Harnessing the potential of neural stem cells for neural regeneration
By: Cindi M. Morshead, PhD
The discovery of neural stem cells in the adult brain in 1992 (1) had a profound impact on the way that we think about repairing the damaged nervous system. The dogma of the time was that there was no way to replace lost neurons following injury or disease, so therapies were based on the development of neuroprotective strategies—to save the cells from dying. The existence of neural stem cells, first in the adult brain and soon thereafter in the spinal cord (2), opened the door for the development of cell-based therapies to replace lost and damaged cells. Much excitement has been generated regarding two distinct stem cell based therapeutic interventions; 1) neural stem cell transplantation following their isolation and expansion in culture, and 2) stimulation of resident neural stem cells and their progeny to get them to contribute to neural repair following injury or disease. Our lab is interested in utilizing both of these strategies to promote tissue repair and functional recovery in animal models of injury including stroke and spinal cord injury.
Adult brain neural stem cells reside in a very well defined periventricular region lining the lateral ventricles (3-4). Similar to other stem cells found throughout the body, neural stem cells comprise a rare population of cells in the brain and they proliferate to give rise to a more abundant population of progenitor cells. In the adult brain, the progeny of adult brain stem cells contribute to neurogenesis, the formation of new neurons, throughout life. The newly born neurons migrate to the olfactory bulb where they integrate into the neural circuitry and play a role in the sensation of smell (5-6). These rare neural stem cells can be dissected from the periventricular region and placed in culture in the presence of growth factors, where they proliferate to form clonally derived, free-floating colonies of cells consisting of stem and progenitor cells (together termed “precursor cells”). When exposed to differentiation conditions, the cells within the colonies give rise to the mature neurons and glia that make up the central nervous system (1). The ability to isolate and expand the population of stem cells and their progeny provide an excellent source of cells for transplantation.
When considering transplantation as a strategy for cell replacement, a number of hurdles need to be overcome. For instance, the survival of transplanted cells is exceedingly low (<5%) (7). With the goal of enhancing cell survival, we have developed combinatorial strategies including the co-delivery of cells in biomaterials that promote their survival (7), and the co-delivery of cells with factors that promote neural precursor cell survival and differentiation(8). The exciting, collaborative efforts with bioengineers and neurosurgeons alike have demonstrated success in rodent models of spinal cord injury, a devastating injury with major social and economic implications, where injured rodents have shown some functional improvement and new tissue formation (8). Keeping in mind the ultimate goal of therapeutic application, ongoing studies in our lab are examining the effects of drugs already used in a clinical setting, to enhance the cell transplantation efforts. In this regard, we have demonstrated that the commonly used immunosuppressant molecule, Cyclosporin A, has direct effects on neural precursor cells and promotes their survival without modifying their proliferation kinetics (9-10). We are now testing the co-delivery of cells and Cyclosporin A in models of spinal cord injury.
Some of the most exciting work we are pursuing is the activation of resident neural precursor cells to enhance the “self-repair” mechanisms in the injured brain. Previous studies have demonstrated that injury alone is able to activate the resident neural precursor cells, inducing them to proliferate and undergo limited migration towards the injury site (11-12). However, it is clear that this response to injury is insufficient to promote repair. Towards the goal of facilitating self-repair, it is essential that we understand the mechanisms and factors that regulate neural precursor cell behaviour including their survival, proliferation, migration and differentiation. All of these behaviours need to be exploited when developing activation strategies. For instance, an expansion of the rare fraction of precursor cells will permit a larger population of cells to contribute to the repair and the neural precursor cells must migrate from their periventricular niche to the injury site where they will differentiate to replace the lost cells. To understand the regulation of these behaviours, we first look to tissue culture; what we learn from the dish, we then apply to animal models. Using this strategy, we successfully demonstrated that the same factors that promote neural precursor cell proliferation, survival and differentiation into neurons in a dish (namely epidermal growth factor, Cyclosporin A and erythropoietin), are able to stimulate cells in the adult brain to induce tissue regeneration and functional recovery following a stroke injury (13-14). These compelling findings were just the beginning and we have since applied our activation strategy to different models of stroke, in both rats and mice, with similar promising outcomes. Most important, we have discovered that a number of therapeutically relevant drugs currently used in the clinical setting are able to activate resident neural precursor cells, taking us one step closer to moving this regenerative strategy to the clinic.
These compelling findings also form the basis for some of our current work exploring the effects of aging on the regenerative capacity of the brain. Indeed, stroke is more prevalent in the aged population, making an examination of the neural precursor cell pool an important consideration when moving stem cell based therapies towards the clinic. To date, little work has been done using old-age animals and we are just now beginning to understand the fundamental biology of neural stem and progenitor cells in the old-age brain. The regenerative capacity of the aged brain was thought to be compromised by the fact that there appeared to be a significant loss of neural stem cells with age (15). Again, starting in the dish, we have learned that factors from the young brain can dramatically enhance the survival of old age stem cells, and that these “young” factors can activate the neural stem cell pool when delivered directly to the brain of old-age mice. What this tells us is that neural precursor cells in the aged brain need an extra “push” to become activated. This finding has enormous implications for developing clinically relevant neuroregenerative strategies.
To enhance the efficacy of any cell replacement approach to neural repair, it is critical that neural precursor cells migrate to the site of injury. Accordingly, we have been developing novel approaches to enhance the directed and rapid migration of neural precursors cells. Based on previous studies reporting that electric fields play a critical role in the development of the central nervous system (16), as well as cell migration during wound healing (17), we asked whether externally applied direct current electric fields would be an effective way to promote adult derived neural precursor cell migration. Using time-lapse imaging microscopy to visualize the behavior of neural precursor cells when exposed to a physiologically relevant direct current electric field, we demonstrated that undifferentiated precursors, but not differentiated cells, migrated rapidly and directly towards the cathode. Moreover, if the direction of the cathode was changed during the cell migration, the cells would quickly respond and reverse their direction. This dramatic effect of migration is observed in virtually 100% of the precursor cells and did not change the proliferation kinetics of the cells or induce cell death (18). We are currently examining this phenomenon in brain slices with the ultimate goal of applying these physiologically relevant electric fields to resident neural precursor cells in the brain to thereby promote self-repair.
Understanding the fundamental biology of neural precursor cells is an essential first step to harnessing the potential of adult neural precursor cells and ultimately utilizing these cells in regenerative medicine strategies.
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