ABOVE: © iStock.com, filo

Aweekly drug treatment strengthens neural connections and enhances neuron regeneration in mice with chronic spinal cord injuries, according to a study published in PLOS Biology last week (September 20).

The spinal cord is made up of bundles of lengthy axons that carry either movement information from the brain to muscles via descending axons, or sensory information from the body to the brain. In both mice and humans, spinal cord injury causes spasticity, pain, and loss of voluntary movement below the injury site. For years, researchers have sought a way to reconnect the severed neural connections above and below the lesion, both immediately and long after the damage has set in. “What is really important is to provide a tissue bridge that can provide traffic and structural support across the cavity that’s formed after spinal cord injury,” explains study coauthor Simone Di Giovanni, a neuroscientist at the Imperial College London. “And hopefully restore function.”

But so far, few effective treatments exist. The new study finds that activating the CREB-binding protein (CBP) and a related protein called p300 can promote axon regeneration at the lesion site long after the initial injury. CBP/p300 are histone acetyltransferases, meaning they can modify histones and unwind DNA, promoting the transcription of a number of genes, including growth-associated ones, by making them more physically accessible to cellular machinery. But the study, which involved treating mice with a drug that activates CBP/p300, did not find that the proteins’ effects led to any improvements in the mice’s ability to move or walk.

fluorescence image of spinal cord in purple with axons going horizontally in green 
Fluorescence image of spinal cord lesion with axons in green
Mueller et al., PLOS Biology

“In the spinal cord injury field, we generally consider two groups of patients: those that are in the early, acute stage of the disease and those that are in a chronic stage of the disease, meaning a long time after having the injury,” says Mónica Sousa, a neuroscientist at the University of Porto in Portugal who was not involved in the study. “And it is thought that the therapeutical strategies . . . will function better in the acute patients [because] once injury sets in, there’s nothing to be done.” This study suggests otherwise, and “In that sense, it’s a great finding.

Prior to this study, Di Giovanni and his colleagues had reported that CBP/p300 spurs axonal growth in the spinal cords of partially injured mice, and epigenetically enhances the transcription of several regeneration-associated genes. But this is the first study to show that epigenetic activation can boost neural growth in a near-complete spinal lesion long after the initial injury.

The team induced injuries in the upper back (thoracic) portion of mice’s spinal cords. Nearly all of the nerves in the animals’ spines were destroyed due to the injury. One week after inducing spinal cord injury, the researchers placed the mice in what they called an enriched environment, where mice have access to wheels, toys, and tubes. The researchers previously found that this enriched environment also improves mice’s recovery from partial spinal cord injury, although Di Giovanni says he’s unsure how much the mice with near-total injuries can take advantage of their environment, given their limited mobility. 

Twelve weeks after injury, the researchers began treatment. Once a week for 10 weeks, one group of mice received an injection of CSP-TTK21, a molecule that activates the naturally occurring protein pair CBP/p300, while another group remained untreated. 

After 22 weeks, the researchers searched for signatures of neural regeneration and synaptic plasticity—the ability for neural connections to strengthen or weaken over time—throughout the spinal cord. Such regeneration could bridge the gap left by the injury, and these stronger connections, the researchers posited, would enable mice to have better control over their limbs. Using fluorescent markers, the team labeled descending and ascending axons coming from the motor cortex in the brain and from the spinal cord, respectively, both of which had been severed by the injury. Looking through a microscope, the team found that in mice that received CBP/p300, the few undamaged axons at the site of the lesion had started to regenerate and sprout more connections. “We saw an increase in sprouting around the bit of spare tissue that was present in the lesion,” explains Di Giovanni. In particular, he and his team saw more sprouting of serotonin-containing axons, which Di Giovanni says are important for locomotion. No such regrowth was seen in mice that received no treatment. 

The study also pointed to a significant increase in the plasticity of the excitatory synapses, says Di Giovanni: After TTK21 delivery, motor neurons that control the lower back of injured mice had higher levels of specific neurotransmitter receptors that are known to increase in number after connections between neurons strengthen.

The researchers also assessed the mice’s ability to walk and perform sensorimotor tasks, but found no difference between the group treated with CBP/p300 and the controls. They say that while the treatment is not enough to restore function, it does begin to provide connection and structural support across the cavity caused by the injury, which could lead to functional recovery in combination with other therapies.

“It’s a very beautiful paper,” says Martin Oudega, a neuroscientist at Northwestern University who was not involved in the study. While it’s “unfortunate” that promoting axon growth is not enough to promote functional recovery, he says that more steps—a more complicated experimental process—are probably needed to ensure the new growth is “integrated into the circuitry” of the central nervous system. He suggests that one way to achieve this might be targeted therapy of the forelimbs or hindlimbs. 

In the future, Di Giovanni says he hopes to study other treatments in combination with CBP/p300, such administering other pharmaceuticals or implanting biomaterials to bridge the gap between severed nerves, in a bid to induce “much better growth and functional recovery.” Another question for future research is how CBP/p300 affects the environment around growing axons, which are usually stunted and retract after injury. 

“Most patients are at the chronic stage of the disease,” says Sousa. “In most papers in the spinal cord injury field, most therapies are given shortly after or at the same time as the injury. And that’s not what patients need.”