Prothoracicotropic hormone ofRhodnius prolixus: partial characterization and rhythmic release of neuropeptides related toBombyxPTTH and bombyxin

The prothoracicotropic hormone (PTTH) of Rhodnius prolixus (Hemiptera) is noggin-like: Molecular characterisation, functional analysis and evolutionary implications

Prothoracicotropic hormone (PTTH) is a central regulator of insect development that regulates the production of the steroid moulting hormones (ecdysteroids) from the prothoracic glands (PGs). Rhodnius PTTH was the first brain neurohormone discovered in any animal almost 100 years ago but has eluded identification and no homologue of Bombyx mori PTTH occurs in its genome. Here, we report Rhodnius PTTH is the first noggin-like PTTH found. It differs in important respects from known PTTHs and is the first PTTH from the Hemimetabola (Exopterygota) to be fully analysed. Recorded PTTHs are widespread in Holometabola but close to absent in hemimetabolous orders. We concluded Rhodnius PTTH likely differed substantially from the known ones. We identified one Rhodnius gene that coded a noggin-like protein (as defined by Molina et al., 2009) that had extensive similarities with known PTTHs but also had two additional cysteines. Sequence and structural analysis showed known PTTHs are closely related to noggin-like proteins, as both possess a growth factor cystine knot preceded by a potential cleavage site. The gene is significantly expressed only in the brain, in a few cells of the dorsal protocerebrum. We vector-expressed the sequence from the potential cleavage site to the C-terminus. This protein was strongly steroidogenic on PGs in vitro. An antiserum to the protein removed the steroidogenic protein released by the brain. RNAi performed on brains in vitro showed profound suppression of transcription of the gene and of production and release of PTTH and thus of ecdysteroid production by PGs. In vivo, the gene is expressed throughout development, in close synchrony with PTTH release, ecdysteroid production by PGs and the ecdysteroid titre. The Rhodnius PTTH monomer is 17kDa and immunoreactive to anti-PTTH of Bombyx mori (a holometabolan). Bombyx PTTH also mildly stimulated Rhodnius PGs. The two additional cysteines form a disulfide at the tip of finger 2, causing a loop of residues to protrude from the finger. A PTTH variant without this loop failed to stimulate PGs, showing the loop is essential for PTTH activity. It is considered that PTTHs of Holometabola evolved from a noggin-like protein in the ancestor of Holometabola and Hemiptera, c.400ma, explaining the absence of holometabolous-type PTTHs from hemimetabolous orders and the differences of Rhodnius PTTH from them. Noggin-like proteins studied from Hemiptera to Arachnida were homologous with Rhodnius PTTH and may be common as PTTHs or other hormones in lower insects.

Time- and temperature-dependent dynamics of prothoracicotropic hormone and ecdysone sensitivity co-regulate pupal diapause in the green-veined white butterfly Pieris napi

Diapause, a general shutdown of developmental pathways, is a vital adaptation allowing insects to adjust their life cycle to adverse environmental conditions such as winter. Diapause in the pupal stage is regulated by the major developmental hormones prothoracicotropic hormone (PTTH) and ecdysone. Termination of pupal diapause in the butterfly Pieris napi depends on low temperatures; therefore, we study the temperature-dependence of PTTH secretion and ecdysone sensitivity dynamics throughout diapause, with a focus on diapause termination. While PTTH is present throughout diapause in the cell bodies of two pairs of neurosecretory cells in the brain, it is absent in the axons, and the PTTH concentration in the haemolymph is significantly lower during diapause than during post diapause development, indicating that the PTTH signaling is reduced during diapause. The sensitivity of pupae to ecdysone injections is dependent on diapause stage. While pupae are sensitive to ecdysone during early diapause initiation, they gradually lose this sensitivity and become insensitive to non-lethal concentrations of ecdysone about 30 days into diapause. At low temperatures, reflecting natural overwintering conditions, diapause termination propensity after ecdysone injection is precocious compared to controls. In stark contrast, at high temperatures reflecting late summer and early autumn conditions, sensitivity to ecdysone does not return. Thus, here we show that PTTH secretion is reduced during diapause, and additionally, that the low ecdysone sensitivity of early diapause maintenance is lost during termination in a temperature dependent manner. The link between ecdysone sensitivity and low-temperature dependence reveals a putative mechanism of how diapause termination operates in insects that is in line with adaptive expectations for diapause.

The involvement of insulin/ToR signaling pathway in reproductive performance of Rhodnius prolixus

Insulins are peptide hormones widely studied for their important regulatory roles in metabolism, growth and development. In insects, insulin signaling along with the target of rapamycin (ToR) are involved in detecting and interpreting nutrient levels. Recently, by transcriptome analysis we reported an up-regulation of transcripts involved in insulin/ToR signaling in unfed Rhodnius prolixus; however, this signaling pathway is only activated in fed insects. Here, continuing with the blood-gorging triatomine R. prolixus as a model, we report the direct effect of insulin/ToR signaling on reproductive performance. By immunofluorescence we identified cells in the brain with positive signal to the R. prolixus ILP (Rhopr-ILP1) and show that the insulin receptor and protein effectors downstream of insulin/ToR signaling activation, are differentially expressed in ovarian follicles dependent on their developmental stage. Using qPCR we find that the expression of transcripts involved in insulin signaling in the central nervous system (CNS), fat body and ovaries increase as the state of starvation progresses, promoting a more highly sensitized state to respond rapidly to ILP/IGF levels. In addition, using dsRNA injection and in vivo and ex vivo assays to promote signaling activation we demonstrate a direct participation of insulin/ToR signaling in coordinating the synthesis of the main yolk protein precursor, vitellogenin, thereby influencing the numbers of eggs laid per female. We thereby show a mechanism by which nutritional signaling regulates reproductive performance in a vector of Chagas disease. As reproduction is responsible for propagation of insect populations, this work is important for the development of innovative biocontrol methods.

Neuropeptide- and serotonin- cells in the brain of Rhodnius prolixus (Hemiptera) associated with the circadian clock

The neuronal pathways of the circadian clock in the brain of R. prolixus have been described in detail previously, but there is no information concerning the cells or their pathways which relay either inputs to the clock (e.g. for light entrainment), or outputs from it to driven rhythms. Here, we employ antisera to three neuropeptides (type A allatostatin-7, crustacean cardioactive peptide and FMRFamide), and serotonin in confocal laser scanning immunohistochemistry to analyze the distribution of cell bodies and their projections in relation to the principle circadian clock cells (lateral cells, LNs) for all four neuron types. LNs are revealed following labelling with anti- pigment dispersing factor in double labelled preparations. Regions of potential communication between ramifications of the LNs and each of the four other neuron types is described (identified by close superposition of their neurites in various brain regions), as is their detailed projections within the brain. Neuromodulation is sometimes suggested by close, but not intimate, proximity of varicosities of neurites. We infer that some neuron types comprise input pathways to the LNs, some are outputs to neuroendocrine or behavioral rhythms, and others participate in both input and output pathways, sometimes by the same neuron type but in different locations. For example, one retinula cell in each ommatidium is immunoreactive for allatostatin A; its axon projects to the medulla making superpositions with LNs, as do serotonin cells in the optic lobe, indicating roles of both neuron types in light input (entrainment) to the clock. But in other brain areas, these same types appear to mediate outputs from the clock. The accessory medulla has been widely reported as the principle center of integration in other insects; but we found sparse evidence of this in R. prolixus as it contains few neurites other than those from the clock cells. Rather, the importance of neural pathways involving the medulla and the superior protocerebrum is emphasized. We conclude that there is a vast and complex web of interactions in the brain with the LNs, which potentially receive multiple pathways of inputs and outputs that could drive rhythmicity in a multitude of downstream cells, rendering a host of output pathways rhythmic, notably hormone release from neurosecretory cells and behaviors.

Circadian control of prothoracicotropic hormone release in an adult insect and the induction of its rhythmicity by light cues

The insect neuropeptide prothoracicotropic hormone (PTTH) is a critical regulator of larval development. We recently demonstrated that PTTH is also present in adult Rhodnius prolixus and is released by adult brains in vitro with a clear daily rhythm during egg development. Here, we employ a well-established in vitro bioassay, to show that the daily rhythm of PTTH release by brains in vitro is under circadian control since it persists in aperiodic conditions with a free running period of around 24h that is temperature compensated. Prolonged exposure (3weeks) of insects to continuous constant light (LL) completely eliminated PTTH release. Subsequent transfer of such insects from LL to constant darkness (DD) rapidly induced rhythmic PTTH release, indicating that the circadian rhythm of PTTH release is induced by photic cues. Western analysis identified PTTH in the adult hemolymph, suggesting that PTTH acts as a functional neurohormone in the adult insect. Dot blot analysis revealed that PTTH levels in the hemolymph also cycled with a daily rhythm that persisted in DD and was synchronous with the rhythm of PTTH release by brains in vitro. We conclude that the previously documented photosensitive clock in the brain regulates rhythmic PTTH release and thus generates the rhythm seen in the hemolymph. These results emphasize the importance of rhythmic PTTH release in the adult insect and support a role for PTTH in adult physiology and possibly within the adult circadian system.

Insulin-like and testis ecdysiotropin neuropeptides are regulated by the circadian timing system in the brain during larval-adult development in the insect Rhodnius prolixus (Hemiptera)

Insulin-like peptides (ILPs) regulate numerous functions in insects including growth, development, carbohydrate metabolism and female reproduction. This paper reports the immunohistochemical localization of ILPs in brain neurons of Rhodnius prolixus and their intimate associations with the brain circadian clock system. In larvae, three groups of neurons in the protocerebrum are ILP-positive, and testis ecdysiotropin (TE) is co-localized in two of them. During adult development, the number of ILP groups increased to four. A blood meal initiates transport and release of ILPs, indicating that release is nutrient dependent. Both production and axonal transport of ILPs continue during adult development with clear cytological evidence of a daily rhythm that closely correlates with the daily rhythm of ILPs release from brains in vitro. The same phenomena were observed with TE previously. Double labeling for ILPs and pigment dispersing factor (PDF) (contained in the brain lateral clock cells, LNs) revealed intimate associations between axons of the ILP/TE cells and PDF-positive axons in both central brain and retrocerebral complex, revealing potential neuronal pathways for circadian regulation of ILPs and TE. Similar close associations were found previously between LN axons and axons of the brain neurons producing the neuropeptide prothoracicotropic hormone. Thus, the brain clock system controls rhythmicity in multiple brain neurohormones. It is suggested that rhythms in circulating ILPs and TE act in concert with known rhythms of circulating ecdysteroids in both larvae and adults to orchestrate the timing of cellular responses in diverse tissues of the animal, thereby generating internal temporal order within it.

Tissue signaling pathways in the regulation of life-span and reproduction in females of the linden bug, Pyrrhocoris apterus

Molecular studies on Drosophila melanogaster do not provide consistent results with regard to the hormonal regulation of a trade-off between life-span and fecundity. To unravel the physiological basis of the cost of reproduction without affecting animal's genotype, a new insect model, Pyrrhocoris apterus, was employed. Reproduction was manipulated by surgical ablations of tissues implicated in reproductive endocrinology, namely the pars intercerebralis (PI) of the brain, the corpus allatum (CA) and the ovary, and the response of life-span to these interventions under diapause-promoting short days and reproduction-promoting long days was measured. Life-span of long-day females increased in the following order: control (high fecundity)=ovary-ablation (no egg production)<PI-ablation (low fecundity)<CA-ablation=ovary+CA-ablation (no reproduction)<CA+PI-ablation (no reproduction). These results show that: (1) PI-signaling (presumably insulin-like peptides) and CA-signaling (juvenile hormone) reduce life-span of long-day females in additive manner and (2) the ovary has no effect on life-span. Life-span of short-day females increased in the following order: PI-ablation (low fecundity)<CA+PI-ablation (no reproduction)<control=CA-ablation (no reproduction). These results implicate factors from the PI that actively extend life-span of short-day females via down-regulation of CA-signaling and also via CA-independent pathway(s). Overall, the data indicate that life-span and reproduction are linked by signals from PI and CA, but, in contrast with the widely held view, neither production of eggs nor gonad-signaling are costly to female longevity.

Neuroanatomical relations of prothoracicotropic hormone neurons with the circadian timekeeping system in the brain of larval and adultRhodnius prolixus (Hemiptera)

This paper reports the localization in the Rhodnius prolixus brain of neurons producing the key neuropeptide that regulates insect development, prothoracicotropic hormone (PTTH) and describes intimate associations of the PTTH neurons with the brain circadian timekeeping system. Immunohistochemistry and confocal laser scanning microscopy revealed that the PTTH-positive neurons in larvae are located in a single group in the lateral protocerebrum. Their number increases from two in the last larval instar to five during larval-adult development. In adults, there are two distinct groups of these neurons composed of two cells each. A daily rhythm in content of PTTH-positive material occurs in both the somata and the axons in both larval and adult stages. These rhythms correlate with previous evidence of a circadian rhythm of PTTH release from brains in vitro. The key circadian clock cells of Rhodnius are eight neurons, which co-express pigment-dispersing factor (PDF) and the canonical clock proteins PER and TIM; PDF fills the axons. Equivalent cells control behavioral rhythms in other insects. Double labeling revealed intimate associations between axons of larval PTTH neurons and clock neurons, indicating a neuronal pathway from the brain timekeeping system for circadian control of PTTH release. Additional PDF neurons appear in the adult, associated with the second group of PTTH neurons. These findings provide the first direct evidence that neurons of the insect brain timekeeping system control hormone rhythms. The range of functions regulated by this timekeeping system is quite similar to those of the vertebrate suprachiasmatic nucleus, for which the insect system is a valuable model.

Circadian orchestration of developmental hormones in the insect, Rhodnius prolixus

This review presents a new perspective on the circadian regulation and functions of insect developmental hormones. In Rhodnius prolixus (Hemiptera), the brain neuropeptide prothoracicotropic hormone (PTTH) is released with a circadian rhythm that is controlled by paired photosensitive clocks in the brain. These clocks comprise the dorsal and lateral PER/TIM clock neurons known to regulate behavioral rhythms in Drosophila. Axons of PTTH and clock cells make close contact. Photosensitive PER/TIM clocks also reside in the paired prothoracic glands (PGs), which generate rhythmic synthesis and release of the ecdysteroid molting hormones. The PG clocks are entrained by both light and PTTH. These four clocks are coupled together by both nerves and hormones into a timing system whose primary regulated output is the circadian rhythm of ecdysteroids in the hemolymph. This complex timing system appears necessary to ensure circadian organization of the gene expression that is induced in target cells by ecdysteroids via circadian cycling of the nuclear ecdysteroid receptor (EcR). This multioscillator system serves to transduce 'the day outside' into endocrine rhythms that orchestrate 'the day inside'. It has many functional similarities with vertebrate circadian systems.

Testis ecdysiotropic peptides in Rhodnius prolixus: biological activity and distribution in the nervous system and testis

In Rhodnius prolixus, testes from both pharate adult and adult males are shown to produce and release ecdysteroids in vitro. Proteinaceous brain extracts from these stages caused stimulation of ecdysteroid production by testes of unfed adults. Therefore, the brain of Rhodnius contains peptides with testis ecdysiotropic activity. The Lymantria testis ecdysiotropin (LTE) also stimulated the in vitro production of ecdysteroids by unfed adult testis but had no stimulatory effect on prothoracic glands. Western blot analysis of brain peptides using anti-LTE revealed the presence of several medium to small size immunoreactive peptides. Two of these peptides with sizes of 16.8 and 11.0 kDa were present only during pharate adult development and the adult stage. Immunohistochemical analysis using confocal laser scanning microscopy revealed abundant LTE-immunoreactive material in cytoplasmic granules of specific neurosecretory cells in the brain and suboesophageal ganglion and the epithelium of the testis sheath. Clusters of two cytologically distinct cell types were seen within the medial neurosecretory cells (MNC) and also a pair of neurons in the posterior protocerebrum. Feeding in both larvae and adult males resulted in massive release of LTE-immunoreactive material from the MNC cells, suggesting a role of LTE-related peptides in both larval-adult development and in male reproductive development. Release from the MNC cells of LTE-immunoreactive material exhibited a clear daily cycling during larval-adult development, which was synchronous with the rhythms of release of prothoracicotropic hormone and bombyxin reported previously. The testis sheath exhibited intense immunofluorescence in pharate adults and unfed adults, which disappeared following a blood meal. It is concluded that LTE-related peptides are developmentally regulated in several locations and may act as ecdysiotropins in Rhodnius. Those in the MNC cells are very probably classical hormones, i.e. are transported to their target sites via the insect haemolymph.