Noninvasive imaging of transplanted living functional cells transfected with a reporter estrogen receptor gene

Abstract

The transplantation of functional cells such as dopaminergic cells into damaged tissue is now clinically ongoing, but at present the population of surviving cells at the transplantation site mostly cannot be noninvasively examined. To visualize surviving transplanted functional cells using a noninvasive method, we chose the estrogen receptor ligand binding domain (ERL) as a reporter molecule and 16alpha-[18F]-fluoro-17beta-estradiol (FES) for its ligand. We used a mouse embryonic stem (ES) cell line for recipient cells as a model. To obtain ES cells that constitutively or inducibly express ERL, we transfected two types of expression vectors into EB5 parental ES cell line using the lipofection method and obtained about 30 clones for each of the two types of transfectants. Then, to examine the expression level of ERL, we performed Western blotting analysis. Ligand uptake experiments were carried out using [3H]-estradiol with or without excessive unlabeled estradiol for control cells and ERL transfectants. Each selected clone was also used for in vivo positron emission tomography (PET) imaging studies involving FES in nude mice transplanted with control cells and ERL transfectants. In some of the clones transfected with the inducible-type ERL gene, protein was expressed much higher than in the controls. However, constitutive-type ERL gene-transfected ES cells showed no protein production in spite of their gene expression activity being considerably high. All clones also expressed equal levels of the Oct-3/4 gene, a marker of pluripotency, in comparison with the parental cells. Also, the specific uptake of [3H]-estradiol was over 30 times higher in inducer-treated ERL-expressing ES cells compared to untreated control cells. Finally, by performing dynamic PET imaging, we successfully visualized ERL-expressing teratomas using FES.