Guidance of neuronal growth cones in the grasshopper embryo. IV. Temporal delay experiments

Journal ArticleIn the previous 3 papers, we demonstrated that neuronal growth cones display selective affinities for both specific axonal and glial pathways in the grasshopper embryo; for example, the pCC growth cone selectively recognizes the MPl/dMP2 axons, while the aCC growth cone selectively recognizes the U axons and a specific glial cell (the segment boundary cell)

Drosophila type II neuroblast lineages keep Prospero levels low to generate large clones that contribute to the adult brain central complex

Tissue homeostasis depends on the ability of stem cells to properly regulate self-renewal versus differentiation. Drosophila neural stem cells (neuroblasts) are a model system to study self-renewal and differentiation. Recent work has identified two types of larval neuroblasts that have different self-renewal/differentiation properties. Type I neuroblasts bud off a series of small basal daughter cells (ganglion mother cells) that each generate two neurons. Type II neuroblasts bud off small basal daughter cells called intermediate progenitors (INPs), with each INP generating 6 to 12 neurons. Type I neuroblasts and INPs have nuclear Asense and cytoplasmic Prospero, whereas type II neuroblasts lack both these transcription factors. Here we test whether Prospero distinguishes type I/II neuroblast identity or proliferation profile, using several newly characterized Gal4 lines. We misexpress prospero using the 19H09-Gal4 line (expressed in type II neuroblasts but no adjacent type I neuroblasts) or 9D11-Gal4 line (expressed in INPs but not type II neuroblasts). We find that differential prospero expression does not distinguish type I and type II neuroblast identities, but Prospero regulates proliferation in both type I and type II neuroblast lineages. In addition, we use 9D11 lineage tracing to show that type II lineages generate both small-field and large-field neurons within the adult central complex, a brain region required for locomotion, flight, and visual pattern memory

Regulation of spindle orientation and neural stem cell fate in the Drosophila optic lobe.

BACKGROUND: The choice of a stem cell to divide symmetrically or asymmetrically has profound consequences for development and disease. Unregulated symmetric division promotes tumor formation, whereas inappropriate asymmetric division affects organ morphogenesis. Despite its importance, little is known about how spindle positioning is regulated. In some tissues cell fate appears to dictate the type of cell division, whereas in other tissues it is thought that stochastic variation in spindle position dictates subsequent sibling cell fate. RESULTS: Here we investigate the relationship between neural progenitor identity and spindle positioning in the Drosophila optic lobe. We use molecular markers and live imaging to show that there are two populations of progenitors in the optic lobe: symmetrically dividing neuroepithelial cells and asymmetrically dividing neuroblasts. We use genetically marked single cell clones to show that neuroepithelial cells give rise to neuroblasts. To determine if a change in spindle orientation can trigger a neuroepithelial to neuroblast transition, we force neuroepithelial cells to divide along their apical/basal axis by misexpressing Inscuteable. We find that this does not induce neuroblasts, nor does it promote premature neuronal differentiation. CONCLUSION: We show that symmetrically dividing neuroepithelial cells give rise to asymmetrically dividing neuroblasts in the optic lobe, and that regulation of spindle orientation and division symmetry is a consequence of cell type specification, rather than a mechanism for generating cell type diversity.RIGHTS : This article is licensed under the BioMed Central licence at http://www.biomedcentral.com/about/license which is similar to the 'Creative Commons Attribution Licence'. In brief you may : copy, distribute, and display the work; make derivative works; or make commercial use of the work - under the following conditions: the original author must be given credit; for any reuse or distribution, it must be made clear to others what the license terms of this work are

Regulation of spindle orientation and neural stem cell fate in the Drosophila optic lobe

BACKGROUND: The choice of a stem cell to divide symmetrically or asymmetrically has profound consequences for development and disease. Unregulated symmetric division promotes tumor formation, whereas inappropriate asymmetric division affects organ morphogenesis. Despite its importance, little is known about how spindle positioning is regulated. In some tissues cell fate appears to dictate the type of cell division, whereas in other tissues it is thought that stochastic variation in spindle position dictates subsequent sibling cell fate. RESULTS: Here we investigate the relationship between neural progenitor identity and spindle positioning in the Drosophila optic lobe. We use molecular markers and live imaging to show that there are two populations of progenitors in the optic lobe: symmetrically dividing neuroepithelial cells and asymmetrically dividing neuroblasts. We use genetically marked single cell clones to show that neuroepithelial cells give rise to neuroblasts. To determine if a change in spindle orientation can trigger a neuroepithelial to neuroblast transition, we force neuroepithelial cells to divide along their apical/basal axis by misexpressing Inscuteable. We find that this does not induce neuroblasts, nor does it promote premature neuronal differentiation. CONCLUSION: We show that symmetrically dividing neuroepithelial cells give rise to asymmetrically dividing neuroblasts in the optic lobe, and that regulation of spindle orientation and division symmetry is a consequence of cell type specification, rather than a mechanism for generating cell type diversity

A multilayer circuit architecture for the generation of distinct locomotor behaviors in Drosophila.

Animals generate diverse motor behaviors, yet how the same motor neurons (MNs) generate two distinct or antagonistic behaviors remains an open question. Here, we characterize Drosophila larval muscle activity patterns and premotor/motor circuits to understand how they generate forward and backward locomotion. We show that all body wall MNs are activated during both behaviors, but a subset of MNs change recruitment timing for each behavior. We used TEM to reconstruct a full segment of all 60 MNs and 236 premotor neurons (PMNs), including differentially-recruited MNs. Analysis of this comprehensive connectome identified PMN-MN 'labeled line' connectivity; PMN-MN combinatorial connectivity; asymmetric neuronal morphology; and PMN-MN circuit motifs that could all contribute to generating distinct behaviors. We generated a recurrent network model that reproduced the observed behaviors, and used functional optogenetics to validate selected model predictions. This PMN-MN connectome will provide a foundation for analyzing the full suite of larval behaviors

A command-like descending neuron that coordinately activates backward and inhibits forward locomotion

Command-like descending neurons can induce many behaviors, such as backward locomotion, escape, feeding, courtship, egg-laying, or grooming. In most animals it remains unknown how neural circuits switch between these antagonistic behaviors: via top-down activation/inhibition of antagonistic circuits or via reciprocal inhibition between antagonistic circuits. Here we use genetic screens, intersectional genetics, circuit reconstruction by electron microscopy, and functional optogenetics to identify a bilateral pair of larval “mooncrawler descending neurons” (MDNs) with command-like ability to coordinately induce backward locomotion and block forward locomotion; the former by activating a backward-specific premotor neuron, and the latter by disynaptic inhibition of a forward-specific premotor neuron. In contrast, direct reciprocal inhibition between forward and backward circuits was not observed. Thus, MDNs coordinate a transition between antagonistic larval locomotor behaviors. Interestingly, larval MDNs persist into adulthood, where they can trigger backward walking. Thus, MDNs induce backward locomotion in both limbless and limbed animals

Zfh1, a somatic motor neuron transcription factor, regulates axon exit from the CNS

AbstractMotor neurons are defined by their axon projections, which exit the CNS to innervate somatic or visceral musculature, yet remarkably little is known about how motor axons are programmed to exit the CNS. Here, we describe the role of the Drosophila Zfh1 transcription factor in promoting axon exit from the CNS. Zfh1 is detected in all embryonic somatic motor neurons, glia associated with the CNS surface and motor axons, and one identified interneuron. In zfh1 mutants, ventral projecting motor axons often stall at the edge of the CNS, failing to enter the muscle field, despite having normal motor neuron identity. Conversely, ectopic Zfh1 induces a subset of interneurons—all normally expressing two or more “ventral motor neuron transcription factors” (e.g. Islet, Hb9, Nkx6, Lim3)—to project laterally and exit the CNS. We conclude that Zfh1 is required for ventral motor axon exit from the CNS

Neural circuits driving larval locomotion in Drosophila

More than 30 years of studies into Drosophila melanogaster neurogenesis have revealed fundamental insights into our understanding of axon guidance mechanisms, neural differentiation, and early cell fate decisions. What is less understood is how a group of neurons from disparate anterior-posterior axial positions, lineages and developmental periods of neurogenesis coalesce to form a functional circuit. Using neurogenetic techniques developed in Drosophila it is now possible to study the neural substrates of behavior at single cell resolution. New mapping tools described in this review, allow researchers to chart neural connectivity to better understand how an anatomically simple organism performs complex behaviors