Yesterday, CIRM-funded scientists at UCLA published an interesting study that sheds light on the development of sensory neurons, a type of nerve cell that is damaged in patients with spinal cord injury. Their early-stage findings could potentially, down the road, lead to the development of stem cell-based treatments that rebuild the sensory nervous system in paralyzed people that have lost their sense of touch.

Dr. Samantha Butler, a CIRM grantee and professor at the Eli and Edythe Broad Center of Regenerative Medicine and Stem Cell Research at UCLA, led the study, which was published in the journal eLife.

Restoring sensation

Butler and her team were interested in understanding the basic development of sensory interneurons in the spinal cord. These are nerve cells in the spinal cord that receive sensory signals from the environment outside the body (like heat, pain and touch) and relay these signals to the brain where the senses are then perceived.

Developing spinal cord injury treatments often focus on the loss of movement caused by damage to the motor neurons in the spine that control our muscles. However, the damage caused to sensory neurons in the spine can be just as debilitating to people with paralysis. Without being able to feel whether a surface is hot or cold, paralyzed patients can sustain serious burn injuries.

Butler commented in a UCLA news release that attempting to restoring sensation in paralyzed patients is just as important as restoring movement:

Samantha Butler

“The understanding of sensory interneuron development has lagged far behind that of another class of neurons—called motor neurons—which control the body’s ability to move. This lack in understanding belies the importance of sensation: it is at the core of human experience. Some patients faced with the reality of paralysis place the recovery of the sense of touch above movement.”

BMPs are important for sensory neuron development

To restore sensation in paralyzed patients, scientists need to replace the sensory neurons that are damaged in the spine. To create these neurons, Butler looked to proteins involved in the early development of the spinal cord called bone morphogenetic proteins or BMPs.

BMPs are an important family of signaling proteins that influence development of the embryo. Their signaling can determine the fate or identity of cells including cells that make up the developing spinal cord.

It was previously thought that the concentration of BMPs determined what type of sensory neuron a stem cell would develop into, but Butler’s team found the opposite in their research. By studying developing chick embryos, they discovered that the type, not the concentration, of BMP matters when determining what subtype of sensory neuron is produced. Increasing the amount of a particular BMP in the chick spinal cord only produced more of the same type of sensory interneuron rather than creating a different type.

Increasing the concentration of a certain type of BMP increases the production of the same categories of sensory interneurons (red and green). (Image credit: UCLA)

The scientists confirmed these findings using mouse embryonic stem cells grown in the lab. Interestingly a different set of BMPs were responsible for deciding sensory neuron fate in the mouse stem cell model compared to the chick embryo. But the finding that different BMPs determine sensory neuron identity was consistent.

So what and what’s next?

While this research is still in its early stages, the findings are important because they offer a better understanding of sensory neuron development in the spinal cord. This research also hints at the potential for stem cell-based therapies that replace or restore the function of sensory neurons in paralyzed patients.

Madeline Andrews, the first author of the study, concluded:

“Central nervous system injuries and diseases are particularly devastating because the brain and spinal cord are unable to regenerate. Replacing damaged tissue with sensory interneurons derived from stem cells is a promising therapeutic strategy. Our research, which provides key insights into how sensory interneurons naturally develop, gets us one step closer to that goal.”

The next stop on the team’s research journey is to understand how BMPs influence sensory neuron development in a human stem cell model. The UCLA news release gave a sneak preview of their plans in the coming years.

“Butler’s team now plans to apply their findings to human stem cells as well as drug testing platforms that target diseased sensory interneurons. They also hope to investigate the feasibility of using sensory interneurons in cellular replacement therapies that may one day restore sensation to paralyzed patients.”