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The collaborating groups of Professor Stephen Goodwin and Professor Scott Waddell based in The Centre for Neural Circuits and Behaviour (CNCB) are shedding new light on what precisely constitutes a neuronal cell type.

Along the body axis, the nervous systems of vertebrates and insects like Drosophila show common features: a complex brain is formed in the head region, whereas the nervous system of the body region consists of segmental units with segmental nerves. In vertebrates, this is known as the spinal cord; the equivalent in insects is the ventral nerve cord (VNC).

The study of the neural mechanisms of sensorimotor transformation in the insect VNC has a rich historyThe VNC plays a major role in the motor control of walking and flying, mechanosensory processing of touch and proprioception, and descending and ascending communication with the central brain. These different functions are orchestrated by anatomically discrete segments of the VNC, called neuromeres. Cell types within each neuromere are genetically encoded by developmental programmes. Although a large body of work has investigated how the adult Drosophila VNC is established, many outstanding questions remain. It is, for example, unknown whether the transcription factors involved in establishing the cellular diversity of the VNC also play a role in the form and function of these same neurons in the adult. Although specific features of individual neurons can be plastic, the identity of a terminally differentiated neuron is likely to remain stable throughout the animal’s life. It is therefore essential to understand gene expression and regulation that permit mature neurons to preserve their subtype identity, morphology and connectivity, and maintain neural circuit function throughout adulthood. 

The VNC of the genetically tractable vinegar fly, Drosophila melanogaster, is ideally suited for these studies. Cell diversity in the nervous system and specific connections between cell types are the basis for information processing underlying diverse nervous system functions. Single-cell RNA sequencing is a powerful technology that circumvents the problems associated with tissue heterogeneity by revealing the expression profile of individual cells. It can be used to identify different neuronal types and subtypes and to discover novel cell-specific markers.

In a new CNCB study between the groups of Professor Stephen Goodwin and Professor Scott WaddellAaron AllenMegan NevilleSebastian BirtlesVincent Croset and Christoph Treiber generated a single-cell atlas of the adult fly VNC to chart the types and properties of cells for future studies of brain development and function. They used single-cell RNA-sequencing to characterise the transcriptomes of individual cells in a 5-day old adult Drosophila VNC.

The team generated an atlas of the VNC with 26,768 single-cell transcriptional profiles that define more than 100 cell types. This analysis reveals that the VNC has a roughly equal number of inhibitory GABAergic neurons and excitatory cholinergic neurons, and that genes encoding preproneuropeptides are amongst the most highly expressed. The segmentally repeating nature and developmental history of the VNC is born out in the cellular transcriptomes, as maintained expression of Hox and several other neuronal lineage marker genes persist. In addition, these profiles provide many novel markers for known and new cell types. This single-cell atlas of the adult Drosophila VNC provides a useful resource to help connect cellular identity to behaviourally relevant neural circuit function.

“Our results suggest many new directions for further investigation,” said Aaron Allen. “For example, understanding the correspondence and potential causal relationships between transcriptomic signatures and specific anatomical, physiological and functional properties, and how these relationships change depending on cell state.”

The full paper, "A single-cell transcriptomic atlas of the adult Drosophila ventral nerve cord" is available to read in eLife.

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