Drosophila larvae sense and react to a wide range of stimuli and carry out many motor behaviors. These abilities are controlled by a relatively small number of neurons (about 10,000) that can be grouped into about 300 morphologically distinct neuron classes. Using the remarkable genetic toolkit generated by the Rubin lab at Janelia, we can selectively and reproducibly label and manipulate each of these neuron classes.
Our first goal is to investigate the effect of activating and inactivating single neuron classes on larval sensory processing, decision making, and motor production. For this purpose we have developed a set of automated high-throughput behavioral assays.
Our second goal is to target expression of genetically encoded Ca2+ indicators to specific neuron classes and to monitor Ca2+ signals in behaving animals. This should allow us to correlate Ca2+ signals in a neuron class with "perception" of specific stimuli and "generation" of specific reactions.
Our third goal is to identify molecules that are required in specific neuron classes for specific behaviors. Again, we can do this by selectively targeting RNAi (RNA interference) against candidate genes to specific neuron classes and testing the animal's performance in behavioral assays.
Together these approaches should provide insights into how the 300 neuron classes in this little animal generate a remarkable set of behaviors. Larval behavior may be simple when compared to human behavior, but we humans (despite the remarkable complexity of our nervous systems) do not yet understand even the behavior of a little larva, let alone that of humans. We hope that these studies of the little larva will bring us a step closer to understanding the neural and genetic basis of behavior in general.
All organisms react to noxious and mechanical stimuli but we still lack a complete understanding of cellular and molecular mechanisms by which somatosensory information is transformed into appropriate motor outputs. The small number of neurons and excellent genetic tools make Drosophila larva an especially tractable model system in which to address this problem. We developed high throughput assays with which we can simultaneously expose more than 1,000 larvae per man-hour to precisely timed noxious heat, vibration, air current, or optogenetic stimuli. Using this hardware in combination with custom software we characterized larval reactions to somatosensory stimuli in far greater detail than possible previously. Each stimulus evoked a distinctive escape strategy that consisted of multiple actions. The escape strategy was context-dependent. Using our system we confirmed that the nociceptive class IV multidendritic neurons were involved in the reactions to noxious heat. Chordotonal (ch) neurons were necessary for normal modulation of head casting, crawling and hunching, in response to mechanical stimuli. Consistent with this we observed increases in calcium transients in response to vibration in ch neurons. Optogenetic activation of ch neurons was sufficient to evoke head casting and crawling. These studies significantly increase our understanding of the functional roles of larval ch neurons. More generally, our system and the detailed description of wild type reactions to somatosensory stimuli provide a basis for systematic identification of neurons and genes underlying these behaviors.
Longitudinal axon fascicles within the Drosophila embryonic CNS provide connections between body segments and are required for coordinated neural signaling along the anterior-posterior axis. We show here that establishment of select CNS longitudinal tracts and formation of precise mechanosensory afferent innervation to the same CNS region are coordinately regulated by the secreted semaphorins Sema-2a and Sema-2b. Both Sema-2a and Sema-2b utilize the same neuronal receptor, plexin B (PlexB), but serve distinct guidance functions. Localized Sema-2b attraction promotes the initial assembly of a subset of CNS longitudinal projections and subsequent targeting of chordotonal sensory afferent axons to these same longitudinal connectives, whereas broader Sema-2a repulsion serves to prevent aberrant innervation. In the absence of Sema-2b or PlexB, chordotonal afferent connectivity within the CNS is severely disrupted, resulting in specific larval behavioral deficits. These results reveal that distinct semaphorin-mediated guidance functions converge at PlexB and are critical for functional neural circuit assembly.
Prior Publications (2 of 1)
During the development of neural circuitry, neurons of different kinds establish specific synaptic connections by selecting appropriate targets from large numbers of alternatives. The range of alternative targets is reduced by well organised patterns of growth, termination, and branching that deliver the terminals of appropriate pre- and postsynaptic partners to restricted volumes of the developing nervous system. We use the axons of embryonic Drosophila sensory neurons as a model system in which to study the way in which growing neurons are guided to terminate in specific volumes of the developing nervous system. The mediolateral positions of sensory arbors are controlled by the response of Robo receptors to a Slit gradient. Here we make a genetic analysis of factors regulating position in the dorso-ventral axis. We find that dorso-ventral layers of neuropile contain different levels and combinations of Semaphorins. We demonstrate the existence of a central to dorsal and central to ventral gradient of Sema 2a, perpendicular to the Slit gradient. We show that a combination of Plexin A (Plex A) and Plexin B (Plex B) receptors specifies the ventral projection of sensory neurons by responding to high concentrations of Semaphorin 1a (Sema 1a) and Semaphorin 2a (Sema 2a). Together our findings support the idea that axons are delivered to particular regions of the neuropile by their responses to systems of positional cues in each dimension.
Drosophila sensory neurons form distinctive terminal branch patterns in the developing neuropile of the embryonic central nervous system. In this paper we make a genetic analysis of factors regulating arbor position. We show that mediolateral position is determined in a binary fashion by expression (chordotonal neurons) or nonexpression (multidendritic neurons) of the Robo3 receptor for the midline repellent Slit. Robo3 expression is one of a suite of chordotonal neuron properties that depend on expression of the proneural gene atonal. Different features of terminal branches are separately regulated: an arbor can be shifted mediolaterally without affecting its dorsoventral location, and the distinctive remodeling of one arbor continues as normal despite this arbor shifting to an abnormal position in the neuropile.