Morphogenesis and cellular proliferation pattern in the developing antennal lobe of Drosophila melanogaster

Retinal homeobox promotes cell growth, proliferation and survival of mushroom body neuroblasts in the Drosophila brain

The Drosophila mushroom bodies, centers of olfactory learning and memory in the fly 'forebrain', develop from a set of neural stem cells (neuroblasts) that generate a large number of Kenyon cells (KCs) during sustained cell divisions from embryonic to late pupal stage. We show that retinal homeobox (rx), encoding for an evolutionarily conserved transcription factor, is required for proper development of the mushroom bodies. Throughout development rx is expressed in mushroom body neuroblasts (MBNBs), their ganglion mother cells (MB-GMCs) and young KCs. In the absence of rx function, MBNBs form correctly but exhibit a reduction in cell size and mitotic activity, whereas overexpression of rx increases growth of MBNBs. These data suggest that Rx is involved in the control of MBNB growth and proliferation. Rx also promotes cell cycling of MB-GMCs. Moreover, we show that Rx is important for the survival of MBNBs and Kenyon cells which undergo premature cell death in the absence of rx function. Simultaneous blocking of cell death restores the normal set of MBNBs and part of the KCs, demonstrating that both, impaired proliferation and premature cell death (of MBNBs and KCs) account for the observed defects in mushroom body development. We then show that Rx controls proliferation within the MBNB clones independently of Tailless (Tll) and Prospero (Pros), and does not regulate the expression of other key regulators of MB development, Eyeless (Ey) and Dachshund (Dac). Our data support that the role of Rx in forebrain development is conserved between vertebrates and fly.

Origin of Drosophila mushroom body neuroblasts and generation of divergent embryonic lineages

Key to understanding the mechanisms that underlie the specification of divergent cell types in the brain is knowledge about the neurectodermal origin and lineages of their stem cells. Here, we focus on the origin and embryonic development of the four neuroblasts (NBs) per hemisphere in Drosophila that give rise to the mushroom bodies (MBs), which are central brain structures essential for olfactory learning and memory. We show that these MBNBs originate from a single field of proneural gene expression within a specific mitotic domain of procephalic neuroectoderm, and that Notch signaling is not needed for their formation. Subsequently, each MBNB occupies a distinct position in the developing MB cortex and expresses a specific combination of transcription factors by which they are individually identifiable in the brain NB map. During embryonic development each MBNB generates an individual cell lineage comprising different numbers of neurons, including intrinsic γ-neurons and various types of non-intrinsic neurons that do not contribute to the MB neuropil. This contrasts with the postembryonic phase of MBNB development during which they have been shown to produce identical populations of intrinsic neurons. We show that different neuron types are produced in a lineage-specific temporal order and that neuron numbers are regulated by differential mitotic activity of the MBNBs. Finally, we demonstrate that γ-neuron axonal outgrowth and spatiotemporal innervation of the MB lobes follows a lineage-specific mode. The MBNBs are the first stem cells of the Drosophila CNS for which the origin and complete cell lineages have been determined.

Glial investment of the adult and developing antennal lobe of Drosophila

In recent years the Drosophila olfactory system, with its unparalleled opportunities for genetic dissection of development and functional organization, has been used to study the development of central olfactory neurons and the molecular basis of olfactory coding. The results of these studies have been interpreted in the absence of a detailed understanding of the steps in maturation of glial cells in the antennal lobe. Here we present a high-resolution study of the glia associated with olfactory glomeruli in adult and developing antennal lobes. The study provides a basis for comparison of findings in Drosophila with those in the moth Manduca sexta that indicate a critical role for glia in antennal lobe development. Using flies expressing GFP under a Nervana2 driver to visualize glia for confocal microscopy, and probing at higher resolution with the electron microscope, we find that glial development in Drosophila differs markedly from that in moths: glial cell bodies remain in a rind around the glomerular neuropil; glial processes ensheathe axon bundles in the nerve layer but likely contribute little to axonal sorting; their processes insinuate between glomeruli only very late and then form only a sparse, open network around each glomerulus; and glial processes invade the synaptic neuropil. Taking our results in the context of previous studies, we conclude that glial cells in the developing Drosophila antennal lobe are unlikely to play a strong role in either axonal sorting or glomerulus stabilization and that in the adult, glial processes do not electrically isolate glomeruli from their neighbors.

Clonal analysis of Drosophila antennal lobe neurons: diverse neuronal architectures in the lateral neuroblast lineage

The antennal lobe (AL) is the primary structure in the Drosophila brain that relays odor information from the antennae to higher brain centers. The characterization of uniglomerular projection neurons (PNs) and some local interneurons has facilitated our understanding of olfaction; however, many other AL neurons remain unidentified. Because neuron types are mostly specified by lineage and temporal origins, we use the MARCM techniques with a set of enhancer-trap GAL4 lines to perform systematical lineage analysis to characterize neuron morphologies, lineage origin and birth timing in the three AL neuron lineages that contain GAL4-GH146-positive PNs: anterodorsal, lateral and ventral lineages. The results show that the anterodorsal lineage is composed of pure uniglomerular PNs that project through the inner antennocerebral tract. The ventral lineage produces uniglomerular and multiglomerular PNs that project through the middle antennocerebral tract. The lateral lineage generates multiple types of neurons, including uniglomeurlar PNs, diverse atypical PNs, various types of AL local interneurons and the neurons that make no connection within the ALs. Specific neuron types in all three lineages are produced in specific time windows, although multiple neuron types in the lateral lineage are made simultaneously. These systematic cell lineage analyses have not only filled gaps in the olfactory map, but have also exemplified additional strategies used in the brain to increase neuronal diversity.

Quantitative analysis of antennal mosaic generation in Drosophila melanogaster by the MARCM system

Mosaics have been used in Drosophila to study development and to generate mutant structures when a mutant allele is homozygous lethal. New approaches of directed somatic recombination based on FRT/FLP methods, have increased mosaicism rates but likewise multiple clones in the same individual appeared more frequently. Production of single clones could be essential for developmental studies; however, for cell-autonomous gene function studies only the presence of homozygous cells for the target recessive allele is relevant. Herein, we report the number and extension of antennal mosaics generated by the MARCM system at different ages. This information is directed to obtain the appropriated mosaic type for the intended application. By applying heat shock at 10 different developmental stages from 0-12 h to 6-7 days after egg laying, more than 50% of mosaics were obtained from 5,028 adults. Single recombinant clones appeared mainly at early stages while massive recombinant areas were observed with late treatments.

The regenerative cells during the metamorphosis in the midgut of bees

The midgut epithelium of bees is formed by the digestive cells, responsible for enzyme secretion and nutrient absorption and for small regenerative cells that are placed in nests scattered among the digestive cells. During metamorphosis, the larval midgut epithelium degenerates and a new adult midgut epithelium is built during larval differentiation of regenerative cells. The present work focuses on the midgut epithelial modifications during the post-embryonic development of the stingless bee Melipona quadrifasciata anthidioides worker and the occurrence of regenerative cell proliferation during midgut metamorphosis in order to test the hypothesis that adult midgut epithelium of worker bees results from regenerative cell proliferation during the pupal stage. Regenerative cell proliferation was detected during larval lifespan. Larval aging is followed by an increase in the number and the size of the nests of regenerative cells. Larval epithelium degeneration begins 2 days after the start of defecation process and in this period the nests of regenerative cells are in contact by means of cytoplasmic extension which have many septate desmosomes and gap junctions. The BrdU immunoreactive regenerative cells were found in the prepupae 12 h after BrdU injection, suggesting that regenerative cell population increase during this larval period. Regenerative cell proliferation results in the increase of the regenerative cell population and not in the formation of new digestive cells because the proliferation of regenerative cells would not be enough to reestablish the nests of regenerative cells and at the same time form new adult digestive cells. In this sense the hypothesis that digestive adult cells originate from regenerative cell proliferation during midgut metamorphosis in M. quadrifasciata anthidioides was rejected.

A Drosophila nonvisual arrestin is required for the maintenance of olfactory sensitivity

Nonvisual arrestins are a family of multifunctional adaptor molecules that regulate the activities of diverse families of receptors including G protein-coupled receptors, frizzled, and transforming growth factor-beta receptors. These activities indicate broad roles in both physiology and development for nonvisual arrestins. Drosophila melanogaster has a single nonvisual arrestin, kurtz, which is found at high levels within the adult olfactory receptor neurons (ORNs), suggesting a role for this gene in modulating olfactory sensitivity. Using heat-induced expression of a krz cDNA through development, we rescued krz(1) lethality. The resulting adults lacked detectable levels of krz in the olfactory system. The rescued krz(1) homozygotes have an incompletely penetrant antennal structural defect that was completely rescued by the neural expression of a krz cDNA. The krz(1) loss-of-function adults without visible antennal defects displayed diminished behavioral responsiveness to both aversive and attractive odors and also demonstrated reduced olfactory receptor potentials. Both the behavioral and electrophysiological phenotypes were rescued by the targeted expression of the krz cDNA within postdevelopmental ORNs. Thus, krz is required within the nervous system for antennal development and is required later in the ORNs for the maintenance of olfactory sensitivity in Drosophila. The reduced receptor potentials in krz(1) antenna indicate that nonvisual arrestins are required for the early odor-induced signaling events within the ORNs.

Integration of complex larval chemosensory organs into the adult nervous system ofDrosophila

The sense organs of adult Drosophila, and holometabolous insects in general, derive essentially from imaginal discs and hence are adult specific. Experimental evidence presented here, however, suggests a different developmental design for the three largely gustatory sense organs located along the pharynx. In a comprehensive cellular analysis, we show that the posteriormost of the three organs derives directly from a similar larval organ and that the two other organs arise by splitting of a second larval organ. Interestingly, these two larval organs persist despite extensive reorganization of the pharynx. Thus, most of the neurons of the three adult organs are surviving larval neurons. However, the anterior organ includes some sensilla that are generated during pupal stages. Also, we observe apoptosis in a third larval pharyngeal organ. Hence, our experimental data show for the first time the integration of complex, fully differentiated larval sense organs into the nervous system of the adult fly and demonstrate the embryonic origin of their neurons. Moreover, they identify metamorphosis of this sensory system as a complex process involving neuronal persistence, generation of additional neurons and neuronal death. Our conclusions are based on combined analysis of reporter expression from P[GAL4] driver lines, horseradish peroxidase injections into blastoderm stage embryos, cell labeling via heat-shock-induced flip-out in the embryo, bromodeoxyuridine birth dating and staining for programmed cell death. They challenge the general view that sense organs are replaced during metamorphosis.

Early steps in building the insect brain: neuroblast formation and segmental patterning in the developing brain of different insect species

In insects, morphological, molecular and genetic studies have provided a detailed insight into the ontogenetic processes that shape the ventral nerve cord. On the other hand, owing to its complexity and less obvious segmental composition, the knowledge about the development of the brain is still fragmentary. A promising approach towards gaining insight into fundamental processes underlying brain development is the comparison of embryonic brain development among different insect species. However, so far such comparative analyses are scarce. In this review, we summarize and compare data on the early steps in brain formation in different hemi- and holometabolous insects. We show that basic aspects of the spatial and temporal development of the embryonic brain neuroblast pattern are conserved among insects. Furthermore, we compare the number and proliferation patterns of neuroblasts related to major neuropil structures such as mushroom bodies, central complex, and antennal lobe. Finally, comparing the expression patterns of engrailed in different species, and considering new data from Drosophila melanogaster, we discuss the segmental organization of the insect brain.

Spatial and temporal pattern of neuroblasts, proliferation, and Engrailed expression during early brain development in Tenebrio molitor L. (Coleoptera)

In insects, the knowledge of embryonic brain development is still fragmentary, and comparative data are scarce. In this study, we explored aspects of embryonic brain development in the coleopteran Tenebrio molitor. A detailed description is provided of the spatial and temporal pattern of the embryonic brain neuroblasts during 18-60% of embryonic development. Approximately 125 brain NBs have been identified in each hemisphere of the brain at about 40% of embryonic development. A subset of five neuroblasts, among them the two progenitors of the mushroom bodies and two progenitors of the larval antennal lobe, are morphologically identifiable by their larger size. As revealed by incorporation of BrdU, their mitotic behaviour is distinct from that of all other brain NBs, exhibiting an extended period of proliferation into postembryonic stages, and a significantly higher rate of division. To gain insight into the segmental organization of the T. molitor brain, Engrailed expression was examined in the head ectoderm and the deriving components of the CNS (including neuroblasts and their progeny) at different stages of embryonic development.

Adult-like complexity of the larval antennal lobe of D. melanogaster despite markedly low numbers of odorant receptor neurons

We provide a detailed analysis of the larval head chemosensory system of Drosophila melanogaster, based on confocal microscopy of cell-specific reporter gene expression in P[GAL4] enhancer trap lines. In particular, we describe the neuronal composition of three external and three pharyngeal chemosensory organs, the nerve tracts chosen by their afferents, and their central target regions. With a total of 21 olfactory and 80 gustatory neurons, the sensory level is numerically much simpler than that of the adult. Moreover, its design is different than in the adult, showing an association between smell and taste sensilla. In contrast, the first-order relay of the olfactory afferents, the larval antennal lobe (LAL), exhibits adult-like features both in terms of structure and cell number. It shows a division into approximately 30 subunits, reminiscent of glomeruli in the adult antennal lobe. Taken together, the design of the larval chemosensory system is a "hybrid," with larval-specific features in the periphery and central characteristics in common with the adult. The largely reduced numbers of afferents and the similar architecture of the LAL and the adult antennal lobe, render the larval chemosensory system of Drosophila a valuable model system, both for studying smell and taste and for examining the development of its adult organization.

Glia in development, function, and neurodegeneration of the adult insect brain

Glial cells have long been viewed as a passive framework for neurons but in the meanwhile were shown to play a much more active role in brain function and development. Several reviews have described the function of glia in the insect embryo. The focus of this review is the role of glial cells in the development and function of the normal and diseased adult brain. In different insect species, a considerable variety of central nervous system glia has been described indicating adaptation to different functional requirements. In the development of the adult visual and olfactory system, glial cells guide incoming axons acting as intermediate targets. Glia are part of the insect blood-brain barrier, provide nourishment for neurons, and help to regulate the extracellular concentration of ions and neurotransmitters. To fulfill these tasks insect glial cells, like vertebrate glia, interact with each other and with neurons, thus influencing neural activity. The examples presented suggest that crosstalk between all brain cells is necessary not only to develop and maintain the complex insect brain but also to endow it with the capacity to respond and adapt to the changing environment.

Distribution of metabotropic glutamate receptor DmGlu-A in Drosophila melanogaster central nervous system

L-glutamate is the excitatory neurotransmitter at neuromuscular junctions in insects. It may also be involved in neurotransmission within the central nervous system (CNS), but its function therein remains elusive. The roles of glutamatergic synapses in the Drosophila melanogaster CNS were investigated, with focus on the study of DmGluRA, a G-protein-coupled glutamate receptor. In a first attempt to determine the function of this receptor, we describe its distribution in the larval and adult Drosophila CNS, using a polyclonal antibody raised against the C-terminal sequence of the protein. DmGluRA is expressed in a reproducible pattern both in the larva and in the adult. In particular, DmGluRA can be found in the antennal lobes, the optic lobes, the central complex, and the median bundle in the adult CNS. However, DmGluRA-containing neurons represented only a small fraction of all CNS neurons. DmGluRA immunoreactivity was not detected at the larval neuromuscular junction nor in the body wall muscles. The correlations between DmGluRA distribution and previously described glutamate-like immunoreactivity patterns, as well as the implications of these observations concerning the possible functions of DmGluRA in the Drosophila CNS, are discussed.

Putative Drosophila odor receptor OR43b localizes to dendrites of olfactory neurons

To gain insight into the role of the recently identified Drosophila seven transmembrane receptor family, we analyzed the cellular and subcellular localization of a member of this family, OR43b. The OR43b receptor is expressed exclusively in a subset of olfactory neurons in the third antennal segment. Consistent with a direct role in odorant transduction, receptor protein is concentrated within the dendrites, but is also present in the axons of the olfactory neurons in which it is expressed. OR43b protein is only detectable relatively late in development suggesting it may not be required for synaptic target choice of the olfactory neurons in which it is expressed. Flies carrying deletions removing one copy of OR43b have the same number of OR43b positive cells in the antenna as flies with two copies, suggesting that simple allelic exclusion of odor receptors may not occur in Drosophila. We show the OR43b gene on the balancer chromosome SM5 is expressed at reduced levels and contains nucleotide polymorphisms predicted to alter two amino acids in the receptor, including an arginine(128) to proline substitution in the first extracellular loop. The subcellular localization of OR43b in olfactory neurons supports the idea that some of the recently identified family of seven transmembrane receptors are odor receptors, and that Drosophila and vertebrates may differ in the developmental processes used to establish the neuronal architecture of the olfactory system.

Behavioral transformations during metamorphosis: remodeling of neural and motor systems

During insect metamorphosis, neural and motor systems are remodeled to accommodate behavioral transformations. Nerve and muscle cells that are required for larval behavior, such as crawling, feeding and ecdysis, must either be replaced or respecified to allow adult emergence, walking, flight, mating and egg-laying. This review describes the types of cellular changes that occur during metamorphosis, as well as recent attempts to understand how they are related to behavioral changes and how they are regulated. Within the periphery, many larval muscles degenerate at the onset of metamorphosis and are replaced by adult muscles, which are derived from myoblasts and, in some cases, remnants of the larval muscle fibers. The terminal processes of many larval motoneurons persist within the periphery and are essential for the formation of adult muscle fibers. Although most adult sensory neurons are born postembryonically, a subset of larval proprioceptive neurons persist to participate in adult behavior. Within the central nervous system, larval neurons that will no longer be necessary die and some adult interneurons are born postembryonically. By contrast, all of the adult motoneurons, as well as some interneurons and modulatory neurons, are persistent larval cells. In accordance with their new behavioral roles, these neurons undergo striking changes in dendritic morphology, intrinsic biophysical properties, and synaptic interactions.

Metamorphosis in drosophila and other insects: the fate of neurons throughout the stages

The nervous system of insects is profoundly reorganised during metamorphosis, affecting the fate of different types of neuron in different ways. Almost all adult motor neurons derive from larval motor neurons that are respecified for adult functions. A subset of larval motor neurons, those which mediate larval- or ecdysis-specific behaviours, die before and immediately after eclosion, respectively. Many adult interneurons develop from larval interneurons, whereas those related to complex adult sense organs originate during larval life from persisting embryonic neuroblasts. Sensory neurons of larvae and adults derive from essentially two distinct sources. Larval sensory neurons are formed in the embryonic integument and - with few exceptions - die during metamorphosis. Their adult counterparts, on the other hand, arise from imaginal discs. Special emphasis is given in this review to the metamorphic remodelling of persisting neurons, both at the input and output levels, and to the associated behavioural changes. Other sections deal with the programmed death of motor neurons and its causes, as well as with the metamorphic interactions between motor neurons and their target muscles. Remodelling and apoptosis of these two elements appear to be under independent ecdysteroid control. This review focusses on the two most thoroughly studied holometabolous species, the fruitfly Drosophila melanogaster and the tobacco hornworm moth Manduca sexta. While Manduca has a long tradition in neurodevelopmental studies due to the identification of many of its neurons, Drosophila has been increasingly used to investigate neural reorganisation thanks to neurogenetic tools and molecular approaches. The wealth of information available emphasises the strength of the insect model system used in developmental studies, rendering it clearly the most important system for studies at the cellular level.

Building the antennal lobe: engrailed expression reveals a contribution from protocerebral neuroblasts in the grasshopper Schistocerca gregaria

The expression pattern of the engrailed protein was studied in neuroblasts which delaminate at the border of the protocerebrum and antennal lobe of the deutocerebrum in the early embryonic brain of the grasshopper. The antennal lobe is a complex structure comprising both glomerular and non-glomerular components, a cellular organization which distinguishes it from the striate-like neuropil comprising the remainder of the deutocerebrum. Early in embryogenesis engrailed expression in the protocerebrum is restricted to a compact block of neuroblasts located at its interface with the antennal lobe. Subsequently engrailed expression in these cells disappears in a stepwise manner from anterior to posterior so that by 37% of embryogenesis only a single row of three engrailed positive neuroblasts and their progeny remains. Contemporaneously engrailed expression reappears in a group of more anterior progeny deriving from neuroblasts which are no longer immunoreactive. The three remaining engrailed positive neuroblasts then become separated from their non-immunoreactive neighbours by an invagination of the perineurium called the lateral cleft and come to lie completely within the developing antennal lobe. These cells then direct columns of immunoreactive progeny centrifugally towards the centre of the lobe. Such a protocerebral contribution to the antennal lobe suggests that the evolution and ontogeny of this brain region need to be reconsidered.

Formation of antennal lobe and mushroom body neuropils during metamorphosis in the honeybee, apis mellifera

The projections to the mushroom bodies (mbs) have been clearly described in the brain of adult honeybees (Apis mellifera). Olfactory projection neurons arborize in the lip of the calyceal neuropil, whereas visual projection neurons project to the collar. To study the maturation of this pattern of innervation, as well as the development of uniglomerular projection neurons within the antennal lobes (als), we conducted the following three studies focused on the first four stages of pupal development: mass staining of olfactory projection neurons, single cell labeling of olfactory projection neurons, and simultaneous labeling of olfactory projection neurons and visual projection neurons. Examination of whole-mount preparations with the confocal laser scanning microscope revealed that the olfactory projection neurons achieved their adult arborization pattern within their main output region, the lip of the mb calyces, earlier during development (pupal stage 1) than their dendritic processes within their main input region, the al (pupal stage 2). Simultaneous labeling experiments showed further that the fiber terminals of olfactory projection neurons and visual projection neurons did not overlap but instead occupied their respective projection areas within the mb calyces as early as pupal stage 1. We conclude that selective innervation of different subregions of the calycal neuropil precedes the segregation of glomerular units within the antennal lobe neuropil, and that the Kenyon cells themselves provide a template for the innervation of olfactory and visual projection neurons.

A novel family of divergent seven-transmembrane proteins: candidate odorant receptors in Drosophila

Although insects have proven to be valuable models for exploring the function, organization, and development of the olfactory system, the receptor molecules that bind odors have not been identified in any insect. We have developed a novel search algorithm, used it to search the Drosophila genomic sequence database, and identified a large multigene family encoding seven transmembrane domain proteins that are expressed in olfactory organs. We show that expression is restricted to subsets of olfactory receptor neurons (ORNs) for a number of these genes. Different members of the family initiate expression at different times during antennal development. Some of the genes are not expressed in a mutant of the Acj6 POU-domain transcription factor, a mutant in which a subset of ORNs show abnormal odorant specificities.

Neuroblast ablation in Drosophila P[GAL4] lines reveals origins of olfactory interneurons

Hydroxyurea (HU) treatment of early first instar larvae in Drosophila was previously shown to ablate a single dividing lateral neuroblast (LNb) in the brain. Early larval HU application to P[GAL4] strains that label specific neuron types enabled us to identify the origins of the two major classes of interneurons in the olfactory system. HU treatment resulted in the loss of antennal lobe local interneurons and of a subset of relay interneurons (RI), elements usually projecting to the calyx and the lateral protocerebrum (LPR). Other RI were resistant to HU and still projected to the LPR. However, they formed no collaterals in the calyx region (which was also ablated), suggesting that their survival does not depend on targets in the calyx. Hence, the ablated interneurons were derived from the LNb, whereas the HU-resistant elements originated from neuroblasts which begin to divide later in larval life. Developmental GAL4 expression patterns suggested that differentiated RI are present at the larval stage already and may be retained through metamorphosis.

Temporal and spatial expression patterns of two G-protein coupled receptors in Drosophila melanogaster

Temporal and spatial expression patterns of a muscarinic acetylcholine receptor (Acr60C) and an octopamine/tyramine receptor (Octyr) were determined in Drosophila melanogaster using quantitative Northern analysis and in situ hybridization to tissue sections. Expression of mRNA encoding both of these G-protein coupled receptors peaks initially in 18 to 21 hour embryos following the formation of the mature larval nervous system. Levels of mRNA then decline during larval stages, rising to a second peak in 3 to 4-day-old pupae after a period of major nervous system reorganization. The muscarinic acetylcholine receptor mRNA is expressed throughout the cortical regions of the central nervous system in adults and embryos. Particularly high levels of expression of Acr60C are observed in cell bodies adjacent to the antennal lobes, suggesting a major role for this muscarinic receptor in the processing of olfactory information. In contrast, the octopamine/tyramine receptor mRNA is distributed diffusely throughout the adult brain, with patches of signal concentrated in the cortex of the dorsal protocerebrum near the mushroom bodies. These patches may represent individual cells expressing Octyr receptors.