Human neuronal stem cells extend axons (green). (Image UCSD)

An exciting development for spinal cord injury research was published this week in the journal Nature Medicine. Scientists from the University of San Diego School of Medicine transplanted human neural progenitor cells (NPCs) into rhesus monkeys that had spinal cord injuries. These cells, which are capable of turning into other cells in the brain, survived and robustly developed into nerve cells that improved the monkeys’ use of their hands and arms.

The scientists grafted 20 million human NPCs derived from embryonic stem cells into two-week-old spinal cord lesions in the monkeys. These stem cells were delivered with growth factors to improve their survival and growth. The monkeys were also treated with immunosuppressive drugs to prevent their immune system from rejecting the human cells.

After nine months, they discovered that the NPCs had developed into nerve cells within the injury site that extended past the injury into healthy tissue. These nerve extensions are called axons, which allow nerves to transmit electrical signals and instructions to other brain cells. During spinal cord injury, nerve cells and their axon extensions are damaged. Scientists have found it difficult to regenerate these damaged cells because of the inhibitory growth environment created at the injury site. You can compare it to the build-up of scar tissue after a heart attack. The heart has difficulty regenerating healthy heart muscle, which is instead replaced by fibrous scar tissue.

Excitingly, the UCSD team was able to overcome this hurdle in their current study. When they transplanted human NPCs with growth factors into the monkeys, they found that the cells were not affected by the inhibitory environment of the injury and were able to robustly develop into nerve cells and send out axon extensions.

Large numbers of human axons (green) emerge from a lesion/graft sites. Many axons travel along the interface (indicated by arrows) between spinal cord white matter (nerve fibers covered with myelin) and spinal cord gray matter (nerves without the whitish myelin sheathing). Image courtesy of Mark Tuszynski, UC San Diego School of Medicine.

The senior scientist on the study, Dr. Mark Tuszynski, explained how their findings in a large animal model are a huge step forward for the field in a UCSD Health news release:

“While there was real progress in research using small animal models, there were also enormous uncertainties that we felt could only be addressed by progressing to models more like humans before we conduct trials with people. We discovered that the grafting methods used with rodents didn’t work in larger, non-human primates. There were critical issues of scale, immunosuppression, timing and other features of methodology that had to be altered or invented. Had we attempted human transplantation without prior large animal testing, there would have been substantial risk of clinical trial failure, not because neural stem cells failed to reach their biological potential but because of things we did not know in terms of grafting and supporting the grafted cells.”

Dr. Tuszynski is a CIRM-grantee whose earlier research involved optimizing stem cell treatments for rodent models of spinal cord injury. We’ve blogged about that research previously on the Stem Cellar here and here.

Tuszynski recently was awarded a CIRM discovery stage research grant to develop a candidate human neural stem cell line that is optimized to repair the injured spinal cord and can be used in human clinical trials. He expressed cautious optimism about the future of this treatment for spinal cord injury patients emphasizing the need for patience and more research before arriving at clinical trials:

“We seem to have overcome some major barriers, including the inhibitory nature of adult myelin against axon growth. Our work has taught us that stem cells will take a long time to mature after transplantation to an injury site, and that patience will be required when moving to humans. Still, the growth we observe from these cells is remarkable — and unlike anything I thought possible even ten years ago. There is clearly significant potential here that we hope will benefit humans with spinal cord injury.”

Related Links:

• UCSD News Release: Researchers Use Human Neural Stem Cell Grafts to Repair Spinal Cord Injuries in Monkeys
• Extra dose of patience needed for spinal cord injury stem cell therapies, rat study suggests
• Getting On Tract: Stem Cells Regenerate Injured Spinal Cord in Rats