Could rewiring the brain’s “brakes” improve movement in Huntington’s disease?

For decades, Huntington’s disease (HD) research has focused on what goes wrong inside vulnerable brain cells. But neurons don’t work alone, they constantly communicate with one another as part of intricate networks called neural circuits. Understanding how these circuits malfunction could reveal entirely new ways to treat the disease.

A new study published in Nature takes exactly this approach. Rather than looking directly at expanded huntingtin or dying neurons, the researchers asked a different question: what if the movement problems in HD arise because the brain’s communication networks have become unbalanced?

Although these findings are still limited to mice that model HD, they suggest that correcting abnormal brain circuit activity could become a possible therapeutic strategy for HD.

The brain keeps activity in balance

Every thought, memory, and movement depends on vast networks of neurons talking to one another using tiny electrical signals. The billions of neurons that make up the brain are organized into circuits, groups of connected cells that pass information back and forth to control everything we do.

For these circuits to work properly, there needs to be a careful balance between neurons that increase activity (excitatory neurons) and those that hold it back (inhibitory neurons). We can think of them as the brain’s accelerators and brakes. If the balance tips too far in either direction, the messages travelling through the circuit become distorted, and behaviour can suffer.

*For brain circuits to work properly, there needs to be a careful balance between neurons that increase activity (excitatory neurons) and those that hold it back (inhibitory neurons). Image credit:* *Pexels*

One of the brain regions responsible for planning and controlling movement is the motor cortex. Excitatory neurons in the cortex send movement commands to another brain region called the striatum, which is particularly vulnerable in HD. Surrounding these excitatory neurons are inhibitory neurons, which act as local regulators, fine-tuning the strength and timing of movement signals.

A group of researchers from the US and Germany set out to investigate whether changes in the activity of inhibitory neurons can help explain why movements become impaired in HD.

Watching brain circuits at work

To answer these questions, the researchers needed to watch neurons communicating in real time while animals were moving!

They used two-photon calcium imaging, a lab technique that allows scientists to monitor the activity of hundreds of individual neurons simultaneously by detecting brief changes in calcium levels that occur when neurons communicate. To make it possible, the researchers implanted a tiny transparent window over the motor cortex of each mouse. This let them observe how different brain cells behaved while mice performed a walking task on a motorized ladder.

The team focused on four key populations of neurons in the motor cortex. Three were different types of inhibitory neurons that regulate the activity of nearby cells. The fourth population consisted of corticostriatal neurons, the excitatory neurons that send movement-related signals from the cortex to the striatum. Together, these four cell types form a circuit that helps coordinate smooth, precise movement.

A circuit out of balance

The researchers discovered a striking pattern.

As HD mice developed movement problems, one group of inhibitory neurons, called VIP, became much less active during movement. At the same time, two other inhibitory neuron types, SST and PV interneurons, became unusually active. Meanwhile, the excitatory neurons that send movement commands to the striatum became less active.

At first glance, this might seem confusing. Why would one inhibitory neuron become quieter while two others become more active? The answer lies in the unique role of VIP neurons.

Unlike most inhibitory neurons, VIP neurons don’t mainly inhibit excitatory neurons. Instead, they inhibit other inhibitory neurons. In effect, they temporarily release the brakes, allowing movement-related signals to flow through the circuit at just the right moment.

This work opens a new way of thinking about HD, not simply as the loss of neurons, but as a disorder of brain circuits.

If VIP neurons become less active, those brakes are never fully released. This weakens the movement-related signals sent from the motor cortex to the striatum, making purposeful movements harder to initiate, coordinate, and control, consistent with the movement challenges of HD.

The researchers observed the same general pattern when the mice were allowed to move freely, groom themselves, or rest. This suggests that the imbalance was not unique to the ladder task. They saw this pattern in two different types of HD mice.

Can the circuit be repaired?

Having identified VIP interneurons as a potential weak link in the circuit, the researchers asked: Could restoring the activity of these neurons rebalance the circuit and improve movement in a mouse that shows very severe deficits?

To answer this, they turned to optogenetics, a technique that allows scientists to activate specific neurons using light.

The researchers genetically engineered VIP interneurons to produce a light-sensitive protein called ChrimsonR. By delivering brief pulses of red light through the same cranial window used for imaging, they could activate just the VIP neurons, while simultaneously monitoring the activity of the surrounding circuit.

*Circuit repair could be a powerful approach! Activating VIP interneurons restored their electrical activity and brought the rest of the circuit back into balance.* Image credit: Bulat843

The results were striking. Activating VIP interneurons restored their electrical activity to levels seen in healthy mice. Just as importantly, it also brought the rest of the circuit back into balance. The overactive SST and PV interneurons became less active, while the underactive corticostriatal neurons regained much of their normal movement-related activity.

In other words, restoring activity in a single type of neuron was enough to normalize communication across the wider motor circuit.

Better circuits, better movement

Did these changes in brain activity actually matter for behaviour? The answer was yes.

After VIP interneurons were stimulated, HD mice walked better. Their movements became more precise, and many of the movement problems seen before stimulation were significantly improved.

Perhaps the most surprising finding was that these improvements lasted for several days after the light stimulation had stopped. Rather than producing only a temporary boost in activity, activating VIP interneurons appeared to leave the circuit in a healthier state, suggesting that even brief interventions may promote longer-lasting changes in how the brain functions.

Caveats and Takeaways

Mice are not people. However, watching the activity of specific types of brain cells in real time is not possible in humans, making mouse models particularly useful for understanding how HD disrupts brain circuits.

Importantly, the way researchers activated VIP neurons—genetic engineering combined with light stimulation—is an experimental tool, not currently a treatment that could be used in people.

Still, this work offers a different perspective on HD. While much research focuses on what goes wrong with DNA, RNA and proteins inside cells, this study zooms out to show how changes in the way brain cells communicate with each other can shape behaviour—and that restoring balance to these networks can make a difference.

Summary

A specific group of inhibitory brain cells (VIP interneurons) become unusually quiet in mice that model HD, disrupting the balance of activity in motor circuits.
This imbalance causes other inhibitory neurons to become overactive, preventing movement-promoting neurons from sending strong signals to the striatum.
Using optogenetics, a technique that activates selected neurons with light, the researchers restored VIP neuron activity, rebalanced the circuit, and normalized communication between brain cells.
HD mice showed improved movement for several days after treatment, suggesting that restoring healthy brain circuit activity could become a new therapeutic strategy, although much more work is needed before this could be applied to people.

Could rewiring the brain’s “brakes” improve movement in Huntington’s disease?

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