Transplanted stem cells undergo cellular fusion in experimental brain trauma

Recent in vitro studies showed that stem cells might undergo cell fusion ¹ , however, there is no in vivo evidence for this phenomenon in the brain. Our goal was to investigate the possibility of cell fusion in a model of traumatic brain injury followed by grafting of embryonic cortical cells.

Cold lesion was applied to induce brain trauma in halothane anesthetized adult male Wistar rats as described ² . Six days later, BrDU-labelled (50 mg/kg ip 24 hours before harvesting) E14 embryonic brain cell suspension was grafted into the penumbra in 3 deposits (3 μl each; 50 000 cells/μl). Six days later the brain was removed for immunohistochemical processing. Characterization of the grafted cell suspension was performed on methanol-fixed smears, a BrDU antibody was applied to evaluate the effectiveness of labeling and an anti-Nestin antibody was used to calculate the proportion of stem cells. Investigation of the grafted cells was achieved by confocal fluorescent microscopy in triple-labeled sections. Lineage specific markers were labeled with red flurescence [glial fibrillary acidic protein (GFAP) for astrocytes, myelin basic protein (MBP) for oligodendrocytes and neurofilament 68 (NF68) for neurons]. The BrDU antibody was labeled with a green fluorescent tag and the nuclei were counterstained with the far-red fluorescent To-Pro. Nine sequential sections covering the center of the lesion were evaluated to allow for quantitation.

Nuclear labeling with BrDU was detected in 26% of the donor cell suspension, while the stem cell marker nestin labeled 34% of the cells. Six days after grafting, the surviving labeled cells were distributed on the surface of the lesion and in the penumbra. The majority of these cells did not show any morphological signs of differentiation and did not express cell-line specific markers. A small percentage of surviving BrDU positive cells (0.5%) had two distinct nuclei, one was unlabelled and one was BrDU positive, which is a sign of cell fusion. These cells were similar in appearance and size to the astrocytes in the vicinity and expressed the astrocytic marker GFAP. None of the double-nuclei positive cells were MBP or NF68 positive.

We conclude that spontaneous fusion involving grafted embryonic cells can occur in the regenerating brain tissue following traumatic injury. This is the first in vivo report that shows possible cellular fusion involving transplanted embryonic cells in the brain.