Identification of the G protein-activating sequence of the single-transmembrane natriuretic peptide receptor C (NPR-C)

Unraveling the role of natriuretic peptide clearance receptor (NPR3) in glomerular diseases

Natriuretic peptides (NPs) are cardio-derived hormones that have a crucial role in maintaining cardiovascular homeostasis. Physiological effects of NPs are mediated by binding to natriuretic peptide receptors 1 and 2 (NPR1/2), whereas natriuretic peptide receptor 3 (NPR3) acts as a clearance receptor that removes NPs from the circulation. Mouse studies have shown that local NP-signaling in the kidney glomerulus is important for the maintenance of renal homeostasis. In this study we examined the expression of NPR3 in kidney tissue and explored its involvement in renal physiology and disease by generating podocyte-specific knockout mice (NPR3podKO) as well as by using an NPR3 inhibitor (NPR3i) in rodent models of kidney disease. NPR3 was highly expressed by podocytes. NPR3podKO animals showed no renal abnormalities under healthy conditions and responded similarly to nephrotoxic serum (NTS) induced glomerular injury. However, NPR3i showed reno-protective effects in the NTS-induced model evidenced by decreased glomerulosclerosis and reduced podocyte loss. In a ZSF1 rat model of diabetic kidney injury, therapy alone with NPR3i did not have beneficial effects on renal function/histology, but when combined with losartan (angiotensin receptor blocker), NPR3i potentiated its ameliorative effects on albuminuria. In conclusion, these results suggest that NPR3 may contribute to kidney disease progression.

Guanylyl cyclase/natriuretic peptide receptor-A: Identification, molecular characterization, and physiological genomics

The natriuretic peptides (NPs) hormone family, which consists mainly of atrial, brain, and C-type NPs (ANP, BNP, and CNP), play diverse roles in mammalian species, ranging from renal, cardiac, endocrine, neural, and vascular hemodynamics to metabolic regulations, immune responsiveness, and energy distributions. Over the last four decades, new data has transpired regarding the biochemical and molecular compositions, signaling mechanisms, and physiological and pathophysiological functions of NPs and their receptors. NPs are incremented mainly in eliciting natriuretic, diuretic, endocrine, vasodilatory, and neurological activities, along with antiproliferative, antimitogenic, antiinflammatory, and antifibrotic responses. The main locus responsible in the biological and physiological regulatory actions of NPs (ANP and BNP) is the plasma membrane guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), a member of the growing multi-limbed GC family of receptors. Advances in this field have provided tremendous insights into the critical role of Npr1 (encoding GC-A/NPRA) in the reduction of fluid volume and blood pressure homeostasis, protection against renal and cardiac remodeling, and moderation and mediation of neurological disorders. The generation and use of genetically engineered animals, including gene-targeted (gene-knockout and gene-duplication) and transgenic mutant mouse models has revealed and clarified the varied roles and pleiotropic functions of GC-A/NPRA in vivo in intact animals. This review provides a chronological development of the biochemical, molecular, physiological, and pathophysiological functions of GC-A/NPRA, including signaling pathways, genomics, and gene regulation in both normal and disease states.

C Type Natriuretic Peptide Receptor Activation Inhibits Sodium Channel Activity in Human Aortic Endothelial Cells by Activating the Diacylglycerol-Protein Kinase C Pathway

The C-type natriuretic peptide receptor (NPRC) is expressed in many cell types and binds all natriuretic peptides with high affinity. Ligand binding results in the activation or inhibition of various intracellular signaling pathways. Although NPRC ligand binding has been shown to regulate various ion channels, the regulation of endothelial sodium channel (EnNaC) activity by NPRC activation has not been studied. The objective of this study was to investigate mechanisms of EnNaC regulation associated with NPRC activation in human aortic endothelial cells (hAoEC). EnNaC protein expression and activity was attenuated after treating hAoEC with the NPRC agonist cANF compared to vehicle, as demonstrated by Western blotting and patch clamping studies, respectively. NPRC knockdown studies using siRNA's corroborated the specificity of EnNaC regulation by NPRC activation mediated by ligand binding. The concentration of multiple diacylglycerols (DAG) and the activity of protein kinase C (PKC) was augmented after treating hAoEC with cANF compared to vehicle, suggesting EnNaC activity is down-regulated upon NPRC ligand binding in a DAG-PKC dependent manner. The reciprocal cross-talk between NPRC activation and EnNaC inhibition represents a feedback mechanism that presumably is involved in the regulation of endothelial function and aortic stiffness.

Oxygen and steroids affect the regulatory role of natriuretic peptide receptor-C on surfactant secretion by type II cells

Atrial natriuretic peptide (ANP) and its receptors natriuretic peptide receptor (NPR)-A and NPR-C are all highly expressed in alveolar epithelial type II cells (AEC2s) in the late-gestation ovine fetal lung and are dramatically decreased postnatally. However, of all the components, NPR-C stimulation inhibits ANP-mediated surfactant secretion. Since alveolar oxygen increases dramatically after birth, and steroids are administered to mothers antenatally to enhance surfactant lung maturity, we investigated the effects of O₂ concentration and steroids on NPR-C-mediated surfactant secretion in AEC2s. NPR-C expression was highest at 5% O₂ while being suppressed by 21% O₂, in cultured mouse lung epithelial cells (MLE-15s) and/or human primary AEC2s. Surfactant protein-B (SP-B) was significantly elevated in media from both in vitro and ex vivo culture at 13% O₂ versus 21% O₂ in the presence of ANP or terbutaline (TER). Both ANP and C-ANP (an NPR-C agonist) attenuated TER-induced SP-B secretion; this effect was reversed by dexamethasone (DEX) pretreatment in AEC2s and by transfection with NPR-C siRNA in MLE-15 cells. DEX markedly reduced AEC2 NPR-C expression, and pregnant ewes treated with betamethasone showed reduced ANP in fetal sheep lung fluid. These data suggest that elevated O₂ downregulates AEC2 NPR-C and that steroid-mediated NPR-C downregulation in neonatal lungs may provide a novel mechanism for their effect on perinatal surfactant production.

C-Type Natriuretic Peptide Ameliorates Vascular Injury and Improves Neurological Outcomes in Neonatal Hypoxic-Ischemic Brain Injury in Mice

C-type natriuretic peptide (CNP) is an important vascular regulator that is present in the brain. Our previous study demonstrated the innate neuroprotectant role of CNP in the neonatal brain after hypoxic-ischemic (HI) insults. In this study, we further explored the role of CNP in cerebrovascular pathology using both in vivo and in vitro models. In a neonatal mouse HI brain injury model, we found that intracerebroventricular administration of recombinant CNP dose-dependently reduces brain infarct size. CNP significantly decreases brain edema and immunoglobulin G (IgG) extravasation into the brain tissue, suggesting a vasculoprotective effect of CNP. Moreover, in primary brain microvascular endothelial cells (BMECs), CNP dose-dependently protects BMEC survival and monolayer integrity against oxygen-glucose deprivation (OGD). The vasculoprotective effect of CNP is mediated by its innate receptors NPR2 and NPR3, in that inhibition of either NPR2 or NPR3 counteracts the protective effect of CNP on IgG leakage after HI insult and BMEC survival under OGD. Of importance, CNP significantly ameliorates brain atrophy and improves neurological deficits after HI insults. Altogether, the present study indicates that recombinant CNP exerts vascular protection in neonatal HI brain injury via its innate receptors, suggesting a potential therapeutic target for the treatment of neonatal HI brain injury.

Natriuretic Peptide Clearance Receptor (NPR-C) Pathway as a Novel Therapeutic Target in Obesity-Related Heart Failure With Preserved Ejection Fraction (HFpEF)

Heart failure (HF) with preserved ejection fraction (HFpEF) is a major public health problem with cases projected to double over the next two decades. There are currently no US Food and Drug Administration–approved therapies for the health-related outcomes of HFpEF. However, considering the high prevalence of this heterogeneous syndrome, a directed therapy for HFpEF is one the greatest unmet needs in cardiovascular medicine. Additionally, there is currently a lack of mechanistic understanding about the pathobiology of HFpEF. The phenotyping of HFpEF patients into pathobiological homogenous groups may not only be the first step in understanding the molecular mechanism but may also enable the development of novel targeted therapies. As obesity is one of the most common comorbidities found in HFpEF patients and is associated with many cardiovascular effects, it is a viable candidate for phenotyping. Large outcome trials and registries reveal that being obese is one of the strongest independent risk factors for developing HFpEF and that this excess risk may not be explained by traditional cardiovascular risk factors. Recently, there has been increased interest in the intertissue communication between adipose tissue and the heart. Evidence suggests that the natriuretic peptide clearance receptor (NPR-C) pathway may play a role in the development and pathobiology of obesity-related HFpEF. Therefore, therapeutic manipulations of the NPR-C pathway may represent a new pharmacological strategy in the context of underlying molecular mechanisms.

Evolving use of natriuretic peptide receptor type-C as part of strategies for the treatment of pulmonary hypertension due to left ventricle heart failure

Pulmonary hypertension (PH) due to left ventricular heart failure (LV-HF) is a disabling and life-threatening disease for which there is currently no single marketed pharmacological agent approved. Despite recent advances in the pathophysiological understanding, there is as yet no prospect of cure, and the majority of patients continue to progress to right ventricular failure and die. There is, therefore an urgent unmet need to identify novel pharmacological agents that will prevent or reverse the increase in pulmonary artery pressures while enhancing cardiac performance in PH due to LV-HF. In the present article, we first focused on the Natriuretic Peptide Receptor type C (NPR-C) based therapeutic strategies aimed at lowering pulmonary artery pressure. Second, we reviewed potential NPR-C therapeutic strategies to reverse or least halt the detrimental effects of diastolic dysfunction and impaired nitic oxide signalling pathways, as well as possibilities for neurohumoral modulation.

C-type natriuretic peptide functions as an innate neuroprotectant in neonatal hypoxic-ischemic brain injury in mouse via natriuretic peptide receptor 2

Neonatal hypoxia-ischemia (HI) is the most common cause of brain injury in neonates, which leads to high neonatal mortality and severe neurological morbidity in later life (Vannucci, 2000; Volpe, 2001). Yet the molecular mechanisms of neuronal death and brain damage induced by neonatal HI remain largely elusive. Herein, using both in vivo and in vitro models, we determine an endogenous neuroprotectant role of c-type natriuretic peptide (CNP) in preserving neuronal survival after HI brain injury in mouse pups. Postnatal day 7 (P7) mouse pups with CNP deficiency (Nppc^lbab/lbab) exhibit increased brain infarct size and worsened long-term locomotor function after neonatal HI compared with wildtype control (Nppc^+/+). In isolated primary cortical neurons, recombinant CNP dose-dependently protects primary neurons from oxygen-glucose deprivation (OGD) insult. This neuroprotective effect appears to be mediated through its cognate natriuretic peptide receptor 2 (NPR2), in that antagonization of NPR2, but not NPR3, exacerbates neuronal death and counteracts the protective effect of CNP on primary neurons exposed to OGD insult. Immunoblot and confocal microscopy demonstrate the abundant expression of NPR2 in neurons of the neonatal brain and in isolated primary cortical neurons as well. Moreover, similar to CNP deficiency, administration of NPR2 antagonist P19 via intracerebroventricular injection prior to HI results in exacerbated neuronal death and brain injury after HI. Altogether, the present study indicates that CNP and its cognate receptor NPR2 mainly expressed in neurons represent an innate neuroprotective mechanism in neonatal HI brain injury.

Natriuretic peptide C receptor in the developing sheep lung: role in perinatal transition

Background: At birth, the release of surfactant from alveolar type II cells (ATIIs) is stimulated by increased activity of the beta-adrenergic/adenylyl cyclase/cyclic 3'-5' adenosine monophosphate-signaling cascade. Atrial natriuretic peptide (ANP) stimulates surfactant secretion through natriuretic peptide receptor A (NPR-A). ANP inhibits adenylyl cyclase activity through its binding to NPR-C. We wished to further understand the role of the NPR-C in perinatal transition. Methods: We studied ATII expression of NPR-C in fetal and newborn sheep using immunohistochemistry, and surfactant secretion in isolated ATIIs by measuring ³[H] choline release into the media. Results: ANP induced surfactant secretion, and, at higher doses, it inhibits the stimulatory effect of the secretagogue terbutaline. ATII NPR-C expression decreased significantly after birth. Premature delivery also markedly decreased ANP and NPR-C in ATIIs. Co-incubation of terbutaline (10^-4 M) with ANP (10^-6 M) significantly decreased ³[H] choline release from isolated newborn ATII cells when compared with terbutaline alone; this inhibitory effect was mimicked by the specific NPR-C agonist, C-ANP (10^-10 M). Conclusion: ANP may act as an important epithelial-derived inhibitor of surfactant release in the fetal lung, and downregulation of ANP and NPR-C following birth may sensitize ATII cells to the effects of circulating catecholamines, thus facilitating surfactant secretion.

A natriuretic peptides clearance receptor's agonist reduces pulmonary artery pressures and enhances cardiac performance in preclinical models: New hope for patients with pulmonary hypertension due to left ventricular heart failure

BACKGROUND

In patients with left ventricular heart failure (HF), the development of pulmonary hypertension (PH) is common and represents a strong predictor of death. Despite recent advances in the pathophysiological understanding there is as yet no prospect of cure of this deadly clinical entity and the majority of patients continue to progress to right ventricular failure and die. Furthermore, there is no single medical treatment currently approved for PH related to HF. There is, therefore an urgent unmet need to identify novel pharmacological agents that will prevent the progressive increased or reverse the elevated pulmonary arterial pressures while enhancing cardiac performance in HF.

METHOD AND RESULTS

We here reported, for the first time, using a pressure-loop (P-V) conductance catheter system, that a specific natriuretic peptides clearance receptors' agonist, the ring-deleted atrial natriuretic peptide analogue, cANF4-23 (cANF) reduces pulmonary artery pressures. Strikingly, the administration of the cANF in these mice decreased the RVSP by 50% (n=5, F 25.687, DF 14, p<0.001) and heart rate (HR) by 11% (n=5, F 25.69, DF 14, p<0.001) as well as enhancing cardiac performance including left ventricular contractility in mice. Most strikingly, mice lacking NPR-C were much more susceptible to develop HF, indicating that NPR-C is a critical protective receptor in the heart.

CONCLUSION

Natriuretic peptides clearance receptors' agonists may, therefore represent a novel and attractive therapeutic strategy for PH related to HF, and ultimately improves the life expectancy and quality for millions of people around the planet.

Effects of Wild-Type and Mutant Forms of Atrial Natriuretic Peptide on Atrial Electrophysiology and Arrhythmogenesis

Background—

Atrial natriuretic peptide (ANP) is a hormone with numerous beneficial cardiovascular effects. Recently, a mutation in the ANP gene, which results in the generation of a mutant form of ANP (mANP), was identified and shown to cause atrial fibrillation in people. The mechanism(s) through which mANP causes atrial fibrillation is unknown. Our objective was to compare the effects of wild-type ANP and mANP on atrial electrophysiology in mice and humans.

Methods and Results—

Action potentials (APs), L-type Ca 2+ currents ( I Ca,L ), and Na + current were recorded in atrial myocytes from wild-type or natriuretic peptide receptor C knockout (NPR-C −/− ) mice. In mice, ANP and mANP (10–100 nmol/L) had opposing effects on atrial myocyte AP morphology and I Ca,L . ANP increased AP upstroke velocity ( V max ), AP duration, and I Ca,L similarly in wild-type and NPR-C −/− myocytes. In contrast, mANP decreased V max , AP duration, and I Ca,L , and these effects were completely absent in NPR-C −/− myocytes. ANP and mANP also had opposing effects on I Ca,L in human atrial myocytes. In contrast, neither ANP nor mANP had any effect on Na + current in mouse atrial myocytes. Optical mapping studies in mice demonstrate that ANP sped electric conduction in the atria, whereas mANP did the opposite and slowed atrial conduction. Atrial pacing in the presence of mANP induced arrhythmias in 62.5% of hearts, whereas treatment with ANP completely prevented the occurrence of arrhythmias.

Conclusions—

These findings provide mechanistic insight into how mANP causes atrial fibrillation and demonstrate that wild-type ANP is antiarrhythmic.

Impaired sinoatrial node function and increased susceptibility to atrial fibrillation in mice lacking natriuretic peptide receptor C

Natriuretic peptides (NPs) are critical regulators of the cardiovascular system that are currently viewed as possible therapeutic targets for the treatment of heart disease. Recent work demonstrates potent NP effects on cardiac electrophysiology, including in the sinoatrial node (SAN) and atria. NPs elicit their effects via three NP receptors (NPR-A, NPR-B and NPR-C). Among these receptors, NPR-C is poorly understood. Accordingly, the goal of this study was to determine the effects of NPR-C ablation on cardiac structure and arrhythmogenesis. Cardiac structure and function were assessed in wild-type (NPR-C(+/+)) and NPR-C knockout (NPR-C(-/-)) mice using echocardiography, intracardiac programmed stimulation, patch clamping, high-resolution optical mapping, quantitative polymerase chain reaction and histology. These studies demonstrate that NPR-C(-/-) mice display SAN dysfunction, as indicated by a prolongation (30%) of corrected SAN recovery time, as well as an increased susceptibility to atrial fibrillation (6% in NPR-C(+/+) vs. 47% in NPR-C(-/-)). There were no differences in SAN or atrial action potential morphology in NPR-C(-/-) mice; however, increased atrial arrhythmogenesis in NPR-C(-/-) mice was associated with reductions in SAN (20%) and atrial (15%) conduction velocity, as well as increases in expression and deposition of collagen in the atrial myocardium. No differences were seen in ventricular arrhythmogenesis or fibrosis in NPR-C(-/-) mice. This study demonstrates that loss of NPR-C results in SAN dysfunction and increased susceptibility to atrial arrhythmias in association with structural remodelling and fibrosis in the atrial myocardium. These findings indicate a critical protective role for NPR-C in the heart.

Guanylyl cyclase/natriuretic peptide receptor-A signaling antagonizes phosphoinositide hydrolysis, Ca(2+) release, and activation of protein kinase C

Thus far, three related natriuretic peptides (NPs) and three distinct sub-types of cognate NP receptors have been identified and characterized based on the specific ligand binding affinities, guanylyl cyclase activity, and generation of intracellular cGMP. Atrial and brain natriuretic peptides (ANP and BNP) specifically bind and activate guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), and C-type natriuretic peptide (CNP) shows specificity to activate guanylyl cyclase/natriuretic peptide receptor-B (GC-B/NPRB). All three NPs bind to natriuretic peptide receptor-C (NPRC), which is also known as clearance or silent receptor. The NPRA is considered the principal biologically active receptor of NP family; however, the molecular signaling mechanisms of NP receptors are not well understood. The activation of NPRA and NPRB produces the intracellular second messenger cGMP, which serves as the major signaling molecule of all three NPs. The activation of NPRB in response to CNP also produces the intracellular cGMP; however, at lower magnitude than that of NPRA, which is activated by ANP and BNP. In addition to enhanced accumulation of intracellular cGMP in response to all three NPs, the levels of cAMP, Ca²⁺ and inositol triphosphate (IP₃) have also been reported to be altered in different cells and tissue types. Interestingly, ANP has been found to lower the concentrations of cAMP, Ca²⁺, and IP₃; however, NPRC has been proposed to increase the levels of these metabolic signaling molecules. The mechanistic studies of decreased and/or increased levels of cAMP, Ca²⁺, and IP₃ in response to NPs and their receptors have not yet been clearly established. This review focuses on the signaling mechanisms of ANP/NPRA and their biological effects involving an increased level of intracellular accumulation of cGMP and a decreased level of cAMP, Ca²⁺, and IP₃ in different cells and tissue systems.

Inhibition of Gαi activity by Gβγ is mediated by PI 3-kinase-γ- and cSrc-dependent tyrosine phosphorylation of Gαi and recruitment of RGS12

Others and we have characterized several Gβγ-dependent effectors in smooth muscle, including G protein-coupled receptor kinase 2 (GRK2), PLCβ3, and phosphatidylinositol (PI) 3-kinase-γ, and have identified various signaling targets downstream of PI 3-kinase-γ, including cSrc, integrin-linked kinase, and Rac1-Cdc42/p21-activated kinase/p38 MAP kinase. This study identified a novel mechanism whereby Gβγ acting via PI 3-kinase-γ and cSrc exerts an inhibitory influence on Gαi activity. The Gi2-coupled δ-opioid receptor agonist d-penicillamine (2,5)-enkephalin (DPDPE) activated cSrc, stimulated tyrosine phosphorylation of Gαi2, and induced regulator of G protein signaling 12 (RGS12) association; all three events were blocked by PI 3-kinase (LY294002) and cSrc (PP2) inhibitors and by expression of the COOH-terminal sequence of GRK2-(495-689), a Gβγ-scavenging peptide. Inhibition of forskolin-stimulated cAMP and muscle relaxation by DPDPE was augmented by PP2, LY294002, and a selective PI 3-kinase-γ inhibitor, AS-605420. Expression of tyrosine-deficient (Y69F, Y231F, or Y321F) Gαi2 mutant or knockdown of RGS12 blocked Gαi2 phosphorylation and Gαi2-RGS12 association and caused greater inhibition of cAMP. Parallel studies using somatostatin, cyclopentyl adenosine, or ACh to activate, respectively, Gi1-coupled somatostatin (sstr3) receptors, and Gi3-coupled adenosine A1 or muscarinic m2 receptors elicited cSrc activation, Gαi1 or Gαi3 phosphorylation, Gαi1-RGS12 or Gαi3-RGS12 association, and inhibition of cAMP. Inhibition of cAMP and muscle relaxation was greatly increased by AS-605240 and PP2. The results demonstrate that Gβγ-dependent tyrosine phosphorylation of Gαi1/2/3 by cSrc facilitated recruitment of RGS12, a Gαi-specific RGS protein with a unique phosphotyrosine-binding domain, resulting in rapid deactivation of Gαi and facilitation of smooth muscle relaxation.

Effects of natriuretic peptides on electrical conduction in the sinoatrial node and atrial myocardium of the heart

Natriuretic peptides, including B-type and C-type natriuretic peptide (BNP and CNP), are powerful regulators of the cardiovascular system; however, their electrophysiological effects in the heart, particularly in the sinoatrial node (SAN), are incompletely understood. We have used high-resolution optical mapping to measure the effects of BNP and CNP, and the roles of natriuretic peptide receptors (NPR-A, NPR-B and NPR-C), on electrical conduction within the SAN and atrial myocardium. In basal conditions BNP and CNP (50-500 nm) increased conduction velocity (CV) within the SAN by ∼30% at the high dose and shifted the initial exit site superiorly. These effects sped conduction from the SAN to the surrounding atrial myocardium and were mediated by the NPR-A and NPR-B receptors. In the presence of isoproterenol (1 μm) the NPR-C receptor made a major contribution to the effects of BNP and CNP in the heart. In these conditions BNP, CNP and the NPR-C agonist cANF each decreased SAN CV and shifted the initial exit site inferiorly. The effects of cANF (30% reduction) were larger than BNP or CNP (∼15% reduction), indicating that BNP and CNP activate multiple natriuretic peptide receptors. In support of this, the inhibitory effects of BNP were absent in NPR-C knockout mice, where BNP instead elicited a further increase (∼25%) in CV. Measurements in externally paced atrial preparations demonstrate that the effects of natriuretic peptides on CV are partially independent of changes in cycle length. These data provide detailed novel insight into the complex effects of natriuretic peptides and their receptors on electrical conduction in the heart.

Natriuretic peptide receptor-3 gene (NPR3): nonsynonymous polymorphism results in significant reduction in protein expression because of accelerated degradation

BACKGROUND- The primary role of natriuretic peptide receptor-3 (NPR3) or NPR-C is in the clearance of natriuretic peptides that play an important role in modulating intravascular volume and vascular tone. Genetic variation in NPR3 has been associated with variation in blood pressure and obesity. Despite the importance of NPR3, sequence variation in the gene has not been addressed using DNA from different ethnic populations. We set out to identify and functionally characterize genetic variation in NPR3 in 3 ethnic groups. METHODS AND RESULTS- DNA samples from 96 European American, 96 African American, and 96 Han Chinese American healthy subjects were used to resequence NPR3 exons, splice junctions, and flanking regions. We identified 105 polymorphisms, 50 of which were novel, including 8 nonsynonymous single-nucleotide polymorphisms, 7 were novel. Expression constructs were created for the nonsynonymous single-nucleotide polymorphisms. HEK293 cells were transfected with constructs for wild type and variant allozymes; and recombinant proteins were measured by quantitative Western blot analysis. The most significant change in NPR3 protein was observed for the Arg146 variant allozyme, with 20% of wild-type protein, primarily because of autophagy-dependent degradation. NPR3 structural modeling confirmed that the Arg146 variant protein was not compatible with wild-type conformation and could result in protein misfolding or instability. CONCLUSIONS- Multiple novel NPR3 genetic polymorphisms were identified in 3 ethnic groups. The Arg146 allozyme displayed a significant decrease in protein quantity because of degradation mediated predominantly by autophagy. This genetic variation could have a significant effect on the metabolism of natriuretic peptides with potential clinical implications.

Modeling the assembly of the multiple domains of α-actinin-4 and its role in actin cross-linking

The assembly of proteins into multidomain complexes is critical for their function. In eukaryotic nonmuscle cells, regulation of the homodimeric actin cross-linking protein α-actinin-4 (ACTN4) during cell migration involves signaling receptors with intrinsic tyrosine kinase activity, yet the underlying molecular mechanisms are poorly understood. As a first step to address the latter, we validate here an atomic model for the ACTN4 end region, which corresponds to a ternary complex between the N-terminal actin-binding domain (ABD) and an adjacent helical neck region of one monomer, and the C-terminal calmodulin-like domain of the opposite antiparallel monomer. Mutagenesis experiments designed to disrupt this ternary complex confirm that its formation reduces binding to F-actin. Molecular dynamics simulations show that the phosphomimic mutation Y265E increases actin binding by breaking several interactions that tether the two calponin homology domains into a closed ABD conformation. Simulations also show a disorder-to-order transition in the double phosphomimic mutant Y4E/Y31E of the 45-residue ACTN4 N-terminal region, which can inhibit actin binding by latching both calponin homology domains more tightly. Collectively, these studies provide a starting point for understanding the role of external cues in regulating ACTN4, with different phenotypes resulting from changes in the multidomain assembly of the protein.

Emerging Roles of Natriuretic Peptides and their Receptors in Pathophysiology of Hypertension and Cardiovascular Regulation

Thus far, three related natriuretic peptides (NPs) and three distinct receptors have been identified, which have advanced our knowledge towards understanding the control of high blood pressure, hypertension, and cardiovascular disorders to a great extent. Biochemical and molecular studies have been advanced to examine receptor function and signaling mechanisms and the role of second messenger cGMP in pathophysiology of hypertension, renal hemodynamics, and cardiovascular functions. The development of gene-knockout and gene-duplication mouse models along with transgenic mice have provided a framework for understanding the importance of the antagonistic actions of natriuretic peptides receptor in cardiovascular events at the molecular level. Now, NPs are considered as circulating markers of congestive heart failure, however, their therapeutic potential for the treatment of cardiovascular diseases such as hypertension, renal insufficiency, cardiac hypertrophy, congestive heart failure, and stroke has just begun to unfold. Indeed, the alternative avenues of investigations in this important are need to be undertaken, as we are at the initial stage of the molecular therapeutic and pharmacogenomic implications.

Effects of forward and backward transitions in light intensities in tau-illuminance curves of the rat motor activity rhythm under constant dim light

Circadian rhythms are strongly influenced by light intensity, the effects of which may persist beyond the duration of light exposure (aftereffects). Here, the authors constructed period-illuminance curves for the motor activity circadian rhythm of male and female rats by recording the effects of a series of small upward and downward steps in light intensity (illuminance ranging between .01 lux of dim red light and 1 lux of white light) on their activity. In all cases, stepwise changes were made in five logarithmic steps (irradiance: dim red light: .692 µW/cm(2) and white light: .006, .016, .044, .12, and .315 µW/cm(2), corresponding, respectively, to .02, .05, .14, .13, and 1 lux measured at cage level), with changes in intensity every 2 wks. One group of rats (DLD) started in dim red light, moved up to 1 lux white light, and then back down to the original light intensity. Another group (LDL) started at 1 lux, moved down to .01 lux, and then back up to the original intensity. Motor activity data were recorded throughout the experiment and tau values, the percentage of variance explained by the rhythm, and the mean motor activity for each stage and group were calculated. The results show differences in the dynamics of tau values between the DLD and LDL groups and between males and females. In the LDL group, the tau values of both males and females were dependent on light intensity, and were similar for the forward and backward transitions. In other words, no aftereffects were found, and no differences were detected between males and females. In the DLD group, however, differences were found between males and females. Males had a tau value of 24 h 20 min under dim red light, 25 h 40 min under 1 lux, and 24 h 50 min on return to dim red light. It is noticeable that the tau values of the backward branch of the illuminance curve contradicted classical predictions, since at .38 and .14 lux the tau values were shorter than those found under the same intensities after exposure to 1 lux. Females became arrhythmic at 1 lux, and only one half of them recovered their circadian rhythm at .02 lux. The other one half remained arrhythmic even under dim red light. Thus, some of the results of this paper contradict the predictions of standard descriptions of the functioning of the circadian clock, possibly due to the effects of dim light.

Novel GαS-protein signaling associated with membrane-tethered amyloid precursor protein intracellular domain

Numerous physiological functions, including a role as a cell surface receptor, have been ascribed to Alzheimer's disease-associated amyloid precursor protein (APP). However, detailed analysis of intracellular signaling mediated by APP in neurons has been lacking. Here, we characterized intrinsic signaling associated with membrane-bound APP C-terminal fragments, which are generated following APP ectodomain release by α- or β-secretase cleavage. We found that accumulation of APP C-terminal fragments or expression of membrane-tethered APP intracellular domain results in adenylate cyclase-dependent activation of PKA (protein kinase A) and inhibition of GSK3β signaling cascades, and enhancement of axodendritic arborization in rat immortalized hippocampal neurons, mouse primary cortical neurons, and mouse neuroblastoma. We discovered an interaction between BBXXB motif of APP intracellular domain and the heterotrimeric G-protein subunit Gα(S), and demonstrate that Gα(S) coupling to adenylate cyclase mediates membrane-tethered APP intracellular domain-induced neurite outgrowth. Our study provides clear evidence that APP intracellular domain can have a nontranscriptional role in regulating neurite outgrowth through its membrane association. The novel functional coupling of membrane-bound APP C-terminal fragments with Gα(S) signaling identified in this study could impact several brain functions such as synaptic plasticity and memory formation.

The natriuretic peptides BNP and CNP increase heart rate and electrical conduction by stimulating ionic currents in the sinoatrial node and atrial myocardium following activation of guanylyl cyclase-linked natriuretic peptide receptors

Natriuretic peptides (NPs) are best known for their ability to regulate blood vessel tone and kidney function whereas their electrophysiological effects on the heart are less clear. Here, we measured the effects of BNP and CNP on sinoatrial node (SAN) and atrial electrophysiology in isolated hearts as well as isolated SAN and right atrial myocytes from mice. BNP and CNP dose-dependently increased heart rate and conduction through the heart as indicated by reductions in R-R interval, P wave duration and P-R interval on ECGs. In conjunction with these ECG changes BNP and CNP (100 nM) increased spontaneous action potential frequency in isolated SAN myocytes by increasing L-type Ca(2+) current (I(Ca,L)) and the hyperpolarization-activated current (I(f)). BNP had no effect on right atrial myocyte APs in basal conditions; however, in the presence of isoproterenol (10nM), BNP increased atrial AP duration and I(Ca,L). Quantitative gene expression and immunocytochemistry data show that all three NP receptors (NPR-A, NPR-B and NPR-C) are expressed in the SAN and atrium. The effects of BNP and CNP on SAN and right atrial myocytes were maintained in mutant mice lacking functional NPR-C receptors and blocked by the NPR-A antagonist A71915 indicating that BNP and CNP function through their guanylyl cyclase-linked receptors. Our data also show that the effects of BNP and CNP are completely absent in the presence of the phosphodiesterase 3 inhibitor milrinone. Based on these data we conclude that NPs can increase heart rate and electrical conduction by activating the guanylyl cyclase-linked NPR-A and NPR-B receptors and inhibiting PDE3 activity.

Signal protein-derived peptides as functional probes and regulators of intracellular signaling

The functionally important regions of signal proteins participating in their specific interaction and responsible for transduction of hormonal signal into cell are rather short in length, having, as a rule, 8 to 20 amino acid residues. Synthetic peptides corresponding to these regions are able to mimic the activated form of full-size signal protein and to trigger signaling cascades in the absence of hormonal stimulus. They modulate protein-protein interaction and influence the activity of signal proteins followed by changes in their regulatory and catalytic sites. The present review is devoted to the achievements and perspectives of the study of signal protein-derived peptides and to their application as selective and effective regulators of hormonal signaling systems in vitro and in vivo. Attention is focused on the structure, biological activity, and molecular mechanisms of action of peptides, derivatives of the receptors, G protein α subunits, and the enzymes generating second messengers.

Impact of natriuretic peptide clearance receptor (NPR3) gene variants on blood pressure in type 2 diabetes

OBJECTIVE

Hypertension in diabetes is characterized by abnormal sodium homeostasis, suggesting a particular role of natriuretic peptide pathway. Natriuretic peptides can affect blood pressure (BP) through their plasma concentrations, which are dependent on their receptor activities. We thus assessed the association between nine NPR3 gene polymorphisms and BP levels in patients with type 2 diabetes.

RESEARCH DESIGN AND METHODS

Nine single nucleotide polymorphisms (SNPs) tagging the haplotype structure of the NPR3 gene were genotyped in the 3,126 French Non-insulin-dependent Diabetes, Hypertension, Microalbuminuria or Proteinuria, Cardiovascular Events, and Ramipril (DIABHYCAR) trial participants. We then used a second population (Diabete de type 2, Nephropathie et Genetique [DIAB2NEPHROGENE]/Survie, Diabete de type 2 et Genetique [SURDIAGENE] study) of 2,452 patients for the purpose of replication. Finally, we separately investigated subjects selected according to their rs 2270915SNP genotypes for their BP response to salt restriction.

RESULTS

In DIABHYCAR patients, three SNPs (rs6889608, rs1173773, and rs2270915) were significantly associated with systolic BP (SBP). The effect of the rs2270915 was replicated in the second step population: AA homozygotes had a lower SBP than G carriers (137.4 ± 19.1 vs. 140.0 ± 20.2 mmHg, P = 0.004). The rs2270915 influenced the response of SBP to salt reduction, with AA homozygous patients showing greater reductions after restriction of salt intake compared with G carriers: -20 mmHg (-43 to -8) vs. -3 (-20 to +7); P = 0.006.

CONCLUSIONS

We found a consistent and significant association between the rs2270915 polymorphism of the NPR3 gene and SBP in diabetic patients. This genetic variation may affect pressure response to changes in dietary sodium.

Dual pathways of carbon monoxide-mediated vasoregulation: modulation by redox mechanisms

RATIONALE

Vascular tissues produce carbon monoxide (CO) via HO-dependent and HO-independent mechanisms; the former in tandem with biliverdin and iron and the latter as a lone product. CO has been shown to function as both a vasoconstrictor and vasodilator; however, factors that dictate the vasoregulatory phenotype of this gas are unknown.

OBJECTIVE

We investigated whether CO-mediated vasoconstriction is mechanistically linked to enhanced reactive oxygen species production that masks vasodilatory pathways.

METHODS AND RESULTS

Sprague-Dawley rat interlobar and interlobular arteries were examined in terms of superoxide (O2*-) generation and vascular reactivity in the absence and presence of antioxidants. Both authentic CO and the CO-releasing molecule (CORM)-3 constricted renal arteries and increased O2*- production in a dose-dependent manner. The antioxidants tempol, ebselen, and deferoxamine inhibited CO-induced O2*- production and converted CO from constrictor to dilator. CO-induced O2*- generation was found to involve the activity of multiple oxidases including nitric oxide synthase, NADPH oxidase, xanthine oxidase, and complex IV of the mitochondrial electron chain. Furthermore, inhibition of these enzymes converted CO from constrictor to dilator. Similarly, biliverdin and bilirubin inhibited CO-induced O2*- production and vasoconstriction, allowing for a vasodilatory response to CO to be expressed. CO-induced vasoconstriction was dependent on a non-thromboxane agonist of the thromboxane receptor, whereas vasodilatory mechanisms of CO relied on the activation of soluble guanylate cyclase and calcium-gated potassium channels.

CONCLUSIONS

CO-induced vasoconstriction involves the generation of reactive oxygen species, which, when negated, allows for the expression of vasodilatory pathways which are masked by the primary oxidative stress response to this gas.

The C type natriuretic peptide receptor tethers AHNAK1 at the plasma membrane to potentiate arachidonic acid-induced calcium mobilization

Arachidonic acid (AA) liberated from membrane phospholipids is known to activate phospholipase C gamma1 (PLCgamma1) concurrently with AHNAK in nonneuronal cells. The recruitment of AHNAK from the nucleus is required for it to activate PLCgamma1 at the plasma membrane. Here, we identify the C-type natriuretic peptide receptor (NPR-C), an atypical G protein-coupled receptor, as a protein binding partner for AHNAK1 in various cell types. Mass spectrometry and MASCOT analysis of excised bands from NPR-C immunoprecipitation studies revealed multiple signature peptides corresponding to AHNAK1. Glutathione S-transferase (GST) pulldown assays using GST- AHNAK1 fusion proteins corresponding to each of the distinct domains of AHNAK1 showed the C1 domain of AHNAK1 associates with NPR-C. The role of NPR-C in mediating AA-dependent AHNAK1 calcium signaling was explored in various cell types, including 3T3-L1 preadipocytes during the early stages of differentiation. Sucrose density gradient centrifugation studies showed AHNAK1 resides in the nucleus, cytoplasm, and at the plasma membrane, but small interfering RNA (siRNA)-mediated knockdown of NPR-C resulted in AHNAK1 accumulation in the nucleus. Overexpression of a portion of AHNAK1 resulted in augmentation of intracellular calcium mobilization, whereas siRNA-mediated knockdown of NPR-C or AHNAK1 protein resulted in attenuation of intracellular calcium mobilization in response to phorbol 12-myristate 13-acetate. We characterize the novel association between AHNAK1 and NPR-C and provide evidence that this association potentiates the AA-induced mobilization of intracellular calcium. We address the role of intracellular calcium in the various cell types that AHNAK1 and NPR-C were found to associate.

Heme oxygenase induction attenuates afferent arteriolar autoregulatory responses

Heme oxygenases (HO-1, HO-2) catalyze conversion of heme to iron, carbon monoxide (CO), and biliverdin/bilirubin. We studied the effects of renal HO-1 induction on afferent arteriole (Aff-Art) autoregulatory responses to increases in renal perfusion pressure (RPP). Rats were treated with hemin and SnCl2 to induce HO-1, and Aff-Art autoregulatory responses were evaluated using the rat blood-perfused juxtamedullary nephron preparation. Renal HO-1 expression was significantly increased in hemin- and SnCl2-treated rats, while HO-2 was not altered. Aff-Art autoregulatory constrictor responses to increases in RPP from 100 to 150 mmHg were attenuated in hemin- and SnCl2-treated rats compared with control rats (+1.1+/-3.3, n=9 and +4.4+/-5.3, n=9 vs. -14.2+/-1.5%, n=10, respectively) (P<0.05). Acute HO inhibition with chromium mesoporphyrin (CrMP; 15 micromol/l) restored Aff-Art autoregulatory responses in hemin- and SnCl2-treated rats. Superfusing Aff-Arts from control rats with 100 micromol/l biliverdin did not alter autoregulatory responses; however, superfusion with 1 mmol/l CO significantly attenuated autoregulatory responses to increases in RPP from 100 to 150 mmHg (+3.3+/-5.4 vs. -16.6+/-3.8%, n=6) (P<0.05). Acute soluble guanylate cyclase inhibition with 10 micromol/l ODQ restored Aff-Art autoregulatory responses in hemin-treated rats. Immunohistochemistry shows HO-2 to be expressed mainly in epithelial cells with weak staining in proximal tubules, interlobular arteries, and Aff-Arts. In hemin- and SnCl2-treated rats, HO-1 was induced in tubular epithelial cells but not interlobular arteries and Aff-Arts. We conclude that induction of renal HO-1 attenuates Aff-Art constrictor responses to increases in RPP via increasing CO production from tubular epithelial cells, suggesting that an augmented HO system in pathophysiological conditions modulates renal autoregulation.

Natriuretic peptides and their receptors in the central nervous system

Natriuretic peptides (NPs), including atrial, brain and C-type NPs, are a family of structurally related but genetically distinct peptides. These peptides, along with their receptors (NPRs), are long known to be involved in the regulation of various physiological functions, such as diuresis, natriuresis, and blood flow. Recently, abundant evidence shows that NPs and NPRs are widely distributed in the central nervous system (CNS), suggesting possible roles of NPs in modulating physiological functions of the CNS. This review starts with a brief summary of relevant background information, such as molecular structures of NPs and NPRs and general intracellular mechanisms after activation of NPRs. We then provide a detailed description of the expression profiles of NPs and NPRs in the CNS and an in-depth discussion of how NPs are involved in neural development, neurotransmitter release, synaptic transmission and neuroprotection through activation of NPRs.

Phosphorylation of GRK2 by PKA augments GRK2-mediated phosphorylation, internalization, and desensitization of VPAC2 receptors in smooth muscle

The smooth muscle of the gut expresses mainly G(s) protein-coupled vasoactive intestinal peptide (VIP)/pituitary adenylate cyclase-activating peptide receptors (VPAC(2) receptors), which belong to the secretin family of G protein-coupled receptors. The extent to which PKA and G protein-coupled receptor kinases (GRKs) participate in homologous desensitization varies greatly among the secretin family of receptors. The present study identified the novel role of PKA in homologous desensitization of VPAC(2) receptors via the phosphorylation of GRK2 at Ser(685). VIP induced phosphorylation of GRK2 in a concentration-dependent fashion, and the phosphorylation was abolished by blockade of PKA with cell-permeable myristoylated protein kinase inhibitor (PKI) or in cells expressing PKA phosphorylation-site deficient GRK2(S685A). Phosphorylation of GRK2 increased its activity and binding to G betagamma. VIP-induced phosphorylation of VPAC(2) receptors was abolished in muscle cells expressing kinase-deficient GRK2(K220R) and attenuated in cells expressing GRK2(S685A) or by PKI. VPAC(2) receptor internalization (determined from residual (125)I-labeled VIP binding and receptor biotinylation after a 30-min exposure to VIP) was blocked in cells expressing GRK2(K220R) and attenuated in cells expressing GRK2(S685A) or by PKI. Finally, VPAC(2) receptor degradation (determined from residual (125)I-labeled VIP binding and receptor expression after a prolonged exposure to VIP) and functional VPAC(2) receptor desensitization (determined from the decrease in adenylyl cyclase activity and cAMP formation after a 30-min exposure to VIP) were abolished in cells expressing GRK2(K220R) and attenuated in cells expressing GRK2(S685A). These results demonstrate that in gastric smooth muscle VPAC(2) receptor phosphorylation is mediated by GRK2. Phosphorylation of GRK2 by PKA enhances GRK2 activity and its ability to induce VPAC(2) receptor phosphorylation, internalization, desensitization, and degradation.

Natriuretic peptide C receptor signalling in the heart and vasculature

Natriuretic peptides (NPs), including atrial, brain and C-type natriuretic peptides (ANP, BNP and CNP), bind two classes of cell surface receptors: the guanylyl cyclase-linked A and B receptors (NPR-A and NPR-B) and the C receptor (NPR-C). The biological effects of NPs have been mainly attributed to changes in intracellular cGMP following their binding to NPR-A and NPR-B. NPR-C does not include a guanylyl cyclase domain. It has been denoted as a clearance receptor and is thought to bind and internalize NPs for ultimate degradation. However, a substantial body of biochemical work has demonstrated the ability of NPR-C to couple to inhibitory G proteins (Gi) and cause inhibition of adenylyl cyclase and activation of phospholipase-C. Recently, novel physiological effects of NPs, mediated specifically by NPR-C, have been discovered in the heart and vasculature. We have described the ability of CNP, acting via NPR-C, to selectively inhibit L-type calcium currents in atrial and ventricular myocytes, as well as in pacemaker cells (sinoatrial node myocytes). In contrast, our studies of the electrophysiological effects of CNP on cardiac fibroblasts demonstrated an NPR-C-Gi-phospholipase-C-dependent activation of a non-selective cation current mediated by transient receptor potential (TRP) channels. It is also known that CNP and BNP have important anti-proliferative effects in cardiac fibroblasts that appear to involve NPR-C. In the mammalian resistance vessels, including mesenteric and coronary arteries, CNP has been found to function as an NPR-C-dependent endothelium-derived hyperpolarizing factor that regulates local blood flow and systemic blood pressure by hyperpolarizing smooth muscle cells. In this review we highlight the role of NPR-C in mediating these NP effects in myocytes and fibroblasts from the heart as well as in vascular smooth muscle cells.

Acute antihypertensive action of Tempol in the spontaneously hypertensive rat

Acute intravenous Tempol reduces mean arterial pressure (MAP) and heart rate (HR) in spontaneously hypertensive rats. We investigated the hypothesis that the antihypertensive action depends on generation of hydrogen peroxide, activation of heme oxygenase, glutathione peroxidase or potassium conductances, nitric oxide synthase, and/or the peripheral or central sympathetic nervous systems (SNSs). Tempol caused dose-dependent reductions in MAP and HR (at 174 micromol/kg; DeltaMAP, -57+/- 3 mmHg; and DeltaHR, -50 +/- 4 beats/min). The antihypertensive response was unaffected by the infusion of a pegylated catalase or by the inhibition of catalase with 3-aminotriazole, inhibition of glutathione peroxidase with buthionine sulfoximine, inhibition of heme oxygenase with tin mesoporphyrin, or inhibition of large-conductance Ca(2+)-activated potassium channels with iberiotoxin. However, the antihypertensive response was significantly (P < 0.01) blunted by 48% by the activation of adenosine 5'-triphosphate-sensitive potassium (K(ATP)) channels with cromakalim during maintenance of blood pressure with norepinephrine and by 31% by the blockade of these channels with glibenclamide, by 40% by the blockade of nitric oxide synthase with N(omega)-nitro-L-arginine methyl ester (L-NAME), and by 40% by the blockade of ganglionic autonomic neurotransmission with hexamethonium. L-NAME and hexamethonium were additive, but glibenclamide and hexamethonium were less than additive. The central administration of Tempol was ineffective. The acute antihypertensive action of Tempol depends on the independent effects of potentiation of nitric oxide and inhibition of the peripheral SNS that involves the activation of K(ATP) channels.

Coupling of M3 acetylcholine receptor to Gq16 activates a natriuretic peptide receptor guanylyl cyclase

Muscarinic activation of tracheal smooth muscle (TSM) involves a M(3)AChR/heterotrimeric-G protein/NPR-GC coupling mechanism. G protein activators Mastoparan (MAS) and Mastoparan-7 stimulated 4- and 10-fold the NPR-GC respectively, being insensitive to PTX and antibodies against Galpha(i/o) subfamily. Muscarinic and MAS stimulation of NPR-GC was blocked by antibodies against C-terminal of Galpha(q16), whose expression was confirmed by RT-PCR. However, synthetic peptides from C-terminal of Galpha(q15/16) stimulated the NPR-GC. Coupling of alpha(q16) to M(3)AChR is supported by MAS decreased [(3)H]QNB binding, being abolished after M(3)AChR-4-DAMP-alkylation. Anti-i(3)M(3)AChR antibodies blocked the muscarinic activation of NPR-GC, and synthetic peptide from i(3)M(3)AChR (M(3)P) was more potent than MAS increasing GTPgamma [(35)S] and decreasing the [(3)H]QNB activities. Coupling between NPR-GC and Galpha(q16) was evaluated by using trypsin-solubilized-fraction from TSM membranes, which displayed a MAS-sensitive-NPR-GC activity, being immunoprecipitated with anti-Galpha(q16), also showing an immunoreactive heterotrimeric-G-beta-subunit. These data support the existence of a novel transducing cascade, involving Galpha(q16)beta gamma coupling M(3)AChR to NPR-GC.

C-type natriuretic peptide activates a non-selective cation current in acutely isolated rat cardiac fibroblasts via natriuretic peptide C receptor-mediated signalling

In the heart, fibroblasts play an essential role in the deposition of the extracellular matrix and they also secrete a number of hormonal factors. Although natriuretic peptides, including C-type natriuretic peptide (CNP) and brain natriuretic peptide, have antifibrotic effects on cardiac fibroblasts, the effects of CNP on fibroblast electrophysiology have not been examined. In this study, acutely isolated ventricular fibroblasts from the adult rat were used to measure the effects of CNP (2 x 10(-8) M) under whole-cell voltage-clamp conditions. CNP, as well as the natriuretic peptide C receptor (NPR-C) agonist cANF (2 x 10(-8) M), significantly increased an outwardly rectifying non-selective cation current (NSCC). This current has a reversal potential near 0 mV. Activation of this NSCC by cANF was abolished by pre-treating fibroblasts with pertussis toxin, indicating the involvement of G(i) proteins. The cANF-activated NSCC was inhibited by the compounds Gd(3+), SKF 96365 and 2-aminoethoxydiphenyl borate. Quantitative RT-PCR analysis of mRNA from rat ventricular fibroblasts revealed the expression of several transient receptor potential (TRP) channel transcripts. Additional electrophysiological analysis showed that U73122, a phospholipase C antagonist, inhibited the cANF-activated NSCC. Furthermore, the effects of CNP and cANF were mimicked by the diacylglycerol analogue 1-oleoyl-2-acetyl-sn-glycerol (OAG), independently of protein kinase C activity. These are defining characteristics of specific TRPC channels. More detailed molecular analysis confirmed the expression of full-length TRPC2, TRPC3 and TRPC5 transcripts. These data indicate that CNP, acting via the NPR-C receptor, activates a NSCC that is at least partially carried by TRPC channels in cardiac fibroblasts.

Transcriptome-based identification of pro- and antioxidative gene expression in kidney cortex of nitric oxide-depleted rats

Nitric oxide (NO) depletion in rats induces severe endothelial dysfunction within 4 days. Subsequently, hypertension and renal injury develop, which are ameliorated by α-tocopherol (VitE) cotreatment. The hypothesis of the present study was that NO synthase (NOS) inhibition induces a renal cortical antioxidative transcriptional response and invokes pro-oxidative and proinflammatory gene expression due to elimination of dampening effects of NO and enhanced oxidative stress. Male Sprague-Dawley rats received NOS inhibitor Nω-nitro-l-arginine (l-NNA, 500 mg/l water) for 4 (4d-LNNA), 21 (21d-LNNA), or 21 days with VitE in chow (0.7 g/kg body wt/day). Renal cortical RNA was applied to oligonucleotide rat arrays. In 4d-LNNA, 21d-LNNA, and 21d-LNNA+VitE, 120, 320, and 184 genes were differentially expressed, respectively. Genes related to glutathione and bilirubin synthesis were suppressed during 4d and 21d-LNNA and not corrected by VitE. Proteinuria, tubulointerstitial macrophages, and heme-oxygenase-1 (HO-1) expression were strongly correlated. Remarkably, pro-oxidative genes were not induced. Inflammation- and injury-related genes, including kidney injury molecule-1 and osteopontin, were unchanged at day 4, induced at 21d, and partly corrected by VitE. Superimposing HO-1 inhibition on NOS inhibition had no impact on the development of hypertension. To summarize, renal expression of genes involved in synthesis of the antioxidants glutathione and bilirubin seemed directly NO dependent, but there were no direct effects of NO depletion on pro-oxidant systems. This indicates that renal transcriptional regulation of two defense systems, glutathione and bilirubin syntheses, seems to depend upon adequate NO synthesis. Interaction between NO synthesis and heme degradation pathways for blood pressure regulation was not found.

Autonomic Cardiovascular Responses to Heme Oxygenase Inhibition in Conscious Rats

Carbon monoxide (CO) is produced in the course of heme degradation from biliverdin by heme oxygenase (HO) in various tissues, including the central nervous system. Recent studies suggest the inhibition of HO activity increases arterial pressure mediated by the autonomic nervous system. The present study was designed to investigate the autonomic regulation of cardiovascular responses to inhibition of endogenous CO production by the HO inhibitor Zinc deuteroporphyrin 2, 4-bis glycol (ZnDPBG) by using direct sympathetic nerve recordings in conscious, chronically instrumented rats. ZnDPBG induced increases in mean arterial pressure (MAP) (P<0.05) and renal sympathetic nerve activity (RSNA) (P<0.05) but no significant change in heart rate (P>0.05) in intact rats. In atropine-treated rats, ZnDPBG also induced increases in MAP (P<0.05) and RSNA (P<0.05) but no change in heart rate (P>0.05). In sinoaortic denervated rats, ZnDPBG induced increases in MAP (P<0.05), heart rate (P<0.05), and RSNA (P<0.05). ZnDPBG shifted the baroreflex curve for RSNA upward and to the right, which was characterized by increases in the maximum and minimum response and midpoint pressure without altering the maximum gain. These results indicate that inhibition of HO activity within the central nervous system causes sympathoexcitation, resulting in an increase in arterial pressure. We conclude that the CO/HO system plays an important role in cardiovascular regulation by modulating sympathetic tone.

Molecular cloning and functional expression of a VIP-specific receptor

Three receptors for VIP and pituitary adenylate cyclase-activating peptide (PACAP) have been cloned and characterized: PAC(1), with high affinity for PACAP, and VPAC(1) and VPAC(2) with equally high affinity for VIP and PACAP. The existence of a VIP-specific receptor (VIP(s)) in guinea pig (GP) teniae coli smooth muscle was previously surmised on the basis of functional studies, and its existence was confirmed by cloning of a partial NH(2)-terminal sequence. Here we report the cloning of the full-length cDNAs of two receptors, a VPAC(2) receptor from GP gastric smooth muscle and VIP(s) from GP teniae coli smooth muscle. The cDNA sequence of the VIP(s) encodes a 437-amino acid protein (M(r) 49,560) that possesses 87% similarity to VPAC(2) receptors in rat and mouse and differs from the VPAC(2) receptor in GP gastric smooth muscle by only two amino-acid residues, F(40)F(41) in lieu of L(40)L(41). In COS-1 cells transfected with the GP teniae coli smooth muscle receptor, only VIP bound with high affinity (IC(50) 1.4 nM) and stimulated cAMP formation with high potency (EC(50) 1 nM). In contrast, in COS-1 cells transfected with the GP gastric smooth muscle receptor, both VIP and PACAP bound with equally high affinity (IC(50) 2.3 nM) and stimulated cAMP with equally high potency (EC(50) 1.5 nM). We conclude that the receptor cloned from GP teniae coli smooth muscle is a VIP(s) distinct from VPAC(1) and VPAC(2) receptors. The ligand specificity in this species is determined by a pair of adjacent phenylalanine residues (L(40)L(41)) in the NH(2)-terminal ligand-binding domain.

Characterization of a G protein-coupled guanylyl cyclase-B receptor from bovine tracheal smooth muscle

A G protein-coupled natriuretic peptide-guanylyl cyclase receptor-B (NPR-B) located in plasma membranes from bovine tracheal smooth muscle shows complex kinetics and regulation. NPR-B was activated by natriuretic peptides (CNP-53 > ANP-28) at the ligand extracellular domain, stimulated by Gq-protein activators, such as mastoparan, and inhibited by Gi-sensitive chloride, interacting at the juxtamembrane domain. The kinase homology domain was evaluated by the ATP inhibition of Mn2+-activated NPR-B, which was partially reversed by mastoparan. The catalytic domain was studied by kinetics of Mn2+/Mg2+ and GTP, and the catalytic effect with GTP analogues with modifications of the /gamma phosphates and ribose moieties. Most NPR-B biochemical properties remained after detergent solubilization but the mastoparan activation and chloride inhibition of NPR-B disappeared. Our results indicate that NPR-B is a highly regulated nano-machinery with domains acting at cross-talk points with other signal transducing cascades initiated by G protein-coupled receptors and affected by intracellular ligands such as chloride, Mn2+, Mg2+, ATP, and GTP.

Interaction between endogenously produced carbon monoxide and nitric oxide in regulation of renal afferent arterioles

Heme oxygenases (HO-1 and HO-2) catalyze the conversion of heme to carbon monoxide (CO), iron, and biliverdin. CO causes vasorelaxation via stimulation of soluble guanylate cyclase (sGC) and/or activation of calcium-activated potassium channels. Because nitric oxide (NO) exerts effects via the same pathways, we tested the interaction between CO and NO on rat afferent arterioles (AAs) using the blood-perfused juxtamedullary nephron preparation. AAs were superfused with either tricarbonyldichlororuthenium (II) dimer, known as CO releasing molecule (CORM-2), 10 micromol/l CO solution, or 15 micromol/l chromium mesoporphyrin (CrMP, HO inhibitor). AAs were also superfused with 1 mmol/l N(omega)-nitro-L-arginine (L-NNA) to inhibit NO synthase (NOS) or 10 micromol/l 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one to inhibit sGC, and then CrMP was superfused during NOS inhibition or sGC inhibition. Treatment with 150 and 300 micromol/l CORM-2 or with CO (10 micromol/l) significantly dilated AAs (22.0 +/- 0.9 and 22.8 +/- 0.9 vs. 18.3 +/- 0.9 microm, n = 5, P < 0.05; and 26.0 +/- 1.4 vs. 18.8 +/- 0.7 microm, n = 5, P < 0.05). In untreated vessels, HO inhibition did not alter AA diameter (17.5 +/- 0.7 vs. 17.2 +/- 0.6 microm, n = 7, P > 0.05); however, during inhibition of NO production, which constricted arterioles to 14.6 +/- 1.2 microm, n = 6, P < 0.05, concurrent HO inhibition led to further vasoconstriction (11.7 +/- 1.6 microm, n = 6, P < 0.05). CORM-2 attenuated the L-NNA-induced vasoconstriction. Inhibition of sGC caused significant constriction (15.7 +/- 0.4 vs. 18.8 +/- 0.4 microm, n = 6, P < 0.05). HO inhibition during sGC inhibition did not cause further change in AAs (15.5 +/- 0.7 microm, n = 6). We conclude that endogenously produced CO does not exert a perceptible influence on AA diameter in the presence of intact NO system; however, when NO production is inhibited, CO serves as an important renoprotective reserve mechanism to prevent excess afferent arteriolar constriction.

Signaling for contraction and relaxation in smooth muscle of the gut

Phosphorylation of Ser19 on the 20-kDa regulatory light chain of myosin II (MLC20) by Ca2+/calmodulin-dependent myosin light-chain kinase (MLCK) is essential for initiation of smooth muscle contraction. The initial [Ca2+]i transient is rapidly dissipated and MLCK inactivated, whereas MLC20 and muscle contraction are well maintained. Sustained contraction does not reflect Ca2+ sensitization because complete inhibition of MLC phosphatase activity in the absence of Ca2+ induces smooth muscle contraction. This contraction is suppressed by staurosporine, implying participation of a Ca2+-independent MLCK. Thus, sustained contraction, as with agonist-induced contraction at experimentally fixed Ca2+ concentrations, involves (a) G protein activation, (b) regulated inhibition of MLC phosphatase, and (c) MLC20 phosphorylation via a Ca2+-independent MLCK. The pathways that lead to inhibition of MLC phosphatase by G(q/13)-coupled receptors are initiated by sequential activation of Galpha(q)/alpha13, RhoGEF, and RhoA, and involve Rho kinase-mediated phosphorylation of the regulatory subunit of MLC phosphatase (MYPT1) and/or PKC-mediated phosphorylation of CPI-17, an endogenous inhibitor of MLC phosphatase. Sustained MLC20 phosphorylation is probably induced by the Ca2+-independent MLCK, ZIP kinase. The pathways initiated by G(i)-coupled receptors involve sequential activation of Gbetagamma(i), PI 3-kinase, and the Ca2+-independent MLCK, integrin-linked kinase. The last phosphorylates MLC20 directly and inhibits MLC phosphatase by phosphorylating CPI-17. PKA and PKG, which mediate relaxation, act upstream to desensitize the receptors (VPAC2 and NPR-C), inhibit adenylyl and guanylyl cyclase activities, and stimulate cAMP-specific PDE3 and PDE4 and cGMP-specific PDE5 activities. These kinases also act downstream to inhibit (a) initial contraction by inhibiting Ca2+ mobilization and (b) sustained contraction by inhibiting RhoA and targets downstream of RhoA. This increases MLC phosphatase activity and induces MLC20 dephosphorylation and muscle relaxation.

Metabolic syndrome increases endogenous carbon monoxide production to promote hypertension and endothelial dysfunction in obese Zucker rats

Vascular heme oxygenase (HO) metabolizes heme to form carbon monoxide (CO). Increased heme-derived CO inhibits nitric oxide synthase and can contribute to hypertension via endothelial dysfunction in Dahl salt-sensitive rats. Obese Zucker rats (ZR) are models of metabolic syndrome. This study tests the hypothesis that endogenous CO formation is increased and contributes to hypertension and endothelial dysfunction in obese ZR. Awake obese ZR showed increased respiratory CO excretion, which was lowered by HO inhibitor administration [zinc deuteroporphyrin 2,4-bis glycol (ZnDPBG) 25 micromol.kg(-1).24 h(-1) ip]. In awake obese ZR, chronically instrumented with femoral arterial catheters, blood pressure was elevated but was decreased by the HO inhibitor ZnDPBG. Body weight, blood glucose, glycated hemoglobin, plasma insulin, total and LDL cholesterol, oxidized LDL, and triglyceride levels were elevated in obese ZR, and, except for LDL cholesterol, were unchanged by HO inhibition. Total HO-1 protein levels were not different between lean and obese ZR aortas. In vitro experiments used isolated skeletal muscle arterioles with constant pressure and no flow, or constant midpoint, but altered endpoint pressures to establish graded levels of luminal flow. In obese ZR arterioles, responses to ACh and flow were attenuated. Acute in vitro pretreatment with an HO inhibitor, chromium mesoporphyrin, enhanced ACh and flow-induced dilation and abolished the differences between groups. Furthermore, exogenous CO prevented the restoration of flow-induced dilation by the HO inhibitor in obese ZR arterioles. These results suggest that HO-derived CO production is increased and promotes hypertension and arteriolar endothelial dysfunction in obese ZR with metabolic syndrome independent of affecting metabolic parameters.

Biology of natriuretic peptides and their receptors

Increasing evidence suggests that natriuretic peptides (NPs) play diverse roles in mammals, including renal hemodynamics, neuroendocrine, and cardiovascular functions. Collectively, NPs are classified as hypotensive hormones; the main actions of NPs are implicated in eliciting natriuretic, diuretic, steroidogenic, antiproliferative, and vasorelaxant effects, important factors in the control of body fluid volume and blood pressure homeostasis. One of the principal loci involved in the regulatory actions of NPs is their cognate plasma membrane receptor molecules, which are activated by binding with specific NPs. Interaction of NPs with their receptors plays a central role in physiology and pathophysiology of hypertension and cardiovascular disorders. Gaining insight into the intricacies of NPs-specific receptor signaling pathways is of pivotal importance for understanding both hormone-receptor biology and the disease states arising from abnormal hormone receptor interplay. During the last decade there has been a surge in interest in NP receptors; consequently, a wealth of information has emerged concerning molecular structure and function, signaling mechanisms, and use of transgenics and gene-targeted mouse models. The objective of this present review is to summarize and document the previous findings and recent discoveries in the field of the natriuretic peptide hormone family and receptor systems with emphasis on the structure-function relationship, signaling mechanisms, and the physiological and pathophysiological significance in health and disease.

Neuronal regulation and function of natriuretic peptide receptor C

The neuromodulatory effect of natriuretic peptides is probably the best example of a whole organ response mediated by natriuretic peptide C receptors (NPR-Cs). Both NPR-C specific agonists and ablation experiments utilizing NPR-C specific antibodies or antisense oligonucleotides demonstrated the essential signaling role of the NPR-C in these neuromodulatory responses. Our most recent studies utilize peptides representing the NPR-C intracellular region to elucidate the specific signaling region of the NPR-C. These studies have identified an inhibitory influence of NPR-C on adrenergic neurotransmission by a calcium-dependent process.

Natriuretic peptide receptor-C signaling and regulation

The natriuretic peptides (NP) are a family of three polypeptide hormones termed atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP). ANP regulates a variety of physiological parameters by interacting with its receptors present on the plasma membrane. These are of three subtypes NPR-A, NPR-B, and NPR-C. NPR-A and NPR-B are guanylyl cyclase receptors, whereas NPR-C is non-guanylyl cyclase receptor and is coupled to adenylyl cyclase inhibition or phospholipase C activation through inhibitory guanine nucleotide regulatory protein (Gi). ANP, BNP, CNP, as well as C-ANP(4-23), a ring deleted peptide that specifically interacts with NPR-C receptor inhibit adenylyl cyclase activity through Gi protein. Unlike other G-protein-coupled receptors, NPR-C receptors have a single transmembrane domain and a short cytoplasmic domain of 37 amino acids, which has a structural specificity like those of other single transmembrane domain receptors. A 37 amino acid cytoplasmic peptide is sufficient to inhibit adenylyl cyclase activity with an apparent Ki similar to that of ANP(99-126) or C-ANP(4-23). In addition, C-ANP(4-23) also stimulates phosphatidyl inositol (PI) turnover in vascular smooth muscle cells (VSMC) which is attenuated by dbcAMP and cAMP-stimulatory agonists, suggesting that NPR-C receptor-mediated inhibition of adenylyl cyclase and resultant decreased levels of cAMP may be responsible for NPR-C-mediated stimulation of PI turnover. Furthermore, the activation of NPR-C receptor by C-ANP(4-23) and CNP inhibits the mitogen-activated protein kinase activity stimulated by endothelin-3, platelet-derived growth factor, phorbol-12 myristate 13-acetate, suggesting that NPR-C receptor might also be coupled to other signal transduction system or that there may be an interaction of the NPR-C receptor and some other signaling pathways. In this review article, NPR-C receptor coupling to different signaling pathways and their regulation will be discussed.

A new paradigm for hormone recognition and allosteric receptor activation revealed from structural studies of NPR-C

The natriuretic peptide system of hormones and receptors poses an abundance of interesting biophysical questions regarding receptor structure, hormone recognition, and receptor activation. Functional and biochemical data have implicated a series of conformational changes as the mechanism by which NP receptor activation is achieved. We have explored the structural basis of hormone recognition by the NP clearance receptor, termed NPR-C. While NPR-C does not contain the classical guanylyl-cyclase activity in its intracellular domains, its extracellular domain is highly similar to the GC-coupled members of this family. The 1:2 stoichiometry of hormone binding to NPR-C is also used by NPR-A and -B to bind hormones. The structure of NPR-C in both quiescent and hormone-bound forms reveals the hormone intercalates within the interface of a receptor dimer, inducing a large-scale conformational change in the membrane proximal regions. This mechanism of hormone recognition will be conserved across the entire NPR family. The allosteric response of the NPR-C ectodomain to ligand binding is likely a glimpse of the general activation signal of these receptors, despite their differing downstream signaling cascades. In this review, we discuss our results on NPR-C and their relevance to the NPR family as a whole, as well as its place as a basic new paradigm for receptor activation.

C-type natriuretic peptide inhibits L-type Ca2+ current in rat magnocellular neurosecretory cells by activating the NPR-C receptor

Magnocellular neurosecretory cells (MNCs), of the paraventricular and supraoptic nuclei of the hypothalamus, secrete the hormones vasopressin and oxytocin. As a result, they have an essential role in fundamental physiological responses including regulation of blood volume and fluid homeostasis. C-type natriuretic peptide (CNP) is present at high levels in the hypothalamus. Although CNP is known to decrease hormone secretion from MNCs, no studies have examined the role of the natriuretic peptide C receptor (NPR-C) in these neurons. In this study, whole cell recordings from acutely isolated MNCs, and MNCs in a coronal slice preparation, show that CNP (2 x 10(-8) M) and the selective NPR-C agonist, cANF (2 x 10(-8) M), significantly inhibit L-type Ca2+ current (I(Ca(L))) by approximately 50%. This effect on I(Ca(L)) is mimicked by dialyzing a G(i)-activator peptide (10(-7) M) into these cells, implicating a role for the inhibitory G protein, G(i). These NPR-C-mediated effects were specific to I(Ca(L)). T-type Ca2+ channels were unaffected by CNP. Current-clamp experiments revealed the ability of CNP, acting via the NPR-C receptor, to decrease (approximately 25%) the number of action potentials elicited during a 500 ms depolarizing stimulus. Analysis of action potential duration revealed that CNP and cANF significantly decreased 50% repolarization time (APD50) in MNCs. In summary, our findings show that CNP has a potent and selective inhibitory effect on I(Ca(L)) and on excitability in MNCs that is mediated by the NPR-C receptor. These data represent the first electrophysiological evidence of a functional role for the NPR-C receptor in the mammalian hypothalamus.