Scientists create single-cell atlas of spinal neurons involved in motor coordination

Scientists at St. Jude Children's Research Hospital created a single-cell atlas of a key class of spinal neurons involved in motor coordination. From that resource, they identified a small subgroup of interneurons associated with the speed of rhythmic movements such as walking. The findings, which have implications for understanding spinal cord function and recovery from injury, were published today in Nature Communications.

A group of neurons that runs along the length of the spinal cord is responsible for processing and coordinating information coming from the brain and peripheral nervous system and going to motor neurons. Called interneurons, these cells play a crucial role in directing the activity of motor neurons that drive muscle contraction and control our body's movement. One population of these cells, called V1 interneurons, is intimately involved in motor control, but the roles of specific subgroups of V1 cells have remained unclear. By comprehensively documenting V1 interneurons in a mouse model, the current study clarifies how V1 cells carry out multiple motor functions.

Understanding the role of different neurons gives us better insights into how these brain-to-muscle neuronal circuits work, which may be important for helping aid recovery after spinal cord damage. Providing this single-cell database and showing how it can be used to understand function is a first step toward that goal."

Jay Bikoff, PhD, corresponding author, St. Jude Department of Developmental Neurobiology

To create the cellular atlas, the scientists used single-nucleus sequencing to measure gene expression in V1 interneurons from a mouse model. They then organized the cells into smaller subgroups based on their molecular features and created an online database for other researchers to explore. With the atlas in hand, the team addressed a longstanding question about how the spinal cord maintains rhythmic movement.

Singling out neurons that control speed

For many processes, such as walking or breathing, the nervous system creates a central pattern, which allows people to move rhythmically without focusing on their movement. Previous studies showed that V1 interneurons play an important role in controlling the speed of certain rhythmic behaviors like locomotion. Removing them in model systems slows the rhythm and causes additional problems with flexing and extending limbs. However, because V1 interneurons contain many different subgroups, scientists did not know whether the same cells controlled both movement speed and flexion-extension patterns, or whether those functions were divided among different cells.

To find the answer, Bikoff's group compared their single-cell atlas to a similar single-cell screen from mice with a key gene removed from V1 cells. Those mice had slower movement rhythms but no sign of hyperflexion. The comparison revealed that one, and only one, group of cells was missing in these mice.

"We found that slowed locomotor speed and hyperflexion were separable, suggesting they may be controlled by different cells," Bikoff said. "Intriguingly, we found that loss of one small subpopulation of V1 interneurons, V1^Pou6f2, was associated only with slowed speed, implicating these neurons in controlling the speed of our body's rhythmic movements but not flexion or extension."

Breaking down big problems into single cells

The study shows how a single-cell atlas can be used to subdivide a large group of neurons and gain a better understanding of the individual parts that make up the whole. By connecting gene expression patterns to specific cells that are candidates for governing different functions, the database gives researchers a way to generate and test new hypotheses about spinal cord circuitry, which may have implications for treating spinal cord injuries or neurodegenerative diseases.

"We showed that this single-cell interneuron database can be used as a resource to tease apart how a multifunctional population of cells can be separated into its individual components," Bikoff said. "We showed it can reveal as-yet unknown biology, giving it the potential to discover new parts of neuronal circuitry that may help us find new ways to improve recovery for patients with spinal cord damage."

Authors and funding

The study's first author is Alexandra Trevisan, St. Jude. The study's other authors are Katie Han, Phillip Chapman, Anand Kulkarni, Jennifer Hinton, Cody Ramirez and Alfonso Lavado, St. Jude; Ines Klein and Graziana Gatto, University Hospital of Cologne; Mariano Gabitto, Allen Institute for Brain Science; and Vilas Menon, Columbia University.

The study was supported by grants from the National Cancer Institute (P30 CA021765), National Institutes of Health (R01NS123116), the German Research Foundation (CRC1451 - 431549029-Z02) and the American Lebanese Syrian Associated Charities (ALSAC), the fundraising and awareness organization of St. Jude.