Isoform-specific expression of the neuropeptide orcokinin in Drosophila melanogaster

The Integrative Physiology of Hormone Signaling: Insights from Insect Models

Hormones orchestrate virtually all physiological processes in animals and enable them to adjust internal responses to meet diverse physiological demands. Studies in both vertebrates and insects have uncovered many novel hormones and dissected the physiological mechanisms they regulate, demonstrating a remarkable conservation in endocrine signaling across the tree of life. In this review, we focus on recent advances in insect research, which have provided a more integrative view of the conserved interorgan communication networks that control physiology. These new insights have been driven by experimental advantages inherent to insects, which over the past decades have aligned with new technologies and sophisticated genetic tools, to transform insect genetic models into a powerful testbed for posing new questions and exploring longstanding issues in endocrine research. Here, we illustrate how insect studies have addressed classic questions in three main areas, hormonal control of growth and development, neuroendocrine regulation of ion and water balance, and hormonal regulation of behavior and metabolism, and how these discoveries have illuminated our fundamental understanding of endocrine signaling in animals. The application of integrative physiology in insect systems to questions in endocrinology and physiology is expanding and is poised to be a crucible of discovery, revealing fundamental mechanisms of hormonal regulation that underlie animal adaptations to their environments.

What Drosophila can tell us about state-dependent peptidergic signaling in insects

Plasticity in animal behavior and physiology is largely due to modulatory and regulatory signaling with neuropeptides and peptide hormones (collectively abbreviated NPHs). The NPHs constitute a very large and versatile group of signaling substances that partake at different regulatory levels in most daily activities of an organism. This review summarizes key principles in NPH actions in the brain and in interorgan signaling, with focus on Drosophila. NPHs are produced by neurons, neurosecretory cells (NSCs) and other endocrine cells in NPH-specific and stereotypic patterns. Most of the NPHs have multiple (pleiotropic) functions and target several different neuronal circuits and/or peripheral tissues. Such divergent NPH signaling ensures orchestration of behavior and physiology in state-dependent manners. Conversely, many neurons, circuits, NSCs, or other cells, are targeted by multiple NPHs. This convergent signaling commonly conveys various signals reporting changes in the external and internal environment to central neurons/circuits. As an example of wider functional convergence, 26 different Drosophila NPHs act at many different levels to regulate food search and feeding. Convergence is also seen in hormonal regulation of peripheral functions. For instance, multiple NPHs target renal tubules to ensure osmotic homeostasis. Interestingly, several of the same osmoregulatory NPHs also regulate feeding, metabolism and stress. However, for some NPHs the cellular distribution and functions suggests multiple unrelated functions that are restricted to specific circuits. Thus, NPH signaling follows distinct patterns for each specific NPH, but taken together they form overlapping networks that modulate behavior and physiology.

Transcriptome analysis reveals salivary gland-specific neuropeptide signaling genes in the predatory stink bug, Picromerus lewisi

Predatory stink bugs derive from phytophagous stink bugs and evolved enhanced predation skills. Neuropeptides are a diverse class of ancient signaling molecules that regulate physiological processes and behavior in animals, including stink bugs. Neuropeptide evolution might be important for the development of predation because neuropeptides can be converted to venoms that impact prey. However, information on neuropeptide signaling genes in predatory stink bugs is lacking. In the present study, neuropeptide signaling genes of Picromerus lewisi, an important predatory stink bug and an effective biological agent, were comprehensively identified by transcriptome analysis, with a total of 59 neuropeptide precursor genes and 58 potential neuropeptide receptor genes found. In addition, several neuropeptides and their receptors enriched in salivary glands of P. lewisi were identified. The present study and subsequent functional research contribute to an in-depth understanding of the biology and behavior of the predatory bugs and can provide basic information for the development of better pest management strategies, possibly including neuropeptide receptors as insecticide targets and salivary gland derived venom toxins as novel killing moleculars.

Specification of the Drosophila Orcokinin A neurons by combinatorial coding

The central nervous system contains a daunting number of different cell types. Understanding how each cell acquires its fate remains a major challenge for neurobiology. The developing embryonic ventral nerve cord (VNC) of Drosophila melanogaster has been a powerful model system for unraveling the basic principles of cell fate specification. This pertains specifically to neuropeptide neurons, which typically are stereotypically generated in discrete subsets, allowing for unambiguous single-cell resolution in different genetic contexts. Here, we study the specification of the OrcoA-LA neurons, characterized by the expression of the neuropeptide Orcokinin A and located laterally in the A1-A5 abdominal segments of the VNC. We identified the progenitor neuroblast (NB; NB5-3) and the temporal window (castor/grainyhead) that generate the OrcoA-LA neurons. We also describe the role of the Ubx, abd-A, and Abd-B Hox genes in the segment-specific generation of these neurons. Additionally, our results indicate that the OrcoA-LA neurons are "Notch Off" cells, and neither programmed cell death nor the BMP pathway appears to be involved in their specification. Finally, we performed a targeted genetic screen of 485 genes known to be expressed in the CNS and identified nab, vg, and tsh as crucial determinists for OrcoA-LA neurons. This work provides a new neuropeptidergic model that will allow for addressing new questions related to neuronal specification mechanisms in the future.

Interorgan communication through peripherally derived peptide hormones in Drosophila

In multicellular organisms, endocrine factors such as hormones and cytokines regulate development and homoeostasis through communication between different organs. For understanding such interorgan communications through endocrine factors, the fruit fly Drosophila melanogaster serves as an excellent model system due to conservation of essential endocrine systems between flies and mammals and availability of powerful genetic tools. In Drosophila and other insects, functions of neuropeptides or peptide hormones from the central nervous system have been extensively studied. However, a series of recent studies conducted in Drosophila revealed that peptide hormones derived from peripheral tissues also play critical roles in regulating multiple biological processes, including growth, metabolism, reproduction, and behaviour. Here, we summarise recent advances in understanding target organs/tissues and functions of peripherally derived peptide hormones in Drosophila and describe how these hormones contribute to various biological events through interorgan communications.

A Review of Pedal Peptide/Orcokinin-type Neuropeptides

Neuropeptides are endogenous active substances that play important roles in a number of physiological processes and are ubiquitous in the nervous tissue in vivo. The gene encoding pedal peptide/orcokinin-type (PP/OK-type) neuropeptide is an important member of the neuropeptide gene family and is ubiquitous in invertebrates of Bilateria; orcokinin (OK) is mainly found in Arthropoda, while pedal peptide (PP) is mainly found in Mollusca. OK and PP are also present in other animals. PP/OK-type neuropeptides are a kind of multifunctional neuropeptides predominantly expressed in the nervous tissue and play important roles in the nerve regulation of movement. Moreover, OK has a number of other physiological functions. This review describes the distribution, expression, function and maturation of PP/OK-type neuropeptides to facilitate investigations of new functions and receptors of PP/OK-type neuropeptides, providing the theoretical foundation for the potential use of PP/OK-type neuropeptides in the prevention and control of agricultural and forestry pests, as an additive for skin care products and in the screening of drugs for the treatment of diabetes.

Filling in the gaps: A reevaluation of the Lygus hesperus peptidome using an expanded de novo assembled transcriptome and molecular cloning

Peptides are the largest and most diverse class of molecules modulating physiology and behavior. Previously, we predicted a peptidome for the western tarnished plant bug, Lygus hesperus, using transcriptomic data produced from whole individuals. A potential limitation of that analysis was the masking of underrepresented genes, in particular tissue-specific transcripts. Here, we reassessed the L. hesperus peptidome using a more comprehensive dataset comprised of the previous transcriptomic data as well as tissue-specific reads produced from heads and accessory glands. This augmented assembly significantly improves coverage depth providing confirmatory transcripts for essentially all of the previously identified families and new transcripts encoding a number of new peptide precursors corresponding to 14 peptide families. Several families not targeted in our initial study were identified in the expanded assembly, including agatoxin-like peptide, CNMamide, neuropeptide-like precursor 1, and periviscerokinin. To increase confidence in the in silico data, open reading frames of a subset of the newly identified transcripts were amplified using RT-PCR and sequence validated. Further PCR-based profiling of the putative L. hesperus agatoxin-like peptide precursor revealed evidence of alternative splicing with near ubiquitous expression across L. hesperus development, suggesting the peptide serves functional roles beyond that of a toxin. The peptides predicted here, in combination with those identified in our earlier study, expand the L. hesperus peptidome to 42 family members and provide an improved platform for initiating molecular and physiological investigations into peptidergic functionality in this non-model agricultural pest.

Genome-wide identification of neuropeptides and their receptor genes in Bemisia tabaci and their transcript accumulation change in response to temperature stresses

Insect neuropeptides play an important role in regulating physiological functions such as growth, development, behavior and reproduction. We identified temperature-sensitive neuropeptides and receptor genes of the cotton whitefly, Bemisia tabaci. We identified 38 neuropeptide precursor genes and 35 neuropeptide receptors and constructed a phylogenetic tree using additional data from other insects. As temperature adaptability enables B. tabaci to colonize a diversity of habitats, we performed quantitative polymerase chain reaction with two temperature stresses (low = 4 °C and high = 40 °C) to screen for temperature-sensitive neuropeptides. We found many neuropeptides and receptors that may be involved in the temperature adaptability of B. tabaci. This study is the first to identify B. tabaci neuropeptides and their receptors, and it will help to reveal the roles of neuropeptides in temperature adaptation of B. tabaci.

Hormonal axes in Drosophila: regulation of hormone release and multiplicity of actions

Hormones regulate development, as well as many vital processes in the daily life of an animal. Many of these hormones are peptides that act at a higher hierarchical level in the animal with roles as organizers that globally orchestrate metabolism, physiology and behavior. Peptide hormones can act on multiple peripheral targets and simultaneously convey basal states, such as metabolic status and sleep-awake or arousal across many central neuronal circuits. Thereby, they coordinate responses to changing internal and external environments. The activity of neurosecretory cells is controlled either by (1) cell autonomous sensors, or (2) by other neurons that relay signals from sensors in peripheral tissues and (3) by feedback from target cells. Thus, a hormonal signaling axis commonly comprises several components. In mammals and other vertebrates, several hormonal axes are known, such as the hypothalamic-pituitary-gonad axis or the hypothalamic-pituitary-thyroid axis that regulate reproduction and metabolism, respectively. It has been proposed that the basic organization of such hormonal axes is evolutionarily old and that cellular homologs of the hypothalamic-pituitary system can be found for instance in insects. To obtain an appreciation of the similarities between insect and vertebrate neurosecretory axes, we review the organization of neurosecretory cell systems in Drosophila. Our review outlines the major peptidergic hormonal pathways known in Drosophila and presents a set of schemes of hormonal axes and orchestrating peptidergic systems. The detailed organization of the larval and adult Drosophila neurosecretory systems displays only very basic similarities to those in other arthropods and vertebrates.

Orcokinin in the central complex of the locust Schistocerca gregaria: Identification of immunostained neurons and colocalization with other neuroactive substances

The central complex is a group of highly interconnected neuropils in the insect brain. It is involved in the control of spatial orientation, based on external compass cues and various internal needs. The functional and neurochemical organization of the central complex has been studied in detail in the desert locust Schistocerca gregaria. In addition to classical neurotransmitters, immunocytochemistry has provided evidence for a major contribution of neuropeptides to neural signaling within the central complex. To complement these data, we have identified all orcokinin-immunoreactive neurons in the locust central complex and associated brain areas. About 50 bilateral pairs of neurons innervating all substructures of the central complex exhibit orcokinin immunoreactivity. Among these were about 20 columnar neurons, 33 bilateral pairs of tangential neurons of the central body, and seven pairs of tangential neurons of the protocerebral bridge. In silico transcript analysis suggests the presence of eight different orcokinin-A type peptides in the desert locust. Double label experiments showed that all orcokinin-immunostained tangential neurons of the lateral accessory lobe cluster were also immunoreactive for GABA and the GABA-synthesizing enzyme glutamic acid decarboxylase. Two types of tangential neurons of the upper division of the central body were, furthermore, also labeled with an antiserum against Dip-allatostatin I. No colocalization was found with serotonin immunostaining. The data provide additional insights into the neurochemical organization of the locust central complex and suggest that orcokinin-peptides of the orcokinin-A gene act as neuroactive substances at all stages of signal processing in this brain area.

Metabolism and growth adaptation to environmental conditions in Drosophila

Organisms adapt to changing environments by adjusting their development, metabolism, and behavior to improve their chances of survival and reproduction. To achieve such flexibility, organisms must be able to sense and respond to changes in external environmental conditions and their internal state. Metabolic adaptation in response to altered nutrient availability is key to maintaining energy homeostasis and sustaining developmental growth. Furthermore, environmental variables exert major influences on growth and final adult body size in animals. This developmental plasticity depends on adaptive responses to internal state and external cues that are essential for developmental processes. Genetic studies have shown that the fruit fly Drosophila, similarly to mammals, regulates its metabolism, growth, and behavior in response to the environment through several key hormones including insulin, peptides with glucagon-like function, and steroid hormones. Here we review emerging evidence showing that various environmental cues and internal conditions are sensed in different organs that, via inter-organ communication, relay information to neuroendocrine centers that control insulin and steroid signaling. This review focuses on endocrine regulation of development, metabolism, and behavior in Drosophila, highlighting recent advances in the role of the neuroendocrine system as a signaling hub that integrates environmental inputs and drives adaptive responses.

Orcokinin neuropeptides regulate sleep in Caenorhabditis elegans

Orcokinin neuropeptides are conserved among ecdysozoans, but their functions are incompletely understood. Here, we report a role for orcokinin neuropeptides in the regulation of sleep in the nematode Caenorhabditis elegans. The C. elegans orcokinin peptides, which are encoded by the nlp-14 and nlp-15 genes, are necessary and sufficient for quiescent behaviors during developmentally timed sleep (DTS) as well as during stress-induced sleep (SIS). The five orcokinin neuropeptides encoded by nlp-14 have distinct but overlapping functions in the regulation of movement and defecation quiescence during SIS. We suggest that orcokinins may regulate behavioral components of sleep-like states in nematodes and other ecdysozoans.

Functional morphology of the primary olfactory centers in the brain of the hermit crab Coenobita clypeatus (Anomala, Coenobitidae)

Terrestrial hermit crabs of the genus Coenobita display strong behavioral responses to volatile odors and are attracted by chemical cues of various potential food sources. Several aspects of their sense of aerial olfaction have been explored in recent years including behavioral aspects and structure of their peripheral and central olfactory pathway. Here, we use classical histological methods and immunohistochemistry against the neuropeptides orcokinin and allatostatin as well as synaptic proteins and serotonin to provide insights into the functional organization of their primary olfactory centers in the brain, the paired olfactory lobes. Our results show that orcokinin is present in the axons of olfactory sensory neurons, which target the olfactory lobe. Orcokinin is also present in a population of local olfactory interneurons, which may relay lateral inhibition across the array of olfactory glomeruli within the lobes. Extensive lateral connections of the glomeruli were also visualized using the histological silver impregnation method according to Holmes-Blest. This technique also revealed the structural organization of the output pathway of the olfactory system, the olfactory projection neurons, the axons of which target the lateral protocerebrum. Within the lobes, the course of their axons seems to be reorganized in an axon-sorting zone before they exit the system. Together with previous results, we combine our findings into a model on the functional organization of the olfactory system in these animals.

A single-cell transcriptomic atlas of the adult Drosophila ventral nerve cord

The Drosophila ventral nerve cord (VNC) receives and processes descending signals from the brain to produce a variety of coordinated locomotor outputs. It also integrates sensory information from the periphery and sends ascending signals to the brain. We used single-cell transcriptomics to generate an unbiased classification of cellular diversity in the VNC of five-day old adult flies. We produced an atlas of 26,000 high-quality cells, representing more than 100 transcriptionally distinct cell types. The predominant gene signatures defining neuronal cell types reflect shared developmental histories based on the neuroblast from which cells were derived, as well as their birth order. The relative position of cells along the anterior-posterior axis could also be assigned using adult Hox gene expression. This single-cell transcriptional atlas of the adult fly VNC will be a valuable resource for future studies of neurodevelopment and behavior.

Peptide Hormones in the Insect Midgut

Insects produce many peptide hormones that play important roles in regulating growth, development, immunity, homeostasis, stress, and other processes to maintain normal life. As part of the digestive system, the insect midgut is also affected by hormones secreted from the prothoracic gland, corpus allatum, and various neuronal cells; these hormones regulate the secretion and activity of insects’ digestive enzymes and change their feeding behaviors. In addition, the insect midgut produces certain hormones when it recognizes various components or pathogenic bacteria in ingested foods; concurrently, the hormones regulate other tissues and organs. In addition, intestinal symbiotic bacteria can produce hormones that influence insect signaling pathways to promote host growth and development; this interaction is the result of long-term evolution. In this review, the types, functions, and mechanisms of hormones working on the insect midgut, as well as hormones produced therein, are reviewed for future reference in biological pest control.

A cell atlas of the adult Drosophila midgut

Studies of the adult Drosophila midgut have led to many insights in our understanding of cell-type diversity, stem cell regeneration, tissue homeostasis, and cell fate decision. Advances in single-cell RNA sequencing provide opportunities to identify new cell types and molecular features. We used single-cell RNA sequencing to characterize the transcriptome of midgut epithelial cells and identified 22 distinct clusters representing intestinal stem cells, enteroblasts, enteroendocrine cells (EEs), and enterocytes. This unbiased approach recovered most of the known intestinal stem cells/enteroblast and EE markers, highlighting the high quality of the dataset, and led to insights on intestinal stem cell biology, cell type-specific organelle features, the roles of new transcription factors in progenitors and regional variation along the gut, 5 additional EE gut hormones, EE hormonal expression diversity, and paracrine function of EEs. To facilitate mining of this rich dataset, we provide a web-based resource for visualization of gene expression in single cells. Altogether, our study provides a comprehensive resource for addressing functions of genes in the midgut epithelium.

The silkworm (Bombyx mori) neuropeptide orcokinin is involved in the regulation of pigmentation

The natural colorful cuticles of insects play important roles in many physiological processes. Pigmentation is a physiological process with a complex regulatory network whose regulatory mechanism remains unclear. Bombyx mori pigmentation mutants are ideal materials for research on pigmentation mechanisms. The purple quail-like (q-l^p) and brown quail-like (q-l^b) mutants originated from plain silkworm breeds 932VR and 0223JH respectively exhibit similar cuticle pigmentation to that of the quail mutant. The q-l^p mutant also presents a developmental abnormality. In this study, genes controlling q-l^p and q-l^b mutants were located on chromosome 8 by positional cloning. Then the neuropeptide gene orcokinin (OK) was identified to be the major gene responsible for two quail-like mutants. The B. mori orcokinin gene (BommoOK) produces two transcripts, BommoOKA and BommoOKB, by alternative splicing. The CRISPR/Cas9 system and orcokinin peptides injection were used for further functional verification. We show a novel function of BommoOKA in inhibiting pigmentation, and one mature peptide of orcokinin A, OKA_type2, is the key factor in pigmentation inhibition. These results provide a reference for studying the function of orcokinin and are of theoretical importance for studying the regulatory mechanism of pigmentation.

Recent advances in neuropeptide signaling in Drosophila, from genes to physiology and behavior

This review focuses on neuropeptides and peptide hormones, the largest and most diverse class of neuroactive substances, known in Drosophila and other animals to play roles in almost all aspects of daily life, as w;1;ell as in developmental processes. We provide an update on novel neuropeptides and receptors identified in the last decade, and highlight progress in analysis of neuropeptide signaling in Drosophila. Especially exciting is the huge amount of work published on novel functions of neuropeptides and peptide hormones in Drosophila, largely due to the rapid developments of powerful genetic methods, imaging techniques and innovative assays. We critically discuss the roles of peptides in olfaction, taste, foraging, feeding, clock function/sleep, aggression, mating/reproduction, learning and other behaviors, as well as in regulation of development, growth, metabolic and water homeostasis, stress responses, fecundity, and lifespan. We furthermore provide novel information on neuropeptide distribution and organization of peptidergic systems, as well as the phylogenetic relations between Drosophila neuropeptides and those of other phyla, including mammals. As will be shown, neuropeptide signaling is phylogenetically ancient, and not only are the structures of the peptides, precursors and receptors conserved over evolution, but also many functions of neuropeptide signaling in physiology and behavior.

Substrates for Neuronal Cotransmission With Neuropeptides and Small Molecule Neurotransmitters in Drosophila

It has been known for more than 40 years that individual neurons can produce more than one neurotransmitter and that neuropeptides often are colocalized with small molecule neurotransmitters (SMNs). Over the years much progress has been made in understanding the functional consequences of cotransmission in the nervous system of mammals. There are also some excellent invertebrate models that have revealed roles of coexpressed neuropeptides and SMNs in increasing complexity, flexibility, and dynamics in neuronal signaling. However, for the fly Drosophila there are surprisingly few functional studies on cotransmission, although there is ample evidence for colocalization of neuroactive compounds in neurons of the CNS, based both on traditional techniques and novel single cell transcriptome analysis. With the hope to trigger interest in initiating cotransmission studies, this review summarizes what is known about Drosophila neurons and neuronal circuits where different neuropeptides and SMNs are colocalized. Coexistence of neuroactive substances has been recorded in different neuron types such as neuroendocrine cells, interneurons, sensory cells and motor neurons. Some of the circuits highlighted here are well established in the analysis of learning and memory, circadian clock networks regulating rhythmic activity and sleep, as well as neurons and neuroendocrine cells regulating olfaction, nociception, feeding, metabolic homeostasis, diuretic functions, reproduction, and developmental processes. One emerging trait is the broad role of short neuropeptide F in cotransmission and presynaptic facilitation in a number of different neuronal circuits. This review also discusses the functional relevance of coexisting peptides in the intestine. Based on recent single cell transcriptomics data, it is likely that the neuronal systems discussed in this review are just a fraction of the total set of circuits where cotransmission occurs in Drosophila. Thus, a systematic search for colocalized neuroactive compounds in further neurons in anatomically defined circuits is of interest for the near future.

The long and the short of it - a perspective on peptidergic regulation of circuits and behaviour

Neuropeptides are the most diverse class of chemical modulators in nervous systems. They contribute to extensive modulation of circuit activity and have profound influences on animal physiology. Studies on invertebrate model organisms, including the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans, have enabled the genetic manipulation of peptidergic signalling, contributing to an understanding of how neuropeptides pattern the output of neural circuits to underpin behavioural adaptation. Electrophysiological and pharmacological analyses of well-defined microcircuits, such as the crustacean stomatogastric ganglion, have provided detailed insights into neuropeptide functions at a cellular and circuit level. These approaches can be increasingly applied in the mammalian brain by focusing on circuits with a defined and identifiable sub-population of neurons. Functional analyses of neuropeptide systems have been underpinned by systematic studies to map peptidergic networks. Here, we review the general principles and mechanistic insights that have emerged from these studies. We also highlight some of the challenges that remain for furthering our understanding of the functional relevance of peptidergic modulation.

The neuropeptides and protein hormones of the agricultural pest fruit fly Bactrocera dorsalis: What do we learn from the genome sequencing and tissue-specific transcriptomes?

Neuropeptides and protein hormones are very important signaling molecules, and are involved in the regulation and coordination of various physiological processes in invertebrates and vertebrates. Using a bioinformatics approach, we screened the recently sequenced genome and six tissue-specific transcriptome databases (central nervous system, fat body, ovary, testes, male accessory glands, antennae) of the oriental fruit fly (Bactrocera dorsalis) that is economically one of the most important pest insects of tropical and subtropical fruit. Thirty-nine candidate genes were found to encode neuropeptides or protein hormones. These include most of the known insect neuropeptides and protein hormones, with the exception of adipokinetic hormone-corazonin-related peptide, allatropin, diuretic hormone 34, diuretic hormone 45, IMFamide, inotocin, and sex peptide. Our results showed the neuropeptides and protein hormones of Diptera insects appear to have a reduced repertoire compared to some other insects. Moreover, there are also differences between B. dorsalis and the super-model of Drosophila melanogaster. Interesting features of the oriental fruit fly are the absence of genes coding for sex peptide and the presence of neuroparsin and two genes coding neuropeptide F. The majority of the identified neuropeptides and protein hormones is present in the central nervous system, with only a limited number of these in the other tissues. Moreover, we predicted their physiological functions via comparing with data of FlyBase and FlyAtlas. Taken together, owing to the large number of identified peptides, this study can be used as a reference about structure, tissue distribution and physiological functions for comparative studies in other model and important pest insects.

bHLH proneural genes as cell fate determinants of entero-endocrine cells, an evolutionarily conserved lineage sharing a common root with sensory neurons

Entero-endocrine cells involved in the regulation of digestive function form a large and diverse cell population within the intestinal epithelium of all animals. Together with absorptive enterocytes and secretory gland cells, entero-endocrine cells are generated by the embryonic endoderm and, in the mature animal, from a pool of endoderm derived, self-renewing stem cells. Entero-endocrine cells share many structural/functional and developmental properties with sensory neurons, which hints at the possibility of an ancient evolutionary relationship between these two cell types. We will survey in this article recent findings that emphasize the similarities between entero-endocrine cells and sensory neurons in vertebrates and insects, for which a substantial volume of data pertaining to the entero-endocrine system has been compiled. We will then report new findings that shed light on the specification and morphogenesis of entero-endocrine cells in Drosophila. In this system, presumptive intestinal stem cells (pISCs), generated during early metamorphosis, undergo several rounds of mitosis that produce the endocrine cells and stem cells (ISCs) with which the fly is born. Clonal analysis demonstrated that individual pISCs can give rise to endocrine cells expressing different types of peptides. Immature endocrine cells start out as unpolarized cells located basally of the gut epithelium; they each extend an apical process into the epithelium which establishes a junctional complex and apical membrane specializations contacting the lumen of the gut. Finally, we show that the Drosophila homolog of ngn3, a bHLH gene that defines the entero-endocrine lineage in mammals, is expressed and required for the differentiation of this cell type in the fly gut.

Orcokinin neuropeptides regulate ecdysis in the hemimetabolous insect Rhodnius prolixus

To grow and develop insects must undergo ecdysis. During this process, the individual sheds the old cuticle to emerge as the following developmental stage. During ecdysis, different programed behaviors are regulated by neuropeptidergic pathways. In general, components of these pathways are better characterized in crustacean and holometabolous insects than in hemimetabola. In insects, the orkoninin gene produces two different neuropeptide precursors by alternative splicing: orcokinin A and orcokinin B. Although orcokinins are well conserved in insect species, their physiological role remains elusive. Here we describe a new splicing variant of the orcokinin gene in the hemimetabolous triatomine Rhodnius prolixus. We further analyze the expression pattern and the function of the alternatively spliced RhoprOK transcripts by means of immunohistochemistry and RNAi-mediated gene silencing. Our results indicate that orkoninis play an essential role in the peptidergic signaling pathway regulating ecdysis in the hemimetabolous insect Rhodnius prolixus.

Developmental and sex-specific differences in expression of neuropeptides derived from allatotropin gene in the silkmoth Bombyx mori

Allatotropin (AT) and related neuropeptides are widespread bioactive molecules that regulate development, food intake and muscle contractions in insects and other invertebrates. In moths, alternative splicing of the at gene generates three mRNA precursors encoding AT with different combinations of three structurally similar AT-like peptides (ATLI-III). We used in situ hybridization and immunohistochemistry to map the differential expression of these transcripts during the postembryonic development of Bombyx mori. Transcript encoding AT alone was expressed in numerous neurons of the central nervous system and frontal ganglion, whereas transcripts encoding AT with ATLs were produced by smaller specific subgroups of neurons in larval stages. Metamorphosis was associated with considerable developmental changes and sex-specific differences in the expression of all transcripts. The most notable was the appearance of AT/ATL transcripts (1) in the brain lateral neurosecretory cells producing prothoracicotropic hormone; (2) in the male-specific cluster of about 20 neurons in the posterior region of the terminal abdominal ganglion; (3) in the female-specific medial neurons in the abdominal ganglia AG2-7. Immunohistochemical staining showed that these neurons produced a mixture of various neuropeptides and innervated diverse peripheral organs. Our data suggest that AT/ATL neuropeptides are involved in multiple stage- and sex-specific functions during the development of B. mori.

Maintenance of the adult Drosophila intestine: all roads lead to homeostasis

Maintenance of tissue homeostasis is critical in tissues with high turnover such as the intestinal epithelium. The intestinal epithelium is under constant cellular assault due to its digestive functions and its function as a barrier to chemical and bacterial insults. The resulting high rate of cellular turnover necessitates highly controlled mechanisms of regeneration to maintain the integrity of the tissue over the lifetime of the organism. Transient increase in stem cell proliferation is a commonly used and elaborate mechanism to ensure fast and efficient repair of the gut. However, tissue repair is not limited to regulating ISC proliferation, as emerging evidence demonstrates that the Drosophila intestine uses multiple strategies to ensure proper tissue homeostasis that may also extend to other tissues.

A Systematic Analysis of Drosophila Regulatory Peptide Expression in Enteroendocrine Cells

The digestive system is gaining interest as a major regulator of various functions including immune defense, nutrient accumulation, and regulation of feeding behavior, aside from its conventional function as a digestive organ. The Drosophila midgut epithelium is completely renewed every 1-2 weeks due to differentiation of pluripotent intestinal stem cells in the midgut. Intestinal stem cells constantly divide and differentiate into enterocytes that secrete digestive enzymes and absorb nutrients, or enteroendocrine cells that secrete regulatory peptides. Regulatory peptides have important roles in development and metabolism, but study has mainly focused on expression and functions in the nervous system, and not much is known about the roles in endocrine functions of enteroendocrine cells. We systemically examined the expression of 45 regulatory peptide genes in the Drosophila midgut, and verified that at least 10 genes are expressed in the midgut enteroendocrine cells through RT-PCR, in situ hybridization, antisera, and 25 regulatory peptide-GAL transgenes. The Drosophila midgut is highly compartmentalized, and individual peptides in enteroendocrine cells were observed to express in specific regions of the midgut. We also confirmed that some peptides expressed in the same region of the midgut are expressed in mutually exclusive enteroendocrine cells. These results indicate that the midgut enteroendocrine cells are functionally differentiated into different subgroups. Through this study, we have established a basis to study regulatory peptide functions in enteroendocrine cells as well as the complex organization of enteroendocrine cells in the Drosophila midgut.

Neuropeptidome of the Cephalopod Sepia officinalis: Identification, Tissue Mapping, and Expression Pattern of Neuropeptides and Neurohormones during Egg Laying

Cephalopods exhibit a wide variety of behaviors such as prey capture, communication, camouflage, and reproduction thanks to a complex central nervous system (CNS) divided into several functional lobes that express a wide range of neuropeptides involved in the modulation of behaviors and physiological mechanisms associated with the main stages of their life cycle. This work focuses on the neuropeptidome expressed during egg-laying through de novo construction of the CNS transcriptome using an RNAseq approach (Illumina sequencing). Then, we completed the in silico analysis of the transcriptome by characterizing and tissue-mapping neuropeptides by mass spectrometry. To identify neuropeptides involved in the egg-laying process, we determined (1) the neuropeptide contents of the neurohemal area, hemolymph (blood), and nerve endings in mature females and (2) the expression levels of these peptides. Among the 38 neuropeptide families identified from 55 transcripts, 30 were described for the first time in Sepia officinalis, 5 were described for the first time in the animal kingdom, and 14 were strongly overexpressed in egg-laying females as compared with mature males. Mass spectrometry screening of hemolymph and nerve ending contents allowed us to clarify the status of many neuropeptides, that is, to determine whether they were neuromodulators or neurohormones.

The power of next-generation sequencing as illustrated by the neuropeptidome of the crayfish Procambarus clarkii

Transcriptomes of the crayfish Procambarus clarkii were analyzed for the presence of transcripts encoding neurohormones, neuropeptides and their receptors. A total of 58 different transcripts were found to encode such ligands and another 82 for their receptors. A very large number of the neuropeptide transcripts appeared to be complete and for those that were not only small parts seemed to be lacking. Transcripts for the neuropeptide GPCRs as well as for the putative receptors for insulin, neuroparsin and eclosion hormone were often also complete or almost so. Of particular interest is the presence of three different neuroparsin genes and two putative neuroparsin receptors. There are also three pigment dispersing hormones as well three likely receptors for these neuropeptides. CNMamide, calcitonin, CCRFamide, natalisin, trissin and relaxin appear to be new crustacean neuropeptides. The recently identified crustacean female sex hormone was also found and in the crayfish appears to be not only expressed in the eyestalk, but in the ovary as well (though not in the testis). Interestingly, there are two other proteins in the crayfish with a structure similar to crustacean female sex hormone, that could be precursors of neurohormones, but these are not expressed by the ovary. The ovary also appears to contain significant numbers of transcripts encoding pigment dispersing hormones, CNMamide as well as glycoprotein B5, but not glycoprotein A2.

Orcokinins contribute to the regulation of vitellogenin transcription in the cockroach Blattella germanica

Orcokinins (OKs) are neuropeptides that were first identified in crustacean through their myotropic activity. In insects, the OK gene gives rise to two mRNAs coding for two different families of conserved mature neuropeptides: OKA and OKB. Although OKs are conserved in many insect species, its physiological role in this animal class is not fully understood. Until now prothoracicotropic, regulatory of light entrainment to the circadian clock and "awakening" activities have been reported for these peptides in different insect species. Here we report the identification of OKA and OKB precursors in the cockroach Blattella germanica. OKA mRNA was detected in brain, whereas OKB mRNA was detected both in brain and midgut. In vivo silencing of OK precursors suggests the involvement of OK gene products in the regulation of vitellogenin expression in the fat body, an action that appears to be independent of juvenile hormone. This is the first time that a function of this kind has been reported for OKs.

Alternatively spliced orcokinin isoforms and their functions in Tribolium castaneum

Orcokinin and orcomyotropin were originally described as neuropeptides in crustaceans but have now been uncovered in many species of insects in which they are called orcokinin-A (OK-A) and orcokinin-B (OK-B), respectively. The two groups of mature peptides are products of alternatively spliced transcripts of the single copy gene orcokinin in insects. We investigated the expression patterns and the functions of OK-A and OK-B in the red flour beetle Tribolium castaneum. In situ hybridization and immunohistochemistry using isoform-specific probes and antibodies for each OK-A and OK-B suggests that both peptides are co-expressed in 5-7 pairs of brain cells and in the midgut enteroendocrine cells, which contrasts to expression patterns in other insects in which the two peptides are expressed in different cells. We developed a novel behavioral assay to assess the phenotypes of orcokinin RNA interference (RNAi) in T. castaneum. RNAi of ok-a and ok-b alone or in combination resulted in higher frequencies and longer durations of death feigning in response to mechanical stimulation in the adult assay. In the larval behavioral assays, we observed longer recovery times from knockout induced by water submergence in the insects treated with RNAi for ok-a and ok-b alone or in combination. We conclude that both OK-A and OK-B have "awakening" activities and are potentially involved in the control of circadian rhythms.