Theoretically, the TRE promoter should be silent in the absence of tTA, and no GFP was produced in hfMSCs even when they were transdifferentiated to PCs. only in the presence of Cre recombinase and tTA. We observed a significant emergence of GFP-expressing PCs and interneurons in symptomatic, but not non-symptomatic, SCA1 mice 2 weeks after the MSC injection. These results, together with the results obtained using age-matched wild-type mice, led us to conclude that hfMSCs have the potential to preferentially fuse with degenerating PCs and interneurons but not with healthy neurons. Introduction Injured tissues generally release cytokines and other growth factors that induce immune responses and chemotaxis of various cell types Secretin (human) [1,2]. Mesenchymal stem cells (MSCs) are attracted to the damaged tissues by the cytokines and exert therapeutic influence by releasing trophic factors [3C6] or transdifferentiating into the cell types in the tissue [7,8]. MSCs are readily obtained from various species and a variety of tissues (including bone marrow, adipose tissue, brain tissue and dental pulp) by using fluorescence-activated cell sorting followed by cultivation to isolate proliferative cells with adhesive properties. Whether the obtained cells are really MSCs is verified by the expression of standard mesenchymal surface antigens and trilineage differentiation into chondrocytes, adipocytes and osteoblasts. The biological properties of MSCs, which are defined only by cell surface antigens and trilineage differentiation potential, substantially Secretin (human) differs among laboratories, depending on the species, tissue source and age of the animal from which the cells were obtained [9C11]. As a result, published results may not be reproducible if different MSCs are used . Therefore, the biological quality of MSCs is critical for the success of cell therapy. Human fetal MSCs (hfMSCs) has intermediate properties between those of adult and embryonic stem cells. The advantages of hfMSCs over adult MSCs are their faster proliferative rate, higher differentiation capacity and longer telomeres with reduced senescence . Therefore, high quality hfMSC lines can be expanded several log-fold and may potentially be used for many patients with various diseases, such as osteogenesis imperfecta, as allogeneic transplants . Spinocerebellar ataxia type 1 (SCA1) is an inherited neurodegenerative disease caused by abnormal expansion of trinucleotide CAG repeats in the coding sequence of a causative gene [15C17]. FAM194B Affected individuals demonstrate neurodegeneration in multiple Central Nervous System (CNS) regions, Secretin (human) including the cerebellum and brain stem . There is no effective therapy for SCA1, and the current management approach is largely empirical and palliative. Recently, using a murine model of SCA1 mice, we have shown that intrathecal injection of wild-type murine MSCs significantly suppresses degeneration of cerebellar Purkinje cells (PCs) and alleviates progressive ataxia in the mice . More recently, we have demonstrated that injection of MSCs into the intrathecal space of SCA1 knock-in mice suppresses degeneration of motor neuron axons , thus suggesting the therapeutic potential of MSCs against SCA1 and potentially other neurodegenerative disorders. Although the mechanism by which intrathecally administered MSCs ameliorate murine SCA1 pathology [18,19] remains unexplored, there are several possibilities. These include the release of trophic factors from the MSCs and the replacement of damaged neurons after their transdifferentiation into neurons. However, a different intriguing phenomenon may be the fusion of the MSCs with degenerating neurons in the cerebellum, which rescues the damaged neurons. Kemp and colleagues have previously demonstrated the presence of GFP-labeled and binucleated PCs after intravenous or intracortical administration of GFP-expressing MSCs, thus indicating that MSC fusion with PCs occurred [20,21]. However, because binucleated PCs spontaneously occur in both mice  and humans , the GFP-positive binucleated PCs found by Kemp and colleagues may have been the result of a transfer of the GFP protein or the mRNA from neighboring MSCs to binucleated PCs through gap junctions or tunneling nanotubules [24,25] without fusion. Alternatively, MSCs may be transdifferentiated into PCs, because MSCs have been shown to transdifferentiate into neuronal cells in different brain regions [26C28]. Thus, it is Secretin (human) still unknown whether MSCs fuse with PCs. In this study, we sought to unequivocally demonstrate the fusion of hfMSCs with mouse PCs by using a genetic device that initiated GFP expression after hfMSC Secretin (human) fusion.