Effects of Kifunensine on Production and N-glycosylation Modification of Butyrylcholinesterase in a Transgenic Rice Cell Culture Bioreactor

The production and N-glycosylation of recombinant human butyrylcholinesterase (BChE), a model highly glycosylated therapeutic protein, in a transgenic rice cell suspension culture treated with kifunensine, a strong α-mannosidase I inhibitor, was studied in a 5 L bioreactor. A media exchange was performed at day 7 of cultivation by removing spent sugar rich media (NB+S) and adding fresh sugar free (NB-S) media to induce the rice α-amylase 3D (RAmy3D) promoter to produce rice recombinant human BChE (rrBChE). Using a 1.25X concentrated sugar-free medium together with an 80% reduced working volume during the media exchange lead to a total active rrBChE production level of 79 ± 2 µg (g FW)-1 or 7.5 ± 0.4 mg L-1 in the presence of kifunensine, which is 1.5-times higher than our previous bioreactor runs using normal sugar free medium with no kifunensine treatment. Importantly, the amount of secreted active rrBChE in culture medium was enhanced in the presence of kifunensine, comprising 44% of the total active rrBChE at day 5 post-induction. Coomassie stained SDS-PAGE gel and Western blot analyses reveal different electrophoretic migration of purified rrBChE bands with and without kifunensine treatment, which is attributed to different N-glycoforms. N-Glycosylation analysis shows substantial increase of oligomannose glycans (Man5/6/7/8) in rrBChE treated with kifunensine compared to controls. However, mass transfer limitation of kifunensine is likely the major reason for incomplete inhibition of α-mannosidase I in this bioreactor study.