Expression pattern of CAPA/pyrokinin neuropeptide genes in Remipedia and silverfish: Rapid differentiation after gene duplication in early Hexapoda, followed by strong conservation of newly established features in insects

CAPA Neuropeptide and Its Receptor in Insects: A Mini Review

A neuropeptide, the CAPA, and its cognate receptor have been diversely characterized in different orders of class Insecta. CAPA peptides are synthesized in the abdominal neurohemal system and activate their corresponding receptor, CAPA receptor (CAPA-R), a type of G protein-coupled receptor (GPCR), to initiate cellular signals for diverse physiological functions in insects. Activation of the CAPA-R in Malpighian tubules results in ion-water homeostasis via antidiuresis in the majority of insect species; however, diuresis and myotropic activities are also known to result. Antidiuretic activity of CAPA peptides has been reported from mosquitoes, assassin bugs, spotted wing drosophila, and more; hence, this group of peptides also holds importance as potential targets when it comes to medical and agricultural entomology. GPCRs form a diverse family of cell membrane receptors responsible for signal transduction across the cell membrane in humans as well as in insects. With the advances in knowledge of human GPCRs, their physiological functions in agriculturally important insects have offered an opportunity for designing and implementing GPCR-targeting compounds in integrated pest management programs. In this review, we present a comprehensive view on physiological factors and peptides responsible for the diuresis/anti-diuresis in insects with special reference to the CAPA peptide-receptor interaction. The major focus is on the role of CAPA peptides in fluid and energy homeostasis, stress tolerance, muscle functioning, regulation of reproduction, and diapause-related processes. We end by outlining the significance of insect excretion with respect to the capa-r gene silencing and pest management.

Neuropeptides from a praying mantis: what the loss of pyrokinins and tryptopyrokinins suggests about the endocrine functions of these peptides

Background

Neuropeptides play important roles in insects, but in many cases their functions remain obscure. Comparative neuropeptidome analyses may provide clues to what these functions might be. Praying mantises are predators and close relatives of cockroaches that are scavengers. Cockroach neuropeptidomes are well established, but little is known about mantis neuropeptides. The recently published genome assembly of the praying mantis Tenodera sinensis makes it Possible to change that.

Methods

The genome assembly from T. sinensis was analyzed for the presence of genes coding neuropeptides. For comparison, publicly available short read archives from this and other mantis species were also examined for the presence and expression of neuropeptides.

Results

As a rule, the neuropeptidomes of the Mantodea and Blattodea are almost identical; praying mantises and cockroaches use very similar neuropeptides. However, there is one surprising exception. Praying mantises lack the receptors for pyrokinins, including those for the tryptopyrokinins. No typical pyrokinin genes were found, but some species do have a tryptopyrokinin gene, in others this has also been lost and, in one species it is a speudogene. For most praying mantises there is no information where tryptopyrokinin is expressed, but in Deroplatys truncata it is in the thorax and thus not in the suboesophageal ganglion, as in other insects. In the genomic short read archives of two species-out of 52-sequences were found for a tryptopyrokinin specific receptor. The phylogenetic position of those two species implies that the receptor gene was independently lost on multiple occasions. The loss of the tryptopyrokinin gene also happened more than once.

Discussion

The multiple independent losses of the pyrokinin receptors in mantises suggests that these receptors are irrelevant in praying mantises. This is very surprising, since expression of tryptopyrokinin is very strongly conserved in two neuroendocrine cells in the suboeosphageal ganglion. In those species for which this is known, the expression of its receptor is in the salivary gland. As a neuroendocrine, tryptopyrokinin is unlikely to acutely regulate salivation, which in other insects is regulated by well characterized neurons. If the action of tryptopyrokinin were to prime the salivary gland for subsequent salivation, it would make perfect sense for a praying mantis to lose this capacity, as they can not anticipate when they will catch their next prey. Priming the salivary gland days before it is actually needed would be energetically costly. The other pyrokinins are known to facilitate feeding and may in a similar fashion prime muscles needed for moving to the food source and digesting it. This hypothesis provides a good explanation as to why praying mantises do not need pyrokinins, and also what the function of these ubiquitous arthropod neuropeptides may be.

Functional expression and characterization of CAPA receptor in the digestive tract and life stages of Drosophila suzukii, and differential activities with insect PRXamide peptides

Spotted-wing drosophila, Drosophila suzukii (Matsumura), is an invasive vinegar fly that is a major threat to the small fruits industries globally. Insect capa genes encode multiple neuropeptides, including CAPA-periviscerokinin (CAPA-PVK) peptides, that are specifically known to cause diuresis or anti-diuresis in various organisms. Here we identified and characterized a corresponding G protein-coupled receptor (GPCR) of the D. suzukii CAPA-PVK peptides: CAPA receptor (CAPA-R). To better characterize the behavior of D. suzukii CAPA-R, we used insect cell-based functional expression assays to evaluate responses of CAPA-R against D. suzukii CAPA-PVKs, CAPA-PVKs from five species in Insecta, one species from Mollusca, modified CAPA-PVK peptides, and some PRXamide family peptides: pyrokinin (PK), diapause hormone (DH), and ecdysis-triggering hormone (ETH). Functional studies revealed that the D. suzukii CAPA-R is strongly activated by both of its own natural D. suzukii CAPA-PVKs, and interestingly, it was strongly activated by other CAPA-PVK peptides from Frankliniella occidentallis (Thysanoptera), Solenopsis invicta (Hymenoptera), Helicoverpa zea (Lepidoptera) and Plutella xylostella (Lepidoptera). However, D. suzukii CAPA-R was not activated by Mollusca CAPA-PVK or the other PRXamide peptides. Gene expression analyses showed that the CAPA-R was highly expressed in the Malpighian tubules and moderately in hindgut compared to other digestive organs or the rest of body, supporting diuretic/antidiuretic functionality. When compared across life stages of D. suzukii, expression of CAPA-R was approximately 1.5x greater in the third instar than the other stages and minimally detected in the eggs, 4-day old pupae and 3-day old adults. Our results functionally characterized the D. suzukii CAPA-R and a few short peptides were identified as potential biological targets to exploit the CAPA-R for D. suzukii management.

Pheromone biosynthesis activating neuropeptide family in insects: a review

Neuropeptides are involved in almost all physiological activities of insects. Their classification is based on physiological function and the primary amino acid sequence. The pyrokinin (PK)/pheromone biosynthesis activating neuropeptides (PBAN) are one of the largest neuropeptide families in insects, with a conserved C-terminal domain of FXPRLamide. The peptide family is divided into two groups, PK1/diapause hormone (DH) with a WFGPRLa C-terminal ending and PK2/PBAN with FXPRLamide C-terminal ending. Since the development of cutting-edge technology, an increasing number of peptides have been sequenced primarily through genomic, transcriptomics, and proteomics, and their functions discovered using gene editing tools. In this review, we discussed newly discovered functions, and analyzed the distribution of genes encoding these peptides throughout different insect orders. In addition, the location of the peptides that were confirmed by PCR or immunocytochemistry is also described. A phylogenetic tree was constructed according to the sequences of the receptors of most insect orders. This review offers an understanding of the significance of this conserved peptide family in insects.

Identification and characterisation of PRXamide peptides in the western flower thrips, Frankliniella occidentalis

Insect CAPA-PVK (periviscerokinin) and pyrokinin (PK) neuropeptides belong to the PRX family peptides and are produced from capa and pyrokinin genes. We identified and characterised the two genes from the western flower thrips, Frankliniella occidentalis. The capa gene transcribes three splice variants, capa-a, -b, and -c, encoding two CAPA-PVKs (EVQGLFPFPRVamide; QGLIPFPRVamide) and two PKs (ASWMPSSSPRLamide; DSASFTPRLamide). The pyrokinin mRNA encodes three PKs: DLVTQVLQPGQTGMWFGPRLamide, SEGNLVNFTPRLamide, and ESGEQPEDLEGSMGGAATSRQLRTDSEPTWGFSPRLamide, the most extended pheromone biosynthesis activating neuropeptide (PBAN) ortholog in insects. Multiple potential endoproteolytic cleavage sites were presented in the prepropeptides from the pyrokinin gene, creating ambiguity to predict mature peptides. To solve this difficulty, we used three G protein-coupled receptors (GPCRs) for CAPA-PVK, tryptophan PK (trpPK), and PK peptides, and evaluated the binding affinities of the peptides. The binding activities revealed each subfamily of peptides exclusively bind to their corresponding receptors, and were significant for determining the CAPA-PVK and PK peptides. Our biological method using specific GPCRs would be a valuable tool for determining mature peptides, particularly with multiple and ambiguous cleavage sites in those prepropeptides. Both capa and pyrokinin mRNAs were strongly expressed in the head/thorax, but minimally expressed in the abdomen. The two genes also were clearly expressed during most of the life stages. Whole-mounting immunocytochemistry revealed that neurons contained PRXamide peptides throughout the whole-body: four to six neurosecretory cells in the head, and three and seven pairs of immunostained cells in the thorax and abdomen, respectively. Notably, the unusual PRXamide profiles of Thysanoptera are different from the other insect groups.

Insect PRXamides: Evolutionary Divergence, Novelty, and Loss in a Conserved Neuropeptide System

The PRXamide neuropeptides have been described in both protostome and deuterostome species, including all major groups of the Panarthropoda. Best studied are the insect PRXamides consisting of three genes: pk/pban, capa, and eth, each encoding multiple short peptides that are cleaved post-translationally. Comparisons of genome and transcriptome sequences reveal that while retaining its fundamental ancestral organization, the products of the pk/pban gene have undergone significant change in the insect Order Diptera. Basal dipteran pk/pban genes are much like those of other holometabolous insects, while more crown species have lost two peptide coding sequences including the otherwise ubiquitous pheromone biosynthesis activating neuropeptide (PBAN). In the genomic model species Drosophila melanogaster, one of the remaining peptides (hugin) plays a potentially novel role in feeding and locomotor regulation tied to circadian rhythms. Comparison of peptide coding sequences of pk/pban across the Diptera pinpoints the acquisition or loss of the hugin and PBAN peptide sequences respectively, and provides clues to associated changes in life history, physiology, and/or behavior. Interestingly, the neural circuitry underlying pk/pban function is highly conserved across the insects regardless of the composition of the pk/pban gene. The rapid evolution and diversification of the Diptera provide many instances of adaptive novelties from genes to behavior that can be placed in the context of emerging selective pressures at key points in their phylogeny; further study of changing functional roles of pk/pban may then be facilitated by the high-resolution genetic tools available in Drosophila melanogaster.

Myotropic Activities of Tick Pyrokinin Neuropeptides and Analog in Feeding Tissues of Hard Ticks (Ixodidae)

Neuropeptides regulate many important physiological processes in animals. The G protein-coupled receptors of corresponding small neuropeptide ligands are considered promising targets for controlling arthropod pests. Pyrokinins (PKs) are pleiotropic neuropeptides that, in some insect species, stimulate muscle contraction and modulate pheromone biosynthesis, embryonic diapause, and feeding behavior. However, their function remains unknown in ticks. In this study, we reported the myotropic activity of tick endogenous PKs and a PK agonist analog, PK-PEG₈ (MS[PEG₈]-YFTPRLa), on feeding tissues of two tick species representing the family Ixodidae lineages, namely, Prostriata (Ixodes scapularis) and Metastriata (Rhipicephalus sanguineus). First, we predicted the sequences of two periviscerokinins (PVK), one with a derived ending RNa and five PKs encoded by the CAPA peptide precursor from R. sanguineus and found the encoded PKs were identical to those of R. microplus identified previously. The pharynx-esophagus of both tick species responded with increased contractions to 10 μM of the endogenous PK as well as to PK-PEG₈ but not to the scrambled PK peptide, as expected. A dose-dependent myotropic activity of the PK-PEG₈ was found for both tick species, validating the analog activity previously found in the pyrokinin recombinant receptor assay. In agreement with the tissue activity elicited, we quantified the relative transcript abundance of R. sanguineus PK receptor in unfed female ticks and found it was the highest in the feeding tissues extracted from the capitulum and lowest in the reproductive tissue. This is the first report of the activity of pyrokinins in ticks. These findings strongly indicate the potential role of PKs in regulating tick blood feeding and therefore, making the tick PK receptor a potential target for interference.

Schistocerca neuropeptides - An update

We compiled a comprehensive list of 67 precursor genes encoding neuropeptides and neuropeptide-like peptides using the Schistocerca gregaria genome and several transcriptome datasets. 11 of these 67 precursor genes have alternative transcripts, bringing the total number of S. gregaria precursors identified in this study to 81. Based on this precursor information, we used different mass spectrometry approaches to identify the putative mature, bioactive peptides processed in the nervous system of S. gregaria. The thereby generated dataset for S. gregaria confirms significant conservation of the entire neuropeptidergic gene set typical of insects and also contains precursors typical of Polyneoptera only. This is in striking contrast to the substantial losses of peptidergic systems in some holometabolous species. The neuropeptidome of S. gregaria, apart from species-specific sequences within the known range of variation, is quite similar to that of Locusta migratoria and even to that of less closely related Polyneoptera. With the S. gregaria peptidomics data presented here, we have thus generated a very useful source of information that could also be relevant for the study of other polyneopteran species.