Heinrich Reichert

Amplification of neural stem cell proliferation by intermediate progenitor cells in Drosophila brain development

BackgroundIn the mammalian brain, neural stem cells divide asymmetrically and often amplify the number of progeny they generate via symmetrically dividing intermediate progenitors. Here we investigate whether specific neural stem cell-like neuroblasts in the brain of Drosophila might also amplify neuronal proliferation by generating symmetrically dividing intermediate progenitors.ResultsCell lineage-tracing and genetic marker analysis show that remarkably large neuroblast lineages exist in the dorsomedial larval brain of Drosophila. These lineages are generated by brain neuroblasts that divide asymmetrically to self renew but, unlike other brain neuroblasts, do not segregate the differentiating cell fate determinant Prospero to their smaller daughter cells. These daughter cells continue to express neuroblast-specific molecular markers and divide repeatedly to produce neural progeny, demonstrating that they are proliferating intermediate progenitors. The proliferative divisions of these intermediate progenitors have novel cellular and molecular features; they are morphologically symmetrical, but molecularly asymmetrical in that key differentiating cell fate determinants are segregated into only one of the two daughter cells.ConclusionOur findings provide cellular and molecular evidence for a new mode of neurogenesis in the larval brain of Drosophila that involves the amplification of neuroblast proliferation through intermediate progenitors. This type of neurogenesis bears remarkable similarities to neurogenesis in the mammalian brain, where neural stem cells as primary progenitors amplify the number of progeny they generate through generation of secondary progenitors. This suggests that key aspects of neural stem cell biology might be conserved in brain development of insects and mammals.

The brain tumor gene negatively regulates neural progenitor cell proliferation in the larval central brain of Drosophila

Brain development in Drosophila is characterized by two neurogenic periods, one during embryogenesis and a second during larval life. Although much is known about embryonic neurogenesis, little is known about the genetic control of postembryonic brain development. Here we use mosaic analysis with a repressible cell marker (MARCM) to study the role of the brain tumor (brat) gene in neural proliferation control and tumour suppression in postembryonic brain development of Drosophila. Our findings indicate that overproliferation in brat mutants is due to loss of proliferation control in the larval central brain and not in the optic lobe. Clonal analysis indicates that the brat mutation affects cell proliferation in a cell-autonomous manner and cell cycle marker expression shows that cells of brat mutant clones show uncontrolled proliferation, which persists into adulthood. Analysis of the expression of molecular markers, which characterize cell types in wild-type neural lineages, indicates that brat mutant clones comprise an excessive number of cells, which have molecular features of undifferentiated progenitor cells that lack nuclear Prospero (Pros). pros mutant clones phenocopy brat mutant clones in the larval central brain, and targeted expression of wild-type pros in brat mutant clones promotes cell cycle exit and differentiation of brat mutant cells, thereby abrogating brain tumour formation. Taken together, our results provide evidence that the tumour suppressor brat negatively regulates cell proliferation during larval central brain development of Drosophila, and suggest that Prospero acts as a key downstream effector of brat in cell fate specification and proliferation control.

Developmental defects in brain segmentation caused by mutations of the homeobox genes orthodenticle and empty spiracles in Drosophila

We have studied the roles of the homeobox genes orthodenticle (otd) and empty spiracles (ems) in embryonic brain development of Drosophila. The embryonic brain is composed of three segmental neuromeres. The otd gene is expressed predominantly in the anterior neuromere; expression of ems is restricted to the two posterior neuromeres. Mutation of otd eliminates the first (protocerebral) brain neuromere. Mutation of ems eliminates the second (deutocerebral) and third (tritocerebral) neuromeres. otd is also necessary for development of the dorsal protocerebrum of the adult brain. We conclude that these homeobox genes are required for the development of specific brain segments in Drosophila, and that the regionalized expression of their homologs in vertebrate brains suggests an evolutionarily conserved program for brain development.

Evolution of Nervous Systems

Based on classical neuroanatomy, the bilaterians have been divided into two major groups: the Gastroneuralia, such as arthropods and annelids, characterized by a ventral central nervous system (CNS) and the Notoneuralia, such as chordates, characterized by a dorsal CNS. In contrast, molecular genetic studies based on the expression and function of conserved developmental control genes in neurogenesis have revealed striking similarities in the developmental organization of the brain in animals as diverse as flies and mammals. Comparison of the expression, function, and regulation of genes and genetic networks involved in anteroposterior, dorsoventral, and midline patterning of the insect and vertebrate CNS suggests that orthologous genes were already involved in neural specification in a common ancestor, indicating that insect and vertebrate brains evolved from an ancestral urbilaterian brain. The notion that the brains of diverse animal groups are homologous is supported by molecular genetic data that evoke the hypothesis of a dorsoventral body and neuraxis inversion in animal evolution and revive the historic debate about a common Bauplan underlying development in bilaterians. Whereas the evolutionary occurrence and mode of nervous system centralization are still debated, available data suggest that one ancestral and complex nervous system type was at the origin of bilaterian CNS evolution.

An urbilaterian origin of the tripartite brain: developmental genetic insights from Drosophila.

Studies on expression and function of key developmental control genes suggest that the embryonic vertebrate brain has a tripartite ground plan that consists of a forebrain/midbrain, a hindbrain and an intervening midbrain/hindbrain boundary region, which are characterized by the specific expression of the Otx, Hox and Pax2/5/8 genes, respectively. We show that the embryonic brain of the fruitfly Drosophila melanogaster expresses all three sets of homologous genes in a similar tripartite pattern. Thus, a Pax2/5/8 expression domain is located at the interface of brain-specific otd/Otx2 and unpg/Gbx2 expression domains anterior to Hox expression regions. We identify this territory as the deutocerebral/tritocerebral boundary region in the embryonic Drosophila brain. Mutational inactivation of otd/Otx2 and unpg/Gbx2 result in the loss or misplacement of the brain-specific expression domains of Pax2/5/8 and Hox genes. In addition, otd/Otx2 and unpg/Gbx2 appear to negatively regulate each other at the interface of their brain-specific expression domains. Our studies demonstrate that the deutocerebral/tritocerebral boundary region in the embryonic Drosophila brain displays developmental genetic features similar to those observed for the midbrain/hindbrain boundary region in vertebrate brain development. This suggests that a tripartite organization of the embryonic brain was already established in the last common urbilaterian ancestor of protostomes and deuterostomes.

Conserved genetic programs in insect and mammalian brain development

In recent years it has become evident that the developmental regulatory genes involved in patterning the embryonic body plan are conserved throughout the animal kingdom. Striking examples are the orthodenticle (otd/Otx) gene family and the Hox gene family, both of which act in the specification of anteroposterior polarity along the embryonic body axis. Studies carried out in Drosophila and mouse now demonstrate that these genes are also involved in the formation of the insect and mammalian brain; the otd/Otx genes are involved in rostral brain development and the Hox genes are involved in caudal brain development. These studies also show that the genes of the otd/Otx family can functionally replace each other in cross‐phylum rescue experiments and indicate that the genetic mechanisms underlying pattern formation in insect and mammalian brain development are evolutionarily conserved. BioEssays 21:677–684, 1999.

Neuronal circuits controlling flight in the locust: how sensory information is processed for motor control

Abstract Animals abstract relevant information from a profusion of diverse stimuli and then organize appropriate behavioral responses. How is this done? A partial answer comes from studies on the flight steering system of the locust where multiple sensory systems have been shown to interact and shape the response properties of deviation-specific detector neurons. These sensory detector neurons project to a population of segmental premotor interneurons, which, gated by the flight central oscillator, then phase-couple and distribute integrated sensorimotor information to specific flight motoneurons, thus producing the motor program modifications which underly corrective steering responses.

Early development of the Drosophila mushroom bodies, brain centres for associative learning and memory.

Abstract We have studied the formation of Drosophila mushroom bodies using enhancer detector techniques to visualize specific components of these complex intrinsic brain structures. During embryogenesis, neuronal proliferation begins in four mushroom body neuroblasts and the major axonal pathways of the mushroom bodies are pioneered. During larval development, neuronal proliferation continues and further axonal projections in the pedunculus and lobes are formed in a highly structured manner characterized by spatial heterogeneity of reporter gene expression. Enhancer detector analysis identifies many genomic locations that are specifically activated in mushroom body intrinsic neurons (Kenyon cells) during the transition from embryonic to postembryonic development and during metamorphosis.

Building a brain: developmental insights in insects

Understanding the cellular, molecular and genetic mechanisms involved in building the brain remains one of the most challenging problems of neurobiology. In this article, we review recent work on the developmental mechanisms that generate the embryonic brain in insects. We compare some of the early developmental events that occur in the insect brain with those that operate during brain development in vertebrates and find that numerous parallels are present at both the cellular and the molecular levels. Thus, the roles of glial cells in prefiguring axon pathways, the function of pioneer neurons in establishing axon pathways, and the formation of a primary axon scaffolding are features of embryonic brain development in both insects and vertebrates. Moreover, at the molecular genetic level homologous regulatory genes control morphogenesis, regionalization and patterning during embryonic brain development in both insects and vertebrates. This indicates that there might be universal mechanisms for brain development, and that knowledge gained from Drosophila and other insects is relevant to our understanding of brain development in other more complex organisms, including man.

Postembryonic development of transit amplifying neuroblast lineages in the Drosophila brain.

BackgroundSpecific dorsomedial (DM) neuroblast lineages of the Drosophila brain amplify their proliferation through generation of transit amplifying intermediate progenitor cells. Together, these DM neuroblast lineages comprise over 5,000 adult-specific neural cells and thus represent a substantial part of the brain. However, no information is currently available about the structure or function of any of the neural cells in these DM lineages. In this report we use MARCM-based clonal analysis together with immunocytochemical labeling techniques to investigate the type and fate of neural cells generated in the DM lineages.ResultsGenetic cell lineage-tracing and immunocytochemical marker analysis reveal that DM neuroblasts are multipotent progenitors that produce a set of postembryonic brain glia as well as a large number of adult-specific protocerebral neurons. During larval development the adult-specific neurons of each DM lineage form several spatially separated axonal fascicles, some of which project along larval brain commissural structures that are primordia of midline neuropile. By taking advantage of a specific Gal4 reporter line, the DM-derived neuronal cells can be identified and followed into early pupal stages. During pupal development the neurons of the DM lineages arborize in many parts of the brain and contribute to neuropile substructures of the developing central complex, such as the fan-shaped body, noduli and protocerebral bridge.ConclusionsOur findings provide cellular and molecular evidence for the fact that DM neuroblasts are multipotent progenitors; thus, they represent the first identified progenitor cells in the fly brain that have neuroglioblast functions during postembryonic development. Moreover, our results demonstrate that the adult-specific neurons of the DM lineages arborize widely in the brain and also make a major contribution to the developing central complex. These findings suggest that the amplification of proliferation that characterizes DM lineages may be an important requirement for generating the large number of neurons required in highly complex neuropile structures such as the central complex in the Drosophila brain.