Molecular cloning and characterization of a putative cyclic nucleotide-gated channel from Drosophila melanogaster

Function and regulation of nitric oxide signaling in Drosophila

Nitric oxide (NO) serves as an evolutionarily conserved signaling molecule that plays an important role in a wide variety of cellular processes. Extensive studies in Drosophila melanogaster have revealed that NO signaling is required for development, physiology, and stress responses in many different types of cells. In neuronal cells, multiple NO signaling pathways appear to operate in different combinations to regulate learning and memory formation, synaptic transmission, selective synaptic connections, axon degeneration, and axon regrowth. During organ development, elevated NO signaling suppresses cell cycle progression, whereas downregulated NO leads to an increase in larval body size via modulation of hormone signaling. The most striking feature of the Drosophila NO synthase is that various stressors, such as neuropeptides, aberrant proteins, hypoxia, bacterial infection, and mechanical injury, can activate Drosophila NO synthase, initially regulating cellular physiology to enable cells to survive. However, under severe stress or pathophysiological conditions, high levels of NO promote regulated cell death and the development of neurodegenerative diseases. In this review, I highlight and discuss the current understanding of molecular mechanisms by which NO signaling regulates distinct cellular functions and behaviors.

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.

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.

A cyclic nucleotide-gated channel in the brain regulates olfactory modulation through neuropeptide F in fruit fly Drosophila melanogaster

Olfactory sensing and its modulation are important for the insects in recognizing diverse odors from the environment and in making correct decisions to survive. Identifying new genes involved in olfactory modulation and unveiling their mechanisms may lead us to understand decision making processes in the central nervous system. Here, we report a novel olfactory function of the cyclic nucleotide-gated (CNG) channel CG42260 in modulating ab3A olfactory sensory neurons, which specifically respond to food-derived odors in fruit fly Drosophila melanogaster. We found that two independent CG42260 mutants show reduced responses in the ab3A neurons. Unlike mammalian CNGs, CG42260 is not expressed in the odorant sensory neurons but broadly in the central nervous system including neuropeptide-producing cells. By using molecular genetic tools, we identified CG42260 expression in one pair of neuropeptide F (NPF) positive L1-l cells known to modulate food odor responsiveness. Knockdown of CG42260 in the NPF neurons reduced production of NPF in Ll-1 cells, which in turn, led to reduction of neuronal responses of the ab3A neurons. Our findings show the novel biological function of CG42260 in modulating olfactory responses to food odor through NPF.

Genetic coupling of signal and preference facilitates sexual isolation during rapid speciation

The divergence of sexual signals is ultimately a coevolutionary process: while signals and preferences diverge between lineages, they must remain coordinated within lineages for matings to occur. Divergence in sexual signals makes a major contribution to evolving species barriers. Therefore, the genetic architecture underlying signal-preference coevolution is essential to understanding speciation but remains largely unknown. In Laupala crickets where male song pulse rate and female pulse rate preferences have coevolved repeatedly and rapidly, we tested two contrasting hypotheses for the genetic architecture underlying signal-preference coevolution: linkage disequilibrium between unlinked loci and genetic coupling (linkage disequilibrium resulting from pleiotropy of a shared locus or tight physical linkage). Through selective introgression and quantitative trait locus (QTL) fine mapping, we estimated the location of QTL underlying interspecific variation in both female preference and male pulse rate from the same mapping populations. Remarkably, map estimates of the pulse rate and preference loci are as close as 0.06 cM apart, the strongest evidence to date for genetic coupling between signal and preference loci. As the second pair of colocalizing signal and preference loci in the Laupala genome, our finding supports an intriguing pattern, pointing to a major role for genetic coupling in the quantitative evolution of a reproductive barrier and rapid speciation in Laupala. Owing to its effect on suppressing recombination, a coupled, quantitative genetic architecture offers a powerful and parsimonious genetic mechanism for signal-preference coevolution and the establishment of positive genetic covariance on which the Fisherian runaway process of sexual selection relies.

The Genetics of Mating Song Evolution Underlying Rapid Speciation: Linking Quantitative Variation to Candidate Genes for Behavioral Isolation

Differences in mating behaviors evolve early during speciation, eventually contributing to reproductive barriers between species. Knowledge of the genetic and genomic basis of these behaviors is therefore integral to a causal understanding of speciation. Acoustic behaviors are often part of the mating ritual in animal species. The temporal rhythms of mating songs are notably species-specific in many vertebrates and arthropods and often underlie assortative mating. Despite discoveries of mutations that disrupt the temporal rhythm of these songs, we know surprisingly little about genes affecting naturally occurring variation in the temporal pattern of singing behavior. In the rapidly speciating Hawaiian cricket genus Laupala, the striking species variation in song rhythms constitutes a behavioral barrier to reproduction between species. Here, we mapped the largest-effect locus underlying interspecific variation in song rhythm between two Laupala species to a narrow genomic region, wherein we find no known candidate genes affecting song temporal rhythm in Drosophila Whole-genome sequencing, gene prediction, and functional annotation of this region reveal an exciting and promising candidate gene, the putative cyclic nucleotide-gated ion channel-like gene, for natural variation in mating behavior. Identification and molecular characterization of the candidate gene reveals a nonsynonymous mutation in a conserved binding domain, suggesting that ion channels are important targets of selection on rhythmic signaling during establishment of behavioral isolation and rapid speciation.

Histone acetyltransferase CBP-related H3K23 acetylation contributes to courtship learning in Drosophila

Background

Histone modifications are critical in regulating neuronal processes. However, the impacts of individual histone modifications on learning and memory are elusive. Here, we investigated the contributions of histone H3 lysine modifications to learning and memory in Drosophila by using histone lysine-to-alanine mutants.

Results

Behavioural analysis indicated that compared to the H3WT group, mutants overexpressing H3K23A displayed impaired courtship learning. Chromatin immunoprecipitation analysis of H3K23A mutants showed that H3K23 acetylation (H3K23ac) levels were decreased on learning-related genes. Knockdown of CREB-binding protein (CBP) decreased H3K23ac levels, attenuated the expression of learning-related genes, led to a courtship learning defect and altered development of the mushroom bodies. A decline in courtship learning ability was observed in both larvae and adult treatments with ICG-001. Furthermore, treatment of Drosophila overexpressing mutated H3K23A with a CBP inhibitor did not aggravate the learning defect.

Conclusions

H3K23ac, catalysed by the acetyltransferases dCBP, contributes to Drosophila learning, likely by controlling the expression of specific genes. This is a novel epigenetic regulatory mechanism underlying neuronal behaviours.

Electronic supplementary material

The online version of this article (10.1186/s12861-018-0179-z) contains supplementary material, which is available to authorized users.

Visual Working Memory Requires Permissive and Instructive NO/cGMP Signaling at Presynapses in the Drosophila Central Brain

The gaseous second messenger nitric oxide (NO) has been shown to regulate memory formation by activating retrograde signaling cascades from post- to presynapse that involve cyclic guanosine monophosphate (cGMP) production to induce synaptic plasticity and transcriptional changes. In this study, we analyzed the role of NO in the formation of a visual working memory that lasts only a few seconds. This memory is encoded in a subset of ring neurons that form the ellipsoid body in the Drosophila brain. Using genetic and pharmacological manipulations, we show that NO signaling is required for cGMP-mediated CREB activation, leading to the expression of competence factors like the synaptic homer protein. Interestingly, this cell-autonomous function can also be fulfilled by hydrogen sulfide (H₂S) through a converging pathway, revealing for the first time that endogenously produced H₂S has a role in memory processes. Notably, the NO synthase is strictly localized to the axonal output branches of the ring neurons, and this localization seems to be necessary for a second, phasic role of NO signaling. We provide evidence for a model where NO modulates the opening of cGMP-regulated cation channels to encode a short-term memory trace. Local production of NO/cGMP in restricted branches of ring neurons seems to represent the engram for objects, and comparing signal levels between individual ring neurons is used to orient the fly during search behavior. Due to its short half-life, NO seems to be a uniquely suited second messenger to encode working memories that have to be restricted in their duration.

PKA and cAMP/CNG Channels Independently Regulate the Cholinergic Ca(2+)-Response of Drosophila Mushroom Body Neurons

The mushroom bodies (MBs) are the most prominent structures in adult Drosophila brain. They have been involved in several crucial functions, such as learning and memory, sleep, locomotor activity, and decision making.

The mushroom bodies (MBs), one of the main structures in the adult insect brain, play a critical role in olfactory learning and memory. Though historical genes such as dunce and rutabaga, which regulate the level of cAMP, were identified more than 30 years ago, their in vivo effects on cellular and physiological mechanisms and particularly on the Ca²⁺-responses still remain largely unknown. In this work, performed in Drosophila, we took advantage of in vivo bioluminescence imaging, which allowed real-time monitoring of the entire MBs (both the calyx/cell-bodies and the lobes) simultaneously. We imaged neuronal Ca²⁺-activity continuously, over a long time period, and characterized the nicotine-evoked Ca²⁺-response. Using both genetics and pharmacological approaches to interfere with different components of the cAMP signaling pathway, we first show that the Ca²⁺-response is proportional to the levels of cAMP. Second, we reveal that an acute change in cAMP levels is sufficient to trigger a Ca²⁺-response. Third, genetic manipulation of protein kinase A (PKA), a direct effector of cAMP, suggests that cAMP also has PKA-independent effects through the cyclic nucleotide-gated Ca²⁺-channel (CNG). Finally, the disruption of calmodulin, one of the main regulators of the rutabaga adenylate cyclase (AC), yields different effects in the calyx/cell-bodies and in the lobes, suggesting a differential and regionalized regulation of AC. Our results provide insights into the complex Ca²⁺-response in the MBs, leading to the conclusion that cAMP modulates the Ca²⁺-responses through both PKA-dependent and -independent mechanisms, the latter through CNG-channels.

Cyclic nucleotide-gated channels, calmodulin, adenylyl cyclase, and calcium/calmodulin-dependent protein kinase II are required for late, but not early, long-term memory formation in the honeybee

Memory is a dynamic process that allows encoding, storage, and retrieval of information acquired through individual experience. In the honeybee Apis mellifera, olfactory conditioning of the proboscis extension response (PER) has shown that besides short-term memory (STM) and mid-term memory (MTM), two phases of long-term memory (LTM) are formed upon multiple-trial conditioning: an early phase (e-LTM) which depends on translation from already available mRNA, and a late phase (l-LTM) which requires de novo transcription and translation. Here we combined olfactory PER conditioning and neuropharmacological inhibition and studied the involvement of the NO-cGMP pathway, and of specific molecules, such as cyclic nucleotide-gated channels (CNG), calmodulin (CaM), adenylyl cyclase (AC), and Ca(2+)/calmodulin-dependent protein kinase (CaMKII), in the formation of olfactory LTM in bees. We show that in addition to NO-cGMP and cAMP-PKA, CNG channels, CaM, AC, and CaMKII also participate in the formation of a l-LTM (72-h post-conditioning) that is specific for the learned odor. Importantly, the same molecules are dispensable for olfactory learning and for the formation of both MTM (in the minute and hour range) and e-LTM (24-h post-conditioning), thus suggesting that the signaling pathways leading to l-LTM or e-LTM involve different molecular actors.

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.

Dynamics of learning-related cAMP signaling and stimulus integration in the Drosophila olfactory pathway

Functional imaging with genetically encoded calcium and cAMP reporters was used to examine the signal integration underlying learning in Drosophila. Dopamine and octopamine modulated intracellular cAMP in spatially distinct patterns in mushroom body neurons. Pairing of neuronal depolarization with subsequent dopamine application revealed a synergistic increase in cAMP in the mushroom body lobes, which was dependent on the rutabaga adenylyl cyclase. This synergy was restricted to the axons of mushroom body neurons, and occurred only following forward pairing with time intervals similar to those required for behavioral conditioning. In contrast, forward pairing of neuronal depolarization and octopamine produced a subadditive effect on cAMP. Finally, elevating intracellular cAMP facilitated calcium transients in mushroom body neurons, suggesting that cAMP elevation is sufficient to induce presynaptic plasticity. These data suggest that rutabaga functions as a coincidence detector in an intact neuronal circuit, with dopamine and octopamine bidirectionally influencing the generation of cAMP.

Cyclic nucleotide-activated currents in cultured olfactory receptor neurons of the hawkmoth Manduca sexta

Moth pheromones cause rises in intracellular Ca(2+) concentrations that activate Ca(2+)-dependent cation channels in antennal olfactory receptor neurons. In addition, mechanisms of adaptation and sensitization depend on changes in cyclic nucleotide concentrations. Here, cyclic nucleotide-activated currents in cultured olfactory receptor neurons of the moth Manduca sexta are described, which share properties with currents through vertebrate cyclic nucleotide-gated channels. The cyclic nucleotide-activated currents of M. sexta carried Ca(2+) and monovalent cations. They were directly activated by cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), modulated by Ca(2+)/calmodulin, and inhibited by lanthanum. M. sexta cyclic nucleotide-activated currents developed in an all-or-none manner, which suggests that the underlying channels are coupled and act coordinately. At least one cAMP- and two cGMP-activated nonselective cation currents could be distinguished. Compared with the cAMP-activated current, both cGMP-activated currents appeared to conduct more Ca(2+) and showed a stronger down-regulation by Ca(2+)/calmodulin-dependent negative feedback. Furthermore, both cGMP-activated currents differed in their Ca(2+)-dependent inhibition. Thus M. sexta olfactory receptor neurons, like vertebrate sensory neurons, appear to express nonselective cyclic nucleotide-activated cation channels with different subunit compositions. Besides the nonselective cyclic nucleotide-activated cation currents, olfactory receptor neurons express a cAMP-dependent current. This current resembled a protein kinase-modulated low-voltage-activated Ca(2+) current.

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.

Cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) ion channels were first discovered in rod photoreceptors, where they are responsible for the primary electrical signal of the photoreceptor in response to light. CNG channels are highly specialized membrane proteins that open an ion-permeable pore across the membrane in response to the direct binding of intracellular cyclic nucleotides. CNG channels have been identified in a number of other tissues, including the brain, where their roles are only beginning to be appreciated. Recently, significant progress has been made in understanding the molecular mechanisms underlying their functional specializations. From these studies, a picture is beginning to emerge for how the binding of cyclic nucleotide is transduced into the opening of the pore and how this allosteric transition is modulated by various physiological effectors.

Cyclic nucleotide-gated ion channels

Cyclic nucleotide-gated (CNG) channels are nonselective cation channels first identified in retinal photoreceptors and olfactory sensory neurons (OSNs). They are opened by the direct binding of cyclic nucleotides, cAMP and cGMP. Although their activity shows very little voltage dependence, CNG channels belong to the superfamily of voltage-gated ion channels. Like their cousins the voltage-gated K+ channels, CNG channels form heterotetrameric complexes consisting of two or three different types of subunits. Six different genes encoding CNG channels, four A subunits (A1 to A4) and two B subunits (B1 and B3), give rise to three different channels in rod and cone photoreceptors and in OSNs. Important functional features of these channels, i.e., ligand sensitivity and selectivity, ion permeation, and gating, are determined by the subunit composition of the respective channel complex. The function of CNG channels has been firmly established in retinal photoreceptors and in OSNs. Studies on their presence in other sensory and nonsensory cells have produced mixed results, and their purported roles in neuronal pathfinding or synaptic plasticity are not as well understood as their role in sensory neurons. Similarly, the function of invertebrate homologs found in Caenorhabditis elegans, Drosophila, and Limulus is largely unknown, except for two subunits of C. elegans that play a role in chemosensation. CNG channels are nonselective cation channels that do not discriminate well between alkali ions and even pass divalent cations, in particular Ca2+. Ca2+ entry through CNG channels is important for both excitation and adaptation of sensory cells. CNG channel activity is modulated by Ca2+/calmodulin and by phosphorylation. Other factors may also be involved in channel regulation. Mutations in CNG channel genes give rise to retinal degeneration and color blindness. In particular, mutations in the A and B subunits of the CNG channel expressed in human cones cause various forms of complete and incomplete achromatopsia.

Model organisms: new insights into ion channel and transporter function. L-type calcium channels regulate epithelial fluid transport in Drosophila melanogaster

The neuropeptide CAP2b stimulates fluid transport obligatorily via calcium entry, nitric oxide, and cGMP in Drosophila melanogaster Malpighian (renal) tubules. We have shown by RT-PCR that the Drosophila L-type calcium channel alpha1-subunit genes Dmca1D and Dmca1A (nbA) are both expressed in tubules. CAP2b-stimulated fluid transport and cytosolic calcium concentration ([Ca2+]i) increases are inhibited by the L-type calcium channel blockers verapamil and nifedipine. cGMP-stimulated fluid transport is verapamil and nifedipine sensitive. Furthermore, cGMP induces a slow [Ca2+]i increase in tubule principal cells via verapamil- and nifedipine-sensitive calcium entry; RT-PCR shows that tubules express Drosophila cyclic nucleotide-gated channel (cng). Additionally, thapsigargin-induced [Ca2+]i increase is verapamil sensitive. Phenylalkylamines bind with differing affinities to the basolateral and apical surfaces of principal cells in the main segment; however, dihydropyridine binds apically in the tubule initial segment. Immunocytochemical evidence suggests localization of alpha1-subunits to both basolateral and apical surfaces of principal cells in the tubule main segment. We suggest roles for L-type calcium channels and cGMP-mediated calcium influx in both calcium signaling and fluid transport mechanisms in Drosophila.