Atypical soluble guanylyl cyclases in Drosophila can function as molecular oxygen sensors

Mechanisms of gas sensing by internal sensory neurons in Drosophila larvae

Internal sensory neurons monitor the chemical and physical state of the body, providing critical information to the central nervous system for maintaining homeostasis and survival. A population of larval Drosophila sensory neurons, tracheal dendrite (td) neurons, elaborate dendrites along respiratory organs and may serve as a model for elucidating the cellular and molecular basis of chemosensation by internal neurons. We find that td neurons respond to decreases in O2 levels and increases in CO2 levels. We assessed the roles of atypical soluble guanylyl cyclases (Gycs) and a gustatory receptor (Gr) in mediating these responses. We found that Gyc88E/Gyc89Db were necessary for responses to hypoxia, and that Gr28b was necessary for responses to CO2. Targeted expression of Gr28b isoform c in td neurons rescued responses to CO2 in mutant larvae and also induced ectopic sensitivity to CO2 in the td network. Gas-sensitive td neurons were activated when larvae burrowed for a prolonged duration, demonstrating a natural-like feeding condition in which td neurons are activated. Together, our work identifies two gaseous stimuli that are detected by partially overlapping subsets of internal sensory neurons, and establishes roles for Gyc88E/Gyc89Db in the detection of hypoxia, and Gr28b in the detection of CO2.

Modulation of the NO-cGMP pathway has no effect on olfactory responses in the Drosophila antenna

Olfaction is a crucial sensory modality in insects and is underpinned by odor-sensitive sensory neurons expressing odorant receptors that function in the dendrites as odorant-gated ion channels. Along with expression, trafficking, and receptor complexing, the regulation of odorant receptor function is paramount to ensure the extraordinary sensory abilities of insects. However, the full extent of regulation of sensory neuron activity remains to be elucidated. For instance, our understanding of the intracellular effectors that mediate signaling pathways within antennal cells is incomplete within the context of olfaction in vivo. Here, with the use of optical and electrophysiological techniques in live antennal tissue, we investigate whether nitric oxide signaling occurs in the sensory periphery of Drosophila. To answer this, we first query antennal transcriptomic datasets to demonstrate the presence of nitric oxide signaling machinery in antennal tissue. Next, by applying various modulators of the NO-cGMP pathway in open antennal preparations, we show that olfactory responses are unaffected by a wide panel of NO-cGMP pathway inhibitors and activators over short and long timescales. We further examine the action of cAMP and cGMP, cyclic nucleotides previously linked to olfactory processes as intracellular potentiators of receptor functioning, and find that both long-term and short-term applications or microinjections of cGMP have no effect on olfactory responses in vivo as measured by calcium imaging and single sensillum recording. The absence of the effect of cGMP is shown in contrast to cAMP, which elicits increased responses when perfused shortly before olfactory responses in OSNs. Taken together, the apparent absence of nitric oxide signaling in olfactory neurons indicates that this gaseous messenger may play no role as a regulator of olfactory transduction in insects, though may play other physiological roles at the sensory periphery of the antenna.

ROS and cGMP signaling modulate persistent escape from hypoxia in Caenorhabditis elegans

The ability to detect and respond to acute oxygen (O₂) shortages is indispensable to aerobic life. The molecular mechanisms and circuits underlying this capacity are poorly understood. Here, we characterize the behavioral responses of feeding Caenorhabditis elegans to approximately 1% O₂. Acute hypoxia triggers a bout of turning maneuvers followed by a persistent switch to rapid forward movement as animals seek to avoid and escape hypoxia. While the behavioral responses to 1% O₂ closely resemble those evoked by 21% O₂, they have distinct molecular and circuit underpinnings. Disrupting phosphodiesterases (PDEs), specific G proteins, or BBSome function inhibits escape from 1% O₂ due to increased cGMP signaling. A primary source of cGMP is GCY-28, the ortholog of the atrial natriuretic peptide (ANP) receptor. cGMP activates the protein kinase G EGL-4 and enhances neuroendocrine secretion to inhibit acute responses to 1% O₂. Triggering a rise in cGMP optogenetically in multiple neurons, including AIA interneurons, rapidly and reversibly inhibits escape from 1% O₂. Ca²⁺ imaging reveals that a 7% to 1% O₂ stimulus evokes a Ca²⁺ decrease in several neurons. Defects in mitochondrial complex I (MCI) and mitochondrial complex I (MCIII), which lead to persistently high reactive oxygen species (ROS), abrogate acute hypoxia responses. In particular, repressing the expression of isp-1, which encodes the iron sulfur protein of MCIII, inhibits escape from 1% O₂ without affecting responses to 21% O₂. Both genetic and pharmacological up-regulation of mitochondrial ROS increase cGMP levels, which contribute to the reduced hypoxia responses. Our results implicate ROS and precise regulation of intracellular cGMP in the modulation of acute responses to hypoxia by C. elegans.

The ability to detect and respond to acute oxygen shortages is indispensable to aerobic life, but the molecular mechanisms underlying this capacity are poorly understood. This study reveals that high levels of cGMP and reactive oxygen species (ROS) prevent the nematode Caenorhabditis elegans from escaping hypoxia.

Knockdown of a Cyclic Nucleotide-Gated Ion Channel Impairs Locomotor Activity and Recovery From Hypoxia in Adult Drosophila melanogaster

Cyclic guanosine monophosphate (cGMP) modulates the speed of recovery from anoxia in adult Drosophila and mediates hypoxia-related behaviors in larvae. Cyclic nucleotide-gated channels (CNG) and cGMP-activated protein kinase (PKG) are two cGMP downstream targets. PKG is involved in behavioral tolerance to hypoxia and anoxia in adults, however little is known about a role for CNG channels. We used a CNGL (CNG-like) mutant with reduced CNGL transcripts to investigate the contribution of CNGL to the hypoxia response. CNGL mutants had reduced locomotor activity under normoxia. A shorter distance travelled in a standard locomotor assay was due to a slower walking speed and more frequent stops. In control flies, hypoxia immediately reduced path length per minute. Flies took 30–40 min in normoxia for >90% recovery of path length per minute from 15 min hypoxia. CNGL mutants had impaired recovery from hypoxia; 40 min for ∼10% recovery of walking speed. The effects of CNGL mutation on locomotor activity and recovery from hypoxia were recapitulated by pan-neuronal CNGL knockdown. Genetic manipulation to increase cGMP in the CNGL mutants increased locomotor activity under normoxia and eliminated the impairment of recovery from hypoxia. We conclude that CNGL channels and cGMP signaling are involved in the control of locomotor activity and the hypoxic response of adult Drosophila.

From chemoreception to regulation: filling the gaps in understanding how insects control gas exchange

Insects coordinate the opening and closing of spiracles with convective ventilatory movements to produce considerable intraspecific and interspecific variation in gas exchange patterns. But fundamental questions remain regarding how these movements are coordinated and modulated by central and peripheral respiratory chemoreceptors, and where these chemoreceptors are located and how they function. Recent findings have revealed regions of the CNS that generate coordinated respiratory motor activity, while peripheral neurons sensitive to respiratory gases have been identified in Drosophila. Importantly, plasticity in structure and function of neural elements of respiratory control indicate the need for caution when generalizing the mechanistic basis for breathing in insects, and an adaptive explanation for breathing pattern variability.

Chemosensory signal transduction in Caenorhabditis elegans

Chemosensory neurons translate perception of external chemical cues, including odorants, tastants, and pheromones, into information that drives attraction or avoidance motor programs. In the laboratory, robust behavioral assays, coupled with powerful genetic, molecular and optical tools, have made Caenorhabditis elegans an ideal experimental system in which to dissect the contributions of individual genes and neurons to ethologically relevant chemosensory behaviors. Here, we review current knowledge of the neurons, signal transduction molecules and regulatory mechanisms that underlie the response of C. elegans to chemicals, including pheromones. The majority of identified molecules and pathways share remarkable homology with sensory mechanisms in other organisms. With the development of new tools and technologies, we anticipate that continued study of chemosensory signal transduction and processing in C. elegans will yield additional new insights into the mechanisms by which this animal is able to detect and discriminate among thousands of chemical cues with a limited sensory neuron repertoire.

A review of the actions of Nitric Oxide in development and neuronal function in major invertebrate model systems

Ever since the late-eighties when endothelium-derived relaxing factor was found to be the gas nitric oxide, endogenous nitric oxide production has been observed in virtually all animal groups tested and additionally in plants, diatoms, slime molds and bacteria. The fact that this new messenger was actually a gas and therefore didn't obey the established rules of neurotransmission made it even more intriguing. In just 30 years there is now too much information for useful comprehensive reviews even if limited to animals alone. Therefore this review attempts to survey the actions of nitric oxide on development and neuronal function in selected major invertebrate models only so allowing some detailed discussion but still covering most of the primary references. Invertebrate model systems have some very useful advantages over more expensive and demanding animal models such as large, easily identifiable neurons and simple circuits in tissues that are typically far easier to keep viable. A table summarizing this information along with the major relevant references has been included for convenience.

Identification of possible hypoxia sensor for behavioral responses in a marine annelid, Capitella teleta

Hypoxia often occurs in summer and causes deleterious effects on marine benthic animals. A marine annelid, Capitella teleta, is tolerant to hypoxia, as shown by the fact that it inhabits organically polluted areas, where severe hypoxia is often observed. To understand how this species adapts to the environment, we focused on its hypoxia sensor, and we showed that TRPAbasal was a possible contributor to hypoxia detection in C. teleta. To examine the involvement of TRPA1 in the response of C. teleta to hypoxia, we exposed C. teleta to hypoxic water with or without a TRPA1-specific inhibitor, A-967079. Hypoxic stimulation induced escape behavior in C. teleta from the sediment, and this behavior was suppressed by the inhibitor. The cloned TRPA gene from C. teleta was phylogenetically categorized into TRPAbasal, and contains an oxygen-dependent degradation domain, which is important for the detection of hypoxia. Whole-mount in situ hybridization analysis showed that the gene was transcribed in the prostomium, where sensing functions are localized. These results suggested that the worm has a hypoxia-sensing system possibly utilizing CtTRPAbasal, and this system contributes to expanding the organism's niches in hypoxic environments by detecting whether hypoxia exceeds a level that would imperil its survival.

Summary: The hypoxia-avoidance behavior of a marine annelid was suppressed by a TRPA1-specific inhibitor. Our dataset suggests that the possible hypoxia sensor for hypoxia-inducing behaviors of this marine annelid is TRPAbasal.

Role of Modified Atmosphere in Pest Control and Mechanism of Its Effect on Insects

Pests not only attack field crops during the growing season, but also damage grains and other food products stored in granaries. Modified or controlled atmospheres (MAs or CAs) with higher or lower concentrations of atmospheric gases, mainly oxygen (O₂), carbon dioxide (CO₂), ozone (O₃), and nitric oxide (NO), provide a cost-effective method to kill target pests and protect stored products. In this review, the most recent discoveries in the field of MAs are discussed, with a focus on pest control as well as current MA technologies. Although MAs have been used for more than 30 years in pest control and play a role in storage pest management, the specific mechanisms by which insects are affected by and adapt to low O₂ (hypoxia) and high carbon CO₂ (hypercapnia) are not completely understood. Insect tolerance to hypoxia/anoxia and hypercapnia involves a decrease in aerobic metabolism, including decreased NADPH enzyme activity, and subsequently, decreases in glutathione production and catalase, superoxide dismutase, glutathione-S-transferase, and glutathione peroxidase activities, as well as increases in carboxyl esterase and phosphatase activities. In addition, hypoxia induces energy and nutrient production, and in adapted insects, glycolysis and pyruvate carboxylase fluxes are downregulated, accompanied with O₂ consumption and acetate production. Consequently, genes encoding various signal transduction pathway components, including epidermal growth factor, insulin, Notch, and Toll/Imd signaling, are downregulated. We review the changes in insect energy and nutrient sources, metabolic enzymes, and molecular pathways in response to modified O₂, CO₂, NO, and O₃ concentrations, as well as the role of MAs in pest control. This knowledge will be useful for applying MAs in combination with temperature control for pest control in stored food products.

Drosophila as a Genetic Model for Hematopoiesis

In this FlyBook chapter, we present a survey of the current literature on the development of the hematopoietic system in Drosophila The Drosophila blood system consists entirely of cells that function in innate immunity, tissue integrity, wound healing, and various forms of stress response, and are therefore functionally similar to myeloid cells in mammals. The primary cell types are specialized for phagocytic, melanization, and encapsulation functions. As in mammalian systems, multiple sites of hematopoiesis are evident in Drosophila and the mechanisms involved in this process employ many of the same molecular strategies that exemplify blood development in humans. Drosophila blood progenitors respond to internal and external stress by coopting developmental pathways that involve both local and systemic signals. An important goal of these Drosophila studies is to develop the tools and mechanisms critical to further our understanding of human hematopoiesis during homeostasis and dysfunction.

Systemic control of immune cell development by integrated carbon dioxide and hypoxia chemosensation in Drosophila

Drosophila hemocytes are akin to mammalian myeloid blood cells that function in stress and innate immune-related responses. A multi-potent progenitor population responds to local signals and to systemic stress by expanding the number of functional blood cells. Here we show mechanisms that demonstrate an integration of environmental carbon dioxide (CO2) and oxygen (O2) inputs that initiate a cascade of signaling events, involving multiple organs, as a stress response when the levels of these two important respiratory gases fall below a threshold. The CO2 and hypoxia-sensing neurons interact at the synaptic level in the brain sending a systemic signal via the fat body to modulate differentiation of a specific class of immune cells. Our findings establish a link between environmental gas sensation and myeloid cell development in Drosophila. A similar relationship exists in humans, but the underlying mechanisms remain to be established.

White - cGMP Interaction Promotes Fast Locomotor Recovery from Anoxia in Adult Drosophila

Increasing evidence indicates that the white (w) gene in Drosophila possesses extra-retinal functions in addition to its classical role in eye pigmentation. We have previously shown that w+ promotes fast and consistent locomotor recovery from anoxia, but how w+ modulates locomotor recovery is largely unknown. Here we show that in the absence of w+, several PDE mutants, especially cyclic guanosine monophosphate (cGMP)-specific PDE mutants, display wildtype-like fast locomotor recovery from anoxia, and that during the night time, locomotor recovery was light-sensitive in white-eyed mutant w1118, and light-insensitive in PDE mutants under w1118 background. Data indicate the involvement of cGMP in the modulation of recovery timing and presumably, light-evoked cGMP fluctuation is associated with light sensitivity of locomotor recovery. This was further supported by the observations that w-RNAi-induced delay of locomotor recovery was completely eliminated by upregulation of cGMP through multiple approaches, including PDE mutation, simultaneous overexpression of an atypical soluble guanylyl cyclase Gyc88E, or sildenafil feeding. Lastly, prolonged sildenafil feeding promoted fast locomotor recovery from anoxia in w1118. Taken together, these data suggest that a White-cGMP interaction modulates the timing of locomotor recovery from anoxia.

A Sensor for Low Environmental Oxygen in the Mouse Main Olfactory Epithelium

Sensing the level of oxygen in the external and internal environments is essential for survival. Organisms have evolved multiple mechanisms to sense oxygen. No function in oxygen sensing has been attributed to any mammalian olfactory system. Here, we demonstrate that low environmental oxygen directly activates a subpopulation of sensory neurons in the mouse main olfactory epithelium. These neurons express the soluble guanylate cyclase Gucy1b2 and the cation channel Trpc2. Low oxygen induces calcium influx in these neurons, and Gucy1b2 and Trpc2 are required for these responses. In vivo exposure of a mouse to low environmental oxygen causes Gucy1b2-dependent activation of olfactory bulb neurons in the vicinity of the glomeruli formed by axons of Gucy1b2+ sensory neurons. Low environmental oxygen also induces conditioned place aversion, for which Gucy1b2 and Trpc2 are required. We propose that this chemosensory function enables a mouse to rapidly assess the oxygen level in the external environment.

Stunted by Developing in Hypoxia: Linking Comparative and Model Organism Studies

Animals develop in atmospheric hypoxia in a wide range of habitats, and tissues may experience O2 limitation of ATP production during postembryonic development if O2 supply structures do not keep pace with growing O2 demand during ontogeny. Most animal species are stunted by postembryonic development in hypoxia, showing reduced growth rates and size in moderate hypoxia (5-15 kPa Po2). In mammals, the critical Po2 that limits resting metabolic rate also falls in this same moderate hypoxic range, so stunted growth may simply be due to hypoxic limits on ATP production. However, in most invertebrates and at least some lower vertebrates, hypoxic stunting occurs at Po2 values well above those that limit resting metabolism. Studies with diverse model organisms have identified multiple homologous O2-sensing signaling pathways that can inhibit feeding and growth during moderate hypoxia. Together, these comparative and model organism-based studies suggest that hypoxic stunting of growth and size can occur as programmed inhibition of growth, often by inhibition of insulin stimulation of growth processes. Furthermore, there is increasing evidence that these same O2 signaling pathways can be utilized during normal animal development to ensure matching of O2 supply and demand structures and in mediation of variation in animal performance.

Trpc2-expressing sensory neurons in the mouse main olfactory epithelium of type B express the soluble guanylate cyclase Gucy1b2

Chemoreception in the mouse olfactory system occurs primarily at two chemosensory epithelia in the nasal cavity: the main olfactory epithelium (MOE) and the vomeronasal epithelium. The canonical chemosensory neurons in the MOE, the olfactory sensory neurons (OSNs), express the odorant receptor (OR) gene repertoire, and depend on Adcy3 and Cnga2 for chemosensory signal transduction. The canonical chemosensory neurons in the vomeronasal epithelium, the vomeronasal sensory neurons (VSNs), express two unrelated vomeronasal receptor (VR) gene repertoires, and involve Trpc2 for chemosensory signal transduction. Recently we reported the discovery of two types of neurons in the mouse MOE that express Trcp2 in addition to Cnga2. These cell types can be distinguished at the single-cell level by expression of Adcy3: positive, type A and negative, type B. Some type A cells express OR genes. Thus far there is no specific gene or marker for type B cells, hampering further analyses such as physiological recordings. Here, we show that among MOE cells, type B cells are unique in their expression of the soluble guanylate cyclase Gucy1b2. We came across Gucy1b2 in an explorative approach based on Long Serial Analysis of Gene Expression (LongSAGE) that we applied to single red-fluorescent cells isolated from whole olfactory mucosa and vomeronasal organ of mice of a novel Trcp2-IRES-taumCherry gene-targeted strain. The generation of a novel Gucy1b2-IRES-tauGFP gene-targeted strain enabled us to visualize coalescence of axons of type B cells into glomeruli in the main olfactory bulb. Our molecular and anatomical analyses define Gucy1b2 as a marker for type B cells within the MOE. The Gucy1b2-IRES-tauGFP strain will be useful for physiological, molecular, cellular, and anatomical studies of this newly described chemosensory subsystem.

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Trpc2 + cells exist as type A and type B in the mouse main olfactory epithelium.

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We find no evidence for expression of chemosensory GPCR genes in type B cells.

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We identify the soluble guanylate cyclase Gucy1b2 as a marker for type B cells.

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Gucy1b2-IRES-tauGFP knockin mice will be useful for physiological studies.

Dynamic change of heme environment in soluble guanylate cyclase and complexation of NO-independent drug agents with H-NOX domain

Soluble guanylate cyclase is a heterodimer receptor that functions in several signal transduction pathways. Conversion of guanosine 5'-triphosphate to 3',5'-cyclic monophosphate second messenger at the catalytic domain is regulated by the changes at heme nitric oxide/oxygen domain of the β-subunit. To better understand conformational changes at heme site that may impact on activities of catalytic domain, three soluble guanylate cyclase homolog proteins with heme at Fe-His state were investigated, and their dynamic behaviors were monitored in both unliganded (apo) and complex with heme. As a result of dynamic conformational changes, Lys110, Asp45, Arg135, and Glu41 were found interacting with the site gate, which may interfere with transportation of small molecules in and out of the heme site. An alternative binding site adjacent to that of heme was identified. Binding affinity of several nitric oxide-independent activators and heme-dependent stimulators was examined, and their binding modes in the heme site and in the alternative binding site in the human soluble guanylate cyclase enzyme were computationally simulated. The calculated binding energies were used as criteria to filter results of virtual high-throughput screenings based on FlexX ligand-docking algorithm and absorption, distribution, metabolism, excretion, and toxicity properties on databases of available drugs. The identified drugs from virtual high-throughput screening have been suggested for experimental investigations, based on which they may either be directly repurposed or require structural modifications for better physico-chemical and pharmacological properties.

The neurobiology of sensing respiratory gases for the control of animal behavior

Aerobic metabolism is fundamental for almost all animal life. Cellular consumption of oxygen (O(2)) and production of carbon dioxide (CO(2)) signal metabolic states and physiological stresses. These respiratory gases are also detected as environmental cues that can signal external food quality and the presence of prey, predators and mates. In both contexts, animal nervous systems are endowed with mechanisms for sensing O(2)/CO(2) to trigger appropriate behaviors and maintain homeostasis of internal O(2)/CO(2). Although different animal species show different behavioral responses to O(2)/CO(2), some underlying molecular mechanisms and pathways that function in the detection of respiratory gases are fundamentally similar and evolutionarily conserved. Studies of Caenorhabditis elegans and Drosophila melanogaster have identified roles for cyclic nucleotide signaling and the hypoxia inducible factor (HIF) transcriptional pathway in mediating behavioral responses to respiratory gases. Understanding how simple invertebrate nervous systems detect respiratory gases to control behavior might reveal general principles common to nematodes, insects and vertebrates that function in the molecular sensing of respiratory gases and the neural control of animal behaviors.

Structure and regulation of soluble guanylate cyclase

Nitric oxide (NO) is an essential signaling molecule in biological systems. In mammals, the diatomic gas is critical to the cyclic guanosine monophosphate (cGMP) pathway as it functions as the primary activator of soluble guanylate cyclase (sGC). NO is synthesized from l-arginine and oxygen (O(2)) by the enzyme nitric oxide synthase (NOS). Once produced, NO rapidly diffuses across cell membranes and binds to the heme cofactor of sGC. sGC forms a stable complex with NO and carbon monoxide (CO), but not with O(2). The binding of NO to sGC leads to significant increases in cGMP levels. The second messenger then directly modulates phosphodiesterases (PDEs), ion-gated channels, or cGMP-dependent protein kinases to regulate physiological functions, including vasodilation, platelet aggregation, and neurotransmission. Many studies are focused on elucidating the molecular mechanism of sGC activation and deactivation with a goal of therapeutic intervention in diseases involving the NO/cGMP-signaling pathway. This review summarizes the current understanding of sGC structure and regulation as well as recent developments in NO signaling.

Behavioral responses to hypoxia and hyperoxia in Drosophila larvae: molecular and neuronal sensors

The ability to detect changes in oxygen concentration in the environment is critical to the survival of all animals. This requires cells to express a molecular oxygen sensor that can detect shifts in oxygen levels and transmit a signal that leads to the appropriate cellular response. Recent biochemical, genetic and behavioral studies have shown that the atypical soluble guanylyl cyclases function as oxygen detectors in Drosophila larvae triggering a behavioral escape response when exposed to hypoxia. These studies also identified the sensory neurons that innervate the terminal sensory cones as likely chemosensors that mediate this response. Here I summarize the data that led to these conclusions and also highlight evidence that suggests additional, as yet unidentified, proteins are also required for detecting increases and decreases in oxygen concentrations.

Out of thin air: sensory detection of oxygen and carbon dioxide

Oxygen (O₂) and carbon dioxide (CO₂) levels vary in different environments and locally fluctuate during respiration and photosynthesis. Recent studies in diverse animals have identified sensory neurons that detect these external variations and direct a variety of behaviors. Detection allows animals to stay within a preferred environment as well as identify potential food or dangers. The complexity of sensation is reflected in the fact that neurons compartmentalize detection into increases, decreases, and short-range and long-range cues. Animals also adjust their responses to these prevalent signals in the context of other cues, allowing for flexible behaviors. In general, the molecular mechanisms for detection suggest that sensory neurons adopted ancient strategies for cellular detection and coupled them to brain activity and behavior. This review highlights the multiple strategies that animals use to extract information about their environment from variations in O₂ and CO₂.

Drosophila gustatory preference behaviors require the atypical soluble guanylyl cyclases

The intracellular messenger cGMP has been suggested to play a role in taste signal transduction in both vertebrates and invertebrates. In the present study, we have examined the role of the Drosophila atypical soluble guanylyl cyclases (sGCs), Gyc-89Da and Gyc-89Db, in larval and adult gustatory preference behaviors. We showed that in larvae, sucrose attraction requires Gyc-89Db and caffeine avoidance requires Gyc-89Da. In adult flies, sucrose attraction is unaffected by mutations in either gene whereas avoidance of low concentrations of caffeine is eliminated by loss of either gene. Similar defective behaviors were observed when cGMP increases were prevented by the expression of a cGMP-specific phosphodiesterase. We also showed that both genes were expressed in gustatory receptor neurons (GRNs) in larval and adult gustatory organs, primarily in a non-overlapping pattern, with the exception of a small group of cells in the adult labellum. In addition, in adults, several cells co-expressed the bitter taste receptor, Gr66a, with either Gyc-89Da or Gyc-89Db. We also showed that the electrophysiological responses of a GRN to caffeine were significantly reduced in flies mutant for the atypical sGCs, suggesting that at least part of the adult behavioral defects were due to a reduced ability to detect caffeine.

Behavioral responses to hypoxia in Drosophila larvae are mediated by atypical soluble guanylyl cyclases

The three Drosophila atypical soluble guanylyl cyclases, Gyc-89Da, Gyc-89Db, and Gyc-88E, have been proposed to act as oxygen detectors mediating behavioral responses to hypoxia. Drosophila larvae mutant in any of these subunits were defective in their hypoxia escape response-a rapid cessation of feeding and withdrawal from their food. This response required cGMP and the cyclic nucleotide-gated ion channel, cng, but did not appear to be dependent on either of the cGMP-dependent protein kinases, dg1 and dg2. Specific activation of the Gyc-89Da neurons using channel rhodopsin showed that activation of these neurons was sufficient to trigger the escape behavior. The hypoxia escape response was restored by reintroducing either Gyc-89Da or Gyc-89Db into either Gyc-89Da or Gyc-89Db neurons in either mutation. This suggests that neurons that co-express both Gyc-89Da and Gyc-89Db subunits are primarily responsible for activating this behavior. These include sensory neurons that innervate the terminal sensory cones. Although the roles of Gyc-89Da and Gyc-89Db in the hypoxia escape behavior appeared to be identical, we also showed that changes in larval crawling behavior in response to either hypoxia or hyperoxia differed in their requirements for these two atypical sGCs, with responses to 15% oxygen requiring Gyc-89Da and responses to 19 and 25% requiring Gyc-89Db. For this behavior, the identity of the neurons appeared to be critical in determining the ability to respond appropriately.

Incorporation of tyrosine and glutamine residues into the soluble guanylate cyclase heme distal pocket alters NO and O2 binding

Nitric oxide (NO) is the physiologically relevant activator of the mammalian hemoprotein soluble guanylate cyclase (sGC). The heme cofactor of alpha1beta1 sGC has a high affinity for NO but has never been observed to form a complex with oxygen. Introduction of a key tyrosine residue in the sGC heme binding domain beta1(1-385) is sufficient to produce an oxygen-binding protein, but this mutation in the full-length enzyme did not alter oxygen affinity. To evaluate ligand binding specificity in full-length sGC we mutated several conserved distal heme pocket residues (beta1 Val-5, Phe-74, Ile-145, and Ile-149) to introduce a hydrogen bond donor in proximity to the heme ligand. We found that the NO coordination state, NO dissociation, and enzyme activation were significantly affected by the presence of a tyrosine in the distal heme pocket; however, the stability of the reduced porphyrin and the proteins affinity for oxygen were unaltered. Recently, an atypical sGC from Drosophila, Gyc-88E, was shown to form a stable complex with oxygen. Sequence analysis of this protein identified two residues in the predicted heme pocket (tyrosine and glutamine) that may function to stabilize oxygen binding in the atypical cyclase. The introduction of these residues into the rat beta1 distal heme pocket (Ile-145 --> Tyr and Ile-149 --> Gln) resulted in an sGC construct that oxidized via an intermediate with an absorbance maximum at 417 nm. This absorbance maximum is consistent with globin Fe(II)-O(2) complexes and is likely the first observation of a Fe(II)-O(2) complex in the full-length alpha1beta1 protein. Additionally, these data suggest that atypical sGCs stabilize O(2) binding by a hydrogen bonding network involving tyrosine and glutamine.

Neural detection of gases--carbon dioxide, oxygen--in vertebrates and invertebrates

Carbon dioxide (CO(2)) and oxygen (O(2)) are important cues that can signal the presence of food, predators, and environmental stress. Here we will review recent studies on the mechanisms of how the olfactory system detects these two molecules. In both vertebrates and invertebrates, the two molecules are detected by subsets of specialized olfactory neurons. In addition, the signal transduction cascades for sensing these two gases appear to be different from those for sensing typical odorants. CO(2) and O(2) signals can evoke stereotypical innate behaviors such as attraction and avoidance in many animal species. Future studies on the neural pathways underlying CO(2) and O(2) sensing may shed light on the circuit mechanisms of these behaviors.

Dissection of a hypoxia-induced, nitric oxide-mediated signaling cascade

Befitting oxygen's key role in life's processes, hypoxia engages multiple signaling systems that evoke pervasive adaptations. Using surrogate genetics in a powerful biological model, we dissect a poorly understood hypoxia-sensing and signal transduction system. Hypoxia triggers NO-dependent accumulation of cyclic GMP and translocation of cytoplasmic GFP-Relish (an NFkappaB/Rel transcription factor) to the nucleus in Drosophila S2 cells. An enzyme capable of eliminating NO interrupted signaling specifically when it was targeted to the mitochondria, arguing for a mitochondrial NO signal. Long pretreatment with an inhibitor of nitric oxide synthase (NOS), L-NAME, blocked signaling. However, addition shortly before hypoxia was without effect, suggesting that signaling is supported by the prior action of NOS and is independent of NOS action during hypoxia. We implicated the glutathione adduct, GSNO, as a signaling mediator by showing that overexpression of the cytoplasmic enzyme catalyzing its destruction, GSNOR, blocks signaling, whereas knockdown of this activity caused reporter translocation in the absence of hypoxia. In downstream steps, cGMP accumulated, and calcium-dependent signaling was subsequently activated via cGMP-dependent channels. These findings reveal the use of unconventional steps in an NO pathway involved in sensing hypoxia and initiating signaling.

Neurons detect increases and decreases in oxygen levels using distinct guanylate cyclases

Homeostatic sensory systems detect small deviations in temperature, water balance, pH, and energy needs to regulate adaptive behavior and physiology. In C. elegans, a homeostatic preference for intermediate oxygen (O2) levels requires cGMP signaling through soluble guanylate cyclases (sGCs), proteins that bind gases through an associated heme group. Here we use behavioral analysis, functional imaging, and genetics to show that reciprocal changes in O2 levels are encoded by sensory neurons that express alternative sets of sGCs. URX sensory neurons are activated by increases in O2 levels, and require the sGCs gcy-35 and gcy-36. BAG sensory neurons are activated by decreases in O2 levels, and require the sGCs gcy-31 and gcy-33. The sGCs are instructive O2 sensors, as forced expression of URX sGC genes causes BAG neurons to detect O2 increases. Both sGC expression and cell-intrinsic dynamics contribute to the differential roles of URX and BAG in O2-dependent behaviors.

Biochemistry of soluble guanylate cyclase

Nitric oxide (NO) functions in biology as both a critical cytotoxic agent and an essential signaling molecule. The toxicity of the diatomic gas has long been accepted; however, it was not known to be a signaling molecule until it was identified as the endothelium-derived relaxing factor (EDRF). Since this discovery, the physiological signaling pathways that are regulated by NO have been the focus of numerous studies. Many of the cellular responses that NO modulates are mediated by the heme protein soluble guanylate cyclase (sGC). NO binds to sGC at a diffusion controlled rate, and leads to a several 100-fold increase in the synthesis of the second messenger cGMP from GTP. Other diatomic gases either do not bind (dioxygen), or do not significantly activate (carbon monoxide) sGC. This provides selectivity and efficiency for NO even in an aerobic environment, which is critical due to the high reactivity of NO. Several biochemical studies have focused on elucidating the mechanism of NO activation and O(2) discrimination. Significant advances in our understanding of these topics have occurred with the identification and characterization of the sGC-like homologues termed Heme-Nitric oxide and OXygen binding (H-NOX) proteins.

Synaptic transmission in neurons that express the Drosophila atypical soluble guanylyl cyclases, Gyc-89Da and Gyc-89Db, is necessary for the successful completion of larval and adult ecdysis

Insect ecdysis is a precisely coordinated series of behavioral and hormonal events that occur at the end of each molt. A great deal is known about the hormonal events that underlie this process, although less is known about the neuronal circuitry involved. In this study we identified two populations of neurons that are required for larval and adult ecdyses in the fruit fly, Drosophila melanogaster (Meigen). These neurons were identified by using the upstream region of two genes that code for atypical soluble guanylyl cyclases to drive tetanus toxin in the neurons that express these cyclases to block their synaptic activity. Expression of tetanus toxin in neurons that express Gyc-89Da blocked adult eclosion whereas expression of tetanus toxin in neurons that express Gyc-89Db prevented the initiation of the first larval ecdysis. Expression of tetanus toxin in the Gyc-89Da neurons also resulted in about 50% lethality just prior to pupariation; however, this was probably due to suffocation in the food as lethality was prevented by stopping the larvae from burrowing deep within the food. This result is consistent with our model that the atypical soluble guanylyl cyclases can act as molecular oxygen detectors. The expression pattern of these cyclases did not overlap with any of the neurons containing peptides known to regulate ecdysis and eclosion behaviors. By using the conditional expression of tetanus toxin we were also able to demonstrate that synaptic activity in the Gyc-89Da and Gyc-89Db neurons is required during early adult development for adult eclosion.

Gaseous neuromodulator-related genes expressed in the brain of honeybee Apis mellifera

Nitric oxide (NO), hydrogen sulfide (H2S), and carbon monoxide (CO) are thought to act as gaseous neuromodulators in the brain across species. For example, in the brain of honeybee Apis mellifera, NO plays important roles in olfactory learning and discrimination, but the existence of H2S- and CO-mediated signaling pathways remains unknown. In the present study, we identified the genes of nitric oxide synthase (NOS), soluble guanylyl cyclase (sGC), cystathionine beta-synthase (CBS), and heme oxygenase (HO) from the honeybee brain. The honeybee brain contains at least one gene for each of NOS, CBS, and HO. The deduced proteins for NOS, CBS, and HO are thought to contain domains to generate NO, H2S, and CO, respectively, and to contain putative Ca2+/calmodulin-binding domains. On the other hand, the honeybee brain contains three subunits of sGC: sGCalpha1, sGCbeta1, and sGCbeta3. Phylogenetic analysis of sGC revealed that Apis sGCalpha1 and sGCbeta1 are closely related to NO- and CO-sensitive sGC subunits, whereas Apis sGCbeta3 is closely related to insect O2-sensitive sGC subunits. In addition, we performed in situ hybridization for Apis NOS mRNA and NADPH-diaphorase histochemistry in the honeybee brain. The NOS gene was strongly expressed in the optic lobes and in the Kenyon cells of the mushroom bodies. NOS activity was detected in the optic lobes, the mushroom bodies, the central body complex, the lateral protocerebral lobes, and the antennal lobes. These findings suggest that NO is involved in various brain functions and that H2S and CO can be endogenously produced in the honeybee brain.

Pharmacological approaches to nitric oxide signalling during neural development of locusts and other model insects

A novel aspect of cellular signalling during the formation of the nervous system is the involvement of the messenger molecule nitric oxide (NO), which has been discovered in the mammalian vascular system as mediator of smooth muscle relaxation. NO is a membrane-permeant molecule, which activates soluble guanylyl cyclase (sGC) and leads to the formation of cyclic GMP (cGMP) in target cells. The analysis of specific cell types in model insects such as Locusta, Schistocerca, Acheta, Manduca, and Drosophila shows that the NO/cGMP pathway is required for the stabilization of photoreceptor growth cones at the start of synaptic assembly in the optic lobe, for regulation of cell proliferation, and for correct outgrowth of pioneer neurons. Inhibition of the NOS and sGC enzymes combined with rescue experiments show that NO, and potentially also another atypical messenger, carbon monoxide (CO), orchestrate cell migration of enteric neurons. Cultured insect embryos are accessible model systems in which the molecular pathways linking cytoskeletal rearrangement to directed cell movements can be analyzed in natural settings. Based on the results obtained from the insect models, I discuss current evidence for NO and cGMP as essential signalling molecules for the development of vertebrate brains.

Glyceraldehyde-3-phosphate dehydrogenase mediates anoxia response and survival in Caenorhabditis elegans

Oxygen deprivation has a role in the pathology of many human diseases. Thus it is of interest in understanding the genetic and cellular responses to hypoxia or anoxia in oxygen-deprivation-tolerant organisms such as Caenorhabditis elegans. In C. elegans the DAF-2/DAF-16 pathway, an IGF-1/insulin-like signaling pathway, is involved with dauer formation, longevity, and stress resistance. In this report we compared the response of wild-type and daf-2(e1370) animals to anoxia. Unlike wild-type animals, the daf-2(e1370) animals have an enhanced anoxia-survival phenotype in that they survive long-term anoxia and high-temperature anoxia, do not accumulate significant tissue damage in either of these conditions, and are motile after 24 hr of anoxia. RNA interference was used to screen DAF-16-regulated genes that suppress the daf-2(e1370)-enhanced anoxia-survival phenotype. We identified gpd-2 and gpd-3, two nearly identical genes in an operon that encode the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase. We found that not only is the daf-2(e1370)-enhanced anoxia phenotype dependent upon gpd-2 and gpd-3, but also the motility of animals exposed to brief periods of anoxia is prematurely arrested in gpd-2/3(RNAi) and daf-2(e1370);gpd-2/3(RNAi) animals. These data suggest that gpd-2 and gpd-3 may serve a protective role in tissue exposed to oxygen deprivation.

Nitric oxide signaling: no longer simply on or off

Nitric oxide (NO) triggers various physiological responses in numerous tissues by binding and activating soluble guanylate cyclase (sGC) to produce the second messenger cGMP. In vivo, basal NO/cGMP signaling maintains a resting state in target cells (for example, resting tone in smooth muscle), but an acute burst of NO/cGMP signaling triggers rapid responses (such as smooth muscle relaxation). Recent studies have shown that the sGC heterodimer comprises at least four modular domains per subunit. The N-terminal heme domain is a member of the H-NOX family of domains that bind O(2) and/or NO and are conserved in prokaryotes and higher eukaryotes. Studies of these domains have uncovered the molecular basis for ligand discrimination by sGC. Other work has identified two temporally distinct states of sGC activation by NO: formation of a stable NO-heme complex results in a low-activity species, and additional NO produces a transient fully active enzyme. Nucleotides also allosterically modulate the duration and intensity of enzyme activity. Together, these studies suggest a biochemical basis for the two distinct types of NO/cGMP signal observed in vivo.

Oxygen-sensitive guanylyl cyclases in insects and their potential roles in oxygen detection and in feeding behaviors

Responses to hypoxia and hyperoxia depend critically on the ability of the animal to detect changes in O2 levels. However, it has only been recently that an O2-sensing system has been identified in invertebrates. Evidence is accumulating that this molecular O2 sensor is, surprisingly, a class of soluble guanylyl cyclase (sGC) known as atypical sGCs. It has long been known that the conventional sGC alpha and beta subunits form heterodimeric enzymes that are potently activated by NO, but do not bind O2. By contrast, the Drosophila melanogaster atypical sGC subunits, Gyc-88E, Gyc-89Da and Gyc-89Db, are only slightly sensitive to NO, but are potently activated under hypoxic conditions. Here we review evidence that suggests that the atypical sGCs can function as molecular O2 sensors mediating behavioral responses to hypoxia. Sequence comparisons of other predicted O2-sensitive sGCs suggest that most, if not all, insects express two heterodimeric sGCs; an NO-sensitive isoform and a separate O2-sensitive isoform. Expression data and recent experiments that block the function of cells that express the atypical sGCs and experiments that reduce the cGMP levels in these cells also suggest a role in behavioral responses to sweet tastants.

Multiple lineage specific expansions within the guanylyl cyclase gene family

BACKGROUND

Guanylyl cyclases (GCs) are responsible for the production of the secondary messenger cyclic guanosine monophosphate, which plays important roles in a variety of physiological responses such as vision, olfaction, muscle contraction, homeostatic regulation, cardiovascular and nervous function. There are two types of GCs in animals, soluble (sGCs) which are found ubiquitously in cell cytoplasm, and receptor (rGC) forms which span cell membranes. The complete genomes of several vertebrate and invertebrate species are now available. These data provide a platform to investigate the evolution of GCs across a diverse range of animal phyla.

RESULTS

In this analysis we located GC genes from a broad spectrum of vertebrate and invertebrate animals and reconstructed molecular phylogenies for both sGC and rGC proteins. The most notable features of the resulting phylogenies are the number of lineage specific rGC and sGC expansions that have occurred during metazoan evolution. Among these expansions is a large nematode specific rGC clade comprising 21 genes in C. elegans alone; a vertebrate specific expansion in the natriuretic receptors GC-A and GC-B; a vertebrate specific expansion in the guanylyl GC-C receptors, an echinoderm specific expansion in the sperm rGC genes and a nematode specific sGC clade. Our phylogenetic reconstruction also shows the existence of a basal group of nitric oxide (NO) insensitive insect and nematode sGCs which are regulated by O2. This suggests that the primordial eukaryotes probably utilized sGC as an O2 sensor, with the ligand specificity of sGC later switching to NO which provides a very effective local cell-to-cell signalling system. Phylogenetic analysis of the sGC and bacterial heme nitric oxide/oxygen binding protein domain supports the hypothesis that this domain originated from a cyanobacterial source.

CONCLUSION

The most salient feature of our phylogenies is the number of lineage specific expansions, which have occurred within the GC gene family during metazoan evolution. Our phylogenetic analyses reveal that the rGC and sGC multi-domain proteins evolved early in eumetazoan evolution. Subsequent gene duplications, tissue specific expression patterns and lineage specific expansions resulted in the evolution of new networks of interaction and new biological functions associated with the maintenance of organismal complexity and homeostasis.

Characterization of sub-nuclear changes in Caenorhabditis elegans embryos exposed to brief, intermediate and long-term anoxia to analyze anoxia-induced cell cycle arrest

Background

The soil nematode C. elegans survives oxygen-deprived conditions (anoxia; <.001 kPa O₂) by entering into a state of suspended animation in which cell cycle progression reversibly arrests. The majority of blastomeres of embryos exposed to anoxia arrest at interphase, prophase and metaphase. The spindle checkpoint proteins SAN-1 and MDF-2 are required for embryos to survive 24 hours of anoxia. To further investigate the mechanism of cell-cycle arrest we examined and compared sub-nuclear changes such as chromatin localization pattern, post-translational modification of histone H3, spindle microtubules, and localization of the spindle checkpoint protein SAN-1 with respect to various anoxia exposure time points. To ensure analysis of embryos exposed to anoxia and not post-anoxic recovery we fixed all embryos in an anoxia glove box chamber.

Results

Embryos exposed to brief periods to anoxia (30 minutes) contain prophase blastomeres with chromosomes in close proximity to the nuclear membrane, condensation of interphase chromatin and metaphase blastomeres with reduced spindle microtubules density. Embryos exposed to longer periods of anoxia (1–3 days) display several characteristics including interphase chromatin that is further condensed and in close proximity to the nuclear membrane, reduction in spindle structure perimeter and reduced localization of SAN-1 at the kinetochore. Additionally, we show that the spindle checkpoint protein SAN-1 is required for brief periods of anoxia-induced cell cycle arrest, thus demonstrating that this gene product is vital for early anoxia responses. In this report we suggest that the events that occur as an immediate response to brief periods of anoxia directs cell cycle arrest.

Conclusion

From our results we conclude that the sub-nuclear characteristics of embryos exposed to anoxia depends upon exposure time as assayed using brief (30 minutes), intermediate (6 or 12 hours) or long-term (24 or 72 hours) exposures. Analyzing these changes will lead to an understanding of the mechanisms required for initiation and maintenance of cell cycle arrest in respect to anoxia exposure time as well as order the events that occur to bring about anoxia-induced cell cycle arrest.

Signalling via cGMP: lessons from Drosophila

Guanosine 3', 5'-cyclic monophosphate (cGMP) signalling has received increasing attention over the last decade, since the discovery of the gaseous signalling molecule, nitric oxide, which activates cGMP synthesis. Furthermore, research into cGMP signalling has also been stimulated by the development of Viagra and pharmacologically active related compounds, which act to prevent cGMP breakdown. While much is known about the biochemical aspects of components of the cGMP signalling pathway, the precise in vivo roles of such components have only recently come to light through work in model organisms. This review outlines recent work utilising the genetic model organism Drosophila melanogaster in studies of organotypic cGMP signalling. While organisms such as Drosophila may not be the obvious choice for such studies, use of this model has proved that unique and detailed insights for cGMP signalling can be achieved.

Drosophila soluble guanylyl cyclase mutants exhibit increased foraging locomotion: behavioral and genomic investigations

Genetic variation in the gene foraging (for) is associated with a natural behavioral dimorphism in the fruit fly, Drosophila melanogaster. Some larvae, called 'rovers', have increased foraging locomotion compared to others, called 'sitters', and this difference is directly related to for-encoded cGMP-dependent protein kinase (PKG) activity. Here we report that larvae with mutations in the gene dgcalpha1, which encodes a soluble guanylyl cyclase (sGC) subunit, have increases in both PKG activity and foraging locomotion. This is contrary to our original prediction that, based on the role of sGC in the synthesis of cGMP, dgcalpha1 mutant larvae would have deficient cGMP production leading to decreased PKG activation and thus reduced larval foraging locomotion. We performed DNA microarray analyses to compare transcriptional changes induced by a dgcalpha1 mutation in both rover and sitter wildtype genetic backgrounds. In either background, we identified many genes that are differentially transcribed, and interestingly, relatively few are affected in both backgrounds. Furthermore, several of these commonly affected genes are enhanced or suppressed in a background-dependent manner. Thus, genetic background has a critical influence on the molecular effects of this mutation. These findings will support future investigations of Drosophila foraging behavior.

Ligand discrimination in soluble guanylate cyclase and the H-NOX family of heme sensor proteins

Soluble guanylate cyclases (s GC s) are eukaryotic heme sensor proteins that selectively bind NO in the presence of a large excess of the similar diatomic gas, O(2); this discrimination is essential for NO signaling. Recent discoveries place sGC in the H-NOX (heme nitric oxide and/or oxygen binding domain) family that includes bacterial proteins. The defining characteristic of this family is that some H-NOX proteins tightly bind O(2) whereas others, such as sGC, show no measurable affinity for O(2). A molecular basis for this ligand selectivity has now been established. A distal pocket tyrosine is requisite for O(2) binding and is used to kinetically distinguish between NO and O(2). In the absence of this tyrosine, the O(2) dissociation rate is so fast that the O(2) complex is never formed, whereas the rate of NO dissociation remains essentially unchanged, thus providing discrimination.

Experience-dependent modulation of C. elegans behavior by ambient oxygen

BACKGROUND

Ambient oxygen (O2) influences the behavior of organisms from bacteria to man. In C. elegans, an atypical O2 binding soluble guanylate cyclase (sGC), GCY-35, regulates O2 responses. However, how acute and chronic changes in O2 modify behavior is poorly understood.

RESULTS

Aggregating C. elegans strains can respond to a reduction in ambient O2 by a rapid, reversible, and graded inhibition of roaming behavior. This aerokinetic response is mediated by GCY-35 and GCY-36 sGCs, which appear to become activated as O2 levels drop and to depolarize the AQR, PQR, and URX neurons. Coexpression of GCY-35 and GCY-36 is sufficient to transform olfactory neurons into O2 sensors. Natural variation at the npr-1 neuropeptide receptor alters both food-sensing and O2-sensing circuits to reconfigure the salient features of the C. elegans environment. When cultivated in 1% O2 for a few hours, C. elegans reset their preferred ambient O2, seeking instead of avoiding 0%-5% O2. This plasticity involves reprogramming the AQR, PQR, and URX neurons.

CONCLUSIONS

To navigate O2 gradients, C. elegans can modulate turning rates and speed of movement. Aerotaxis can be reprogrammed by experience or engineered artificially. We propose a model in which prolonged activation of the AQR, PQR, and URX neurons by low O2 switches on previously inactive O2 sensors. This enables aerotaxis to low O2 environments and may encode a "memory" of previous cultivation in low O2.

A molecular basis for NO selectivity in soluble guanylate cyclase

Soluble guanylate cyclases (sGCs) function as heme sensors that selectively bind nitric oxide (NO), triggering reactions essential to animal physiology. Recent discoveries place sGCs in the H-NOX family (heme nitric oxide/oxygen-binding domain), which includes bacterial proteins from aerobic and anaerobic organisms. Some H-NOX proteins tightly bind oxygen (O2), whereas others show no measurable affinity for O2, providing the basis for selective NO signaling in aerobic cells. Using a series of wild-type and mutant H-NOXs, we established a molecular basis for ligand discrimination. A distal pocket tyrosine is requisite for O2 binding in the H-NOX family. These data suggest that sGC uses a kinetic selection against O2; we propose that the O2 dissociation rate in the absence of this tyrosine is fast and that a stable O2 complex does not form.

Insect Neuropeptide and Peptide Hormone Receptors: Current Knowledge and Future Directions

Peptides form a very versatile class of extracellular messenger molecules that function as chemical communication signals between the cells of an organism. Molecular diversity is created at different levels of the peptide synthesis scheme. Peptide messengers exert their biological functions via specific signal-transducing membrane receptors. The evolutionary origin of several peptide precursor and receptor gene families precedes the divergence of the important animal Phyla. In this chapter, current knowledge is reviewed with respect to the analysis of peptide receptors from insects, incorporating many recent data that result from the sequencing of different insect genomes. Therefore, detailed information is provided on six different peptide receptor families belonging to two distinct receptor categories (i.e., the heptahelical and the single transmembrane receptors). In addition, the remaining problems, the emerging concepts, and the future prospects in this area of research are discussed.