Littérature scientifique sur le sujet « Stem cells ; Drosophila ; Optic lobes »

ABSTRACTEnvironmental factors such as the availability of oxygen are instructive cues that regulate stem cell maintenance and differentiation. We used a genetically encoded biosensor to monitor the hypoxic state of neural cells in the larval brain of Drosophila. The biosensor reveals brain compartment and cell-type specific levels of hypoxia. The values correlate with differential tracheolation that is observed throughout development between the central brain and the optic lobe. Neural stem cells in both compartments show the strongest hypoxia response while intermediate progenitors, neurons and glial cells reveal weaker responses. We demonstrate that the distance between a cell and the next closest tracheole is a good predictor of the hypoxic state of that cell. Our study indicates that oxygen availability appears to be the major factor controlling the hypoxia response in the developing Drosophila brain and that cell intrinsic and cell-type specific factors contribute to modulate the response in an unexpected manner.This article has an associated First Person interview with the first author of the paper.

Stem cell proliferation is controlled through cell cycle arrest and activation. In the central nervous system of Drosophila melanogaster, neuroblast quiescence and activation takes place in defined spatial and temporal patterns. Two genes have been identified that regulate the pattern of neuroblast quiescence and proliferation. ana, which has been previously described by Ebens and coworkers (Ebens, A., Garren, H., Cheyette, B. N. R. and Zipursky, S. L. (1993). Cell 74, 15–28), encodes a secreted glial glycoprotein that inhibits premature neuroblast proliferation. We previously showed that trolsd causes a dramatic drop in the number of dividing cells in the larval brain late in development. This study presents evidence that this decrease results from a failure to activate proliferation in the quiescent neuroblast population at the appropriate time. However, trolsd does not affect the maintenance of cell division in already dividing mushroom body neuroblasts. The quiescent optic lobe and thoracic neuroblasts affected by trolsd proliferate in a trol mutant background if they have been activated by a lack of the ana proliferation repressor, demonstrating that trolsd does not affect cellular viability, nor does trol represent a celltype-specific mitotic factor. This also shows that trol acts downstream of ana to activate proliferation of quiescent neuroblasts in an ana-dependent pathway, possibly by inactivating or bypassing the ana repressor. These results suggest that trol and ana are components of a novel developmental pathway for the control of cell cycle activation in quiescent neuroblasts.

In insects, the ecdysteroids act to transform the CNS from its larval to its adult form. A key gene in this response is the ecdysone receptor (EcR), which has been shown in Drosophila to code for 3 protein isoforms. Two of these isoforms, EcR-A and EcR-B1, are prominently expressed in the CNS and we have used isoform-specific antibodies to examine their fluctuations through postembryonic life. EcR expression at the onset of metamorphosis is extremely diverse but specific patterns of EcR expression correlate with distinct patterns of steroid response. Most larval neurons show high levels of EcR-B1 at the start of metamorphosis, a time when they lose larval features in response to ecdysteroids. Earlier, during the larval molts, the same cells have no detectable receptors and show no response to circulating ecdysteroids; later, during the pupal-adult transformation, they switch to EcR-A expression and respond by maturing to their adult form. During the latter period, a subset of the larval neurons hyperexpress EcR-A and these cells are fated to die after the emergence of the adult. The stem cells for the imaginal neurons show prominent EcR-B1 expression during the last larval stage correlated with their main proliferative period. Most imaginal neurons, by contrast, express only EcR-A when they subsequently initiate maturation at the start of metamorphosis. The imaginal neurons of the mushroom bodies are unusual amongst imaginal neurons in expressing the B1 isoform at the start of metamorphosis but they also show regressive changes at this time as they lose their larval axons. Imaginal neurons of the optic lobe show a delayed expression of EcR-B1 through the period when cell-cell interactions are important for establishing connections within this region of the CNS. Overall, the appearance of the two receptor isoforms in cells correlates with different types of steroid responses: EcR-A predominates when cells are undergoing maturational responses whereas EcR-B1 predominates during proliferative activity or regressive responses. The heterogeneity of EcR expression at the start of metamorphosis presumably reflects the diverse origins and requirements of the neurons that nevertheless are all exposed to a common hormonal signal.

We have identified a set of larval neurones in the developing adult optic lobes of Drosophila by selectively labelling cells that have undergone only a few mitoses. A cluster of three cells is located in each of the optic lobes near the insertion site of the optic stalk. Their axons fasciculate with fibres of the larval optic nerve, the Bolwig's nerve, and then form part of the posterior optic tract. These cells are likely to be first order interneurones of the larval visual system. Unlike the Bolwig's nerve, they persist into the adult stage. The possibility of a pioneering function of the larval visual system during formation of the adult optic lobe neuropil is discussed.

Mutations in the complex gene locus optomotor-blind (omb) can lead to defects in the development of both the optic lobes and external features of the adult fly. We describe here the expression of omb in the developing and adult nervous system using in situ hybridization. During embryogenesis, omb expression is first observed in the optic lobe anlagen. It later expands to a larger part of the developing larval brain and to the gnathal lobes. Cells in the ventral and peripheral nervous systems begin to express omb after completion of germ band extension. Later in embryonic development, expression declines and only persists in the antennomaxillary complex and in part of the brain hemispheres. During the larval and pupal stages, omb expression in the brain is confined to the developing optic lobes and contiguous regions of the central brain. At these stages, only a few cells show expression in the ventral ganglion. In the eye imaginal disc, transcript accumulation is most conspicuous in a group of presumptive glia precursor cells posterior to the morphogenetic furrow and in the optic stalk. In the adult brain, expression is prominent in several regions of the optic lobe cortex and along the border between central brain and optic lobes. In the mutation In(1)ombH31, 40 kb of regulatory DNA, downstream from the transcription unit, are removed from the omb gene. In(1)ombH31 is characterized by the lack of a set of giant interneurons from the lobula plate of the adult optic lobes. We find that, already during embryogenesis, there is a drastic difference between wild type and In(1)ombH31 in the level of the omb transcript in the optic lobe primordia. The adult mutant phenotype may thus be caused by omb misexpression during embryonic development.

The importance of retinal innervation for the normal development of the optic ganglia in Drosophila is well documented. However, little is known about retrograde effects of the optic lobe on the adult photoreceptor cells (R-cells). We addressed this question by examining the survival of R-cells in mutant flies where R-cells do not connect to the brain. Although imaginal R-cells develop normally in the absence of connections to the optic lobes, we find that their continued survival requires these connections. Genetic mosaic studies with the disconnected (disco) mutation demonstrate that survival of R-cells does not depend on the genotype of the eye, but is correlated with the presence of connections to the optic ganglia. These results suggest the existence of retrograde interactions in the Drosophila visual system reminiscent of trophic interactions found in vertebrates.

Previous studies demonstrated that Fasciclin II and Beaten path are necessary for regulating cell adhesion events that are important for motoneuron development in Drosophila. We observe that the cell adhesion molecule Fasciclin II and the secreted anti-adhesion molecule Beaten path have additional critical roles in the development of at least one set of sensory organs, the larval visual organs. Taken together, phenotypic analysis, genetic interactions, expression studies and rescue experiments suggest that, in normal development, secretion of Beaten path by cells of the optic lobes allows the Fasciclin II-expressing larval visual organ cells to detach from the optic lobes as a cohesive cell cluster. Our results also demonstrate that mechanisms guiding neuronal development may be shared between motoneurons and sensory organs, and provide evidence that titration of adhesion and anti-adhesion is critical for early steps in development of the larval visual system.

Abstract The somda (sine oculis-medusa) mutant is the result of a P element insertion at position 43C on the second chromosome. somda causes aberrant development of the larval photoreceptor (Bolwig's) organ and the optic lobe primordium in the embryo. Later in development, adult photoreceptors fail to project axons into the optic ganglion. Consequently optic lobe development is aborted and photoreceptor cells show age-dependent retinal degeneration. The so gene was isolated and characterized. The gene encodes a homeodomain protein expressed in the optic lobe primordium and Bolwig's organ of embryos, in the developing adult visual system of larvae, and in photoreceptor cells and optic lobes of adults. In addition, the SO product is found at invagination sites during embryonic development: at the stomadeal invagination, the cephalic furrow, and at segmental boundaries. The mutant somda allele causes severe reduction of SO embryonic expression but maintains adult visual system expression. Ubiquitous expression of the SO gene product in 4-8-hr embryos rescues all somda mutant abnormalities, including the adult phenotypes. Thus, all deficits in adult visual system development and function results from failure to properly express the so gene during embryonic development. This analysis shows that the homeodomain containing SO gene product is involved in the specification of the larval and adult visual system development during embryogenesis.

Dissertação de Mestrado apresentada no ISPA - Instituto Universitário para obtenção de grau de Mestre na especialidade em Neurociências Cognitivas e Comportamentais.
Sendo o Encéfalo um dos órgãos mais importantes do organismo, a descoberta de que o processo de neurogénese continuava presente durante a fase adulta do animal foi uma das grandes revelações científcas da última metade do século XX. Assim, uma das vantagens na investigação deste processo: Neurogénese Adulta, é a sua aplicação em diversos organismos, que posteriormente possibilitará uma compreensão mais completa e aprofundada do mesmo. A Drosophila melanogaster foi recentemente apresentada como um óptimo modelo de estudo devido à sua acessibilidade genética e grande capacidade neuroregenerativa após lesão, contribuindo para o acesso a aspectos até então inexplorados. O objectivo principal deste trabalho foi o estudo da regulação do processo de Neurogénese regenerativa com foco na acção de dois factores: a Idade e a Actividade. Tal foi possível através da aplicação de um sistema de sensível de “lineage tracing”, o qual permite a visualização e quantificação de neurónios recentemente produzidos após uma lesão. Nos resultados obtidos, destacou-se a evidência de que o envelhecimento não promove uma diminuição na capacidade regenerativa no cérebro adulto da mosca, e persiste num nível constante até às 6 semanas de idade, o é próximo da vida inteira. Diferentes protocolos foram realizados para estudar o efeito da actividade física na neurogénese regenerativa. As experiências optimizadas ainda estão a decorrer e não poderam ser concluídas, porém dados preliminares indicam, que as moscas que têm um maior espaço envolvente anteriormente à lesão podem regenerar mais facilmente comparado com moscas que estão confinadas num espaço reduzido. No futuro, será importante será importante investigar quais os genes que regulam a ativação de células progenitoras neurais adultas induzida através de lesão, e ainda quais os fatores que controlam a diferenciação neuronal para obter uma compreensão mais detalhada de como a idade e a atividade influenciam a regeneração no cérebro adulto.
Being the brain one of the most important organs, the discovery that neurogenesis was continuous even in the adult phases was one of the great scientific revelations of the last century. The research of adult neurogenesis in several model systems will allow a better and complete understanding of this process. Drosophila melanogaster has been proposed as a novel model due to its genetic accessibility and its ability to regenerate neurons after injury, thereby opening the way to unexplored aspects. The main goal of the present project was to study the regulation of regenerative neurogenesis with respect to Age and Activity. This was achieved by applying sensitive lineage tracing, which allowed the visualization and quantification of the newly generated neurons upon injury. The observed results demonstrate that the regenerative capacity in adult fly brains does not decrease with age and persists on a constant level up to 6 weeks of age, which is close to the entire life span. Different paradigms were tested to study the effect of physical activity on regenerative neurogenesis. Experiments with the optimized set-up are still ongoing and could not be concluded, but preliminary data indicates, that flies that could move in a large compartment before injury may regenerate more favorably compared to flies that were kept in a more confined space. In the future, it will be important to gain further insight into which genes regulate injury-induced activation of adult neural progenitor cells and which factors control neuronal differentiation to gain a more detailed understanding of how age and activity impinge on regeneration.