Detecting disulfide-bound complexes and the oxidative regulation of cyclic nucleotide-dependent protein kinases by H2O2

The activity of cGMP-dependent protein kinase Iα is not directly regulated by oxidation-induced disulfide formation at cysteine 43

The type I cGMP-dependent protein kinases (PKGs) are key regulators of smooth muscle tone, cardiac hypertrophy, and other physiological processes. The two isoforms PKGIα and PKGIβ are thought to have unique functions because of their tissue-specific expression, different cGMP affinities, and isoform-specific protein-protein interactions. Recently, a non-canonical pathway of PKGIα activation has been proposed, in which PKGIα is activated in a cGMP-independent fashion via oxidation of Cys⁴³, resulting in disulfide formation within the PKGIα N-terminal dimerization domain. A "redox-dead" knock-in mouse containing a C43S mutation exhibits phenotypes consistent with decreased PKGIα signaling, but the detailed mechanism of oxidation-induced PKGIα activation is unknown. Therefore, we examined oxidation-induced activation of PKGIα, and in contrast to previous findings, we observed that disulfide formation at Cys⁴³ does not directly activate PKGIα in vitro or in intact cells. In transfected cells, phosphorylation of Ras homolog gene family member A (RhoA) and vasodilator-stimulated phosphoprotein was increased in response to 8-CPT-cGMP treatment, but not when disulfide formation in PKGIα was induced by H₂O₂ Using purified enzymes, we found that the Cys⁴³ oxidation had no effect on basal kinase activity or K_m and V_max values; however, PKGIα containing the C43S mutation was less responsive to cGMP-induced activation. This reduction in cGMP affinity may in part explain the PKGIα loss-of-function phenotype of the C43S knock-in mouse. In conclusion, disulfide formation at Cys⁴³ does not directly activate PKGIα, and the C43S-mutant PKGIα has a higher K_a for cGMP. Our results highlight that mutant enzymes should be carefully biochemically characterized before making in vivo inferences.

Thiol-Based Redox Modulation of Soluble Guanylyl Cyclase, the Nitric Oxide Receptor

SIGNIFICANCE

Soluble guanylyl cyclase (sGC), which produces the second messenger cyclic guanosine 3', 5'-monophosphate (cGMP), is at the crossroads of nitric oxide (NO) signaling: sGC catalytic activity is both stimulated by NO binding to the heme and inhibited by NO modification of its cysteine (Cys) thiols (S-nitrosation). Modulation of sGC activity by thiol oxidation makes sGC a therapeutic target for pathologies originating from oxidative or nitrosative stress. sGC has an unusually high percentage of Cys for a cytosolic protein, the majority solvent exposed and therefore accessible modulatory targets for biological and pathophysiological signaling. Recent Advances: Thiol oxidation of sGC contributes to the development of cardiovascular diseases by decreasing NO-dependent cGMP production and thereby vascular reactivity. This thiol-based resistance to NO (e.g., increase in peripheral resistance) is observed in hypertension and hyperaldosteronism.

CRITICAL ISSUES

Some roles of specific Cys thiols have been identified in vitro. So far, it has not been possible to pinpoint the roles of specific Cys of sGC in vivo and to investigate the molecular mechanisms in an animal model.

FUTURE DIRECTIONS

The role of Cys as redox sensors, intermediates of activation, and mediators of change in sGC conformation, activity, and dimerization remains largely unexplored. To understand modulation of sGC activity, it is critical to investigate the roles of specific oxidative thiol modifications that are formed during these processes. Where the redox state of sGC thiols contribute to pathologies (vascular resistance and sGC desensitization by NO donors), it becomes crucial to design therapeutic strategies to restore sGC to its normal, physiological thiol redox state. Antioxid. Redox Signal. 26, 137-149.

ROS signaling and redox biology in endothelial cells

The purpose of this review is to provide an overview of redox mechanisms, sources and antioxidants that control signaling events in ECs. In particular, we describe which molecules are involved in redox signaling and how they influence the relationship between ECs and other vascular component with regard to angiogenesis. Recent and new tools to investigate physiological ROS signaling will be also discussed. Such findings are providing an overview of the ROS biology relevant for endothelial cells in the context of normal and pathological angiogenic conditions.

Ca(2+) signaling in the myocardium by (redox) regulation of PKA/CaMKII

Homeostatic cardiac function is maintained by a complex network of interdependent signaling pathways which become compromised during disease progression. Excitation-contraction-coupling, the translation of an electrical signal to a contractile response is critically dependent on a tightly controlled sequence of events culminating in a rise in intracellular Ca(2+) and subsequent contraction of the myocardium. Dysregulation of this Ca(2+) handling system as well as increases in the production of reactive oxygen species (ROS) are two major contributing factors to myocardial disease progression. ROS, generated by cellular oxidases and by-products of cellular metabolism, are highly reactive oxygen derivatives that function as key secondary messengers within the heart and contribute to normal homeostatic function. However, excessive production of ROS, as in disease, can directly interact with kinases critical for Ca(2+) regulation. This post-translational oxidative modification therefore links changes in the redox status of the myocardium to phospho-regulated pathways essential for its function. This review aims to describe the oxidative regulation of the Ca(2+)/calmodulin-dependent kinase II (CaMKII) and cAMP-dependent protein kinase A (PKA), and the subsequent impact this has on Ca(2+) handling within the myocardium. Elucidating the impact of alterations in intracellular ROS production on Ca(2+) dynamics through oxidative modification of key ROS sensing kinases, may provide novel therapeutic targets for preventing myocardial disease progression.

Deficient angiogenesis in redox-dead Cys17Ser PKARIα knock-in mice

Angiogenesis is essential for tissue development, wound healing and tissue perfusion, with its dysregulation linked to tumorigenesis, rheumatoid arthritis and heart disease. Here we show that pro-angiogenic stimuli couple to NADPH oxidase-dependent generation of oxidants that catalyse an activating intermolecular-disulphide between regulatory-RIα subunits of protein kinase A (PKA), which stimulates PKA-dependent ERK signalling. This is crucial to blood vessel growth as 'redox-dead' Cys17Ser RIα knock-in mice fully resistant to PKA disulphide-activation have deficient angiogenesis in models of hind limb ischaemia and tumour-implant growth. Disulphide-activation of PKA represents a new therapeutic target in diseases with aberrant angiogenesis.

Analysis of erectile responses to H2S donors in the anesthetized rat

Hydrogen sulfide (H2S) is a biologically active endogenous gasotransmitter formed in penile tissue that has been shown to relax isolated cavernosal smooth muscle. In the present study, erectile responses to the H2S donors sodium sulfide (Na2S) and sodium hydrosulfide (NaHS) were investigated in the anesthetized rat. Intracavernosal injections of Na2S in doses of 0.03-1 mg/kg increased intracavernosal pressure and transiently decreased mean arterial pressure in a dose-dependent manner. Blood pressure responses to Na2S were rapid in onset and short in duration. Responses to Na2S and NaHS were similar at doses up to 0.3 mg/kg, after which a plateau in the erectile response to NaHS was reached. Increases in intracavernosal pressure in response to Na2S were attenuated by tetraethylammonium (K(+) channel inhibitor) and iberiotoxin (large-conductance Ca(2+)-activated K(+) channel inhibitor), whereas glybenclamide [ATP-sensitive K(+) (KATP) channel inhibitor] and inhibitors of nitric oxide (NO) synthase, cyclooxygenase, and cytochrome P-450 epoxygenase had no effect. These data indicate that erectile responses to Na2S are mediated by a tetraethylammonium- and iberiotoxin-sensitive mechanism and that KATP channels, NO, or arachidonic acid metabolites are not involved. Na2S did not alter erectile responses to sodium nitroprusside (NO donor) or cavernosal nerve stimulation, indicating that neither NO nor cGMP metabolism are altered. Thus, Na2S has erectile activity mediated by large-conductance Ca(2+)-activated K(+) channels. It is suggested that strategies that increase H2S formation in penile tissue may be useful in the treatment of erectile dysfunction when NO bioavailability, KATP channel function, or poor responses to PGE1 are present.

Analysis of cardiovascular responses to the H2S donors Na2S and NaHS in the rat

Hydrogen sulfide (H2S) is an endogenous gaseous molecule formed from L-cysteine in vascular tissue. In the present study, cardiovascular responses to the H2S donors Na2S and NaHS were investigated in the anesthetized rat. The intravenous injections of Na2S and NaHS 0.03-0.5 mg/kg produced dose-related decreases in systemic arterial pressure and heart rate, and at higher doses decreases in cardiac output, pulmonary arterial pressure, and systemic vascular resistance. H2S infusion studies show that decreases in systemic arterial pressure, heart rate, cardiac output, and systemic vascular resistance are well-maintained, and responses to Na2S are reversible. Decreases in heart rate were not blocked by atropine, suggesting that the bradycardia was independent of parasympathetic activation and was mediated by an effect on the sinus node. The decreases in systemic arterial pressure were not attenuated by hexamethonium, glybenclamide, N(w)-nitro-L-arginine methyl ester hydrochloride, sodium meclofenamate, ODQ, miconazole, 5-hydroxydecanoate, or tetraethylammonium, suggesting that ATP-sensitive potassium channels, nitric oxide, arachidonic acid metabolites, cyclic GMP, p450 epoxygenase metabolites, or large conductance calcium-activated potassium channels are not involved in mediating hypotensive responses to the H2S donors in the rat and that responses are not centrally mediated. The present data indicate that decreases in systemic arterial pressure in response to the H2S donors can be mediated by decreases in vascular resistance and cardiac output and that the donors have an effect on the sinus node independent of the parasympathetic system. The present data indicate that the mechanism of the peripherally mediated hypotensive response to the H2S donors is uncertain in the intact rat.

Soluble guanylate cyclase is required for systemic vasodilation but not positive inotropy induced by nitroxyl in the mouse

Nitroxyl (HNO), the reduced and protonated form of nitric oxide (NO·), confers unique physiological effects including vasorelaxation and enhanced cardiac contractility. These features have spawned current pharmaceutical development of HNO donors as heart failure therapeutics. HNO interacts with selective redox sensitive cysteines to effect signaling but is also proposed to activate soluble guanylate cyclase (sGC) in vitro to induce vasodilation and potentially enhance contractility. Here, we tested whether sGC stimulation is required for these HNO effects in vivo and if HNO also modifies a redox-sensitive cysteine (C42) in protein kinase G-1α to control vasorelaxation. Intact mice and isolated arteries lacking the sGC-β subunit (sGCKO, results in full sGC deficiency) or expressing solely a redox-dead C42S mutant protein kinase G-1α were exposed to the pure HNO donor, CXL-1020. CXL-1020 induced dose-dependent systemic vasodilation while increasing contractility in controls; however, vasodilator effects were absent in sGCKO mice whereas contractility response remained. The CXL-1020 dose reversing 50% of preconstricted force in aortic rings was ≈400-fold greater in sGCKO than controls. Cyclic-GMP and cAMP levels were unaltered in myocardium exposed to CXL-1020, despite its inotropic-vasodilator activity. In protein kinase G-1α(C42S) mice, CXL-1020 induced identical vasorelaxation in vivo and in isolated aortic and mesenteric vessels as in littermate controls. In both groups, dilation was near fully blocked by pharmacologically inhibiting sGC. Thus, sGC and cGMP-dependent signaling are necessary and sufficient for HNO-induced vasodilation in vivo but are not required for positive inotropic action. Redox modulation of protein kinase G-1α is not a mechanism for HNO-mediated vasodilation.

cAMP signaling in subcellular compartments

In the complex microcosm of a cell, information security and its faithful transmission are critical for maintaining internal stability. To achieve a coordinated response of all its parts to any stimulus the cell must protect the information received from potentially confounding signals. Physical segregation of the information transmission chain ensures that only the entities able to perform the encoded task have access to the relevant information. The cAMP intracellular signaling pathway is an important system for signal transmission responsible for the ancestral 'flight or fight' response and involved in the control of critical functions including frequency and strength of heart contraction, energy metabolism and gene transcription. It is becoming increasingly apparent that the cAMP signaling pathway uses compartmentalization as a strategy for coordinating the large number of key cellular functions under its control. Spatial confinement allows the formation of cAMP signaling "hot spots" at discrete subcellular domains in response to specific stimuli, bringing the information in proximity to the relevant effectors and their recipients, thus achieving specificity of action. In this report we discuss how the different constituents of the cAMP pathway are targeted and participate in the formation of cAMP compartmentalized signaling events. We illustrate a few examples of localized cAMP signaling, with a particular focus on the nucleus, the sarcoplasmic reticulum and the mitochondria. Finally, we discuss the therapeutic potential of interventions designed to perturb specific cAMP cascades locally.

Biochemical methods for monitoring protein thiol redox states in biological systems

Oxidative post-translational modifications of proteins resulting from events that increase cellular oxidant levels play important roles in physiological and pathophysiological processes. Evaluation of alterations to protein redox states is increasingly common place because of methodological advances that have enabled detection, quantification and identification of such changes in cells and tissues. This mini-review provides a synopsis of biochemical methods that can be utilized to monitor the array of different oxidative and electrophilic modifications that can occur to protein thiols and can be important in the regulatory or maladaptive impact oxidants can have on biological systems. Several of the methods discussed are valuable for monitoring the redox state of established redox sensing proteins such as Keap1.