Phosphorylation-Dependent Regulation of Guanylyl Cyclase (GC)-A and Other Membrane GC Receptors

Receptor guanylyl cyclases (GCs) are single membrane spanning, multidomain enzymes, that synthesize cGMP in response to natriuretic peptides or other ligands. They are evolutionarily conserved from sea urchins to humans and regulate diverse physiologies. Most family members are phosphorylated on four to seven conserved serines or threonines at the beginning of their kinase homology domains. This review describes studies that demonstrate that phosphorylation and dephosphorylation are required for activation and inactivation of these enzymes, respectively. Phosphorylation sites in GC-A, GC-B, GC-E and sea urchin receptors are discussed as are mutant receptors that mimic the dephosphorylated, inactive or phosphorylated, active forms of GC-A and GC-B, respectively. A salt bridge model is described that explains why phosphorylation is required for enzyme activation. Potential kinases, phosphatases and ATP regulation of GC receptors are also discussed. Critically, knock-in mice with glutamate substitutions for receptor phosphorylation sites are described. The inability of opposing signaling pathways to inhibit cGMP synthesis in mice where GC-A or GC-B cannot be dephosphorylated demonstrates the necessity of receptor dephosphorylation in vivo. Cardiac hypertrophy, oocyte meiosis, long bone growth/achondroplasia, and bone density are regulated by GC phosphorylation, but additional processes are likely to be identified in the future.

Novel enhancers of guanylyl cyclase-A activity acting via allosteric modulation

BACKGROUND AND PURPOSE

Guanylyl cyclase-A (GC-A), activated by endogenous atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), plays an important role in the regulation of cardiovascular and renal homeostasis and is an attractive drug target. Even though small molecule modulators allow oral administration and longer half-life, drug targeting of GC-A has so far been limited to peptides. Thus, in this study we aimed to develop small molecular activators of GC-A.

EXPERIMENTAL APPROACH

Hits were identified through high-throughput screening and optimized by in silico design. Cyclic GMP was measured in QBIHEK293A cells expressing GC-A, GC-B or chimerae of the two receptors using AlphaScreen technology. Binding assays were performed in membrane preparations or whole cells using 125 I-ANP. Vasorelaxation was measured in aortic rings isolated from Wistar rats.

KEY RESULTS

We have identified small molecular allosteric enhancers of GC-A, which enhanced ANP or BNP effects in cellular systems and ANP-induced vasorelaxation in rat aortic rings. The mechanism of action appears novel and not mediated through previously described allosteric binding sites. In addition, the selectivity and activity depend on a single amino acid residue that differs between the two similar receptors GC-A and GC-B.

CONCLUSION AND IMPLICATIONS

We describe a novel allosteric binding site on GC-A, which can be targeted by small molecules to enhance ANP and BNP effects. These compounds will be valuable tools in further development and proof-of-concept of GC-A enhancement for the potential use in cardiovascular therapy.

The Concise Guide to PHARMACOLOGY 2023/24: Catalytic receptors

The Concise Guide to PHARMACOLOGY 2023/24 is the sixth in this series of biennial publications. The Concise Guide provides concise overviews, mostly in tabular format, of the key properties of approximately 1800 drug targets, and nearly 6000 interactions with about 3900 ligands. There is an emphasis on selective pharmacology (where available), plus links to the open access knowledgebase source of drug targets and their ligands (https://www.guidetopharmacology.org/), which provides more detailed views of target and ligand properties. Although the Concise Guide constitutes almost 500 pages, the material presented is substantially reduced compared to information and links presented on the website. It provides a permanent, citable, point-in-time record that will survive database updates. The full contents of this section can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.16180. Catalytic receptors are one of the six major pharmacological targets into which the Guide is divided, with the others being: G protein-coupled receptors, ion channels, nuclear hormone receptors, enzymes and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. The landscape format of the Concise Guide is designed to facilitate comparison of related targets from material contemporary to mid-2023, and supersedes data presented in the 2021/22, 2019/20, 2017/18, 2015/16 and 2013/14 Concise Guides and previous Guides to Receptors and Channels. It is produced in close conjunction with the Nomenclature and Standards Committee of the International Union of Basic and Clinical Pharmacology (NC-IUPHAR), therefore, providing official IUPHAR classification and nomenclature for human drug targets, where appropriate.

Vicinal glutamates are better phosphomimetics: Phosphorylation is required for allosteric activation of guanylyl cyclase-A

Multisite phosphorylation of guanylyl cyclase (GC)-A, also known as NPR-A or NPR1, is required for receptor activation by natriuretic peptides (NPs) because alanine substitutions for the first four GC-A phosphorylation sites produce an enzyme that cannot be stimulated by NPs. In contrast, single Glu substitutions for the first six chemically identified GC-A phosphorylation sites to mimic the negative charge of phosphate produced an enzyme that is activated by NPs but had an elevated Michaelis constant (Km), resulting in low activity. Here, we show that vicinal (double adjacent) Glu substitutions for the same sites to mimic the two negative charges of phosphate produced a near wild type (WT) enzyme with a low Km. Unlike the enzyme with single glutamate substitutions, the vicinally substituted enzyme did not require the functionally identified Ser-473-Glu substitution to achieve WT-like activity. Importantly, the negative charge associated with either phosphorylation or glutamate substitutions was required for allosteric activation of GC-A by ATP. We conclude that vicinal Glu substitutions are better phosphomimetics than single Glu substitutions and that phosphorylation is required for allosteric activation of GC-A in the absence and presence of NP. Finally, we suggest that the putative functionally identified phosphorylation sites, Ser-473 in GC-A and Ser-489 in GC-B, are not phosphorylation sites at all.

Guanylyl Cyclase-B Dependent Bone Formation in Mice is Associated with Youth, Increased Osteoblasts, and Decreased Osteoclasts

C-type natriuretic peptide (CNP) activation of guanylyl cyclase-B (GC-B) catalyzes the synthesis of cGMP in chondrocytes and osteoblasts. Elevated cGMP stimulates long bone growth, and inactivating mutations in CNP or GC-B reduce cGMP, which causes dwarfism. GC-B^7E/7E mice that express a GC-B mutant that cannot be inactivated by dephosphorylation exhibit increased CNP-dependent GC-B activity, which increases bone length, as well as bone mass and strength. Importantly, how GC-B increases bone mass is not known. Here, we injected 12-week-old, wild type mice once daily for 28 days with or without BMN-111 (Vosoritide), a proteolytically resistant CNP analog. We found that BMN-111 treated mice had elevated levels of osteocalcin and collagen 1 C-terminal telopeptide (CTX) as well as increased osteoblasts and osteoclasts. In BMN-111 injected mice, tibial mRNAs for Rank ligand and osteoprotegrin were increased and decreased, respectively, whereas sclerostin mRNA was elevated 400-fold, consistent with increased osteoclast activity and decreased osteoblast activity. Mineral apposition rates and trabecular bone mass were not elevated in response to BMN-111. Because 9-week-old male GC-B^7E/7E mice have increased bone mass but do not exhibit increased mineral apposition rates, we examined 4-week-old male GC-B^7E/7E mice and found that these animals had increased serum osteocalcin, but not CTX. Importantly, tibias from these mice had 37% more osteoblasts, 26% fewer osteoclasts as well as 36% and 40% higher mineral apposition and bone formation rates, respectively. We conclude that GC-B-dependent bone formation is coupled to an early juvenile process that requires both increased osteoblasts and decreased osteoclasts.

Epitope-tagged and phosphomimetic mouse models for investigating natriuretic peptide-stimulated receptor guanylyl cyclases

The natriuretic peptide receptors NPR1 and NPR2, also known as guanylyl cyclase A and guanylyl cyclase B, have critical functions in many signaling pathways, but much remains unknown about their localization and function in vivo. To facilitate studies of these proteins, we developed genetically modified mouse lines in which endogenous NPR1 and NPR2 were tagged with the HA epitope. To investigate the role of phosphorylation in regulating NPR1 and NPR2 guanylyl cyclase activity, we developed mouse lines in which regulatory serines and threonines were substituted with glutamates, to mimic the negative charge of the phosphorylated forms (NPR1-8E and NPR2-7E). Here we describe the generation and applications of these mice. We show that the HA-NPR1 and HA-NPR2 mice can be used to characterize the relative expression levels of these proteins in different tissues. We describe studies using the NPR2-7E mice that indicate that dephosphorylation of NPR2 transduces signaling pathways in ovary and bone, and studies using the NPR1-8E mice that indicate that the phosphorylation state of NPR1 is a regulator of heart, testis, and adrenal function.

Guanylyl cyclase-A phosphorylation decreases cardiac hypertrophy and improves systolic function in male, but not female, mice

Atrial natriuretic peptide (NP) and BNP increase cGMP, which reduces blood pressure and cardiac hypertrophy by activating guanylyl cyclase (GC)-A, also known as NPR-A or Npr1. Although GC-A is highly phosphorylated, and dephosphorylation inactivates the enzyme, the significance of GC-A phosphorylation to heart structure and function remains unknown. To identify in vivo processes that are regulated by GC-A phosphorylation, we substituted glutamates for known phosphorylation sites to make GC-A^8E/8E mice that express an enzyme that cannot be inactivated by dephosphorylation. GC-A activity, but not protein, was increased in heart and kidney membranes from GC-A^8E/8E mice. Activities were threefold higher in female compared to male cardiac ventricles. Plasma cGMP and testosterone were elevated in male and female GC-A^8E/8E mice, but aldosterone was only increased in mutant male mice. Plasma and urinary creatinine concentrations were decreased and increased, respectively, but blood pressure and heart rate were unchanged in male GC-A^8E/8E mice. Heart weight to body weight ratios for GC-A^8E/8E male, but not female, mice were 12% lower with a 14% reduction in cardiomyocyte cross-sectional area. Subcutaneous injection of fsANP, a long-lived ANP analog, increased plasma cGMP and decreased aldosterone in male GC-A^WT/WT and GC-A^8E/8E mice at 15 min, but only GC-A^8E/8E mice had elevated levels of plasma cGMP and aldosterone at 60 min. fsANP reduced ventricular ERK1/2 phosphorylation to a greater extent and for a longer time in the male mutant compared to WT mice. Finally, ejection fractions were increased in male but not female hearts from GC-A^8E/8E mice. We conclude that increased phosphorylation-dependent GC-A activity decreases cardiac ERK activity, which results in smaller male hearts with improved systolic function.

THE CONCISE GUIDE TO PHARMACOLOGY 2021/22: Catalytic receptors

The Concise Guide to PHARMACOLOGY 2021/22 is the fifth in this series of biennial publications. The Concise Guide provides concise overviews, mostly in tabular format, of the key properties of nearly 1900 human drug targets with an emphasis on selective pharmacology (where available), plus links to the open access knowledgebase source of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. Although the Concise Guide constitutes over 500 pages, the material presented is substantially reduced compared to information and links presented on the website. It provides a permanent, citable, point-in-time record that will survive database updates. The full contents of this section can be found at http://onlinelibrary.wiley.com/doi/bph.15541. Catalytic receptors are one of the six major pharmacological targets into which the Guide is divided, with the others being: G protein-coupled receptors, ion channels, nuclear hormone receptors, enzymes and transporters. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. The landscape format of the Concise Guide is designed to facilitate comparison of related targets from material contemporary to mid-2021, and supersedes data presented in the 2019/20, 2017/18, 2015/16 and 2013/14 Concise Guides and previous Guides to Receptors and Channels. It is produced in close conjunction with the Nomenclature and Standards Committee of the International Union of Basic and Clinical Pharmacology (NC-IUPHAR), therefore, providing official IUPHAR classification and nomenclature for human drug targets, where appropriate.

Prevention of guanylyl cyclase-B dephosphorylation rescues achondroplastic dwarfism

Activating mutations in the fibroblast growth factor receptor 3 (FGFR3) or inactivating mutations in guanylyl cyclase-B (GC-B), also known as NPR-B or Npr2, cause short-limbed dwarfism. FGFR3 activation causes dephosphorylation and inactivation of GC-B, but the contribution of GC-B dephosphorylation to achondroplasia (ACH) is unknown. GC-B7E/7E mice that express a glutamate-substituted version of GC-B that cannot be inactivated by dephosphorylation were bred with mice expressing FGFR3-G380R, the most common human ACH mutation, to determine if GC-B dephosphorylation is required for ACH. Crossing GC-B7E/7E mice with FGFR3G380R/G380R mice increased naso-anal and long (tibia and femur), but not cranial, bone length twice as much as crossing GC-B7E/7E mice with FGFR3WT/WT mice from 4 to 16 weeks of age. Consistent with increased GC-B activity rescuing ACH, long bones from the GC-B7E/7E/FGFR3G380R/G380R mice were not shorter than those from GC-BWT/WT/FGFR3WT/WT mice. At 2 weeks of age, male but not female FGFR3G380R/G380R mice had shorter long bones and smaller growth plate hypertrophic zones, whereas female but not male GC-B7E/7E mice had longer bones and larger hypertrophic zones. In 2-week-old males, crossing FGFR3G380R/G380R mice with GC-B7E/7E mice increased long bone length and hypertrophic zone area to levels observed in mice expressing WT versions of both receptors. We conclude that preventing GC-B dephosphorylation rescues reduced axial and appendicular skeleton growth in a mouse model of achondroplasia.

Receptor guanylyl cyclase (RGC) family (version 2020.3) in the IUPHAR/BPS Guide to Pharmacology Database

The mammalian genome encodes seven guanylyl cyclases, GC-A to GC-G, that are homodimeric transmembrane receptors activated by a diverse range of endogenous ligands. These enzymes convert guanosine-5'-triphosphate to the intracellular second messenger cyclic guanosine-3',5'-monophosphate (cyclic GMP). GC-A, GC-B and GC-C are expressed predominantly in the cardiovascular system, skeletal system and intestinal epithelium, respectively. GC-D and GC-G are found in the olfactory neuropepithelium and Grueneberg ganglion of rodents, respectively. GC-E and GC-F are expressed in retinal photoreceptors.

Male mice with elevated C-type natriuretic peptide-dependent guanylyl cyclase-B activity have increased osteoblasts, bone mass and bone strength

C-type natriuretic peptide (CNP) activation of guanylyl cyclase (GC)-B, also known as NPR2, stimulates cGMP synthesis and bone elongation. CNP activation requires the phosphorylation of multiple GC-B residues and dephosphorylation inactivates the receptor. GC-B7E/7E knockin mice, expressing a glutamate-substituted, "pseudophosphorylated," form of GC-B, exhibit increased CNP-dependent GC activity. Since mutations that constitutively activate GC-B in the absence of CNP result in low bone mineral density in humans, we determined the skeletal phenotype of 9-week old male GC-B7E/7E mice. Unexpectedly, GC-B7E/7E mice have significantly greater tibial and L5 vertebral trabecular bone volume fraction, tibial trabecular number, and tibial bone mineral density. Cortical cross-sectional area, cortical thickness, periosteal diameter and cortical cross-sectional moment of inertia were also significantly increased in GC-B7E/7E tibiae. Three-point bending measurements demonstrated that the mutant tibias and femurs had greater ultimate load, stiffness, energy to ultimate load, and energy to failure. No differences in microhardness indicated similar bone quality at the tissue level between the mutant and wildtype bones. Procollagen 1 N-terminal propeptide and osteocalcin were elevated in serum, and osteoblast number per bone perimeter and osteoid width per bone perimeter were elevated in tibias from the mutant mice. In contrast to mutations that constitutively activate GC-B, we report that mutations that enhance GC-B activity only in the presence of its natural ligand, increase bone mass, bone strength, and the number of active osteoblasts at the bone surface.

The pseudokinase domains of guanylyl cyclase-A and -B allosterically increase the affinity of their catalytic domains for substrate

Natriuretic peptides regulate multiple physiologic systems by activating transmembrane receptors containing intracellular guanylyl cyclase domains, such as GC-A and GC-B, also known as Npr1 and Npr2, respectively. Both enzymes contain an intracellular, phosphorylated pseudokinase domain (PKD) critical for activation of the C-terminal cGMP-synthesizing guanylyl cyclase domain. Because ATP allosterically activates GC-A and GC-B, we investigated how ATP binding to the PKD influenced guanylyl cyclase activity. Molecular modeling indicated that all the residues of the ATP-binding site of the prototypical kinase PKA, except the catalytic aspartate, are conserved in the PKDs of GC-A and GC-B. Kinase-inactivating alanine substitutions for the invariant lysine in subdomain II or the aspartate in the DYG-loop of GC-A and GC-B failed to decrease enzyme phosphate content, consistent with the PKDs lacking kinase activity. In contrast, both mutations reduced enzyme activation by blocking the ability of ATP to decrease the Michaelis constant without affecting peptide-dependent activation. The analogous lysine-to-alanine substitution in a glutamate-substituted phosphomimetic mutant form of GC-B also reduced enzyme activity, consistent with ATP stimulating guanylyl cyclase activity through an allosteric, phosphorylation-independent mechanism. Mutations designed to rigidify the conserved regulatory or catalytic spines within the PKDs increased guanylyl cyclase activity, increased sensitivity to natriuretic peptide, or reduced the Michaelis constant in the absence of ATP, consistent with ATP binding stabilizing the PKD in a conformation analogous to that of catalytically active kinases. We conclude that allosteric mechanisms evolutionarily conserved in the PKDs promote the catalytic activation of transmembrane guanylyl cyclases.

Dephosphorylation of the NPR2 guanylyl cyclase contributes to inhibition of bone growth by fibroblast growth factor

Activating mutations in fibroblast growth factor (FGF) receptor 3 and inactivating mutations in the NPR2 guanylyl cyclase both cause severe short stature, but how these two signaling systems interact to regulate bone growth is poorly understood. Here, we show that bone elongation is increased when NPR2 cannot be dephosphorylated and thus produces more cyclic GMP. By developing an in vivo imaging system to measure cyclic GMP production in intact tibia, we show that FGF-induced dephosphorylation of NPR2 decreases its guanylyl cyclase activity in growth plate chondrocytes in living bone. The dephosphorylation requires a PPP-family phosphatase. Thus FGF signaling lowers cyclic GMP production in the growth plate, which counteracts bone elongation. These results define a new component of the signaling network by which activating mutations in the FGF receptor inhibit bone growth.

Between birth and puberty, the bones of mammals grow drastically in length. This process is controlled by many proteins, and mutations affecting these proteins can cause bones to either be too long or too short. For example, mutations of a protein called the fibroblast growth factor receptor, or FGF for short, and a protein called NPR2, can cause similar forms of dwarfism – a condition characterized by short stature.

The FGF protein controls bone growth, and people with overactive receptors for FGF suffer from a form of dwarfism known as achondroplasia, while people that lack FGF receptors have longer bones. The NPR2 protein, on the other hand, produces a molecule called cGMP, which is necessary for the bones to grow. When NPR2 is blocked, less cGMP is produced, which results in shorter limbs.

Previous studies of bone cells grown in the laboratory have shown that these two proteins are linked by a chain of chemical messages. When the FGF receptor is active, phosphate molecules are removed from the NPR2 protein, which reduces the amount of GMP produced. However, until now it was not known whether this mechanism also controls growth in actual bones.

Here, Shuhaibar et al. used genetically modified mice in which the phosphate group could not be removed from their NPR2 enzyme. As a result, the bones of these mice were longer than usual. Shuhaibar et al. then developed an imaging technique to examine the region in the bone were growth happens. To see whether FGF reduces the amount of cGMP produced by NPR2 in these areas, cGMP was detected with a fluorescent sensor in order to be tracked.

In normal mice, the FGF receptor reduced the rate at which cGMP was produced, but in mice with mutated NPR2, this did not happen. When the cells could not remove the phosphates from NPR2, cGMP levels stayed high and the bones grew longer.

These findings reveal new insights into the molecular causes of dwarfism. The next step will be to identify the enzyme responsible for removing phosphate from NPR2. Blocking its activity could help to enhance bone growth. In the future, this could lead to new drug treatments for achondroplasia.

Skeletal overgrowth-causing mutations mimic an allosterically activated conformation of guanylyl cyclase-B that is inhibited by 2,4,6,-trinitrophenyl ATP

Activating mutations in the receptor for C-type natriuretic peptide (CNP), guanylyl cyclase B (GC-B, also known as Npr2 or NPR-B), increase cellular cGMP and cause skeletal overgrowth, but how these mutations affect GTP catalysis is poorly understood. The A488P and R655C mutations were compared with the known mutation V883M. Neither mutation affected GC-B concentrations. The A488P mutation decreased the EC₅₀ 5-fold, increased V_max 2.6-fold, and decreased the K_m 13-fold, whereas the R655C mutation decreased the EC₅₀ 5-fold, increased the V_max 2.1-fold, and decreased the K_m 4.7-fold. Neither mutation affected maximum activity at saturating CNP concentrations. Activation by R655C did not require disulfide bond formation. Surprisingly, the A488P mutant only activated the receptor when it was phosphorylated. In contrast, the R655C mutation converted GC-B-7A from CNP-unresponsive to CNP-responsive. Interestingly, neither mutant was activated by ATP, and the K_m and Hill coefficient of each mutant assayed in the absence of ATP were similar to those of wild-type GC-B assayed in the presence of ATP. Finally, 1 mm 2,4,6,-trinitrophenyl ATP inhibited all three mutants by as much as 80% but failed to inhibit WT-GC-B. We conclude that 1) the A488P and R655C missense mutations result in a GC-B conformation that mimics the allosterically activated conformation, 2) GC-B phosphorylation is required for CNP-dependent activation by the A488P mutation, 3) the R655C mutation abrogates the need for phosphorylation in receptor activation, and 4) an ATP analog selectively inhibits the GC-B mutants, indicating that a pharmacologic approach could reduce GC-B dependent human skeletal overgrowth.

A Glutamate-Substituted Mutant Mimics the Phosphorylated and Active Form of Guanylyl Cyclase-A

Multisite phosphorylation is required for activation of guanylyl cyclase (GC)-A, also known as NPR-A or NPR1, by cardiac natriuretic peptides (NPs). Seven chemically identified sites (Ser-487, Ser-497, Thr-500, Ser-502, Ser-506, Ser-510, and Thr-513) and one functionally identified putative site (Ser-473) were reported. Single alanine substitutions for Ser-497, Thr-500, Ser-502, Ser-506, and Ser-510 reduced maximal velocity (V_max), whereas glutamate substitutions had no effect or increased V_max Ala but not Glu substitution for Ser-497 increased the Michaelis constant (K_m) approximately 400%. A GC-A mutant containing Glu substitutions for all seven chemically identified sites (GC-A-7E) had a K_m approximately 10-fold higher than phosphorylated wild-type (WT) GC-A, but one additional substitution for Ser-473 to make GC-A-8E resulted in the same V_max, K_m, and EC₅₀ as the phosphorylated WT enzyme. Adding more glutamates to make GC-A-9E or GC-A-10E had little effect on activity, and sequential deletion of individual glutamates in GC-A-8E progressively increased the K_m Double Ala substitutions for Ser-497 and either Thr-500, Ser-510 or Thr-513 in WT-GC-A increased the K_m 23- to 70-fold but the same mutations in GC-A-8E only increased the K_m 8-fold, consistent with one site affecting the phosphorylation of other sites. Phosphate measurements confirmed that single-site Ala substitutions reduced receptor phosphate levels more than expected for the loss of a single site. We conclude that a concentrated region of negative charge, not steric properties, resulting from multiple interdependent phosphorylation sites is required for a GC-A conformation capable of transmitting the hormone binding signal to the catalytic domain.

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.

Heterozygous mutations in natriuretic peptide receptor-B (NPR2) gene as a cause of short stature

Based on the observation of reduced stature in relatives of patients with acromesomelic dysplasia, Maroteaux type (AMDM), caused by homozygous or compound heterozygous mutations in natriuretic peptide receptor-B gene (NPR2), it has been suggested that heterozygous mutations in this gene could be responsible for the growth impairment observed in some cases of idiopathic short stature (ISS). We enrolled 192 unrelated patients with short stature and 192 controls of normal height and identified seven heterozygous NPR2 missense or splice site mutations all in the short stature patients, including one de novo splice site variant. Three of the six inherited variants segregated with short stature in the family. Nine additional rare nonsynonymous NPR2 variants were found in three additional cohorts. Functional studies identified eight loss-of-function mutations in short individuals and one gain-of-function mutation in tall individuals. With these data, we were able to rigorously verify that NPR2 functional haploinsufficiency contributes to short stature. We estimate a prevalence of NPR2 haploinsufficiency of between 0 and 1/26 in people with ISS. We suggest that NPR2 gain of function may be a more common cause of tall stature than previously recognized.

Dephosphorylation and inactivation of NPR2 guanylyl cyclase in granulosa cells contributes to the LH-induced decrease in cGMP that causes resumption of meiosis in rat oocytes

In mammals, the meiotic cell cycle of oocytes starts during embryogenesis and then pauses. Much later, in preparation for fertilization, oocytes within preovulatory follicles resume meiosis in response to luteinizing hormone (LH). Before LH stimulation, the arrest is maintained by diffusion of cyclic (c)GMP into the oocyte from the surrounding granulosa cells, where it is produced by the guanylyl cyclase natriuretic peptide receptor 2 (NPR2). LH rapidly reduces the production of cGMP, but how this occurs is unknown. Here, using rat follicles, we show that within 10 min, LH signaling causes dephosphorylation and inactivation of NPR2 through a process that requires the activity of phosphoprotein phosphatase (PPP)-family members. The rapid dephosphorylation of NPR2 is accompanied by a rapid phosphorylation of the cGMP phosphodiesterase PDE5, an enzyme whose activity is increased upon phosphorylation. Later, levels of the NPR2 agonist C-type natriuretic peptide decrease in the follicle, and these sequential events contribute to the decrease in cGMP that causes meiosis to resume in the oocyte.

Regulation of intraocular pressure by soluble and membrane guanylate cyclases and their role in glaucoma

Glaucoma is a progressive optic neuropathy characterized by visual field defects that ultimately lead to irreversible blindness (Alward, 2000; Anderson et al., 2006). By the year 2020, an estimated 80 million people will have glaucoma, 11 million of which will be bilaterally blind. Primary open-angle glaucoma (POAG) is the most common type of glaucoma. Elevated intraocular pressure (IOP) is currently the only risk factor amenable to treatment. How IOP is regulated and can be modulated remains a topic of active investigation. Available therapies, mostly geared toward lowering IOP, offer incomplete protection, and POAG often goes undetected until irreparable damage has been done, highlighting the need for novel therapeutic approaches, drug targets, and biomarkers (Heijl et al., 2002; Quigley, 2011). In this review, the role of soluble (nitric oxide (NO)-activated) and membrane-bound, natriuretic peptide (NP)-activated guanylate cyclases that generate the secondary signaling molecule cyclic guanosine monophosphate (cGMP) in the regulation of IOP and in the pathophysiology of POAG will be discussed.

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.

Guanylyl cyclases A and B are asymmetric dimers that are allosterically activated by ATP binding to the catalytic domain

It is not known how natriuretic peptides and adenosine triphosphate (ATP) activate guanylyl cyclase A (GC-A) and GC-B, which generate the second messenger cyclic guanosine monophosphate. We determined that natriuretic peptides increased the maximum rate of these enzymes >10-fold in a positive cooperative manner in the absence of ATP. In the absence of natriuretic peptides, ATP shifted substrate-velocity profiles from cooperative to linear but did not increase the affinity of GCs for the substrate guanosine triphosphate (GTP) since the Michaelis constant was unchanged. However, in the presence of natriuretic peptides, ATP competed with GTP for binding to an allosteric site, which enhanced the activation of GCs by decreasing the Michaelis constant. Thus, natriuretic peptide binding was required for communication of the allosteric activation signal to the catalytic site. The ability of ATP to activate GCs decreased and enzyme potency (a measure of sensitivity to stimulation) increased with increasing GTP concentrations. Point mutations in the purine-binding site of the catalytic domain abolished GC activity but not allosteric activation. Coexpression of inactive mutants produced half the activity expected for symmetric catalytic dimers. 2'-Deoxy-ATP and 2'-deoxy-GTP were poor allosteric activators, but 2'-deoxy-GTP was an effective substrate, consistent with distinct binding requirements for the allosteric and catalytic sites. We conclude that membrane GC domains are asymmetric homodimers with distinct and reciprocally regulated catalytic and allosteric sites that bind to GTP and ATP, respectively. These data define a new membrane GC activation model and provide evidence of a previously unidentified GC drug interaction site.

A functional screen provides evidence for a conserved, regulatory, juxtamembrane phosphorylation site in guanylyl cyclase a and B

Kinase homology domain (KHD) phosphorylation is required for activation of guanylyl cyclase (GC)-A and -B. Phosphopeptide mapping identified multiple phosphorylation sites in GC-A and GC-B, but these approaches have difficulty identifying sites in poorly detected peptides. Here, a functional screen was conducted to identify novel sites. Conserved serines or threonines in the KHDs of phosphorylated receptor GCs were mutated to alanine and tested for reduced hormone to detergent activity ratios. Mutation of Ser-489 in GC-B to alanine but not glutamate reduced the activity ratio to 60% of wild type (WT) levels. Similar results were observed with Ser-473, the homologous site in GC-A. Receptors containing glutamates for previously identified phosphorylation sites (GC-A-6E and GC-B-6E) were activated to ∼20% of WT levels but the additional glutamate substitution for S473 or S489 increased activity to near WT levels. Substrate-velocity assays indicated that GC-B-WT-S489E and GC-B-6E-S489E had lower Km values and that WT-GC-B-S489A, GC-B-6E and GC-B-6E-S489A had higher Km values than WT-GC-B. Homologous desensitization was enhanced when GC-A contained the S473E substitution, and GC-B-6E-S489E was resistant to inhibition by a calcium elevating treatment or protein kinase C activation – processes that dephosphorylate GC-B. Mass spectrometric detection of a synthetic phospho-Ser-473 containing peptide was 200–1300-fold less sensitive than other phosphorylated peptides and neither mass spectrometric nor ³²PO₄ co-migration studies detected phospho-Ser-473 or phospho-Ser-489 in cells. We conclude that Ser-473 and Ser-489 are Km-regulating phosphorylation sites that are difficult to detect using current methods.

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.

The indolocarbazole, Gö6976, inhibits guanylyl cyclase-A and -B

BACKGROUND AND PURPOSE

Atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) decrease vascular volume and pressure by activating guanylyl cyclase-A (GC-A). C-type natriuretic peptide (CNP) activation of guanylyl cyclase-B (GC-B) stimulates long bone growth. This study investigated the effects of the indolocarbazole, Gö6976, on the guanylyl cyclase activity of GC-A and GC-B as a first step towards developing small molecule regulators of these enzymes.

EXPERIMENTAL APPROACH

Whole cell cGMP concentrations or ³²P-cGMP accumulation in membrane preparations measured the effects of indolocarbazoles on the enzymatic activity GC-A and GC-B from transfected 293T or endogenously expressing 3T3-L1 cells.

KEY RESULTS

Gö6976 blocked cellular CNP-dependent cGMP elevations in 293T-GC-B cells. The t(½) for Gö6976 inhibition was 7 s and IC₅₀ was 380 nM. Gö6976 increased the EC₅₀ for CNP 4.5-fold, but increasing the CNP concentration did not overcome the inhibition. Half of the inhibition was lost 1 h after removal of Gö6976 from the medium. Cellular exposure to Gö6976 reduced basal and natriuretic peptide-dependent, but not detergent-dependent, GC-A and GC-B activity. Inhibition was also observed when Gö6976 was added directly to the cyclase assay. A constitutively phosphorylated form of GC-B was similarly inhibited.

CONCLUSIONS AND IMPLICATIONS

These data demonstrate that Gö6976 potently, rapidly and reversibly inhibited GC-A and GC-B via a process that did not require intact cells, known phosphorylation sites or inactivation of all catalytic sites. This is the first report of an intracellular inhibitor of a transmembrane guanylyl cyclase and the first report of a non-kinase target for Gö6976.

Guanylyl cyclase structure, function and regulation

Nitric oxide, bicarbonate, natriuretic peptides (ANP, BNP and CNP), guanylins, uroguanylins and guanylyl cyclase activating proteins (GCAPs) activate a family of enzymes variously called guanyl, guanylyl or guanylate cyclases that catalyze the conversion of guanosine triphosphate to cyclic guanosine monophosphate (cGMP) and pyrophosphate. Intracellular cyclic GMP is a second messenger that modulates: platelet aggregation, neurotransmission, sexual arousal, gut peristalsis, blood pressure, long bone growth, intestinal fluid secretion, lipolysis, phototransduction, cardiac hypertrophy and oocyte maturation. This review briefly discusses the discovery of cGMP and guanylyl cyclases, then nitric oxide, nitric oxide synthase and soluble guanylyl cyclase are described in slightly greater detail. Finally, the structure, function, and regulation of the individual mammalian single membrane-spanning guanylyl cyclases GC-A, GC-B, GC-C, GC-D, GC-E, GC-F and GC-G are described in greatest detail as determined by biochemical, cell biological and gene-deletion studies.

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.

ProBNP(1-108) is resistant to degradation and activates guanylyl cyclase-A with reduced potency

BACKGROUND

B-type natriuretic peptide (BNP) compensates for the failing heart and is synthesized as a 108-residue prohormone that is cleaved to a 32-residue C-terminal maximally active peptide. During heart failure, serum concentrations of proBNP(1-108) exceed concentrations of BNP(1-32). The aim of this study was to determine why the proBNP(1-108)/BNP(1-32) ratio increases and whether proBNP(1-108) is bioactive.

METHODS

Using cGMP elevation and (125)I-ANP binding assays, we measured binding and activation of individual human natriuretic peptide receptor populations by recombinant human proBNP(1-108) and human synthetic BNP(1-32). Using receptor bioassays, we measured degradation of recombinant proBNP(1-108) and BNP(1-32) by human kidney membranes.

RESULTS

ProBNP(1-108) stimulated guanylyl cyclase-A (GC-A) to near-maximum activities but was 13-fold less potent than BNP(1-32). ProBNP(1-108) bound human GC-A 35-fold less tightly than BNP(1-32). Neither proBNP(1-108) nor BNP(1-32) activated GC-B. The natriuretic peptide clearance receptor bound proBNP(1-108) 3-fold less tightly than BNP(1-32). The half time for degradation of proBNP(1-108) by human kidney membranes was 2.7-fold longer than for BNP(1-32), and the time required for complete degradation was 6-fold longer. BNP(1-32) and proBNP(1-108) were best fitted by first- and second-order exponential decay models, respectively.

CONCLUSIONS

ProBNP(1-108) activates GC-A with reduced potency and is resistant to degradation. Reduced degradation of proBNP(1-108) may contribute to the increased ratio of serum proBNP(1-108) to BNP(1-32) observed in patients with congestive heart failure.

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.

Natriuretic peptide metabolism, clearance and degradation

Atrial natriuretic peptide, B-type natriuretic peptide and C-type natriuretic peptide constitute a family of three structurally related, but genetically distinct, signaling molecules that regulate the cardiovascular, skeletal, nervous, reproductive and other systems by activating transmembrane guanylyl cyclases and elevating intracellular cGMP concentrations. This review broadly discusses the general characteristics of natriuretic peptides and their cognate signaling receptors, and then specifically discusses the tissue-specific metabolism of natriuretic peptides and their degradation by neprilysin, insulin-degrading enzyme, and natriuretic peptide receptor-C.

Dendroaspis natriuretic peptide and the designer natriuretic peptide, CD-NP, are resistant to proteolytic inactivation

Designer natriuretic peptides (NPs) represent an active area of drug development. In canine and human studies, the designer natriuretic peptide CD-NP demonstrated more desirable therapeutic potential than recombinant B-type NP (BNP), which is known as nesiritide and is approved for treatment of acute decompensated heart failure. However, why CD-NP is more effective than BNP is not known. We previously reported that CD-NP is a poorer activator of human guanylyl cyclase-A (GC-A) and a better activator of human guanylyl cyclase-B than BNP. Here, guanylyl cyclase bioassays were used to compare the susceptibility of CD-NP verses ANP, BNP, CNP and DNP to inactivation by human kidney membranes. The half time (t(1/2)) for CD-NP inactivation was increased by factors of 13, 3 and 4 compared to ANP, BNP and CNP, respectively, when measured in the same assay. Surprisingly, DNP failed to undergo complete inactivation and was the most degradation resistant of the peptides tested. The neutral endopeptidase (NEP) inhibitor, phosphoramidon, blocked inactivation of CNP and CD-NP, but not BNP or DNP. In contrast, the general serine and cysteine protease inhibitor, leupeptin, completely blocked the degradation of BNP and CD-NP, but did not block CNP inactivation unless phosphoramidon was included in the assay. Thus, NPs with shorter carboxyl tails (ANP and CNP) are degraded by phosphoramidon-sensitive proteases and NPs with extended carboxyl tails (BNP, DNP and CD-NP) are resistant to NEP degradation and degraded by leupeptin-sensitive proteases. We conclude that DNP and CD-NP are highly resistant to proteolysis and that proteolytic resistance contributes to the beneficial cardiovascular properties of CD-NP. We suggest that this property may be exploited to increase the half-life of NP-based drugs.

Insulin-degrading enzyme modulates the natriuretic peptide-mediated signaling response

Natriuretic peptides (NPs) are cyclic vasoactive peptide hormones with high therapeutic potential. Three distinct NPs (ANP, BNP, and CNP) can selectively activate natriuretic peptide receptors, NPR-A and NPR-B, raising the cyclic GMP (cGMP) levels. Insulin-degrading enzyme (IDE) was found to rapidly cleave ANP, but the functional consequences of such cleavages in the cellular environment and the molecular mechanism of recognition and cleavage remain unknown. Here, we show that reducing expression levels of IDE profoundly alters the response of NPR-A and NPR-B to the stimulation of ANP, BNP, and CNP in cultured cells. IDE rapidly cleaves ANP and CNP, thus inactivating their ability to raise intracellular cGMP. Conversely, reduced IDE expression enhances the stimulation of NPR-A and NPR-B by ANP and CNP, respectively. Instead of proteolytic inactivation, IDE cleavage can lead to hyperactivation of BNP toward NPR-A. Conversely, decreasing IDE expression reduces BNP-mediated signaling. Additionally, the cleavages of ANP and BNP by IDE render them active with NPR-B and a reduction of IDE expression diminishes the ability of ANP and BNP to stimulate NPR-B. Our kinetic and crystallographic analyses offer the molecular basis for the selective degradation of NPs and their variants by IDE. Furthermore, our studies reveal how IDE utilizes its catalytic chamber and exosite to engulf and bind up to two NPs leading to biased stochastic, non-sequential cleavages and the ability of IDE to switch its substrate selectivity. Thus, the evolutionarily conserved IDE may play a key role in modulating and reshaping the strength and duration of NP-mediated signaling.

Mass spectrometric identification of phosphorylation sites in guanylyl cyclase A and B

Guanylyl cyclase A and B (GC-A and GC-B) are transmembrane guanylyl cyclase receptors that mediate the physiologic effects of natriuretic peptides. Some sites of phosphorylation are known for rat GC-A and GC-B, but no phosphorylation site information is available for the human homologues. Here, we used mass spectrometry to identify phosphorylation sites in GC-A and GC-B from both species. Tryptic digests of receptors purified from HEK293 cells were separated and analyzed by nLC-MS-MS. Seven sites of phosphorylation were identified in rat GC-A (S497, T500, S502, S506, S510, T513, and S487), and all of these sites except S510 and T513 were observed in human GC-A. Six phosphorylation sites were identified in rat GC-B (S513, T516, S518, S523, S526, and T529), and all six sites were also identified in human GC-B. Five sites are identical between GC-A and GC-B. S487 in GC-A and T529 in GC-B are novel, uncharacterized sites. Substitution of alanine for S487 did not affect initial ligand-dependent GC-A activity, but a glutamate substitution reduced activity 20%. Similar levels of ANP-dependent desensitization were observed for the wild-type, S487A, and S487E forms of GC-A. Substitution of glutamate or alanine for T529 increased or decreased ligand-dependent cyclase activity of GC-B, respectively, and T529E increased cyclase activity in a GC-B mutant containing glutamates for all five previously identified sites as well. In conclusion, we identified and characterized new phosphorylation sites in GC-A and GC-B and provide the first evidence of phosphorylation sites within human guanylyl cyclases.

Human B-type natriuretic peptide is not degraded by meprin A

B-type natriuretic peptide (BNP) combats cardiac stress by reducing blood pressure and ventricular fibrosis. Human BNP is inactivated by unknown cell surface proteases. N-terminal cleavage of mouse BNP by the renal protease meprin A was reported to increase inactivating degradation by a second protease named neprilysin. Since the sequence surrounding the meprin A cleavage site in BNP differs between species, we tested whether meprin A degrades human BNP. Using a recently developed proteolytic bioassay, the ability of various protease inhibitors to block the inactivation of BNP was measured. In rat kidney membranes, inhibitors of meprin A or neprilysin partially or completely blocked inactivation of rat BNP(1-32) when added individually or in combination, respectively. In contrast, neither inhibitor alone or in combination prevented the inactivation of human BNP(1-32) by human kidney membranes. Leupeptin, a serine protease inhibitor, totally blocked inactivation of human BNP by human membranes, substantially blocked the inactivation of rat BNP(1-32) by human membranes, but had no effect on the inactivation of rat BNP(1-32) by rat kidney membranes. Purified neprilysin reduced the bioactivity of rat BNP(1-32) and human BNP. Digestion with both meprin and neprilysis caused the greatest reduction in rat BNP(1-32) but had no effect on the bioactivity of human BNP(1-32). We conclude that meprin A does not degrade BNP in humans and should not be considered a pharmacologic target of the natriuretic peptide system.

Arg13 of B-type natriuretic Peptide reciprocally modulates binding to guanylyl cyclase but not clearance receptors

B-type natriuretic peptide (BNP) decreases cardiac preload and hypertrophy. As such, synthetic BNP, nesiritide, was approved for the treatment of acutely decompensated heart failure. However, two problems limit its therapeutic potential. First, ensuing hypertension decreases urine output, and second, guanylyl cyclase-A (GC-A), the primary signaling receptor for BNP, is down-regulated in heart failure. Thus, alternative or chimeric natriuretic peptides maintaining the renal but lacking the vasorelaxation properties of BNP provide an alternative approach. Here, we examined the ability of single amino acid substitutions in the conserved 17-amino acid disulfide ring structure of human BNP to activate GC-A and guanylyl cyclase-B (GC-B), which is not reduced in heart failure. We hypothesized that substitution of highly conserved residues in BNP with highly conserved residues from a GC-B-specific peptide would yield BNP variants with increased and decreased potency for human GC-B and GC-A, respectively. Substitution of Leu for Arg13 (l-bnp) yielded a 5-fold more potent activator of GC-B and 7-fold less potent activator of GC-A compared with wild type. l-bnp also bound GC-A 4.5-fold less tightly than wild type. In contrast, substitution of Met for Ser21 (M-BNP) had no effect. A peptide containing both the Leu and Met substitutions behaved similarly to l-bnp. Meanwhile, wild-type and l-bnp bound the natriuretic peptide clearance receptor with similar affinities. These data indicate that Arg13 of BNP is a critical discriminator of binding to guanylyl cyclase-linked but not clearance natriuretic peptide receptors, supporting designer natriuretic peptides as an alternative to wild-type BNP for the treatment of heart failure.

Prolonged atrial natriuretic peptide exposure stimulates guanylyl cyclase-a degradation

Natriuretic peptide receptor-A (NPR-A), also known as guanylyl cyclase-A, is a transmembrane receptor guanylyl cyclase that is activated by the cardiac hormones atrial natriuretic peptide and B-type natriuretic peptide. Although ligand-dependent NPR-A degradation (also known as down-regulation) is widely acknowledged in human and animal models of volume overload, down-regulation in cultured cells is controversial. Here, we examined the effect of ANP exposure on cellular NPR-A levels as a function of time. Relative receptor concentrations were estimated using guanylyl cyclase and immunoblot assays in a wide variety of cell lines that endogenously or exogenously expressed low or high numbers of receptors. ANP exposures of 1 h markedly reduced hormone-dependent but not detergent-dependent guanylyl cyclase activities in membranes from exposed cells. However, 1-h ANP exposures did not significantly reduce NPR-A concentrations in any cell line. In contrast, exposures of greater than 1 h reduced receptor concentrations in a time-dependent manner. The time required for half of the receptors to be degraded (t(1/2)) in primary bovine aortic endothelial and immortalized HeLa cells was approximately 8 h. In contrast, a 24-h exposure of ANP to 293T cells stably overexpressing NPR-A caused less than half of the receptors to be degraded. To our knowledge, this is the first report to directly measure NPR-A down-regulation in endogenously expressing cells. We conclude that down-regulation is a universal property of NPR-A but is relatively slow and varies with receptor expression levels and cell type.

A familial mutation renders atrial natriuretic Peptide resistant to proteolytic degradation

A heterozygous frameshift mutation causing a 12-amino acid extension to the C terminus of atrial natriuretic peptide (ANP) was recently genetically linked to patients with familial atrial fibrillation (Hodgson-Zingman, D. M., Karst, M. L., Zingman, L. V., Heublein, D. M., Darbar, D., Herron, K. J., Ballew, J. D., de Andrade, M., Burnett, J. C., Jr., and Olson, T. M. (2008) N. Engl. J. Med. 359, 158-165). The frameshift product (fsANP), but not wild-type ANP (wtANP), was elevated in the serum of affected patients, but the molecular basis for the elevated peptide concentrations was not determined. Here, we measured the ability of fsANP to interact with natriuretic peptide receptors and to be proteolytically degraded. fsANP and wtANP bound and activated human NPR-A and NPR-C similarly, whereas fsANP had a slightly increased efficacy for human NPR-B. Proteolytic susceptibility was addressed with novel bioassays that measure the time required for kidney membranes or purified neutral endopeptidase to abolish ANP-dependent activation of NPR-A. The half-life of fsANP was markedly greater than that of wtANP in both assays. Additional membrane proteolysis studies indicated that wtANP and fsANP are preferentially degraded by neutral endopeptidase and serine peptidases, respectively. These data indicate that the familial ANP mutation associated with atrial fibrillation has only minor effects on natriuretic peptide receptor interactions but markedly modifies peptide proteolysis.

Natriuretic peptides: their structures, receptors, physiologic functions and therapeutic applications

Natriuretic peptides are a family of three structurally related hormone/ paracrine factors. Atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) are secreted from the cardiac atria and ventricles, respectively. ANP signals in an endocrine and paracrine manner to decrease blood pressure and cardiac hypertrophy. BNP acts locally to reduce ventricular fibrosis. C-type natriuretic peptide (CNP) primarily stimulates long bone growth but likely serves unappreciated functions as well. ANP and BNP activate the transmembrane guanylyl cyclase, natriuretic peptide receptor-A (NPR-A). CNP activates a related cyclase, natriuretic peptide receptor-B (NPR-B). Both receptors catalyze the synthesis of cGMP, which mediates most known effects of natriuretic peptides. A third natriuretic peptide receptor, natriuretic peptide receptor-C (NPR-C), clears natriuretic peptides from the circulation through receptor-mediated internalization and degradation. However, a signaling function for the receptor has been suggested as well. Targeted disruptions of the genes encoding all natriuretic peptides and their receptors have been generated in mice, which display unique physiologies. A few mutations in these proteins have been reported in humans. Synthetic analogs of ANP (anaritide and carperitide) and BNP (nesiritide) have been investigated as potential therapies for the treatment of decompensated heart failure and other diseases. Anaritide and nesiritide are approved for use in acute decompensated heart failure, but recent studies have cast doubt on their safety and effectiveness. New clinical trials are examining the effect of nesiritide and novel peptides, like CD-NP, on these critical parameters. In this review, the history, structure, function, and clinical applications of natriuretic peptides and their receptors are discussed.

Novel bifunctional natriuretic peptides as potential therapeutics

Synthetic atrial natriuretic peptide (carperitide) and B-type natriuretic peptide (BNP; nesiritide) are used to treat congestive heart failure. However, despite beneficial cardiac unloading properties, reductions in renal perfusion pressures limit their clinical effectiveness. Recently, CD-NP, a chimeric peptide composed of C-type natriuretic peptide (CNP) fused to the C-terminal tail of Dendroaspis natriuretic peptide (DNP), was shown to be more glomerular filtration rate-enhancing than BNP in dogs. However, the molecular basis for the increased responsiveness was not determined. Here, we show that the DNP tail has a striking effect on CNP, converting it from a non-agonist to a partial agonist of natriuretic peptide receptor (NPR)-A while maintaining the ability to activate NPR-B. This effect is specific for human receptors because CD-NP was only a slightly better activator of rat NPR-A due to the promiscuous nature of CNP in this species. Interesting, the DNP tail alone had no effect on any NPR even though it is effective in vivo. To further increase the potency of CD-NP for NPR-A, we converted two different triplet sequences within the CNP ring to their corresponding residues in BNP. Both variants demonstrated increased affinity and full agonist activity for NPR-A, whereas one was as potent as any NPR-A activator known. In contrast to a previous report, we found that DNP binds the natriuretic peptide clearance receptor (NPR-C). However, none of the chimeric peptides bound NPR-C with significantly higher affinity than endogenous ligands. We suggest that bifunctional chimeric peptides represent a new generation of natriuretic peptide therapeutics.

Reduced ability of C-type natriuretic peptide (CNP) to activate natriuretic peptide receptor B (NPR-B) causes dwarfism in lbab -/- mice

C-type natriuretic peptide (CNP) stimulates endochondrial ossification by activating the transmembrane guanylyl cyclase, natriuretic peptide receptor-B (NPR-B). Recently, a spontaneous autosomal recessive mutation that causes severe dwarfism in mice was identified. The mutant, called long bone abnormality (lbab), contains a single point mutation that converts an arginine to a glycine in a conserved coding region of the CNP gene, but how this mutation affects CNP activity has not been reported. Here, we determined that 30-fold to greater than 100-fold more CNP(lbab) was required to activate NPR-B as compared to wild-type CNP in whole cell cGMP elevation and membrane guanylyl cyclase assays. The reduced ability of CNP(lbab) to activate NPR-B was explained, at least in part, by decreased binding since 10-fold more CNP(lbab) than wild-type CNP was required to compete with [125I][Tyr0]CNP for receptor binding. Molecular modeling suggested that the conserved arginine is critical for binding to an equally conserved acidic pocket in NPR-B. These results indicate that reduced binding to and activation of NPR-B causes dwarfism in lbab(-/-) mice.