Kate WilletteWhat if it were possible to replace lost neurons by combining superfast, individual-specific 3D printing with cellular therapy designed to promote axon growth? Turns out that it is possible.

This is another story about the holy grail of spinal cord injury repair: regeneration — the process by which cells of the spinal cord grow back again across the damaged section, or gap. During my own adult life, the prospect of regeneration has gone from “impossible, don’t even bother” to “oh wait, I see how this might work.” Since the 1970s, scientists struggling to repair damaged cords have faced a long and frustrating set of harsh surprises, with each small success revealing yet more obstacles to over¬come. Regeneration is not a project for the faint of heart or for those in a hurry — but it is marked with milestones like the one I’m about to describe.

In the February edition of the journal Nature Medicine, a team of research¬ers led by Dr. Mark Tuszynski reported that they’d found a new way to fill a spinal cord gap. Their paper is called “Biomimetic 3D printed scaffolds for spinal cord injury repair.”

Details will follow, but first a little history: It used to be thought that once broken, the delicate thread-like axons that carry messages from brain to body and body to brain simply could not regrow. Ever. Then scientists learned that axons could in fact grow under the right conditions, but they kept running up against “glial scarring” — the physical and chemical barrier that walls off damaged cells in the cord and protects the healthy parts above and below an injury site. Axons that grew fine in petri dishes were stopped cold in living creatures when they bumped up against the glial scar. Researchers thus began a long string of attempts to fashion a way to go around or through it.

One idea was to use bits of borrowed peripheral nerves to form living bridges that would give axons a path to avoid the scar, but it turned out that axons needed more guidance and more incentive to grow. Experiments with all sorts of bridges and tunnels, some of them with promising results, proved to be unrepeatable. Paths through or around the injury site were very hard to create and often required one-in-a-million surgical skills. The axons stubbornly refused to grow, at least in meaningful numbers.

Setting bridges aside, perhaps the trick would be to simply replace all the damaged cells with new ones. Over the last few decades, many variants on this idea have been tried, with a variety of candidate cells being implanted into the damaged cords of rodents — and a not insignificant number of human volunteers and customers. So far, and in spite of much hype, it’s only recently that cells from any source have been shown to reliably lead to regeneration in large mammals. We looked at two instances of recent regeneration successes in New  Mobility’s October 2018 and January 2019 issues. One involved a bridge, and the other involved neuroprogenitor cells (NPCs).

Tuszynski’s team used both NPCs and a bridge that relies on new technology. For the first time, 3D printing is now producing a very promising version of the long-awaited bridge. Even more intriguing, when seeded with NPCs, this bridge consistently gets closer to the goal than anything tried so far. Axons are crossing the injury and coming out the other side to form strong new connections. What makes NPCs special is that they’re exactly like the cells in a developing embryo that can only differentiate into the cells of the central nervous system.

Can 3D Technology Simplify the Complexity of the Spinal Cord?

If you could slice through the human spinal cord like a sushi roll and look down at the round surface you created, you’d see a sort of gray butterfly shape surrounded by a lighter-colored field. The butterfly shape is the central part of the cord. The outer field is where the axons are. These axons — and this is important — are arranged according to what kinds of information they pass along and which direction it goes.  The critical point is that the arrangement of groups of axons inside the cord is parallel. The tracts are a bit like bundles of extremely fine hairs, all placed neatly alongside one another and symmetrically on either side of the gray matter.

I have to pause here in sheer amazement that all of this can be known in such detail. When surgeons operate to expose the spinal cord, these structures aren’t even visible. What they see there is just a soft, semi-liquid mesh.

At the site of an injury, all of this meticulous biology, consisting of millions and millions of individual cells, each with a particular set of connections, is destroyed.

Tuszynski’s team worked with rats, trying to establish proof of the concept that a tiny scaffold printed to match the precise dimensions of an injury and then loaded with NPCs could serve as both bridge and living relay switch. The paper lays out the process by which they were able to prove their point. There were four sets of rats, all with spinal cords completely severed at the thoracic level.

• One of the groups was a control — no further intervention was done.
• One was given just the cells, with no scaffold of any kind.
• One was given a different scaffold model without the cells.
• One was given the scaffold built by a 3D printer and loaded with cells.

The 3D scaffold, which can be printed in 1.6 seconds, is both flexible and sturdy and made from a material that does not appear in nature. Think of a tiny, very firm Jell-O made from a mold and you’ll be close.

Designed to mimic the shape and structure of a cord cross section, the scaffold has microscopic channels built in to guide the complex arrangement of axons along their original paths and link up with their counterparts on the other side of the injury site.

And that’s exactly what the axons did. The host axons entered the bridge and formed synapses with some of the NPCs that had been implanted there. Some of the NPCs then differentiated and grew their own axons out of the bridge and into the healthy cord below, where more synapses were formed. And all of that translated into measurable improvement in movement for the rats — and another step toward the once-thought-impossible goal of regeneration.

What’s exciting about this technology is that it’s both scalable and adaptable to different injury geometries. It’s likely that every person reading this has been told at some point that “every injury is different.” The complexity of the cord makes it impossible to predict with any specificity what patient will get what kind of outcome after damage to the cord. With 3D technology, that could change. An MRI of the injury can produce a precise set of dimensions of the lesion. That set of dimensions can then be fed into digital design software, which can in turn be used to create a soft gel scaffold that’s exactly like the injury, no matter how misshapen it is. Even better, the fact that this is a digitized process means it’s very fast and requires none of the extraordinary surgical transplant skills that made earlier attempts at bridging techniques so difficult to reproduce. The scaffold is placed into the lesion, where technology has ensured that it fits perfectly.

Someone from Tuszynski’s lab at this fall’s Working 2 Walk lab in Cleveland, Ohio, will be prepared to explain how this technology works and describe the path forward. I intend to be there.

Resources
• “Biomimetic 3D printed scaffolds for spinal cord injury repair,” Nature Medicine, nature.com/articles/s41591-018-0296-z
• Working 2 Walk conference, u2fp.org/working-2-walk/this-years-symposium.html