The spinal cord is made up of many varieties of neural cells. After spinal cord trauma, any cellular repair strategy must, therefore, address this specificity.

Spinal cord cells called V2a interneurons have been the focus of recent studies in several labs. This cell is not what you’d think of as a main highway for nerve signals; these are the intricate side roads that form the network known as the Central Pattern Generator (CPG). The CPG handles a lot of body functions we don’t consciously think about – including breathing and walking. As we’ve seen from spinal cord stimulation studies, the CPG can be rebooted to some extent after SCI, and is the reason the spinal cord is thought be “smart” on its own.

V2a interneurons are essential for both respiration and locomotion. Taking away the V2a cells in mice disrupts normal breathing patterns, impairs reaching, and disrupts left-right hindlimb coordination.

I learned about V2a cells earlier this year from Sam Pfaff at the Salk Institute (a member of the Reeve International Research Consortium on Spinal Cord Injury). The Pfaff lab coaxed embryonic stem cells from mice to cluster into what they called circuitoids -- 50,000 cells, barely large enough to see unaided. Pfaff showed that these aggregates of V2a cells began to pulse rhythmically – with a spontaneous, coordinated activity linked to repetitive movement.

Says Pfaff, “ ... we think that developing this kind of simple circuitry in a dish will allow us to extract some of the principles of how real brain circuits operate. With that basic information maybe we can begin to understand how things go awry in disease.”

Here are two other recent research publications about V2a cells, both looking at the potential for using V2a neural subtypes as potential therapies for spinal cord injury.

Todd McDevitt lab/Gladstone

Scientists from the Gladstone Institutes and University of California labs in both San Francisco and Berkeley cultured V2a interneurons from human stem cells – a first. They used complex biochemical signaling cues to direct growth – the same recipe as when the nervous system first develops. Once the lab propagated large quantities of V2a cells, they injected them into healthy mice spinal cords. The scientists, led by bioengineer Todd McDevitt from UCSF and Gladstone, weren’t sure what to expect.

“To our pleasant surprise,” said McDevitt, “The cells extended over long distances and connected with other neurons. They seemed pretty robust over pretty long distances.”

The work, Differentiation of V2a interneurons from human pluripotent stem cells, was published the Proceedings of the National Academy of Sciences. First author is Jessica Butts.

McDevitt describes V2a interneurons as “long relay cables -- stretching out up to 1,000 times longer than a normal human cell.” They connect brain neurons to the motor neurons that connect directly to muscles.

McDevitt wondered whether replacing V2a cells in the spinal cord could reconnect spinal circuitry. “Our goal is to rewire the impaired circuitry by replacing damaged interneurons to create new pathways for signal transmission around the site of the injury.”

From a Gladstone press release:

After experimenting with round after round of chemical combinations, the researchers landed on a process that can now produce a sizable batch of human V2a interneurons in a little over two weeks. The first step was to inject the cells into the spinal cords of healthy mice and see if the cells survived. They did even better.

“Within two weeks, we saw a number of these cells extend their axons over long distances — five millimeters reliably, but some even longer than that,” McDevitt said, adding that the wiry cells are also making important connections. “Even though they’re mice, we see these human cells that appear to be connecting to other neurons.”

Not quite to the stage of clinical relevance. A forthcoming mouse injury studies will treat acute injuries. McDevitt speculated that the V2a interneurons might have utility in chronic injury. More from the news release:

Trials will first need to be run with injured mice before any human subjects can be tested. Plus, it’s entirely possible that V2a interneurons only fix very specific types of spinal injuries, or none at all. It might require the production of other spinal cord neurons, or a combination of several, to find the most effective treatment.

“At the most basic level, this work shows that we can successfully introduce a new type of spinal neuron made from human pluripotent stem cells,” McDevitt said. “I see it as a step in what’s probably going to be a much bigger effort by the field.”

Michael Lane Lab/Drexel

This lab’s focus is on boosting plasticity -- the naturally occurring recovery of damaged spinal circuitry. Such recovery is limited but the thinking here is that there may be ways to harness and strengthen plasticity and thus optimize functional recovery. Says the Lane lab website: “We believe that spinal interneurons are a key therapeutic target for optimizing anatomical repair and functional recovery following SCI.”

The Lane group just published a paper that considers how V2a interneurons in cervical spinal cord injury affect phrenic nerve activity, and thus breathing.

The work was published in the Journal of Neurotrauma: “Anatomical recruitment of spinal V2a interneurons into phrenic motor circuitry after high cervical spinal cord injury.”

From the paper:

These experiments provide the first evidence for the involvement of a specific population of spinal interneurons in anatomical plasticity post-SCI, capable of modulating motor output and functional plasticity following spinal cord injury.

With an increasing appreciation for the neuroplastic potential of the injured spinal cord, there is a greater need for identifying the neural components that contribute to spontaneous change. The identified anatomical components may also represent targets for harnessing and amplifying functional plasticity, and promoting lasting improvement in outcome post-SCI.

First author on the paper is graduate student Lana Zholudeva. She framed the work for me.

What we know from our data (and from work by other laboratories) is that V2a cells play an important role in normal respiratory (and locomotion) circuits, but also an important role in plasticity that occurs after injury or disease. There is a growing interest in the spinal cord injury community in using these cells as a potential therapeutic target for transplantation. Of course, there is still a lot of work to be done in terms of characterizing how these cells develop after transplantation into an inhibitory environment of an injury and how we can best optimize their growth and connectivity to appropriate neural circuitry – using experimental models first, prior to thinking about translation.

Moving forward, we would like to amplify plasticity that naturally occurs by transplanting V2a cells into more clinically relevant models of spinal cord injury, such as a contusion model, which results in neuronal death.

Principal investigator is Michael Lane, a professor at Drexel and part of the Spinal Cord Research Center there:

It is quite possible that the potential contribution of V2a cells to functional recovery could be enhanced with rehabilitation, neural interfacing or other activity-based therapies. This is something that our team has considered in light of our new results. A key focus of our research is to explore whether transplanting populations of V2a neurons can contribute to spinal cord repair and enhanced plasticity. We are hoping to publish the first set of results from this ongoing work soon.