Imagine your brain wiring itself wrong – suddenly, the scent of roses could trigger disgust, or the smell of danger could become irresistibly alluring. That's the high-stakes game of neural circuit formation, and scientists are just beginning to understand the rules. Without this fundamental knowledge, we're left in the dark about how our brains build the very circuits that drive our behavior.
But here's some exciting news: researchers at the Wu Tsai Neurosciences Institute at Stanford have made significant strides in decoding this intricate process. Their groundbreaking work, published in two papers in Nature (November 19, 2025), sheds light on the push-and-pull forces that guide neurons to form the correct connections. Led by neurobiologist Liqun Luo, the team first investigated the mechanisms guiding neuron wiring in the olfactory system of fruit flies – a region of the brain dedicated to the sense of smell. Then, in a stunning display of control, they successfully rewired these circuits, directly altering the flies' behavior.
"As Richard Feynman famously said, 'What I cannot create, I do not understand,'" explains postdoctoral fellow Cheng Lyu, who co-led the research with graduate student Zhuoran Li. "Now that we've demonstrated the ability to manipulate these connections, we're one giant leap closer to fully understanding the entire system."
The Old Wiring Puzzle: More Complicated Than We Thought
Neuroscientists have already uncovered a wealth of information about how neurons establish synaptic links to build functional circuits. New discoveries are constantly emerging. However, the initial process of neurons finding their correct partners – sometimes across surprisingly long distances – has remained a major challenge.
Think of it like this: even in a tiny insect brain, there are thousands of neurons, each belonging to one of dozens of different types. If these neurons don't connect in the proper way, the resulting circuits might not function as intended, leading to unpredictable and potentially harmful behaviors.
Take the fruit fly olfactory circuit as an example. Roughly 50 distinct types of neurons receive smell signals from the antennae, while another 50 or so types relay these signals deeper into the brain. If these neurons pair up incorrectly, a fruit fly might mistakenly be drawn to decaying matter instead of ripe fruit.
(Why fruit flies, you ask? Because their brains are relatively simple compared to mammals like mice or humans, and because researchers have a vast toolkit for genetically manipulating individual neuron types. This makes it far easier to observe and understand what's happening inside the fly brain.)
Over six decades ago, neurobiologist Roger Sperry proposed a revolutionary idea: neurons might possess chemical "tags" – molecules expressed on their surfaces – that allow them to recognize and connect with their appropriate partners.
This hypothesis has largely been proven correct, but it's not the whole story. The sheer number of neurons in the brain far exceeds the number of chemical tags identified so far, suggesting that there must be other mechanisms at play.
Earlier this year, Luo, Lyu, Li, and their colleagues uncovered one crucial piece of the puzzle. They found that neurons extend their axons – the long, slender branches that transmit signals to downstream neurons – along predetermined pathways, rather than randomly searching the entire brain region. This significantly narrows down the possibilities. But here's where it gets controversial... Even with chemical tags and restricted search spaces, each neuron still encounters a multitude of potential partners. So, what determines the final choice?
A Neuron's "Gag Reflex": The Power of Repulsion
In the first of the two new Nature papers, Li and her team explored whether the nature of the chemical tags themselves could provide the missing piece. Sperry's original hypothesis focused on "attractive" chemical tags, where a neuron's axon grows toward others with matching tags.
But there's another equally important possibility: repulsion. Previous research has demonstrated that both attraction and repulsion play a role in guiding axon growth. Repulsive tags can prevent a neuron from forming synapses with itself.
However, the role of repulsion in the final stages of synaptic partner matching was less clear. So, Li set out to identify some repulsive tags. The team focused on two types of olfactory neurons that sense different smells but share the same attractive tags. This meant that attraction alone couldn't explain how these neurons distinguished between their correct partners.
Leveraging a single-cell RNA sequencing database – created as part of the Neuro-Omics Initiative, a 2018 Wu Tsai Neuro Big Ideas project – Li and her colleagues identified three genes that produce previously unknown chemical tags. When they deactivated these genes, they observed cross-wiring in the brain circuits: axons that normally connected to only one of the two neuron types now connected to both. This strongly suggested that the newly identified tags acted as repulsive signals, preventing certain neurons from forming connections with each other.
Creating is Understanding: Rewiring the Brain
To truly validate their understanding of the roles of attraction and repulsion in synaptic partner matching, the team decided to demonstrate that they could control how olfactory circuits formed in the fruit fly brain.
In the second Nature paper, Lyu and the team did just that. By manipulating gene expression, they altered a specific type of olfactory receptor neuron in three key ways: increasing repulsion between its usual partners, decreasing repulsion between new partners, and increasing attraction between new partners.
The results were remarkable. Lyu and his colleagues showed that these changes physically rewired the fruit flies' brain circuits. And this is the part most people miss... This rewiring directly altered the flies' behavior. Normally, the receptor neuron they studied plays a crucial role in mating behavior, specifically discouraging male flies from attempting to mate with other males.
However, the rewired male flies now attempted to court both male and female partners. They chased other males, vibrated their wings to produce courtship songs, and displayed other signs of attempted mating.
Still More to Learn: A Glimmer of Hope
These findings represent significant progress in unraveling the mysteries of brain circuit formation, Luo emphasizes. By demonstrating the ability to control which neurons connect with each other and, consequently, to influence behavior, they have gained a detailed understanding of how neurons form the links that underlie brain circuits – at least in the fruit fly olfactory system.
Now, the team plans to investigate how other types of neurons wire up both within the fly olfactory system and throughout the rest of the fly brain. They are also eager to explore whether the wiring principles they have discovered apply to other animals, such as mice.
"This is an important milestone in one part of one circuit," Luo concludes. "Now, the burning 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
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What do you think? Could these findings revolutionize our understanding of brain disorders linked to faulty wiring, such as autism or schizophrenia? Do you believe that the principles discovered in fruit flies will hold true in more complex brains like our own? Share your thoughts and opinions in the comments below!
