Neural Stem Cells and Gene Therapy: Prospects for Repairing the Injured Spinal Cord

Because of the adult central nervous system's (CNS's) limited ability to repair itself following traumatic injury, spinal cord injuries can be devastating, and the prospects for recovery are generally grim. However, the observation that a few regions in the CNS continue to produce neurons throughout life offers exciting prospects for repairing an injured spinal cord. Considerable progress has been made in developing efficient methods for culturing the neural stem cells of rodents, genetically modifying them to produce therapeutic genes, and transplanting them into animal models of brain diseases. These same gene therapy and grafting methods are now being pursued for restoring function following traumatic spinal cord injury.

Stem cells are multipotential cells that have the capacity to proliferate in an undifferentiated state, to self-renew, and to give rise to all the cell types of a particular tissue.¹ In the developing embryo, neuroepithelial cells of the neural tube generate a variety of lineage-restricted precursor cells that migrate and differentiate into neurons, astrocytes, and oligodendrocytes (Figure 1).² CNS stem cells have now been discovered in the human CNS and appear to behave similarly to their rodent counterparts.³ These stem cells could potentially be used to promote neurogenesis following injury and disease.

Transplantation studies have demonstrated that neural stem cells and precursors have the capacity to alter their fate in response to the environment into which they are reintroduced and to integrate appropriately with the host tissue.⁴ Neural stem cells can be isolated from different areas and propagated for long periods in culture without losing their multipotentiality. Thus, when transplanted back into the CNS, these stem cells have the capacity to migrate, to integrate with the host tissue, and to respond to local cues for differentiation.

Neural stem cell grafts have been studied in a variety of animal models. One application involves grafting neural stem cells into a specific area of degeneration to replace a missing or deficient product. For example, in an animal model of Parkinson disease, precursor cells grafted into the striatum can replace degenerated dopamine-producing neurons in the nigrostriatal pathway and promote limited functional recovery.⁵ Grafts of neural stem cells may also be effective in cases of widespread neural degeneration. For example, in a genetic model of demyelination, both the pathology and symptoms can be reversed by transplantation of neural stem cells into the cerebral ventricles at birth.⁶ The grafted stem cells migrate extensively throughout the brain, integrate into the host cytoarchitecture, and correct the myelination process during subsequent developmental stages.

Grafted neural stem cells could potentially replace cells lost to injury, reconstitute the neuronal circuitry, and provide a relay station between the injured pathways above and below the lesion. Furthermore, intraspinal stem cell transplants can be genetically modified to provide therapeutic factors that prevent cell death and promote regeneration.

Cells to be transplanted into the injured spinal cord need to be readily obtained, easily expanded and stored, and amenable to genetic modification. They should also be able to survive for extended periods within the injury site, to integrate with host tissue, to rescue injured neurons from cell death and atrophy, to promote axonal regeneration, and, ultimately, to restore function. Neural stem cells and neural precursors theoretically fit many of the above requirements; the challenge is to demonstrate their efficacy and safety for clinical applications.

Among the most promising sources of cells for spinal cord repair are neuronal-restricted precursors (NRPs) derived from the developing spinal cord. These cells can be expanded in vitro and have the potential to differentiate into numerous neuronal types (Figure 1), including motoneurons.⁷ In the ex vivo modality of gene therapy, therapeutic genes are introduced into cultured cells that are subsequently transplanted into the CNS. Researchers in our laboratory, in collaboration with Mahendra Rao, MBBS, PhD, at the University of Utah School of Medicine, are studying the developmental potential of NRP cells and plan to use the ex vivo approach to examine the therapeutic potential of these cells grafted into a rat model of spinal cord injury. Preliminary observations demonstrate survival of grafted NRP cells in the lesion site for at least 1 month (Figure 2).

Genetically modified stem cells have not yet been grafted into the injured spinal cord; however, transplantation of brain-derived neurotropic factor–producing fibroblasts has been carried out in our laboratory using a rat spinal cord injury model of partial cervical hemisection. These grafts resulted in long distance regeneration of axons from brainstem neurons and partial recovery of motor function.⁸ Ongoing experiments with genetically modified fibroblasts are examining the effects of other growth factors, as well as adhesion molecules and growth-associated genes.

Transplantation of neural stem cells and precursor cells together with gene therapy offers great promise for spinal cord repair. Specific research goals include improving neuronal survival, promoting functional recovery through axonal regeneration, compensating for demyelination, and replacing lost cells.⁹ Many issues will need to be resolved before stem cells can be considered for use in human subjects, but continued basic research on the properties of these cells and development of appropriate animal models of repair will pave the way for successful clinical application.