor 2 at Ser-2808 and Ser-2814. Specifically, Ser-2808 phosphorylation was mediated by muscarinic receptor subtype 2 and activation of PKG (protein kinase G), whereas dephosphorylation of Ser-2814 involved activation of muscarinic receptor subtype 3 and decreased reactive oxygen species-dependent activation of CaMKII (Ca/calmodulin-dependent protein kinase II). The overall effect of these changes in phosphorylation of ryanodine receptor 2 is an increase in systolic Ca release at the low sarcoplasmic reticulum Ca content and a paradoxical reduction in aberrant Ca leak. Accordingly, cholinergic stimulation of cardiomyocytes isolated from failing hearts improved Ca cycling efficiency by restoring altered ryanodine receptor 2 phosphorylation balance.

Mechanisms of spreading depolarization in vertebrate and insect central nervous systems

Spreading depolarization (SD) is generated in the central nervous systems of both vertebrates and invertebrates. SD manifests as a propagating wave of electrical depression caused by a massive redistribution of ions. Mammalian SD underlies a continuum of human pathologies from migraine to stroke damage, whereas insect SD is associated with environmental stress-induced neural shutdown. The general cellular mechanisms underlying SD seem to be evolutionarily conserved throughout the animal kingdom. In particular, SD in the central nervous system of Locusta migratoria and Drosophila melanogaster has all the hallmarks of mammalian SD. Locust SD is easily induced and monitored within the metathoracic ganglion (MTG) and can be modulated both pharmacologically and by preconditioning treatments. The finding that the fly brain supports repetitive waves of SD is relatively recent but noteworthy, since it provides a genetically tractable model system. Due to the human suffering caused by SD manifestations, elucidating control mechanisms that could ultimately attenuate brain susceptibility is essential. Here we review mechanisms of SD focusing on the similarities between mammalian and insect systems. Additionally we discuss advantages of using invertebrate model systems and propose insect SD as a valuable model for providing new insights to mammalian SD.

Soluble guanylate cyclase activation during ischemic injury in mice protects against postischemic inflammation at the mitochondrial level

The aim was to determine whether treatment with BAY 60-2770, a selective activator of oxidized soluble guanylate cyclase (sGC), near the end of an ischemic event would prevent postischemic inflammation and mitochondrial dysfunction in wild-type (WT) and heme oxygenase-1 KO (HO-1(-/-)) mice. This protocol prevented increases in leukocyte rolling (LR) and adhesion (LA) to intestinal venules along with elevated TNFα and circulating neutrophil levels that accompany ischemia-reperfusion (I/R) in both animal models. We further hypothesized that a component of BAY 60-2770 treatment involves maintenance of mitochondrial membrane integrity during I/R. Measurements on isolated enterocytes of calcein fluorescence (mitochondrial permeability) and JC-1 fluorescence ratio (mitochondrial membrane potential) were reduced by I/R, indicating formation of mitochondrial permeability transition pores (mPTP). These effects were abrogated by BAY 60-2770 as well as cyclosporin A and SB-216763, which prevented mPTP opening and inhibited glycogen synthase kinase-3β (GSK-3β), respectively. Western blots of WT and HO-1(-/-) enterocytes indicated that GSK-3β phosphorylation on Ser(9) (inhibitory site) was reduced by half following I/R alone (increased GSK-3β activity) and increased by one-third (reduced GSK-3β activity) following BAY 60-2770. Other investigators have associated phosphorylation of the GSK-3β substrate cyclophilin D (pCyPD) with mPTP formation. We observed a 60% increase in pCyPD after I/R, whereas BAY 60-2770 treatment of sham and I/R groups reduced pCyPD by about 20%. In conclusion, selective activation of oxidized sGC of WT and HO-1(-/-) during ischemia protects against I/R-induced inflammation and preserves mucosal integrity in part by reducing pCyPD production and mPTP formation.

The Role of cGMP on Adenosine A 1 Receptor-mediated Inhibition of Synaptic Transmission at the Hippocampus

Both adenosine A₁ receptor and cGMP inhibit synaptic transmission at the hippocampus and recently it was found that A₁ receptor increased cGMP levels in hippocampus, but the role of cGMP on A₁ receptor-mediated inhibition of synaptic transmission remains to be established. In the present work we investigated if blocking the NOS/sGC/cGMP/PKG pathway using nitric oxide synthase (NOS), protein kinase G (PKG), and soluble guanylyl cyclase (sGC) inhibitors modify the A₁ receptor effect on synaptic transmission. Neurotransmission was evaluated by measuring the slope of field excitatory postsynaptic potentials (fEPSPs) evoked by electrical stimulation at hippocampal slices. N6-cyclopentyladenosine (CPA, 15 nM), a selective A₁ receptor agonist, reversibly decreased the fEPSPs by 54 ± 5%. Incubation of the slices with an inhibitor of NOS (L-NAME, 200 μM) decreased the CPA effect on fEPSPs by 57 ± 9% in female rats. In males, ODQ (10 μM), an sGC inhibitor, decreased the CPA inhibitory effect on fEPSPs by 23 ± 6%, but only when adenosine deaminase (ADA,1 U/ml) was present; similar results were found in females, where ODQ decreased CPA-induced inhibition of fEPSP slope by 23 ± 7%. In male rats, the presence of the PKG inhibitor (KT5823, 1 nM) decreased the CPA effect by 45.0 ± 9%; similar results were obtained in females, where KT5823 caused a 32 ± 9% decrease on the CPA effect. In conclusion, the results suggest that the inhibitory action of adenosine A₁ receptors on synaptic transmission at hippocampus is, in part, mediated by the NOS/sGC/cGMP/PKG pathway.

Differential effects of superoxide and hydrogen peroxide on myogenic signaling, membrane potential, and contractions of mouse renal afferent arterioles

Myogenic contraction is the principal component of renal autoregulation that protects the kidney from hypertensive barotrauma. Contractions are initiated by a rise in perfusion pressure that signals a reduction in membrane potential (Em) of vascular smooth muscle cells to activate voltage-operated Ca(2+) channels. Since ROS have variable effects on myogenic tone, we investigated the hypothesis that superoxide (O2 (·-)) and H2O2 differentially impact myogenic contractions. The myogenic contractions of mouse isolated and perfused single afferent arterioles were assessed from changes in luminal diameter with increasing perfusion pressure (40-80 mmHg). O2 (·-), H2O2, and Em were assessed by fluorescence microscopy during incubation with paraquat to increase O2 (·-) or with H2O2 Paraquat enhanced O2 (·-) generation and myogenic contractions (-42 ± 4% vs. -19 ± 4%, P < 0.005) that were blocked by SOD but not by catalase and signaled via PKC. In contrast, H2O2 inhibited the effects of paraquat and reduced myogenic contractions (-10 ± 1% vs. -19 ± 2%, P < 0.005) and signaled via PKG. O2 (·-) activated Ca(2+)-activated Cl(-) channels that reduced Em, whereas H2O2 activated Ca(2+)-activated and voltage-gated K(+) channels that increased Em Blockade of voltage-operated Ca(2+) channels prevented the enhanced myogenic contractions with paraquat without preventing the reduction in Em Myogenic contractions were independent of the endothelium and largely independent of nitric oxide. We conclude that O2 (·-) and H2O2 activate different signaling pathways in vascular smooth muscle cells linked to discreet membrane channels with opposite effects on Em and voltage-operated Ca(2+) channels and therefore have opposite effects on myogenic contractions.

20-HETE-induced mitochondrial superoxide production and inflammatory phenotype in vascular smooth muscle is prevented by glucose-6-phosphate dehydrogenase inhibition

20-Hydroxyeicosatetraeonic acid (20-HETE) produced by cytochrome P-450 monooxygenases in NADPH-dependent manner is proinflammatory, and it contributes to the pathogenesis of systemic and pulmonary hypertension. In this study, we tested the hypothesis that inhibition of glucose-6-phosphate dehydrogenase (G6PD), a major source of NADPH in the cell, prevents 20-HETE synthesis and 20-HETE-induced proinflammatory signaling that promotes secretory phenotype of vascular smooth muscle cells. Lipidomic analysis indicated that G6PD inhibition and knockdown decreased 20-HETE levels in pulmonary arteries as well as 20-HETE-induced 1) mitochondrial superoxide production, 2) activation of mitogen-activated protein kinase 1 and 3, 3) phosphorylation of ETS domain-containing protein Elk-1 that activate transcription of tumor necrosis factor-α gene (Tnfa), and 4) expression of tumor necrosis factor-α (TNF-α). Moreover, inhibition of G6PD increased protein kinase G1α activity, which, at least partially, mitigated superoxide production and Elk-1 and TNF-α expression. Additionally, we report here for the first time that 20-HETE repressed miR-143, which suppresses Elk-1 expression, and miR-133a, which is known to suppress synthetic/secretory phenotype of vascular smooth muscle cells. In summary, our findings indicate that 20-HETE elicited mitochondrial superoxide production and promoted secretory phenotype of vascular smooth muscle cells by activating MAPK1-Elk-1, all of which are blocked by inhibition of G6PD.

Nitric oxide inhibits basolateral 10-pS Cl- channels through the cGMP/PKG signaling pathway in the thick ascending limb of C57BL/6 mice

We used patch-clamp techniques to examine whether nitric oxide (NO) decreases NaCl reabsorption by suppressing basolateral 10-pS Cl^- channels in the thick ascending limb (TAL). Both the NO synthase substrate l-arginine (l-Arg) and the NO donor S-nitroso-N-acetylpenicillamine significantly inhibited 10-pS Cl^- channel activity in the TAL. The inhibitory effect of l-Arg on Cl^- channels was completely abolished in the presence of the NO synthase inhibitor or NO scavenger. Moreover, inhibition of soluble guanylyl cyclase abrogated the effect of l-Arg on Cl^- channels, whereas the cGMP analog 8-bromo-cGMP (8-BrcGMP) mimicked the effect of l-Arg and significantly decreased 10-pS Cl^- channel activity, indicating that NO inhibits basolateral Cl^- channels by increasing cGMP production. Furthermore, treatment of the TAL with a PKG inhibitor blocked the effect of l-Arg and 8-BrcGMP on Cl^- channels, respectively. In contrast, a phosphodiesterase 2 inhibitor had no significant effect on l-Arg or 8-BrcGMP-induced inhibition of Cl^- channels. Therefore, we conclude that NO decreases basolateral 10-pS Cl^- channel activity through a cGMP-dependent PKG pathway, which may contribute to the natriuretic and diuretic effects of NO in vivo.

Leukotriene D4 and prostaglandin E2 signals synergize and potentiate vascular inflammation in a mast cell-dependent manner through cysteinyl leukotriene receptor 1 and E-prostanoid receptor 3

BACKGROUND

Although arachidonic acid metabolites, cysteinyl leukotrienes (cys-LTs; leukotriene [LT] C4, LTD4, and LTE4), and prostaglandin (PG) E2 are generated at the site of inflammation, it is not known whether crosstalk exists between these 2 classes of inflammatory mediators.

OBJECTIVE

We sought to determine the role of LTD4-PGE2 crosstalk in inducing vascular inflammation in vivo, identify effector cells, and ascertain specific receptors and pathways involved in vitro.

METHODS

Vascular (ear) inflammation was assessed by injecting agonists into mouse ears, followed by measuring ear thickness and histology, calcium influx with Fura-2, phosphorylation and expression of signaling molecules by means of immunoblotting, PGD2 and macrophage inflammatory protein 1β generation by using ELISA, and expression of transcripts by using RT-PCR. Candidate receptors and signaling molecules were identified by using antagonists and inhibitors and confirmed by using small interfering RNA.

RESULTS

LTD4 plus PGE2 potentiated vascular permeability and edema, gearing the system toward proinflammation in wild-type mice but not in Kit(W-sh) mice. Furthermore, LTD4 plus PGE2, through cysteinyl leukotriene receptor 1 (CysLT1R) and E-prostanoid receptor (EP) 3, enhanced extracellular signal-regulated kinase (Erk) and c-fos phosphorylation, inflammatory gene expression, macrophage inflammatory protein 1β secretion, COX-2 upregulation, and PGD2 generation in mast cells. Additionally, we uncovered that this synergism is mediated through Gi, protein kinase G, and Erk signaling. LTD4 plus PGE2-potentiated effects are partially sensitive to CysLT1R or EP3 antagonists but completely abolished by simultaneous treatment both in vitro and in vivo.

CONCLUSIONS

Our results unravel a unique LTD4-PGE2 interaction affecting mast cells through CysLT1R and EP3 involving Gi, protein kinase G, and Erk and contributing to vascular inflammation in vivo. Furthermore, current results also suggest an advantage of targeting both CysLT1R and EP3 in attenuating inflammation.

Redox regulation of cGMP-dependent protein kinase Iα in the cardiovascular system

Elevated levels of oxidants in biological systems have been historically referred to as “oxidative stress,” a choice of words that perhaps conveys an imbalanced view of reactive oxygen species in cells and tissues. The term stress suggests a harmful role, whereas a contemporary view is that oxidants are also crucial for the maintenance of homeostasis or adaptive signaling that can actually limit injury. This regulatory role for oxidants is achieved in part by them inducing oxidative post-translational modifications of proteins which may alter their function or interactions. Such mechanisms allow changes in cell oxidant levels to be coupled to regulated alterations in enzymatic function (i.e., signal transduction), which enables “redox signaling.” In this review we focus on the role of cGMP-dependent protein kinase (PKG) Ia disulfide dimerisation, an oxidative modification that is induced by oxidants that directly activates the enzyme, discussing how this impacts on the cardiovascular system. Additionally, how this oxidative activation of PKG may coordinate with or differ from classical activation of this kinase by cGMP is also considered.

B-Type natriuretic peptide suppression of neutrophil superoxide generation: mechanistic studies in normal subjects

Many acute cardiovascular disease states are associated with neutrophil infiltration of myocardium and subsequent release of superoxide (O2 (-) ) and myeloperoxidase (MPO), which contribute to inflammatory reactions. B-Type natriuretic peptide (BNP) is known to exert anti-inflammatory and antifibrotic effects, but it is not known whether these may include interactions with neutrophils. In neutrophils isolated from 20 healthy subjects, we assessed the effect of BNP on the 'neutrophil burst' (O2 (-) production and MPO release) stimulated by phorbol myristate acetate (PMA) and N-formyl-methionyl-leucyl-phenylalanine (fMLP), respectively. Effects of BNP on cGMP accumulation, and the effects of the cell-permeable cGMP analogue 8-(4-chlorophenylthio) guanosine-cGMP (8-p-CPT-cGMP) and protein kinase G (PKG) inhibition with KT5823 on the neutrophil-BNP interaction were also evaluated. B-Type natriuretic peptide suppressed O2 (-) release from neutrophils by 23 ± 6% (P < 0.001) and 24 ± 8% (P < 0.05) following PMA and fMLP stimulation, respectively. Although BNP did not significantly increase cGMP formation, 8-p-CPT-cGMP suppressed both PMA- and fMLP-induced neutrophil O2 (-) release by 16% and 28%, respectively (P < 0.05). The PKG inhibitor KT5823 attenuated the effects of BNP on both fMLP- and PMA-associated O2 (-) production. Neither BNP nor 8-p-CPT-cGMP significantly affected MPO release from neutrophils. Suppression of O2 (-) release from neutrophils by BNP may contribute to its anti-inflammatory and antifibrotic actions.

Oestrogen upregulates the sarcoplasmic reticulum Ca(2+) ATPase pump in coronary arteries

The presence of circulating plasma 17β-oestradiol (E2) is beneficial in women against abnormal vascular tone development, such as coronary arterial vasospasms. Several vascular diseases have demonstrated that increased expression of the sarcoplasmic reticulum Ca(2+) -ATPase pump (SERCA2b) serves to limit the excessive accumulation of intracellular Ca(2+) . Therefore, the hypothesis of the present study was that E2 would increase SERCA2b expression in the coronary vasculature. Coronary arteries were dissected from hearts obtained from mature female pigs. Artery segments were cultured for 24 h in E2 (1 pmol/L or 1 nmol/L) and homogenized for western blot analysis. At 1 nmol/L, E2 induced an approximate 50% increase in immunoreactivity for SERCA2b. In addition, E2 increased the protein expression of the known SERCA regulatory proteins, protein kinase A (PKA) and protein kinase G (PKG). The E2-induced increase in SERCA2b was attenuated when the culture medium was supplemented with the oestrogen receptor (ER) α/β antagonist ICI 182,780 and the PKG antagonist KT5823 (10 μmol/L, 24 h for both). The PKA antagonist (KT5720; 10 μmol/L, 24 h) had no effect on SERCA2b expression. Removal of the endothelium (using a wooden toothpick) from artery segments prior to culture decreased the E2-mediated increase in SERCA2b and PKG expression by 45% and 47%, respectively. Overall, the findings suggest that one of the potential cardiovascular benefits of E2 in women is upregulation of SERCA2b, via activation of the classic ERα and ERβ pathway.

PKG in honey bees: spatial expression, Amfor gene expression, sucrose responsiveness, and division of labor

Division of labor is a hallmark of social insects. In honey bees, division of labor involves transition of female workers from one task to the next. The most distinct tasks are nursing (providing food for the brood) and foraging (collecting pollen and nectar). The brain mechanisms regulating this form of behavioral plasticity have largely remained elusive. Recently, it was suggested that division of labor is based on nutrition-associated signaling pathways. One highly conserved gene associated with food-related behavior across species is the foraging gene, which encodes a cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG). Our analysis of this gene reveals the presence of alternative splicing in the honey bee. One isoform is expressed in the brain. Expression of this isoform is most pronounced in the mushroom bodies, the subesophageal ganglion, and the corpora allata. Division of labor and sucrose responsiveness in honey bees correlate significantly with foraging gene expression in distinct brain regions. Activating PKG selectively increases sucrose responsiveness in nurse bees to the level of foragers, whereas the same treatment does not affect responsiveness to light. These findings demonstrate a direct link between PKG signaling in distinct brain areas and division of labor. Furthermore, they demonstrate that the difference in sensory responsiveness between nurse bees and foragers can be compensated for by activating PKG. Our findings on the function of PKG in regulating specific sensory responsiveness and social organization offer valuable indications for the function of the cGMP/PKG pathway in many other insects and vertebrates.

Turning on cGMP-dependent pathways to treat cardiac dysfunctions: boom, bust, and beyond

cGMP inhibits hypertrophy, decreases fibrosis, and protects against cardiac ischemia-reperfusion (I/R) injury. Gene-targeting studies have not defined a clear role for its major downstream effector, cGMP-dependent protein kinase I (cGKI), in cardiac hypertrophy, but do implicate cGMP-cGKI signaling in fibrosis and I/R injury. No direct cGKI activators have advanced to clinical trials, whereas cardiac trials of agents that modulate cGMP via particulate or soluble guanylyl cyclases (GCs) and phosphodiesterase 5 (PDE5) are ongoing. Here we review concerns arising from preclinical and clinical studies that question whether targeting the cGMP pathway remains an encouraging concept for management of heart dysfunction. So far, trial results for GC modulators are inconclusive, and sildenafil, a PDE5 inhibitor, although cardioprotective in mouse models, has not shown positive clinical results. Preclinical cardioprotection observed for sildenafil may result from inhibition of PDE5 in non-cardiomyocytes or off-target effects, possibly on PDE1C. On the basis of such mechanistic considerations, re-evaluation of the cellular localization of drug target(s) and intervention protocols for cGMP-elevating agents may be needed.

Exposure of mice to chronic hypoxia attenuates pulmonary arterial contractile responses to acute hypoxia by increases in extracellular hydrogen peroxide

Exposing mice to a chronic hypoxic treatment (10% oxygen, 21 days) that promotes pulmonary hypertension was observed to attenuate the pulmonary vasoconstriction response to acute hypoxia (HPV) both in vivo and in isolated pulmonary arteries. Since catalase restored the HPV response in isolated arteries, it appeared to be attenuated by extracellular hydrogen peroxide. Chronic hypoxia promoted the detection of elevated lung superoxide, extracellular peroxide, extracellular SOD expression, and protein kinase G (PKG) activation [based on PKG dimerization and vasodilator-stimulated phosphoprotein (VASP) phosphorylation], suggesting increased generation of extracellular peroxide and PKG activation may contribute to the suppression of HPV. Aorta from mice exposed to 21 days of hypoxia also showed evidence for extracellular hydrogen peroxide, suppressing the relaxation response to acute hypoxia. Peroxide appeared to partially suppress contractions to phenylephrine used in the study of in vitro hypoxic responses. Treatment of mice with the heme precursor δ-aminolevulinic acid (ALA; 50 mg·kg(-1)·day(-1)) during exposure to chronic hypoxia was examined as a pulmonary hypertension therapy because it could potentially activate beneficial cGMP-mediated effects through promoting a prolonged protoporphyrin IX (PpIX)-elicited activation of soluble guanylate cyclase. ALA attenuated pulmonary hypertension, increases in both superoxide and peroxide, and the suppression of in vitro and in vivo HPV responses. ALA generated prolonged detectible increases in PpIX and PKG-associated phosphorylation of VASP, suggesting PKG activation may contribute to suppression of pulmonary hypertension and prevention of alterations in extracellular peroxide that appear to be attenuating HPV responses caused by chronic hypoxia.

HIV-1 protein Tat produces biphasic changes in NMDA-evoked increases in intracellular Ca2+ concentration via activation of Src kinase and nitric oxide signaling pathways

HIV-associated neurocognitive disorders afflict about half of HIV-infected patients. HIV-infected cells shed viral proteins, such as the transactivator of transcription (Tat), which can cause neurotoxicity by over activation of NMDA receptors. Here, we show that Tat causes a time-dependent, biphasic change in NMDA-evoked increases in intracellular Ca(2+) concentration ([Ca(2+)]i). NMDA-evoked responses were potentiated following 2-h exposure to Tat (50 ng/mL). Tat-induced potentiation of NMDA-evoked increases in [Ca(2+)]i peaked by 8 h and then adapted by gradually reversing to baseline by 24 h and eventually dropping below control by 48 h. Tat-induced potentiation of NMDA-evoked responses was blocked by inhibition of lipoprotein receptor-related protein (LRP) or Src tyrosine kinase. Potentiation was unaffected by inhibition of nitric oxide synthase (NOS). However, NOS activity was required for adaptation. Adaptation was also prevented by inhibition of soluble guanylate cyclase (sGC) and cyclic guanosine monophosphate-dependent protein kinase G (PKG). Together, these findings indicate that Tat potentiates NMDA-evoked increases in [Ca(2+)]i via LRP-dependent activation of Src and that this potentiation adapts via activation of the NOS/sGC/PKG pathway. Adaptation may protect neurons from excessive Ca(2+) influx and could reveal targets for the treatment of HIV-associated neurocognitive disorders. HIV-associated neurocognitive disorders (HAND) afflict about half of HIV-infected patients. HIV-infected cells shed viral proteins, such as the transactivator of transcription (Tat), which can cause neurotoxicity by over activation of NMDA receptors (NMDARs). We show that HIV-1 Tat evoked biphasic changes in NMDA-evoked [Ca(2+) ]i responses. Initially, Tat potentiated NMDA-evoked responses following LRP-mediated activation of Src kinase. Subsequently, Tat-induced NMDAR potentiation adapted by activation of a NOS/sGC/PKG pathway that attenuated NMDA-evoked increases in [Ca(2+)]i . Adaptation may be a novel neuroprotective mechanism to prevent excessive Ca(2+) influx. Solid and dashed arrows represent direct and potentially indirect connections, respectively.

Phosphodiesterase 5 inhibition ameliorates angiontensin II-induced podocyte dysmotility via the protein kinase G-mediated downregulation of TRPC6 activity

The emerging role of the transient receptor potential cation channel isotype 6 (TRPC6) as a central contributor to various pathological processes affecting podocytes has generated interest in the development of therapeutics to modulate its function. Recent insights into the regulation of TRPC6 have revealed PKG as a potent negative modulator of TRPC6 conductance and associated signaling via its phosphorylation at two highly conserved amino acid residues: Thr(69)/Thr(70) (Thr(69) in mice and Thr(70) in humans) and Ser(321)/Ser(322) (Ser(321) in mice and Ser(322) in humans). Here, we tested the role of PKG in modulating TRPC6-dependent responses in primary and conditionally immortalized mouse podocytes. TRPC6 was phosphorylated at Thr(69) in nonstimulated podocytes, but this declined upon ANG II stimulation or overexpression of constitutively active calcineurin phosphatase. ANG II induced podocyte motility in an in vitro wound assay, and this was reduced 30-60% in cells overexpressing a phosphomimetic mutant TRPC6 (TRPC6T70E/S322E) or activated PKG (P < 0.05). Pretreatment of podocytes with the PKG agonists S-nitroso-N-acetyl-dl-penicillamine (nitric oxide donor), 8-bromo-cGMP, Bay 41-2772 (soluble guanylate cyclase activator), or phosphodiesterase 5 (PDE5) inhibitor 4-{[3',4'-(methylenedioxy)benzyl]amino}[7]-6-methoxyquinazoline attenuated ANG II-induced Thr(69) dephosphorylation and also inhibited TRPC6-dependent podocyte motility by 30-60%. These data reveal that PKG activation strategies, including PDE5 inhibition, ameliorate ANG II-induced podocyte dysmotility by targeting TRPC6 in podocytes, highlighting the potential therapeutic utility of these approaches to treat hyperactive TRPC6-dependent glomerular disease.

Dysfunctional survival-signaling and stress-intolerance in aged murine and human myocardium

Changes in cytoprotective signaling may influence cardiac aging, and underpin sensitization to ischemic insult and desensitization to 'anti-ischemic' therapies. We tested whether age-dependent shifts in ischemia-reperfusion (I-R) tolerance in murine and human myocardium are associated with reduced efficacies and coupling of membrane, cytoplasmic and mitochondrial survival-signaling. Hormesis (exemplified in ischemic preconditioning; IPC) and expression of proteins influencing signaling/stress-resistance were also assessed in mice. Mouse hearts (18 vs. 2-4 mo) and human atrial tissue (75±2 vs. 55±2 yrs) exhibited profound age-dependent reductions in I-R tolerance. In mice aging negated cardioprotection via IPC, G-protein coupled receptor (GPCR) agonism (opioid, A1 and A3 adenosine receptors) and distal protein kinase c (PKC) activation (4 nM phorbol 12-myristate 13-acetate; PMA). In contrast, p38-mitogen activated protein kinase (p38-MAPK) activation (1 μM anisomycin), mitochondrial ATP-sensitive K(+) channel (mKATP) opening (50 μM diazoxide) and permeability transition pore (mPTP) inhibition (0.2 μM cyclosporin A) retained protective efficacies in older hearts (though failed to eliminate I-R tolerance differences). A similar pattern of change in protective efficacies was observed in human tissue. Murine hearts exhibited molecular changes consistent with altered membrane control (reduced caveolin-3, cholesterol and caveolae), kinase signaling (reduced p70 ribosomal s6 kinase; p70s6K) and stress-resistance (increased G-protein receptor kinase 2, GRK2; glycogen synthase kinase 3β, GSK3β; and cytosolic cytochrome c). In summary, myocardial I-R tolerance declines with age in association with dysfunctional hormesis and transduction of survival signals from GPCRs/PKC to mitochondrial effectors. Differential changes in proteins governing caveolar and mitochondrial function may contribute to signal dysfunction and stress-intolerance.

Protein kinase G increases antioxidant function in lung microvascular endothelial cells by inhibiting the c-Abl tyrosine kinase

Oxidant injury contributes to acute lung injury (ALI). We previously reported that activation of protein kinase GI (PKGI) posttranscriptionally increased the key antioxidant enzymes catalase and glutathione peroxidase 1 (Gpx-1) and attenuated oxidant-induced cytotoxicity in mouse lung microvascular endothelial cells (MLMVEC). The present studies tested the hypothesis that the antioxidant effect of PKGI is mediated via inhibition of the c-Abl tyrosine kinase. We found that activation of PKGI with the cGMP analog 8pCPT-cGMP inhibited c-Abl activity and decreased c-Abl expression in wild-type but not PKGI(-/-) MLMVEC. Treatment of wild-type MLMVEC with atrial natriuretic peptide also inhibited c-Abl activation. Moreover, treatment of MLMVEC with the c-Abl inhibitor imatinib increased catalase and GPx-1 protein in a posttranscriptional fashion. In imatinib-treated MLMVEC, there was no additional effect of 8pCPT-cGMP on catalase or GPx-1. The imatinib-induced increase in antioxidant proteins was associated with an increase in extracellular H2O2 scavenging by MLMVEC, attenuation of oxidant-induced endothelial barrier dysfunction, and prevention of oxidant-induced endothelial cell death. Finally, in the isolated perfused lung, imatinib prevented oxidant-induced endothelial toxicity. We conclude that cGMP, through activation of PKGI, inhibits c-Abl, leading to increased key antioxidant enzymes and resistance to lung endothelial oxidant injury. Inhibition of c-Abl by active PKGI may be the downstream mechanism underlying PKGI-mediated antioxidant signaling. Tyrosine kinase inhibitors may represent a novel therapeutic approach in oxidant-induced ALI.