Transplantation of neural stem cells in ischemia-damaged brains is an attractive novel therapeutic strategy. Still, at present it is unclear how these cells promote behavioral recovery. The dynamic nature of the processes involved in lesion evolution and graft-induced recovery necessitate a repeated assessment of an individual subject over time. The use of contrast agent-loaded cells followed serially over time with MRI provides the opportunity to assess the relevance of the site of these cells to behavior. We here report the use of serial MRI over 1 year with concomitant behavior to evaluate for how long contrast agent-loaded cells were visible on MRI scans and if the presence of the bimodal contrast agent GRID inside transplanted cells affected their ability to promote behavioral improvements after a stroke.
Methods
Sprague-Dawley rats underwent 60 minutes of right intraluminal thread middle cerebral artery occlusion 2 weeks prior to transplantation of 100 000 neural stem cells from the MHP36 stem cell line into the contralateral hemisphere. MHP36 cells were either labeled with the bimodal contrast agent GRID or with the non-MRI enhancing fluorescent dye PKH26 prior to grafting. Animals were assessed on the bilateral asymmetry test concomitant to MRI examinations 1 week prior to transplantation and 1, 4, 12, 26, 39, and 52 weeks post-implantation. MRI examinations were conducted on a 4.7 T Varian system affording a spatial resolution of 128 μm in plane with a thickness of 600 μm.
Results
GRID-labeled cells were clearly visible on the MRI scans and by 1 week already appeared to have crossed the corpus callosum into the damaged hemisphere. At 4 weeks following transplantion, GRID-labeled transplants indicate that MHP36 delineate the area around the lesion and potentially participate in processes of recovery. No behavioral recovery, however, was observed at this time point, but previous studies suggest that between 4 and 6 weeks a definite change in behavior can be observed in these animals. Behavioural recovery therefore appears to lag behind the arrival of transplanted cells in the peri-infarct area. By 3 months following implantation, only cells in the injection tract were still visible on MRI scans suggesting that either the label inside the cells was degraded or that cells have died. Although no behavioral improvement in the removal of ‘sticky tape’ from the animals' forepaw was observed in animals with GRID-labeled transplants, a definite improvement of animals with PKH26-labeled transplants was observed.
Conclusion
These results suggest that loading of cells with contrast agents to assess their in vivo survival provides invaluable information about their migration and location over extended time periods, but will need further investigations to allow contrast-agent labeled cells to fulfill their full potential for brain repair.