Constitutive activation and uncoupling of the atrial natriuretic peptide receptor by mutations at the dimer interface. Role of the dimer structure in signalling

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

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

Natriuretic peptides and neprilysin inhibition in hypertension and hypertensive organ damage

The family of natriuretic peptides (NPs) discovered in mammalian tissues including cardiac atrium and brain consists of three members, namely, atrial, B- and C-type natriuretic peptides (ANP, BNP, CNP). Since the discovery, basic and clinical studies have been vigorously performed to explore the biological functions and pathophysiological roles of NPs in a wide range of diseases including hypertension and heart failure. These studies revealed that ANP and BNP are hormones secreted from the heart into the blood stream in response to pre- or after-load, counteracting blood pressure (BP) elevation and fluid retention through specific receptors. Meanwhile, CNP was found to be produced by the vascular endothelium, acting as a local mediator potentially serving protective functions for the blood vessels. Because NPs not only exert blood pressure lowering actions but also alleviate hypertensive organ damage, attempts have been made to develop therapeutic agents for hypertension by utilizing this family of NPs. One strategy is to inhibit neprilysin, an enzyme degrading NPs, thereby enhancing the actions of endogenous peptides. Recently, a dual inhibitor of angiotensin receptor-neprilysin was approved for heart failure, and neprilysin inhibition has also been shown to be beneficial in treating patients with hypertension. This review summarizes the roles of NPs in regulating BP, with special references to hypertension and hypertensive organ damage, and discusses the therapeutic implications of neprilysin inhibition.

An Activating Deletion Variant in the Submembrane Region of Natriuretic Peptide Receptor-B Causes Tall Stature

CONTEXT

C-type natriuretic peptide (CNP) is critically involved in endochondral bone growth. Variants in the genes encoding CNP or its cyclic guanosine monophosphate (cGMP)-forming receptor (natriuretic peptide receptor-B [NPR-B], gene NPR2) cause monogenic growth disorders. Here we describe a novel gain-of-function variant of NPR-B associated with tall stature and macrodactyly of the great toes (epiphyseal chondrodysplasia, Miura type).

DESIGN

History and clinical characteristics of 3 family members were collected. NPR2 was selected for sequencing. Skin fibroblasts and transfected HEK-293 cells were used to compare mutant versus wild-type NPR-B activities. Homology modeling was applied to understand the molecular consequences of the variant.

RESULTS

Mother's height was +2.77 standard deviation scores (SDS). The heights of her 2 daughters were +1.96 SDS at 7 years and +1.30 SDS at 4 years of age. Skeletal surveys showed macrodactyly of the great toes and pseudo-epiphyses of the mid- and proximal phalanges. Sequencing identified a novel heterozygous variant c.1444_1449delATGCTG in exon 8 of NPR2, predicted to result in deletion of 2 amino acids Met482-Leu483 within the submembrane region of NPR-B. In proband's skin fibroblasts, basal cGMP levels and CNP-stimulated cGMP production were markedly increased compared with controls. Consistently, assays with transfected HEK-293 cells showed markedly augmented baseline and ligand-dependent activity of mutant NPR-B.

CONCLUSIONS

We report the second activating variant within the intracellular submembrane region of NPR-B resulting in tall stature and macrodactyly. Our functional and modeling studies suggest that this domain plays a critical role in the baseline conformation and ligand-dependent structural rearrangement of NPR-B required for cGMP production.

Molecular Physiology of Membrane Guanylyl Cyclase Receptors

cGMP controls many cellular functions ranging from growth, viability, and differentiation to contractility, secretion, and ion transport. The mammalian genome encodes seven transmembrane guanylyl cyclases (GCs), GC-A to GC-G, which mainly modulate submembrane cGMP microdomains. These GCs share a unique topology comprising an extracellular domain, a short transmembrane region, and an intracellular COOH-terminal catalytic (cGMP synthesizing) region. GC-A mediates the endocrine effects of atrial and B-type natriuretic peptides regulating arterial blood pressure/volume and energy balance. GC-B is activated by C-type natriuretic peptide, stimulating endochondral ossification in autocrine way. GC-C mediates the paracrine effects of guanylins on intestinal ion transport and epithelial turnover. GC-E and GC-F are expressed in photoreceptor cells of the retina, and their activation by intracellular Ca(2+)-regulated proteins is essential for vision. Finally, in the rodent system two olfactorial GCs, GC-D and GC-G, are activated by low concentrations of CO2and by peptidergic (guanylins) and nonpeptidergic odorants as well as by coolness, which has implications for social behaviors. In the past years advances in human and mouse genetics as well as the development of sensitive biosensors monitoring the spatiotemporal dynamics of cGMP in living cells have provided novel relevant information about this receptor family. This increased our understanding of the mechanisms of signal transduction, regulation, and (dys)function of the membrane GCs, clarified their relevance for genetic and acquired diseases and, importantly, has revealed novel targets for therapies. The present review aims to illustrate these different features of membrane GCs and the main open questions in this field.

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.

Natriuretic peptide receptor A as a novel target for cancer

The receptor for the cardiac hormone atrial natriuretic peptide (ANP), natriuretic peptide receptor A (NPR-A), has been reported to be expressed in lung cancer, prostate cancer and ovarian cancer. NPR-A expression and signaling is important for tumor growth; its deficiency protects C57BL/6 mice from lung, skin and ovarian cancers. This suggests that NPR-A is a new marker and a new target for cancer therapy. Recently, NPR-A has been demonstrated to be expressed in pre-implantation embryos and in embryonic stem cells, which has a novel role in the maintenance of self-renewal and pluripotency of embryonic stem cells. A nanoparticle-formulated interfering RNA for NPR-A attenuated B16 melanoma tumors in mice. Ectopic expression of a plasmid encoding NP73-102, the NH2-terminal peptide of the ANP prohormone which downregulates NPR-A expression, also suppressed lung metastasis of A549 cells in nude mice and tumorigenesis of Line 1 cells in immunocompetent BALB/c mice. These results suggest that NPR-A is involved in tumorigenesis and a new target for cancer therapy. This review focuses on structure, abnormal functions and carcinogenic mechanisms of NPR-A to investigate its role in tumorigenesis.

Structure, signaling mechanism and regulation of the natriuretic peptide receptor guanylate cyclase

Atrial natriuretic peptide (ANP) and the homologous B-type natriuretic peptide are cardiac hormones that dilate blood vessels and stimulate natriuresis and diuresis, thereby lowering blood pressure and blood volume. ANP and B-type natriuretic peptide counterbalance the actions of the renin-angiotensin-aldosterone and neurohormonal systems, and play a central role in cardiovascular regulation. These activities are mediated by natriuretic peptide receptor-A (NPRA), a single transmembrane segment, guanylyl cyclase (GC)-linked receptor that occurs as a homodimer. Here, we present an overview of the structure, possible chloride-mediated regulation and signaling mechanism of NPRA and other receptor GCs. Earlier, we determined the crystal structures of the NPRA extracellular domain with and without bound ANP. Their structural comparison has revealed a novel ANP-induced rotation mechanism occurring in the juxtamembrane region that apparently triggers transmembrane signal transduction. More recently, the crystal structures of the dimerized catalytic domain of green algae GC Cyg12 and that of cyanobacterium GC Cya2 have been reported. These structures closely resemble that of the adenylyl cyclase catalytic domain, consisting of a C1 and C2 subdomain heterodimer. Adenylyl cyclase is activated by binding of G(s)α to C2 and the ensuing 7° rotation of C1 around an axis parallel to the central cleft, thereby inducing the heterodimer to adopt a catalytically active conformation. We speculate that, in NPRA, the ANP-induced rotation of the juxtamembrane domains, transmitted across the transmembrane helices, may induce a similar rotation in each of the dimerized GC catalytic domains, leading to the stimulation of the GC catalytic activity.

Reversibly bound chloride in the atrial natriuretic peptide receptor hormone-binding domain: possible allosteric regulation and a conserved structural motif for the chloride-binding site

The binding of atrial natriuretic peptide (ANP) to its receptor requires chloride, and it is chloride concentration dependent. The extracellular domain (ECD) of the ANP receptor (ANPR) contains a chloride near the ANP-binding site, suggesting a possible regulatory role. The bound chloride, however, is completely buried in the polypeptide fold, and its functional role has remained unclear. Here, we have confirmed that chloride is necessary for ANP binding to the recombinant ECD or the full-length ANPR expressed in CHO cells. ECD without chloride (ECD(-)) did not bind ANP. Its binding activity was fully restored by bromide or chloride addition. A new X-ray structure of the bromide-bound ECD is essentially identical to that of the chloride-bound ECD. Furthermore, bromide atoms are localized at the same positions as chloride atoms both in the apo and in the ANP-bound structures, indicating exchangeable and reversible halide binding. Far-UV CD and thermal unfolding data show that ECD(-) largely retains the native structure. Sedimentation equilibrium in the absence of chloride shows that ECD(-) forms a strongly associated dimer, possibly preventing the structural rearrangement of the two monomers that is necessary for ANP binding. The primary and tertiary structures of the chloride-binding site in ANPR are highly conserved among receptor-guanylate cyclases and metabotropic glutamate receptors. The chloride-dependent ANP binding, reversible chloride binding, and the highly conserved chloride-binding site motif suggest a regulatory role for the receptor bound chloride. Chloride-dependent regulation of ANPR may operate in the kidney, modulating ANP-induced natriuresis.

Ligand-mediated endocytosis and intracellular sequestration of guanylyl cyclase/natriuretic peptide receptors: role of GDAY motif

The guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), also referred to as GC-A, is a single polypeptide molecule having a critical function in blood pressure regulation and cardiovascular homeostasis. GC-A/NPRA, which resides in the plasma membrane, consists of an extracellular ligand-binding domain, a single transmembrane domain, and an intracellular cytoplasmic region containing a protein kinase-like homology domain (KHD) and a guanylyl cyclase (GC) catalytic domain. After binding with atrial and brain natriuretic peptides (ANP and BNP), GC-A/NPRA is internalized and sequestered into intracellular compartments. Therefore, GC-A/NPRA is a dynamic cellular macromolecule that traverses different subcellular compartments through its lifetime. This review describes the roles of short-signal sequences in the internalization, trafficking, and intracellular redistribution of GC-A/NPRA from cell surface to cell interior. Evidence indicates that, after internalization, the ligand-receptor complexes dissociate inside the cell and a population of GC-A/NPRA recycles back to the plasma membrane. Subsequently, the disassociated ligands are degraded in the lysosomes. However, a small percentage of the ligand escapes the lysosomal degradative pathway, and is released intact into culture medium. Using pharmacologic and molecular perturbants, emphasis has been placed on the cellular regulation and processing of ligand-bound GC-A/NPRA in terms of receptor trafficking and down-regulation in intact cells. The discussion is concluded by examining the functions of short-signal sequence motifs in the cellular life-cycle of GC-A/NPRA, including endocytosis, trafficking, metabolic processing, inactivation, and/or down-regulation in model cell systems.

GCC signaling in colorectal cancer: Is colorectal cancer a paracrine deficiency syndrome?

Guanylyl cyclase C (GCC) is the receptor expressed by intestinal cells for the paracrine hormones guanylin and uroguanylin that coordinate mucosal homeostasis and its silencing contributes to intestinal transformation. It orchestrates proliferative and metabolic circuits by limiting the cell cycle and programming metabolic transitions central to regeneration along the crypt-villus axis. Mice deficient in GCC are more susceptible to colon cancer induced by germline mutations or carcinogens. Moreover, guanylin and uroguanylin are the most commonly lost gene products in colon cancer. The role of GCC as a tumor suppressor and the universal loss of its hormones in transformation suggest a paradigm in which colorectal cancer is a disease of paracrine hormone insufficiency. Indeed, GCC signaling reverses the tumorigenic phenotype of human colon cancer cells by regulating proliferation and metabolism. These data suggest a pathophysiological hypothesis in which GCC is a tumor suppressor coordinating proliferative homeostasis whose silencing through hormone loss initiates transformation. The correlative therapeutic hypothesis suggests that colorectal cancer is a disease of hormone insufficiency that can be prevented or treated by oral hormone replacement therapy employing GCC ligands.

Expression, purification, and characterization of the intra-cellular domain of the ANP receptor

The membrane-bound atrial natriuretic peptide receptor (GCA) catalyzes the formation of cGMP from GTP in response to natriuretic peptide hormones. Previous structural studies have focused on the extra-cellular hormone binding domain of this receptor whereas its intra-cellular domain has not yet been amenable to such studies. We report here the baculovirus expression and purification of the GCA intra-cellular domain construct GCA(ID) comprising the complete intra-cellular region which includes the kinase-homology domain, coiled-coil region, and catalytic cyclase domain. The intra-cellular domain was enzymatically characterized in terms of guanylyl cyclase activity and the effects of ATP, manganese, and Triton X-100. Our results indicate that the activity of the intra-cellular domain of the ANP receptor is about 2 fold less active compared to a truncated cyclase domain construct lacking the kinase-like domain that was also expressed and purified. In addition, unlike the full length receptor, the intra-cellular domain could not be activated by Triton X-100/Mn(2+) or its activity stimulated by ATP. These data therefore indicate that the major part of the transition from the basal state to the fully, ANP/ATP-dependent, activated state as well its stimulation/enhancement by Triton X-100/Mn(2+) requires the full length receptor. These receptor insights could aid in the development of novel therapeutics as the GCA receptor is a key drug target for cardiovascular diseases.

Structure of the atrial natriuretic peptide receptor extracellular domain in the unbound and hormone-bound states by single-particle electron microscopy

Atrial natriuretic peptide (ANP) plays a major role in blood pressure and volume regulation. ANP activities are mediated by a cell surface, single-span transmembrane receptor linked to its intrinsic guanylate cyclase activity. The crystal structures of the dimerized ANP receptor extracellular domain (ECD) with and without ANP have revealed a novel hormone-induced rotation mechanism occurring in the juxtamembrane region that appears to mediate signal transduction [Ogawa H, Qiu Y, Ogata CM & Misono KS (2004) J Biol Chem 279, 28625-28631]. However, the ECD crystal packing contains two major intermolecular contacts that suggest two possible dimer pairs: 'head-to-head' (hh) and 'tail-to-tail' (tt) dimers associated via the membrane-distal and membrane-proximal subdomains, respectively. The existence of these two potential dimer forms challenges the proposed signaling mechanism. In this study, we performed single-particle electron microscopy (EM) to determine the ECD dimer structures occurring in the absence of crystal contacts. EM reconstruction yielded the dimer structures with and without ANP in only the hh dimer forms. We further performed steady-state fluorescence spectroscopy of Trp residues, one of which (Trp74) occurs in the hh dimer interface and none of which occurs in the tt dimer interface. ANP binding caused a time-dependent decrease in Trp emission at 350 nm that was attributable to partially buried Trp74 in the unbound hh dimer interface becoming exposed to solvent water upon ANP binding. Thus, the results of single-particle EM and Trp fluorescence studies have provided direct evidence for hh dimer structures for unbound and ANP-bound receptor. The results also support the proposed rotation mechanism for transmembrane signaling by the ANP receptor.

Design and characterization of a soluble fragment of the extracellular ligand-binding domain of the peptide hormone receptor guanylyl cyclase-C

The intestinal guanylyl cyclase-C (GC-C) was originally identified as an Escherichia coli heat-stable enterotoxin (STa) receptor. STa stimulates GC-C to much higher activity than the endogenous ligands guanylin and uroguanylin, causing severe diarrhea. To investigate the interactions of the endogenous and bacterial ligands with GC-C, we designed and characterized a soluble and properly folded fragment of the extracellular ligand-binding domain of GC-C. The membrane-bound guanylyl cyclases exhibit a single transmembrane spanning helix and a globularly folded extracellular ligand-binding domain that comprises about 410 of 1050 residues. Based on the crystal structure of the dimerized-binding domain of the guanylyl cyclase-coupled atrial natriuretic peptide receptor and a secondary structure-guided sequence alignment, we generated a model of the extracellular domain of GC-C comprised of two subdomains. Mapping of mutational and cross-link data onto this structural model restricts the ligand-binding region to the membrane proximal subdomain. We thus designed miniGC-C, a 197 amino acid fragment that mimics the ligand-binding membrane proximal subdomain. Cloning, expression and spectroscopic studies reveal miniGC-C to be a soluble and properly folded protein with a distinct secondary and tertiary structure. MiniGC-C binds STa with nanomolar affinity.

Conserved sequences of sperm-activating peptide and its receptor throughout evolution, despite speciation in the sea star Asterias amurensis and closely related species

The asteroidal sperm-activating peptides (asterosaps) from the egg jelly bind to their sperm receptor, a membrane-bound guanylate cyclase, on the tail to activate sperm in sea stars. Asterosaps are produced as single peptides and then cleaved into shorter peptides. Sperm activation is followed by the acrosome reaction, which is subfamily specific. In order to investigate the molecular details of the asterosap-receptor interaction, corresponding cDNAs have been cloned, sequenced and analysed from the Asteriinae subfamily including Asterias amurensis, A. rubens, A. forbesi and Aphelasterias japonica, as well as Distolasterias nipon from the Coscinasteriinae subfamily. Averages of 29% and 86% identity were found from the deduced amino acid sequences in asterosap and its receptor extracellular domains, respectively, across all species examined. The phylogenic tree topology for asterosap and its receptor was similar to that of the mitochondrial cytochrome c oxidase subunit I. In spite of a certain homology, the amino acid sequences exhibited speciation. Conservation was found in the asterosap residues involved in disulphide bonding and proteinase-cleaving sites. Conversely, similarities were detected between potential asterosap-binding sites and the structure of the atrial natriuretic peptide receptor. Although the sperm-activating peptide and its receptor share certain common sequences, they may serve as barriers that ensure speciation in the sea star A. amurensis and closely related species.

Adenine nucleotides decrease the apparent Km of endogenous natriuretic peptide receptors for GTP

Natriuretic peptide receptors A (NPR-A) and B (NPR-B) mediate most effects of natriuretic peptides by synthesizing cGMP. ATP increases the activity of these receptors by an unknown mechanism. We recently reported that a nonhydrolyzable form of ATP, adenylyl imidodiphosphate (AMPPNP), stabilizes but is not required for the activation of NPR-A and NPR-B in membranes from highly overexpressing cells. Here, we repeated these studies on receptors expressed in endogenous settings. Kinetic analysis indicated that both AMPPNP and ATP dramatically decrease the apparent K(m) of both receptors for GTP but had little effect on the V(max). The EC(50) for AMPPNP decreased as substrate concentration increased whereas the magnitude of the effect was greater at lower GTP concentrations. ATP increased the activity of a mutant receptor containing glutamates substituted for all known phosphorylation sites similarly to the wild-type receptor, consistent with a phosphorylation independent mechanism. Finally, the putative ATP binding sites were investigated. Mutation of the ATP modulatory domain region had no effect, but mutation of K535A dramatically diminished ANP-dependent cyclase activity in a manner that was unresponsive to ATP. Mutation of the highly conserved 630-KSS to AAA (all alanines) resulted in an expressed receptor that had no detectable guanylyl cyclase activity. We conclude that ATP is not required for the initial activation of NPRs but does increase activity over time by reducing the apparent K(m) for GTP.

Natriuretic peptides, their receptors, and cyclic guanosine monophosphate-dependent signaling functions

Natriuretic peptides are a family of structurally related but genetically distinct hormones/paracrine factors that regulate blood volume, blood pressure, ventricular hypertrophy, pulmonary hypertension, fat metabolism, and long bone growth. The mammalian members are atrial natriuretic peptide, B-type natriuretic peptide, C-type natriuretic peptide, and possibly osteocrin/musclin. Three single membrane-spanning natriuretic peptide receptors (NPRs) have been identified. Two, NPR-A/GC-A/NPR1 and NPR-B/GC-B/NPR2, are transmembrane guanylyl cyclases, enzymes that catalyze the synthesis of cGMP. One, NPR-C/NPR3, lacks intrinsic enzymatic activity and controls the local concentrations of natriuretic peptides through constitutive receptor-mediated internalization and degradation. Single allele-inactivating mutations in the promoter of human NPR-A are associated with hypertension and heart failure, whereas homozygous inactivating mutations in human NPR-B cause a form of short-limbed dwarfism known as acromesomelic dysplasia type Maroteaux. The physiological effects of natriuretic peptides are elicited through three classes of cGMP binding proteins: cGMP-dependent protein kinases, cGMP-regulated phosphodiesterases, and cyclic nucleotide-gated ion channels. In this comprehensive review, the structure, function, regulation, and biological consequences of natriuretic peptides and their associated signaling proteins are described.

Atrial natriuretic peptide-dependent photolabeling of a regulatory ATP-binding site on the natriuretic peptide receptor-A

The natriuretic peptide receptor-A (NPR-A) is composed of an extracellular ligand-binding domain, a transmembrane-spanning domain, a kinase homology domain (KHD) and a guanylyl cyclase domain. Because the presence of ATP or adenylylimidodiphosphate reduces atrial natriuretic peptide (ANP) binding and is required for maximal guanylyl cyclase activity, a direct interaction of ATP with the receptor KHD domain is plausible. Therefore, we investigated whether ATP interacts directly with a binding site on the receptor by analyzing the binding of a photoaffinity analog of ATP to membranes from human embryonic kidney 293 cells expressing the NPR-A receptor lacking the guanylyl cyclase moiety (DeltaGC). We demonstrate that this receptor (NPR-A-DeltaGC) can be directly labeled by 8-azido-3'-biotinyl-ATP and that labeling is highly increased following ANP treatment. The mutant receptor DeltaKC, which does not contain the KHD, is not labeled. Photoaffinity labeling of the NPR-A-DeltaGC is reduced by 50% in the presence of 550 microm ATP, and competition curve fitting studies indicate a Hill slope of 2.2, suggestive of cooperative binding. This approach demonstrates directly that the interaction of ANP with its receptor modulates the binding of ATP to the KHD, probably through a conformational change in the KHD. In turn, this conformational change is essential for maximal activity. In addition, the ATP analog, 8-azido-adenylylimidodiphosphate, inhibits guanylyl cyclase activity but increases ANP binding to the extracellular domain. These results suggest that the KHD regulates ANP binding and guanylyl cyclase activity independently.

Biology of natriuretic peptides and their receptors

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

ATP-regulated module (ARM) of the atrial natriuretic factor receptor guanylate cyclase

ATP is an obligatory agent for the atrial natriuretic factor (ANF) and the type C natriuretic peptide (CNP) signaling of their respective receptor guanylate cyclases, ANF-RGC and CNP-RGC. Through a common mechanism, it binds to a defined ARM domain of the cyclase, activates the cyclase and transduces the signal into generation of the second messenger cyclic GMP. In this presentation, the authors review the ATP-regulated transduction mechanism and refine the previously simulated three-dimensional ARM model (Duda T, Yadav P, Jankowska A, Venkataraman V, Sharma RK. Three dimensional atomic model and experimental validation for the ATP-regulated module (ARM) of the atrial natriuretic factor receptor guanylate cyclase. Mol Cell Biochem 2000;214:7-14; reviewed in: Sharma RK, Yadav P, Duda T. Allosteric regulatory step and configuration of the ATP-binding pocket in atrial natriuretic factor receptor guanylate cyclase transduction mechanism. Can J Physiol Pharmacol 2001;79: 682-91; Sharma RK. Evolution of the membrane guanylate cyclase transduction system. Mol Cell Biochem 2002;230:3-30). The model depicts the ATP-binding dependent configurational changes in the ARM and supports the concept that in the first step, ATP partially activates the cyclase and primes it for the subsequent transduction steps, resulting in full activation of the cyclase.

Internalization and trafficking of guanylyl cyclase/natriuretic peptide receptor-A

One of the principal loci involved in the regulatory action of atrial and brain natriuretic peptides (ANP and BNP) is guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA), whose ligand-binding efficiency and GC catalytic activity vary remarkably in different target cells and tissues. In its mature form, NPRA resides in the plasma membrane and contains an extracellular ligand-binding domain, a single transmembrane region, and the intracellular protein kinase-like homology domain (KHD) and guanylyl cyclase (GC) catalytic domain. NPRA is a dynamic cellular macromolecule that traverses through different compartments of the cell through its lifetime. Binding of ligand to NPRA triggers a complex array of signal transduction events and accelerates the endocytosis. The endocytic transport is important in regulating signal transduction, formation of specialized signaling complexes, and modulation of specific components of internalization events. The present review describes the experiments which reveal the internalization of ligand-receptor complexes of NPRA, receptor trafficking and recycling, and delivery of both ligand-receptor molecules into subcellular compartments. The ligand-receptor complexes of NPRA are finally degraded within the lysosomes. The experimental evidence provides a consensus forum, which establishes the endocytosis, cellular trafficking, sequestration, and metabolic processing of ANP/NPRA complexes in the intact cells. The discussion is afforded to address the experimental insights into the mechanisms that cells utilize in modulating the delivery and metabolic processing of ligand-bound NPRA into the cell interior.

Structural studies of the natriuretic peptide receptor: a novel hormone-induced rotation mechanism for transmembrane signal transduction

The atrial natriuretic peptide (ANP) receptor is a single-span transmembrane receptor that is coupled to its intrinsic intracellular guanylate cyclase (GCase) catalytic activity. To investigate the mechanisms of hormone binding and signal transduction, we have expressed the extracellular hormone-binding domain of the ANP receptor (ANPR) and characterized its structure and function. The disulfide-bond structure, state of glycosylation, binding-site residues, chloride-dependence of ANP binding, dimerization, and binding stoichiometry have been determined. More recently, the crystal structures of both the apoANPR dimer and ANP-bound complex have been determined. The structural comparison between the two has shown that, upon ANP binding, two ANPR molecules in the dimer undergo an inter-molecular twist with little intra-molecular conformational change. This motion produces a Ferris wheel-like translocation of two juxtamembrane domains with essentially no change in the inter-domain distance. This movement alters the relative orientation of the two domains equivalent to counter-clockwise rotation of each by 24 degrees . These results suggest that transmembrane signaling by the ANP receptor is mediated by a novel hormone-induced rotation mechanism.

Backbone dynamics of oxidised and reduced forms of human atrial natriuretic peptide

The backbone dynamics of the 28 residue 15N-labelled human atrial natriuretic peptide have been examined by 15N NMR methods. 15N R1, R2 and [1H]-15N NOE values were determined for the oxidised and reduced forms of the peptide (ANPox and ANPrd, respectively), and analysed using reduced spectral density mapping and an extended model-free approach. The two forms possessed correlation times for overall molecular motion of 4.7 ns and were highly flexible, with substantial contributions to relaxation processes from internal motions on picosecond to nanosecond time scales. Reduction of the Cys7-Cys23 disulphide bond to form ANPrd produced a very dynamic linear peptide with a mean overall order parameter of 0.2; the intramolecular cross-link in ANPox increased this to a mean value of 0.4. A simple model for segmental backbone motion accounted for the R2 values of both species using only two variable parameters, indicating that relaxation is dominated by interactions with sites <7 residues distant in the covalent network and that changes in the conformation of the disulphide bond lead to significant chemical exchange broadening in ANPox. The contributions of backbone dynamics to configurational entropy were determined and accounted for the different receptor binding affinities of cyclised and linear natriuretic peptides.

Crystal structure of hormone-bound atrial natriuretic peptide receptor extracellular domain: rotation mechanism for transmembrane signal transduction

A cardiac hormone, atrial natriuretic peptide (ANP), plays a major role in blood pressure and volume regulation. ANP activities are mediated by a single span transmembrane receptor carrying intrinsic guanylate cyclase activity. ANP binding to its extracellular domain stimulates guanylate cyclase activity by an as yet unknown mechanism. Here we report the crystal structure of dimerized extracellular hormone-binding domain in complex with ANP. The structural comparison with the unliganded receptor reveals that hormone binding causes the two receptor monomers to undergo an intermolecular twist with little intramolecular conformational change. This motion produces a Ferris wheel-like translocation of two juxtamembrane domains in the dimer with essentially no change in the interdomain distance. This movement alters the relative orientation of the two domains by a shift equivalent to counterclockwise rotation of each by 24 degrees. These results suggest that transmembrane signaling by the ANP receptor is initiated via a hormone-induced rotation mechanism.