Fruit flies show US scientists how brain connections work

One hundred thirteen years after Thomas Hunt Morgan, a professor of Zoology at Columbia University started breeding fruit flies in his laboratory in a bid to understand the then elusive transmitter of hereditary information – the gene – the fruit fly is still the means of unravelling the mysteries of the living system. This time it is about complex neural connections in the brain.

Researchers at California Institute of Technology (Caltech) in the US have developed a method to easily see neural connections and the flow of communications in the brain in real time within living fruit flies – Drosophila malanogaster. The human brain is composed of billions of neurons wired together in intricate webs and communicating through electrical pulses and chemical signals. Although neuroscientists have made progress in understanding the brain’s many functions – such as regulating sleep, storing memories, and making decisions – visualising the entire “wiring diagram” of neural connections throughout a brain is not possible using currently available methods.

The new fruit fly research, published in the journal eLife, is a step forward toward creating a map of the entire fly brain’s many connections, which could help scientists understand the neural circuits within human brains as well. “If an electrical engineer wants to understand how a computer works, the first thing that he or she would want to figure out is how the different components are wired to each other. Similarly, we must know how neurons are wired together in order to understand how brains work,” said Carlos Lois, research professor at Caltech.

When two neurons connect, they link together with a structure called a synapse, a space through which one neuron can send and receive electrical and chemical signals to or from another neuron. Even if multiple neurons are very close together, they need synapses to truly communicate. Researchers developed a method for tracing the flow of information across synapses, called TRACT (Transneuronal Control of Transcription). Using genetically engineered Drosophila fruit flies, TRACT allows researchers to observe which neurons are “talking” and which neurons are “listening” by prompting the connected neurons to produce glowing proteins.

With TRACT, when a neuron “talks” – or transmits a chemical or electrical signal across a synapse – it will also produce and send along a fluorescent protein that lights up both the talking neuron and its synapses with a particular colour. Any neurons “listening” to the signal receive this protein, which binds to a so-called receptor molecule – genetically built-in by the researchers – on the receiving neuron’s surface. The binding of the signal protein activates the receptor and triggers the neuron it is attached to in order to produce its own, differently coloured fluorescent protein.

Using a type of microscope that can peer through a thin window installed on the fly’s head, the researchers can observe the colourful glow of neural connections in real time as the fly grows, moves, and experiences changes in its environment.