New neuron connections exposed

Editorial

Rebecca Pool

Monday, November 13, 2017 - 15:15
Image: A reconstruction of neurons in the adult fly mushroom body.
 
Using high resolution electron microscopy, US-based researchers have discovered new neural connections in the brain of a fruit fly.
 
Two teams of researchers from the Janelia Research Campus have independently mapped a brain region critical for memory and learning in the fruit fly, Drosophila melanogaster, in the larval and adult life stages.
 
Their findings have added some unexpected neural connections, or synapses, to a seemingly solved circuit.
 
A projection neuron (blue) makes contact with Kenyon cells (stars) in the mushroom body of larval flies. [Eichler et al./Nature 2017]
 
"These results underscore the need for deeply detailed brain mapping, and make a case for using high-resolution electron microscopy to do it," says Janelia neurobiologist, Albert Cardona from one of the research teams.
 
"Even in the most well-studied parts of the fly brain, there’s still a lot more for scientists to see," he adds.
 
The latest research forms part of a larger effort to map the entire Drosophila nervous system.
 
Cardona and his team from Janelia capture a complete reconstruction of the Drosophila larva mushroom body. [Eichler et al.]
 
Cardona and collaborators are looking at the nervous system in larvae.
 
Meanwhile, the second team of researchers from Janelia's 'FlyEM project' are focusing on adult flies.
 
The studies employed two different EM methods: TEM in the larvae and focused-ion beam scanning electron microscopy in the adult.
 
A reconstruction of neurons in the adult fly mushroom body – including Kenyon cells (yellow), mushroom body output neurons (purple), and dopaminergic neurons (green) – highlights unexpected connections. [Takemura et al./eLife 2017]
 
Both studies targeted the mushroom body, the area of the brain responsible for associative learning.
 
The mushroom body consists of multiple compartments, three of which make up the 'alpha lobe' in the adult. FlyEM zeroed in on this region, mapping a total of 983 neurons.
 
Cardona’s approach differed slightly. Neurons in the larva’s nervous system are smaller compared to the adult fly, so he was able to trace all 300 neurons of the larval mushroom bodies.
 
Much to their surprise, FIB-SEM revealed three classes of never-before-seen synapses in the mushroom body, which were observed in both the larva and the adult.
 
These synaptic connections were formed among three neuron types: Kenyon cells, which report sensory information; dopaminergic neurons, which provide details about the sensory information; and output neurons, which relay information beyond the mushroom body.
 
Three types of Kenyon cells (shown in this reconstruction) stretch throughout the mushroom body in the adult fly brain. [Takemura et al./eLife 2017]
 
"The fact that these synapses were seen in young and full-grown flies implies that the connections are a critical part of the learning and memory circuit," highlights Gerry Rubin, Janelia’s executive director.
 
Janelia’s FlyEM team confirmed the neurons’ existence in adults, showing that the synapses contributed to the function of the mushroom body.
 
Now, the teams are continuing to map the fly and larva nervous systems in full.
 
FlyEM, with ten-fold more neurons to chart, is applying the latest machine vision and artificial intelligence methods in collaboration with Google, as well as using computer algorithms and software developed in-house at Janelia.
 
“This region of the brain had been picked over for decades, and people had already described the major circuits,” says Rubin. “The models had everything all neat and tidy; our research papers say, ‘Well, not so quick.’”
 
Research is published in Nature and eLife.
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