A novel calcium-regulated membrane guanylate cyclase transduction system in the olfactory neuroepithelium

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.

Structural basis of retinal membrane guanylate cyclase regulation by GCAP1 and RD3

Retinal membrane guanylate cyclases (RetGC1 and RetGC2) are expressed in photoreceptor rod and cone cells, where they promote the onset of visual recovery during phototransduction. The catalytic activity of RetGCs is regulated by their binding to regulatory proteins, guanylate cyclase activating proteins (GCAP1-5) and the retinal degeneration 3 protein (RD3). RetGC1 is activated by its binding to Ca2+-free/Mg2+-bound GCAP1 at low cytosolic Ca2+ levels in light-activated photoreceptors. By contrast, RetGC1 is inactivated by its binding to Ca2+-bound GCAP1 and/or RD3 at elevated Ca2+ levels in dark-adapted photoreceptors. The Ca2+ sensitive cyclase activation helps to replenish the cytosolic cGMP levels in photoreceptors during visual recovery. Mutations in RetGC1, GCAP1 or RD3 that disable the Ca2+-dependent regulation of cyclase activity are genetically linked to rod/cone dystrophies and other inherited forms of blindness. Here I review the structural interaction of RetGC1 with GCAP1 and RD3. I propose a two-state concerted model in which the dimeric RetGC1 allosterically switches between active and inactive conformational states with distinct quaternary structures that are oppositely stabilized by the binding of GCAP1 and RD3. The binding of Ca2+-free/Mg2+-bound GCAP1 is proposed to activate the cyclase by stabilizing RetGC1 in an active conformation (R-state), whereas Ca2+-bound GCAP1 and/or RD3 inhibit the cyclase by locking RetGC1 in an inactive conformation (T-state). Exposed hydrophobic residues in GCAP1 (residues H19, Y22, M26, F73, V77, W94) are essential for cyclase activation and could be targeted by rational drug design for the possible treatment of rod/cone dystrophies.

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.

Dimerization of Neuronal Calcium Sensor Proteins

Neuronal calcium sensor (NCS) proteins are EF-hand containing Ca²⁺ binding proteins that regulate sensory signal transduction. Many NCS proteins (recoverin, GCAPs, neurocalcin and visinin-like protein 1 (VILIP1)) form functional dimers under physiological conditions. The dimeric NCS proteins have similar amino acid sequences (50% homology) but each bind to and regulate very different physiological targets. Retinal recoverin binds to rhodopsin kinase and promotes Ca²⁺-dependent desensitization of light-excited rhodopsin during visual phototransduction. The guanylyl cyclase activating proteins (GCAP1–5) each bind and activate retinal guanylyl cyclases (RetGCs) in light-adapted photoreceptors. VILIP1 binds to membrane targets that modulate neuronal secretion. Here, I review atomic-level structures of dimeric forms of recoverin, GCAPs and VILIP1. The distinct dimeric structures in each case suggest that NCS dimerization may play a role in modulating specific target recognition. The dimerization of recoverin and VILIP1 is Ca²⁺-dependent and enhances their membrane-targeting Ca²⁺-myristoyl switch function. The dimerization of GCAP1 and GCAP2 facilitate their binding to dimeric RetGCs and may allosterically control the Ca²⁺-dependent activation of RetGCs.

CO2/bicarbonate modulates cone photoreceptor ROS-GC1 and restores its CORD6-linked catalytic activity

This study with recombinant reconstituted system mimicking the cellular conditions of the native cones documents that photoreceptor ROS-GC1 is modulated by gaseous CO₂. Mechanistically, CO₂ is sensed by carbonic anhydrase (CAII), generates bicarbonate that, in turn, directly targets the core catalytic domain of ROS-GC1, and activates it to increased synthesis of cyclic GMP. This, then, functions as a second messenger for the cone phototransduction. The study demonstrates that, in contrast to the Ca²⁺-modulated phototransduction, the CO₂ pathway is Ca²⁺-independent, yet is linked with it and synergizes it. It, through R⁷⁸⁷C mutation in the third heptad of the signal helix domain of ROS-GC1, affects cone-rod dystrophy, CORD6. CORD6 is caused firstly by lowered basal and GCAP1-dependent ROS-GC1 activity and secondly, by a shift in Ca²⁺ sensitivity of the ROS-GC1/GCAP1 complex that remains active in darkness. Remarkably, the first but not the second defect disappears with bicarbonate thus explaining the basis for CORD6 pathological severity. Because cones, but not rods, express CAII, the excessive synthesis of cyclic GMP would be most acute in cones.

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.

Molecular and neuronal homology between the olfactory systems of zebrafish and mouse

Studies of the two major olfactory organs of rodents, the olfactory mucosa (OM) and the vomeronasal organ (VNO), unraveled the molecular basis of smell in vertebrates. However, some vertebrates lack a VNO. Here we generated and analyzed the olfactory transcriptome of the zebrafish and compared it to the olfactory transcriptomes of mouse to investigate the evolutionary and molecular relationship between single and dual olfactory systems. Our analyses revealed a high degree of molecular conservation, with orthologs of mouse olfactory cell-specific markers and all but one of their chemosensory receptor classes expressed in the single zebrafish olfactory organ. Zebrafish chemosensory receptor genes are expressed across a large dynamic range and their RNA abundance correlates positively with the number of neurons expressing that RNA. Thus we estimate the relative proportions of neuronal sub-types expressing different chemosensory receptors. Receptor repertoire size drives the absolute abundance of different classes of neurons, but we find similar underlying patterns in both species. Finally, we identified novel marker genes that characterize rare neuronal populations in both mouse and zebrafish. In sum, we find that the molecular and cellular mechanisms underpinning olfaction in teleosts and mammals are similar despite 430 million years of evolutionary divergence.

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.

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.

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.

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.

Ca(2+) sensor GCAP1: A constitutive element of the ONE-GC-modulated odorant signal transduction pathway

In a small subset of the olfactory sensory neurons, the odorant receptor ONE-GC guanylate cyclase is a central transduction component of the cyclic GMP signaling pathway. In a two-step transduction model, the odorant, uroguanylin, binds to the extracellular domain and activates its intracellular domain to generate the odorant second messenger, cyclic GMP. This study via comprehensive technology, including gene deletion, live cell Forster resonance energy transfer (FRET), and surface plasmon resonance (SPR) spectroscopy, documents the identity of a remarkably intriguing operation of a Ca(2+) sensor component of the ONE-GC transduction machinery, GCAP1. In the ciliary membranes, the sites of odorant transduction, GCAP1 is biochemically and physiologically coupled to ONE-GC. Strikingly, this coupling reverses its well- established function in ROS-GC1 signaling, linked with phototransduction. In response to the free Ca(2+) range from nanomolar to semimicromolar, it inhibits ROS-GC1, yet in this range, it incrementally stimulates ONE-GC. These two opposite modes of signaling two SENSORY processes by a single Ca(2+) sensor define a new transduction paradigm of membrane guanylate cyclases. This paradigm is pictorially presented.

Neurocalcin delta modulation of ROS-GC1, a new model of Ca(2+) signaling

ROS-GC1 membrane guanylate cyclase is a Ca(2+) bimodal signal transduction switch. It is turned "off" by a rise in free Ca(2+) from nanomolar to the semicromolar range in the photoreceptor outer segments and the olfactory bulb neurons; by a similar rise in the bipolar and ganglion retinal neurons it is turned "on". These opposite operational modes of the switch are specified by its Ca(2+) sensing devices, respectively termed GCAPs and CD-GCAPs. Neurocalcin delta is a CD-GCAP. In the present study, the neurocalcin delta-modulated site, V(837)-L(858), in ROS-GC1 has been mapped. The location and properties of this site are unique. It resides within the core domain of the catalytic module and does not require the alpha-helical dimerization domain structural element (amino acids 767-811) for activating the catalytic module. Contrary to the current beliefs, the catalytic module is intrinsically active; it is directly regulated by the neurocalcin delta-modulated Ca(2+) signal and is dimeric in nature. A fold recognition based model of the catalytic domain of ROS-GC1 was built, and neurocalcin delta docking simulations were carried out to define the three-dimensional features of the interacting domains of the two molecules. These findings define a new transduction model for the Ca(2+) signaling of ROS-GC1.

Distinct ONE-GC transduction modes and motifs of the odorants: Uroguanylin and CO(2)

In a subset of the olfactory sensory neurons ONE-GC($) membrane guanylate cyclase is a central component of two odorant-dependent cyclic GMP signaling pathways. These odorants are uroguanylin and CO(2). The present study was designed to decipher the biochemical and molecular differences between these two odorant signaling mechanisms. The study shows (1) in contrast to uroguanylin, CO(2) transduction mechanism is Ca(2+)-independent. (2) CO(2) transduction site, like that of uroguanylin-neurocalcin delta, resides in the core catalytic domain, aa 880-1028, of ONE-GC. (3) The site, however, does not overlap the signature neurocalcin delta signal transduction domain, (908)LSEPIE(913). Finally, (4) this study negates the prevailing concept that CO(2) uniquely signals ONE-GC activity (Sun et al. [19]; Guo et al. [21]). It demonstrates that it also signals the activation of photoreceptor membrane guanylate cyclase ROS-GC1. These results show an additional new transduction mechanism of the membrane guanylate cyclases and broaden our understanding of the molecular mechanisms by which different odorants using a single guanylate cyclase can regulate diverse cyclic GMP signaling pathways.

Membrane guanylate cyclase is a beautiful signal transduction machine: overview

This article is a sequel to the four earlier comprehensive reviews which covered the field of membrane guanylate cyclase from its origin to the year 2002 (Sharma in Mol Cell Biochem 230:3-30, 2002) and then to the year 2004 (Duda et al. in Peptides 26:969-984, 2005); and of the Ca(2+)-modulated membrane guanylate cyclase to the year 1997 (Pugh et al. in Biosci Rep 17:429-473, 1997) and then to 2004 (Sharma et al. in Curr Top Biochem Res 6:111-144, 2004). This article contains three parts. The first part is "Historical"; it is brief, general, and freely borrowed from the earlier reviews, covering the field from its origin to the year 2004 (Sharma in Mol Cell Biochem, 230:3-30, 2002; Duda et al. in Peptides 26:969-984, 2005). The second part focuses on the "Ca(2+)-modulated ROS-GC membrane guanylate cyclase subfamily". It is divided into two sections. Section "Historical" and covers the area from its inception to the year 2004. It is also freely borrowed from an earlier review (Sharma et al. in Curr Top Biochem Res 6:111-144, 2004). Section "Ca(2+)-modulated ROS-GC membrane guanylate cyclase subfamily" covers the area from the year 2004 to May 2009. The objective is to focus on the chronological development, recognize major contributions of the original investigators, correct misplaced facts, and project on the future trend of the field of mammalian membrane guanylate cyclase. The third portion covers the present status and concludes with future directions in the field.

Receptor guanylyl cyclases in mammalian olfactory function

The contributions of guanylyl cyclases to sensory signaling in the olfactory system have been unclear. Recently, studies of a specialized subpopulation of olfactory sensory neurons (OSNs) located in the main olfactory epithelium have provided important insights into the neuronal function of one receptor guanylyl cyclase, GC-D. Mice expressing reporters such as beta-galactosidase and green fluorescent protein in OSNs that normally express GC-D have allowed investigators to identify these neurons in situ, facilitating anatomical and physiological studies of this sparse neuronal population. The specific perturbation of GC-D function in vivo has helped to resolve the role of this guanylyl cyclase in the transduction of olfactory stimuli. Similar approaches could be useful for the study of the orphan receptor GC-G, which is expressed in another distinct subpopulation of sensory neurons located in the Grueneberg ganglion. In this review, we discuss key findings that have reinvigorated the study of guanylyl cyclase function in the olfactory system.

Odorant-linked ROS-GC subfamily membrane guanylate cyclase transduction system

This review focuses on the principles of the Ca(2+)-modulated ROS-GC subfamily transduction system linked with the mammalian olfactory transduction field, its historical development, and the present day status on its constitution and operational mechanisms controlling the process of olfactory-transduction. Beginning parts of this article are freely borrowed from the earlier reviews of the authors (Sharma RK, Duda T, Venkataraman V, Koch KW, Curr Topics Biochem Res 6:111-144, 2004; Duda T, Venkataraman V, Sharma RK, Neuronal calcium sensor proteins, pp 91-113, Nova Science Publishers, Inc., 2007).

Ca(2+)-modulated membrane guanylate cyclase in the testes

To date, the calcium-regulated membrane guanylate cyclase Rod Outer Segment Guanylate Cyclase type 1 (ROS-GC1) transduction system in addition to photoreceptors is known to be expressed in three other types of neuronal cells: in the pinealocytes, mitral cells of the olfactory bulb and the gustatory epithelium of tongue. Very recent studies from our laboratory show that expression of ROS-GC1 is not restricted to the neuronal cells; the male gonads and the spermatozoa also express ROS-GC1. In this presentation, the authors review the existing information on the localization and function of guanylate cyclase with special emphasis on Ca(2+)-modulated membrane guanylate cyclase, ROS-GC1, in the testes. The role of ROS-GC1 and its Ca(2+)-sensing modulators in the processes of spermatogenesis and fertilization are discussed.

Diversity of sensory guanylate cyclases in teleost fishes

Teleost fishes like medaka fish (Oryzias latipes), zebrafish (Danio rerio), and pufferfish (Fugu rubripes) contain in their genomes a larger number of guanylate cyclases and guanylate cyclase-activating proteins than mammals. Based on amino acid sequence alignments a group of transmembrane sensory guanylate cyclases can be identified, which are mainly if not exclusively expressed in sensory organs like the retina and olfactory tissue. Retina specific guanylate cyclases and guanylate cyclase-activating proteins in the zebrafish show dynamic changes in their spatial-temporal expression patterns and transcripts of the corresponding genes appear coincidently with the beginning of cone cell maturation at 3 days post-fertilization. Expression patterns of the guanylate cyclase signaling systems during larval development are correlated with the special habitat challenges of zebrafishes in the wild.

Olfactory receptors: G protein-coupled receptors and beyond

Sensing the chemical environment is critical for all organisms. Diverse animals from insects to mammals utilize highly organized olfactory system to detect, encode, and process chemostimuli that may carry important information critical for health, survival, social interactions and reproduction. Therefore, for animals to properly interpret and react to their environment it is imperative that the olfactory system recognizes chemical stimuli with appropriate selectivity and sensitivity. Because olfactory receptor proteins play such an essential role in the specific recognition of diverse stimuli, understanding how they interact with and transduce their cognate ligands is a high priority. In the nearly two decades since the discovery that the mammalian odorant receptor gene family constitutes the largest group of G protein-coupled receptor (GPCR) genes, much attention has been focused on the roles of GPCRs in vertebrate and invertebrate olfaction. However, is has become clear that the 'family' of olfactory receptors is highly diverse, with roles for enzymes and ligand-gated ion channels as well as GPCRs in the primary detection of olfactory stimuli.

Ca2+-modulated ONE-GC odorant signal transduction

In a subset of olfactory epithelium the odorant receptor guanylate cyclase, ONE-GC, is a central transduction component of the cyclic GMP signaling pathway. The odorant binds to the extracellular domain and activates its intracellular catalytic domain to generate the odorant second messenger, cyclic GMP. The present study demonstrates that it is a two-step, Ca(2+)-independent and Ca(2+)-dependent, sequential process. In step one, the odorant, uroguanylin, binds ONE-GC and primes it for stimulation. In step two, Ca(2+)-bound neurocalcin delta binds to the defined intracellular domain and saturates ONE-GC activity. A prototype model is proposed that depicts this signal transduction process.

Hippocalcin, new Ca(2+) sensor of a ROS-GC subfamily member, ONE-GC, membrane guanylate cyclase transduction system

Hippocalcin is a member of the neuronal Ca(2+) sensor protein family. Among its many biochemical functions, its established physiological function is that via neuronal apoptosis inhibitory protein it protects the neurons from Ca(2+)-induced cell death. The precise biochemical mechanism/s, through which hippocalcin functions, is not clear. In the present study, a new mechanism by which it functions is defined. The bovine form of hippocalcin (BovHpca) native to the hippocampus has been purified, sequenced, cloned, and studied. The findings show that there is the evolutionary conservation of its structure. It is a Ca(2+)-sensor of a variant form of the ROS-GC subfamily of membrane guanylate cyclases, ONE-GC. It senses physiological increments of Ca(2+) with a K(1/2) of 0.5 microM and stimulates ONE-GC or ONE-GC-like membrane guanylate cyclase. The Hpca-modulated ONE-GC-like transduction system exists in the hippocampal neurons. And hippocalcin-modulated ONE-GC transduction system exists in the olfactory receptor neuroepithelium. The Hpca-gene knock out studies demonstrate that the portion of this is about 30% of the total membrane guanylate cyclase transduction system. The findings establish Hpca as a new Ca(2+) sensor modulator of the ROS-GC membrane guanylate cyclase transduction subfamily. They support the concept on universality of the presence and operation of the ROS-GC transduction system in the sensory and sensory-linked neurons. They validate that the ROS-GC transduction system exists in multiple forms. And they provide an additional mechanism by which ROS-GC subfamily acts as a transducer of the Ca(2+) signals originating in the neurons.

Rod outer segment membrane guanylate cyclase type 1 (ROS-GC1) calcium-modulated transduction system in the sperm

OBJECTIVE

Evaluation of the presence of a Ca(2+)-regulated membrane guanylate cyclase signal transudation system in the spermatozoa.

DESIGN

Experimental study.

SETTING

Research university laboratory.

PATIENT(S)

Human sperm obtained from healthy donors who met the criteria of the World Health Organization for normozoospermia and bovine semen collected from bulls of proven fertility.

INTERVENTION(S)

Radioimmunoassay and immunohistochemistry of human and bovine spermatozoa.

MAIN OUTCOME MEASURE(S)

The membrane guanylate cyclase activity and the presence of membrane guanylate cyclase transduction machinery components in the spermatozoa.

RESULT(S)

The identity of a Ca(2+)-modulated membrane guanylate cyclase transduction machinery in human and bovine spermatozoa has been documented. The machinery is both inhibited and stimulated within nanomolar to semimicromolar range of free Ca(2+). The transduction component of this machinery is the rod outer segment membrane guanylate cyclase type 1 (ROS-GC1). The enzyme coexists with three Ca(2+)-dependent modulators: guanylate cyclase activating protein type 1 (GCAP1), S100B and neurocalcin delta. ROS-GC1 and its modulators are present in the heads and tails of both species' spermatozoa.

CONCLUSION(S)

The coexpression of ROS-GC1 and its activators in spermatozoa suggests that the Ca(2+)-modulated ROS-GC1 transduction system may be a part of the fertilization machinery.

Visinin-like proteins (VSNLs): interaction partners and emerging functions in signal transduction of a subfamily of neuronal Ca2+ -sensor proteins

The visinin-like protein (VSNL) subfamily, including VILIP-1 (the founder protein), VILIP-2, VILIP-3, hippocalcin, and neurocalcin delta, constitute a highly homologous subfamily of neuronal calcium sensor (NCS) proteins. Comparative studies have shown that VSNLs are expressed predominantly in the brain with restricted expression patterns in various subsets of neurons but are also found in peripheral organs. In addition, the proteins display differences in their calcium affinities, in their membrane-binding kinetics, and in the intracellular targets to which they associate after calcium binding. Even though the proteins use a similar calcium-myristoyl switch mechanism to translocate to cellular membranes, they show calcium-dependent localization to various subcellular compartments when expressed in the same neuron. These distinct calcium-myristoyl switch properties might be explained by specificity for defined phospholipids and membrane-bound targets; this enables VSNLs to modulate various cellular signal transduction pathways, including cyclic nucleotide and MAPK signaling. An emerging theme is the direct or indirect effect of VSNLs on gene expression and their interaction with components of membrane trafficking complexes, with a possible role in membrane trafficking of different receptors and ion channels, such as glutamate receptors of the kainate and AMPA subtype, nicotinic acetylcholine receptors, and Ca(2+)-channels. One hypothesis is that the highly homologous VSNLs have evolved to fulfil specialized functions in membrane trafficking and thereby affect neuronal signaling and differentiation in defined subsets of neurons. VSNLs are involved in differentiation processes showing a tumor-invasion-suppressor function in peripheral organs. Finally, VSNLs play neuroprotective and neurotoxic roles and have been implicated in neurodegenerative diseases.

ATP signaling site in the ARM domain of atrial natriuretic factor receptor guanylate cyclase

Atrial natriuretic factor (ANF) receptor guanylate cyclase (ANF-RGC) is a single transmembrane spanning modular protein. It binds ANF to its extracellular module and activates its intracellular catalytic module located at its carboxyl end. This results in the accelerated production of cyclic GMP, which acts as a critical second messenger in decreasing blood pressure. Two mechanistic models have been proposed for the ANF signaling of ANF-RGC. One is ATP-dependent and the other ATP-independent. In the former, ATP works through the ARM (ATP-regulated transduction module) of ANF-RGC. This model has recently been challenged [Antos et al. (2005) J Biol Chem 280:26928-26932] in support of the ATP-independent model. The present in-depth study analyzes the major principles of this challenge and concludes that the challenge lacks merit. The study then moves on to dissect the ATP mechanism of ANF signaling of ANF-RGC. It shows that the ATP photoaffinity probe, [gamma(32)P]-8-azido-ATP, reacts with Cys(634) residue in the ATP-binding pocket of ARM, and also signals the ANF-dependent activation of ANF-RGC. The target site of the 8-azido (nitrene) group is between the Cys(634) and Val(635) bond of the ATP-binding pocket. Thus, the study experimentally validates the ARM model-predicted role of Val(635) in the folding pattern of the ATP-binding pocket. And, it also identifies another residue Cys(634) that along with eight already identified residues is a part of the fold around the adenine ring of the ATP pocket. This information establishes the direct role of ATP in ANF signal transduction model of ANF-RGC, and provides a significant advancement on the mechanism by which the ATP-dependent transduction model operates.

Novel frequenin-modulated Ca2+-signaling membrane guanylate cyclase (ROS-GC) transduction pathway in bovine hippocampus

Frequenin is a member of the neuronal Ca(2+) sensor protein family, implicated in being the modulator of the neurotransmitter release, potassium channels, phosphatidylinositol signaling pathway and the Ca(2+)-dependent exocytosis of dense-core granules in the PC12 cells. Frequenin exhibits these biological activities through its Ca(2+) myristoyl switch, yet the switch is functionally inactive. These structural and functional traits of frequenin have been derived through the use of recombinant frequenin. In the present study, frequenin (BovFrq) native to the bovine hippocampus has been purified, sequenced for its 9 internal fragments, cloned, and studied. The findings show that structure of the BovFrq is identical to its form present in chicken, rat, mouse and human, indicating its evolutionary conservation. Its Ca(2+) myristoyl switch is active in the hippocampus. And, BovFrq physically interacts and turns on yet undisclosed ONE-GC-like ROS-GC membrane guanylate cyclase transduction machinery in the hippocampal neurons. This makes BovFrq a new Ca(2+)-sensor modulator of a novel ROS-GC transduction machinery. The study demonstrates the presence and mechanistic features of this cyclic GMP signaling pathway in the hippocampal neurons, and also provides one more support for the evolving concept where the Ca(2+)-modulated membrane guanylate cyclase transduction machinery in its variant forms is a central operational component of all neurons.

Two membrane juxtaposed signaling modules in ANF-RGC are interlocked

Atrial natriuretic factor (ANF) receptor guanylate cyclase ANF-RGC is a single transmembrane spanning modular protein. Juxtaposed to each side of the transmembrane module is a Cys423-Cys432 disulfide ANF signaling module motif and the ATP-regulated transduction module (ARM) motif. The signaling module motif is conserved in nearly all membrane guanylate cyclases and is believed to be critical in the signaling activities of all membrane guanylate cyclases. The present study with the model system of the olfactory membrane guanylate cyclase shows that this concept is not valid. Furthermore, the study shows that in ANF-GC the signaling motif works through the ARM domain. A new signaling model is proposed where in its natural state the disulfide structural motif represses the ARM domain activity, which, in turn, represses the catalytic module activity of ANF-RGC. ANF signaling relieves the disulfide structural motif restraint on the ARM inhibition and stimulates the catalytic module of the cyclase.

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.

Calcium-modulated ciliary membrane guanylate cyclase transduction machinery: constitution and operational principles

Odorant transduction is a biochemical process by which the odorant signal generates the electric signal. The cilia of the olfactory neuroepithelium are the sites of this process. This study documents the detailed biochemical, structural and functional description of an odorant-responsive Ca2+ -modulated membrane guanylate cyclase transduction machinery in the cilia. Myristoylated (myr)-neurocalcin delta is the Ca2+ -sensor component and the cyclase, ONE-GC, the transduction component of the machinery. Myr-neurocalcin delta senses increments in free Ca2+, binds to a defined domain of ONE-GC and stimulates the cyclase. The findings enable the formulation of an odorant transduction model in which three pivotal signaling components--Ca2+, myr-neurocalcin delta and ONE-GC--of the transduction machinery are locked. A glaring feature of the model is that its Ca2+ -dependent operational principle is opposite to the phototransduction model.

S100B-modulated Ca2+-dependent ROS-GC1 transduction machinery in the gustatory epithelium: a new mechanism in gustatory transduction

Gustatory transduction is a biochemical process by which the gustatory signal generates the electric signal. The microvilli of the taste cells in the gustatory epithelium are the sites of gustatory transduction. This study documents the biochemical, molecular, and functional identity of the Ca2+-modulated membrane guanylate cyclase transduction machinery in the bovine gustatory epithelium. The machinery is a two-component system: the Ca2+-sensor protein, S100B; and the transducer, ROS-GC1. S100B senses increments in free Ca2+, undergoes conformational change, binds to the domain amino acids (aa) Gly962-Asn981 and via the transduction domain aa Ile1030-Gln1041 activates ROS-GC1, generating the second messenger, cyclic GMP. In a recent study, operational presence of this machinery has been demonstrated in the photoreceptor bipolar synapse [Duda et al., EMBO J. 21 (2002) 2547]. Thus, the machinery has a broader role in sensory perceptions, vision in the retinal neurons and gustation in the tongue. The entry of the ROS-GC transduction machinery defines the beginning of a new paradigm of Ca2+ signaling in the tongue.

Ca(2+) sensor S100beta-modulated sites of membrane guanylate cyclase in the photoreceptor-bipolar synapse

This study documents the identity of a calcium- regulated membrane guanylate cyclase transduction system in the photoreceptor-bipolar synaptic region. The guanylate cyclase is the previously characterized ROS-GC1 from the rod outer segments and its modulator is S100beta. S100beta senses increments in free Ca(2+) and stimulates the cyclase. Specificity of photoreceptor guanylate cyclase activation by S100beta is validated by the identification of two S100beta-regulatory sites. A combination of peptide competition, surface plasmon resonance binding and deletion mutation studies has been used to show that these sites are specific for S100beta and not for another regulator of ROS-GC1, guanylate cyclase-activating protein 1. One site comprises amino acids (aa) Gly962-Asn981, the other, aa Ile1030-Gln1041. The former represents the binding site. The latter binds S100beta only marginally, yet it is critical for control of maximal cyclase activity. The findings provide evidence for a new cyclic GMP transduction system in synaptic layers and thereby extend existing concepts of how a membrane-bound guanylate cyclase is regulated by small Ca(2+)-sensor proteins.