Corticospinal Tract Regeneration

Posted by Sam Maddox in Research News on May 20, 2016

The corticospinal tract (CST) is the Mother Road of motor function from the brain to the spinal cord. Damage to this is bundle of long nerve fibers is the reason people with spinal cord injuries lose voluntary control of movement.

Here’s a very cool paper that came out a few weeks ago that offers a promising regenerative strategy to repair the CST in a chronic animal model, using a type of stem cell. The research, “Spinal cord reconstitution with homologous neural grafts enables robust corticospinal regeneration,” comes from the Mark Tuszynski lab at UC San Diego.

“The corticospinal projection is the most important motor system in humans," says Tuszynski, a physician scientist who directs the UCSD Translational Neuroscience Institute. “It has not been successfully regenerated before. Many have tried, many have failed -- including us, in previous efforts.

“The new thing here was that we used neural stem cells for the first time to determine whether they, unlike any other cell type tested, would support regeneration. And to our surprise, they did.”

From the paper:

In murine [rat] models of SCI, we report robust corticospinal axon regeneration, functional synapse formation and improved skilled forelimb function after grafting multipotent neural progenitor cells into sites of SCI.

Lead author (he originated the research and performed the experiments) of the CST study is Ken Kadoya, also an M.D./Ph.D. who has an affiliation with both UCSD and Hokkaido University, Sapporo, Japan. Co-authors include Paul Lu, whose work with stem cell transplants (unprecedented growth of spinal cord axons above and below a cord lesion) we have discussed here, and here.

I asked Kadoya to help us understand his work, and its significance.

Nerve regeneration in the chronic stage of spinal cord injury has been “a formidable challenge,” said Kadoya. “Our study provides evidence that transplanted early stage nerve cells can overcome the limitations of the chronically injured environment, grow robustly, and make new connections.”

Kadoya explained that the cells used in the study are embryonic rat stem cells that have been coaxed one step toward being spinal cord nerve cells. These so-called neural progenitor cells (NPC) were transplanted into injury sites two weeks after a complete T3 spinal cord transection injury or six months after the injury. In both cases, says Kadoya, the transplanted cells developed normally and generated new neurons.

“The clinical significance of this,” says Kadoya, “is first, the achievement of CST regeneration, which has been the most refractory system in the spinal cord. Second, we observed improvement of fine hand function in the test animals. This is important because most cases of SCI [human, of course] involve cervical cord injury, and fine hand functions are often impaired.”

Kadoya said from a technical perspective, the study showed the importance of cell type. “Previous studies did not look at the importance of the specific type of NPCs. We show that spinal cord type NPCs induce CST regeneration, whereas brain type NPCs do not.”

Further, the study showed that human NPC grafts support corticospinal axon regeneration in rats, “indicating conservation of cell-cell interactions across species that enable the growth of this system.”

Is “regeneration” the right word for what’s happening here after transplantation? Yes, says Kadoya. “We are very sure about that.” The NPC grafts replaced lost cells. “After SCI, the lesion site becomes empty. Our NPCs filled up the space. We labeled them with green and observed that green cells filled up lesion sites with neurons and other neural cells.”

What about the roadblock of the spinal cord scar? The NPCs, says Kadoya, “attenuated” scar formation. That’s a Ph.D. way of saying the cells weakened the scar enough to allow CST growth, which was observed, he says, as long as 3.2 mm away from the lesion.

(The Tuszynski lab reported 15 years ago that the spinal cord lesion scar is not an impenetrable barrier to axon growth, especially if axons are nourished with growth factors. Paul Lu’s remarkable axon growth studies relied on generous application of several such factors; in this new study, Kadoya says smaller lesions do not appear to need growth, or trophic, support. “We will need to figure out if a contusion injury [the typical bruise injury that occurs in human SCI] requires growth factor or not.”)

Kadoya says his CST axons formed connections (synapses) with spinal cord nerve networks. “We confirmed synapse formation by two methods, electron microscopy and optogenetics. To perform the optogenetics study, a virus carrying the genes for channelrhodopsin (ChR) and green protein was injected into the area where the CST neurons are located. The infected CST neurons start expressing ChR and green.

“Then we removed the spinal cord and kept it alive in a special environment. We saw green axons (CST axons) in the cord growing into the NPC grafts. We inserted a very fine needle into one neuron in the graft to record its activity. When we applied blue light to the cord, which stimulated axons infected with ChR, we observed that grafted neurons were activated. This is a solid way to confirm that CST axons made functional synapses onto grafted neurons.”

How long after graft did it take for changes to occur in CST function? “For a new synapse formation, we saw it at four weeks after grafts. We saw treatment effects at five weeks after grafts.”

And what were these treatment effects? Says Kadoya, functional changes -- improved skilled forelimb function – were observed using what it called a staircase test. Animals were pre-trained on reaching for a food pellets on multiple levels. Injured animals that had the NPC transplants did a better job reaching pellets than those that did not get cell transplants.

What’s next? There is a lot of work to do out before the clinical promise of neural precursor stem cells arrives. Kadoya says testing in a larger animal model is necessary. He also wants to better understand the precise mechanism that underlies CST regeneration, and to determine the idea type of cell to use.

From the paper:

Recent reports have emphasized the importance of amplifying intrinsic neuronal signaling to promote the growth of refractory corticospinal axons after SCI [e.g. deleting PTEN genes to boost axon growth]. However, these efforts have not been successful in generating extensive corticospinal regeneration into large, clinically relevant lesion sites that lack small bridges of host tissue.

We now find that NPC grafts enable the extensive and consistent regeneration of corticospinal axons into sites of severe SCI. Indeed, neural grafts achieve extensive regeneration of corticospinal axons into complete spinal cord transection sites, rather than growth only through spared tissue bridges, which is an important practical milestone for their application to the vast majority of injured humans who have lesion cavitation and relatively little spared tissue.

Successful regeneration does not require the therapeutic activation of the corticospinal neuron per se; provided solely with a permissive graft milieu, we show for the first time that corticospinal axons are capable of extensive regeneration into a lesion site.