Spinal cord injury typically causes permanent paralysis and is currently a condition without a cure. Could stem cell therapy provide hope?
American actor and activist Christopher Reeve will be remembered for his leading role in the 1978 blockbuster movie Superman. Sadly, he will also be remembered as a man whose tremendously active life, both on and off screen, was shattered by a catastrophic injury that left him paralysed from the neck downwards – a state in which he remained until he died in 2004.
In May 1995, during an equestrian competition, Reeve was thrown headfirst off his horse. The weight of his body was thrust through his spine, breaking two of the vertebrae in his neck and causing extensive damage to his spinal cordw1.
What happened during his accident – at the level of blood, bones, cells and molecules – to cause his life-long paralysis? And how might research into new treatments based on stem cells offer hope for people paralysed by spinal cord injury? Could it help them to regain some control over their bodies and their lives?
Your spinal cord is an information highway connecting your brain to the rest of your body (figure 1). Injuries to it are usually caused by sudden trauma, such as that sustained in sports or car accidents, and result in dislocation and / or breakage of vertebrae, which rip into the spinal cord tissue, damaging or severing axons. Sensation and motor control are lost below the level of the injury (figure 2).
Multiple cell types die at or near the site of the spinal cord injury, due tosecondary effects of the trauma, such as changes in blood supply, immune responses and an increase in free radicals and excitatory neurotransmitters (see box on the secondary effects of spinal cord injury).
Secondary effects that take place during spinal cord injury lead to death of multiple cell types, including neurons and oligodendrocytes. These secondary effects include:
On its own, the body cannot replace the cells that are lost as a result of spinal cord injury, so its function is permanently impaired. Current short-term treatments consist merely of damage limitation: surgery to relieve the pressure on the spinal cord and to stabilise the spinal column, and immobilisation (by bracing) to prevent further damage. In the long term, patients are given physiotherapy and treatment to relieve injury-related symptoms, such as pain, as well as counselling to help them cope with their new disability (figure 3).
There is currently no cure for spinal cord injury, which causes such immense physical and emotional suffering. Several groups of scientists, however, are investigating the potential of stem cells to provide a cure.
Stem cells are cells that can both differentiate into specialised cell types and self-renew to produce more stem cells. Broadly, there are two types: pluripotent stem cells, which can differentiate into any of the body’s cells, and adult stem cells, which can differentiate into only certain cell types. Pluripotent stem cells can be either extracted from embryos (embryonic stem cells) or generated in the lab (induced pluripotent stem cells) from specialised cells, such as skin cellsw2. Adult stem cells are found in various tissues, such as bone marrow.
There are two potential approaches to the use of stem cells in spinal cord injury:
In both approaches, the intention is for stem cells either to replace lost or damaged spinal cord cells, or to promote recovery indirectly. This indirect benefit may come from the stem cells themselves, or from the cells into which they differentiate.
Studies of spinal cord injury performed on animal models (mainly rodents) have shown that transplantation therapy can help recovery, although it is often difficult to know which mechanisms are responsible. In these studies, three types of stem cell have been used: pluripotent stem cells, non-neural adult stem cells and neural stems cells (from either adults or embryos).
Transplantation of pluripotent stem cells into animals has led to severe side effects, such as tumour formation. Before they are used to effectively treat spinal cord injury, therefore, these cells are treated to make them differentiate into neural progenitor cells – cells that have the potential to differentiate into particular types of specialised neural cells. One study showed that seven days after transplantation of oligodendrocyte progenitor cells into rats with spinal cord injury, axons were remyelinated and the rats were better able to move their legs (Keirstead et al., 2005).
Non-neural adult stem cells are extracted from various tissues, such as bone marrow, adipose tissue and the placenta. It is thought that these stem cells help repair the injured spinal cord indirectly. Many studies show that animals are able to better move and feel after transplantation of these cells (Parr et al., 2007).
Neural stem cells are extracted from certain parts of the nervous systems of embryos or adults. Many studies have shown that the transplanted cells differentiate into astrocytes that help new neural cells to grow (Enzmann et al., 2006; Pfeifer et al., 2006). Research published last year suggests that neural stem cells can re-programme the local inflammatory response to injury, reducing the proportion of harmful immune cells (such as macrophages), while promoting healing of the injured spinal cord (Cusimano et al., 2012). In most studies that have looked at recovery after neural stem cell transplantation, the animals were better able to move their legs.
Transplanting stem cells is risky: it requires a delicate surgical procedure, and (in the case of embryonic stem cells and neural stem cells) the immune system may reject the newly introduced cells. These risks could potentially be avoided using an alternative approach in which drugs instruct the injured spinal cord’s resident neural stem cells to promote recovery (Barnabé-Heider & Frisén, 2008).
Even if these risks can be overcome, research into this potential treatment, known as stem cell recruitment therapy, is still at an early stage and it remains to be seen whether it can aid recovery from spinal cord injury in animals.
Understanding the mechanisms involved, testing the effectiveness in animals, running clinical trials in humans – the development of a treatment is a slow and complicated process. For now at least, transplantation therapy holds the greatest promise for the treatment of spinal cord injury with stem cells. In 2010, the Californian company Geron started a clinical trialw3 based on this approach, although it was halted at an early stage for financial reasons. Currently, Balgrist University Hospital in Zurich, Switzerland, is running a trialw4 using cells derived from human brain tissue. The hope is that when transplanted into the injured spinal cord, these cells may re-establish some of the circuitry important for the network of nerves that carry information around the body.
Given the multifaceted nature of spinal cord injury, it is unlikely that any one treatment will provide a cure, but even small improvements would make a big difference to patients’ lives. Imagine if, like Christopher Reeve, you were paralysed from the neck downwards: being able to move your arms and grip with your hands could make the difference between living a dependent or independent life.
The author would like to thank Kate Doherty from Eurostemcell for her help in planning and writing the article. Thanks also go to Dr Stefano Pluchino of the Department of Clinical Neurosciences, University of Cambridge, UK, for his expert advice.
The website includes videos of people with paralysis talking about their lives.
Read more about embryonic and induced pluripotent stem cells on Nature Education’s Scitable website.
In an online video on the Eurostemcell website, ‘Stem cells – the future: an introduction to iPS cells’, leading scientists tell the story of induced pluripotent stem cells.
Eurostemcell’s stem cell toolkit is a set of downloadable, Creative Commons-licensed resources and activities suitable for a variety of educational settings. For example:
Hope Beyond Hype is a story about stem cell therapies, from discovery to therapy, in the form of an interactive comic book.
Ready or not? is a role play about spinal cord injuries, designed for classroom use.
Gebhardt P (2006) Review of A Stem Cell Story. Science in School 3: 88.