Miniature lab grown circuits unlock hidden recovery paths for paralysis

Cambridge scientists have grown miniature circuits in the lab that mimic how the brain and spinal cord connect up, which underlies our movements. They used this model to show how damage to these connections previously considered 'irreversible' could, in fact, be reversible.

As we develop and grow from embryo to fetus to infant, our nerve cells (neurons) form connections, allowing information to be transmitted between the brain and the spinal cord. A key component of each neuron is the axon – the nerve fibre 'cable' that transmits information to other neurons to activate muscle contractions.

At some point, we lose the ability to grow axons in the central nervous system, or this ability is at least greatly impaired or slowed down. This means that damage to the brain and spinal cord becomes permanent, leading to devastating disabilities, such as the inability to grasp or walk. This is often the case for traumatic spinal cord injury and can be a feature of many neurological diseases, including motor neurone disease or multiple sclerosis.

In 2021, Dr András Lakatos and colleagues at the University of Cambridge developed 'mini brains' using human patient-derived stem cells – special cells that have the potential to develop into most human cell types – which they guided to grow into pea-sized brain 'organoids'. These organoids were 3D models that resemble parts of the human cerebral cortex. The team used these to demonstrate molecular problems in motor neurone disease and potential ways to prevent them.

Now, in research published in Cell Reports, Dr Lakatos's team has taken its research a step further, building a mini version of the connected human brain and spinal cord system in the lab by recreating these tissues using organoids.

In the human body, the brain and spinal cord tissues are distinct but connected by axons, so the researchers kept the brain and spinal cord organoids apart. They saw that nerve fibres from the brain tissue grew across the gap to connect to the spinal cord, forming a working circuit that could even cause tiny muscle clusters to contract.

By growing this human system in the dish for more than a year, they found that up until around day 150 – which corresponds to the mid-trimester of pregnancy – the axons were able to regrow after damage, but after this time, their growth was greatly impaired.

Neurons taken from less mature organoids regrew long fibres after injury, but those from more mature organoids showed a sharp drop in their ability to regrow. In other words, poor regeneration is built into human neurons as they mature in the central nervous system."

George Gibbons, Study First Author, Department of Clinical Neurosciences, University of Cambridge

By analysing the gene expression – a sign of how active the genes are – in neurons that connect the brain and the spinal cord, they were able to identify a network of genes that acts as a 'switch' restricting the axon growth ability while the neurons mature to form connections (synapses). Amazingly, blocking key regulators of this network switched back on the ability of axons to grow.

The team then scanned a database of drug compounds to search for those that act on the genes in this network and identified as a candidate lynestrenol, a hormone drug licensed for managing certain menstrual disorders and as a contraceptive. When they tried this drug on damaged neurons, they found that it significantly boosted axon regrowth.

While scar tissue and inflammation may also restrict axon repair, exploring and tackling neuron-specific causes – the subject of this study – is very important. This is supported by evidence that axons of less mature neurons can grow through non-permissive environments that characterise injury sites.

Senior author Dr András Lakatos, who led the project at the Department of Clinical Neurosciences, said: "When the brain and spinal cord are damaged, the nerve fibres that carry movement signals from the brain to the spinal cord rarely grow back. That's why paralysis is usually permanent. But we didn't know exactly when the ability of axons to regenerate becomes limited. Our model provides a good indication that this block happens during development, and it can still be reversed after this point.

"Lynestrenol itself may not be the answer to spinal cord repair, but it shows us that, in principle, it should be possible to directly target human neurons and regenerate their axons. Although we still need to show that this strategy will also help to re-establish appropriate connections between the brain and spinal cord cells, this gives us hope that one day we may be able to treat conditions previously thought untreatable."

Organoid models are an important way of understanding human biology. While animal models – for example, mice and rats – are useful for studying our biology as they share some similarities with humans, their differences ultimately limit what we can learn. Organoids grown from human stem cells can more closely mimic human biology.

Dr Lakatos added: "Much of what we know about nerve regeneration comes from rodents, whose neurons behave differently from human neurons. Our sophisticated organoid models help bridge the knowledge gap from animal models to what we see in patients. They are also an important contribution to efforts to reduce the use of animals in research."

Organoids, often referred to as 'mini organs', are being used increasingly to model human biology and disease. At the University of Cambridge alone, researchers use them to repair damaged livers, understand Crohn's disease in children, and model the early stages of pregnancy, among many other applications.