Biochemical and physiological evidence that calmodulin is involved in the taste response of the sugar receptor cells of the blowfly, Phormia regina

An upstream signaling gene calmodulin regulates the synthesis of insect wax via activating fatty acid biosynthesis pathway

Insect wax accumulates on the surface of insect cuticle, which acts as an important protective barrier against rain, ultraviolet light radiation, pathogens, etc. The waxing behavior, wax composition and molecular mechanism underling wax biosynthesis are unclear in dustywings. Herein, the current study determined the vital developmental stage for waxing behavior in dustywings, examined the components of waxy secretions, and identified key regulatory genes for wax biosynthesis. The wax glands were mainly located on the thorax and abdomen of dustywing adults. The adults spread the waxy secretions over their entire body surface. The metabolomics analysis identified 32 lipids and lipid-like molecules, 15 organic acids and derivatives, 7 benzenoids, etc. as the main components of waxy secretions. The fatty acids represented the largest proportion of the category of lipid and lipid-like molecules. The conjoint analysis of metabolomics and transcriptomics identified two crucial genes fatty acyl-CoA reductase (CsFAR) and calmodulin (CsCaM) for wax biosynthesis. The down-regulation of these genes via nanocarrier-mediated RNA interference technology significantly reduced the amount of wax particles. Notably, the RNAi of CsCaM apparently suppressed the expression of most genes in fatty acid biosynthesis pathway, indicating the CsCaM might act as a main upstream regulator of fatty acid biosynthesis pathway.

Characterization of calcium signaling proteins from the fat body of the Colorado Potato Beetle, Leptinotarsa decemlineata (Coleoptera: Chrysomelidae): Implications for diapause and lipid metabolism

Calcium (Ca²⁺) regulates many cellular and physiological processes from development to reproduction. Ca²⁺ is also an important factor in the metabolism of lipids, the primary energy source used during insect starvation and diapause. Ca²⁺ signaling proteins bind to Ca²⁺ and maintain intracellular Ca²⁺ levels. However, knowledge about Ca²⁺ signaling proteins is mostly restricted to the model Drosophila melanogaster and the response of Ca²⁺ signaling genes to starvation or diapause is not known. In this study, we identified three Ca²⁺ signaling proteins; the primary Ca²⁺ binding protein Calmodulin (LdCaM), phosphatase Calcineurin B (LdCaNB), and the senescence marker protein Regucalcin (LdRgN), from the fat body of the Colorado Potato Beetle, Leptinotarsa decemlineata (Coleoptera: Chrysomelidae). This insect is a major pest of potato worldwide and overwinters under hibernation diapause as adults while utilizing lipids as the primary energy source. Putative EF-hand domains involved in Ca²⁺ binding were present in LdCaM, LdCaNB, but absent in LdRgN. LdCaM and LdCaNB were expressed in multiple tissues, while LdRgN was primarily expressed in the fat body. LdCaM was constitutively-expressed throughout larval development and at the adult stage. LdCaNB was primarily expressed in feeding larvae, and LdRgN in both feeding larvae and adults at comparable levels; however, both genes were down-regulated by molting. A response to starvation was observed only for LdRgN. Transcript abundance analysis in the entire body in relation to diapause revealed differential regulation with a general suppression during diapause, and higher mRNA levels in favor of females at post-diapause for LdCaM, and in favor of males at non-diapause for LdCaNB. Fat body-specific transcript abundance was not different between non-diapause and post-diapause for LdCaNB, but both LdCaM and LdRgN were down-regulated in males and both sexes, respectively by post-diapause. Silencing LdCaNB or LdRgN in larvae led to decreased fat content, indicating their involvement in lipid accumulation, while RNAi of LdCaM led to lethality.

A journey into the world of insect lipid metabolism

Lipid metabolism is fundamental to life. In insects, it is critical, during reproduction, flight, starvation, and diapause. The coordination center for insect lipid metabolism is the fat body, which is analogous to the vertebrate adipose tissue and liver. Fat body contains various different cell types; however, adipocytes and oenocytes are the primary cells related to lipid metabolism. Lipid metabolism starts with the hydrolysis of dietary lipids, absorption of lipid monomers, followed by lipid transport from midgut to the fat body, lipogenesis or lipolysis in the fat body, and lipid transport from fat body to other sites demanding energy. Lipid metabolism is under the control of hormones, transcription factors, secondary messengers and posttranscriptional modifications. Primarily, lipogenesis is under the control of insulin-like peptides that activate lipogenic transcription factors, such as sterol regulatory element-binding proteins, whereas lipolysis is coordinated by the adipokinetic hormone that activates lipolytic transcription factors, such as forkhead box class O and cAMP-response element-binding protein. Calcium is the primary-secondary messenger affecting lipid metabolism and has different outcomes depending on the site of lipogenesis or lipolysis. Phosphorylation is central to lipid metabolism and multiple phosphorylases are involved in lipid accumulation or hydrolysis. Although most of the knowledge of insect lipid metabolism comes from the studies on the model Drosophila; other insects, in particular those with obligatory or facultative diapause, also have great potential to study lipid metabolism. The use of these models would significantly improve our knowledge of insect lipid metabolism.

Double-stranded RNA targeting calmodulin reveals a potential target for pest management of Nilaparvata lugens

BACKGROUND

Calmodulin (CaM) is an essential protein in cellular activity and plays important roles in many processes in insect development. RNA interference (RNAi) has been hypothesized to be a promising method for pest control. CaM is a good candidate for RNAi target. However, the sequence and function of CaM in Nilaparvata lugens are unknown. Furthermore, the double-stranded RNA (dsRNA) target to CaM gene in pest control is still unavailable.

RESULTS

In the present study, two alternatively spliced variants of CaM transcripts, designated NlCaM1 and NlCaM2, were cloned from N. lugens. The two cDNA sequences exhibited 100% identity to each other in the open reading frame (ORF), and only differed in the 3' untranslated region (UTR). NlCaM including NlCaM1 and NlCaM2 mRNA was detectable in all developmental stages and tissues of N. lugens, with significantly increased expression in the salivary glands. Knockdown of NlCaM expression by RNAi with different dsRNAs led to an inability to molt properly, increased mortality, which ranged from 49.7 to 92.5%, impacted development of the ovaries and led to female infertility. There were no significant reductions in the transcript levels of vitellogenin and its receptor or in the total vitellogenin protein level relative to the control group. However, a significant reduction in vitellogenin protein was detected in ovaries injected with dsNlCaM. In addition, a specific dsRNA of NlCaM for control of N. lugens was designed and tested.

CONCLUSION

NlCaM plays important roles mainly in nymph development and uptake of vitellogenin by ovaries in vitellogenesis in N. lugens. dsRNA derived from the less conserved 3'-UTR of NlCaM shows great potential for RNAi-based N. lugens management. © 2018 Society of Chemical Industry.

Nitric oxide contributes to high-salt perception in a blood-sucking insect model

In all organisms, salts produce either appetitive or aversive responses depending on the concentration. While low-salt concentration in food elicits positive responses to ingest, high-salt triggers aversion. Still the mechanisms involved in this dual behavior have just started to be uncovered in some organisms. In Rhodnius prolixus, using pharmacological and behavioral assays, we demonstrated that upon high-salt detection in food a nitric oxide (NO) dependent cascade is activated. This activation involves a soluble guanylate cyclase (sGC) and the production of cyclic guanosine monophosphate (cGMP). Thus, appetitive responses to low-salt diets turn to aversion whenever this cascade is activated. Conversely, insects feed over aversive high-salt solutions when it is blocked by reducing NO levels or by affecting the sGC activity. The activation of NO/sGC/cGMP cascade commands the avoidance feeding behavior in R. prolixus. Investigations in other insect species should examine the possibility that high-salt aversion is mediated by NO/sSG/cGMP signaling.

Transduction mechanism(s) of Na-saccharin in the blowfly Protophormia terraenovae: evidence for potassium and calcium conductance involvement

The study on transduction mechanisms underlying bitter stimuli is a particularly intriguing challenge for taste researchers. The present study investigates, in the labellar chemosensilla of the blowfly Protophormia terraenovae, the transduction mechanism by which saccharin evokes the response of the "deterrent" cell, with particular attention to the contribution of K(+) and Ca(2+) current and the role of cyclic nucleotides, since second messengers modulate Ca(2+), Cl(-) and K(+) currents to different extents. As assessed by extracellular single-sensillum recordings, our results show that the addition of a Ca(2+) chelator such as EGTA or the Ca(2+) current blockers SK&F-96365, Mibefradil, Nifedipine and W-7 decrease the response of the "deterrent" cell to saccharin. A similar decreasing effect was also obtained following the addition of 4-aminopyridine, a K(+) current blocker. On the contrary, the membrane-permeable cyclic nucleotide 8-bromoguanosine 3',5'-cyclic monophosphate (8Br-cGMP) activates this cell and shows an additive effect when presented mixed with saccharin. Our results are consistent with the hypothesis that in the labellar chemosensilla of the blowfly both Ca(2+) and K(+) ions are involved in the transduction mechanism of the "deterrent" cell in response to saccharin. Our results also suggest a possible pathway common to saccharin and 8Br-cGMP.

Water taste transduction pathway is calcium dependent in Drosophila

In mammals, detection of osmolarity by the gustatory system was overlooked until recently. In insects, specific taste receptor neurons detect hypoosmotic stimuli and are commonly called "W" (water) cells. W cells are easy to access in vivo and represent a good model to study the transduction of hypoosmotic stimuli. Using pharmacological and genetic approaches in Drosophila, we show that tarsal W cell firing activity depends on the concentration of external calcium bathing the dendrite. This dependence was confirmed by the strong inhibition of W cell responses to hypoosmotic stimuli by lanthanum (IC(50) = 8 nM), an ion known to inhibit calcium-permeable channels. Downstream, the transduction pathway likely involves calmodulin because calmodulin antagonists such as W-7 (IC(50) = 2 microM) and fluphenazine (IC(50) = 30 microM) prevented the activation of the W cell by hypoosmotic stimuli. A protein kinase C (PKC) may also be involved as W cell responses were blocked by PKC inhibitors, chelerythrine (IC(50) = 20 microM) and staurosporine (IC(50) = 30 microM). It was also reduced when expressing an inhibitory pseudosubstrate of PKC in gustatory receptor neurons. In the rat, the transduction pathway underlying low osmolarity detection involves aquaporin and swelling-activated ion channels. Our study suggests that the transduction pathway of hypoosmotic stimuli in insects differs from mammals.

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.

Transduction pathways mediated by second messengers including cAMP in the sugar receptor cell of the blow fly: study by the whole cell clamp method

The whole cell clamp method was directly applied to the sensory receptor neurons isolated from the adult labellar hair of the blow fly Phormia regina to locate the signal transduction pathways mediated by second messengers. First, the cAMP-mediated transduction pathway was examined to specify its location in the sugar receptor cell. Sugar receptor cell was identified by recording inward current flow under the voltage clamp applying sucrose solution to the surface of the taste neurons. When cyclic nucleotides, such as cGMP and cAMP, were introduced into the sugar receptor cell, inward current was observed (cGMP, 70pA; cAMP, 300pA at 350microM). Inhibitors and activators for the second messengers (GDPbetaS and forskolin) and non-cyclic nucleotides were also examined. Second, non-nucleotide second messengers (IP3 and Ca2+) were examined. The sugar receptor cell was activated when it was injected with IP3 or Ca2+. All the obtained results suggest that the cAMP-mediated signal transduction pathway plays a major role in the sugar receptor cell. The possibility of other transduction pathways mediated by IP3 or Ca2+ was not excluded.

Ca2+ stabilizes the membrane potential of moth olfactory receptor neurons at rest and is essential for their fast repolarization

The role of Ca(2+) in insect olfactory transduction was studied in the moth Spodoptera littoralis. Single sensillum recordings were made to investigate in vivo the role of sensillar Ca(2+) on the electrophysiological properties of sex pheromone responsive olfactory receptor neurons (ORNs). Lowering the sensillar Ca(2+) concentration to 2 x 10(-8) M increased ORN spontaneous firing activity and induced long bursts of action potentials (APs) superimposed on spontaneous negative deflections of the transepithelial potential. We inferred that Ca(2+) stabilizes the membrane potential of ORNs, keeping the spontaneous firing activity at a low and regular level. Neither the amplitude and kinetics of the rising phase of sensillar potentials (SPs) recorded in response to pheromone stimuli nor the AP generation during stimulation depended on the extracellular Ca(2+) concentration. Thus, extracellular Ca(2+) is not absolutely necessary for ORN response. Partial inhibition of responses with a calmodulin antagonist, W-7, also indicates that intracellular Ca(2+) contributes to the ORN response and suggests that Ca(2+) release from internal stores is involved. In 2 x 10(-8) M Ca(2+), the repolarization of the SP was delayed when compared with higher Ca(2+) concentrations. Therefore, in contrast to depolarization, ORN repolarization depends on extracellular Ca(2+). Ca(2+)-gated K(+) channels identified from cultured ORNs with whole-cell recordings are good candidates to mediate ORN repolarization.