Survival and differentiation within the adult mouse striatum of grafted rat pheochromocytoma cells (PC12) genetically modified to express recombinant β-NGF

Rat pheochromocytoma PC12 cells were genetically modified in vitro to express recombinant β-nerve growth factor (β-NGF) using a replication-deficient retroviral vector carrying the mouse β-NGF gene and subsequently implanted into the striatum of a mouse model of Parkinson's disease. The fate of the genetically modified PC12 cells (PC12N.8) was assessed at varying times postimplantation by studying immunoreactivity (IR) to tyrosine hydroxylase (TH) or the rat NGF receptor (NGFR). In vitro, the genetically modified PC12 cells displayed a neuronal morphology in the absence of exogenous NGF which was characterized by extensive neurite outgrowth. In addition, the genetically modified PC12 displayed a catecholaminergic phenotype in vitro as assessed by TH-IR. Following implantation into the striatum, the survival of PC12N.8 cells was limited. Surviving cells could be identified by NGFR-IR, but not by TH-IR. In addition, PC12N.8 cells with a neuronal morphology similar to that observed in vitro were only rarely observed in vivo. No tumors were observed in PC12N.8 graft recipients up to 30 days postimplantation. In contrast, intrastriatal tumors were observed in 50% of the PC12 cell recipients. These data demonstrate that PC12 cells genetically modified in vitro to synthesize β-NGF do not revert to the mitotic phenotype of the parent PC12 cell line following implantation into the adult striatum, an observation that suggests that these cells may continue to express recombinant β-NGF in vivo. The data further suggest that the genetically modified PC12 cells lose the catecholaminergic phenotype following implantation into the striatal parenchyma. However, the limited survival and neuronal differentation of the genetically modified PC12 within the striatum, together with their decreased TH-IR in vivo, suggests that these cells witll not be useful for replacement of nigrostriatal dopamine neurons.