Take-off! Researchers map fly neural networks controlling wing and leg movement

One of the main functions of the neural system is to coordinate movements of the body. In order to understand how brain controls adaptive motor behaviours, scientists have long been trying to decipher the neural circuit map down to the muscles.

Now researchers have connected the neurons from the fruit fly nerve cord, similar to the vertebrate spinal cord, with the muscles controlling the legs and wings. This sheds light on how the fly senses and controls movements of the legs and wings. Whilst connectomes in small animals have been mapped before, it is the first time that scientists find the synapse-level wiring diagram of motor circuits for a limbed animal.

Why the fruit fly? Drosophila melanogaster has a compact nervous system with sophisticated genetic tools and identified neurons with stereotyped function across individuals. “They are marvelously skilled at motor control, including walking and flying, despite their tiny nervous system”, explains John Tuthill, researcher at the University of Washington and corresponding author of the publication. Indeed, each of the flies’ legs is controlled only by 70 motor neurons (compared to 600 in a cat’s calf muscle) and only 29 motor neurons controlling each wing.

The fly has specialised muscles for power and steering in the wings. These wing muscles attach to different body parts: power muscles to the thorax and steering muscles to the wing hinge.  

The team has now determined which pre-motor neurons in the fly’s version of the spinal cord, known as ventral nerve cord (VNC), coordinate motor neurons controlling the leg and wing. To achieve this, they used several techniques: electron microscopy, sparse genetic labelling, and X-ray holographic nanotomography (XNH) at the ESRF.

Connectomics using X-rays

Electron microscopy showed the VNC network, with 45 million synapses and 14,600 neuronal cell bodies. They paired this data with maps of leg and wing motor neurons using XNH at the ESRF. “Mapping motor neurons to their muscles with XNH was essential to interpreting premotor network organization in the context of motor neurons function,” says Wei-Chung Lee, principal investigator at Harvard University and one of the corresponding authors. He adds: “ESRF ID16A is currently the only beamline in the world with the combination of imaging field-of-view and resolution to densely reconstruct neuronal wiring at such scales.”

Over the last few years the ESRF team led by scientist Alexandra Pacureanu has been developing X-ray holographic nanotomography to address the specific challenges posed by the connectomics field. Pacureanu is supported by an ERC Starting Grant to pursue this research. “Connectomics based on X-ray microscopy is an emerging field that stemmed at ESRF thanks to my ERC grant. Recently we have improved both the spatial resolution and the scalability of this technology to enable the exploration of meaningful neural circuits”, she says.

Alexandra Pacureanu on beamline ID16A. Credits: S. Candé.

She adds: “The collaboration with neuroscientists has been crucial to leverage X-ray microscopy for pushing the frontiers of understanding how the nervous system functions”. 

Future work includes identifying differences in neuronal network wiring between individuals, sexes, over development, across species, and in response to injury or disease.

Reference:

Azevedo A., et al, Nature, 26 June 2024. https://doi.org/10.1038/s41586-024-07389-x

Text by Montserrat Capellas Espuny

Top image: The circuits controlling the wing steering (left) and the wing power (right) of the fruit fly. Credits: Azevedo et al., Nature 2024.