Neuronal Computations Made Visible with Subcellular Resolution.

Sensory information is gradually processed within dedicated neural circuits to generate specific behaviors. In this issue, Yang et al. push technology boundaries to measure both voltage and calcium signals from subcellular compartments of genetically defined interconnected neurons and shed light on local neural computations critical for motion detection.

Analysis of the genetically tractable crustacean Parhyale hawaiensis reveals the organisation of a sensory system for low-resolution vision.

BACKGROUND: Arthropod eyes have diversified during evolution to serve multiple needs, such as finding mates, hunting prey and navigating in complex surroundings under varying light conditions. This diversity is reflected in the optical apparatus, photoreceptors and neural circuits that underpin vision. Yet our ability to genetically manipulate the visual system to investigate its function is largely limited to a single species, the fruit fly Drosophila melanogaster. Here, we describe the visual system of Parhyale hawaiensis, an amphipod crustacean for which we have established tailored genetic tools. RESULTS: Adult Parhyale have apposition-type compound eyes made up of ~ 50 ommatidia. Each ommatidium contains four photoreceptor cells with large rhabdomeres (R1-4), expected to be sensitive to the polarisation of light, and one photoreceptor cell with a smaller rhabdomere (R5). The two types of photoreceptors express different opsins, belonging to families with distinct wavelength sensitivities. Using the cis-regulatory regions of opsin genes, we established transgenic reporters expressed in each photoreceptor cell type. Based on these reporters, we show that R1-4 and R5 photoreceptors extend axons to the first optic lobe neuropil, revealing striking differences compared with the photoreceptor projections found in related crustaceans and insects. Investigating visual function, we show that Parhyale have a positive phototactic response and are capable of adapting their eyes to different levels of light intensity. CONCLUSIONS: We propose that the visual system of Parhyale serves low-resolution visual tasks, such as orientation and navigation, based on broad gradients of light intensity and polarisation. Optic lobe structure and photoreceptor projections point to significant divergence from the typical organisation found in other malacostracan crustaceans and insects, which could be associated with a shift to low-resolution vision. Our study provides the foundation for research in the visual system of this genetically tractable species.

Retinal determination genes coordinate neuroepithelial specification and neurogenesis modes in the Drosophila optic lobe.

Differences in neuroepithelial patterning and neurogenesis modes contribute to area-specific diversifications of neural circuits. In the Drosophila visual system, two neuroepithelia, the outer (OPC) and inner (IPC) proliferation centers, generate neuron subtypes for four ganglia in several ways. Whereas neuroepithelial cells in the medial OPC directly convert into neuroblasts, in an IPC subdomain they generate migratory progenitors by epithelial-mesenchymal transition that mature into neuroblasts in a second proliferative zone. The molecular mechanisms that regulate the identity of these neuroepithelia, including their neurogenesis modes, remain poorly understood. Analysis of Polycomblike revealed that loss of Polycomb group-mediated repression of the Hox gene Abdominal-B (Abd-B) caused the transformation of OPC to IPC neuroepithelial identity. This suggests that the neuroepithelial default state is IPC-like, whereas OPC identity is derived. Ectopic Abd-B blocks expression of the highly conserved retinal determination gene network members Eyes absent (Eya), Sine oculis (So) and Homothorax (Hth). These factors are essential for OPC specification and neurogenesis control. Finally, eya and so are also sufficient to confer OPC-like identity, and, in parallel with hth, the OPC-specific neurogenesis mode on the IPC.

Ssrp1a controls organogenesis by promoting cell cycle progression and RNA synthesis.

Tightly controlled DNA replication and RNA transcription are essential for differentiation and tissue growth in multicellular organisms. Histone chaperones, including the FACT (facilitates chromatin transcription) complex, are central for these processes and act by mediating DNA access through nucleosome reorganisation. However, their roles in vertebrate organogenesis are poorly understood. Here, we report the identification of zebrafish mutants for the gene encoding Structure specific recognition protein 1a (Ssrp1a), which, together with Spt16, forms the FACT heterodimer. Focussing on the liver and eye, we show that zygotic Ssrp1a is essential for proliferation and differentiation during organogenesis. Specifically, gene expression indicative of progressive organ differentiation is disrupted and RNA transcription is globally reduced. Ssrp1a-deficient embryos exhibit DNA synthesis defects and prolonged S phase, uncovering a role distinct from that of Spt16, which promotes G1 phase progression. Gene deletion/replacement experiments in Drosophila show that Ssrp1b, Ssrp1a and N-terminal Ssrp1a, equivalent to the yeast homologue Pob3, can substitute Drosophila Ssrp function. These data suggest that (1) Ssrp1b does not compensate for Ssrp1a loss in the zebrafish embryo, probably owing to insufficient expression levels, and (2) despite fundamental structural differences, the mechanisms mediating DNA accessibility by FACT are conserved between yeast and metazoans. We propose that the essential functions of Ssrp1a in DNA replication and gene transcription, together with its dynamic spatiotemporal expression, ensure organ-specific differentiation and proportional growth, which are crucial for the forming embryo.

Flybow: genetic multicolor cell labeling for neural circuit analysis in Drosophila melanogaster.

To facilitate studies of neural network architecture and formation, we generated three Drosophila melanogaster variants of the mouse Brainbow-2 system, called Flybow. Sequences encoding different membrane-tethered fluorescent proteins were arranged in pairs within cassettes flanked by recombination sites. Flybow combines the Gal4-upstream activating sequence binary system to regulate transgene expression and an inducible modified Flp-FRT system to drive inversions and excisions of cassettes. This provides spatial and temporal control over the stochastic expression of one of two or four reporters within one sample. Using the visual system, the embryonic nervous system and the wing imaginal disc, we show that Flybow in conjunction with specific Gal4 drivers can be used to visualize cell morphology with high resolution. Finally, we demonstrate that this labeling approach is compatible with available Flp-FRT-based techniques, such as mosaic analysis with a repressible cell marker; this could further support the genetic analysis of neural circuit assembly and function.

A challenge of numbers and diversity: neurogenesis in the Drosophila optic lobe.

The brain areas that endow insects with the ability to see consist of remarkably complex neural circuits. Reiterated arrays of many diverse neuron subtypes are assembled into modular yet coherent functional retinotopic maps. Tremendous progress in developing genetic tools and cellular markers over the past years advanced our understanding of the mechanisms that control the stepwise production and differentiation of neurons in the visual system of Drosophila melanogaster. The postembryonic optic lobe utilizes at least two modes of neurogenesis that are distinct from other parts of the fly central nervous system. In the first optic ganglion, the lamina, neuroepithelial cells give rise to precursor cells, whose proliferation and differentiation depend on anterograde signals from photoreceptor axons. In the second optic ganglion, the medulla, the coordinated activity of four signaling pathways orchestrates the gradual conversion of neuroepithelial cells into neuroblasts, while a specific cascade of temporal identity transcription factors controls subtype diversification of their progeny.

Regulation of locomotion and motoneuron trajectory selection and targeting by the Drosophila homolog of Olig family transcription factors.

During the development of locomotion circuits it is essential that motoneurons with distinct subtype identities select the correct trajectories and target muscles. In vertebrates, the generation of motoneurons and myelinating glia depends on Olig2, one of the five Olig family bHLH transcription factors. We investigated the so far unknown function of the single Drosophila homolog Oli. Combining behavioral and genetic approaches, we demonstrate that oli is not required for gliogenesis, but plays pivotal roles in regulating larval and adult locomotion, and axon pathfinding and targeting of embryonic motoneurons. In the embryonic nervous system, Oli is primarily expressed in postmitotic progeny, and in particular, in distinct ventral motoneuron subtypes. oli mediates axonal trajectory selection of these motoneurons within the ventral nerve cord and targeting to specific muscles. Genetic interaction assays suggest that oli acts as part of a conserved transcription factor ensemble including Lim3, Islet and Hb9. Moreover, oli is expressed in postembryonic leg-innervating motoneuron lineages and required in glutamatergic neurons for walking. Finally, over-expression of vertebrate Olig2 partially rescues the walking defects of oli-deficient flies. Thus, our findings reveal a remarkably conserved role of Drosophila Oli and vertebrate family members in regulating motoneuron development, while the steps that require their function differ in detail.

HOW to wrap axons with crooked neck.

How can RNA processing direct specific aspects of nervous system development? In this issue of Neuron, Edenfeld et al. identified a novel function for two regulators of mRNA splicing in Drosophila: peripheral glial cells require Crooked neck (Crn) and Held out wings (HOW) to mediate migration and ensheathment of peripheral axons.

Flybow to Dissect Circuit Assembly in the Drosophila Brain: An Update.

Visualization of single neurons and glia, as well as neural lineages within their complex environment is a pivotal step towards uncovering the mechanisms that control neural circuit development and function. This chapter provides detailed technical information on how to use Drosophila variants of the mouse Brainbow-2 system, called Flybow, for stochastic labeling of individual cells or lineages with different fluorescent proteins in one sample. We describe the genetic strategies and the heat shock regime required for induction of recombination events. Furthermore, we explain how Flybow and the mosaic analysis with a repressible cell marker (MARCM) approach can be combined to generate wild-type or homozygous mutant clones that are positively labeled in multiple colors. This is followed by a detailed protocol as to how to prepare samples for imaging. Finally, we provide specifications to facilitate multichannel image acquisition using confocal microscopy.

glial cells missing and gcm2 cell autonomously regulate both glial and neuronal development in the visual system of Drosophila.

The transcription factors Glial cells missing (Gcm) and Gcm2 are known to play a crucial role in promoting glial-cell differentiation during Drosophila embryogenesis. Our findings reveal a central function for gcm genes in regulating neuronal development in the postembryonic visual system. We demonstrate that Gcm and Gcm2 are expressed in both glial and neuronal precursors within the optic lobe. Removal of gcm and gcm2 function shows that the two genes act redundantly and are required for the formation of a subset of glial cells. They also cell-autonomously control the differentiation and proliferation of specific neurons. We show that the transcriptional regulator Dachshund acts downstream of gcm genes and is required to make lamina precursor cells and lamina neurons competent for neuronal differentiation through regulation of epidermal growth factor receptor levels. Our findings further suggest that gcm genes regulate neurogenesis through collaboration with the Hedgehog-signaling pathway.

Control of photoreceptor axon target choice by transcriptional repression of Runt.

Drosophila photoreceptor neurons (R cells) project their axons to one of two layers in the optic lobe, the lamina or the medulla. The transcription factor Runt (Run) is normally expressed in the two inner R cells (R7 and R8) that project their axons to the medulla. Here we examine the relationship between Run and the ubiquitously expressed nuclear protein Brakeless (Bks), which has previously been shown to be important for axon termination in the lamina. We report that Bks represses Run in two of the outer R cells: R2 and R5. Expression of Run in R2 and R5 causes axonal mistargeting of all six outer R cells (R1-R6) to the inappropriate layer, without altering expression of cell-specific developmental markers.

Spatio-temporal relays control layer identity of direction-selective neuron subtypes in Drosophila.

Visual motion detection in sighted animals is essential to guide behavioral actions ensuring their survival. In Drosophila, motion direction is first detected by T4/T5 neurons. Their axons innervate one of the four lobula plate layers. How T4/T5 neurons with layer-specific representation of motion-direction preferences are specified during development is unknown. We show that diffusible Wingless (Wg) between adjacent neuroepithelia induces its own expression to form secondary signaling centers. These activate Decapentaplegic (Dpp) signaling in adjacent lateral tertiary neuroepithelial domains dedicated to producing layer 3/4-specific T4/T5 neurons. T4/T5 neurons derived from the core domain devoid of Dpp signaling adopt the default layer 1/2 fate. Dpp signaling induces the expression of the T-box transcription factor Optomotor-blind (Omb), serving as a relay to postmitotic neurons. Omb-mediated repression of Dachshund transforms layer 1/2- into layer 3/4-specific neurons. Hence, spatio-temporal relay mechanisms, bridging the distances between neuroepithelial domains and their postmitotic progeny, implement T4/T5 neuron-subtype identity.

Lapsyn controls branch extension and positioning of astrocyte-like glia in the Drosophila optic lobe.

Astrocytes have diverse, remarkably complex shapes in different brain regions. Their branches closely associate with neurons. Despite the importance of this heterogeneous glial cell type for brain development and function, the molecular cues controlling astrocyte branch morphogenesis and positioning during neural circuit assembly remain largely unknown. We found that in the Drosophila visual system, astrocyte-like medulla neuropil glia (mng) variants acquire stereotypic morphologies with columnar and layered branching patterns in a stepwise fashion from mid-metamorphosis onwards. Using knockdown and loss-of-function analyses, we uncovered a previously unrecognized role for the transmembrane leucine-rich repeat protein Lapsyn in regulating mng development. lapsyn is expressed in mng and cell-autonomously required for branch extension into the synaptic neuropil and anchoring of cell bodies at the neuropil border. Lapsyn works in concert with the fibroblast growth factor (FGF) pathway to promote branch morphogenesis, while correct positioning is essential for mng survival mediated by gliotrophic FGF signaling.How glial cells, such as astrocytes, acquire their characteristic morphology during development is poorly understood. Here the authors describe the morphogenesis of astrocyte-like glia in the Drosophila optic lobe, and through a RNAi screen, they identify a transmembrane LRR protein-Lapsyn-that plays a critical role in this process.

Neurons and glia: team players in axon guidance.

In the developing nervous system, growth cones follow specific trajectories to reach their target area and ultimately connect with their correct postsynaptic partners. This review focuses on studies in both Drosophila and vertebrates to highlight that mutual interactions between neurons and glia are essential in forming specific neuronal connections. Glia signal to neurons to direct pathfinding and targeting of axons, as well as to stabilize and refine axonal branches within the target area. Equally, neurons provide crucial information to glia, supporting their migration and correct positioning.

Versatile genetic paintbrushes: Brainbow technologies.

UNLABELLED: Advances in labeling technologies are instrumental to study the developmental mechanisms that control organ formation and function at the cellular level. Until recently, genetic tools relied on the expression of single markers to visualize individual cells or lineages in developing and adult animals. Exploiting the expanding color palette of fluorescent proteins and the power of site-specific recombinases in rearranging DNA fragments, the development of Brainbow strategies in mice made it possible to stochastically label many cells in different colors within the same sample. Over the past years, these pioneering approaches have been adapted for other experimental model organisms, including Drosophila melanogaster, zebrafish, and chicken. Balancing the distinct requirements of single cell and clonal analyses, adjustments were made that both enhance and expand the functionality of these tools. Multicolor cell labeling techniques have been successfully applied in studies analyzing the cellular components of neural circuits and other tissues, and the compositions and interactions of lineages. While being continuously refined, Brainbow technologies have thus found a firm place in the genetic toolboxes of developmental and neurobiologists. For further resources related to this article, please visit the WIREs website. CONFLICT OF INTEREST: The authors have declared no conflicts of interest for this article.

A region-specific neurogenesis mode requires migratory progenitors in the Drosophila visual system.

Brain areas each generate specific neuron subtypes during development. However, underlying regional variations in neurogenesis strategies and regulatory mechanisms remain poorly understood. In Drosophila, neurons in four optic lobe ganglia originate from two neuroepithelia, the outer (OPC) and inner (IPC) proliferation centers. Using genetic manipulations, we found that one IPC neuroepithelial domain progressively transformed into migratory progenitors that matured into neural stem cells (neuroblasts) in a second domain. Progenitors emerged by an epithelial-mesenchymal transition-like mechanism that required the Snail-family member Escargot and, in subdomains, Decapentaplegic signaling. The proneural factors Lethal of scute and Asense differentially controlled progenitor supply and maturation into neuroblasts. These switched expression from Asense to a third proneural protein, Atonal. Dichaete and Tailless mediated this transition, which was essential for generating two neuron populations at defined positions. We propose that this neurogenesis mode is central for setting up a new proliferative zone to facilitate spatio-temporal matching of neurogenesis and connectivity across ganglia.

Neural circuit assembly: economically wired by a single cadherin.

Neurons are thought to acquire shapes and configurations consistent with the wiring optimization principle. A new study sheds light on the underlying molecular mechanisms by demonstrating that N-cadherin-mediated differential adhesion determines relative neurite positioning in developing columnar synaptic modules.

Flybow to dissect circuit assembly in the Drosophila brain.

Visualization of single neurons within their complex environment is a pivotal step towards uncovering the mechanisms that control neural circuit development and function. This chapter provides detailed technical information on how to use Drosophila variants of the mouse Brainbow-2 system, called Flybow, for stochastic labeling of cells with different fluorescent proteins in one sample. We first describe the genetic strategies and the heat shock regime required for induction of recombination events. This is followed by a detailed protocol as to how to prepare samples for imaging. Finally, we provide specifications to facilitate multichannel image acquisition using confocal microscopy.

Localized netrins act as positional cues to control layer-specific targeting of photoreceptor axons in Drosophila.

A shared feature of many neural circuits is their organization into synaptic layers. However, the mechanisms that direct neurites to distinct layers remain poorly understood. We identified a central role for Netrins and their receptor Frazzled in mediating layer-specific axon targeting in the Drosophila visual system. Frazzled is expressed and cell autonomously required in R8 photoreceptors for directing their axons to the medulla-neuropil layer M3. Netrin-B is specifically localized in this layer owing to axonal release by lamina neurons L3 and capture by target neuron-associated Frazzled. Ligand expression in L3 is sufficient to rescue R8 axon-targeting defects of Netrin mutants. R8 axons target normally despite replacement of diffusible Netrin-B by membrane-tethered ligands. Finally, Netrin localization is instructive because expression in ectopic layers can retarget R8 axons. We propose that provision of localized chemoattractants by intermediate target neurons represents a highly precise strategy to direct axons to a positionally defined layer.