Isolation, cloning, and tissue expression of a putative octopamine/tyramine receptor from locust visceral muscle tissues

Identification of G protein-coupled receptors required for vitellogenesis and egg development in an insect with panoistic ovary

G protein-coupled receptors (GPCRs), a superfamily of integral transmembrane proteins regulate a variety of physiological processes in insects. Juvenile hormone (JH) is known to stimulate Vitellogenin (Vg) synthesis in the fat body, secretion into the hemolymph and uptake by developing oocytes. However, the role of GPCRs in JH-dependent insect vitellogenesis and oocyte maturation remains elusive. In the present study, we performed transcriptomic analysis and RNA interference (RNAi) screening in vitellogenic females of the migratory locust Locusta migratoria. Of 22 GPCRs identified in ovarian transcriptome, LGR4, OR-A1, OR-A2, Mthl1, Mthl5 and Smo were most abundant in the ovary. By comparison, mAChR-C expressed at higher levels in the fat body, whereas Oct/TyrR, OARβ, AdoR and ADGRA3 were at higher expression levels in the brain. Our RNAi screening demonstrated that knockdown of six GPCRs resulted in defective phenotypes of Vg accumulation in developing oocytes, accompanied by blocked ovarian development and impaired oocyte maturation. While LGR4 and Oct/TyrR appeared to control Vg synthesis in the fat body, OR-A1, OR-A2, mAChR-C and CirlL regulated Vg transportation and uptake. The findings provide fundamental evidence for deciphering the regulatory mechanisms of GPCRs in JH-stimulated insect reproduction.

Corrigendum: Cloning and Functional Characterization of Octβ2-Receptor and Tyr1-Receptor in the Chagas Disease Vector, Rhodnius prolixus

[This corrects the article DOI: 10.3389/fphys.2017.00744.].

Pharmacological Properties of the Type 1 Tyramine Receptor in the Diamondback Moth, Plutella xylostella

Tyramine receptors (TARs) can be activated by tyramine (TA) or octopamine (OA) and have been shown to be related to physiological regulation (e.g., gustatory responsiveness, social organization, and learning behavior) in a range of insect species. A tyramine receptor gene in Plutella xylostella, Pxtar1, was cloned and stably expressed in the HEK-293 cell line. Pharmacological properties and expression profile of Pxtar1 were also analyzed. Tyramine could activate the PxTAR1 receptor, increasing the intracellular Ca²⁺ concentration ((Ca²⁺)i) at an EC50 of 13.1 nM and reducing forskolin (10 μM)-stimulated intracellular cAMP concentration ((cAMP)i) at an IC₅₀ of 446 nM. DPMF (a metabolite of amitraz) and L(-)-carvone (an essential oil) were found to act as PxTAR1 receptor agonists. Conversely, yohimbine and mianserin had significant antagonistic effects on PxTAR1. In both larvae and adults, Pxtar1 had the highest expression in the head capsule and expression of Pxtar1 was higher in male than in female reproductive organs. This study reveals the temporal and spatial differences and pharmacological properties of Pxtar1 in P. xylostella and provides a strategy for screening insecticidal compounds that target PxTAR1.

Improving Soluble Expression of Tyrosine Decarboxylase from Lactobacillus brevis for Tyramine Synthesis with High Total Turnover Number

The soluble expression of tyrosine decarboxylase (TDC) in heterologous host is often challenging. Here, acidic condition was found to be favorable for improving the soluble expression of TDC from Lactobacillus brevis in Escherichia coli, while addition of carbohydrates (such as glucose, arabinose, and fructose) was vital for decreasing the insoluble fraction. By simple pH control and addition of glucose, the specific activity of TDC in crude extract was enhanced to 46.3 U mg^-1, 3.67-fold of that produced from LB medium. Optimization of the reaction conditions revealed that Tween-80 was effective in improving the tyramine production catalyzed by TDC, especially at high tyrosine loadings. As much as 400 mM tyrosine could be completely converted into tyramine with a substrate to catalyst ratio of 29.0 g g^-1 and total turnover number of 23,300. This study provides efficient strategies for the highly soluble expression of TDC and biocatalytic production of tyramine.

Pharmacological characterization of a β-adrenergic-like octopamine receptor in Plutella xylostella

The β-adrenergic-like octopamine receptor (OA2B2) belongs to the class of G-protein coupled receptors. It regulates important physiological functions in insects, thus is potentially a good target for insecticides. In this study, the putative open reading frame sequence of the Pxoa2b2 gene in Plutella xylostella was cloned. Orthologous sequence alignment, phylogenetic tree analysis, and protein sequence analysis all showed that the cloned receptor belongs to the OA2B2 protein family. PxOA2B2 was transiently expressed in HEK-293 cells. It was found that PxOA2B2 could be activated by both octopamine and tyramine, resulting in increased intracellular cyclic AMP (cAMP) levels, whereas dopamine and serotonin were not effective in eliciting cAMP production. Further studies with series of PxOA2B2 agonists and antagonists showed that all four tested agonists (e.g., naphazoline, clonidine, 2-phenylethylamine, and amitraz) could activate the PxOA2B2 receptor, and two of tested antagonists (e.g., phentolamine and mianserin) had significant antagonistic effects. However, antagonist of yohimbine had no effects. Quantitative real-time polymerase chain reaction analysis showed that Pxoa2b2 gene was expressed in all developmental stages of P. xylostella and that the highest expression occurred in male adults. Further analysis with fourth-instar P. xylostella larvae showed that the Pxoa2b2 gene was mainly expressed in Malpighian tubule, epidermal, and head tissues. This study provides both a pharmacological characterization and the gene expression patterns of the OA2B2 in P. xylostella, facilitating further research for insecticides using PxOA2B2 as a target.

Cloning and Functional Characterization of Octβ2-Receptor and Tyr1-Receptor in the Chagas Disease Vector, Rhodnius prolixus

Octopamine and tyramine, both biogenic amines, are bioactive chemicals important in diverse physiological processes in invertebrates. In insects, octopamine and tyramine operate analogously to epinephrine and norepinephrine in the vertebrates. Octopamine and tyramine bind to G-protein coupled receptors (GPCRs) leading to changes in second messenger levels and thereby modifying the function in target tissues and insect behavior. In this paper, we report the cDNA sequences of two GPCRs, RhoprOctβ2-R, and RhoprTyr1-R, have been cloned and functionally characterized from Rhodnius prolixus. Octopamine and tyramine each activate RhoprOctβ2-R and RhoprTyr1-R in a dose-dependent manner. Octopamine is one order of magnitude more potent than tyramine in activating RhoprOctβ2-R. Tyramine is two orders of magnitude more potent than octopamine in activating RhoprTyr1-R. Phentolamine and gramine significantly antagonize RhoprOctβ2-R, whereas yohimbine and phenoxybenzamine are effective blockers of RhoprTyr1-R. The transcripts of both receptors are enriched in the central nervous system (CNS) and are expressed throughout the adult female reproductive system. It has been shown in other insects that Octβ2-R is essential for processes such as ovulation and fertilization. We previously reported that octopamine and tyramine modulate oviducts and bursa contractions in R. prolixus. Our data confirm the importance of octopamine and tyramine signaling in the reproductive system of R. prolixus.

Identification of multiple functional receptors for tyramine on an insect secretory epithelium

The biogenic amine tyramine (TA) regulates many aspects of invertebrate physiology and development. Although three TA receptor subtypes have been identified (TAR1-3), specific receptors have not been linked to physiological responses in native tissue. In the Malpighian (renal) tubule of Drosophila melanogaster, TA activates a transepithelial chloride conductance, resulting in diuresis and depolarization of the transepithelial potential. In the current work, mutation or RNAi-mediated knockdown in the stellate cells of the tubule of TAR2 (tyrR, CG7431) resulted in a dramatic reduction, but not elimination, of the TA-mediated depolarization. Mutation or knockdown of TAR3 (tyrRII, CG16766) had no effect. However, deletion of both genes, or knockdown of TAR3 on a TAR2 mutant background, eliminated the TA responses. Thus while TAR2 is responsible for the majority of the TA sensitivity of the tubule, TAR3 also contributes to the response. Knockdown or mutation of TAR2 also eliminated the response of tubules to the related amine octopamine (OA), indicating that OA can activate TAR2. This finding contrasts to reports that heterologously expressed TAR2 is highly selective for TA over OA. This is the first report of TA receptor function in a native tissue and indicates unexpected complexity in the physiology of the Malpighian tubule.

Octopamine and tyramine regulate the activity of reproductive visceral muscles in the adult female blood-feeding bug, Rhodnius prolixus

The role of octopamine and tyramine in regulating spontaneous contractions of reproductive tissues was examined in the female Rhodnius prolixus Octopamine decreased the amplitude of spontaneous contractions of the oviducts and reduced RhoprFIRFa-induced contractions in a dose-dependent manner, whereas tyramine only reduced the RhoprFIRFa-induced contractions. Both octopamine and tyramine decreased the frequency of spontaneous bursal contractions and completely abolished the contractions at 5×10^-7 mol l^-1 and above. Phentolamine, an octopamine receptor antagonist, attenuated the inhibition induced by octopamine on the oviducts and the bursa. Octopamine also increased the levels of cAMP in the oviducts, and this effect was blocked by phentolamine. Dibutyryl cyclic AMP mimicked the effects of octopamine by reducing the frequency of bursal contractions, suggesting that the octopamine receptor may act by an Octβ receptor. The tyramine receptor antagonist yohimbine failed to block the inhibition of contractions induced by tyramine on the bursa, suggesting that tyramine may be acting on the Octβ receptor in the bursa.

Crystal structure of tyrosine decarboxylase and identification of key residues involved in conformational swing and substrate binding

Tyrosine decarboxylase (TDC) is a pyridoxal 5-phosphate (PLP)-dependent enzyme and is mainly responsible for the synthesis of tyramine, an important biogenic amine. In this study, the crystal structures of the apo and holo forms of Lactobacillus brevis TDC (LbTDC) were determined. The LbTDC displays only 25% sequence identity with the only reported TDC structure. Site-directed mutagenesis of the conformationally flexible sites and catalytic center was performed to investigate the potential catalytic mechanism. It was found that H241 in the active site plays an important role in PLP binding because it has different conformations in the apo and holo structures of LbTDC. After binding to PLP, H241 rotated to the position adjacent to the PLP pyridine ring. Alanine scanning mutagenesis revealed several crucial regions that determine the substrate specificity and catalytic activity. Among the mutants, the S586A variant displayed increased catalytic efficiency and substrate affinity, which is attributed to decreased steric hindrance and increased hydrophobicity, as verified by the saturation mutagenesis at S586. Our results provide structural information about the residues important for the protein engineering of TDC to improve catalytic efficiency in the green manufacturing of tyramine.

Octopamine receptor gene expression in three lepidopteran species of insect

The invertebrate octopaminergic system affects many diverse processes and represents the counterpart to the vertebrate adrenergic/noradrenergic system with the classes of octopamine receptor (OAR) being homologous to those of vertebrate adrenergic receptors. However, there is still little information on the OARs present in different insect species, and the levels and distribution of these receptors throughout the body. cDNAs sharing high similarity with known insect OARs were cloned in three lepidopteran species: the cabbage looper, Trichoplusia ni; the true armyworm, Pseudaletia unipuncta; and the cabbage white, Pieris rapae. Seven major larval tissues and one adult tissue were examined in T. ni using quantitative real-time PCR to determine the relative expression levels of each receptor transcript across different tissues, as well as of all receptor transcripts within individual tissues. A subset of these tissues was also examined in P. unipuncta and P. rapae. All receptor transcripts were expressed in the nervous system of all three species, however, the distribution of the different receptor types varied between species. In all tissues, the OARbeta2 transcript was the most highly expressed, except in the Malpighian tubules where OARbeta1 was highest, and the midgut where there was no significant difference in receptor transcript levels.

Cloning of an octopamine/tyramine receptor and plasticity of its expression as a function of adult sexual maturation in the male moth Agrotis ipsilon

In the male moth Agrotis ipsilon behavioural response and antennal lobe (AL) neuron sensitivity to the female-produced sex pheromone increase with age and juvenile hormone (JH) level. We recently showed that the neuromodulator, octopamine (OA), interacts with JH in this age-dependent olfactory plasticity. To further elucidate its role, we cloned a full cDNA encoding a protein that presents biochemical features essential to OA/tyramine receptor (AipsOAR/TAR) function. The AipsOAR/TAR transcript was detected predominantly in the antennae, the brain and, more specifically, in ALs where its expression level varied concomitantly with age. This expression plasticity indicates that AipsOAR/TAR might be involved in central processing of the pheromone signal during maturation of sexual behaviour in A. ipsilon.

The role of octopamine in locusts and other arthropods

The biogenic amine octopamine and its biological precursor tyramine are thought to be the invertebrate functional homologues of the vertebrate adrenergic transmitters. Octopamine functions as a neuromodulator, neurotransmitter and neurohormone in insect nervous systems and prompts the whole organism to "dynamic action". A growing number of studies suggest a prominent role for octopamine in modulating multiple physiological and behavioural processes in invertebrates, as for example the phase transition in Schistocerca gregaria. Both octopamine and tyramine exert their effects by binding to specific receptor proteins that belong to the superfamily of G protein-coupled receptors. Since these receptors do not appear to be present in vertebrates, they may present very suitable and specific insecticide and acaricide targets.

Octopamine receptors from the barnacle balanus improvisus are activated by the alpha2-adrenoceptor agonist medetomidine

G protein-coupled octopamine receptors of insects and other invertebrates represent counterparts of adrenoceptors in vertebrate animals. The alpha(2)-adrenoceptor agonist medetomidine, which is in clinical use as a veterinary sedative agent, was discovered to inhibit the settling process of barnacles, an important step in the ontogeny of this crustacean species. Settling of barnacles onto ship hulls leads to biofouling that has many harmful practical consequences, and medetomidine is currently under development as a novel type of antifouling agent. We now report that medetomidine induces hyperactivity in the barnacle larvae to disrupt the settling process. To identify the molecular targets of medetomidine, we cloned five octopamine receptors from the barnacle Balanus improvisus. We show by phylogenetic analyses that one receptor (BiOctalpha) belongs to the alpha-adrenoceptor-like subfamily, and the other four (BiOctbeta-R1, BiOctbeta-R2, BiOctbeta-R3, and BiOctbeta-R4) belong to the beta-adrenoceptor-like octopamine receptor subfamily. Phylogenetic analyses also indicated that B. improvisus has a different repertoire of beta-adrenoceptor-like octopamine receptors than insects. When expressed in CHO cells, the cloned receptors were activated by both octopamine and medetomidine, resulting in increased intracellular cAMP or calcium levels. Tyramine activated the receptors but with much lesser potency than octopamine. A hypothesis for receptor discrimination between tyramine and octopamine was generated from a homology three-dimensional model. The characterization of B. improvisus octopamine receptors is important for a better functional understanding of these receptors in crustaceans as well as for practical applications in development of environmentally sustainable antifouling agents.

Characterization of a novel octopamine receptor expressed in the surf clam Spisula solidissima

We have cloned and sequenced a cDNA from the surf clam (Spisula solidissima, a pelecypod mollusc) that encodes an octopamine receptor which we have named Spi-OAR. The sequence of Spi-OAR shares many similarities with two Aplysia and three Drosophila octopamine receptors belonging to a sub-group of beta-adrenergic-like octopamine receptors. Using an expression vector and transient transfections of Spi-OAR into HEK 293 cells, we observed an increase of cAMP upon addition of octopamine and, to a lesser extent, of tyramine, but not after addition of dopamine, serotonin, or histamine. Using a battery of known agonists and antagonists for octopamine receptors, we observed a rather unique pharmacological profile for Spi-OAR through measurements of cAMP. Spi-OAR exhibited some constitutive activity in HEK 293 cells and no Ca(2+) responses could be detected following addition of octopamine to Spi-OAR-transfected cells. RT-PCR analysis revealed ubiquitous expression of Spi-OAR mRNA in all adult tissues, oocytes and early embryos examined. While addition of serotonin to isolated clam oocytes resulted in meiotic activation, similar additions of octopamine had no effect, suggesting that its potential role in clam reproductive physiology differs significantly from that of serotonin. This work identifies Spi-OAR as a novel mollusc octopamine receptor closely related to other invertebrate beta-adrenergic-like octopamine receptors, with possible reproductive and other physiological functions. This initial characterization of Spi-OAR makes possible further investigations and comparisons with more studied and familiar insect or gastropod mollusc octopamine receptors.

Isoform- and cell-specific function of tyrosine decarboxylase in the Drosophila Malpighian tubule

The biogenic amine tyramine (TA) is a potent diuretic factor when applied to the Malpighian tubule (MT) of Drosophila melanogaster, stimulating both urine production and transepithelial chloride conductance. Isolated MTs can respond not only to TA but also to its precursor, tyrosine; this observation led to the proposal that MTs are able to synthesize TA from applied tyrosine through the action of the enzyme tyrosine decarboxylase (TDC). In the current study it is shown that the non-neuronal isoform of TDC, Tdc1, is expressed in the principal cells of the MT. A mutant allele of Tdc1, Tdc1(f03311), was identified that reduced expression of the mature Tdc1 transcript by greater than 100-fold. MTs isolated from Tdc1(f03311) homozygous flies showed no significant depolarization of their transepithelial potential (TEP) or diuresis in response to tyrosine while retaining normal sensitivity to TA. By contrast, a previously identified null mutant allele of the neuronal TDC isoform Tdc2 had no effect on either tyrosine or TA sensitivity. To determine in which cell type of the MT Tdc1 expression is required, flies were generated carrying a UAS-Tdc1 transgene and cell-type-specific Gal4 drivers on a Tdc1(f03311) homozygous background. Rescue of Tdc1 expression in principal cells fully restored sensitivity to tyrosine whereas expression of Tdc1 in stellate cells had no rescuing effect. It is concluded that synthesis of TA by Tdc1 in the principal cells of the MT is required for physiological responses to tyrosine. TA synthesis in the MT is the first reported physiological role for Drosophila Tdc1.

The female reproductive system and control of oviposition in Locusta migratoria migratorioidesThe present review is the first of a series of occasional review articles that have been invited by the Editors and will feature the broad range of disciplines and expertise represented in our Editorial Advisory Board

The spermatheca acts as a repository for sperm deposited by the male and, in the African migratory locust ( Locusta migratoria migratorioides (Fairmaire and Reiche, 1849)), is situated dorsal to the lateral and common oviducts. In the locust, eggs mature in the ovaries and are ovulated into the lateral oviducts where they are held until a suitable oviposition site is found. At that time, a hole is dug in the soil by the locust and, aided by muscular contractions of the upper lateral oviducts, the eggs are propelled through the common oviduct and genital chamber and deposited in a pod in the soil. Contractions of the spermathecal sac lead to sperm release, resulting in fertilization of eggs in the genital chamber. Coordination of digging and of the oviducts and spermatheca is clearly critical to the production of viable eggs. The muscles responsible for digging and both reproductive structures are under central neuronal control, incorporating neurons that express an array of neuropeptide and amine phenotypes. Many of the phenotypes are common to both reproductive tissues. A neural loop ensures the coordinated release of sperm when an egg passes into the genital chamber. This review will discuss our understanding of the neural control of these reproductive tissues and their coordination with digging.

Tyramine: from octopamine precursor to neuroactive chemical in insects

It is well acknowledged that tyramine acts as the biosynthetic intermediate precursor for octopamine. This fact has biased the interpretation of biological effects of tyramine towards an artifact of it being a partial agonist on octopamine receptors. Over recent years there has been an accumulation of evidence to show that tyramine is in fact a neuroactive chemical in its own right, with diverse physiological/behavioral roles. In addition, tyramine plays a unique role in a non-neuronal tissue, namely the Malpighian tubules. This review examines this evidence, taking into account the criteria that need to be satisfied in order to claim neuroactive chemical status. Thus, the evidence points to tyramine being synthesized by, and present in, neurons; capable of being released from neurons; removed by high affinity plasma membrane transporters; acting upon specific tyramine receptors; and producing physiological/behavioral effects that can be blocked by antagonists. This composite evidence is strong, although the final proof still awaits analysis on a uniquely identifiable tyraminergic neuron as has been possible with octopamine.

Biotransformation of L-tyrosine to tyramine by the growing cells of Lactococcus lactis

The potential of pyridoxal-5-phosphate dependent tyrosine decarboxylase (E.C. 4.1.1.25) of Lactococcus lactis was explored for the biotransformation of L-tyrosine to tyramine. Maximum bioconversion of L-tyrosine to tyramine was achieved in tyramine production medium (pH -5.5) at 30 degrees C after 16 h of incubation with 0.2% L-tyrosine. The yield oftyramine was found to be 11.8 microg/mL by the growing cells of L. lactis at shake flask level. Growth medium and different physico-chemical parameters to maximize the biotransformation of L-tyrosine to tyramine were optimized and yielded 1.9-fold increased synthesis of tyramine.

Trace amine-associated receptor 1-Family archetype or iconoclast?

Interest has recently been rekindled in receptors that are activated by low molecular weight, noncatecholic, biogenic amines that are typically found as trace constituents of various vertebrate and invertebrate tissues and fluids. The timing of this resurgent focus on receptors activated by the "trace amines" (TA) beta-phenylethylamine (PEA), tyramine (TYR), octopamine (OCT), synephrine (SYN), and tryptamine (TRYP) is the direct result of 2 publications that appeared in 2001 describing the cloning of a novel G protein-coupled receptor (GPCR) referred to by their discoverers Borowsky et al. as TA1 and Bunzow et al. as TA receptor 1 (TAR1). When heterologously expressed in Xenopus laevis oocytes and various eukaryotic cell lines, recombinant rodent and human TAR dose-dependently couple to the stimulation of adenosine 3',5'-monophosphate (cAMP) production. Structure-activity profiling based on this functional response has revealed that in addition to the TA, other biologically active compounds containing a 2-carbon aliphatic side chain linking an amino group to at least 1 benzene ring are potent and efficacious TA receptor agonists with amphetamine (AMPH), methamphetamine, 3-iodothyronamine, thyronamine, and dopamine (DA) among the most notable. Almost 100 years after the search for TAR began, numerous TA1/TAR1-related sequences, now called TA-associated receptors (TAAR), have been identified in the genome of every species of vertebrate examined to date. Consequently, even though heterologously expressed TAAR1 fits the pharmacological criteria established for a bona fide TAR, a major challenge for those working in the field is to discern the in vivo pharmacology and physiology of each purported member of this extended family of GPCR. Only then will it be possible to establish whether TAAR1 is the family archetype or an iconoclast.