New tool reveals details about motor neuron circuitry in fruit flies.
How do networks of neurons connect to form functional circuits? To address this long-standing question in neuroscience, researchers from Boston Children's Hospital and Harvard Medical School have developed a new pipeline to study neural circuits and, in the process, learn more about the connections between them.
The technique, an automated electron microscopy system that the researchers dubbed GridTape, enables comprehensive, high-resolution imaging of large neural circuits.
As a proof of principle, the team used GridTape to map more than 1,000 motor and sensory neurons involved in motor function control in fruit flies—a dataset that sheds light on previously unknown neural types and helps provide insights into neural function and behavior.
Their findings are reported in a new paper published in Cell.
“Neural networks are extensive, but the connections between them are really small,” said Wei-Chung Allen Lee, HMS assistant professor of neurology at Boston Children's. “So, we have had to develop techniques to see them in extremely high-resolution over really large areas and volumes.”
For decades, researchers have relied on electron microscopy to visualize extremely small structures. However, electron microscopy provides such high image resolution that it has been difficult to study whole neural circuits, according to Lee, whose lab is interested in learning how neural circuits underlie function and behavior.
“To improve the technique, we developed an automated system to image at high resolution but at the scale to encompass neuronal circuits,” said Lee, a faculty member of the F.M. Kirby Neurobiology Center.
Traditional electron microscopy requires hand collection of thousands of tissue samples that are sliced in 40 nanometer-thick sections, or about a thousand times thinner than a human hair.
In contrast, GridTape automates collection of the samples and assigns a barcode to each section. These samples are then applied onto a conveyor belt that can then be fed through an electron microscope like a movie projector. An advantage of the technique is that every neuron is labeled in each tissue section.
“As the electrons pass through each section, we can image each neuron in fine detail,” Lee explained. “And because all of the sections have a barcode, we know exactly where each of these sections comes from so we can reassemble the sample and subsequently reconstruct the neuronal circuits embedded within.”
With the goal of creating a comprehensive map of the neuronal circuits that control motor function in Drosophila melanogaster fruit flies, the team used GridTape to study the fly’s ventral nerve cord. This structure, analogous to a spinal cord, contains all the neural circuits the fly uses to move its limbs.
“By applying this method to the entire nerve cord, we were able to reconstruct all of its motor neurons, as well as a large population of sensory neurons,” Lee said.
The team created a map of more than 1,000 motor and sensory neuron reconstructions. Their results revealed a specific kind of sensory neuron in the fly thought to detect changes in load, like body weight.
“These neurons are very large, relatively rare in number, and they make direct connections onto motor neurons of the same type on both sides of the body,” Lee said. “We believe this may be a circuit that helps stabilize body position.”
The team has made this map available on an open registry. “It allows anyone in the world to access this data set and look at any neuron that they're interested in and ask what neurons are connected to it,” Lee said.
In their paper, the team provides GridTape instrumentation designs and software to make the technique accessible and affordable to the larger scientific research community.
With the ability to map ever-larger neural circuits, Lee believes this technique could be useful for studying neuronal circuits in larger brains and testing predictions about neural function and behavior.
“This new technique allows us to do electron microscopy faster and in an automated way, with high quality, yet at a reasonable price,” said Lee.
His team and teams at the Allen Institute and at Princeton are using the technique to study mice, and researchers in the U.K. and Japan are applying the technique across multiple animal systems. The technology has broader potential uses where large numbers of samples need to be imaged at very high resolution.
“In principle, various forms of microscopy could be advanced by integrating this technique” Lee said. This includes biomedical diagnostic imaging, spatially resolved sequencing, or any where people need to generate massive amounts of nanoscale data, he added.