598. Fusion of Mesenchymal Stem Cells Exclusively with “Degenerating” Cerebellar Neurons in Spinocerebellar Ataxia Type 1 Model Mice

Abstract

Mesenchymal stem cells (MSCs) have the potential to migrate to damaged tissues where they participate in tissue repair. Previous studies suggest that intravenous or intracortical injections of MSCs results in the emergence of binucleated cerebellar Purkinje cells (PCs) containing a MSC-derived marker protein in mice suggestive of a fusion event. However, transdifferentiation of MSCs into PCs or transfer of a marker protein from a MSC to a PC cannot be completely ruled-out. Here we unequivocally showed a fusion of human fetal MSCs (hMSCs) with mouse PCs by combination of tetracycline-regulated system with Cre-dependent genetic inversion switch (flip-excision; FLEx). We performed intra-cerebellar injection of adeno-associated viral vectors expressing tetracycline transactivator (tTA) and Cre recombinase into either non-symptomatic (4 week-old) or clearly symptomatic (6 – 8 month-old) spinocerebellar ataxia type 1 (SCA1) mice. Two weeks after the treatment, the mice received injection of 50,000 geneticially-engineered hMSCs that expressed GFP only in the presence of Cre recombinase and tTA, which restricted GFP expression exclusively to cerebellar cells fused with hMSCs. We observed GFP-expressing PCs and interneurons in symptomatic, but not non-symptomatic, SCA1 mice as early as 2 weeks after the MSC injection. The frequency of GFP-expressing PCs did not increase when analysed at longer time points (≈23 weeks post hMSC grafting), suggesting that most hMSC fusion events occurred within 2 weeks of delivery. Using this model, we show conclusively that hMSC display a tendency to fuse with degenerating PC and interneurons, and not with normal neurons.