NG2-Expressing Neural Lineage Cells Derived from Embryonic Stem Cells Penetrate Glial Scar and Promote Axonal Outgrowth After Spinal Cord Injury


After a spinal cord injury, resident stem cells in the spinal cord contribute to the production of a glial scar that is rich in chondroitin sulfate proteoglycan (CSPG). The glial scar is a formidable barrier to axonal regeneration in the injured spinal cord, since CSPG actively repels growing axonal growth cones. Even though the glial scar seals off the spinal cord from further damage from inflammation, the long-term effects of the glial scar are to prevent regeneration of spinal nerves, which have the ability to regenerate in culture.

The major components of the site of injury include myelin debris, the scar-forming astrocytes, activated resident microglia and infiltrating blood-borne immune cells, chondroitin sulfate proteoglycans (CSPGs) and other growth-inhibitory matrix components. All of them are potential targets for therapeutic intervention. Many of the interventions can be optimized by considering the beneficial aspects of the scar tissue and fine-tuning the optimal time window for their application. Each target and the strategies directed at its modulation are shown.
The major components of the site of injury include myelin debris, the scar-forming astrocytes, activated resident microglia and infiltrating blood-borne immune cells, chondroitin sulfate proteoglycans (CSPGs) and other growth-inhibitory matrix components. All of them are potential targets for therapeutic intervention. Many of the interventions can be optimized by considering the beneficial aspects of the scar tissue and fine-tuning the optimal time window for their application. Each target and the strategies directed at its modulation are shown.

New work by Sudhakar Vadivelu, in the laboratory of John McDonald at the International Center for Spinal Cord Injury, Hugo W. Moser Research Institute at the Kennedy Krieger Institute, Baltimore, Maryland has discovered new ways to breach the glial scar. Vadivelu and colleagues used a cell culture system that tested the ability of particular cells to help growing axonal growth cones penetrate glial scar material. This culture system showed that embryonic stem cell-derived neural lineage cells (ESNLCs) with prominent expression of nerve glial antigen 2 (NG2) survived, and passed through an increasingly inhibitory gradient of CSPG. These cells also expressed matrix metalloproteinase 9 (MMP-9) at the appropriate stage of their development, which helped poke holes in the CSPG. The outgrowth of axons from ESNLCs followed the NG2-expressing cells because the migrating cells chiseled pathways through the CSPG for the outgrowth of new axons.

To confirm these results in a living animal, Vadivelu and others transplanted embryonic stem cell-derived ESNLCs directly into the cavities of a contused spinal cord of laboratory animals 9 days after injury. One week later, implanted ESNLCs survived and expressed NG2 and MMP-9. The axons of these neurons had grown through long distances (>10 mm), although they preferred to grow across white rather than gray matter.

These data are consistent with CSPG within the injury scar acting as an important impediment to neuronal regeneration, but that NG2+ progenitors derived from ESNLCs can alter the microenvironment within the injured spinal cord to allow axons to grow through such a barrier. This beneficial action seems to be due, in part, at least, to the developmentally-regulated expression of MMP-9. Vadivelu and others conclude from these data that it might be possible to induce axonal regeneration in the human spinal cord by transplanting ESNLCs or other cells that express NG2.

Human Stem Cell-Derived Neurons Grow New Axons In Spinal Cord Injured Rats


A stem cell-based treatment for spinal cord injury took one more baby step forward when scientists from the laboratory of Mark Tuszynski at the at the University of California, San Diego used cells derived from an elderly man’s skin to regrow neural connections in rats with damaged spinal cords.

Tuszynski and others published their results in the Aug. 7 online issue of the journal Neuron. In that paper, Tuszynski and his co-worker report that human stem cells triggered the growth of numerous axons in the damaged spinal cord. Axons are those fibers that extend from the main part or body a neuron (nerve cell) that serve to send electrical impulses away from the body to other cells. Some of these new axons even grew into the animals’ brains.

Axon picture

Dr. Mark Tuszynski is a professor of neurosciences at the University of California, San Diego. “This degree of growth in axons has not been appreciated before,” he said. However, Tuszynski also cautioned that there is still much to be learned about how these newly established nerve fibers behave in laboratory animals. He likened the potential for stem-cell-induced axon growth to nuclear fusion. If it’s contained, you get energy; if it’s not contained, you get an explosion. “Too much axon growth into the wrong places would be a bad thing,” Tuszynski added.

Stem cell researchers have examined the potential for stem cells to restore functioning nerve connections in people with spinal cord injuries. Embryonic stem cells have been used to make new neurons and to also make “oligodendrocyte progenitor cells” or OPCs, which make the insulating myelin sheath that enwraps the axons of spinal nerves. However, several other types of stem cells can make OPCs and new neurons and these stem cells do not come from embryos (for more, see chapter 27 of my book, The Stem Cell Epistles).

In this study, Tuszynski and his team used induced pluripotent stem cells or iPSCs, which are derived from mature adult cells by means of genetic engineering and cell culture techniques. They used cells from a healthy 86-year-old man and genetically reprogrammed so that they were reprogrammed into iPSCs. These iPSCs were then differentiated into neurons that were implanted into a special scaffold embedded with proteins called growth factors, and then grafted into the spinal cords of laboratory rats with spinal cord injuries.

Over the course of several months, these animals showed new, mature neurons and extensive growth in the cells’ axons. These fibers grew through the injury-related scar tissue in the animals’ spinal cords and connected with resident rat neurons.

This is an enormous advance, because the wounded spinal cord creates a “Glial scar” that contains a host of molecules that repel growing axons. Even though this glial scar prevents the immune system from leaking into the spinal cord and destroying it, this same scar prevents the regeneration of damaged neurons and their severed axons.

Glial scar axon repulsion

Dr. David Langer, director of neurosurgery at Lenox Hill Hospital in New York City said: “One of the big obstacles [in this type of research] is this area of scarring in the spinal cord. Getting neurons to traverse it is a real challenge,” said Langer, who was not involved in the research. “The beauty of this study,” he said, “is that they got the neurons to survive and traverse the scar.”

Langer also cautioned, much like Tuszynski, that this experimental success is just a preliminary step. There are, in his words, “huge questions” as to whether or not these axons can make appropriate connections and actually restore function to spine-damaged lab animals. “It’s not just a matter of having the cables,” Langer said. “The wiring has to work.”

And even if this stem cell approach does pan out in animals, Langer added, it would all have to be translated to humans. “We have a long way to go until we’re there,” he said. “It’s not that people shouldn’t have hope. But it should be a realistic hope.”

A few biotech companies have already launched early-stage clinical trials using embryonic (Geron) or fetal stem cells (StemCells Inc) to treat patients with spinal cord injuries. But Tuszynski said his team’s findings offer a cautionary note about moving to human trials too quickly. “We still have a lot to learn,” he said. “We want to be very sure these axons don’t make inappropriate connections. And we need to see if the new connections formed by these axons are stable.”

Ideally, Tuszynski added, if stem cells were to be used in treating spinal cord injuries, they’d be generated as they were in this study — by creating them from a patient’s own cells. That way, he explained, patients would not need immune-suppressing drugs afterward.

Stem Cell-Mediated Scarring of the Spinal Cord Aids in Recovery


After injury to the spinal cord, glial cells and neural stem cells in the spinal cord contribute to the formation of the “glial scar.” This glial scar is rich in molecules known as chondroitin sulfate proteoglycans (CSPGs) that are known to repel growing axons. Therefore, the glial scar is viewed as a major impediment to spinal cord regeneration.

However, new work from the Karolinska Institutet in Solna, Sweden has confirmed that the glial scar actually works to contain the damage within the spinal cord. Far from impairing spinal cord recovery, the stem cell-mediated formation of the glial scar confines the damage to a discrete portion of the spinal cord and prevents it from spreading.

Trauma to the spinal cord can sever those nerve fibers that conduct nerve impulses to from the brain to skeletal muscles below the level of spinal cord injury. Depending on where the spinal cord is injured and the severity of the injury, spinal cord injuries can lead to a various degrees of paralysis. Such paralysis is often permanent, since the severed nerves do not grow back.

The absence of neural regeneration required an explanation, since cultured neurons whose axons are severed can regenerate both in culture and in a living creatures (for an excellent review, see Nishio T. Axonal regeneration and neural network reconstruction in mammalian CNS. J Neurol. 2009 Aug;256 Suppl 3:306-9). Thus, neuroscientists have concluded that the injured spinal contains a variety of molecules that inhibit axonal outgrowth and regeneration.

This hypothesis has been demonstrated since many axon growth inhibitors have been isolated from the injured spinal cord (see Schwab ME (2002) Repairing the injured spinal cord. Science 295:1029–1031). Such molecules include proteins like Nogo, Myelin-Associated Glycoprotein (MAG), and Oligodendrocyte-Myelin Glycoprotein (OMgp). However, as the Nishio review points out, axons from severed nerved have been seen growing throughout the central nervous system. Therefore, most of the blame for a lack of regrowth has been pinned on the glial scar.

A new study by Jonas Frisén of the Department of Cell and Molecular Biology and his colleagues has shown that the neural stem cell population in the spinal cord are the main contributors to the glial scar. However, when glial scar formation was prevented after spinal cord injury, the injured area in the spinal cord expanded and more nerve fibers were severed. Furthermore, in their mouse model, a great number of nerve cells died in those mice that did not make glial scars when compared to those mice that were able to produce a normal glial scar.

Ependymal cell incorporation of 5-ethynyl-2′-deoxyuridine is reduced in the absence of Ras genes in intact spinal cord (A and B) and 7 days after injury (C to E). Arrowheads and arrows point to proliferating recombined (A and C) and unrecombined (C and D) ependymal cells, respectively. Injury-induced migration is blocked in rasless ependymal cells (F). Sagittal view of the lesion site 14 weeks after injury in a FoxJ1 control mouse (G) and FoxJ1-rasless mice (H to J). Recombined ependymal cells express YFP in (A) to (D), and cell nuclei are labeled with 4′,6-diamidino-2-phenylindole (DAPI) and appear blue. *P < 0.05, **P < 0.01; Student’s t test. Error bars show SEM. Scale bars represent 10 μm in (A) to (D) and 200 μm in (G) to (J). GFAP, glial fibrillary acidic protein.
Ependymal cell incorporation of 5-ethynyl-2′-deoxyuridine is reduced in the absence of Ras genes in intact spinal cord (A and B) and 7 days after injury (C to E). Arrowheads and arrows point to proliferating recombined (A and C) and unrecombined (C and D) ependymal cells, respectively. Injury-induced migration is blocked in rasless ependymal cells (F). Sagittal view of the lesion site 14 weeks after injury in a FoxJ1 control mouse (G) and FoxJ1-rasless mice (H to J). Recombined ependymal cells express YFP in (A) to (D), and cell nuclei are labeled with 4′,6-diamidino-2-phenylindole (DAPI) and appear blue. *P < 0.05, **P < 0.01; Student’s t test. Error bars show SEM. Scale bars represent 10 μm in (A) to (D) and 200 μm in (G) to (J). GFAP, glial fibrillary acidic protein.

“It turned out that scarring from stem cells was necessary for stabilizing the injury and preventing it from spreading,” said Frisén. “Scar tissue also facilitated the survival of damaged nerve cells. Our results suggest that more rather than less stem cell scarring could limit the consequences of a spinal cord injury.”

According to earlier animal studies, recovery can be improved by transplanting stem cells to the injured spinal cord. These new findings suggest that stimulating the spinal cord’s own stem cells could offer an alternative to cell transplantation therapies.

This paper appeared in the journal Science, 1 November 2013: 637-640, and the first author was Hanna Sabelström. This interesting paper might be leaving one thing out when it comes to spinal cord regeneration.  Once the acute phase of spinal cord injury is completed and the chronic phase begins, the glial scar does in fact prevent spinal cord regeneration.  This is the main reason Chinese researchers have used chondroitinase enzymes to digest the scar in combination with transplantations on stem cells.  By weakening the repulsive effects of the glial scar, these stem cells can form axons that grow through the scar.  Also, olfactory ensheathing cells or OECs seem to be able to shepherd axons through the scar, although the degree of regeneration with these cells has been modest, but definitely real.  Therefore, negotiating axonal regeneration through the glial scar remains a major challenge of spinal cord injury.  Thus, while the glial scar definitely has short-term benefits, for the purposes or long-term regeneration, it is a barrier all the same.

About that Hold


Geron has revealed the reason for the FDA hold on its Spinal Cord Injury Investigational New Drug application. In an August 27th press release, Geron scientists revealed that the implanted GRNOPC1 cells caused cysts in a small proportion of the animals injected with them. These cysts were not cancerous. The report calls them “non-proliferative,” which simply means that they are not growing. Additionally. the cysts are very small – microscopic in size. Finally, the cysts were confined to the region of the injury and did not adversely affect the laboratory animals.

Why the hub-hub? A recent animal study reported a greater frequency of cysts. Once again, they are non-proliferative (non-growing), restricted in location to the site of injury and do not affect the animals.

What’s going on? Cyst formation is common in spinal cord injury. Once the spinal cord is injured, inflammation ensues and this involves the invasion of the spinal cord by immune cells that mop up the dead cells and debris from the injury. Unfortunately, immune cells are sloppy eaters and they do a great deal of damage to the spinal cord. The damage they cause also tends to summon more immune cells, which come to the scene of the injury and damage the spinal cord even more. the whole thing is a positive feedback mess.

To put an end to it the spinal cord makes a plug called a glial scar. The glial scar comes from the stem cell population in the spinal cord. These stem cells form support cells called “glial cells” and these cells plug the hole in the spine and shut out the immune response, thus saving the spinal cord. The formation of this glial scar, however has a severe downside for spinal cord regeneration: the glial scar is loaded with chemicals that repel growing neurons. Therefore, those neurons that were severed by the injury could not regrow their extensions if they wanted to. The glial scar acts like a bunch of burly security guards that prevent the neuronal extensions from getting to the other side.

These cysts are probably the result of the GRNOPC1 cells forming tiny glial scars to help the injured spinal cord heal. Now they do not seem to affect the laboratory animals, but they are inhibitors of neuronal healing. Therefore, while they may not affect the laboratory animals, they may represent a fix that sentences the spinal cord to never being fixed by anything else again.