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

Clonal identity determines astrocyte cortical heterogeneity

Astrocytes are the most numerous cell type in the brain, where they are known to play multiple important functions. While there is increasing evidence of their morphological, molecular, and functional heterogeneity, it is not clear whether their positional and morphological identities are specified during brain development. We address this problem with a novel strategy to analyze cell lineages through the combinatorial expression of fluorescent proteins. Following in utero electroporation, stochastic expression of these proteins produces inheritable marks that enable the long-term in vivo tracing of glial progenitor lineages. Analyses of clonal dispersion in the adult cortex revealed unanticipated and highly specific clonal distribution patterns. In addition to the existence of clonal arrangements in specific domains, we found that different classes of astrocytes emerge from different clones. This reinforces the view that lineage origin impinges on cell heterogeneity, unveiling a new level of astrocyte diversity likely associated with specific regional functions.

Structure-function relationship of cerebral networks in experimental neuroscience: contribution of magnetic resonance imaging

The analysis of neuronal networks, their interactions in resting condition as well as during brain activation have become of great interest for a better understanding of the signal processing of the brain during sensory stimulus or cognitive tasks. Parallel to the study of the functional networks and their dynamics, the underlying network structure is highly important as it provides the basis of the functional interaction. Moreover, under pathological conditions, some nodes in such a net may be impaired and the function of the whole network affected. Mechanisms such as functional deficit and improvement, and plastic reorganization are increasingly discussed in the context of existing structural and functional networks. While many of these aspects have been followed in human and clinical studies, the experimental range is limited for obvious reasons. Here, animal experimental studies are needed as they permit longer scan times and, moreover, comparison with invasive histology. Experimental non-invasive imaging modalities are now able to perform impressive contributions. In this review we try to highlight most recent new cutting-edge developments and applications in experimental neuroscience of functional and structural networks of the brain, relying on non-invasive imaging. We focus primarily on the potential of experimental Magnetic Resonance Imaging (MRI), but also touch upon micro positron emission tomography (μPET) and optical imaging developments where they are applicable to the topic of the present review.

Sparse and combinatorial neuron labelling

Sparse, random labelling of individual cells is a key approach to study brain circuit organisation and development. An array of methods based on genetic engineering now complements older methods such as Golgi staining, facilitating analysis while providing higher information content. Increasingly refined expression strategies based on transcriptional modulators and site-specific recombinases are used to distribute markers or combinations of markers within specific neuronal subsets. Several trends are emerging: first, increasing labelling density with multiplexed markers to allow more cells to be reliably distinguished; second, using labels to report lineage relationships among defined cells in addition to anatomy; third, coupling cell labelling with genetic manipulations that reveal or perturb cell function. These strategies offer new opportunities for characterizing the fine scale architecture of neuronal circuits, and understanding lineage and functional relations among their cellular components in normal or experimental situations.

Power tools for gene expression and clonal analysis in Drosophila

The development of two-component expression systems in Drosophila melanogaster, one of the most powerful genetic models, has allowed the precise manipulation of gene function in specific cell populations. These expression systems, in combination with site-specific recombination approaches, have also led to the development of new methods for clonal lineage analysis. We present a hands-on user guide to the techniques and approaches that have greatly increased resolution of genetic analysis in the fly, with a special focus on their application for lineage analysis. Our intention is to provide guidance and suggestions regarding which genetic tools are most suitable for addressing different developmental questions.

Tracing cells for tracking cell lineage and clonal behavior

Reconstructing the lineage of cells is central to understanding development and is now also an important issue in stem cell research. Technological advances in genetically engineered permanent cell labeling, together with a multiplicity of fluorescent markers and sophisticated imaging, open new possibilities for prospective and retrospective clonal analysis.

Genetic manipulation of genes and cells in the nervous system of the fruit fly

Research in the fruit fly Drosophila melanogaster has led to insights in neural development, axon guidance, ion channel function, synaptic transmission, learning and memory, diurnal rhythmicity, and neural disease that have had broad implications for neuroscience. Drosophila is currently the eukaryotic model organism that permits the most sophisticated in vivo manipulations to address the function of neurons and neuronally expressed genes. Here, we summarize many of the techniques that help assess the role of specific neurons by labeling, removing, or altering their activity. We also survey genetic manipulations to identify and characterize neural genes by mutation, overexpression, and protein labeling. Here, we attempt to acquaint the reader with available options and contexts to apply these methods.

Pokes, sunburn, and hot sauce: Drosophila as an emerging model for the biology of nociception

The word "nociception" is derived from the Latin "nocere," which means "to harm." Nociception refers to the sensory perception of noxious stimuli that have the potential to cause tissue damage. Since the perception of such potentially harmful stimuli often results in behavioral escape responses, nociception provides a protective mechanism that allows an organism to avoid incipient (or further) damage to the tissue. It appears to be universal in metazoans as a variety of escape responses can be observed in both mammalian and non-mammalian vertebrates, as well as diverse invertebrates such as leeches, nematodes, and fruit flies (Sneddon [2004] Brain Research Review 46:123-130; Tobin and Bargmann [2004] Journal of Neurobiology 61:161-174; Smith and Lewin [2009] Journal of Comparative Physiology 195:1089-1106). Several types of stimuli can trigger nociceptive sensory transduction, including noxious heat, noxious chemicals, and harsh mechanical stimulation. Such high-threshold stimuli induce the firing of action potentials in peripheral nociceptors, the sensory neurons specialized for their detection (Basbaum et al. [2009] Cell 139:267-284). In vertebrates, these action potentials can either be relayed directly to a spinal motor neuron to provoke escape behavior (the so-called monosynaptic reflex) or can travel via spinal cord interneurons to higher-order processing centers in the brain. This review will cover the establishment of Drosophila as a system to study various aspects of nociceptive sensory perception. We will cover development of the neurons responsible for detecting noxious stimuli in larvae, the assays used to assess the function(s) of these neurons, and the genes that have been found to be required for both thermal and mechanical nociception. Along the way, we will highlight some of the genetic tools that make the fly such a powerful system for studies of nociception. Finally, we will cover recent studies that introduce new assays employing adult Drosophila to study both chemical and thermal nociception and provide an overview of important unanswered questions in the field.

Recent advances in imaging embryonic myoblast fusion in Drosophila

Myoblast fusion in the Drosophila embryos is a complex process that includes changes in cell movement, morphology and behavior over time. The advent of fluorescent proteins (FPs) has made it possible to track and image live cells, to capture the process of myoblast fusion, and to carry out quantitative analysis of myoblasts in real time. By tagging proteins with FPs, it is also possible to monitor the subcellular events that accompany the fusion process. Herein, we discuss the recent progress that has been made in imaging myoblast fusion in Drosophila, reagents that are now available, and microscopy conditions to consider. Using an Actin-FP fusion protein along with a membrane marker to outline the cells, we show the dynamic formation and breakdown of F-actin foci at sites of fusion. We also describe the methods used successfully to show that these foci are primarily if not wholly present in the fusion-competent myoblasts.

Multiple new site-specific recombinases for use in manipulating animal genomes

Site-specific recombinases have been used for two decades to manipulate the structure of animal genomes in highly predictable ways and have become major research tools. However, the small number of recombinases demonstrated to have distinct specificities, low toxicity, and sufficient activity to drive reactions to completion in animals has been a limitation. In this report we show that four recombinases derived from yeast--KD, B2, B3, and R--are highly active and nontoxic in Drosophila and that KD, B2, B3, and the widely used FLP recombinase have distinct target specificities. We also show that the KD and B3 recombinases are active in mice.

Neuronal control of Drosophila courtship song

The courtship song of the Drosophila male serves as a genetically tractable model for the investigation of the neural mechanisms of decision-making, action selection, and motor pattern generation. Singing has been causally linked to the activity of the set of neurons that express the sex-specific fru transcripts, but the specific neurons involved have not been identified. Here we identify five distinct classes of fru neuron that trigger or compose the song. Our data suggest that P1 and pIP10 neurons in the brain mediate the decision to sing, and to act upon this decision, while the thoracic neurons dPR1, vPR6, and vMS11 are components of a central pattern generator that times and shapes the song's pulses. These neurons are potentially connected in a functional circuit, with the descending pIP10 neuron linking the brain and thoracic song centers. Sexual dimorphisms in each of these neurons may explain why only males sing.