Several studies in laboratory animals have shown that transplanted neural stem cells have remarkable ability to differentiate into brain and spinal cord cells and replace dead cells. A few clinical trials have also shown that neural stem cells have a great deal of promise to treat neurological diseases.
Unfortunately, getting stem cells into the spinal cord requires injections directly into the spinal cord by highly skilled neurosurgeons using special equipment. Such a procedure is highly invasive and risky. It would be much safer and easier if intravenously administered cells could find their way to the spinal cord.
Giacomo Comi and Stefanie Corti from the University of Milan may have found a way to do just that. With their coworkers, Comi and Corti made neural stem cells from human induced pluripotent stem cells, but they selected their cells in a very unique way. They screened differentiating induced pluripotent stem cells that expressed high levels of an enzyme called aldehyde dehydrogenase, scattered light in a particular way, and expressed the cell adhesion molecule VLA4. Previous experiments showed that neural stem cells made from induced pluripotent stem cells that expressed high levels of aldehyde dehydrogenase with low side scattering of light grew well in the spinal cords of rodents with a neurodegenerative disease, differentiated into nerve cells and relieved symptoms (see Corti S., et al. Hum. Mol. Genet. 2006;15:167–187 and Corti S., et al. J. Clin. Invest. 2008;118:3316–3330). Additional work in other laboratories have shown that cells that express the VLA4 protein on their cell surfaces can enter the central nervous system from the general circulation (see Pluchino S., et al. Nature. 2005;438:266–271; and Winkler E.A., et al. Acta Neuropathol.2013;125:111–120). Thus, Comi and Corti sought to make neural stem cells from induced pluripotent stem cells that had all the qualities they had previously relied upon, but also expressed the cell adhesion molecule VLA4 to determine if such cells could enter the nervous system from the general circulation.
After establishing their desired neural stem cell lines in culture, Comi and Corti and their coworkers transplanted these cells into the spinal cords of mice that suffered from an experimental form of Amyotrophic Lateral Sclerosis (ALS). The implanted cells had previously been labeled with a green-glowing protein, and the presence of green-glowing cells in the spinal cord of the rats was confirmed. However, another set of ALS rats were given these same cells intravenously, and once again green-glowing cells were found in the spinal cord of the ALS rats. Donor cells also reached the brain and were detected in the cortical and subcortical areas of the brain. Even more remarkably, no adverse effects, including tumor formation, abnormal cell growth or inflammation, were detected in any of the recipient animals.
Neural stem cells administered in either manner increased survival in the recipient mice and reduced the loss of neurons and their connections with other cells. Also, the levels of nerve growth factors were increased in the spinal cords of transplanted animals.
Likewise, transplanted animals did not display the massive proliferation of cells known as astrocytes that is so characteristic of ALS spinal cords. As it turns out, the administered neural stem cells prevented the astrocyte explosion by activating an astrocyte cell surface protein called TRPV1. The activation of this cell surface protein prevented the astrocytes from dividing and cluttering up the spinal cord.
These remarkable experiments show, first of all, that neural stem cells can be made that express the VLA4 protein and such cells do not need to be injected into the spinal cord. Instead they can be given intravenously and they will enter the spinal cord on their own, which is a much safer mode of administration. Secondly, neural stem cells made from induced pluripotent stem cells are notorious for being able to cause tumors, but these cells, and the screening method used to select them from differentiating induced pluripotent stem cells, produced cells that apparently do not cause readily cause tumors in laboratory animals. Of course, more intense screening is required to establish the safety of this line, but the initial observations appear hopeful. Thirdly, this shows that we do not need to rip the spinal cords from 10-week old fetuses to make therapeutically useful neural stem cell lines; induced pluripotent stem cell technology will provide the means to do this.