Identification of specific sites in the third intracellular loop and carboxyl terminus of the Bombyx mori pheromone biosynthesis activating neuropeptide receptor crucial for ligand-induced internalization

Functional Analysis of Pheromone Biosynthesis Activating Neuropeptide Receptor Isoforms in Maruca vitrata

Insect sex pheromones are volatile chemicals that induce mating behavior between conspecific individuals. In moths, sex pheromone biosynthesis is initiated when pheromone biosynthesis-activating neuropeptide (PBAN) synthesized in the suboesophageal ganglion binds to its receptor on the epithelial cell membrane of the pheromone gland. To investigate the function of PBAN receptor (PBANR), we identified two PBANR isoforms, MviPBANR-B and MviPBANR-C, in the pheromone glands of Maruca vitrata. These two genes belong to G protein-coupled receptors (GPCRs) and have differences in the C-terminus but share a 7-transmembrane region and GPCR family 1 signature. These isoforms were expressed in all developmental stages and adult tissues. MviPBANR-C had the highest expression level in pheromone glands among the examined tissues. Through in vitro heterologous expression in HeLa cell lines, only MviPBANR-C-transfected cells responded to MviPBAN (≥5 µM MviPBAN), inducing Ca²⁺ influx. Sex pheromone production and mating behavior were investigated using gas chromatography and a bioassay after MviPBANR-C suppression by RNA interference, which resulted in the major sex pheromone component, E10E12-16:Ald, being quantitatively reduced compared to the control, thereby decreasing the mating rate. Our findings indicate that MviPBANR-C is involved in the signal transduction of sex pheromone biosynthesis in M. vitrata and that the C-terminal tail plays an important role in its function.

Identification and functional characterization of the first molluscan neuromedin U receptor in the slug, Deroceras reticulatum

Neuromedin U (NmU) is a neuropeptide regulating diverse physiological processes. The insect homologs of vertebrate NmU are categorized as PRXamide family peptides due to their conserved C-terminal end. However, NmU homologs have been elusive in Mollusca, the second largest phylum in the animal kingdom. Here we report the first molluscan NmU/PRXamide receptor from the slug, Deroceras reticulatum. Two splicing variants of the receptor gene were functionally expressed and tested for binding with ten endogenous peptides from the slug and some insect PRXamide and vertebrate NmU peptides. Three heptapeptides (QPPLPRYa, QPPVPRYa and AVPRPRIa) triggered significant activation of the receptors, suggesting that they are true ligands for the NmU/PRXamide receptor in the slug. Synthetic peptides with structural modifications at different amino acid positions provided important insights on the core moiety of the active peptides. One receptor variant always exhibited higher binding activity than the other variant. The NmU-encoding genes were highly expressed in the slug brain, while the receptor gene was expressed at lower levels in general with relatively higher expression levels in both the brain and foot. Injection of the bioactive peptides into slugs triggered defensive behavior such as copious mucus secretion and a range of other anomalous behaviors including immobilization, suggesting their role in important physiological functions.

Identification and functional characterization of the pheromone biosynthesis activating neuropeptide receptor isoforms from Mamestra brassicae

In most moth species, including Mamestra brassicae, pheromone biosynthesis activating neuropeptide (PBAN) regulates pheromone production. Generally, PBAN acts directly on the pheromone gland (PG) cells via its specific G protein-coupled receptor (i.e. PBANR) with Ca²⁺ as a second messenger. In this study, we identified cDNAs encoding three variants (A, B and C) of the M. brassicae PBANR (Mambr-PBANR). The full-length coding sequences were transiently expressed in cultured Trichoplusia ni cells and Sf9 cells for functional characterization. All three isoforms dose-dependently mobilized extracellular Ca²⁺ in response to PBAN analogs with Mambr-PBANR-C exhibiting the greatest sensitivity. Fluorescent confocal microscopy imaging studies demonstrated binding of a rhodamine red-labeled ligand (RR10CPBAN) to all three Mambr-PBANR isoforms. RR10CPBAN binding did not trigger ligand-induced internalization in cells expressing PBANR-A, but did in cells expressing the PBANR-B and -C isoforms. Furthermore, activation of the PBANR-B and -C isoforms with the 18 amino acid Mambr-pheromonotropin resulted in co-localization with a Drosophila melanogaster arrestin homolog (Kurtz), whereas stimulation with an unrelated peptide had no effect. PCR-based profiling of the three transcripts revealed a basal level of expression throughout development with a dramatic increase in PG transcripts from the day of adult emergence with PBANR-C being the most abundant.

Regulation of pheromone biosynthesis in moths

Female moths release sex pheromones for attracting males from a distance. Most moths are nocturnal so there is a periodicity to the release of sex pheromone. The temporal release of sex pheromone in most moths is regulated by calling behavior and by the biosynthesis of sex pheromone. In most moths, biosynthesis occurs in the pheromone gland and is controlled by the neuropeptide PBAN (pheromone biosynthesis activating neuropeptide). PBAN is produced in the subesophageal ganglion and released into circulation where it travels to the pheromone gland to activate pheromone biosynthesis. The G-protein coupled receptor that binds PBAN has been identified as well as aspects of signal transduction to activate the biosynthetic pathway. This review will highlight recent advances in the study of regulation of pheromone biosynthesis in moths.

Molecular characterization and functional analyses of a diapause hormone receptor-like gene in parthenogenetic Artemia

In arthropods, mature females under certain conditions produce and release encysted gastrula embryos that enter diapause, a state of obligate dormancy. The process is presumably regulated by diapause hormone (DH) and diapause hormone receptor (DHR) that were identified in the silkworm, Bombyx mori and other insects. However, the molecular structure and function of DHR in crustaceans remains unknown. Here, a DHR-like gene from parthenogenetic Artemia (Ar-DHR) was isolated and sequenced. The cDNA sequence consists of 1410bp with a 1260-bp open reading frame encoding a protein consisting of 420 amino acid residues. The results of real-time PCR (qRT-PCR) and Western blot analysis showed that the mRNA and protein of Ar-DHR were mainly expressed at the diapause stage. Furthermore, we found that Ar-DHR was located on the cell membrane of the pre-diapause cyst but in the cytoplasm of the diapause cyst by analysis of immunofluorescence. In vivo knockdown of Ar-DHR by RNA interference (RNAi) and antiserum neutralization consistently inhibited diapause cysts formation. The results indicated that Ar-DHR plays an important role in the induction and maintenance of embryonic diapause in Artemia. Thus, our findings provide an insight into the regulation of diapause formation in Artemia and the function of Ar-DHR.

Disruption of diapause induction by TALEN-based gene mutagenesis in relation to a unique neuropeptide signaling pathway in Bombyx

The insect neuropeptide family FXPRLa, which carries the Phe-Xaa-Pro-Arg-Leu-NH2 sequence at the C-terminus, is involved in many physiological processes. Although ligand-receptor interactions in FXPRLa signaling have been examined using in vitro assays, the correlation between these interactions and in vivo physiological function is unclear. Diapause in the silkworm, Bombyx mori, is thought to be elicited by diapause hormone (DH, an FXPRLa) signaling, which consists of interactions between DH and DH receptor (DHR). Here, we performed transcription activator-like effector nuclease (TALEN)-based mutagenesis of the Bombyx DH-PBAN and DHR genes and isolated the null mutants of these genes in a bivoltine strain. All mutant silkworms were fully viable and showed no abnormalities in the developmental timing of ecdysis or metamorphosis. However, female adults oviposited non-diapause eggs despite diapause-inducing temperature and photoperiod conditions. Therefore, we conclude that DH signaling is essential for diapause induction and consists of highly sensitive and specific interactions between DH and DHR selected during ligand-receptor coevolution in Bombyx mori.

Identification of functionally important residues of the silkmoth pheromone biosynthesis-activating neuropeptide receptor, an insect ortholog of the vertebrate neuromedin U receptor

The biosynthesis of sex pheromone components in many lepidopteran insects is regulated by the interaction between pheromone biosynthesis-activating neuropeptide (PBAN) and the PBAN receptor (PBANR), a class A G-protein-coupled receptor. To identify functionally important amino acid residues in the silkmoth PBANR, a series of 27 alanine substitutions was generated using a PBANR chimera C-terminally fused with enhanced GFP. The PBANR mutants were expressed in Sf9 insect cells, and their ability to bind and be activated by a core PBAN fragment (C10PBAN(R2K)) was monitored. Among the 27 mutants, 23 localized to the cell surface of transfected Sf9 cells, whereas the other four remained intracellular. Reduced binding relative to wild type was observed with 17 mutants, and decreased Ca(2+) mobilization responses were observed with 12 mutants. Ala substitution of Glu-95, Glu-120, Asn-124, Val-195, Phe-276, Trp-280, Phe-283, Arg-287, Tyr-307, Thr-311, and Phe-319 affected both binding and Ca(2+) mobilization. The most pronounced effects were observed with the E120A mutation. A molecular model of PBANR indicated that the functionally important PBANR residues map to the 2nd, 3rd, 6th, and 7th transmembrane helices, implying that the same general region of class A G-protein-coupled receptors recognizes both peptidic and nonpeptidic ligands. Docking simulations suggest similar ligand-receptor recognition interactions for PBAN-PBANR and the orthologous vertebrate pair, neuromedin U (NMU) and NMU receptor (NMUR). The simulations highlight the importance of two glutamate residues, Glu-95 and Glu-120, in silkmoth PBANR and Glu-117 and Glu-142 in human NMUR1, in the recognition of the most functionally critical region of the ligands, the C-terminal residue and amide.

The Arginine Residue within the C-Terminal Active Core of Bombyx mori Pheromone Biosynthesis-Activating Neuropeptide is Essential for Receptor Binding and Activation

In most lepidopteran insects, the biosynthesis of sex pheromones is regulated by pheromone biosynthesis-activating neuropeptide (PBAN). Bombyx mori PBAN (BomPBAN) consists of 33 amino acid residues and contains a C-terminus FSPRLamide motif as the active core. Among neuropeptides containing the FXPRLamide motif, the arginine (Arg, R) residue at the second position from the C-terminus is highly conserved across several neuropeptides, which can be designated as RXamide peptides. The purpose of this study was to clarify the role of the Arg residue in the BomPBAN active core. We synthesized 10-residue peptides corresponding to the C-terminal part of BomPBAN with a series of replacements at the second position from the C-terminus, termed the C2 position, and measured their efficacy in stimulating Ca(2+) influx in insect cells expressing a fluorescent PBAN receptor chimera (PBANR-EGFP) using the fluorescent Ca(2+) indicator, Fura Red-AM. The PBAN analogs with the C2 position replaced with alanine (Ala, A), aspartic acid (Asp, D), serine (Ser, S), or l-2-aminooctanoic acid (Aoc) decreased PBAN-like activity. R(C2)A (SKTRYFSPALamide) and R(C2)D (SKTRYFSPDLamide) had the lowest activity and could not inhibit the activity of PBAN C10 (SKTRYFSPRLamide). We also prepared Rhodamine Red-labeled peptides of the PBAN analogs and examined their ability to bind PBANR. In contrast to Rhodamine Red-PBAN C10 at 100 nM, none of the synthetic analogs exhibited PBANR binding at the same concentration. Taken together, our results demonstrate that the C2 Arg residue in BomPBAN is essential for PBANR binding and activation.

Establishment of Sf9 Transformants Constitutively Expressing PBAN Receptor Variants: Application to Functional Evaluation

To facilitate further evaluation of pheromone biosynthesis activating neuropeptide receptor (PBANR) functionality and regulation, we generated cultured insect cell lines constitutively expressing green fluorescent protein chimeras of the recently identified Bombyx mori PBANR (BommoPBANR) and Pseudaletia separata PBANR (PsesePBANR) variants. Fluorescent chimeras included the BommoPBANR-A, -B, and -C variants and the PsesePBANR-B and -C variants. Cell lines expressing non-chimeric BommoPBANR-B and -C variants were also generated. Functional evaluation of these transformed cell lines using confocal laser microscopy revealed that a Rhodamine Red-labeled PBAN derivative (RR-C10PBAN(R2K)) specifically co-localized with all of the respective PBANR variants at the plasma membrane. Near complete internalization of the fluorescent RR-C10PBAN(R2K) ligand 30 min after binding was observed in all cell lines except those expressing the BommoPBANR-A variant, in which the ligand/receptor complex remained at the plasma membrane. Fluorescent Ca(2+) imaging further showed that the BommoPBANR-A cell line exhibited drastically different Ca(2+) mobilization kinetics at a number of RR-C10PBAN(R2K) concentrations including 10 μM. These observations demonstrate a clear functional difference between the BommoPBANR-A variant and the BommoPBANR-B and -C variants in terms of receptor regulation and activation of downstream effector molecules. We also found that, contrary to previous reports, ligand-induced internalization of BommoPBANR-B and BommoPBANR-C in cell lines stably expressing these variants occurred in the absence of extracellular Ca(2+).

Re-Evaluation of the PBAN Receptor Molecule: Characterization of PBANR Variants Expressed in the Pheromone Glands of Moths

Sex pheromone production in most moths is initiated following pheromone biosynthesis activating neuropeptide receptor (PBANR) activation. PBANR was initially cloned from pheromone glands (PGs) of Helicoverpa zea and Bombyx mori. The B. mori PBANR is characterized by a relatively long C-terminus that is essential for ligand-induced internalization, whereas the H. zea PBANR has a shorter C-terminus that lacks features present in the B. mori PBANR critical for internalization. Multiple PBANRs have been reported to be concurrently expressed in the larval CNS of Heliothis virescens. In the current study, we sought to examine the prevalence of multiple PBANRs in the PGs of three moths and to ascertain their potential functional relevance. Multiple PBANR variants (As, A, B, and C) were cloned from the PGs of all species examined with PBANR-C the most highly expressed. Alternative splicing of the C-terminal coding sequence of the PBAN gene gives rise to the variants, which are distinguishable only by the length and composition of their respective C-terminal tails. Transient expression of fluorescent PBANR chimeras in insect cells revealed that PBANR-B and PBANR-C localized exclusively to the cell surface while PBANR-As and PBANR-A exhibited varying degrees of cytosolic localization. Similarly, only the PBANR-B and PBANR-C variants underwent ligand-induced internalization. Taken together, our results suggest that PBANR-C is the principal receptor molecule involved in PBAN signaling regardless of moth species. The high GC content of the C-terminal coding sequence in the B and C variants, which makes amplification using conventional polymerases difficult, likely accounts for previous "preferential" amplification of PBANR-A like receptors from other species.