Research reportNeural progenitor cell transplants promote long-term functional recovery after traumatic brain injury
Introduction
Traumatic brain injury (TBI) is a significant clinical problem in the United States, yet few effective strategies for treating it have emerged [38]. The disappointing outcomes of numerous clinical trials examining pharmacological treatments after TBI [8] may be due to a number of complex secondary events subsequent to the initial traumatic insult, including elevated inflammation, excitotoxicity, demyelination and ischemia. These secondary modes of damage have been shown to contribute to delayed cell death and prolonged functional deficits [14], [25], [33]. Thus, a long-term strategy may be required to effect a clinically significant solution. In this context of long-term treatment after TBI, strategies incorporating sustained release of trophic factors and encapsulated cell therapy have demonstrated feasibility [2], [21], but may face clinical limitations such as volume constraints and the inability of donor cells to interact with the host tissue. As an alternative to pharmacological strategies and encapsulated cell therapies, direct transplantation of neural cells into the injured brain may provide for long-term survival and integration that could mediate functional recovery through mechanisms such as bulk trophic support, cell–cell mediated repair and replacement of cells lost by injury.
The success of neural transplantation fundamentally hinges on the choice of cell type. Autologous primary cells are perhaps the most attractive choice from an immunological standpoint, but are limited with regard to availability and capacity to generate enough cells for therapy. Many cell lines have been generated to provide a virtually infinite supply of cells for research and clinical therapies [10], [13], but cells lines are associated with concerns about tumorogenesis and potential immune reactions.
Neural progenitor cells (NPCs) present an interesting approach for mediating repair and rescue of the host tissue after injury, because it is relatively easy to generate large quantities of cells in vitro, and because of their inherent ability to adapt to signals from host cells and the extracellular environment [12], [15], [18], [26], [42]. Further, the multipotential nature of NPCs may be of particular importance after trauma, where the introduction of multiple cell types may be necessary for repair and regeneration of injured tissue [20], [28], [36]. Recent studies indicate that transplanted NPCs may respond to signals present in the injured brain by differentiating into neurons and glia according to transplant location [3], [5], and it has been demonstrated that these different phenotypes can promote repair and restoration of function after TBI [23], [37]. However, the functional mechanisms underlying transplant-mediated recovery are not clear and the long-term effects of these NPC transplants following TBI remain unknown. Thus, the objective of the present study was to evaluate the long-term survival, migration, differentiation and functional significance of FGF2-responsive NPCs transplanted into a mouse model of TBI.
Section snippets
Subjects
Male C57BL.6J mice (8 weeks; Jackson, Bar Harbor, ME) were housed individually in plastic cages and kept on a 12-h light–dark cycle (lights on from 0700–1900 h). Food and water were available ad libitum. Thirty-five animals were randomly assigned to the following five groups: (1) injury, no cells (INJURY, n=6), (2) injury+vehicle (INJ/VEH, n=6), (3) injury+neural progenitor cells (INJ/NPC, n=7), (4) craniectomy, no cells (CRAN, n=6) and (5) sham operation, no cells (SHAM, n=6). Behavioral
Rotorod
Functional recovery of motor behavior mediated by the transplants was assessed using a rotorod task. Since our goal was to assess potential NPC-mediated effects for 1 year following transplantation, testing dates were spaced farther apart as the study progressed to minimize potential acclimation effects and optimize task sensitivity. Based on pilot work done in our laboratory, we chose a fixed-speed version of rotorod testing. In the present study, this protocol resulted in sensitive measures
Discussion
An important feature of any potential therapy for TBI is the improvement of functional outcomes following injury. In the present study, we first assessed motor function and showed that (1) our CCI injury model results in motor deficits sustained out to 1 year post-injury and (2) NPC transplants enhanced recovery from injury-induced motor impairment. NPC-mediated motor recovery was observed as early as 1 week and sustained up to 1 year post-transplant, indicating a long-term effect of the grafts
Acknowledgments
We would like to thank Dr. Masaru Okabe (Osaka University, Osaka, Japan) for the generous donation of EGFP transgenic mice, and Dr. Howard Reese (Dept. of Neurology, Emory University) for assistance with confocal microscopy. Funding for this study was provided by Research Gifts from Field Neurosciences Institute and Genre (to DGS), Georgia Tech/Emory Biotechnology Research Center, and a NSF GRF (to DAS).
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We wish to acknowledge that the first and second authors share equal credit for work completed in this study.