Regeneration, Repair, and Developmental NeuroscienceResearch PaperIn vivo GABA release and kinetics of transgene loss in a GABAergic cell line after long-term transplantation into the rat brain

Ex vivo gene therapy uses modified cells to deliver substances into the brain. Cell line M213-2O CL-4 expresses human glutamate decarboxylase (hGAD67) by means of an Epstein-Barr virus-based plasmid. This cell line releases GABA in response to depolarizing stimuli in vitro, and after brain transplantation it modulates seizures in animal models. It is unclear if the functional effects observed can be attributed to GABA release by the grafted cells and if GABA release, in turn, is related to the kinetics of transgene permanence or loss under long-term transplantation conditions. To address these issues, two experiments were performed. The first one evaluated GABA levels in the vicinity of an intranigral transplant by microdialysis followed by high performance liquid chromatography (HPLC) quantification. GABA levels and GAD activity were higher in rats with 8-week-old transplants than in control animals, but this effect was lost in rats with 12-week-old transplants. The second experiment evaluated the number of copies of the plasmid containing the hGAD67 (GAD1) transgene by real-time PCR after transplantation into the hippocampus at the same times. A time-dependent loss of the plasmid in the transplants was observed. The mechanism of plasmid loss was explored in vitro by analyzing the effects of DNA methylation and the absence of selection pressure. The results suggest that the loss of plasmid copies from transplants under long-term conditions may be related to methylation of plasmid regions involved in its nuclear retention. Taking these data together, we propose that the reported long-term functional effects of transplants of cell line M213-2O CL-4 may not be attributed exclusively to increased GABA release in the area of the graft, but that a paracrine-like action of GABA may lead to the remodeling of neural circuits in the host.

▶Transplants of cells expressing hGAD67 release more GABA than those of control cells. ▶GAD activity levels are higher in nigral transplants of transgene-bearing cells. ▶By 12-weeks after transplantation, these differences disappear. ▶The number of plasmid copies in the grafts decrease with time. ▶CpG methylation in plasmid sequence might be involved in its loss.