Prolonged atrial natriuretic peptide exposure stimulates guanylyl cyclase-a degradation

Natriuretic peptides potentiate cardiac hypertrophic response to noradrenaline in rats

Excessive activation of the sympathetic nervous system is involved in cardiovascular damage including cardiac hypertrophy. Natriuretic peptides are assumed to exert protective actions for the heart, alleviating hypertrophy and/or fibrosis of the myocardium. In contrast to this assumption, we show in the present study that both atrial and C-type natriuretic peptides (ANP and CNP) potentiate cardiac hypertrophic response to noradrenaline (NA) in rats. Nine-week-old male Wistar rats were continuously infused with subcutaneous 30 micro-g/h NA without or with persistent intravenous administration of either 1.0 micro-g/h ANP or CNP for 14 days. Blood pressure (BP) was recorded under an unrestrained condition by a radiotelemetry system. Cardiac hypertrophic response to NA was evaluated by heart weight/body weight (HW/BW) ratio and microscopic measurement of myocyte size of the left ventricle. Mean BP levels at the light and dark cycles rose by about 20mmHg following NA infusion for 14 days, with slight increases in HW/BW ratio and ventricular myocyte size. Infusions of ANP and CNP had no significant effects on mean BP in NA-infused rats, while two natriuretic peptides potentiated cardiac hypertrophic response to NA. Cardiac hypertrophy induced by co-administration of NA and ANP was attenuated by treatment with prazosin or atenolol. In summary, both ANP and CNP potentiated cardiac hypertrophic effect of continuously infused NA in rats, suggesting a possible pro-hypertrophic action of natriuretic peptides on the heart.

Therapeutic Progress and Knowledge Basis on the Natriuretic Peptide System in Heart Failure

Notwithstanding substantial improvements in diagnosis and treatment, Heart Failure (HF) remains a major disease burden with high prevalence and poor outcomes worldwide. Natriuretic Peptides (NPs) modulate whole cardiovascular system and exhibit multiple cardio-protective effects, including the counteraction of the Renin-Angiotensin-Aldosterone System (RAAS) and Sympathetic Nervous System (SNS), promotion of vasodilatation and natriuresis, and inhibition of hypertrophy and fibrosis. Novel pharmacological therapies based on NPs may achieve a valuable shift in managing patients with HF from inhibiting RAAS and SNS to a reversal of neurohormonal imbalance. Enhancing NP bioavailability through exogenous NP administration and inhibiting Neutral Endopeptidase (NEP) denotes valuable therapeutic strategies for HF. On the one hand, NEP-resistant NPs may be more specific as therapeutic choices in patients with HF. On the other hand, NEP Inhibitors (NEPIs) combined with RAAS inhibitors have proved to exert beneficial effects and reduce adverse events in patients with HF. Highly effective and potentially safe Angiotensin Receptor Blocker Neprilysin Inhibitors (ARNIs) have been developed after the failure of NEPIs and Vasopeptidase Inhibitors (VPIs) due to lacking efficacy and safety. Therapeutic progress and knowledge basis on the NP system in HF are summarized in the current review.

Human Atrial Natriuretic Peptide in Cold Storage of Donation After Circulatory Death Rat Livers: An Old but New Agent for Protecting Vascular Endothelia?

BACKGROUND

Current critical shortage of donor organs has increased the use of donation after circulatory death (DCD) livers for transplantation, despite higher risk for primary nonfunction or ischemic cholangiopathy. Human atrial natriuretic peptide (hANP) is a cardiovascular hormone that possesses protective action to vascular endothelia. We aimed to clarify the therapeutic potential of hANP in cold storage of DCD livers.

METHODS

Male Wistar rats were exposed to 30-minute warm ischemia in situ. Livers were then retrieved and cold-preserved for 6 hours with or without hANP supplementation. Functional and morphological integrity of the livers was evaluated by oxygenated ex vivo reperfusion at 37°C.

RESULTS

hANP supplementation resulted in significant reduction of portal venous pressure (12.2 ± 0.5 versus 22.5 ± 3.5 mm Hg, P < 0.001). As underlying mechanisms, hANP supplementation significantly increased tissue adenosine concentration (P = 0.008), resulting in significant upregulation of endothelial nitric oxide synthase and significant downregulation of endothelin-1 (P = 0.01 and P = 0.004 vs. the controls, respectively). Consequently, hANP significantly decreased transaminase release (P < 0.001) and increased bile production (96.2 ± 18.2 versus 36.2 ± 15.2 μL/g-liver/h, P < 0.001). Morphologically, hepatocytes and sinusoidal endothelia were both better maintained by hANP (P = 0.021). Electron microscopy also revealed that sinusoidal ultrastructures and microvilli formation in bile canaliculi were both better preserved by hANP supplementation. Silver staining also demonstrated that hANP significantly preserved reticulin fibers in Disse space (P = 0.017), representing significant protection of sinusoidal frameworks/architectures.

CONCLUSIONS

Supplementation of hANP during cold storage significantly attenuated cold ischemia/warm reperfusion injury of DCD livers.

Emerging concepts of receptor endocytosis and concurrent intracellular signaling: Mechanisms of guanylyl cyclase/natriuretic peptide receptor-A activation and trafficking

Endocytosis is a prominent clathrin-mediated mechanism for concentrated uptake and internalization of ligand-receptor complexes, also known as cargo. Internalization of cargo is the fundamental mechanism for receptor-dependent regulation of cell membrane function, intracellular signal transduction, and neurotransmission, as well as other biological and physiological activities. However, the intrinsic mechanisms of receptor endocytosis and contemporaneous intracellular signaling are not well understood. We review emerging concepts of receptor endocytosis with concurrent intracellular signaling, using a typical example of guanylyl cyclase/natriuretic peptide receptor-A (NPRA) internalization, subcellular trafficking, and simultaneous generation of second-messenger cGMP and signaling in intact cells. We highlight the role of short-signal motifs located in the carboxyl-terminal regions of membrane receptors during their internalization and subsequent receptor trafficking in organelles that are not traditionally studied in this context, including nuclei and mitochondria. This review sheds light on the importance of future investigations of receptor endocytosis and trafficking in live cells and intact animals in vivo in physiological context.

Regulation of the Natriuretic Peptide Receptor 2 (Npr2) by Phosphorylation of Juxtamembrane Serine and Threonine Residues Is Essential for Bifurcation of Sensory Axons

cGMP signaling elicited by activation of the transmembrane receptor guanylyl cyclase Npr2 (also known as guanylyl cyclase B) by the ligand CNP controls sensory axon bifurcation of DRG and cranial sensory ganglion (CSG) neurons entering the spinal cord or hindbrain, respectively. Previous studies have shown that Npr2 is phosphorylated on serine and threonine residues in its kinase homology domain (KHD). However, it is unknown whether phosphorylation of Npr2 is essential for axon bifurcation. Here, we generated a knock-in mouse line in which the seven regulatory serine and threonine residues in the KHD of Npr2 were substituted by alanine (Npr2-7A), resulting in a nonphosphorylatable enzyme. Real-time imaging of cGMP in DRG neurons with a genetically encoded fluorescent cGMP sensor or biochemical analysis of guanylyl cyclase activity in brain or lung tissue revealed the absence of CNP-induced cGMP generation in the Npr2^7A/7A mutant. Consequently, bifurcation of axons, but not collateral formation, from DRG or CSG in this mouse mutant was perturbed at embryonic and mature stages. In contrast, axon branching was normal in a mouse mutant in which constitutive phosphorylation of Npr2 is mimicked by a replacement of all of the seven serine and threonine sites by glutamic acid (Npr2-7E). Furthermore, we demonstrate that the Npr2^7A/7A mutation causes dwarfism as described for global Npr2 mutants. In conclusion, our in vivo studies provide strong evidence that phosphorylation of the seven serine and threonine residues in the KHD of Npr2 is an important regulatory element of Npr2-mediated cGMP signaling which affects physiological processes, such as axon bifurcation and bone growth.SIGNIFICANCE STATEMENT The branching of axons is a morphological hallmark of virtually all neurons. It allows an individual neuron to innervate different targets and to communicate with neurons located in different regions of the nervous system. The natriuretic peptide receptor 2 (Npr2), a transmembrane guanylyl cyclase, is essential for the initiation of bifurcation of sensory axons when entering the spinal cord or the hindbrain. By using two genetically engineered mouse lines, we show that phosphorylation of specific serine and threonine residues in juxtamembrane regions of Npr2 are required for its enzymatic activity and for axon bifurcation. These investigations might help to understand the regulation of Npr2 and its integration in intracellular signaling systems.

Synthesis, secretion, function, metabolism and application of natriuretic peptides in heart failure

As a family of hormones with pleiotropic effects, natriuretic peptide (NP) system includes atrial NP (ANP), B-type NP (BNP), C-type NP (CNP), dendroaspis NP and urodilatin, with NP receptor-A (guanylate cyclase-A), NP receptor-B (guanylate cyclase-B) and NP receptor-C (clearance receptor). These peptides are genetically distinct, but structurally and functionally related for regulating circulatory homeostasis in vertebrates. In humans, ANP and BNP are encoded by NP precursor A (NPPA) and NPPB genes on chromosome 1, whereas CNP is encoded by NPPC on chromosome 2. NPs are synthesized and secreted through certain mechanisms by cardiomyocytes, fibroblasts, endotheliocytes, immune cells (neutrophils, T-cells and macrophages) and immature cells (embryonic stem cells, muscle satellite cells and cardiac precursor cells). They are mainly produced by cardiovascular, brain and renal tissues in response to wall stretch and other causes. NPs provide natriuresis, diuresis, vasodilation, antiproliferation, antihypertrophy, antifibrosis and other cardiometabolic protection. NPs represent body's own antihypertensive system, and provide compensatory protection to counterbalance vasoconstrictor-mitogenic-sodium retaining hormones, released by renin-angiotensin-aldosterone system (RAAS) and sympathetic nervous system (SNS). NPs play central roles in regulation of heart failure (HF), and are inactivated through not only NP receptor-C, but also neutral endopeptidase (NEP), dipeptidyl peptidase-4 and insulin degrading enzyme. Both BNP and N-terminal proBNP are useful biomarkers to not only make the diagnosis and assess the severity of HF, but also guide the therapy and predict the prognosis in patients with HF. Current NP-augmenting strategies include the synthesis of NPs or agonists to increase NP bioactivity and inhibition of NEP to reduce NP breakdown. Nesiritide has been established as an available therapy, and angiotensin receptor blocker NEP inhibitor (ARNI, LCZ696) has obtained extremely encouraging results with decreased morbidity and mortality. Novel pharmacological approaches based on NPs may promote a therapeutic shift from suppressing the RAAS and SNS to re-balancing neuroendocrine dysregulation in patients with HF. The current review discussed the synthesis, secretion, function and metabolism of NPs, and their diagnostic, therapeutic and prognostic values in HF.

Catalytically Active Guanylyl Cyclase B Requires Endoplasmic Reticulum-mediated Glycosylation, and Mutations That Inhibit This Process Cause Dwarfism

C-type natriuretic peptide activation of guanylyl cyclase B (GC-B), also known as natriuretic peptide receptor B or NPR2, stimulates long bone growth, and missense mutations in GC-B cause dwarfism. Four such mutants (L658F, Y708C, R776W, and G959A) bound (125)I-C-type natriuretic peptide on the surface of cells but failed to synthesize cGMP in membrane GC assays. Immunofluorescence microscopy also indicated that the mutant receptors were on the cell surface. All mutant proteins were dephosphorylated and incompletely glycosylated, but dephosphorylation did not explain the inactivation because the mutations inactivated a "constitutively phosphorylated" enzyme. Tunicamycin inhibition of glycosylation in the endoplasmic reticulum or mutation of the Asn-24 glycosylation site decreased GC activity, but neither inhibition of glycosylation in the Golgi by N-acetylglucosaminyltransferase I gene inactivation nor PNGase F deglycosylation of fully processed GC-B reduced GC activity. We conclude that endoplasmic reticulum-mediated glycosylation is required for the formation of an active catalytic, but not ligand-binding domain, and that mutations that inhibit this process cause dwarfism.

Role of FQQI motif in the internalization, trafficking, and signaling of guanylyl-cyclase/natriuretic peptide receptor-A in cultured murine mesangial cells

Binding of the cardiac hormone atrial natriuretic peptide (ANP) to transmembrane guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), produces the intracellular second messenger cGMP in target cells. To delineate the critical role of an endocytic signal in intracellular sorting of the receptor, we have identified a FQQI (Phe(790), Gln(791), Gln(792), and Ile(793)) motif in the carboxyl-terminal region of NPRA. Mouse mesangial cells (MMCs) were transiently transfected with the enhanced green fluorescence protein (eGFP)-tagged wild-type (WT) and mutant constructs of eGFP-NPRA. The mutation FQQI/AAAA, in the eGFP-NPRA cDNA sequence, markedly attenuated the internalization of mutant receptors by almost 49% compared with the WT receptor. Interestingly, we show that the μ1B subunit of adaptor protein-1 binds directly to a phenylalanine-based FQQI motif in the cytoplasmic tail of the receptor. However, subcellular trafficking indicated that immunofluorescence colocalization of the mutated receptor with early endosome antigen-1 (EEA-1), lysosome-associated membrane protein-1 (LAMP-1), and Rab 11 marker was decreased by 57% in early endosomes, 48% in lysosomes, and 42% in recycling endosomes, respectively, compared with the WT receptor in MMCs. The receptor containing the mutated motif (FQQI/AAAA) also produced a significantly decreased level of intracellular cGMP during subcellular trafficking than the WT receptor. The coimmunoprecipitation assay confirmed a decreased level of colocalization of the mutant receptor with subcellular compartments during endocytic processes. The results suggest that the FQQI motif is essential for the internalization and subcellular trafficking of NPRA during the hormone signaling process in intact MMCs.

Subcellular trafficking of guanylyl cyclase/natriuretic peptide receptor-A with concurrent generation of intracellular cGMP

Atrial natriuretic peptide (ANP) modulates blood pressure and fluid volume by activation of natriuretic peptide receptor-A (NPRA). Immunofluorescence (IF) studies reveal that NPRA is internalized and redistributed into subcellular compartments with concurrent production of cGMP.

Atrial natriuretic peptide (ANP) activates guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), which lowers blood pressure and blood volume. The objective of the present study was to visualize internalization and trafficking of enhanced GFP (eGFP)-tagged NPRA (eGFP–NPRA) in human embryonic kidney-293 (HEK-293) cells, using immunofluorescence (IF) and co-immunoprecipitation (co-IP) of eGFP–NPRA. Treatment of cells with ANP initiated rapid internalization and co-localization of the receptor with early endosome antigen-1 (EEA-1), which was highest at 5 min and gradually decreased within 30 min. Similarly, co-localization of the receptor was observed with lysosome-associated membrane protein-1 (LAMP-1); however, after treatment with lysosomotropic agents, intracellular accumulation of the receptor gradually increased within 30 min. Co-IP assays confirmed that the localization of internalized receptors occurred with subcellular organelles during the endocytosis of NPRA. Rab 11, which was used as a recycling endosome (Re) marker, indicated that ∼20% of receptors recycled back to the plasma membrane. ANP-treated cells showed a marked increase in the IF of cGMP, whereas receptor was still trafficking into the intracellular compartments. Thus, after ligand binding, NPRA is rapidly internalized and trafficked from the cell surface into endosomes, Res and lysosomes, with concurrent generation of intracellular cGMP.

Endocytosis and Trafficking of Natriuretic Peptide Receptor-A: Potential Role of Short Sequence Motifs

The targeted endocytosis and redistribution of transmembrane receptors among membrane-bound subcellular organelles are vital for their correct signaling and physiological functions. Membrane receptors committed for internalization and trafficking pathways are sorted into coated vesicles. Cardiac hormones, atrial and brain natriuretic peptides (ANP and BNP) bind to guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA) and elicit the generation of intracellular second messenger cyclic guanosine 3',5'-monophosphate (cGMP), which lowers blood pressure and incidence of heart failure. After ligand binding, the receptor is rapidly internalized, sequestrated, and redistributed into intracellular locations. Thus, NPRA is considered a dynamic cellular macromolecule that traverses different subcellular locations through its lifetime. The utilization of pharmacologic and molecular perturbants has helped in delineating the pathways of endocytosis, trafficking, down-regulation, and degradation of membrane receptors in intact cells. This review describes the investigation of the mechanisms of internalization, trafficking, and redistribution of NPRA compared with other cell surface receptors from the plasma membrane into the cell interior. The roles of different short-signal peptide sequence motifs in the internalization and trafficking of other membrane receptors have been briefly reviewed and their potential significance in the internalization and trafficking of NPRA is discussed.

Suppression of neutrophil superoxide generation by BNP is attenuated in acute heart failure: a case for 'BNP resistance'

AIMS

The release of the B-type natriuretic peptide (BNP) is increased in heart failure (HF), a condition associated with oxidative stress. BNP is known to exert anti-inflammatory effects including suppression of neutrophil superoxide (O2(-)) release. However, BNP-based restoration of homeostasis in HF is inadequate, and the equivocal clinical benefit of a recombinant BNP, nesiritide, raises the possibility of attenuated response to BNP. We therefore tested the hypothesis that BNP-induced suppression of neutrophil O2(-) generation is impaired in patients with acute HF.

METHODS AND RESULTS

We have recently characterized suppression of neutrophil O2(-) generation (PMA- or fMLP-stimulated neutrophil burst) by BNP as a measure of its physiological activity. In the present study, BNP response was compared in neutrophils of healthy subjects (n = 29) and HF patients (n = 45). Effects of BNP on fMLP-induced phosphorylation of the NAD(P)H oxidase subunit p47phox were also evaluated. In acute HF patients, the suppressing effect of BNP (1 µmol/L) on O2(-) generation was attenuated relative to that in healthy subjects (P < 0.05 for both PMA and fMLP). Analogously, BNP inhibited p47phox phosphorylation in healthy subjects but not in HF patients (P < 0.05). However, O2(-)-suppressing effects of the cell-permeable cGMP analogue (8-pCPT-cGMP) were preserved in acute HF. Conventional HF treatment for 5 weeks partially restored neutrophil BNP responsiveness (n = 25, P < 0.05), despite no significant decrease in plasma NT-proBNP levels.

CONCLUSIONS

BNP inhibits neutrophil O2(-) generation by suppressing NAD(P)H oxidase assembly. This effect is impaired in acute HF patients, with partial recovery during treatment.

A human skeletal overgrowth mutation increases maximal velocity and blocks desensitization of guanylyl cyclase-B

C-type natriuretic peptide (CNP) increases long bone growth by stimulating guanylyl cyclase (GC)-B/NPR-B/NPR2. Recently, a Val to Met missense mutation at position 883 in the catalytic domain of GC-B was identified in humans with increased blood cGMP levels that cause abnormally long bones. Here, we determined how this mutation activates GC-B. In the absence of CNP, cGMP levels in cells expressing V883M-GC-B were increased more than 20 fold compared to cells expressing wild-type (WT)-GC-B, and the addition of CNP only further increased cGMP levels 2-fold. In the absence of CNP, maximal enzymatic activity (Vmax) of V883M-GC-B was increased 15-fold compared to WT-GC-B but the affinity of the enzymes for substrate as revealed by the Michaelis constant (Km) was unaffected. Surprisingly, CNP decreased the Km of V883M-GC-B 10-fold in a concentration-dependent manner without increasing Vmax. Unlike the WT enzyme the Km reduction of V883M-GC-B did not require ATP. Unexpectedly, V883M-GC-B, but not WT-GC-B, failed to inactivate with time. Phosphorylation elevated but was not required for the activity increase associated with the mutation because the Val to Met substitution also activated a GC-B mutant lacking all known phosphorylation sites. We conclude that the V883M mutation increases maximal velocity in the absence of CNP, eliminates the requirement for ATP in the CNP-dependent Km reduction, and disrupts the normal inactivation process.

Epidermal growth factor receptor signaling-dependent calcium elevation in cumulus cells is required for NPR2 inhibition and meiotic resumption in mouse oocytes

In preovulatory ovarian follicles, the oocyte is maintained in meiotic prophase arrest by natriuretic peptide precursor C (NPPC) and its receptor natriuretic peptide receptor 2 (NPR2). LH treatment results in the decrease of NPR2 guanylyl cyclase activity that promotes resumption of meiosis. We investigated the regulatory mechanism of LH-activated epidermal growth factor (EGF) receptor signaling on NPR2 function. Cumulus cell-oocyte complex is cultured in the medium with 30 nM NPPC to prevent oocyte spontaneous maturation. In this system, EGF could stimulate oocyte meiotic resumption after 4 hours of incubation. Further study showed that EGF elevated intracellular calcium concentrations of cumulus cells and decreased cGMP levels in cumulus cells and oocytes, and calcium-elevating reagents ionomycin and sphingosine-1-phosphate mimicked the effects of EGF on oocyte maturation and cGMP levels. EGF-mediated cGMP levels and meiotic resumption could be reversed by EGF receptor inhibitor AG1478 and the calcium chelator bis-(o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid, tetra(acetoxymethyl)-ester. EGF also decreased the expression of Npr2 mRNA in cumulus cells, which may not be involved in meiotic resumption, because the block of NPR2 protein de novo synthesis by cycloheximide had no effect on NPPC and EGF-mediated oocyte maturation. However, EGF had no effect on oocyte maturation when meiotic arrest was maintained in the present of cGMP analog 8-bromoadenosine-cGMP. These results suggest that EGF receptor signaling induces meiotic resumption by elevating calcium concentrations of cumulus cells to decrease NPR2 guanylyl cyclase activity.

Luteinizing hormone reduces the activity of the NPR2 guanylyl cyclase in mouse ovarian follicles, contributing to the cyclic GMP decrease that promotes resumption of meiosis in oocytes

In preovulatory ovarian follicles of mice, meiotic prophase arrest in the oocyte is maintained by cyclic GMP from the surrounding granulosa cells that diffuses into the oocyte through gap junctions. The cGMP is synthesized in the granulosa cells by the transmembrane guanylyl cyclase natriuretic peptide receptor 2 (NPR2) in response to the agonist C-type natriuretic peptide (CNP). In response to luteinizing hormone (LH), cGMP in the granulosa cells decreases, and as a consequence, oocyte cGMP decreases and meiosis resumes. Here we report that within 20 min, LH treatment results in decreased guanylyl cyclase activity of NPR2, as determined in the presence of a maximally activating concentration of CNP. This occurs by a process that does not reduce the amount of NPR2 protein. We also show that by a slower process, first detected at 2h, LH decreases the amount of CNP available to bind to the receptor. Both of these LH actions contribute to decreasing cGMP in the follicle, thus signaling meiotic resumption in the oocyte.

Guanylyl cyclase (GC)-A and GC-B activities in ventricles and cardiomyocytes from failed and non-failed human hearts: GC-A is inactive in the failed cardiomyocyte

Cardiomyocytes release atrial natriuretic peptide (ANP) and B-type natriuretic peptide to stimulate processes that compensate for the failing heart by activating guanylyl cyclase (GC)-A. C-type natriuretic peptide is also elevated in the failing heart and inhibits cardiac remodeling by activating the homologous receptor, GC-B. We previously reported that GC-A is the most active membrane GC in normal mouse ventricles while GC-B is the most active membrane GC in failing ventricles due to increased GC-B and decreased GC-A activities. Here, we examined ANP and CNP-specific GC activity in membranes obtained from non-failing and failing human left ventricles and in membranes from matched cardiomyocyte-enriched pellet preparations. Similar to our findings in the murine study, we found that CNP-dependent GC activity was about half of the ANP-dependent GC activity in the non-failing ventricular and was increased in the failing ventricle. ANP and CNP increased GC activity 9- and 5-fold in non-failing ventricles, respectively. In contrast to the mouse study, in failing human ventricles, ANP-dependent activity was unchanged compared to non-failing values whereas CNP-dependent activity increased 35% (p=0.005). Compared with ventricular membranes, basal GC activity was reduced an order of magnitude in membranes derived from myocyte-enriched pellets from non-failing ventricles. ANP increased GC activity 2.4-fold but CNP only increased GC activity 1.3-fold. In contrast, neither ANP nor CNP increased GC activity in equivalent preparations from failing ventricles. We conclude that: 1) GC-B activity is increased in non-myocytes from failing human ventricles, possibly as a result of increased fibrosis, 2) human ventricular cardiomyocytes express low levels of GC-A and much lower levels or possibly no GC-B, and 3) GC-A in cardiomyocytes from failing human hearts is refractory to ANP stimulation.

ATP potentiates competitive inhibition of guanylyl cyclase A and B by the staurosporine analog, Gö6976: reciprocal regulation of ATP and GTP binding

Natriuretic peptides and ATP activate and Gö6976 inhibits guanylyl cyclase (GC)-A and GC-B. Here, the mechanism of inhibition was determined. Gö6976 progressively increased the Michaelis-Menten constant and decreased the Hill coefficient without reducing the maximal velocity of GC-A and GC-B. In the presence of 1 mm ATP, the K(i) was 1 μm for both enzymes. Inhibition of GC-B was minimal in the absence of ATP, and 1 mm ATP increased the inhibition 4-fold. In a reciprocal manner, 10 μm Gö6976 increased the potency of ATP for GC-B 4-fold. In contrast to a recent study (Duda, T., Yadav, P., and Sharma, R. K. (2010) FEBS J. 277, 2550-2553), neither staurosporine nor Gö6976 activated GC-A or GC-B. This is the first study to show that Gö6976 reduces GTP binding and the first demonstration of a competitive inhibitor of a receptor guanylyl cyclase. We conclude that Gö6976 reduces GTP binding to the catalytic site of GC-A and GC-B and that ATP increases the magnitude of the inhibition.

Antibody tracking demonstrates cell type-specific and ligand-independent internalization of guanylyl cyclase a and natriuretic peptide receptor C

Atrial natriuretic peptide (ANP) binds guanylyl cyclase-A (GC-A) and natriuretic peptide receptor-C (NPR-C). Internalization of GC-A and NPR-C is poorly understood, in part, because previous studies used (125)I-ANP binding to track these receptors, which are expressed in the same cell. Here, we evaluated GC-A and NPR-C internalization using traditional and novel approaches. Although HeLa cells endogenously express GC-A, (125)I-ANP binding and cross-linking studies only detected NPR-C, raising the possibility that past studies ascribed NPR-C-mediated processes to GC-A. To specifically measure internalization of a single receptor, we developed an (125)I-IgG-binding assay that tracks extracellular FLAG-tagged versions of GC-A and NPR-C independently of each other and ligand for the first time. FLAG-GC-A bound ANP identically with wild-type GC-A and was internalized slowly (0.5%/min), whereas FLAG-NPR-C was internalized rapidly (2.5%/min) in HeLa cells. In 293 cells, (125)I-ANP and (125)I-IgG uptake curves were superimposable because these cells only express a single ANP receptor. Basal internalization of both receptors was 8-fold higher in 293 compared with HeLa cells and ANP did not increase internalization of FLAG-GC-A. For FLAG-NPR-C, neither ANP, BNP, nor CNP increased its internalization in either cell line. Prolonged ANP exposure concomitantly reduced surface and total GC-A levels, consistent with rapid exchange of extracellular and intracellular receptor pools. We conclude that ligand binding does not stimulate natriuretic peptide receptor internalization and that cellular environment determines the rate of this process. We further deduce that NPR-C is internalized faster than GC-A and that increased internalization is not required for GC-A down-regulation.

Regulation and therapeutic targeting of peptide-activated receptor guanylyl cyclases

Cyclic GMP is a ubiquitous second messenger that regulates a wide array of physiologic processes such as blood pressure, long bone growth, intestinal fluid secretion, phototransduction and lipolysis. Soluble and single-membrane-spanning enzymes called guanylyl cyclases (GC) synthesize cGMP. In humans, the latter group consists of GC-A, GC-B, GC-C, GC-E and GC-F, which are also known as NPR-A, NPR-B, StaR, Ret1-GC and Ret2-GC, respectively. Membrane GCs are activated by peptide ligands such as atrial natriuretic peptide (ANP), B-type natriuretic peptide (BNP), C-type natriuretic peptide (CNP), guanylin, uroguanylin, heat stable enterotoxin and GC-activating proteins. Nesiritide and carperitide are clinically approved peptide-based drugs that activate GC-A. CD-NP is an experimental heart failure drug that primarily activates GC-B but also activates GC-A at high concentrations and is resistant to degradation. Inactivating mutations in GC-B cause acromesomelic dysplasia type Maroteaux dwarfism and chromosomal mutations that increase CNP concentrations are associated with Marfanoid-like skeletal overgrowth. Pump-based CNP infusions increase skeletal growth in a mouse model of the most common type of human dwarfism, which supports CNP/GC-B-based therapies for short stature diseases. Linaclotide is a peptide activator of GC-C that stimulates intestinal motility and is in late-stage clinical trials for the treatment of chronic constipation. This review discusses the discovery of cGMP, guanylyl cyclases, the general characteristics and therapeutic applications of GC-A, GC-B and GC-C, and emphasizes the regulation of transmembrane guanylyl cyclases by phosphorylation and ATP.