Tip cell-derived RTK signaling initiates cell movements in the Drosophila stomatogastric nervous system anlage

Spemann organizer gene Goosecoid promotes delamination of neuroblasts from the otic vesicle

Neurons of the Statoacoustic Ganglion (SAG), which innervate the inner ear, originate as neuroblasts in the floor of the otic vesicle and subsequently delaminate and migrate toward the hindbrain before completing differentiation. In all vertebrates, locally expressed Fgf initiates SAG development by inducing expression of Neurogenin1 (Ngn1) in the floor of the otic vesicle. However, not all Ngn1-positive cells undergo delamination, nor has the mechanism controlling SAG delamination been elucidated. Here we report that Goosecoid (Gsc), best known for regulating cellular dynamics in the Spemann organizer, regulates delamination of neuroblasts in the otic vesicle. In zebrafish, Fgf coregulates expression of Gsc and Ngn1 in partially overlapping domains, with delamination occurring primarily in the zone of overlap. Loss of Gsc severely inhibits delamination, whereas overexpression of Gsc greatly increases delamination. Comisexpression of Ngn1 and Gsc induces ectopic delamination of some cells from the medial wall of the otic vesicle but with a low incidence, suggesting the action of a local inhibitor. The medial marker Pax2a is required to restrict the domain of gsc expression, and misexpression of Pax2a is sufficient to block delamination and fully suppress the effects of Gsc The opposing activities of Gsc and Pax2a correlate with repression or up-regulation, respectively, of E-cadherin (cdh1). These data resolve a genetic mechanism controlling delamination of otic neuroblasts. The data also elucidate a developmental role for Gsc consistent with a general function in promoting epithelial-to-mesenchymal transition (EMT).

Genetic Tools for the Analysis of Drosophila Stomatogastric Nervous System Development

The Drosophila stomatogastric nervous system (SNS) is a compact collection of neurons that arises from the migration of neural precursors. Here we describe genetic tools allowing functional analysis of the SNS during the migratory phase of development. We constructed GAL4 lines driven by fragments of the Ret promoter, which yielded expression in a subset of migrating neural SNS precursors and also included a distinct set of midgut associated cells. Screening of additional GAL4 lines driven by fragments of the Gfrl/Munin, forkhead, twist and goosecoid (Gsc) promoters identified a Gsc fragment with expression from initial selection of SNS precursors until the end of embryogenesis. Inhibition of EGFR signaling using three identified lines disrupted the correct patterning of the frontal and recurrent nerves. To manipulate the environment traveled by SNS precursors, a FasII-GAL4 line with strong expression throughout the entire intestinal tract was identified. The transgenic lines described offer the ability to specifically manipulate the migration of SNS precursors and will allow the modeling and in-depth analysis of neuronal migration in ENS disorders such as Hirschsprung’s disease.

Specification of Drosophila corpora cardiaca neuroendocrine cells from mesoderm is regulated by Notch signaling

Drosophila neuroendocrine cells comprising the corpora cardiaca (CC) are essential for systemic glucose regulation and represent functional orthologues of vertebrate pancreatic α-cells. Although Drosophila CC cells have been regarded as developmental orthologues of pituitary gland, the genetic regulation of CC development is poorly understood. From a genetic screen, we identified multiple novel regulators of CC development, including Notch signaling factors. Our studies demonstrate that the disruption of Notch signaling can lead to the expansion of CC cells. Live imaging demonstrates localized emergence of extra precursor cells as the basis of CC expansion in Notch mutants. Contrary to a recent report, we unexpectedly found that CC cells originate from head mesoderm. We show that Tinman expression in head mesoderm is regulated by Notch signaling and that the combination of Daughterless and Tinman is sufficient for ectopic CC specification in mesoderm. Understanding the cellular, genetic, signaling, and transcriptional basis of CC cell specification and expansion should accelerate discovery of molecular mechanisms regulating ontogeny of organs that control metabolism.

The requirement for glucose regulation is conserved in metazoans and crucial for metabolism, growth, and survival. In fruit flies and other insects, neurons secrete insulin-like hormones and neuroendocrine corpora cardiaca cells secrete adipokinetic hormone, a peptide with functional similarities to glucagon. Both hormones are essential for systemic glucose control in Drosophila. To understand the mechanisms governing formation and function of corpora cardiaca cells, we sought to identify their embryonic origin and investigate their developmental genetic regulation. Based on prior reports suggesting a neuroectodermal origin, we were surprised to discover—using genetic lineage tracing methods—that embryonic corpora cardiac progenitors derive from anterior head mesoderm. To our knowledge, this is the first demonstration of neuroendocrine differentiation from mesoderm in Drosophila. Genetic studies reveal that Notch signaling restricts the number of corpora cardiaca progenitors, and we show that Notch signaling inactivation results in significant expansion of corpora cardiac cells. Loss- and gain-of-function studies identified transcription factors both necessary and sufficient for corpora cardiaca development. These and other findings reveal similarities in the development of fly corpora cardiaca cells and mammalian neuroendocrine cells that develop in the pancreas, pituitary, and from neural crest.

The thoracic muscular system and its innervation in third instar Calliphora vicina Larvae. II. Projection patterns of the nerves associated with the pro- and mesothorax and the pharyngeal complex

We describe the anatomy of the nerves that project from the central nervous system (CNS) to the pro- and mesothoracic segments and the cephalopharyngeal skeleton (CPS) for third instar Calliphora larvae. Due to the complex branching pattern we introduce a nomenclature that labels side branches of first and second order. Two fine nerves that were not yet described are briefly introduced. One paired nerve projects to the ventral arms (VAs) of the CPS. The second, an unpaired nerve, projects to the ventral surface of the cibarial part of the esophagus (ES). Both nerves were tentatively labeled after the structures they innervate. The antennal nerve (AN) innervates the olfactory dorsal organ (DO). It contains motor pathways that project through the frontal connectives (FC) to the frontal nerve (FN) and innervate the cibarial dilator muscles (CDM) which mediate food ingestion. The maxillary nerve (MN) innervates the sensory terminal organ (TO), ventral organ (VO), and labial organ (LO) and comprises the motor pathways to the mouth hook (MH) elevator, MH depressor, and the labial retractor (LR) which opens the mouth cavity. An anastomosis of unknown function exists between the AN and MN. The prothoracic accessory nerve (PaN) innervates a dorsal protractor muscle of the CPS and sends side branches to the aorta and the bolwig organ (BO) (stemmata). In its further course, this nerve merges with the prothoracic nerve (PN). The architecture of the PN is extremely complex. It innervates a set of accessory pharyngeal muscles attached to the CPS and the body wall musculature of the prothorax. Several anastomoses exist between side branches of this nerve which were shown to contain motor pathways. The mesothoracic nerve (MeN) innervates a MH accessor and the longitudinal and transversal body wall muscles of the second segment.

Anatomy of the stomatogastric nervous system associated with the foregut in Drosophila melanogaster and Calliphora vicina third instar larvae

The stomatogastric nervous system (SNS) associated with the foregut was studied in 3rd instar larvae of Drosophila melanogaster and Calliphora vicina (blowfly). In both species, the foregut comprises pharynx, esophagus, and proventriculus. Only in Calliphora does the esophagus form a crop. The position of nerves and neurons was investigated with neuronal tracers in both species and GFP expression in Drosophila. The SNS is nearly identical in both species. Neurons are located in the proventricular and the hypocerebral ganglion (HCG), which are connected to each other by the proventricular nerve. Motor neurons for pharyngeal muscles are located in the brain not, as in other insect groups, in the frontal ganglion. The position of the frontal ganglion is taken by a nerve junction devoid of neurons. The junction is composed of four nerves: the frontal connectives that fuse with the antennal nerves (ANs), the frontal nerve innervating the cibarial dilator muscles and the recurrent nerve that innervates the esophagus and projects to the HCG. Differences in the SNS are restricted to a crop nerve only present in Calliphora and an esophageal ganglion that only exists in Drosophila. The ganglia of the dorsal organs give rise to the ANs, which project to the brain. The extensive conformity of the SNS of both species suggests functional parallels. Future electrophysiological studies of the motor circuits in the SNS of Drosophila will profit from parallel studies of the homologous but more accessible structures in Calliphora.

How to innervate a simple gut: familiar themes and unique aspects in the formation of the insect enteric nervous system

Like the vertebrate enteric nervous system (ENS), the insect ENS consists of interconnected ganglia and nerve plexuses that control gut motility. However, the insect ENS lies superficially on the gut musculature, and its component cells can be individually imaged and manipulated within cultured embryos. Enteric neurons and glial precursors arise via epithelial-to-mesenchymal transitions that resemble the generation of neural crest cells and sensory placodes in vertebrates; most cells then migrate extensive distances before differentiating. A balance of proneural and neurogenic genes regulates the morphogenetic programs that produce distinct structures within the insect ENS. In vivo studies have also begun to decipher the mechanisms by which enteric neurons integrate multiple guidance cues to select their pathways. Despite important differences between the ENS of vertebrates and invertebrates, common features in their programs of neurogenesis, migration, and differentiation suggest that these relatively simple preparations may provide insights into similar developmental processes in more complex systems.

Anatomical and functional characterisation of the stomatogastric nervous system of blowfly (Calliphora vicina) larvae

The anatomy and functionality of the stomatogastric nervous system (SNS) of third-instar larvae of Calliphora vicina was characterised. As in other insects, the Calliphora SNS consists of several peripheral ganglia involved in foregut movement regulation. The frontal ganglion gives rise to the frontal nerve and is connected to the brain via the frontal connectives and antennal nerves (ANs). The recurrent nerve connects the frontal- to the hypocerebral ganglion from which the proventricular nerve runs to the proventricular ganglion. Foregut movements include rhythmic contractions of the cibarial dilator muscles (CDM), wavelike movements of crop and oesophagus and contractions of the proventriculus. Transections of SNS nerves indicate mostly myogenic crop and oesophagus movements and suggest modulatory function of the associated nerves. Neural activity in the ANs, correlating with postsynaptic potentials on the CDM, demonstrates a motor pathway from the brain to CDM. Crop volume is monitored by putative stretch receptors. The respective sensory pathway includes the recurrent nerve and the proventricular nerve. The dorsal organs (DOs) are directly connected to the SNS. Mechanical stimulation of the DOs evokes sensory activity in the AN. This suggests the DOs can provide sensory input for temporal coordination of feeding behaviour.

Embryonic development of the Drosophila corpus cardiacum, a neuroendocrine gland with similarity to the vertebrate pituitary, is controlled by sine oculis and glass

We have investigated the development of the Drosophila neuroendocrine gland, the corpus cardiacum (CC), and identified the role of regulatory genes and signaling pathways in CC morphogenesis. CC progenitors segregate from the blastoderm as part of the anterior lip of the ventral furrow. Among the early genetic determinants expressed and required in this domain are the genes giant (gt) and sine oculis (so). During the extended germ band stage, CC progenitor cells form a paired cluster of 6-8 cells sandwiched in between the inner surface of the protocerebrum and the foregut. While flanking the protocerebrum, CC progenitors are in direct contact with the neural precursors that give rise to the pars intercerebralis, the part of the brain whose neurons later innervate the CC. At this stage, the CC progenitors turn on the homeobox gene glass (gl), which is essential for the differentiation of the CC. During germ band retraction, CC progenitors increase in number and migrate posteriorly, passing underneath the brain commissure and attaching themselves to the primordia of the corpora allata (CA). During dorsal closure, the CC and CA move around the anterior aorta to become the ring gland. Signaling pathways that shape the determination and morphogenesis of the CC are decapentaplegic (dpp) and its antagonist short gastrulation (sog), as well as hedgehog (hh) and heartless (htl; a Drosophila FGFR homolog). Sog is expressed in the midventral domain from where CC progenitors originate, and these cells are completely absent in sog mutants. Dpp and hh are expressed in the anterior visceral head mesoderm and the foregut, respectively; both of these tissues flank the CC. Loss of hh and dpp results in defects in CC proliferation and migration. Htl appears in the somatic mesoderm of the head and trunk. Although mutations of htl do not cause direct effects on the early development of the CC, the later formation of the ring gland is highly abnormal due to the absence of the aorta in these mutants. Defects in the CC are also caused by mutations that severely reduce the protocerebrum, including tailless (tll), suggesting that additional signaling events exist between brain and CC progenitors. We discuss the parallels between neuroendocrine development in Drosophila and vertebrates.

A putative glutathione peroxidase of Drosophila encodes a thioredoxin peroxidase that provides resistance against oxidative stress but fails to complement a lack of catalase activity

Cellular defense systems against reactive oxygen species (ROS) include thioredoxin reductase (TrxR) and glutathione reductase (GR). They generate sulfhydryl-reducing systems which are coupled to antioxidant enzymes, the thioredoxin and glutathione peroxidases (TPx and GPx). The fruit fly Drosophila lacks a functional GR, suggesting that the thioredoxin system is the major source for recycling glutathione. Whole genome in silico analysis identified two non-selenium containing putative GPx genes. We examined the biochemical characteristics of one of these gene products and found that it lacks GPx activity and functions as a TPx. Transgene-dependent overexpression of the newly identified Glutathione peroxidase homolog with thioredoxin peroxidase activity (Gtpx-1) gene increases resistance to experimentally induced oxidative stress, but does not compensate for the loss of catalase, an enzyme which, like GTPx-1, functions to eliminate hydrogen peroxide. The results suggest that GTPx-1 is part of the Drosophila Trx antioxidant defense system but acts in a genetically distinct pathway or in a different cellular compartment than catalase.

The locust frontal ganglion: a central pattern generator network controlling foregut rhythmic motor patterns

The frontal ganglion (FG) is part of the insect stomatogastric nervous system and is found in most insect orders. Previous work has shown that in the desert locust, Schistocerca gregaria, the FG constitutes a major source of innervation to the foregut. In an in vitro preparation,isolated from all descending and sensory inputs, the FG spontaneously generated rhythmic multi-unit bursts of action potentials that could be recorded from all its efferent nerves. The consistent endogenous FG rhythmic pattern indicates the presence of a central pattern generator network. We found the appearance of in vitro rhythmic activity to be strongly correlated with the physiological state of the donor locust. A robust pattern emerged only after a period of saline superfusion, if the locust had a very full foregut and crop, or if the animal was close to ecdysis. Accordingly,haemolymph collected at these stages inhibited an ongoing rhythmic pattern when applied onto the ganglion. We present this novel central pattern generating system as a basis for future work on the neural network characterisation and its role in generating and controlling behaviour.

Mitochondrial and cytoplasmic thioredoxin reductase variants encoded by a single Drosophila gene are both essential for viability

Defense against oxidative stress in mammals includes the regeneration of the major thiol reductants glutathione and thioredoxin by glutathione reductase and thioredoxin reductase (TrxR), respectively. In contrast, Drosophila, and possibly insects in general, lacks glutathione reductase and must rely solely on the TrxR system. The mammalian TrxRs described so far are selenoproteins that utilize NADPH to reduce protein as well as nonprotein substrates in mitochondria and cytoplasm of cells. We show that a single Drosophila gene, Trxr-1, encodes non-selenocysteine-containing cytoplasmic and mitochondrial TrxR isoforms that differ with respect to their N termini. We generated transcript-specific mutants and used in vivo approaches to explore the biological functions of the two enzyme variants by introducing the corresponding transgenes into different Trxr-1 mutants. The results show that, although the two TrxR isoforms have similar biochemical properties, their biological functions are not interchangeable.

Cooperative action of antioxidant defense systems in Drosophila

Molecular oxygen is key to aerobic life but is also converted into cytotoxic byproducts referred to as reactive oxygen species (ROS). Intracellular defense systems that protect cells from ROS-induced damage include glutathione reductase (GR), thioredoxin reductase (TrxR), superoxide dismutase (Sod), and catalase (Cat). Sod and Cat constitute an evolutionary conserved ROS defense system against superoxide; Sod converts superoxide anions to H(2)O(2), and Cat prevents free hydroxyl radical formation by breaking down H(2)O(2) into oxygen and water. As a consequence, they are important effectors in the life span determination of the fly Drosophila. ROS defense by TrxR and GR is more indirect. They transfer reducing equivalents from NADPH to thioredoxin (Trx) and glutathione disulfide (GSSG), respectively, resulting in Trx(SH)(2) and glutathione (GSH), which act as effective intracellular antioxidants. TrxR and GR were found to be molecularly conserved. However, the single GR homolog of Drosophila specifies TrxR activity, which compensates for the absence of a true GR system for recycling GSH. We show that TrxR null mutations reduce the capacity to adequately protect cells from cytotoxic damage, resulting in larval death, whereas mutations causing reduced TrxR activity affect pupal eclosion and cause a severe reduction of the adult life span. We also provide genetic evidence for a functional interaction between TrxR, Sod1, and Cat, indicating that the burden of ROS metabolism in Drosophila is shared by the two defense systems.