Human neural precursor cells continue to proliferate and exhibit low cell death after transplantation to the injured rat spinal cord

During the last decade, the interest in stem and progenitor cells, and their applications in spinal cord injuries have steadily increased. However, little is known about proliferation and cell death mechanisms in these cells after transplantation to the spinal cord. The aim of the present project was to study cell turn-over, i.e. total cell number, with time course of proliferation and cell death, in human neural precursor cells (NPCs) after transplantation to the injured rat spinal cord. Immunodeficient rats were subjected to lateral clip compression injuries, transplanted with neurospheres of human forebrain-derived NPCs two weeks after lesion, and sacrificed after 6 h, 1, 3, 10, or 21 days. Cell death was assessed by quantifying human cells immunoreactive for active caspase-3 and calpain 1-dependent fodrin breakdown products (FBDP). The results showed that after an initial drop, the number of implanted cells increased over time after transplantation. Cell proliferation was substantial, with 34% of human cells being immunoreactive for proliferating cell nuclear antigen at 6 h, but which declined over the next few days. The fractions of caspase-3-, and FBDP-immunoreactive cells were remarkably low, together representing 18% of all human cells at 6 h, and rapidly decreasing the next few days. Our results show that already 10 days after spinal cord transplantation of human NPCs as intact neurospheres, the number of human cells exceeded the initially implanted, which was the result of marked cell proliferation in combination with a low rate of apoptotic and non-apoptotic cell death taking place early after transplantation.