Unraveling the Brain's Wiring: A Journey into Neural Circuits
Imagine a world where your sense of smell leads you astray, where a whiff of turpentine becomes an enticing aroma. This is the intriguing puzzle neuroscientists are tackling, and it's a crucial piece of the brain's complex wiring system.
While we've made significant strides in understanding how neurons form connections, the precise mechanisms that guide these connections, especially over long distances, remain elusive. This knowledge gap hinders our understanding of how the brain constructs the circuits that govern our behaviors.
Enter the groundbreaking work of Wu Tsai Neuro scientists, who, in a pair of papers published in Nature on November 19, 2025, have taken us a step closer to unraveling the brain's intricate wiring.
Led by neurobiologist Liqun Luo, the team first delved into the forces that govern neuron wiring in the brain regions responsible for the sense of smell in fruit flies. They then demonstrated their ability to rewire these systems, altering the flies' behavior.
"As Richard Feynman said, 'What I cannot create, I do not understand,'" reflected postdoctoral fellow Cheng Lyu. "By doing this, we've gained a deeper understanding of the system."
An Age-Old Puzzle
Neuroscientists have made remarkable progress in understanding how neurons form synaptic links, and new discoveries continue to emerge. However, the initial process of neurons finding their correct partners, often across vast distances, has proven more challenging.
It's a complex task. Even in the tiny brain of an insect, each region contains thousands of neurons in various types. If these neurons don't match up correctly, the circuits and, consequently, the brain's functioning may be compromised.
Take the fruit fly's olfactory circuit, for instance. Here, approximately 50 different types of neurons receive smell signals from the antennae, and another 50 send these signals into the brain. If these neurons don't pair up as expected, a fruit fly might be drawn to wet concrete instead of its usual tasty bananas.
Why fruit flies? Their brains are relatively simple compared to mammals, and the abundance of genetic tools available to researchers makes it easier to observe and manipulate individual neuron types and gene expression.
Over six decades ago, neurobiologist Roger Sperry proposed a solution: neurons might carry chemical tags on their surfaces, helping them find their perfect matches.
While this hypothesis holds true, it's not the whole story. The brain contains too many neurons for the relatively small number of chemical tags discovered so far to solve the matching problem entirely.
Earlier this year, Luo and his team found a way neurons simplify this problem. Instead of searching the entire brain region, axons (the long branches of neurons that send signals) follow predetermined paths. This significantly narrows the search space, but it doesn't solve the problem entirely, as each neuron still encounters multiple potential matches.
The Role of Attraction and Repulsion
In the first of the two papers, the team explored whether the nature of these chemical tags could address the remaining issues. Sperry's original hypothesis focused on "attractive" tags, where a neuron's axon grows towards other neurons with matching tags.
But there's another force at play: repulsion. Previous research has shown that both attraction and repulsion guide axons' search paths. Repulsive chemical tags have been shown to play a role in other wiring processes, such as preventing a neuron from forming synapses with itself.
To investigate this further, graduate student Zhuoran Li and her colleagues focused on two types of olfactory neurons that sense different smells. Crucially, these two types share the same attractive chemical tags, meaning other neurons couldn't distinguish between them using attraction alone.
By drawing on a single-cell RNA sequencing database from the Neuro-Omics Initiative, a 2018 Wu Tsai Neuro Big Ideas project, Li and her team identified three genes that produce previously unknown chemical tags. When they knocked out these genes, the brain circuits became cross-wired, suggesting that these new tags repelled certain types of neurons.
Understanding Through Creation
To truly understand the role of attraction and repulsion in synaptic partner matching, the team decided to demonstrate their ability to control how olfactory circuits formed in the fruit fly brain.
In the second Nature paper, the team manipulated a specific type of olfactory receptor neuron in three ways: increasing repulsion between usual neuron partners, decreasing repulsion between new partners, and increasing attraction between new partners. They showed that these changes physically rewired the fruit flies' brain circuits and altered their behavior.
Ordinarily, the receptor neuron they studied plays a role in mating behavior, discouraging male flies from attempting to mate with other males. However, when rewired, these male flies attempted to court both male and female partners, displaying typical mating behaviors such as wing vibration and courtship songs.
A Step Towards Understanding Brain Circuits
These results are significant, Luo said, as they demonstrate the ability to control which neurons link up and the subsequent behavior. This detailed understanding of how neurons form connections is a crucial step towards unraveling the brain's wiring.
However, the team acknowledges that this is just the beginning. They now need to study how other types of neurons wire up in different parts of the fly olfactory system and throughout the fly brain. They're also curious to see how these wiring principles apply to other animals, such as mice.
"This is an important milestone in one part of one circuit," Luo said. "Now, the question is, 'Does this generalize?'"
Reference: Li Z, Lyu C, Xu C, et al. Repulsions instruct synaptic partner matching in an olfactory circuit. Nature. 2025. doi:10.1038/s41586-025-09768-4.
