Membrane guanylate cyclase is a beautiful signal transduction machine: overview

Microbiota-derived butyrate dampens linaclotide stimulation of the guanylate cyclase C pathway in patient-derived colonoids

BACKGROUND & AIMS

Disorders of gut-brain interaction (DGBI) are complex conditions that result in decreased quality of life and a significant cost burden. Linaclotide, a guanylin cyclase C (GCC) receptor agonist, is approved as a DGBI treatment. However, its efficacy has been limited and variable across DGBI patients. Microbiota and metabolomic alterations are noted in DGBI patients, provoking the hypothesis that the microbiota may impact the GCC response to current therapeutics.

METHODS

Human-derived intestinal organoids were grown from pediatric DGBI, non-IBD colon biopsies (colonoids). Colonoids were treated with 250 nM linaclotide and assayed for cGMP to develop a model of GCC activity. Butyrate was administered to human colonoids overnight at a concentration of 1 mM. Colonoid lysates were analyzed for cGMP levels by ELISA. For the swelling assay, colonoids were photographed pre- and post-treatment and volume was measured using ImageJ. Principal coordinate analyses (PCoA) were performed on the Bray-Curtis dissimilarity and Jaccard distance to assess differences in the community composition of short-chain fatty acid (SCFA) producing microbial species in the intestinal microbiota from pediatric patients with IBS and healthy control samples.

KEY RESULTS

Linaclotide treatment induced a significant increase in [cGMP] and swelling of patient-derived colonoids, demonstrating a human in vitro model of linaclotide-induced GCC activation. Shotgun sequencing analysis of pediatric IBS patients and healthy controls showed differences in the composition of commensal SCFA-producing bacteria. Butyrate exposure significantly dampened linaclotide-induced cGMP levels and swelling in patient-derived colonoids.

CONCLUSIONS & INFERENCES

Patient-derived colonoids demonstrate that microbiota-derived butyrate can dampen human colonic responses to linaclotide. This study supports incorporation of microbiota and metabolomic assessment to improve precision medicine for DGBI patients.

Multilimbed membrane guanylate cyclase signaling system, evolutionary ladder

One monumental discovery in the field of cell biology is the establishment of the membrane guanylate cyclase signal transduction system. Decoding its fundamental, molecular, biochemical, and genetic features revolutionized the processes of developing therapies for diseases of endocrinology, cardio-vasculature, and sensory neurons; lastly, it has started to leave its imprints with the atmospheric carbon dioxide. The membrane guanylate cyclase does so via its multi-limbed structure. The inter-netted limbs throughout the central, sympathetic, and parasympathetic systems perform these functions. They generate their common second messenger, cyclic GMP to affect the physiology. This review describes an historical account of their sequential evolutionary development, their structural components and their mechanisms of interaction. The foundational principles were laid down by the discovery of its first limb, the ACTH modulated signaling pathway (the companion monograph). It challenged two general existing dogmas at the time. First, there was the question of the existence of a membrane guanylate cyclase independent from a soluble form that was heme-regulated. Second, the sole known cyclic AMP three-component-transduction system was modulated by GTP-binding proteins, so there was the question of whether a one-component transduction system could exclusively modulate cyclic GMP in response to the polypeptide hormone, ACTH. The present review moves past the first question and narrates the evolution and complexity of the cyclic GMP signaling pathway. Besides ACTH, there are at least five additional limbs. Each embodies a unique modular design to perform a specific physiological function; exemplified by ATP binding and phosphorylation, Ca²⁺-sensor proteins that either increase or decrease cyclic GMP synthesis, co-expression of antithetical Ca²⁺ sensors, GCAP1 and S100B, and modulation by atmospheric carbon dioxide and temperature. The complexity provided by these various manners of operation enables membrane guanylate cyclase to conduct diverse functions, exemplified by the control over cardiovasculature, sensory neurons and, endocrine systems.

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.

Retinal degeneration protein 3 controls membrane guanylate cyclase activities in brain tissue

The retinal degeneration protein RD3 is involved in regulatory processes of photoreceptor cells. Among its main functions is the inhibition of photoreceptor specific membrane guanylate cyclases during trafficking from the inner segment to their final destination in the outer segment. However, any physiological role of RD3 in non-retinal tissue is unsolved at present and specific protein targets outside of retinal tissue have not been identified so far. The family of membrane bound guanylate cyclases share a high homology of their amino acid sequences in their cytoplasmic domains. Therefore, we reasoned that membrane guanylate cyclases that are activated by natriuretic peptides are also regulated by RD3. We analyzed transcript levels of the rd3 gene and natriuretic peptide receptor genes Npr1 and Npr2 in the mouse retina, cerebellum, hippocampus, neocortex, and the olfactory bulb during development from the embryonic to the postnatal stage at P60. The rd3 gene showed a lower expression level than Npr1 and Npr2 (encoding for GC-A and GC-B, respectively) in all tested brain tissues, but was at least one order of magnitude higher in the retina. RD3 and natriuretic peptide receptor GCs co-express in the retina and brain tissue leading to functional tests. We expressed GC-A and GC-B in HEK293T cells and measured the inhibition of GCs by RD3 after activation by natriuretic peptides yielding inhibitory constants around 25 nM. Furthermore, endogenous GCs in astrocytes were inhibited by RD3 to a similar extent. We here show for the first time that RD3 can inhibit two hormone-stimulated GCs, namely GC-A and GC-B indicating a new regulatory feature of these hormone receptors.

Purification, characterization, and preliminary serial crystallography diffraction advances structure determination of full-length human particulate guanylyl cyclase A receptor

Particulate Guanylyl Cyclase Receptor A (pGC-A) is a natriuretic peptide membrane receptor, playing a vital role in controlling cardiovascular, renal, and endocrine functions. The extracellular domain interacts with natriuretic peptides and triggers the intracellular guanylyl cyclase domain to convert GTP to cGMP. To effectively develop methods to regulate pGC-A, structural information on the full-length form is needed. However, structural data on the transmembrane and intracellular domains are lacking. This work presents expression and optimization using baculovirus, along with the first purification of functional full-length human pGC-A. In vitro assays revealed the pGC-A tetramer was functional in detergent micelle solution. Based on our purification results and previous findings that dimer formation is required for functionality, we propose a tetramer complex model with two functional subunits. Previous research suggested pGC-A signal transduction is an ATP-dependent, two-step mechanism. Our results show the binding ligand also moderately activates pGC-A, and ATP is not crucial for activation of guanylyl cyclase. Furthermore, crystallization of full-length pGC-A was achieved, toward determination of its structure. Needle-shaped crystals with 3 Å diffraction were observed by serial crystallography. This work paves the road for determination of the full-length pGC-A structure and provides new information on the signal transduction mechanism.

Ca2+-Sensor Neurocalcin δ and Hormone ANF Modulate ANF-RGC Activity by Diverse Pathways: Role of the Signaling Helix Domain

Prototype member of the membrane guanylate cyclase family, ANF-RGC (Atrial Natriuretic Factor Receptor Guanylate Cyclase), is the physiological signal transducer of two most hypotensive hormones ANF and BNP, and of the intracellular free Ca²⁺. Both the hormonal and the Ca²⁺-modulated signals operate through a common second messenger, cyclic GMP; yet, their operational modes are divergent. The hormonal pathways originate at the extracellular domain of the guanylate cyclase; and through a cascade of structural changes in its successive domains activate the C-terminal catalytic domain (CCD). In contrast, the Ca²⁺ signal operating via its sensor, myristoylated neurocalcin δ both originates and is translated directly at the CCD. Through a detailed sequential deletion and expression analyses, the present study examines the role of the signaling helix domain (SHD) in these two transduction pathways. SHD is a conserved 35-amino acid helical region of the guanylate cyclase, composed of five heptads, each meant to tune and transmit the hormonal signals to the CCD for their translation and generation of cyclic GMP. Its structure is homo-dimeric and the molecular docking analyses point out to the possibility of antiparallel arrangement of the helices. Contrary to the hormonal signaling, SHD has no role in regulation of the Ca²⁺- modulated pathway. The findings establish and define in molecular terms the presence of two distinct non-overlapping transduction modes of ANF-RGC, and for the first time demonstrate how differently they operate, and, yet generate cyclic GMP utilizing common CCD machinery.

Molecular and genetic aspects of guanylyl cyclase natriuretic peptide receptor-A in regulation of blood pressure and renal function

Natriuretic peptides (NPs) exert diverse effects on several biological and physiological systems, such as kidney function, neural and endocrine signaling, energy metabolism, and cardiovascular function, playing pivotal roles in the regulation of blood pressure (BP) and cardiac and vascular homeostasis. NPs are collectively known as anti-hypertensive hormones and their main functions are directed toward eliciting natriuretic/diuretic, vasorelaxant, anti-proliferative, anti-inflammatory, and anti-hypertrophic effects, thereby, regulating the fluid volume, BP, and renal and cardiovascular conditions. Interactions of NPs with their cognate receptors display a central role in all aspects of cellular, biochemical, and molecular mechanisms that govern physiology and pathophysiology of BP and cardiovascular events. Among the NPs atrial and brain natriuretic peptides (ANP and BNP) activate guanylyl cyclase/natriuretic peptide receptor-A (GC-A/NPRA) and initiate intracellular signaling. The genetic disruption of Npr1 (encoding GC-A/NPRA) in mice exhibits high BP and hypertensive heart disease that is seen in untreated hypertensive subjects, including high BP and heart failure. There has been a surge of interest in the NPs and their receptors and a wealth of information have emerged in the last four decades, including molecular structure, signaling mechanisms, altered phenotypic characterization of transgenic and gene-targeted animal models, and genetic analyses in humans. The major goal of the present review is to emphasize and summarize the critical findings and recent discoveries regarding the molecular and genetic regulation of NPs, physiological metabolic functions, and the signaling of receptor GC-A/NPRA with emphasis on the BP regulation and renal and cardiovascular disorders.

Membrane Guanylate Cyclase catalytic Subdomain: Structure and Linkage with Calcium Sensors and Bicarbonate

Membrane guanylate cyclase (MGC) is a ubiquitous multi-switching cyclic GMP generating signaling machine linked with countless physiological processes. In mammals it is encoded by seven distinct homologous genes. It is a single transmembrane spanning multi-modular protein; composed of integrated blocks and existing in homo-dimeric form. Its core catalytic domain (CCD) module is a common transduction center where all incoming signals are translated into the production of cyclic GMP, a cellular signal second messenger. Crystal structure of the MGC's CCD does not exist and its precise identity is ill-defined. Here, we define it at a sub-molecular level for the phototransduction-linked MGC, the rod outer segment guanylate cyclase type 1, ROS-GC1. (1) The CCD is a conserved 145-residue structural unit, represented by the segment V820-P964. (2) It exists as a homo-dimer and contains seven conserved catalytic elements (CEs) wedged into seven conserved motifs. (3) It also contains a conserved 21-residue neurocalcin δ-modulated structural domain, V836-L857. (4) Site-directed mutagenesis documents that each of the seven CEs governs the cyclase's catalytic activity. (5) In contrast to the soluble and the bacterium MGC which use Mn2+-GTP substrate for catalysis, MGC CCD uses the natural Mg2+-GTP substrate. (6) Strikingly, the MGC CCD requires anchoring by the Transmembrane Domain (TMD) to exhibit its major (∼92%) catalytic activity; in isolated form the activity is only marginal. This feature is not linked with any unique sequence of the TMD; there is minimal conservation in TMD. Finally, (7) the seven CEs control each of four phototransduction pathways- -two Ca2+-sensor GCAPs-, one Ca2+-sensor, S100B-, and one bicarbonate-modulated. The findings disclose that the CCD of ROS-GC1 has built-in regulatory elements that control its signal translational activity. Due to conservation of these regulatory elements, it is proposed that these elements also control the physiological activity of other members of MGC family.

Integrative Signaling Networks of Membrane Guanylate Cyclases: Biochemistry and Physiology

This monograph presents a historical perspective of cornerstone developments on the biochemistry and physiology of mammalian membrane guanylate cyclases (MGCs), highlighting contributions made by the authors and their collaborators. Upon resolution of early contentious studies, cyclic GMP emerged alongside cyclic AMP, as an important intracellular second messenger for hormonal signaling. However, the two signaling pathways differ in significant ways. In the cyclic AMP pathway, hormone binding to a G protein coupled receptor leads to stimulation or inhibition of an adenylate cyclase, whereas the cyclic GMP pathway dispenses with intermediaries; hormone binds to an MGC to affect its activity. Although the cyclic GMP pathway is direct, it is by no means simple. The modular design of the molecule incorporates regulation by ATP binding and phosphorylation. MGCs can form complexes with Ca²⁺-sensing subunits that either increase or decrease cyclic GMP synthesis, depending on subunit identity. In some systems, co-expression of two Ca²⁺ sensors, GCAP1 and S100B with ROS-GC1 confers bimodal signaling marked by increases in cyclic GMP synthesis when intracellular Ca²⁺ concentration rises or falls. Some MGCs monitor or are modulated by carbon dioxide via its conversion to bicarbonate. One MGC even functions as a thermosensor as well as a chemosensor; activity reaches a maximum with a mild drop in temperature. The complexity afforded by these multiple limbs of operation enables MGC networks to perform transductions traditionally reserved for G protein coupled receptors and Transient Receptor Potential (TRP) ion channels and to serve a diverse array of functions, including control over cardiac vasculature, smooth muscle relaxation, blood pressure regulation, cellular growth, sensory transductions, neural plasticity and memory.

Aversive Behavior in the Nematode C. elegans Is Modulated by cGMP and a Neuronal Gap Junction Network

All animals rely on their ability to sense and respond to their environment to survive. However, the suitability of a behavioral response is context-dependent, and must reflect both an animal's life history and its present internal state. Based on the integration of these variables, an animal's needs can be prioritized to optimize survival strategies. Nociceptive sensory systems detect harmful stimuli and allow for the initiation of protective behavioral responses. The polymodal ASH sensory neurons are the primary nociceptors in C. elegans. We show here that the guanylyl cyclase ODR-1 functions non-cell-autonomously to downregulate ASH-mediated aversive behaviors and that ectopic cGMP generation in ASH is sufficient to dampen ASH sensitivity. We define a gap junction neural network that regulates nociception and propose that decentralized regulation of ASH signaling can allow for rapid correlation between an animal's internal state and its behavioral output, lending modulatory flexibility to this hard-wired nociceptive neural circuit.

Bicarbonate and Ca(2+) Sensing Modulators Activate Photoreceptor ROS-GC1 Synergistically

Photoreceptor ROS-GC1, a prototype subfamily member of the membrane guanylate cyclase family, is a central component of phototransduction. It is a single transmembrane-spanning protein, composed of modular blocks. In rods, guanylate cyclase activating proteins (GCAPs) 1 and 2 bind to its juxtamembrane domain (JMD) and the C-terminal extension, respectively, to accelerate cyclic GMP synthesis when Ca(2+) levels are low. In cones, the additional expression of the Ca(2+)-dependent guanylate cyclase activating protein (CD-GCAP) S100B which binds to its C-terminal extension, supports acceleration of cyclic GMP synthesis at high Ca(2+) levels. Independent of Ca(2+), ROS-GC1 activity is also stimulated directly by bicarbonate binding to the core catalytic domain (CCD). Several enticing molecular features of this transduction system are revealed in the present study. In combination, bicarbonate and Ca(2+)-dependent modulators raised maximal ROS-GC activity to levels that exceeded the sum of their individual effects. The F(514)S mutation in ROS-GC1 that causes blindness in type 1 Leber's congenital amaurosis (LCA) severely reduced basal ROS-GC1 activity. GCAP2 and S100B Ca(2+) signaling modes remained functional, while the GCAP1-modulated mode was diminished. Bicarbonate nearly restored basal activity as well as GCAP2- and S100B-stimulated activities of the F(514)S mutant to normal levels but could not resurrect GCAP1 stimulation. We conclude that GCAP1 and GCAP2 forge distinct pathways through domain-specific modules of ROS-GC1 whereas the S100B and GCAP2 pathways may overlap. The synergistic interlinking of bicarbonate to GCAPs- and S100B-modulated pathways intensifies and tunes the dependence of cyclic GMP synthesis on intracellular Ca(2+). Our study challenges the recently proposed GCAP1 and GCAP2 "overlapping" phototransduction model (Peshenko et al., 2015b).

cGMP in mouse rods: the spatiotemporal dynamics underlying single photon responses

Vertebrate vision begins when retinal photoreceptors transduce photons into electrical signals that are then relayed to other neurons in the eye, and ultimately to the brain. In rod photoreceptors, transduction of single photons is achieved by a well-understood G-protein cascade that modulates cGMP levels, and in turn, cGMP-sensitive inward current. The spatial extent and depth of the decline in cGMP during the single photon response (SPR) have been major issues in phototransduction research since the discovery that single photons elicit substantial and reproducible changes in membrane current. The spatial profile of cGMP decline during the SPR affects signal gain, and thus may contribute to reduction of trial-to-trial fluctuations in the SPR. Here we summarize the general principles of rod phototransduction, emphasizing recent advances in resolving the spatiotemporal dynamics of cGMP during the SPR.

Bimodal oscillations of cyclic nucleotide concentrations in the circadian system of the Madeira cockroach Rhyparobia maderae

Pigment-dispersing factor (PDF) is the most important coupling factor of the circadian system in insects, comparable to its functional ortholog vasoactive intestinal polypeptide of the mammalian circadian clock. In Drosophila melanogaster, PDF signals via activation of adenylyl cyclases, controlling circadian locomotor activity rhythms at dusk and dawn. In addition, PDF mediates circadian rhythms of the visual system and is involved in entrainment to different photoperiods. We examined whether PDF daytime-dependently elevates cAMP levels in the Madeira cockroach Rhyparobia maderae and whether cAMP mimics PDF effects on locomotor activity rhythms. To determine time windows of PDF release, we searched for circadian rhythms in concentrations of cAMP and its functional opponent cGMP in the accessory medulla (AMe), the insect circadian pacemaker controlling locomotor activity rhythms, and in the optic lobes, as the major input and output area of the circadian clock. Enzyme-linked immunosorbent assays detected PDF-dependent increases of cAMP in optic lobes and daytime-dependent oscillations of cAMP and cGMP baseline levels in the AMe, both with maxima at dusk and dawn. Although these rhythms disappeared at the first day in constant conditions (DD1), cAMP but not cGMP oscillations returned at the second day in constant conditions (DD2). Whereas in light-dark cycles the cAMP baseline level remained constant in other optic lobe neuropils, it oscillated in phase with the AMe at DD2. To determine whether cAMP and cGMP mimic PDF-dependent control of locomotor activity rhythms, both cyclic nucleotides were injected at different times of the circadian day using running-wheel assays. Whereas cAMP injections generated delays at dusk and advances at dawn, cGMP only delayed locomotor activity at dusk. For the first time we found PDF-dependent phase advances at dawn in addition to previously described phase delays at dusk. Thus, we hypothesize that PDF release at dusk and dawn controls locomotor activity rhythms and visual system processing cAMP-dependently.

Membrane guanylate cyclase, a multimodal transduction machine: history, present, and future directions

A sequel to these authors' earlier comprehensive reviews which covered the field of mammalian membrane guanylate cyclase (MGC) from its origin to the year 2010, this article contains 13 sections. The first is historical and covers MGC from the year 1963–1987, summarizing its colorful developmental stages from its passionate pursuit to its consolidation. The second deals with the establishment of its biochemical identity. MGC becomes the transducer of a hormonal signal and founder of the peptide hormone receptor family, and creates the notion that hormone signal transduction is its sole physiological function. The third defines its expansion. The discovery of ROS-GC subfamily is made and it links ROS-GC with the physiology of phototransduction. Sections ROS-GC, a Ca²⁺-Modulated Two Component Transduction System to Migration Patterns and Translations of the GCAP Signals Into Production of Cyclic GMP are Different cover its biochemistry and physiology. The noteworthy events are that augmented by GCAPs, ROS-GC proves to be a transducer of the free Ca²⁺ signals generated within neurons; ROS-GC becomes a two-component transduction system and establishes itself as a source of cyclic GMP, the second messenger of phototransduction. Section ROS-GC1 Gene Linked Retinal Dystrophies demonstrates how this knowledge begins to be translated into the diagnosis and providing the molecular definition of retinal dystrophies. Section Controlled By Low and High Levels of [Ca²⁺]_i, ROS-GC1 is a Bimodal Transduction Switch discusses a striking property of ROS-GC where it becomes a “[Ca²⁺]_i bimodal switch” and transcends its signaling role in other neural processes. In this course, discovery of the first CD-GCAP (Ca²⁺-dependent guanylate cyclase activator), the S100B protein, is made. It extends the role of the ROS-GC transduction system beyond the phototransduction to the signaling processes in the synapse region between photoreceptor and cone ON-bipolar cells; in section Ca²⁺-Modulated Neurocalcin δ ROS-GC1 Transduction System Exists in the Inner Plexiform Layer (IPL) of the Retinal Neurons, discovery of another CD-GCAP, NCδ, is made and its linkage with signaling of the inner plexiform layer neurons is established. Section ROS-GC Linkage With Other Than Vision-Linked Neurons discusses linkage of the ROS-GC transduction system with other sensory transduction processes: Pineal gland, Olfaction and Gustation. In the next, section Evolution of a General Ca²⁺-Interlocked ROS-GC Signal Transduction Concept in Sensory and Sensory-Linked Neurons, a theoretical concept is proposed where “Ca²⁺-interlocked ROS-GC signal transduction” machinery becomes a common signaling component of the sensory and sensory-linked neurons. Closure to the review is brought by the conclusion and future directions.

Ca(2+)-modulated ROS-GC1 transduction system in testes and its presence in the spermatogenic cells

ROS-GC1 belongs to the Ca²⁺-modulated sub-family of membrane guanylate cyclases. It primarily exists and is linked with signaling of the sensory neurons – sight, smell, taste, and pinealocytes. Exceptionally, it is also present and is Ca²⁺-modulated in t he non-neuronal cells, the sperm cells in the testes, where S100B protein serves as its Ca²⁺ sensor. The present report demonstrates the identification of an additional Ca²⁺ sensor of ROS-GC1 in the testes, neurocalcin δ. Through mouse molecular genetic models, it compares and quantifies the relative input of the S100B and neurocalcin δ in regulating the Ca²⁺ signaling of ROS-GC1 transduction machinery, and via immunochemistry it demonstrates the co-presence of neurocalcin δ and ROS-GC1 in the spermatogenic cells of the testes. The suggestion is that in more ways than one the Ca²⁺-modulated ROS-GC1 transduction system is linked with the testicular function. This non-neuronal transduction system may represent an illustration of the ROS-GC1 expanding role in the trans-signaling of the neural and non-neural systems.

ROS-GC interlocked Ca(2+)-sensor S100B protein signaling in cone photoreceptors: review

Photoreceptor rod outer segment membrane guanylate cyclase (ROS-GC) is central to visual transduction; it generates cyclic GMP, the second messenger of the photon signal. Photoexcited rhodopsin initiates a biochemical cascade that leads to a drop in the intracellular level of cyclic GMP and closure of cyclic nucleotide gated ion channels. Recovery of the photoresponse requires resynthesis of cyclic GMP, typically by a pair of ROS-GCs, 1 and 2. In rods, ROS-GCs exist as complexes with guanylate cyclase activating proteins (GCAPs), which are Ca(2+)-sensing elements. There is a light-induced fall in intracellular Ca(2+). As Ca(2+) dissociates from GCAPs in the 20-200 nM range, ROS-GC activity rises to quicken the photoresponse recovery. GCAPs then progressively turn down ROS-GC activity as Ca(2+) and cyclic GMP levels return to baseline. To date, GCAPs mediate the only known mechanism of ROS-GC regulation in the photoreceptors. However, in mammalian cone outer segments, cone synapses and ON bipolar cells, another Ca(2+) sensor protein, S100B, complexes with ROS-GC1 and senses the Ca(2+) signal with a K1/2 of 400 nM. Unlike GCAPs, S100B stimulates ROS-GC activity when Ca(2+) is bound. Thus, the ROS-GC system in cones functions as a Ca(2+) bimodal switch; with rising intracellular Ca(2+), its activity is first turned down by GCAPs and then turned up by S100B. This presentation provides a historical perspective on the role of S100B in the photoreceptors, offers a pictorial model for the "bimodal" operation of the ROS-GC switch and projects future tasks that are needed to understand its operation. Some accounts of this review have been adopted from the original publications of these authors.

Atrial natriuretic factor receptor guanylate cyclase, ANF-RGC, transduces two independent signals, ANF and Ca(2+)

Atrial natriuretic factor receptor guanylate cyclase (ANF-RGC), was the first discovered member of the mammalian membrane guanylate cyclase family. The hallmark feature of the family is that a single protein contains both the site for recognition of the regulatory signal and the ability to transduce it into the production of the second messenger, cyclic GMP. For over two decades, the family has been classified into two subfamilies, the hormone receptor subfamily with ANF-RGC being its paramount member, and the Ca²⁺ modulated subfamily, which includes the rod outer segment guanylate cyclases, ROS-GC1 and 2, and the olfactory neuroepithelial guanylate cyclase. ANF-RGC is the receptor and the signal transducer of the most hypotensive hormones, ANF– and B-type natriuretic peptide (BNP). After binding these hormones at the extracellular domain it, at its intracellular domain, signals activation of the C-terminal catalytic module and accelerates the production of cyclic GMP. Cyclic GMP then serves the second messenger role in biological responses of ANF and BNP such as natriuresis, diuresis, vasorelaxation, and anti-proliferation. Very recently another modus operandus for ANF-RGC was revealed. Its crux is that ANF-RGC activity is also regulated by Ca²⁺. The Ca²⁺ sensor neurocalcin d mediates this signaling mechanism. Strikingly, the Ca²⁺ and ANF signaling mechanisms employ separate structural motifs of ANF-RGC in modulating its core catalytic domain in accelerating the production of cyclic GMP. In this review the biochemistry and physiology of these mechanisms with emphasis on cardiovascular regulation will be discussed.

Presynaptic [Ca(2+)] and GCAPs: aspects on the structure and function of photoreceptor ribbon synapses

Changes in intracellular calcium ions [Ca²⁺] play important roles in photoreceptor signaling. Consequently, intracellular [Ca²⁺] levels need to be tightly controlled. In the light-sensitive outer segments (OS) of photoreceptors, Ca²⁺ regulates the activity of retinal guanylate cyclases thus playing a central role in phototransduction and light-adaptation by restoring light-induced decreases in cGMP. In the synaptic terminals, changes of intracellular Ca²⁺ trigger various aspects of neurotransmission. Photoreceptors employ tonically active ribbon synapses that encode light-induced, graded changes of membrane potential into modulation of continuous synaptic vesicle exocytosis. The active zones of ribbon synapses contain large electron-dense structures, synaptic ribbons, that are associated with large numbers of synaptic vesicles. Synaptic coding at ribbon synapses differs from synaptic coding at conventional (phasic) synapses. Recent studies revealed new insights how synaptic ribbons are involved in this process. This review focuses on the regulation of [Ca²⁺] in presynaptic photoreceptor terminals and on the function of a particular Ca²⁺-regulated protein, the neuronal calcium sensor protein GCAP2 (guanylate cyclase-activating protein-2) in the photoreceptor ribbon synapse. GCAP2, an EF-hand-containing protein plays multiple roles in the OS and in the photoreceptor synapse. In the OS, GCAP2 works as a Ca²⁺-sensor within a Ca²⁺-regulated feedback loop that adjusts cGMP levels. In the photoreceptor synapse, GCAP2 binds to RIBEYE, a component of synaptic ribbons, and mediates Ca²⁺-dependent plasticity at that site. Possible mechanisms are discussed.

Structural analysis of guanylyl cyclase-activating protein-2 (GCAP-2) homodimer by stable isotope-labeling, chemical cross-linking, and mass spectrometry

The topology of the GCAP-2 homodimer was investigated by chemical cross-linking and high resolution mass spectrometry. Complementary conducted size-exclusion chromatography and analytical ultracentrifugation studies indicated that GCAP-2 forms a homodimer both in the absence and in the presence of Ca(2+). In-depth MS and MS/MS analysis of the cross-linked products was aided by (15)N-labeled GCAP-2. The use of isotope-labeled protein delivered reliable structural information on the GCAP-2 homodimer, enabling an unambiguous discrimination between cross-links within one monomer (intramolecular) or between two subunits (intermolecular). The limited number of cross-links obtained in the Ca(2+)-bound state allowed us to deduce a defined homodimeric GCAP-2 structure by a docking and molecular dynamics approach. In the Ca(2+)-free state, GCAP-2 is more flexible as indicated by the higher number of cross-links. We consider stable isotope-labeling to be indispensable for deriving reliable structural information from chemical cross-linking data of multi-subunit protein assemblies.

The ANF-RGC gene motif (669)WTAPELL(675) is vital for blood pressure regulation: biochemical mechanism

ANF-RGC is the prototype membrane guanylate cyclase, both the receptor and the signal transducer of the hormones ANF and BNP. After binding them at the extracellular domain, it, at its intracellular domain, signals activation of the C-terminal catalytic module and accelerates production of the second messenger, cyclic GMP. This, in turn, controls the physiological processes of blood pressure, cardiovascular function, fluid secretion, and others: metabolic syndrome, obesity, and apoptosis. The biochemical mechanism by which this single molecule controls these diverse processes, explicitly blood pressure regulation, is the subject of this study. In line with the concept that the structural modules of ANF-RGC are designed to respond to more than one yet distinctive signals, the study demonstrates the construction of a novel ANF-RGC-In-gene-(669)WTAPELL(675) mouse model. Through this model, the study establishes that (669)WTAPELL(675) is a vital ANF signal transducer motif of the guanylate cyclase. Its striking physiological features linked with their biochemistry are the following. (1) It controls the hormonally dependent cyclic GMP production in the kidney and the adrenal gland. Its deletion causes (2) hypertension and (3) cardiac hypertrophy. (4) These mice show higher levels of the plasma aldosterone. For the first time, a mere seven-amino acid-encoded motif of the mouse gene has been directly linked with the physiological control of blood pressure regulation, a detailed biochemistry of this linkage has been established, and a model for this linkage has been described.

Structural and functional characterization of receptor kinases with nucleotide cyclase activity

There has been an increase in the identification and characterization of plant receptor kinases possessing nucleotide cyclase activity. This has necessitated the development of robust methodologies for the structural and functional characterization of this biologically important family of proteins. Here we outline some of the techniques that can be effectively used in the characterization of this bifunctional family of proteins.

Ca(2+) modulation of ANF-RGC: new signaling paradigm interlocked with blood pressure regulation

ANF-RGC is the prototype receptor membrane guanylate cyclase that is both the receptor and the signal transducer of the most hypotensive hormones, ANF and BNP. It is a single-transmembrane protein. After binding these hormones at the extracellular domain, ANF-RGC at its intracellular domain signals the activation of the C-terminal catalytic module and accelerates the production of the second messenger, cyclic GMP, which controls blood pressure, cardiac vasculature, and fluid secretion. At present, this is the sole transduction mechanism and the physiological function of ANF-RGC. Through comprehensive studies involving biochemistry, immunohistochemistry, and blood pressure measurements in mice with targeted gene deletions, this study demonstrates a new signaling model of ANF-RGC that also controls blood pressure. In this model, (1) ANF-RGC is not the transducer of ANF and BNP, (2) its extracellular domain is not used for signaling, and (3) the signal flow is not downstream from the extracellular domain to the core catalytic domain. Instead, the signal is the intracellular Ca(2+), which is translated at the site of its reception, at the core catalytic domain of ANF-RGC. A model for this Ca(2+) signal transduction is diagrammed. It captures Ca(2+) through its Ca(2+) sensor myristoylated neurocalcin δ and upregulates ANF-RGC activity with a K(1/2) of 0.5 μM. The neurocalcin δ-modulated domain resides in the (849)DIVGFTALSAESTPMQVV(866) segment of ANF-RGC, which is a part of the core catalytic domain. Thereby, ANF-RGC is primed to receive, transmit, and translate the Ca(2+) signals into the generation of cyclic GMP at a rapid rate. The study defines a new paradigm of membrane guanylate cyclase signaling, which is linked to the physiology of cardiac vasculature regulation and possibly also to fluid secretion.

Differential Ca(2+) sensor guanylate cyclase activating protein modes of photoreceptor rod outer segment membrane guanylate cyclase signaling

Photoreceptor ROS-GC1 (rod outer segment membrane guanylate cyclase) is a vital component of phototransduction. It is a bimodal Ca(2+) signal transduction switch, operating between 20 and ∼1000 nM. Modulated by Ca(2+) sensors guanylate cyclase activating proteins 1 and 2 (GCAP1 and GCAP2, respectively), decreasing [Ca(2+)](i) from 200 to 20 nM progressively turns it "on", as does the modulation by the Ca(2+) sensor S100B, increasing [Ca(2+)](i) from 100 to 1000 nM. The GCAP mode plays a vital role in phototransduction in both rods and cones and the S100B mode in the transmission of neural signals to cone ON-bipolar cells. Through a programmed domain deletion, expression, in vivo fluorescence spectroscopy, and in vitro reconstitution experiments, this study demonstrates that the biochemical mechanisms modulated by two GCAPs in Ca(2+) signaling of ROS-GC1 activity are totally different. (1) They involve different structural domains of ROS-GC1. (2) Their signal migratory pathways are opposite: GCAP1 downstream and GCAP2 upstream. (3) Importantly, the isolated catalytic domain, translating the GCAP-modulated Ca(2+) signal into the generation of cyclic GMP, in vivo, exists as a homodimer, the two subunits existing in an antiparallel conformation. Furthermore, the findings demonstrate that the N-terminally placed signaling helix domain is not required for the catalytic domain's dimeric state. The upstream GCAP2-modulated pathway is the first of its kind to be observed for any member of the membrane guanylate cyclase family. It defines a new model of Ca(2+) signal transduction.

Ca(2+)-sensors and ROS-GC: interlocked sensory transduction elements: a review

From its initial discovery that ROS-GC membrane guanylate cyclase is a mono-modal Ca(2+)-transduction system linked exclusively with the photo-transduction machinery to the successive finding that it embodies a remarkable bimodal Ca(2+) signaling device, its widened transduction role in the general signaling mechanisms of the sensory neuron cells was envisioned. A theoretical concept was proposed where Ca(2+)-modulates ROS-GC through its generated cyclic GMP via a nearby cyclic nucleotide gated channel and creates a hyper- or depolarized sate in the neuron membrane (Ca(2+) Binding Proteins 1:1, 7-11, 2006). The generated electric potential then becomes a mode of transmission of the parent [Ca(2+)](i) signal. Ca(2+) and ROS-GC are interlocked messengers in multiple sensory transduction mechanisms. This comprehensive review discusses the developmental stages to the present status of this concept and demonstrates how neuronal Ca(2+)-sensor (NCS) proteins are the interconnected elements of this elegant ROS-GC transduction system. The focus is on the dynamism of the structural composition of this system, and how it accommodates selectivity and elasticity for the Ca(2+) signals to perform multiple tasks linked with the SENSES of vision, smell, and possibly of taste and the pineal gland. An intriguing illustration is provided for the Ca(2+) sensor GCAP1 which displays its remarkable ability for its flexibility in function from being a photoreceptor sensor to an odorant receptor sensor. In doing so it reverses its function from an inhibitor of ROS-GC to the stimulator of ONE-GC membrane guanylate cyclase.

Antithetical modes of and the Ca(2+) sensors targeting in ANF-RGC and ROS-GC1 membrane guanylate cyclases

The membrane guanylate cyclase family has been branched into three subfamilies: natriuretic peptide hormone surface receptors, Ca²⁺-modulated neuronal ROS-GC, and Ca²⁺-modulated odorant surface receptor ONE-GC. The first subfamily is solely modulated by the extracellularly generated hormonal signals; the second, by the intracellularly generated sensory and sensory-linked signals; and the third, by combination of these two. The present study defines a new paradigm and a new mechanism of Ca²⁺ signaling. (1) It demonstrates for the first time that ANF-RGC, the prototype member of the surface receptor subfamily, is stimulated by free [Ca²⁺]_i. The stimulation occurs via myristoylated form of neurocalcin δ, and both the guanylate cyclase and the calcium sensor neurocalcin δ are present in the glomerulosa region of the adrenal gland. (2) The EF-2, EF-3 and EF-4 hands of GCAP1 sense the progressive increment of [Ca²⁺]_i and with a K_1/2 of 100 nM turn ROS-GC1 “OFF.” In total reversal, the same EF hands upon sensing the progressive increment of [Ca²⁺]_i with K_1/2 turn ONE-GC “ON.” The findings suggest a universal Ca²⁺-modulated signal transduction theme of the membrane guanylate cyclase family; demonstrate that signaling of ANF-RGC occurs by the peptide hormones and also by [Ca²⁺]_i signals; that for the Ca²⁺ signal transduction, ANF-RGC functions as a two-component transduction system consisting of the Ca²⁺ sensor neurocalcin δ and the transducer ANF-RGC; and that the neurocalcin δ in this case expands beyond its NCS family. Furthermore, the study shows a novel mechanism of the [Ca²⁺]_i sensor GCAP1 where it acts as an antithetical NCS for the signaling mechanisms of ROS-GC1 and ONE-GC.

S100B serves as a Ca(2+) sensor for ROS-GC1 guanylate cyclase in cones but not in rods of the murine retina

Rod outer segment membrane guanylate cyclase (ROS-GC1) is a bimodal Ca(2+) signal transduction switch. Lowering [Ca(2+)](i) from 200 to 20 nM progressively turns it "ON" as does raising [Ca(2+)](i) from 500 to 5000 nM. The mode operating at lower [Ca(2+)](i) plays a vital role in phototransduction in both rods and cones. The physiological function of the mode operating at elevated [Ca(2+)](i) is not known. Through comprehensive studies on mice involving gene deletions, biochemistry, immunohistochemistry, electroretinograms and single cell recordings, the present study demonstrates that the Ca(2+)-sensor S100B coexists with and is physiologically linked to ROS-GC1 in cones but not in rods. It up-regulates ROS-GC1 activity with a K(1/2) for Ca(2+) greater than 500 nM and modulates the transmission of neural signals to cone ON-bipolar cells. Furthermore, a possibility is raised that under pathological conditions where [Ca(2+)](i) levels rise to and perhaps even enter the micromolar range, the S100B signaling switch will be turned "ON" causing an explosive production of CNG channel opening and further rise in [Ca(2+)](i) in cone outer segments. The findings define a new cone-specific Ca(2+)-dependent feature of photoreceptors and expand our understanding of the operational principles of phototransduction machinery.

EF hand-mediated Ca- and cGMP-signaling in photoreceptor synaptic terminals

Photoreceptors, the light-sensitive receptor neurons of the retina, receive and transmit a plethora of visual informations from the surrounding world. Photoreceptors capture light and convert this energy into electrical signals that are conveyed to the inner retina. For synaptic communication with the inner retina, photoreceptors make large active zones that are marked by synaptic ribbons. These unique synapses support continuous vesicle exocytosis that is modulated by light-induced, graded changes of membrane potential. Synaptic transmission can be adjusted in an activity-dependent manner, and at the synaptic ribbons, Ca²⁺- and cGMP-dependent processes appear to play a central role. EF-hand-containing proteins mediate many of these Ca²⁺- and cGMP-dependent functions. Since continuous signaling of photoreceptors appears to be prone to malfunction, disturbances of Ca²⁺- and cGMP-mediated signaling in photoreceptors can lead to visual defects, retinal degeneration (rd), and even blindness. This review summarizes aspects of signal transmission at the photoreceptor presynaptic terminals that involve EF-hand-containing Ca²⁺-binding proteins.

Guanylyl cyclase / atrial natriuretic peptide receptor-A: role in the pathophysiology of cardiovascular regulation

Atrial natriuretic factor (ANF), also known as atrial natriuretic peptide (ANP), is an endogenous and potent hypotensive hormone that elicits natriuretic, diuretic, vasorelaxant, and anti-proliferative effects, which are important in the control of blood pressure and cardiovascular events. One principal locus involved in the regulatory action of ANP and brain natriuretic peptide (BNP) is guanylyl cyclase / natriuretic peptide receptor-A (GC-A/NPRA). Studies on ANP, BNP, and their receptor, GC-A/NPRA, have greatly increased our knowledge of the control of hypertension and cardiovascular disorders. Cellular, biochemical, and molecular studies have helped to delineate the receptor function and signaling mechanisms of NPRA. Gene-targeted and transgenic mouse models have advanced our understanding of the importance of ANP, BNP, and GC-A/NPRA in disease states at the molecular level. Importantly, ANP and BNP are used as critical markers of cardiac events; however, their therapeutic potentials for the diagnosis and treatment of hypertension, heart failure, and stroke have just begun to be realized. We are now just at the initial stage of molecular therapeutics and pharmacogenomic advancement of the natriuretic peptides. More investigations should be undertaken and ongoing ones be extended in this important field.

The functional genomics of guanylyl cyclase/natriuretic peptide receptor-A: perspectives and paradigms

The cardiac hormones atrial natriuretic peptide and B-type natriuretic peptide (brain natriuretic peptide) activate guanylyl cyclase (GC)-A/natriuretic peptide receptor-A (NPRA) and produce the second messenger cGMP. GC-A/NPRA is a member of the growing family of GC receptors. The recent biochemical, molecular and genomic studies on GC-A/NPRA have provided important insights into the regulation and functional activity of this receptor protein, with a particular emphasis on cardiac and renal protective roles in hypertension and cardiovascular disease states. The progress in this field of research has significantly strengthened and advanced our knowledge about the critical roles of Npr1 (coding for GC-A/NPRA) in the control of fluid volume, blood pressure, cardiac remodeling, and other physiological functions and pathological states. Overall, this review attempts to provide insights and to delineate the current concepts in the field of functional genomics and signaling of GC-A/NPRA in hypertension and cardiovascular disease states at the molecular level.

657WTAPELL663 motif of the photoreceptor ROS-GC1: a general phototransduction switch

This study documents the identity of an intriguing transduction mechanism of the [Ca(2+)](i) signals by the photoreceptor ROS-GC1. Despite their distal residences and operational modes in phototransduction, the two GCAPs transmit and activate ROS-GC1 through a common Ca(2+) transmitter switch (Ca(2+)TS). A combination of immunoprecipitation, fluorescent spectroscopy, mutational analyses and reconstitution studies has been used to demonstrate that the structure of this switch is (657)WTAPELL(663). The two Ca(2+) signaling GCAP pathways converge in Ca(2+)TS, get transduced, activate ROS-GC1, generate the LIGHT signal second messenger cyclic GMP and yet functionally perform divergent operations of the phototransduction machinery. The findings define a new Ca(2+)-modulated photoreceptor ROS-GC transduction model; it is depicted and discussed for its application to processing the different shades of LIGHT.

Transmembrane guanylate cyclase in intestinal pathophysiology

PURPOSE OF REVIEW

Production of cyclic guanosine monophosphate (cGMP) by guanylate cyclase is of critical importance to gastrointestinal physiology. Tight regulation of cGMP concentration is necessary for proper intestinal secretion and intestinal epithelial cell proliferative and apoptotic homeostasis. This review focuses on recent work detailing the role of a subset of transmembrane guanylate cyclases in the pathophysiology of intestinal secretory and motility disorders and intestinal epithelial cell transformation. Also considered is the potential for therapeutic manipulation of intestinal guanylate cyclase/cGMP signaling for the correction of chronic constipation and gastrointestinal cancer.

RECENT FINDINGS

Recent work in mice and humans suggests a role for transmembrane guanylate cyclases in intestinal fluid secretion as well as hormonal enteric-renal signaling which mediates postprandial natriuresis. Transmembrane guanylate cyclases are also important in gastrointestinal transit rate and motility. Ongoing clinical trials have found that guanylate cyclase activating peptides are safe and effective in the treatment of constipation-predominant irritable bowel syndrome and chronic constipation. In addition, accumulating evidence indicates that membrane-associated guanylate cyclase receptors regulate intestinal epithelial cell homeostatic proliferation and apoptosis as well as gastrointestinal malignancy. The anticancer activity of cGMP signaling in animal studies suggests additional therapeutic applications for guanylate cyclase agonists.

SUMMARY

Progress toward understanding gastrointestinal transmembrane guanylate cyclase/cGMP physiology has recently accelerated due to definitive in-vitro studies and work using gene-targeted animal models and has facilitated the development of safe and effective drugs designed to regulate cGMP production in the intestine. Current work should be directed toward a detailed understanding of cGMP effector pathways and the manner in which subcellular concentrations of cGMP regulate them to influence intestinal health and disease.

Allosteric modification, the primary ATP activation mechanism of atrial natriuretic factor receptor guanylate cyclase

ANF-RGC is the prototype receptor membrane guanylate cyclase being both the receptor and the signal transducer of the most hypotensive hormones, ANF and BNP. It is a single transmembrane-spanning protein. After binding these hormones at the extracellular domain it at its intracellular domain signals activation of the C-terminal catalytic module and accelerates the production of its second messenger, cyclic GMP, which controls blood pressure, cardiac vasculature, and fluid secretion. ATP is obligatory for the posttransmembrane dynamic events leading to ANF-RGC activation. It functions through the ATP-regulated module, ARM (KHD) domain, of ANF-RGC. In the current over a decade held model "phosphorylation of the KHD is absolutely required for hormone-dependent activation of NPR-A" [Potter, L. R., and Hunter, T. (1998) Mol. Cell. Biol. 18, 2164-2172]. The presented study challenges this concept. It demonstrates that, instead, ATP allosteric modification of ARM is the primary signaling step of ANF-GC activation. In this two-step new dynamic model, ATP in the first step binds ARM. This triggers in it a chain of transduction events, which cause its allosteric modification. The modification partially activates (about 50%) ANF-RGC and, concomitantly, also prepares the ARM for the second successive step. In this second step, ARM is phosphorylated and ANF-RGC achieves additional (∼50%) full catalytic activation. The study defines a new paradigm of the ANF-RGC signaling mechanism.

Cure and curse: E. coli heat-stable enterotoxin and its receptor guanylyl cyclase C

Enterotoxigenic Escherichia coli (ETEC) associated diarrhea is responsible for roughly half a million deaths per year, the majority taking place in developing countries. The main agent responsible for these diseases is the bacterial heat-stable enterotoxin STa. STa is secreted by ETEC and after secretion binds to the intestinal receptor guanylyl cyclase C (GC-C), thus triggering a signaling cascade that eventually leads to the release of electrolytes and water in the intestine. Additionally, GC-C is a specific marker for colorectal carcinoma and STa is suggested to have an inhibitory effect on intestinal carcinogenesis. To understand the conformational events involved in ligand binding to GC-C and to devise therapeutic strategies to treat both diarrheal diseases and colorectal cancer, it is paramount to obtain structural information on the receptor ligand system. Here we summarize the currently available structural data and report on physiological consequences of STa binding to GC-C in intestinal epithelia and colorectal carcinoma cells.

ATP allosteric activation of atrial natriuretic factor receptor guanylate cyclase

Atrial natriuretic factor receptor guanylate cyclase (ANF-RGC) is the receptor and the signal transducer of two natriuretic peptide hormones: atrial natriuretic factor and brain natriuretic peptide. It is a single transmembrane-spanning protein. It binds these hormones at its extracellular domain and activates its intracellular catalytic domain. This results in the accelerated production of cyclic GMP, a second messenger in controlling blood pressure, cardiac vasculature and fluid secretion. ATP is obligatory for the transduction of this hormonal signal. Two models of ATP action have been proposed. In Model 1, it is a direct allosteric transducer. It binds to the defined regulatory domain (ATP-regulated module) juxtaposed to the C-terminal side of the transmembrane domain of ANF-RGC, induces a cascade of temporal and spatial changes and activates the catalytic module residing at the C-terminus of the cyclase. In Model 2, before ATP can exhibit its allosteric effect, ANF-RGC must first be phosphorylated by an as yet unidentified protein kinase. This initial step is obligatory in atrial natriuretic factor signaling of ANF-RGC. Until now, none of these models has been directly validated because it has not been possible to segregate the allosteric and the phosphorylation effects of ATP in ANF-RGC activation. The present study accomplishes this aim through a novel probe, staurosporine. This unequivocally validates Model 1 and settles the over two-decade long debate on the role of ATP in ANF-RGC signaling. In addition, the present study demonstrates that the mechanisms of allosteric modification of ANF-RGC by staurosporine and adenylyl-imidodiphosphate, a nonhydrolyzable analog of ATP, are almost (or totally) identical.