A window into the fruit fly nervous system

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The dynamics and connectivity of neural circuits change continuously on time scales ranging from milliseconds to an animal’s lifespan. Therefore, to understand biological networks, minimally invasive methods are needed to record them in animals that repeatedly behave.

EPFL scientists have developed an implantation technique that allows unprecedented optical access to the “spinal cord” of the fruit fly, Drosophila melanogaster.

Scientists have attempted to numerically recapitulate the principles underlying motor control in Drosophila. In 2019, they developed DeepFly3D– deep learning-based motion capture software that uses multiple camera views to quantify the 3D limb movements of behaving flies. In 2021, they developed Ramdya’s team revealed LiftPose3D– a method to reconstruct 3D animal poses from 2D images taken from a single camera.

These efforts were complemented by their publication in 2022 through NeuroMechFly– the first morphologically accurate digital “twin” of Drosophila.

But there are always more challenges to overcome. The goal is not only to map and understand an organism’s nervous system – an ambitious task in itself – but also to discover how to develop bio-inspired robots as agile as flies.

Ramya said, “The hurdle we had before this work was that we could only record fly motor circuits for a short time before the animal’s health deteriorated.”

Thus, scientists at EPFL’s Faculty of Engineering have developed tools to monitor neuronal activity in Drosophila for longer periods of time.

Laura Hermans, Ph.D. student who led the project, said: “We have developed micro-engineered devices that provide optical access to the animal’s ventral nerve cord. We then surgically implanted these devices into the fly’s thorax.

“One of these devices, an implant, allows us to push the fly’s organs aside to reveal the ventral nerve cord underneath. We then seal the thorax with a transparent microfabricated window. Once we have flies with these devices, we can record the fly’s behavior and neuronal activity across many experiments over long periods of time.

These tools allow the prolonged observation of a single animal by scientists. Now they can conduct studies that last for days or even the life of the fly, rather than just hours.

Hermans said, “For example, we can study how an animal’s biology adapts during disease progression. We can also study changes in the activity and structure of neural circuits during aging. The ventral nerve cord of the fly is ideal because it houses the animal’s motor circuits, allowing us to study the evolution of locomotion over time or after injury.

Selman Sakar said: “As engineers, we look for well-defined technical challenges. Pavan’s group developed a dissection technique to remove organs from the fly that block the field of vision and visualize the ventral nerve cord. However, flies can only survive for a few hours after surgery. We were convinced that an implant should be placed in the chest. Similar techniques exist for visualizing the nervous system of larger animals such as rats. We were inspired by these solutions and started thinking about the issue of miniaturization.

Early designs attempted to solve the problem of safely retaining and removing the fly’s internal organs to expose the ventral nervous system while allowing the fly to survive post-surgery.

sakar said, “For this challenge, you need someone who can approach a problem with both a life science and engineering perspective – this highlights the importance of Laura [Hermans] and that of Murat [Kaynak] work.”

Only a few flies survived the first implants because they were stiff. Several design changes were required to increase survival rates without degrading imaging quality. The winning design – a conforming V-shaped implant that can safely spread the fly’s organs and reveal the ventral cord – is simple but effective. This allowed scientists to seal the hole on the cuticle with a ‘barcoded thoracic window’, allowing them to observe the ventral nerve cord and measure neural activity during the fly’s daily life. .

sakar said, “Given the anatomical variations from animal to animal, we had to find a safe and adaptive solution. Our implant meets this particular need. We provide a versatile toolkit for neuroscience research, as well as the development of appropriate tissue micromanipulation tools and a 3D nanoimprint-compliant step for mounting animals in repeat imaging sessions.

Ramya said, “By studying the fly, we believe that understanding something relatively simple can lay the groundwork for understanding more complex organisms. When you learn math, you don’t dive into linear algebra; you first learn to add and subtract. Also, for robotics, it would be fantastic to understand how even a “simple” insect works. »

“The next step for the team is to use their new methodology to unravel the mechanisms controlling Drosophila movements. Biological systems are unique compared to artificial systems in that they can dynamically modulate, for example, the excitability of neurons or the strength of synapses. So to understand what makes biological systems so agile, we need to be able to observe this dynamism. In our case, we would like to examine how, for example, motor systems react to course of an animal’s life as it ages or during recovery from injury.

Journal reference:

  1. Laura Hermans, Murat Kaynak, Jonas Braun, et al. Micro-engineered devices allow long-term imaging of the ventral nerve cord in behaving adult Drosophila. Nature Communications, August 25, 2022. DOI: 10.1038/s41467-022-32571-y
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