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

Atypical soluble guanylyl cyclases control brain size in Drosophila

Hypoxia-induced proliferation of neural stem cells has a crucial role in brain development. In the brain of Drosophila melanogaster , the optic lobe exhibits progressive hypoxia during larval development. Here, we investigate an alternative oxygen-sensing mechanism within this brain compartment, distinct from the canonical hypoxia signaling pathway mediated by HIF. Using genetic tools, immunostaining, and confocal microscopy, we demonstrate that the loss of the atypical soluble guanylyl cyclase (asGC) subunit Gyc88E , or the ectopic expression of Gyc89Db in neural stem cells leads to increased optic lobe volume. We propose the existence of a link between cGMP signaling and neurogenesis in the developing brain.

Oxygen sensing in crustaceans: functions and mechanisms

Animals that live in changing environments need to adjust their metabolism to maintain body functions, and sensing these changing conditions is essential for mediating the short- and long-term physiological and behavioral responses that make these adjustments. Previous research on nematodes and insects facing changing oxygen levels has shown that these animals rapidly respond using atypical soluble guanylyl cyclases (sGCs) as oxygen sensors connected to downstream cGMP pathways, and they respond more slowly using hypoxia-inducible transcription factors (HIFs) that are further modulated by oxygen-sensing prolyl hydroxylases (PHs). Crustaceans are known to respond in different ways to hypoxia, but the mechanisms responsible for sensing oxygen levels are more poorly understood than in nematodes and insects. Our paper reviews the functions of and mechanisms underlying oxygen sensing in crustaceans. Furthermore, using the oxygen sensing abilities of nematodes and insects as guides in analyzing available crustacean transcriptomes, we identified orthologues of atypical sGCs, HIFs, and PHs in crustaceans, including in their chemosensory organs and neurons. These molecules include atypical sGCs activated by hypoxia (Gyc-88E/GCY-31 and Gyc-89D/GCY-33) but not those activated by hyperoxia (GCY-35, GCY-36), as well as orthologues of HIF-α, HIF-β, and PH. We offer possible directions for future research on oxygen sensing by crustaceans.

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.

A transcriptomics assessment of oxygen-temperature interactions reveals novel candidate genes underlying variation in thermal tolerance and survival

While single stress responses are fairly well researched, multiple, interactive stress responses are not-despite the obvious importance thereof. Here, using D. melanogaster, we investigated the effects of simultaneous exposures to low O₂ (hypoxia) and varying thermal conditions on mortality rates, estimates of thermal tolerance and the transcriptome. We used combinations of 21 (normoxia), 10 or 5kPa O₂ with control (23°C), cold (4°C) or hot (31°C) temperature exposures before assaying chill coma recovery time (CCRT) and heat knock down time (HKDT) as measures of cold and heat tolerance respectively. We found that mortality was significantly affected by temperature, oxygen partial pressure (PO₂) and the interaction between the two. Cold treatments resulted in low mortality (<5%), regardless of PO₂ treatment; while hot treatments resulted in higher mortality (∼20%), especially at 5kPa O₂ which was lethal for most flies (∼80%). Both CCRT and HKDT were significantly affected by temperature, but not PO₂, of the treatments, and the interaction of temperature and PO₂ was non-significant. Hot treatments led to significantly longer CCRT, and shorter HKDT in comparison to cold treatments. Global gene expression profiling provided the first transcriptome level response to the combined stress of PO₂ and temperature, showing that stressful treatments resulted in higher mortality and induced transcripts that were associated with protein kinases, catabolic processes (proteases, hydrolases, peptidases) and membrane function. Several genes and pathways that may be responsible for the protective effects of combined PO₂ and cold treatments were identified. We found that urate oxidase was upregulated in all three cold treatments, regardless of the PO₂. Small heat shock proteins Hsp22 and Hsp23 were upregulated after both 10 and 21kPa O₂-hot treatments. Collectively, the data from PO₂-hot treatments suggests that hypoxia does exacerbate heat stress, through an as yet unidentified mechanism. Hsp70B and an unannotated transcript (CG6733) were significantly differentially expressed after 5kPa O₂-cold and 10kPa O₂-hot treatments relative to their controls. Downregulation of these transcripts was correlated with reduced thermal tolerance (longer CCRT and shorter HKDT), suggesting that these genes may be important candidates for future research.

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.

Evolution and physiology of neural oxygen sensing

Major evolutionary trends in animal physiology have been heavily influenced by atmospheric O2 levels. Amongst other important factors, the increase in atmospheric O2 which occurred in the Pre-Cambrian and the development of aerobic respiration beckoned the evolution of animal organ systems that were dedicated to the absorption and transportation of O2, e.g., the respiratory and cardiovascular systems of vertebrates. Global variations of O2 levels in post-Cambrian periods have also been correlated with evolutionary changes in animal physiology, especially cardiorespiratory function. Oxygen transportation systems are, in our view, ultimately controlled by the brain related mechanisms, which senses changes in O2 availability and regulates autonomic and respiratory responses that ensure the survival of the organism in the face of hypoxic challenges. In vertebrates, the major sensorial system for oxygen sensing and responding to hypoxia is the peripheral chemoreflex neuronal pathways, which includes the oxygen chemosensitive glomus cells and several brainstem regions involved in the autonomic regulation of the cardiovascular system and respiratory control. In this review we discuss the concept that regulating O2 homeostasis was one of the primordial roles of the nervous system. We also review the physiology of the peripheral chemoreflex, focusing on the integrative repercussions of chemoreflex activation and the evolutionary importance of this system, which is essential for the survival of complex organisms such as vertebrates. The contribution of hypoxia and peripheral chemoreflex for the development of diseases associated to the cardiovascular and respiratory systems is also discussed in an evolutionary context.

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.

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.

Water loss and gas exchange by eggs of Manduca sexta: trading off costs and benefits

Like all terrestrial organisms, insect eggs face a tradeoff between exchanging metabolic gases (O(2) and CO(2)) and conserving water. Here I summarize the physiology underlying this tradeoff and the ecological contexts in which it may be important. The ideas are illustrated primarily by work from my laboratory on eggs of the sphingid moth Manduca sexta. In particular, I discuss: (1) dynamic changes in metabolic demand and water loss during development; and (2) how the eggshell layers and embryonic tracheal system control the traffic of gases between the embryo and its environment. Subsequently, I identify three areas with interesting but unresolved issues: (1) what eggs actually experience in their microclimates, focusing particularly on the leaf microclimates relevant to eggs of M. sexta; (2) how egg experience influences whether or not hatchling larvae succeed in establishing feeding sites on host plants; and (3) whether Hetz and Bradley's [Hetz, S.K., Bradley, T.J., 2005. Insects breathe discontinuously to avoid oxygen toxicity. Nature 433, 516-519] oxygen toxicity hypothesis for discontinuous gas-exchange cycles applies to insect eggs.

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.

The crystal structure of the catalytic domain of a eukaryotic guanylate cyclase

BACKGROUND

Soluble guanylate cyclases generate cyclic GMP when bound to nitric oxide, thereby linking nitric oxide levels to the control of processes such as vascular homeostasis and neurotransmission. The guanylate cyclase catalytic module, for which no structure has been determined at present, is a class III nucleotide cyclase domain that is also found in mammalian membrane-bound guanylate and adenylate cyclases.

RESULTS

We have determined the crystal structure of the catalytic domain of a soluble guanylate cyclase from the green algae Chlamydomonas reinhardtii at 2.55 A resolution, and show that it is a dimeric molecule.

CONCLUSION

Comparison of the structure of the guanylate cyclase domain with the known structures of adenylate cyclases confirms the close similarity in architecture between these two enzymes, as expected from their sequence similarity. The comparison also suggests that the crystallized guanylate cyclase is in an inactive conformation, and the structure provides indications as to how activation might occur. We demonstrate that the two active sites in the dimer exhibit positive cooperativity, with a Hill coefficient of approximately 1.5. Positive cooperativity has also been observed in the homodimeric mammalian membrane-bound guanylate cyclases. The structure described here provides a reliable model for functional analysis of mammalian guanylate cyclases, which are closely related in sequence.

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.

Ligand binding and inhibition of an oxygen-sensitive soluble guanylate cyclase, Gyc-88E, from Drosophila

Soluble guanylate cyclase (sGC) uses a ferrous heme cofactor as a receptor for NO and once bound activates the enzyme for the conversion of GTP to cGMP. The heme cofactor in sGC does not bind oxygen, thereby allowing it to selectively bind NO despite a cellular concentration of oxygen (microM) that is much higher than signaling concentrations of nitric oxide (nM). The molecular details of this ligand discrimination against oxygen have emerged and allowed for predictions regarding ligand specificity in the sGC family. The results reported here show that Gyc-88E from Drosophila is a hemoprotein that binds oxygen, as well as NO and CO. All three ligands form 6-coordinate complexes. Gyc-88E is active as a homodimer (5600 +/- 243 nmol min(-1) mg(-1)) and is inhibited by O2, CO, and NO (3.2-, 2.9-, and 2-fold, respectively). The Km for GTP was 0.66 +/- 0.15 mM in air (273 microM oxygen) and 0.82 +/- 0.15 mM under anaerobic conditions. The Ki for oxygen was calculated to be 51 +/- 28 microM. The biochemical properties of Gyc-88E are unique for guanylate cyclases and suggest a possible function as an oxygen sensor.

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.

Developmental expression and hormonal regulation of different isoforms of the transcription factor E75 in the tobacco hornworm Manduca sexta

E75A and E75B, isoforms of the E75 orphan nuclear receptor, are sequentially up-regulated in the abdominal epidermis of the tobacco hornworm Manduca sexta by 20-hydroxyecdysone (20E) during larval and pupal molts, with E75A also increasing at pupal commitment (Zhou et al., Dev. Biol. 193, 127-138, 1998). We have now cloned E75C and show that little is expressed in the epidermis during larval life with trace amounts seen just before ecdysis. Instead, E75C is found in high amounts during the development of the adult wings as the ecdysteroid titer is rising, and this increase was prevented by juvenile hormone (JH) that prevented adult development. By contrast, E75D is expressed transiently during the larval and pupal molts as the ecdysteroid titer begins to decline and again just before ecdysis, but in the developing adult wings is expressed on the rise of 20E. Removal of the source of JH had little effect on either E75C or E75D mRNA expression during the larval and pupal molts. At the time of pupal commitment, in vitro experiments show that 20E up-regulates E75D and JH prevents this increase. Neither E75A nor E75D mRNA was up-regulated by JH alone. Thus, E75C is primarily involved in adult differentiation whereas E75D has roles both during the molt and pupal commitment.