(a) The TRE-GFP gene was introduced into hfMSCs through a lentivirus (1). The transduced TRE-GFP-hfMSCs were injected into the cerebella of 6–8-month-old SCA1 mice (2). Two weeks after the TRE-GFP-hfMSC injection, the SCA1 mice received an injection of AAV9-L7-HA-mtTA vectors expressing HA-tagged mtTA under the control of a PC-specific L7 promoter (3). (b) Schema depicting GFP expression in PCs expressing mtTA. TRE in the nucleus of TRE-GFP-hfMSCs initiated the transcription of the downstream GFP gene in the presence of mtTA, which was produced only in AAV9-L7-HA-mtTA-infected PCs. (c, d) Immunohistochemistry of cerebellar slices from SCA1 mice treated as shown in the schema (a). The mice were sacrificed 10 weeks after the hfMSC grafting. The slices were double immunolabeled for HA and GFP. HA-tagged mtTA (magenta) was efficiently expressed only in PCs, and some PCs co-expressed GFP (green). (e, f) Enlarged images of PCs immunostained for GFP. Scale bars, 200 μm (c) and 50 μm (d-f).